Experimental Study of the Combustion Efficiency in Multi-Element Gas

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Article Experimental Study of the Combustion Eﬃciency in Multi-Element Gas-Centered Swirl Coaxial Injectors

Seongphil Woo 1,* , Jungho Lee 1,2, Yeoungmin Han 2 and Youngbin Yoon 1,3 1 Department of Aerospace Engineering, Seoul National University, Seoul 08826, Korea; [email protected] (J.L.); [email protected] (Y.Y.) 2 Korea Aerospace Research Institute, Daejeon 34133, Korea; [email protected] 3 Institute of Advanced Aerospace Technology, Seoul National University, Seoul 08826, Korea * Correspondence: [email protected]

Received: 27 October 2020; Accepted: 16 November 2020; Published: 19 November 2020

Abstract: The effects of the momentum-flux ratio of propellant upon the combustion efficiency of a gas-centered-swirl-coaxial (GCSC) injector used in the combustion chamber of a full-scale 9-tonf staged-combustion-cycle engine were studied experimentally. In the combustion experiment, liquid oxygen was used as an oxidizer, and kerosene was used as fuel. The liquid oxygen and kerosene burned in the preburner drive the turbine of the turbopump under the oxidizer-rich hot-gas condition before flowing into the GCSC injector of the combustion chamber. The oxidizer-rich hot gas is mixed with liquid kerosene passed through combustion chamber’s cooling channel at the injector outlet. This mixture has a dimensionless momentum-flux ratio that depends upon the dispensing speed of the two fluids. Combustion tests were performed under varying mixture ratios and combustion pressures for different injector shapes and numbers of injectors, and the characteristic velocities and performance efficiencies of the combustion were compared. It was found that, for 61 gas-centered swirl-coaxial injectors, as the moment flux ratio increased from 9 to 23, the combustion-characteristic velocity increased linearly and the performance efficiency increased from 0.904 to 0.938. In addition, excellent combustion efficiency was observed when the combustion chamber had a large number of injectors at the same momentum-flux ratio.

Keywords: injector design; gas-centered swirl coaxial injectors; performance eﬃciency; gas to liquid momentum ﬂux ratio; staged-combustion-cycle rocket engines

1. Introduction Turbopump-driven liquid-rocket engines can generally be divided into gas-generator-cycle, expander-cycle, and staged-combustion-cycle types. In a gas-generator system, a portion of the propellant is burned within the gas generator to drive the turbine and is then discharged to the outside; here, the propellant supply pressure required for turbopump is relatively low, making it easy to develop and manufacture. However, such systems have the disadvantage of a propellant efficiency that is several percentage points lower than that in other systems, as the propellant burned inside the gas generator is dumped outside the turbine [1,2]. Expander-cycle systems are relatively simple, using a structure in which the vaporizing propellant is cooled in a cooling channel before entering the main combustion chamber that drives the turbine; however, it is difficult to increase the combustion-chamber pressure due to the limited degree of propellant heating in the regenerative cooling channel. Staged-combustion-cycle configurations achieve efficiency by sending the turbine-driven gas back into the main combustion chamber for reburn, although the pressure required by the pump is high and the structure is complicated [2,3]. Staged-combustion-cycle engines are the most widely used, due to their ease of re-ignition and high specific impulse. This type of engine can be further

Energies 2020, 13, 6055; doi:10.3390/en13226055 www.mdpi.com/journal/energies Energies 2020, 13, 6055 2 of 14 divided into fuel-rich-combustion and oxidizer-rich-combustion subtypes. An example of fuel-rich combustion is the RS-25 engine used on the space shuttles, with liquid hydrogen and liquid oxygen as propellants [3]. In oxidizer-rich combustion, kerosene and liquid oxygen are mainly used as propellants: examples include Russia’s RD-170, or the American AR-1; engines that use liquid methane and liquid oxygen as propellants, such as the Raptor or BE-4, are also being developed [4–6]. The gas-centered-swirl-coaxial (GCSC) injector used in oxidizer-rich staged-combustion-cycle engines has a structure that mixes the hot gas burned in the oxidizer-rich condition in the preburner with the fuel that cools the combustion chamber as it passes through the cooling channel; the mixture is then sprayed into the combustion chamber. Such injectors have mainly been developed by Russia in the past, but unfortunately, records of their characteristics are hard to find. Recently, with the development of staged-combustion-cycle engines by the United States, China, and other countries, studies of GCSC injectors have been conducted. In particular, the characteristics of the propellant mixture have been determined through spray testing, numerical analysis, and flow-instability modeling according to the shape of the injector. Jeon et al. [7] studied spray characteristics according to the gas–liquid = 2 2 momentum-flux ratio (J ρgUg/ρlUl ) and recess length through cold-flow tests. Kim et al. [8] analyzed the spray characteristics of the fluid at atmospheric and high pressure in terms of the momentum-flux ratio via spray visualization. This showed that, as the momentum-flux ratio increases, the spray angle decreases until a certain limit is reached. Kang et al. [9] experimentally observed that the size of the sprayed droplet breaks up rapidly as the gas-flow rate increases. Balance et al. [10] visualized the flame-stabilization region under high-pressure conditions using gas–liquid propellant. Using numerical analysis, the propellant mixing, flame formation, and flow instability were studied under supercritical conditions for various shapes of 2D and 3D injectors [11,12]. Park et al. [13,14] investigated the instabilities and characteristics of the flow of the gas and the liquid, respectively, for the GCSC injector; when a perturbation was applied to each fluid, the momentum of the gas was transferred to the liquid as the momentum-flux ratio increased in both cases and the gain value increased accordingly. The previous studies have focused mainly on the characteristics of injector mixing in single-injector or non-reacting flows, but it is necessary to study how the momentum-flux ratio relates to combustion efficiency in tests in which a large number of injectors are applied in actual combustion environments. In this paper, the effects of the momentum-flux ratio upon the combustion-performance of a full-scale 9-tonf staged-combustion-cycle engine combustion chamber were analyzed experimentally under variation of the shape of the injector, the mixture ratio, and the number of injectors. Among the various methods used to increase the performance of full-scale rocket engines, there are ways to increase the efficiency within the combustion chamber. This efficiency can be obtained by identifying the combustion’s characteristic velocity, which is the virtual thrust per unit mass with which the pressure in the combustion chamber acts on chamber’s neck. It can be obtained from the pressure of the combustion chamber, the combustion rate of the propellant, and the cross-sectional area of the nozzle as [1,2,15] . c∗ = PCCAt/m (1) To this end, a 9-tonf full-scale staged-combustion-cycle engine was manufactured, and combustion tests were performed according to the momentum-flux ratio of the GCSC injector in an actual combustion environment with various combustion pressures and mixture ratios. The combustion’s characteristic velocity results were calculated and analyzed.

2. Material and Methods As shown in Figure1, The injector used in the experiment is a GCSC injector from which hot gaseous oxygen (GOx) is sprayed from the center and kerosene used to cool the combustion chamber is injected through the annular hole. The injector has three shapes depending on the velocity of GOx and its outer diameter; the A-type injector is designed such that the ﬂow velocity of GOx injected from the center will be 100 m/s; the B-type is designed to have a ﬂow rate of 75 m/s at the center, such that the Energies 2020, 13, x FOR PEER REVIEW 3 of 14

Energiesthat the2020 inner, 13, 6055diameter of the injector from which GOx is sprayed is larger than in the A-type 3case. of 14 TheEnergies C-type 2020, injector13, x FOR isPEER designed REVIEW to have GOx flow rate of 100 m/s, which is the same as the A-type;3 of 14 however, as the total number of injectors used in the full-scale combustor is smaller than the A-type, that the inner diameter of the injector from which GOx is sprayed is larger than in the A-type case. innerthe injector diameter diameter of the injector is designed from whichto be GOxlarger is th sprayedan that is of larger the thanA-type in theto equalize A-type case. the Theamount C-type of The C-type injector is designed to have GOx flow rate of 100 m/s, which is the same as the A-type; injectorpropellant is designedflow injected to have into the GOx combustion ﬂow rate ofchamber. 100 m/s, All which injectors is the have same a recess as the ratio A-type; of 1.0. however, ashowever, the total as number the total of injectorsnumber of used injectors in the used full-scale in the combustor full-scale iscombustor smaller than is smaller the A-type, than thethe injectorA-type, diameterthe injector is designed diameter to is be designed larger than to be that larger of the th A-typean that to of equalize the A-type the amountto equalize of propellant the amount ﬂow of injectedpropellant into flow the combustioninjected into chamber. the combustion All injectors chamber. have All a recess injectors ratio have of 1.0. a recess ratio of 1.0.

Figure 1. Schematic of the gas-centered-swirl-coaxial Injector.

Combustor heads were manufactured with two types of arrangements according to the injectors’ Figure 1. Schematic of the gas-centered-swirl-coaxial Injector. shape as presented in TableFigure 1. 1. FigureSchematic 2 shows of the gas-centered-swirl-coaxial the combustor heads for Injector. the A-type and the B-type configurationsCombustor comprise heads were 61 injectors. manufactured A hypergolic with two injector types of is arrangements placed at the accordingcenter of the to thefaceplate injectors’ of Combustor heads were manufactured with two types of arrangements according to the injectors’ shapethe combustor as presented head, in with Table 6 main1. Figure injectors2 shows in the the first combustor row and heads12 main for injectors the A-type in the and second. the B-type In the shape as presented in Table 1. Figure 2 shows the combustor heads for the A-type and the B-type configurationsthird row, 18 baffle comprise injectors 61 injectors.are arranged A hypergolicto suppress injector combustion is placed instability. at the The center 4th of row the comprises faceplate configurations comprise 61 injectors. A hypergolic injector is placed at the center of the faceplate of of6 sections the combustor of 3 main head, injectors, with 6 mainwith injectorsindividual in thebaffle first injectors row and dividing 12 main each injectors section in theto prevent second. the combustor head, with 6 main injectors in the first row and 12 main injectors in the second. In the Incombustion the third instability. row, 18 ba fflThee injectorsC-type combustor are arranged head to has suppress a total of combustion 37 injectors: instability. a hypergolic The injector 4th row is third row, 18 baffle injectors are arranged to suppress combustion instability. The 4th row comprises comprisesagain centrally6 sections located,of 3 with main 6 injectors,main injectors with individual in the first barow,ffle 12 injectors baffle dividinginjectors eachin the section second to row prevent and 6 sections of 3 main injectors, with individual baffle injectors dividing each section to prevent combustion6 sections of instability.2 main injectors The C-type in the combustorthird row, with head individual has a total baffle of 37 injectors:injectors adividing hypergolic each injector section. is combustion instability. The C-type combustor head has a total of 37 injectors: a hypergolic injector is again centrally located, with 6 main injectors in the first row, 12 baffle injectors in the second row and again centrally located, with 6 main injectors in the first row, 12 baffle injectors in the second row and 6 sections of 2 main injectors in theTable third 1. row, Injector with head individual configuration. baffle injectors dividing each section. 6 sections of 2 main injectors in the third row, with individual baffle injectors dividing each section. Injector Dg Do Dinj Linj Ug Recess Quantity Table 1. Injector head configuration. Head [mm]Table 1. [mm]Injector head[mm] configuration. [mm] [m/s] Ratio

InjectorA-Type Head Quantity61 Dg [mm]7.4 Do [mm]10.4 Dinj [mm]0.8 Linj [mm]39.5 Ug [m100/s] Recess Ratio 1.0 Injector Dg Do Dinj Linj Ug Recess B-Type Quantity61 8.4 11.4 0.8 39.5 75 1.0 HeadA-Type 61[mm] 7.4 10.4[mm] 0.8[mm] 39.5[mm] 100[m/s] 1.0Ratio C-TypeB-Type 36 61 8.49.6 11.413.5 0.80.9 39.539.5 75100 1.0 1.0 A-Type 61 7.4 10.4 0.8 39.5 100 1.0 C-Type 36 9.6 13.5 0.9 39.5 100 1.0 B-Type 61 8.4 11.4 0.8 39.5 75 1.0 C-Type 36 9.6 13.5 0.9 39.5 100 1.0

(a) (b) (c)

Figure 2. Combustor Head: ( a)) A-type, A-type, ( b) B-type, ( c) C-type. (a) (b) (c) The combustor consists of a separate combustor head with injectors injectors and and a a combustion combustion chamber, chamber, as shown shown in in Figure Figure 3.3 .The TheFigure combustion combustion 2. Combustor chamber chamber Head: can (a) can beA-type, reused be reused (b) becauseB-type, because ( cit) C-type.is designed it is designed to have to a havebolting a bolting structure that allows separation and recombination of the combustor heads for various The combustor consists of a separate combustor head with injectors and a combustion chamber, as shown in Figure 3. The combustion chamber can be reused because it is designed to have a bolting EnergiesEnergies 20202020,, 1313,, x 6055 FOR PEER REVIEW 44 of of 14 14 structure that allows separation and recombination of the combustor heads for various injector shapes.injector Within shapes. the Within curved thepart curved of the combustor part of the head combustor through head which through the pre-burned which the GOx pre-burned flows, an oxidation-resistantGOx flows, an oxidation-resistant coating was appliedcoating to protect was applied the wall’s to protect surface the from wall’s the hot surface gas flowing from the from hot thegas turbopump, flowing from and the to turbopump, prevent ignition and to due prevent to particles ignition that due may to particles occur in that the may preburner. occur in The the preburner. The combustion chamber is designed with regeneration and film cooling, and the ZrO2 combustion chamber is designed with regeneration and film cooling, and the ZrO thermal-barrier andthermal-barrier Ni-Cr coatings and are Ni-Cr applied coatings to protect are applied the comb to protectustion chamber’s the combustion inner chamber’swall from the inner combustion wall from flame.the combustion The high-pressure flame. The kerosene high-pressure that is dischargedkerosene that from isdischarged the fuel pump from flows the fuel into pump the nozzle flows end, into coolsthe nozzle the nozzle end, coolsinner the wall,nozzle and innerpasses wall, through and passesa film-cooling through ring a film-cooling located at ringthe bottom located of at the the combustionbottom of the chamber combustion cylinder. chamber After cylinder. cooling After the wa coolingll of the the wallbottom of thecombustion-chamber bottom combustion-chamber cylinder, kerosenecylinder, passes kerosene through passes a through film-cooling a film-cooling ring located ring at located the top at of the the top combustion-chamber of the combustion-chamber cylinder andcylinder then andenters then the enters fuel manifold the fuel manifoldof the combus of thetion-chamber combustion-chamber head before head being before sprayed being into sprayed the injector.into the injector.To measure To measure the temperature the temperature of kerose of kerosenene passing passing through through the the cooling cooling channel channel of of the the combustioncombustion chamber, chamber, temperature sensors werewere installed installed in in the the manifold manifold of of the the combustion-chamber combustion-chamber headhead fuel, fuel, two two film-cooling film-cooling rings, and manifold atat the the end end of of the the nozzle. nozzle. This This combustor combustor was was designed designed toto test test the the performance characteristics characteristics of of the the GCSC GCSC injector; injector; thus, thus, the the length length of of the the combustion chamberchamber and and the the expansion expansion ratio ratio of of the the nozzle nozzle were were designed designed to to be be relatively relatively short short as as 2.3 2.3 to to fit fit the the vertical-combustion-testvertical-combustion-test facility.

(a) (b)

FigureFigure 3. 3. 9Tf9Tf combustion-chamber combustion-chamber test test model: model: ( (aa)) schematic schematic ( (bb)) actual actual shape. shape.

TheThe 9-tonf 9-tonf staged-combustion-cycle staged-combustion-cycle engine engine consis consiststs of of a a preburner, preburner, a a combustor, combustor, a a turbopump, turbopump, pneumatic-controlpneumatic-control valves,valves, and and piping, piping, as shown as shown in Figure in 4Figure. This engine4. This is designedengine is for designed the development for the developmentof preburner of and preburner combustor and combustor technology technology for staged-combustion-cycle for staged-combustion-cycle engines. engines.Unlike generalUnlike generalliquid-rocket liquid-rocket engines thatengines combine that acombine combustor a combustor and a turbopump, and a turbopump, the turbopump the and turbopump the combustor and the are combustorhere assembled are here as separate assembled structures as separate with structures a certain distance with a certain between distance them to between facilitate them disassembly to facilitate and disassemblyreplacement and of the replacement combustor. of In the addition, combustor. to protect In addition, the engine’s to protect components the engine’s from accidentscomponents that from may accidentsoccur during that the may firing occur test, during major parts the suchfiring as test, the servomajor valves parts and such the as hypergolic-propellant the servo valves and ampule the hypergolic-propellantare placed on opposite ampule sides with are placed a thick on steel opposite plate between sides with them. a thick The steel combustion-pressure-control plate between them. The combustion-pressure-controland mixture ratio control valves and of mixture the combustor ratio control are installed valves at of the the exit combustor of the first- are and installed second-stage at the exitfuel of pumps, the first- respectively. and second-stage Liquid oxygen fuel pumps, and kerosene respectively. propellants Liquid supplied oxygen and from kerosene the test facilitypropellants enter suppliedthe turbopump, from the where test facility they are enter highly the pressurized turbopump, and where supplied they are to the highly preburner. pressurizedPropellants and supplied burned toinside the preburner. the preburner Propellants become burned oxygen-rich inside hotthe gasespreburner and enterbecome the oxygen-rich gas injectors hot of gases the combustor. and enter theThe gas fuel injectors supplied of fromthe combustor. the first-stage The fuel suppli pumped of from the turbopump the first-stage passes fuel throughpump of the the combustor’s turbopump passesmixture through ratio control the valvecombustor’s and enters mixture the nozzle ratio end cont ofrol the valve combustor and toenters cool thethe combustion-chambernozzle end of the combustorwall through to coolingcool the channels combustion-chamber and film cooling. wall Thereafter, through the cooling fuel supplied channels to theand combustor film cooling. head Thereafter,is injected intothe fuel the supplied combustion to the chamber combustor through head theis injected annular into injector, the combustion along with chamber the pre-burned through thehigh-temperature annular injector, oxygen-rich along with gases. the pre-burned The engine high is started-temperature by supplying oxygen-rich high-pressure gases. The helium engine to the is started by supplying high-pressure helium to the starting turbine of the turbopump. The engine- Energies 2020, 13, 6055 5 of 14 Energies 2020, 13, x FOR PEER REVIEW 5 of 14 combustionstarting turbinetest is performed of the turbopump. by supplying The engine-combustion hypergolic propellant test is performed to the preburner by supplying and the hypergolic combustor in accordancepropellant with to the a preburner pre-set sequence. and the combustor in accordance with a pre-set sequence.

(a) (b)

FigureFigure 4. 9Tf 4. 9Tfstaged-combustion-cycle-engine staged-combustion-cycle-engine model model forfor thethe ﬁring firing test: test: (a )(a left) left view view (b) ( frontb) front view. view.

Combustion tests were conducted for pressures between 81 bar and 105 bar (i.e., approximately 10 bar Combustion tests were conducted for pressures between 81 bar and 105 bar (i.e., approximately± of the design combustion pressure of 95 bar), and for mixture ratios between 2.2 and 2.75 ±10 bar of the design combustion pressure of 95 bar), and for mixture ratios between 2.2 and 2.75 (i.e., (i.e., approximately 12% of the design mixture ratio of 2.5), as shown in Figure5. The velocities of ± approximatelythe injected ±12% gas were of the calculated design basedmixture on theratio pressure, of 2.5), temperature,as shown in density,Figure and5. The flow velocities rate of the of the injectedinjector, gas aswere obtained calculated from the based gas propertieson the pressure, of the preburner temperature, and the density, combustor and head; flow thisrate was of the done injector, to as obtainedallow analysis from the of the gas combustion properties e ffiofciency the prebur accordingner and to the the momentum-flux combustor head; ratio. this Also, was the done material to allow analysisproperties of the and combustion flow rate of keroseneefficiency flowing according into the to combustor the momentum-flux were measured 1000ratio. times Also, each the second material propertiesbased onand data flow from rate pressure of kerosene and temperature flowing sensorsinto the and combustor flowmeters were installed measured in the engine 1000 andtimes the each secondtest based facility on in data order from to calculate pressure the speedand temperat of the liquidure propellant.sensors and From flowmeters the law of installed mass conservation, in the engine and the velocitytest facility of the in oxygen-rich order to calculate hot gas, U theg, and speed the axial of the liquid liquid velocity, propellant.Ul, were From calculated, the law and of the mass thickness of the liquid film sprayed from the annular fuel injector, tl, was derived using the expression conservation, the velocity of the oxygen-rich hot gas, , and the axial liquid velocity, , were of Suyari and Lefebbre [1,16]: calculated, and the thickness of the liquid film sprayed. from the annular fuel injector, , was derived 4mg using the expression of Suyari and Lefebbre U[1,16]:g = ; (2) πρgdg 4. = ml ; (2) Ul = ; (3) πρ t (d t ) l l l − l . 0.25 tl =3.1= dlmlµl/ρl∆Pl ; . (4) (3) ( ) . =3.1( /∆) . (4)

Figure 5. Operating points of the 9-Tf staged-combustion cycle engine. Energies 2020, 13, x FOR PEER REVIEW 5 of 14 combustion test is performed by supplying hypergolic propellant to the preburner and the combustor in accordance with a pre-set sequence.

(a) (b)

Figure 4. 9Tf staged-combustion-cycle-engine model for the firing test: (a) left view (b) front view.

Combustion tests were conducted for pressures between 81 bar and 105 bar (i.e., approximately ±10 bar of the design combustion pressure of 95 bar), and for mixture ratios between 2.2 and 2.75 (i.e., approximately ±12% of the design mixture ratio of 2.5), as shown in Figure 5. The velocities of the injected gas were calculated based on the pressure, temperature, density, and flow rate of the injector, as obtained from the gas properties of the preburner and the combustor head; this was done to allow analysis of the combustion efficiency according to the momentum-flux ratio. Also, the material properties and flow rate of kerosene flowing into the combustor were measured 1000 times each second based on data from pressure and temperature sensors and flowmeters installed in the engine and the test facility in order to calculate the speed of the liquid propellant. From the law of mass conservation, the velocity of the oxygen-rich hot gas, , and the axial liquid velocity, , were calculated, and the thickness of the liquid film sprayed from the annular fuel injector, , was derived using the expression of Suyari and Lefebbre [1,16]:

4 = ; (2)

= ; (3) ( ) Energies 2020, 13, 6055 . 6 of 14 =3.1( /∆) . (4)

FigureFigure 5. Operating 5. Operating points points of of the the 9-Tf staged-combustionstaged-combustion cycle cycle engine. engine.

3. ResultsEnergies and2020, Discussion13, x FOR PEER REVIEW 6 of 14 Combustion tests were conducted from 2 s up to 114 s for combustor heads applying 3. Results and Discussion with 3 types of injectors as shown in Figure6. The tests were performed while varying the mixture ratioCombustion and combustion tests were conducted pressure from for 2 di sff uperent to 114 injector s for combustor shapes. Theheads engine-start applying with ignition 3 types of injectors as shown in Figure 6. The tests were performed while varying the mixture ratio and sequence, engine-thrust-control-performance verification, and engine-performance tuning were mainly combustion pressure for different injector shapes. The engine-start ignition sequence, engine-thrust- performed in the A-type injector tests. As shown in Figure7, five seconds after the initial start, control-performance verification, and engine-performance tuning were mainly performed in the A- the combustortype injectorpressure tests. As was shown about in Figure 89 bar 7, andfive remainedseconds after stable the initial for about start, 25the s.combustor Although pressure the engine was operatedwas about at 89 a somewhatbar and remained lower pressurestable for than about the 25 design s. Although target duethe engine to the simultaneouswas operated at research a and developmentsomewhat lower of pressure the staged-combustion-cycle than the design target du enginee to the simultaneous and combustor, research it is and meaningful development that the engineof successfullythe staged-combustion-cycle performed the engine firing testand forcombustor, the first it 30 iss meaningful or more without that the damaging engine successfully the hardware throughperformed several the combustion firing test for tests. the Infirst B-type 30 s or injector more without tests with damaging an optimized the hardware engine-starting through several ignition sequence,combustion off-design tests. operating-pointIn B-type injector teststests werewith an performed optimized for engine-starting combustion ignition pressures sequence, between off-83 bar and 105design bar operating-point and mixture tests ratios were of performed 2.2 to 2.75. for combustion Based on thepressures performance between 83 characteristics bar and 105 bar of the and mixture ratios of 2.2 to 2.75. Based on the performance characteristics of the developed staged- developed staged-combustion-cycle engine under A-type injector testing, it is possible to control combustion-cycle engine under A-type injector testing, it is possible to control the propellant-flow the propellant-flow rate, enabling experimentation under the design operating-point and off-design rate, enabling experimentation under the design operating-point and off-design operating-point test operating-pointconditions. For test the conditions. C-type injector, For thetests C-type of up to injector, 114 s were tests performed of up to under 114 s operating were performed conditions under operatingsimilar conditions to those of similar the B-type tothose tests with of the combustion B-type tests pressures with combustion from 85 to 105 pressures bar and frommixture 85 ratios to 105 bar and mixtureof 2.2 to 2.75. ratios of 2.2 to 2.75.

FigureFigure 6. 6.Firing Firing test test of of 9-Tf9-Tf staged-combustion cycle cycle engine. engine.

Figure 7. Time history of the combustion-chamber pressure of the A-type injector. Energies 2020, 13, x FOR PEER REVIEW 6 of 14

3. Results and Discussion Combustion tests were conducted from 2 s up to 114 s for combustor heads applying with 3 types of injectors as shown in Figure 6. The tests were performed while varying the mixture ratio and combustion pressure for different injector shapes. The engine-start ignition sequence, engine-thrust- control-performance verification, and engine-performance tuning were mainly performed in the A- type injector tests. As shown in Figure 7, five seconds after the initial start, the combustor pressure was about 89 bar and remained stable for about 25 s. Although the engine was operated at a somewhat lower pressure than the design target due to the simultaneous research and development of the staged-combustion-cycle engine and combustor, it is meaningful that the engine successfully performed the firing test for the first 30 s or more without damaging the hardware through several combustion tests. In B-type injector tests with an optimized engine-starting ignition sequence, off- design operating-point tests were performed for combustion pressures between 83 bar and 105 bar and mixture ratios of 2.2 to 2.75. Based on the performance characteristics of the developed staged- combustion-cycle engine under A-type injector testing, it is possible to control the propellant-flow rate, enabling experimentation under the design operating-point and off-design operating-point test conditions. For the C-type injector, tests of up to 114 s were performed under operating conditions similar to those of the B-type tests with combustion pressures from 85 to 105 bar and mixture ratios of 2.2 to 2.75.

Energies 2020, 13, 6055 7 of 14 Figure 6. Firing test of 9-Tf staged-combustion cycle engine.

Energies 2020, 13, x FOR PEER REVIEW 7 of 14

Figure 8 shows the characteristic velocity calculated from the measured combustion pressure FigureFigure 7. Time 7. Time history history of of the the combustion-chamber combustion-chamber pressure pressure of of the the A-type A-type injector. injector. and flow rate into the combustor under each test condition for various injector shapes. All data used in the calculationsFigure8 shows represent the characteristic average velocitymeasurements calculated over from theone measured second combustionin an interval pressure with and stable combustionﬂow rate pressure. into the combustor In the A-type under each case test for condition which forthe various tests were injector conducted shapes. All under data used combustion in the pressurescalculations lower representthan the averagedesign measurementspressure, the overcomb oneustion-characteristic second in an interval velocities with stable were combustion distributed at pointspressure. above In 1700 the A-type m/s. In case the for B-type which thecase, tests which were has conducted the same under number combustion of injectors, pressures the lower gas thanvelocity the design pressure, the combustion-characteristic velocities were distributed at points above 1700 m/s. is lower than in the A-type case and the characteristic velocities are distributed from 1665 to a In the B-type case, which has the same number of injectors, the gas velocity is lower than in the A-type maximum of 1690; it can be seen that the higher the gas velocity under the same combustion pressure, case and the characteristic velocities are distributed from 1665 to a maximum of 1690; it can be seen that the higherthe higher the thecharacteristic gas velocity velocity. under the In same the combustion C-type case, pressure, which the had higher the same the characteristic gas velocity velocity. as the A- type Inbut the a C-typesmaller case, number which of had inje thectors, same the gas characteristic velocity as the A-typevelocities but were a smaller distributed number ofbetween injectors, 1640 and the1675. characteristic Thus, at velocitiesconstant weregas distributedvelocity, a between greater 1640 number and 1675. of in Thus,jectors at constant translates gas velocity,into better combustiona greater characteristics. number of injectors Additionally, translates intofor all better injector combustion shapes, characteristics. the higher the Additionally, combustion-chamber for all pressure,injector the shapes, higher the the higher combustion the combustion-chamber efficiency. pressure, the higher the combustion eﬃciency.

FigureFigure 8. Characteristic 8. Characteristic velocities velocities for for different diﬀerent injectorinjector conﬁgurations. configurations.

In a full-sized engine, the momentum-flux ratio can be changed by controlling the propellant In a full-sized engine, the momentum-flux ratio can be changed by controlling the propellant mixture ratio. The characteristic velocity results for each injector shape are shown in Figure9 mixture ratio. The characteristic velocity results for each injector shape are shown in Figure 9 according to the momentum-flux ratio with uncertainty [17–19]. A-type tests were conducted accordingat momentum-flux to the momentum-flux ratios between ratio 20 with and 22.5,uncertainty with an [17–19]. average characteristicA-type tests velocitywere conducted of 1704. at momentum-flux ratios between 20 and 22.5, with an average characteristic velocity of 1704. The B- type combustor was tested at lower momentum-flux ratios due to the relatively slow gas velocity, although the total injector quantity was the same as that of the A-type combustor. The average characteristic velocity of the B-type combustor with an average momentum-flux ratio of 11.9 is 1681. The C-type injector was designed to have the same gas velocity as the A-type injector, but tests were performed at momentum-flux ratios similar to that in the B-type case due to the large fuel flow rate per injector. The average characteristic velocity for the C-type injector was for 1665 for an average momentum-flux ratio of 12. For tests of the A-type and B-type combustors with 61 GCSC injectors, the characteristic velocity tends to increase linearly in proportion to the momentum-flux ratio; however, for the C-type combustor with 37 injectors, the characteristic velocity decreases as the momentum-flux ratio increases. Energies 2020, 13, 6055 8 of 14

The B-type combustor was tested at lower momentum-flux ratios due to the relatively slow gas velocity, although the total injector quantity was the same as that of the A-type combustor. The average characteristic velocity of the B-type combustor with an average momentum-flux ratio of 11.9 is 1681. The C-type injector was designed to have the same gas velocity as the A-type injector, but tests were performed at momentum-flux ratios similar to that in the B-type case due to the large fuel flow rate per injector. The average characteristic velocity for the C-type injector was for 1665 for an average momentum-flux ratio of 12. For tests of the A-type and B-type combustors with 61 GCSC injectors, the characteristic velocity tends to increase linearly in proportion to the momentum-flux ratio; however, for the C-type combustor with 37 injectors, the characteristic velocity decreases as the momentum-flux Energies 2020ratio, 13 increases., x FOR PEER REVIEW 8 of 14

FigureFigure 9. Characteristic 9. Characteristic veloci velocityty as as function of of momentum-flux momentum-flux ratio. ratio. The characteristic velocity and efficiency results at the design pressure of the combustor, 10 bar Thehigher characteristic than the designvelocity pressure, and efficiency and 10 bar results lower thanat the the design design pressure pressure are of shownthe combustor, for each 10 bar higher thaninjector the shapedesign in pressure, Figures 10 and and 1011 .bar The lower B-type than test the results design show pressure a high characteristic are shown for velocity each injector shape inon Figures average 10 under and high-pressure11. The B-type test test conditions, results and show in design a high pressure characteristic and low velocity pressure teston average under high-pressureconditions, the maximum test conditions, characteristic and velocity in design values pressure are 12 and 13and momentum-flux low pressure ratio, test respectively, conditions, the which is the design mixture ratio. Furthermore, the higher the deviation from the design mixture maximumratio, characteristic the lower the characteristicvelocity values velocity. are The12 and characteristic 13 momentum-flux velocity reaches ratio, a maximum respectively, of about which is the design1690 mixture under the ratio. design Furthermore, pressure and designthe higher mixture the ratio. deviation The C-type from test the results design also mixture tended to ratio, the lower theincrease characteristic the combustion-characteristic velocity. The characteristic velocity along velocity with the reaches combustion a maximum pressure. of The about highest 1690 under the designcombustion-characteristic pressure and design velocity mixture was 1676ratio. under The the C-type design pressuretest results and design also tended mixture ratioto increase test the combustion-characteristicconditions, but this velocity velocity tended along to decrease with th ase momentum-fluxcombustion pressure. ratio increased. The Inhighest other words, combustion- for a constant combustion pressure, increasing the momentum-flux ratio does not necessarily affect the characteristicdirection velocity in which was the 1676 combustion-characteristic under the design velocitypressure is increased;and design and mixture it can be seenratio that test there conditions, but this velocityexists an optimum tended pressureto decrease and mixture as momentum-f ratio that yieldlux the ratio maximum increased. mixing In performance other words, depending for a constant combustionon the pressure, GCSC injector increasing shape. the momentum-flux ratio does not necessarily affect the direction in which the combustion-characteristic velocity is increased; and it can be seen that there exists an optimum pressure and mixture ratio that yield the maximum mixing performance depending on the GCSC injector shape.

(a) (b)

Figure 10. Characteristic velocity according to momentum-flux ratio: (a) B-type, (b) C-type.

The actual combustion environment has a lower combustion-characteristic velocity than the ideal situation due to factors such as the degree of mixing of the propellant, incomplete combustion, and heat loss due to heat flux. Thus, we defined the combustion-performance efficiency as ∗ = ∗ ∗ ⁄ [20]. To analyze the combustion-performance efficiency in comparison to the combustion- characteristic velocity in an ideal situation for each test condition, a comparative analysis was Energies 2020, 13, x FOR PEER REVIEW 8 of 14

Figure 9. Characteristic velocity as function of momentum-flux ratio.

The characteristic velocity and efficiency results at the design pressure of the combustor, 10 bar higher than the design pressure, and 10 bar lower than the design pressure are shown for each injector shape in Figures 10 and 11. The B-type test results show a high characteristic velocity on average under high-pressure test conditions, and in design pressure and low pressure test conditions, the maximum characteristic velocity values are 12 and 13 momentum-flux ratio, respectively, which is the design mixture ratio. Furthermore, the higher the deviation from the design mixture ratio, the lower the characteristic velocity. The characteristic velocity reaches a maximum of about 1690 under the design pressure and design mixture ratio. The C-type test results also tended to increase the combustion-characteristic velocity along with the combustion pressure. The highest combustion- characteristic velocity was 1676 under the design pressure and design mixture ratio test conditions, but this velocity tended to decrease as momentum-flux ratio increased. In other words, for a constant combustion pressure, increasing the momentum-flux ratio does not necessarily affect the direction in which the combustion-characteristic velocity is increased; and it can be seen that there exists an Energiesoptimum2020, 13pressure, 6055 and mixture ratio that yield the maximum mixing performance depending on the9 of 14 GCSC injector shape.

Energies 2020, 13, x FOR PEER REVIEW 9 of 14

performed with the values predicted using NASA chemical equilibrium with applications [21]. Hence, for both B- and C-type combustors, higher combustion pressures map to higher performance efficiencies. Additionally, unlike the combustion-characteristic velocity, which decreased as momentum-flux ratio increased, the performance efficiency tends to increase along with the momentum-flux ratio. However, in the case of a C-type combustor, the actual combustion- characteristic velocity is reduced because the increase in performance efficiency is insignificant.

Therefore, the greater the number of injectors placed on the same area of the combustor head’s surface, the greater the increase(a) in the performance efficiency of the momentum-flux(b) ratio, enhancing the combustion-characteristicFigureFigure 10. 10.Characteristic Characteristic velocity. velocityvelocity according to momentum-flux momentum-ﬂux ratio: ratio: (a ()a B-type,) B-type, (b) ( bC-type.) C-type.

The actual combustion environment has a lower combustion-characteristic velocity than the ideal situation due to factors such as the degree of mixing of the propellant, incomplete combustion,

and heat loss due to heat flux. Thus, we defined the combustion-performance efficiency as ∗ = ∗ ∗ ⁄ [20]. To analyze the combustion-performance efficiency in comparison to the combustion- characteristic velocity in an ideal situation for each test condition, a comparative analysis was

(a) (b)

FigureFigure 11. 11.Performance Performance eefficiencyﬃciency basedbased on the momentum-flux momentum-ﬂux ratio: ratio: (a ()a B-type,) B-type, (b ()b C-type.) C-type.

TheFigure actual 12 combustion presents the environment performance has efficiency a lower as combustion-characteristic a function of momentum-flux velocity ratio than for the each ideal situationinjector. due Performance to factors such efficiency as the degreefor the ofA-type mixing inje ofctor the propellant,averaged 0.934, incomplete which combustion,is acceptable andfor heata typical engine [15]. The average efficiency of the B-type injector was 0.915, which was 0.19= c lowerc than loss due to heat flux. Thus, we defined the combustion-performance efficiency as ηc∗ exp∗ / ideal∗ [20]. Tothat analyze of the the A-type, combustion-performance and the average efficiency efficiency of the C-type in comparison injector was to the 0.902, combustion-characteristic which was 0.32 lower velocitythan that in anof idealthe A-type situation injector. for each Thattest is, for condition, the A-type a comparative and B-type combustors analysis was (for performed which 61 GCSC with the valuesinjectors predicted were applied), using NASA it was chemicalfound that, equilibrium if the momentum-flux-ratio with applications value [21 ].increases, Hence, forthe bothefficiency B- and C-typeof the combustors, combustion-characteristic higher combustion velocity pressures also in mapcreases. to higher For the performance C-type combustor efficiencies. with Additionally, 37 GCSC unlikeinjectors, the combustion-characteristicthe performance efficiency increase velocity, was which small decreased compared as to momentum-flux the test results with ratio 61 increased, GCSC injectors. In terms of performance efficiency, the momentum-flux ratio and performance efficiency the performance efficiency tends to increase along with the momentum-flux ratio. However, in the case are proportional, but the number of injectors seems to affect the increase. The spray-visualization of a C-type combustor, the actual combustion-characteristic velocity is reduced because the increase in experiment in a high-pressure environment shows that, if the recess ratio is 1, the value of the spray performance efficiency is insignificant. Therefore, the greater the number of injectors placed on the contraction increases until the momentum-flux ratio is 45. The higher the speed of the injected gas, same area of the combustor head’s surface, the greater the increase in the performance efficiency of the the greater the atomization performance [8,9]. Thus, if the diameter of the oxidizer-injection hole of momentum-fluxthe A-type injector ratio, is enhancingadjusted to theincrease combustion-characteristic the momentum-flux ratio, velocity. we expect that the performance efficiencyFigure and12 presents characteristic the performance velocity will increase efficiency as aspropellant a function mixing of momentum-flux improves until certain ratio forlimits each injector.are reached. Performance Moreover, effi placingciency more for the injectors A-type in injectorthe same averaged area on the 0.934, face which plate of is the acceptable combustor for a typicalhead enginewhile [reducing15]. The averagetheir diameters efficiency and of thekeepin B-typeg the injector momentum-flux was 0.915, whichratio constant was 0.19 will lower also than thatincrease of the the A-type, combustion-characteristic and the average effi ciencyvelocity. of the C-type injector was 0.902, which was 0.32 lower than that of the A-type injector. That is, for the A-type and B-type combustors (for which 61 GCSC injectors were applied), it was found that, if the momentum-flux-ratio value increases, the efficiency of the combustion-characteristic velocity also increases. For the C-type combustor with 37 GCSC Energies 2020, 13, 6055 10 of 14

injectors, the performance efficiency increase was small compared to the test results with 61 GCSC injectors. In terms of performance efficiency, the momentum-flux ratio and performance efficiency are proportional, but the number of injectors seems to affect the increase. The spray-visualization experiment in a high-pressure environment shows that, if the recess ratio is 1, the value of the spray contraction increases until the momentum-flux ratio is 45. The higher the speed of the injected gas, the greater the atomization performance [8,9]. Thus, if the diameter of the oxidizer-injection hole of the A-type injector is adjusted to increase the momentum-flux ratio, we expect that the performance efficiency and characteristic velocity will increase as propellant mixing improves until certain limits are reached. Moreover, placing more injectors in the same area on the face plate of the combustor head while reducing their diameters and keeping the momentum-flux ratio constant will also increase the Energiescombustion-characteristic 2020, 13, x FOR PEER REVIEW velocity. 10 of 14

FigureFigure 12. 12.PerformancePerformance efficiency efficiency as as a functionfunction of of momentum-flux momentum-flux ratio. ratio. Heat Flux Heat Flux Heat flux of the combustion chamber was analyzed by dividing the combustion chamber into threeHeat sectionsflux of tothe understand combustion the chamber effect of the was momentum-flux analyzed by dividing ratio on the the combustion combustion performance chamber into threee ffisectionsciency. Fromto understand the combustor the effect fuel manifold of the momentum-flux to the first film cooling ratio wason th thee combustion head section, performance from the efficiency.first film From cooling the tocombustor the second fuel film manifold cooling was to the the cylinder first film section, cooling and was from the the head second section, film cooling from the first filmto the cooling combustion to the nozzle second end film was cooling the nozzle was section. the cylinder Heat flux section, of each and sections from wasthe second calculated film using cooling the following formula: to the combustion nozzle end was the nozzle section.. Heat flux of each sections was calculated using q = Cp m ∆T /A (5) the following formula: × × . where Cp is the specific heat capacity, m is the flow rate of kerosene flowing into the end of the nozzle, =( × ×∆)/ (5) ∆T is the temperature increase rate of kerosene in each section, and A is the area inside the combustion wherechamber Cp is [the22– 24specific]. heat capacity, is the flow rate of kerosene flowing into the end of the nozzle, ∆FigureT is the 13 temperature shows the total increase heat flux rate inside of kerosene the combustion in each chamber section, according and A is to the the combustionarea inside the 2 combustionpressure. chamber The A-type [22– has24]. a heat flux value of between 11 and 13 MW/m when the combustion 2 pressureFigure 13 is shows between the 81 total bar and heat 89 flux bar. inside The B-type the combustion indicates heat chamber flux of approximately according to the 10 MW combustion/m , 12 MW/m2 under design pressure test conditions of 95 bar, and 13–14 MW/m2 under high-pressure pressure. The A-type has a heat flux value of between 11 and 13 MW⁄ m when the combustion combustion conditions of 105 bar. The C-type has a value between 13 and 16 MW/m2 depending on pressure is between 81 bar and 89 bar. The B-type indicates heat flux of approximately 10 MW⁄ m the mixing ratio in the low pressure combustion condition, 14–19 MW/m2 at the design pressure and MW⁄ m MW⁄ m 12 17–19.5 MW under/m design2 at the high-pressurepressure test conditions combustion of condition. 95 bar, and As the13–14 combustion pressureunder high-pressure increases, combustionthe heat conditions flux increases of with105 bar. a similar The C-type slope for has all a three value types between of injectors. 13 and The 16 C-typeMW⁄ m injectors depending had on the mixingcomparatively ratio in largerthe low total pressure heat flux combustion values than condition, other types 14–19 of injectors. MW⁄ m at the design pressure and 17–19.5 MW⁄ m at the high-pressure combustion condition. As the combustion pressure increases, the heat flux increases with a similar slope for all three types of injectors. The C-type injectors had comparatively larger total heat flux values than other types of injectors.

Figure 13. Total heat flux of the combustion chamber. Energies 2020, 13, x FOR PEER REVIEW 10 of 14

Figure 12. Performance efficiency as a function of momentum-flux ratio.

Heat Flux Heat flux of the combustion chamber was analyzed by dividing the combustion chamber into three sections to understand the effect of the momentum-flux ratio on the combustion performance efficiency. From the combustor fuel manifold to the first film cooling was the head section, from the first film cooling to the second film cooling was the cylinder section, and from the second film cooling to the combustion nozzle end was the nozzle section. Heat flux of each sections was calculated using the following formula:

=( × ×∆)/ (5) where Cp is the specific heat capacity, is the flow rate of kerosene flowing into the end of the nozzle, ∆T is the temperature increase rate of kerosene in each section, and A is the area inside the combustion chamber [22–24]. Figure 13 shows the total heat flux inside the combustion chamber according to the combustion pressure. The A-type has a heat flux value of between 11 and 13 MW⁄ m when the combustion pressure is between 81 bar and 89 bar. The B-type indicates heat flux of approximately 10 MW⁄ m 12 MW⁄ m under design pressure test conditions of 95 bar, and 13–14 MW⁄ m under high-pressure combustion conditions of 105 bar. The C-type has a value between 13 and 16 MW⁄ m depending on the mixing ratio in the low pressure combustion condition, 14–19 MW⁄ m at the design pressure and 17–19.5 MW⁄ m at the high-pressure combustion condition. As the combustion pressure increases, theEnergies heat2020 flux, 13 ,increases 6055 with a similar slope for all three types of injectors. The C-type injectors11 had of 14 comparatively larger total heat flux values than other types of injectors.

Energies 2020, 13, x FOR PEER REVIEW 11 of 14 FigureFigure 13. 13. TotalTotal heat heat flux flux of of the the combustion combustion chamber. The heat flux for each combustion chamber that was divided into three sections was analyzed and shownThe heat in fluxFigures for each 14–16. combustion In the combustion chamber that head was section, divided similar into three to the sections total heat was flux, analyzed the heat and fluxshown of inthe Figures C-type 14 shows–16. In the combustionhighest value head and section, the B-type similar shows to the totalthe lowest heat flux, value. the heatWhen flux the of combustionthe C-type showspressure the is highest less than value 95 bar, and the the heat B-type flux showsincreases the with lowest a slope value. of Whenabout 0.1 the MW combustion⁄ m bar 2 forpressure all three is less injectors. than 95 In bar, thethe high-pressure heat flux increases condition with where a slope the of combustion about 0.1 MW pressure/m bar is around for all three 105 bar,injectors. the heat In theflux high-pressure value of the C-type condition decreased, where the and combustion the difference pressure between is around the heat 105 flux bar, of the the heat A- typeflux and value the of B-type the C-type decreased decreased, to within and 1 theMW di⁄ff merence. In the between cylinder the section, heat fluxthe higher of the the A-type combustion and the 2 pressureB-type decreased for all injector to within shapes, 1 MW the/ mgreater. In the the cylinder heat flux. section, The heat the flux higher of the the A-type combustion is the pressurelargest and for theall injectorheat flux shapes, of the theB and greater C types the heatare similar, flux. The but heat the flux heat of flux the A-typeincrease is according the largest to and the the pressure heat flux is theof the largest B and inC the types C-type. aresimilar, In the low but pressure the heat fluxcombus increasetion conditions accordingto near the 85 pressure bar in the is the nozzle largest section, in the theC-type. heat Influx the of low the pressure A and B combustion types is similar conditions within near the 85 range bar in of the 3–4 nozzle MW⁄ section, m, but thethe heat C-type flux is of 5–8 the 2 2 MWA and⁄ m B, typeswhich is shows similar a within greater the heat range flux of value 3–4 MW compared/m , but to the other C-type injectors. is 5–8 MWIn the/m design, which and shows low mixturea greater ratio heat of flux the value C-type compared injector, toas otherwith other injectors. injectors, In the the design heat andflux lowalso mixtureincreased ratio with of increasing the C-type pressureinjector, aswith with a otherconstant injectors, slope. theHowever, heat flux under also increased the high withmixture increasing ratio test pressure condition with where a constant the momentum-fluxslope. However, ratio under is the>13, high the heat mixture flux ratioof the test nozzle condition section where is 8 MW the⁄ momentum-flux m, which is about ratio twice is > the13, 2 valuethe heat of otherflux of injectors. the nozzle Thus, section for isthe 8 MWC-type,/m the, which heat isflux about of the twice nozzle the valuesection of is other higher injectors. than the Thus, B- typefor the in C-type, which thethe heat momentum-flux flux of the nozzle ratio section is in isa highersimilar than range. the B-typeIn particular, in which in the areas momentum-flux with large momentum-fluxratio is in a similar ratio, range. suffi Incient particular, mixing does in areas not occur with largeinside momentum-flux the combustion-chamber ratio, suffi cylinder,cient mixing and thedoes main not occurcombustion inside theoccurs combustion-chamber at the bottom of the cylinder, cylinder and and the at main the nozzles combustion of the occurs combustor, at the bottomwhich appearsof the cylinder to reduce and combustion at the nozzles efficiency. of the combustor, which appears to reduce combustion efficiency.

FigureFigure 14. 14. HeatHeat flux ﬂux of of a a head head section section in in the the combustion combustion chamber. chamber.

Figure 15. Heat flux of the cylinder section in the combustion chamber. Energies 2020, 13, x FOR PEER REVIEW 11 of 14

The heat flux for each combustion chamber that was divided into three sections was analyzed and shown in Figures 14–16. In the combustion head section, similar to the total heat flux, the heat flux of the C-type shows the highest value and the B-type shows the lowest value. When the combustion pressure is less than 95 bar, the heat flux increases with a slope of about 0.1 MW⁄ m bar for all three injectors. In the high-pressure condition where the combustion pressure is around 105 bar, the heat flux value of the C-type decreased, and the difference between the heat flux of the A- type and the B-type decreased to within 1 MW⁄ m. In the cylinder section, the higher the combustion pressure for all injector shapes, the greater the heat flux. The heat flux of the A-type is the largest and the heat flux of the B and C types are similar, but the heat flux increase according to the pressure is the largest in the C-type. In the low pressure combustion conditions near 85 bar in the nozzle section, the heat flux of the A and B types is similar within the range of 3–4 MW⁄ m, but the C-type is 5–8 MW⁄ m, which shows a greater heat flux value compared to other injectors. In the design and low mixture ratio of the C-type injector, as with other injectors, the heat flux also increased with increasing pressure with a constant slope. However, under the high mixture ratio test condition where the momentum-flux ratio is >13, the heat flux of the nozzle section is 8 MW⁄ m, which is about twice the value of other injectors. Thus, for the C-type, the heat flux of the nozzle section is higher than the B- type in which the momentum-flux ratio is in a similar range. In particular, in areas with large momentum-flux ratio, sufficient mixing does not occur inside the combustion-chamber cylinder, and the main combustion occurs at the bottom of the cylinder and at the nozzles of the combustor, which appears to reduce combustion efficiency.

Energies 2020, 13, 6055 12 of 14 Figure 14. Heat flux of a head section in the combustion chamber.

Energies 2020, 13, x FOR PEER REVIEW 12 of 14 FigureFigure 15. 15.HeatHeat flux ﬂux of ofthe the cylinder cylinder section section in thethe combustioncombustion chamber. chamber.

FigureFigure 16. 16.HeatHeat flux ﬂux of ofthe the nozzle nozzle section section in thethe combustioncombustion chamber. chamber.

The effect of the momentum-flux ratio on heat flux in the same number of injectors was analyzed The effect of the momentum-flux ratio on heat flux in the same number of injectors was analyzed by comparing the test results of the B-type under test conditions similar to the low pressure and by comparing the test results of the B-type under test conditions similar to the low pressure and high high mixture ratio conditions in which the A-type experiments were performed (Figure 17). In the mixturehead ratio section, conditions the A-type in which shows athe heat A-type flux value expe thatriments is about were 1–2 performedMW/m2 larger (Figure than 17). the In B-type, the head section,and the the A-type difference shows is minimized a heat flux under value 0.5 thatMW is/m about2 in the 1–2 cylinder MW⁄ m section. larger Moreover, than the in B-type, the nozzle and the differencesection, is heatminimized flux is equal under to 0.5 4–5 MWMW⁄/ mm2 . in The the momentum-flux cylinder section. ratio Moreover, averaged in 22 the for nozzle the A-type section, heat andflux 15is forequal the to B-type, 4–5 MW and⁄ the m. combustion-characteristic The momentum-flux ratio velocity averaged of the 22 A-type for the increased A-type by and 1.2% 15 for the B-type,compared and to the the combustion-characteristic B-type. In other words,as velocity the momentum-flux of the A-type ratio increased in the by injector 1.2% increases,compared to the B-type.efficient In mixing other words, and main as the combustion momentum-flux of propellant ratio is in obtained the injector at the increases, injector outletefficient and mixing top of and mainthe combustion combustion-chamber of propellant cylinder, is obtained thereby at increasing the injector the combustionoutlet and top efficiency of the of combustion-chamber the combustor in cylinder,proportion thereby to the increasing momentum-flux the combustion ratio. efficiency of the combustor in proportion to the momentum-flux ratio.

Figure 17. Heat flux distributions in the combustion chambers of the A- and B-type injectors.

4. Conclusions We compared and analyzed the combustion-characteristic velocities and performance efficiencies of various numbers and diameters of GCSC injectors as functions of the momentum-flux ratio. Our experiments were conducted in a 9-tonf oxidizer-rich staged-combustion-cycle engine. For this purpose, a replaceable combustion chamber was employed. Since the length of the combustion chamber and the flow rate of kerosene used for regenerative cooling were not optimized, we focused on comparing the relative performances of different combustor heads, rather than comparing absolute combustion-characteristic values. For A-type and B-type combustor heads with 61 GCSC Energies 2020, 13, x FOR PEER REVIEW 12 of 14

Figure 16. Heat flux of the nozzle section in the combustion chamber.

The effect of the momentum-flux ratio on heat flux in the same number of injectors was analyzed by comparing the test results of the B-type under test conditions similar to the low pressure and high mixture ratio conditions in which the A-type experiments were performed (Figure 17). In the head section, the A-type shows a heat flux value that is about 1–2 MW⁄ m larger than the B-type, and the difference is minimized under 0.5 MW⁄ m in the cylinder section. Moreover, in the nozzle section, heat flux is equal to 4–5 MW⁄ m. The momentum-flux ratio averaged 22 for the A-type and 15 for the B-type, and the combustion-characteristic velocity of the A-type increased by 1.2% compared to the B-type. In other words, as the momentum-flux ratio in the injector increases, efficient mixing and main combustion of propellant is obtained at the injector outlet and top of the combustion-chamber Energies 2020, 13, 6055 13 of 14 cylinder, thereby increasing the combustion efficiency of the combustor in proportion to the momentum-flux ratio.

Figure 17. HeatFigure flux 17. Heat distributions flux distributions in thein the combustion combustion chambers chambers of the A- of and the B-type A- and injectors. B-type injectors. 4. Conclusions 4. Conclusions We compared and analyzed the combustion-characteristic velocities and performance We comparedefficiencies and of analyzed various numbers the combustion-characteristicand diameters of GCSC injectors as velocitiesfunctions of the and momentum-flux performance efficiencies ratio. Our experiments were conducted in a 9-tonf oxidizer-rich staged-combustion-cycle engine. For of various numbersthis purpose, and a replaceable diameters combustion of GCSC chamber injectors was employed. as functions Since the length of of the the combustion momentum-flux ratio. Our experimentschamberwere and conducted the flow rate of in kerosene a 9-tonf used oxidizer-richfor regenerative cooling staged-combustion-cycle were not optimized, we focused engine. For this purpose, a replaceableon comparing combustion the relative performances chamber of was different employed. combustor heads, Since rather the lengththan comparing of the combustion absolute combustion-characteristic values. For A-type and B-type combustor heads with 61 GCSC chamber and the flow rate of kerosene used for regenerative cooling were not optimized, we focused on comparing the relative performances of different combustor heads, rather than comparing absolute combustion-characteristic values. For A-type and B-type combustor heads with 61 GCSC injectors, the characteristic velocity and performance efficiency increased proportionally to the momentum-flux ratio with a specific slope. Heat flux analysis shows that the larger the moment flux ratio, the efficient the mixing and combustion of propellant that occurs close to the combustion injector exit, leading to increased combustion efficiency. For all injector types used in this study, the performance efficiency tended to increase along with momentum flux ratio. However, in the case of the same momentum-flux ratio, the efficiency of the propellant mixing reduced in the case of 37 injectors, resulting in the main combustion near the nozzle of the combustion engine, which reduced the characteristic velocity. Hence, to increase the combustion efficiency, it is advantageous to increase the number of injectors per unit area of the combustor’s face plate and to increase each injector’s momentum-flux ratio. We expect that these test results will be used in the future design of GCSC injectors, and applied to the development of 9-tonf oxidizer-rich staged-combustion-cycle engines with injector-optimization designs.

Author Contributions: S.W., J.L. and Y.H. performed the experiments; S.W. analyzed the data; Y.H. supervised the experiments; S.W. writing—original draft; Y.Y. writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Korea Aerospace Research Institute FR20B00. Acknowledgments: This research was supported by the Preceding Technologies Research Program at the Korea Aerospace Research Institute and the Institute of Advanced Aerospace Technology at Seoul National University. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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