Impact of the Application of Fuel and Water Emulsion on CO and Nox Emission and Fuel Consumption in a Miniature Gas Turbine

energies

Article Impact of the Application of Fuel and Water Emulsion on CO and NOx Emission and Fuel Consumption in a Miniature Gas Turbine

Paweł Niszczota * and Marian Gieras

Department of Division of Aircraft Engines, Warsaw University of Technology, 00-661 Warsaw, Poland; [email protected] * Correspondence: [email protected]

Abstract: Miniature gas turbines (MGT) are an important part of the production of electric energy in distributed systems. Due to the growing requirements for lower emissions and the increasing prices of hydrocarbon fuels, it is becoming more and more important to enhance the efﬁciency and improve the quality of the combustion process in gas turbines. One way to reduce NOx emissions is to add water to the fuel in the form of a water-based emulsion (FWE). This article presents the research results and the analysis of the impact of the use of FWE on CO and NOx emissions as well as on fuel consumption in MGT GTM-120. Experimental tests and numerical calculations were carried out using standard fuel (DF) and FWE with water content from 3% to 12%. It was found that the use of FWE leads to a reduction in NOx and CO emissions and reduction in the consumption of basic fuel. The maximum reduction in emissions by 12.32% and 35.16% for CO and NOx, respectively, and a reduction in fuel consumption by 5.46% at the computational operating point of the gas turbine were recorded.

Citation: Niszczota, P.; Gieras, M. Keywords: miniature gas turbine; combustion zone; CO emissions; NOx emissions; water fuel Impact of the Application of Fuel and Water Emulsion on CO and NOx emulsion; fuel consumption Emission and Fuel Consumption in a Miniature Gas Turbine. Energies 2021, 14, 2224. https://doi.org/10.3390/ en14082224 1. Introduction Miniature gas turbines (MGT) are listed as one of the primary ways to produce Academic Editor: Andrea De Pascale electricity in distributed systems [1,2]. They are characterized by numerous advantages. In energy applications used in parallel with the electricity network, they increase system Received: 30 March 2021 reliability, reduce peak load or eliminate the need for a reserve margin [3]. In addition, Accepted: 13 April 2021 MGT is characterized by compact size, high power-to-weight ratio and the ability to supply Published: 16 April 2021 various fuels [4,5]. It can be an important source of energy in areas without access to the central electricity network and in hard-to-reach places [6]. Despite the undoubted Publisher’s Note: MDPI stays neutral advantages of MGT, it has disadvantages, such as relatively low efficiency [7] and high with regard to jurisdictional claims in heat loss through the housing [8]. published maps and institutional affil- Due to the high potential of MGT and their wide application, numerous studies are iations. being conducted on increasing efficiency and reducing emissions [4,5,9–13]. The works approach the problem from different perspectives and include, among others, design changes of MGT and testing the possibility of using alternative fuels. Experimental studies and numerical calculations have shown that, for example, the use of variable geometry of Copyright: © 2021 by the authors. the turbine stator blades in MGT can lead to a reduction in the specific fuel consumption Licensee MDPI, Basel, Switzerland. by 4%, an increase in the unit thrust to 5%, with a simultaneous reduction in NOx and CO This article is an open access article emissions [9]. Studies on the use of variable combustion chamber geometry have shown distributed under the terms and that by controlling the amount of air supplied to the primary zone and the dilution zone of conditions of the Creative Commons the combustion chamber, the efficiency of the turbine can be increased and the emission Attribution (CC BY) license (https:// of undesirable exhaust components can be reduced [10]. It has been shown in [11] that creativecommons.org/licenses/by/ 4.0/). the simultaneous application of the two above-mentioned modifications to the gas turbine

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can potentially lead to a twofold increase in the thermal efficiency of MGT at the design operating point. The second line of research aimed at increasing the efficiency and reducing the emission of undesirable combustion products from MGT is the use of alternative fuels as a fuel for gas turbines. In [12], the positive effect of biofuels on efficiency and emissions was demonstrated in comparison with the case in which pure Jet-A fuel was burned. For the mixture of 70% Jet-A fuel, 20% rapeseed and 10% ethanol, NOx emissions were reduced by 16%, thermal efficiency increased by 35% and CO emissions decreased. On the other hand, [13] achieved a 5% reduction in CO emissions and a 2% reduction in NOx and CO2 emissions due to the addition of 10% butanol to the Jet A-1 fuel. Studies are also being conducted on the effect of using water additive to fuel in the form of FWE. However, most of them concern the combustion of FWE in compression ignition engines, and only a few studies have been carried out on gas turbines. Tests conducted on compression ignition engines clearly show that the use of FWE leads to a reduction in NOx emissions and a lower temperature of the exhaust gases. The influence of the FWE engine power supply on CO emission is not clear. The reviews show cases of both an increase and a decrease in CO emission as a result of the use of water addition to fuel [14–19]. The same applies to research conducted on gas turbines. In [20], a decrease in NOx emissions was recorded with an increase in water content in the FWE, a practically constant temperature at the outlet of the gas turbine and an increase in CO emissions. The increase in CO emissions caused by the addition of water to the fuel was also noted in [21]. In [22], the influence of the use of FWE (up to 5% water addition) on the operating parameters and emissions of the air combustion chamber was investigated. As in [20], the reduction of NOx emissions was achieved, but also of the temperature at the outlet from the combustion chamber and the reduction of CO emissions. Experimental studies do not give a straightforward answer as to the impact of the use of FWE on CO emission and do not explain the reasons for the obtained results of measurements of the amount of carbon monoxide in exhaust gases. The aim of this work was to investigate the influence of the use of an FWE on the operating parameters and emission of a miniature gas turbine, and to explain the mechanisms that led to the obtained experimental results, regarding CO emissions in particular. Exper- imental tests were carried out on a miniature gas turbine GTM-120. Four FWE variants with a mass water content from 3% to 12% with a 2% addition of surfactant were tested and the results were compared with those obtained for the base fuel (Jet A-1). In addition, a CFD calculation was performed for each of the tested cases in order to find out about the changes in the combustion process caused by the use of an alternative fuel.

2. Materials and Methods The test stand of the miniature gas turbine GTM-120 is shown schematically in Figure1. The MGT test stand allows the measurement of temperatures and pressures in characteristic cross-sections and the composition of exhaust gases in a 50 cross-section (Figure1). In the turbine inlet section 20, static and dynamic pressure as well as total temperature were measured. The static pressure and total temperature were measured in 30 and 40 sections. Fuel consumption was measured using a differential pressure sensor which measured the pressure change exerted by the fuel column during the test. A strain gauge beam was used to measure the thrust. The temperature of the exhaust gases was measured in the 50 cross-section. For pressure measurement, analog pressure transducers with an internal ceramic diaphragm were used. The temperature was measured with NiCr-NiAl sheathed thermocouples. The fuel consumption measurement was recorded using a differential pressure sensor. All measurement data were recorded using a computer data acquisition system with the possibility of direct observation of changes in individual signals during MGT operation with a frequency of 40 Hz. The exhaust gas composition was measured using a Testo 350 analyzer equipped with electrochemical measuring sensors. In the initial stage of the turbine start-up, it is fed with propane; then, the control system switches the Energies 2021, 14, x FOR PEER REVIEW 3 of 16

was measured using a Testo 350 analyzer equipped with electrochemical measuring sensors. In the initial stage of the turbine start-up, it is fed with propane; then, the control system switches the turbine to run on liquid fuel. Originally, the primary fuel was a mixture of Jet A-1 fuel (95%) with AeroShell Turbine 500 oil (5%), which is also used to lubricate the bearings. For the purposes of the research, the factory lubrication system of two MGT gas turbine bearings was modified accordingly. Due to the fact that, in the course of the research, ingredients reducing its lubricating properties were added to the standard fuel Energies 2021, 14, 2224 mixture, it was decided to use a proprietary bearing lubrication system, completely3 of in- 15 dependent of the fuel supply system to the combustion chamber. It consists of an additional fuel tank containing a Jet A-1 mixture with 5% oil additive (DF), a valve and a pump. It has been experimentally checked that the amount of DF supplied to the bearing turbinespace through to run on the liquid additional fuel. Originally, lubrication the system primary does fuel not was change a mixture the MGT of Jet operating A-1 fuel (95%)parameters with AeroShell and the exhaust Turbine gas 500 composition. oil (5%), which is also used to lubricate the bearings.

FigureFigure 1.1.Test Test standstand for for the the miniature miniature gas gas turbine turbine GTM-120. GTM-120.

ForThe the FWE, purposes which of was the research,fed with theMGT factory during lubrication the experimental system of tests, two MGT is a heterogene- gas turbine bearingsous mixture was of modiﬁed fuel and accordingly.water. In the Dueanalyz toed the case, fact water that, inis the the dispersed course of phase the research, and the ingredientsfuel is the continuous reducing its phase. lubricating Figure properties 2a shows werea microscopic added to photo the standard of the FWE fuel mixture,with water it wasdroplets decided clearly to use visible. a proprietary Figure 2b bearing shows lubricationthe standard system, fuel mixture, completely i.e., independentJet A-1 with 5% of theoil. fuelIn the supply emulsion system production to the combustion process, a su chamber.rfactant Itwas consists used, ofwhose an additional task was to fuel create tank a containingboundary surface a Jet A-1 between mixture the with dispersed 5% oil additivewater dr (DF),oplets a and valve the and fuel a in pump. order It to has increase been experimentallythe stability of checkedthe mixture that (Figure the amount 2c) [23]. of DF Comparative supplied to studies the bearing carried space out through for various the additionalpercentages lubrication of surfactant system show does that not its changeinfluence the in MGT the amounts operating used parameters in the conducted and the exhausttests on gas the composition. measurement results can be considered negligible. Figure 2d shows a ready-to-useThe FWE, macroemulsion which was fed with MGTa water during content the of experimental 6% with a 2% tests, addition is a heterogeneous of surfactant. mixtureIn order of to fuel obtain and water.the highest In the possible analyzed repeatability case, water isof thethe dispersed burnt emulsion, phase and it was the fuelpre- ispared the continuousnot earlier than phase. 30 min Figure before2a shows the star a microscopict of each test. photo The developed of the FWE process with waterof pre- dropletsparing the clearly FWE visible.required Figure minimum2b shows energy the expenditure. standard fuel mixture, i.e., Jet A-1 with 5% oil. In the emulsion production process, a surfactant was used, whose task was to create a boundary surface between the dispersed water droplets and the fuel in order to increase the stability of the mixture (Figure2c) [ 23]. Comparative studies carried out for various percentages of surfactant show that its inﬂuence in the amounts used in the conducted tests on the measurement results can be considered negligible. Figure2d shows a ready-to-use macroemulsion with a water content of 6% with a 2% addition of surfactant. In order to obtain the highest possible repeatability of the burnt emulsion, it was prepared not earlier than 30 min before the start of each test. The developed process of preparing the FWE required minimum energy expenditure.

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(a) (b) (c) (d)

Figure 2.2. Fuel–waterFuel–water emulsion:emulsion: ( a(a)) FEW FEW microscope microscope photo, photo, scale scale bar bar corresponds corresponds to 50toµ 50m; μ (bm;) DF;(b) (DF;c) a ( surfactant;c) a surfactant; (d) FWE (d) withFWE 6%with water 6% water and 2% and surfactant. 2% surfactant.

3. Experimental Experimental Research Research Forty-two experimental trials trials were were conducte conductedd for for five five fuel fuel configurations configurations in in total. total. The tests were carried out in twotwo researchresearch series.series. The same MGT operatingoperating parameters were measured in both test series. In In the first first series, tests were carried out for fivefive fuel configurations:configurations: default default fuel fuel (DF) (DF) and four FWE variants. Six Six tests were performed for each tested variant. The The tests tests of of the first first se seriesries were carried out at the ambient pressure inin the range ofof 983983 ÷÷ 1007 hPa and atat thethe temperaturetemperature ofof +18+18 ◦°CC ÷÷ +23 °C.◦C. In the second series, tests werewere carriedcarried outout for for DF DF and and FWE FWE with with 3% 3% water water content. content. Six Six trials trials were were carried car- riedout forout each for each of the of two the variants.two variants. The testsThe te werests were carried carried out at out a pressure at a pressure of 987 of÷ 9871010 ÷ 1010hPa andhPa aand temperature a temperature of +19.8 of +19.8◦C ÷ °C+20.7 ÷ +20.7◦C. The °C. compositionThe composition of the of fuel the blendsfuel blends used usedin the in research the research is presented is presented in Table in 1Table. It was 1. decidedIt was decided to carry to out carry tests out for tests FWE for with FWE a withwater a contentwater content in the range in the of range 3% ÷ of12%, 3% ÷ as 12%, this isas thethis range is the that range allows that reductionallows reduction of NOx ofemissions NOx emissions without reducingwithout reduci the thermalng the efficiency thermal ofefficiency the gas turbine.of the gas Thus, turbine. this is Thus, the scope this isin the which scope the in application which the ofapplication the FWE of has the an FWE economic has an rationale. economic This rationale. is confirmed This is bycon- a firmedstudy carried by a study out on carried full-size out gas on turbines, full-size which gas turbines, showed thatwhich the showed range of that 5% ÷the15% range water of 5%content ÷ 15% in water the FWE content is optimal in the [ 24FWE]. Based is optima on thel [24]. tests Based carried on out the for tests 2.5% carried and 5% out water for 2.5%content and in 5% the water FWE, content it was found in the that FWE, reducing it was thefound water that content reducing to 2.5% the inwater the FWEcontent gives to 2.5%the best in the results FWE in gives terms the of reducingbest results emissions in terms and of maintainingreducing emissions efficiency and [22 maintaining]. efficiency [22]. Table 1. Percentage of ingredients in the fuel mixture (by weight). Table 1. Percentage of ingredients in the fuel mixture (by weight). Fuel Mixture Jet A-1 Oil Surfactant Water FuelDefault Mixture fuel Jet A-1 95.00 Oil 5.00 Surfactant 0 Water 0 3% Fuel–WaterDefault fuel Emulsion 95.00 90.25 5.00 4.75 0 2 0 3 3%6% Fuel–Water Fuel–Water Emulsion Emulsion 90.25 87.40 4.75 4.60 2 2 3 6 6%9% Fuel–Water Fuel–Water Emulsion Emulsion 87.40 84.55 4.60 4.45 2 2 6 9 12% Fuel–Water Emulsion 81.70 4.30 2 12 9% Fuel–Water Emulsion 84.55 4.45 2 9 12% Fuel–Water Emulsion 81.70 4.30 2 12 All the experiments in each of the two series of tests were carried out in an identical mannerAll accordingthe experiments to the procedurein each of describedthe two series below. of Thetests MGT were was carried started out in in an an automatic identical mannermode. In according the initial to phase the procedure of this process, described propane below. was The fed toMGT the combustionwas started chamberin an auto- in maticorder tomode. ensure In thatthe theinitial temperature phase of ofthis the pr combustionocess, propane chamber was fed enabled to the the combustion liquid fuel chamberto evaporate in order quickly to ensure enough. that After the temper these conditionsature of the werecombustion achieved, chamber feeding enabled to the the DF liquidcombustion fuel to chamber evaporate was quickly started, enough. and the Af gaster supply these wasconditions cut off. were The automaticachieved, start-upfeeding process is completed when the speed reaches 33 k rpm and the MGT enters the manual to the DF combustion chamber was started, and the gas supply was cut off. The auto- control mode. In the conducted tests, the speed of 40 k rpm was adopted as the first matic start-up process is completed when the speed reaches 33 k rpm and the MGT en- measuring point. After reaching the set rotational speed, the operating parameters and the ters the manual control mode. In the conducted tests, the speed of 40 k rpm was adopted composition of exhaust gases were recorded for 120 s. After this time, the engine speed

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Energies 2021, 14, 2224 as the first measuring point. After reaching the set rotational speed, the operating5 ofpa- 15 rameters and the composition of exhaust gases were recorded for 120 s. After this time, the engine speed was increased by 20 k rpm, all the way to 120 k rpm, each time leaving the MGT at the given rpm for 120 s. The accuracy of the speed control for the 40 k ÷ 100 was increased by 20 k rpm, all the way to 120 k rpm, each time leaving the MGT at the k rpm range was ±600 rpm, while, for the measuring point, it was 120 k rpm, +0 rpm ÷ given rpm for 120 s. The accuracy of the speed control for the 40 k ÷ 100 k rpm range was −10 k rpm. During each of the experimental tests, pressure and temperature courses ±600 rpm, while, for the measuring point, it was 120 k rpm, +0 rpm ÷ −10 k rpm. During were recorded in the given cross-section measurement planes. Moreover, the composi- each of the experimental tests, pressure and temperature courses were recorded in the tion of exhaust gases (NO, NO2, CO) was determined and the engine thrust and fuel given cross-section measurement planes. Moreover, the composition of exhaust gases (NO, consumption were measured. Before the commencement of each of the two test series, NO2, CO) was determined and the engine thrust and fuel consumption were measured. allBefore sensors the commencementand the test stand of each were of secured the two testagainst series, moving. all sensors This andwas the especially test stand true were of thermocouples,secured against moving.for which This even was a especiallyslight change true ofin thermocouples,position can significantly for which even change a slight the temperaturechange in position reading. can The signiﬁcantly sensors of change the exhaus the temperaturet gas analyzer, reading. before The starting sensors the oftests, the wereexhaust calibrated gas analyzer, by the before manufacturer starting the of tests,the apparatus. were calibrated The correctness by the manufacturer of indications of the of otherapparatus. sensors The was correctness checked ofby indications comparing of their other indications sensors was with checked the indications by comparing of refer- their enceindications sensors. with the indications of reference sensors.

Results of Experimental Research Figure3 3 shows shows thethe consumption consumption of of FWE FWE and and DF DF depending depending on on the the thrust thrust in in a minia-a min- iatureture gas gas turbine. turbine. The The standard standard deviation deviation for for each each datadata pointpoint inin thisthis andand otherother graphsgraphs isis denoted the the by by bars. bars. Measurement Measurement accuracy accuracy is isnot not shown shown in inthe the graphs. graphs. Detailed Detailed discus- dis- sioncussion on onthe the measurement measurement accuracy accuracy for for each each of of the the measured measured values values is is performed and presented inin [ 10[10].]. It It can can be be seen seen that that the consumptionthe consumption of FWE of withFWE increasing with increasing water content water contentin the mixture in the increasesmixture increases for all tested for all variants tested (Figure variants3a). (Figure However, 3a). afterHowever, deducting after thede- waterducting contained the water in thecontained FWE (in in the the tested FWE range(in th frome tested 3% torange 12%), from the DF3% consumptionto 12%), the DF for consumptionall measuring for points all measuring is lower than points in the is lower case of than the in unmodiﬁed the case of DF the fuel unmodified mixture withoutDF fuel mixturethe addition without of water the addition (Figure3 ofb). water (Figure 3b).

(a) (b)

Figure 3. Comparison of default fuel and FWE: (a) FWE consumption; (b) DF consumption.

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Energies 2021, 14, 2224 6 of 15 Figure 3. Comparison of default fuel and FWE: (a) FWE consumption; (b) DF consumption.

For example, at a measuring point for a speed of 80 k rpm (thrust value approx. 45 For example, at a measuring point for a speed of 80 k rpm (thrust value approx. 45 N), N), which is the design MGT operating point [25], the decrease in DF consumption with which is the design MGT operating point [25], the decrease in DF consumption with an increasean increase in water in water content content from from 3% to3% 12% to 12% in FWE in FWE is successively is successively 1.28%, 1.28%, 2.94%, 2.94%, 4.1% 4.1% and 5.46%and 5.46% in relation in relation to DF to without DF without the addition the addition of water. of water. TheThe reductionreduction inin thethe maximummaximum thrustthrust ofof thethe GTM-120GTM-120 turbine,turbine, visiblevisible inin FigureFigure3 ,3, in in thethe casecase ofof usingusing FWE,FWE, isis notnot causedcaused byby thethe negativenegative effecteffect ofof thethe additionaddition ofof waterwater toto thethe fuel.fuel. ThisThis isis duedue toto thethe factfact thatthat thethe maximummaximum volumevolume ofof liquidliquid pumpedpumped byby thethe fuelfuel pumppump isis determineddetermined forfor thethe volumevolume DF.DF. FigureFigure4 4shows shows the the positive positive effect effect of of FWE FWE on on NOx NOx emissions. emissions. The The reported reported NOx NOx emissionemission valuesvalues areare thethe sumsum ofof thethe measuredmeasured NONO andand NO2NO2 emissions.emissions. ForFor thethe maximummaximum achievedachieved thrust,thrust, aa decreasedecrease inin NOxNOx emissionsemissions byby 35.16%35.16% waswas achieved,achieved, forfor aa 12%12% waterwater contentcontent inin the the FWE. FWE. However, However, for for the the turbine turbin operatinge operating point point (80 k(80 rpm), k rpm), the decrease the decrease was 17.86%was 17.86% for the for same the same water water content content in the in emulsion. the emulsion. The reduction The reduction in NOx in emissionsNOx emissions with increasingwith increasing water water content content in the FWEin the is FWE due tois thedue extinction to the extinction of “hot spots”of “hot in spots” the primary in the combustionprimary combustion zone [26 ,zone27]. Most[26,27]. of theMost NO of inthe a NO gas in turbine a gas turbine is produced is produced according according to the thermalto the thermal Zeldovich Zeldovich mechanism mechanism [28]. The [28]. production The production of NO inof theNO primaryin the primary combustion com- zonebustion is exponentially zone is exponentiall temperature-dependent,y temperature-dependent, whereby a slightwhereby reduction a slight in temperaturereduction in intemperature the hottest in area the leads hottest to aarea signiﬁcant leads to reduction a significant in thermal reduction NO in production—resulting thermal NO produc- intion—resulting a reduction in in overalla reduction NOx in emissions. overall NOx The emissions. consequence The consequence of reducing of the reducing maximum the temperaturemaximum temperature in the combustion in the chambercombustion is a slightchamber decrease is a slight in the decrease average temperaturein the average at thetemperature outlet from at thethe combustionoutlet from th chambere combustion (Figure chamber5). (Figure 5).

FigureFigure 4.4. NOxNOx emissionsemissions asas aa functionfunction ofof thrustthrust forfor differentdifferent amountsamounts ofofwater waterin inthe theFWE. FWE.

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FigureFigure 5. 5.Combustion Combustion chamberchamber outletoutlet temperature as a fu functionnction of of thrust thrust for for different different content content of of water in the FWE. water in the FWE.

InIn Figure Figure4 ,4, it it can can be be seenseen thatthat atat thethe measuring points points 40 40 k k rpm rpm and and 60 60 k krpm rpm for for 3% 3%water water content content in inthe the few, few, the the results results deviate deviate from from the general trendtrend ofof reducingreducing NOx NOx emissionsemissions with with increasing increasing water water content content in in the the FWE. FWE. The The fact fact is is that that for for such such a a low low water water contentcontent (3%), (3%), the the temperature temperature changes changes caused caused by by the the use use of of FWE FWE for for all all measuring measuring points points werewere relatively relatively small. small. For For example, example, for for the the measurement measurement point point of of 100 100 k k rpm rpm and and 3% 3% water water contentcontent inin thethe FWE, FWE, a a temperature temperature drop drop after after the the combustion combustion chamber chamber by by 0.45% 0.45% was was recorded.recorded. Therefore, Therefore, in in order order to verifyto verify these these results, results, it was it was decided decided to conduct to conduct an additional an addi- seriestional of series experimental of experimental tests covering tests coveri cases inng which cases MGTin which was MGT fed with was DF fed and with FWE DF with and 3%FWE water with content. 3% water In thiscontent. series In of this tests, series the of position tests, the of theposition exhaust of the gas exhaust analyzer gas probe analyzer and theprobe depth and of the thermocoupledepth of the thermocouple measuring the measuring temperature the behind temperature the combustion behind chamberthe com- werebustion slightly chamber changed. were The slightly results changed. of additional The re testssults are of shownadditional in Figures tests 4are and shown5 (marked in Fig- inures the legend4 and 5 with (marked “S2”). in As the can legend be seen, with the “S2”). second As series can be of testsseen, confirmed the second the series decrease of tests in theconfirmed temperature the decrease behind the in combustionthe temperature chamber behind in the the case combustion of using FWEchamber compared in the tocase the of combustionusing FWE ofcompared DF. NOx to emissions the combustion were also of recorded DF. NOx following emissions the were general also trendrecorded for thefol- firstlowing series the of general tests; therefore, trend for the the results firstof series measuring of tests; NOx therefore, emissions the for results the two ofpreviously measuring mentionedNOx emissions measuring for the points two previously can be considered mentioned as a localmeasuring characteristic points can of turbulent be considered flow of as thea local medium, characteristic which does of turbulent not have toflow be representativeof the medium, for which this flow. does not have to be repre- sentativeThe conducted for this flow. research shows that the use of water addition to fuel in the form of an FWEThe reduces conducted CO emissionsresearch shows (Figure that6). the The use maximum of water addition reduction to in fuel CO in emission the form was of an obtainedFWE reduces for 60 CO k rpm emissions and 12% (Figure water 6). addition; The maximum it is 12.32%. reduction On the in CO other emission hand, for was 80 ob- k rpm,tained a reduction for 60 k rpm in CO and emission 12% water by addition; 7.2% was it achieved. is 12.32%. Carbon On the monoxide other hand, oxidizes for 80 k to rpm, CO2 a relativelyreduction slowly. in CO Theemission intensity by 7.2% of this was process achiev ised. positively Carbon influenced monoxide byoxidizes the increase to CO2 in rela- the particletively slowly. residence The time intensity in the of primary this process combustion is positively zone andinfluenced the increase by the in increase temperature. in the Therefore,particle residence one of the time main in the reasons primary for thecombus increasetion zone in CO and emissions the increase is areas in temperature. of reduced temperature,Therefore, one e.g., of zonesthe main close reasons to the cooledfor the wallsincrease of thein CO combustion emissions chamber, is areas whereof reduced the processtemperature, of further e.g., oxidationzones close to to CO2 the slows cooled down walls [29 of,30 the]. Duecombustion to the negative chamber, correlation where the betweenprocess of CO further and NOx oxidation emissions, to CO2 lowering slows do thewn temperature [29,30]. Due in to the the primary negative combustion correlation

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between CO and NOx emissions, lowering the temperature in the primary combustion zone,between which CO reducesand NOx NOx emissions, (Figure 4),lowering should thalsoe temperature lead to an increase in the primary in CO emissions combustion [29]. zone,Inzone, the which whichdescribed reducesreduces case, NOxNOx the emissions (Figure 44),), of shouldshould NOx an alsoalsod CO leadlead were to an reduced increase simultaneously, in in CO CO emissions emissions which [29]. [29]. is InanIn the the“apparent” describeddescribed contradiction case,case, thethe emissions to this of rule. NOx NOx It canan andd be CO CO assumed were were reduced reduced that in simultaneously, simultaneously,this case, the combustion which which is is anofan “apparent”the“apparent” FWE leads, contradictioncontradiction among others, toto thisthis to rule. the It intens can be ification assumed of that thatthe infuel-oxidant in this this case, case, the themixing combustion combustion process of the FWE leads, among others, to the intensiﬁcation of the fuel-oxidant mixing process and,of the consequently, FWE leads, among to the others,combustion to the of intens a moreification homogeneous of the fuel-oxidant mixture withoutmixing process areas of and, consequently, to the combustion of a more homogeneous mixture without areas of increasedand, consequently, (NOx emission to the increase)combustion and of lowe a morered temperature homogeneous (CO mixture emission without increase). areas of increasedincreased (NOx(NOx emissionemission increase) increase) and and lowe loweredred temperature temperature (CO (CO emission emission increase). increase).

Figure 6. CO emissions as a function of thrust for different content of water in the FWE. FigureFigure 6.6. COCO emissionsemissions asas aa function of thrust fo forr different content of of water water in in the the FWE. FWE. 4. Numerical Modeling 4.4. NumericalNumerical ModelingModeling TheThe numericalnumerical modelmodel ofof thethethe reactivereactivereactive ﬂowflowflow throughthrough the the GTM-120 GTM-120 turbine turbineturbine was waswas created createdcreated ininin orderorder toto betterbetter understand understand thethe the changeschanges changes takingtaking taking place place place in in inthe the the combustion combustion combustion process process process due due due to to tothethe the addition addition of water of water toto thethe to fuelfuel the in fuelin thethe in formform the of formof FWE. FWE. of The FWE.The model model The geometry model geometry geometry used used for for usednumer- numer- for numericalicalical calculations calculations hashas beenbeen has simplified.simplified. been simpliﬁed. ForFor ththe Fore purposes purposes the purposes of of numerical numerical of numerical calculations, calculations, calculations, the the mov- mov- the movinginging parts parts of the of engineengine the engine werewere were omitted.omitted. omitted. TheThe comp comp The computingutinguting domain domain domain starts starts startsat at the the atinlet inlet the to inletto the the com- to com- the compressorpressor diffuser diffuser and and endsends ends afterafter after thethe turbine theturbine turbine statstator statoror vanes. vanes. vanes. Moreover, Moreover, Moreover, it it was was it wasdecided decided decided to to ex- ex- to extendtendtend the the outlet outlet from from thethe the engineengine engine inin in orderorder to to avoid avoidavoid possible possiblepossible disturbances disturbancesdisturbances (Figure (Figure(Figure 7).7 ).7). This This methodmethod ofof simplifyingsimplifying geometrygeometry forfor thethe purposes purposes of ofof calculations calculationscalculations is isis consistent consistentconsistent with with [31–33]. [[31–33].31–33 ].

FigureFigure 7.7. SimpliﬁedSimplified modelmodel ofofof thethethe MGTMGT geometry.geometry.

TheThe computationcomputation gridgridgrid isisis composedcomposedcomposed ofof 2.52.5 MM cells.cecells.lls. It It consists consists ofof of tetrahedraltetrahedral tetrahedral elementselements elements withwith compaction compaction with with prismatic prismaticprismatic elements elementselements near nearnear the the the walls. walls. walls. For For aFor more a a more more accurate accurate accurate representation represen- represen- oftationtation the wallof the ﬂow, wall the flow,flow, size thethe of sizesize the ofof boundary thethe boundaryboundary layer layer cellslayer wascells cells selectedwas was selected selected (iteratively) (iteratively) (iteratively) so that so so + thethatthat dimensionless the dimensionless distance distancedistance from fromfrom the wallthethe wallwally was yy was within was within within the the range the range range appropriate appropriate appropriate for for the for turbulence model used. The maximum speed in the combustion chamber was used as a parameter with which the inﬂuence of the mesh size on the calculation results was tested.

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Reducing the number of mesh elements by 20% generates differences in the observed parameter not exceeding 1%. This method of creating a computational mesh is consistent with [27,34]. Numerical calculations were performed using ANSYS Fluent 15. The k-ε turbulence model with the Scalable Wall Function was used in the calculations. The model for heat transport through radiation was the Discrete Ordinates Model. Calculation of the combustion process was performed as a Non-Premixed Combustion model using the Steady Diffusion Flamelet Model. DF and FWE injection were modeled as a Discrete Phase Model. The thermal conditions on the walls of the combustion chamber and evaporators were modeled as coupled two-sided walls. Numerical simulations were carried out for the boundary conditions corresponding to the operating conditions of the turbine for rotational speeds of 40 k, 60 k, 80 k and 100 k rpm. The calculations were performed for the combustion of DF and FWE with a water content of 3%, 6%, 9% and 12%. Boundary conditions were adopted based on the results of experimental studies from the first series, individually for each of the twenty cases. The mass flow of air at the inlet to the engine was calculated on the basis of the knowledge of dynamic pressure, static pressure and total temperature in the inlet section. In addition, the resultant vector of the inlet air flow direction was entered in a cylindrical configuration based on the velocity triangles at the outlet of the compressor centrifugal rotor. DF and water mass expenditure were set according to the results of experimental studies (Figure3). The pressure at the outlet from the computational domain was iteratively selected in such a way that the pressure values behind the compressor and the combustion chamber corresponded to the values obtained in the experiment. The solver settings remained unchanged from those described in [27].

Results of Numerical Calculations The conducted numerical simulations show that adding water to the fuel in the form of a water–fuel emulsion reduces the maximum temperature in the area of the primary combustion chamber, which is consistent with [26,27]. This phenomenon is more intense when there is more water in the FWE (Figure8). At the same time, the presence of water in the combustion chamber enlarges the area where the temperatures exceed 1300 K. This area widens not only towards the exit from the combustion chamber in the central part of the liner, but also along the walls of the liner, which leads to an increase in the average temperature of these walls (Figure9). The maximum increase, by 11 K, in the mean wall temperature was recorded for 100 k rpm and 12% water content in the FWE. Generally, it can be concluded that with increasing water content in the FWE (within a given range), the temperature in the primary combustion zone becomes more even and, at the same time, the area in which the temperature value exceeds 1300 K increases. OH radicals are assumed to be markers of an active reaction zone and are used to identify the area where combustion takes place [35,36]. In Figure 10, it can be seen that the use of FWE in MGT leads to a reduction in the intensity of the chemical reaction in the combustion zone located near the front of the chamber. At the same time, an increased presence of OH radicals is observed in the central part of the combustion chamber, which explains the expansion of the high-temperature region towards the outlet (Figure8). The use of FWE also increases the intensity of the reaction near the liner walls, increasing their average temperature (Figure9). Energies 2021, 14, 2224 10 of 15 EnergiesEnergies 2021 2021, 14, 14, x, xFOR FOR PEER PEER REVIEW REVIEW 1010 of of 16 16

Figure 8. Temperature distribution inside the combustion chamber (60 k rpm): (a) DF; (b) 3% FWE; (c) 6% FWE; (d) 9% FigureFigure 8.8. TemperatureTemperature distributiondistribution insideinside thethe combustioncombustion chamber (60 k rpm): ( a) DF; (b) 3% FWE; (c) 6% FWE;FWE; (d) 9%9% FWE; (e) 12% FWE. FWE;FWE; ((ee)) 12%12% FWE.FWE.

(a()a ) (b(b) ) FigureFigureFigure 9.9. 9. LinerLiner Liner wallwall wall temperature:temperature: temperature: ((a a())a the)the the averageaverage average temperaturetemperature temperature ofof of outerouter outer wallwall wall andand and innerinner inner wall;wall; wall; ((b b()b) temperature)temperature temperature distributiondistribution distribution Energies 2021, 14, x FOR PEER REVIEW 11 of 16 acrossacrossacross thethe the linerliner liner wallswalls walls (3D(3D (3D numericalnumerical numerical simulation).simulation). simulation).

OHOH radicals radicals are are assumed assumed to to be be markers markers of of an an active active reaction reaction zone zone and and are are used used to to identifyidentify the the area area where where combustion combustion takes takes place place [35,36]. [35,36]. In In Figure Figure 10, 10, it it can can be be seen seen that that thethe use use of of FWE FWE in in MGT MGT leads leads to to a areduction reduction in in the the intensity intensity of of the the chemical chemical reaction reaction in in the the combustioncombustion zone zone located located near near the the front front of of the the chamber. chamber. At At the the same same time, time, an an increased increased presencepresence of of OH OH radicals radicals is is observed observed in in the the central central part part of of the the combustion combustion chamber, chamber, which which explainsexplains the the expansion expansion of of the the high-temperature high-temperature region region toward towards sthe the outlet outlet (Figure (Figure 8). 8). The The useuse of of FWE FWE also also increases increases the the intensity intensity of of th the ereaction reaction near near the the liner liner walls, walls, increasing increasing theirtheir average average temperature temperature (Figure (Figure 9). 9).

Figure 10. Contours of mass fraction of OH for 100 k rpm: (a) DF; (b) 6% FWE; (c) 12% FWE.

5. Discussion Experimental studies carried out in the miniature gas turbine GTM-120 have shown

that the addition of water to the fuel in the form of FWE leads to a simultaneous reduction in NOx and CO emissions. The reduction in NOx emissions is due to the extinction of the “hot micro-areas” in the primary combustion zone. The production of thermal NO, according to the Zeldovich theory, depends exponentially on the temperature; therefore, lowering the temperature in the areas with maximum values leads to a significant reduction in the formation of nitrogen monoxides and, as a result, contributes to the reduction of NOx emissions [20]. The reduction of NOx emissions was observed for all tested water contents in the FWE. The NOx reduction mechanism has been described in detail for the 3% water content of the FWE in [27]. The research carried out at MGT also shows that the use of FWE contributes to the reduction of CO emissions (compared to DF) for all measuring points, although, as a result of the reduction of the maximum temperature in the main part of the primary combustion zone, an increase in CO emissions could be expected (Figure 11) [37].

Figure 11. Influence of primary-zone temperature on CO and NOx emission.

Research on the impact of the use of FWE on CO emissions in various combustion engines does not give a clear answer regarding its impact on CO emissions. In compression-ignition reciprocating engines, both an increase and a decrease in CO emissions were achieved after the application of FWE [14–19]. Research conducted in gas turbines also does not clearly define the influence of the use of FWE on CO emissions. In [20,21], the negative impact of the FWE application on CO emission was noted, while in [22], the reduction of carbon monoxide was achieved. The oxidation of CO to CO2 largely proceeds according to the following reaction:

CO + OH ↔ CO2 + H, (1)

Energies 2021, 14, x FOR PEER REVIEW 11 of 16

Energies 2021, 14, 2224 11 of 15 Figure 10. Contours of mass fraction of OH for 100 k rpm: (a) DF; (b) 6% FWE; (c) 12% FWE.

5. Discussion 5. DiscussionExperimental studies carried out in the miniature gas turbine GTM-120 have shown that theExperimental addition of studies water carriedto the fuel out in in the form miniature of FWE gas leads turbine to GTM-120a simultaneous have shownreduc- thattion thein NOx addition and ofCO water emissions. to the fuelThe inreduction the form in of NOx FWE emissions leads to a simultaneousis due to the extinction reduction inof the NOx “hot and micro-areas” CO emissions. in the The primary reduction comb inustion NOx emissionszone. The production is due to the of extinctionthermal NO, of theaccording “hot micro-areas” to the Zeldovich in the theory, primary depends combustion exponentially zone. The on productionthe temperature; of thermal therefore, NO, accordinglowering the to thetemperature Zeldovich in theory, the areas depends with exponentially maximum values on the leads temperature; to a significant therefore, re- loweringduction in the the temperature formation inof the nitrogen areas with monoxide maximums and, values as a leads result, to acontributes signiﬁcant reductionto the re- induction the formation of NOx of emissions nitrogen monoxides[20]. The reduction and, as a result,of NOx contributes emissions to was the reductionobserved of for NOx all emissionstested water [20 ].contents The reduction in the FWE. of NOx The emissions NOx reduction was observed mechanism for all has tested been water described contents in indetail the FWE.for the The 3% NOxwater reduction content of mechanism the FWE in has [27]. been The described research in carried detail forout the at MGT 3% water also contentshows that of the the FWE use in of [ 27FWE]. The contributes research carried to the outreduction at MGT of also CO shows emissions that the (compared use of FWE to contributesDF) for all tomeasuring the reduction points, of COalthough, emissions as a (compared result of the to DF) reduction for all measuringof the maximum points, although,temperature as ain result the main of the part reduction of the ofprimary the maximum combustion temperature zone, an in increase the main in part CO ofemis- the primarysions could combustion be expect zone,ed (Figure an increase 11) [37]. in CO emissions could be expected (Figure 11)[37].

Figure 11. InﬂuenceInfluence ofof primary-zoneprimary-zone temperaturetemperature onon COCO andand NOxNOx emission.emission.

Research on the impactimpact ofof thethe useuse ofof FWEFWE onon COCO emissionsemissions inin variousvarious combustioncombustion engines doesdoes notnot give give a a clear clear answer answer regarding regarding its its impact impact on on CO CO emissions. emissions. In compression- In compres- ignitionsion-ignition reciprocating reciprocating engines, engines, both both an increase an increase and aand decrease a decrease in CO in emissions CO emissions were achievedwere achieved after theafter application the application of FWE of FWE [14–19 [14–19].]. Research Research conducted conducted in gas in turbines gas turbines also doesalso does not clearlynot clearly deﬁne define the inﬂuencethe influence of the of usethe ofuse FWE of FWE on CO on emissions.CO emissions. In [20 In, 21[20,21],], the negativethe negative impact impact of theof the FWE FWE application application on on CO CO emission emission was was noted, noted, while while in in [[22],22], thethe reduction ofof carboncarbon monoxide monoxide was was achieved. achieved. The The oxidation oxidation of COof CO to CO2 to CO2 largely largely proceeds pro- accordingceeds according to the followingto the following reaction: reaction:

COCO + OH+ OH↔ ↔CO2 CO2 + + H, H, (1)(1)

Reaction (1) at the temperature above 1270 K is of dominant importance in the oxidation of CO [29]. Extension of the area in the combustion chamber of the tested MTE, where the temperature exceeds 1300 K, leads to an increase in the residence time of CO particles in this area and thus extends the duration of this reaction. In the studied range of changes in the water content in the fuel, this area increases with the increase in the water content in the FWE (Figure8), and, therefore, despite the reduction of the maximum temperature in the primary combustion zone, the degree of CO oxidation taking place in accordance with reaction (1) increases. At lower temperatures, the dominant role in the oxidation of CO to CO2 is played by the reaction [29]:

CO + H2O ↔ CO2 + H2, (2)

The addition of water to the fuel increases the amount of H2O molecules in the combustion zone, especially in the middle and rear parts of the combustion chamber Energies 2021, 14, x FOR PEER REVIEW 12 of 16

Reaction (1) at the temperature above 1270 K is of dominant importance in the oxidation of CO [29]. Extension of the area in the combustion chamber of the tested MTE, where the temperature exceeds 1300 K, leads to an increase in the residence time of CO particles in this area and thus extends the duration of this reaction. In the studied range of changes in the water content in the fuel, this area increases with the increase in the water content in the FWE (Figure 8), and, therefore, despite the reduction of the maximum temperature in the primary combustion zone, the degree of CO oxidation taking place in accordance with reaction (1) increases. At lower temperatures, the dominant role in the oxidation of CO to CO2 is played by the reaction [29]:

Energies 2021, 14, 2224 CO + H2O ↔ CO2 + H2, 12 of(2) 15

The addition of water to the fuel increases the amount of H2O molecules in the combustion zone, especially in the middle and rear parts of the combustion chamber (Figure (Figure12), creating 12), creatingfavorable favorable conditions conditions for the oxid foration the oxidationprocess in processthese zones, in these in accordance zones, in withaccordance reaction with (2). reaction (2).

Figure 12. Contours of mass fraction of H2O for 80 k rpm: (a) DF; (b) 6% FWE;FWE; ((c)) 12%12% FWE.FWE.

The contact contact of of CO CO particles particles with with the the cool cool wa wallslls of ofthe the liner liner causes causes the theprocess process of ox- of idationoxidation of CO ofCO to CO2 to CO2 to freeze. to freeze. This is This dueis to due the reduction to the reduction in the enthalpy in the enthalpy of the medium of the whilemedium staying while near staying the nearcool walls. the cool This walls. effect This is observed effect is observed in both reciprocating in both reciprocating engines [30]engines and [gas30] andturbines gas turbines[38]. [38]. The performed numerical calculations showed a tendency to increase the averageaverage temperature of of the the liner liner walls walls of of the the GTM-120 GTM-120 turbine turbine with with increasing increasing water water content content in thein the FWE FWE (Figure (Figure 9). 9The). The maximum maximum temperature temperature increase increase in relation in relation to the to case the in case which in whichthe turbine the turbine was powered was powered by DF by was DF was11 K. 11 According K. According to to[38], [38 ],an an increase increase in in the the wall temperature maymay also also have have a certaina certain effect effect on theon reductionthe reduction of CO of emissions. CO emissions. The conducted The con- ductednumerical numerical simulations simulations also show also that show the that use the of FWEuse of in FWE MGT in leads MGT to leads a reduction to a reduction in the inmaximum the maximum temperature temperature in local in micro-zones, local micro-zones, which favors which the favors formation the formation of a more of uniform a more uniformtemperature temperature distribution distribution and expansion and expansion of the area of with the area increased with temperatureincreased temperature(Figure8). (FigureLowering 8). theLowering local maximum the local combustionmaximum combustion temperature temperature results in a results reduction in a inreduction the rate inof thethe chemicalrate of reaction,the chemical the rate reaction, of which the is exponentiallyrate of which temperature-dependent. is exponentially tempera- The ture-dependent.increased population The ofincreased OH radicals population in the central of OH zone radicals of the combustionin the central chamber, zone shownof the combustionin Figure 10 ,chamber, correctly shown correlates in Figure with the 10, temperaturecorrectly correlates distribution. with the The temperature decrease in dis- the tribution.intensity ofThe the decrease reaction in in the the intensity case of using of the FWE reaction compared in the case to DF of is using also observedFWE compared in the tostudies DF is of also burning observed single in emulsionthe studies droplets of burning [39]. single Carbon emulsion monoxide droplets in the [39]. combustion Carbon chamber of a gas turbine is formed in areas with local “enrichment” of the mixture. One of monoxide in the combustion chamber of a gas turbine is formed in areas with local “en- the reasons for such spots is that the fuel is applied to the combustion chamber in a way richment” of the mixture. One of the reasons for such spots is that the fuel is applied to that prevents the fuel from mixing quickly enough with the oxidant. In order to avoid the combustion chamber in a way that prevents the fuel from mixing quickly enough such spots, it is necessary to increase the fragmentation of the droplets injected into the with the oxidant. In order to avoid such spots, it is necessary to increase the fragmenta- combustion chamber and allow them to premix with the oxidant. In compression-ignition tion of the droplets injected into the combustion chamber and allow them to premix with engines, a reduction in CO emissions is observed with increasing fuel injection pressure [40]. the oxidant. In compression-ignition engines, a reduction in CO emissions is observed Increasing the injection pressure leads to a reduction in the Sauter mean diameter (SMD) of fuel droplets and, as a consequence, it increases the homogenization of the fuel and oxidant mixture and leads to a reduction in CO emissions [41]. Combustion FWE is accompanied by micro-explosion and pufﬁng phenomena, which lead to increased atomization of fuel droplets [42–45]. The occurrence of these phenomena results in better mixing of the burst fuel droplets with air, which reduces the occurrence of locally rich zones. In addition, the broken fuel droplets are characterized by a larger evaporation surface, which should shorten the combustion process. In order to illustrate this process, an experiment was carried out in which a droplet of Jet-A1 fuel emulsion with water with a diameter of approx. 1 mm, suspended on a quartz needle, was quickly immersed in a stream of ﬂowing hot exhaust gas at a temperature of approx. 1200 K. The course of the droplet combustion process is shown in Figure 13. It can be seen that the Energies 2021, 14, x FOR PEER REVIEW 13 of 16

with increasing fuel injection pressure [40]. Increasing the injection pressure leads to a reduction in the Sauter mean diameter (SMD) of fuel droplets and, as a consequence, it increases the homogenization of the fuel and oxidant mixture and leads to a reduction in CO emissions [41]. Combustion FWE is accompanied by micro-explosion and puffing phenomena, which lead to increased atomization of fuel droplets [42–45]. The occurrence of these phenomena results in better mixing of the burst fuel droplets with air, which reduces the occurrence of locally rich zones. In addition, the broken fuel droplets are characterized by a larger evaporation surface, which should shorten the combustion process. In order to illustrate this Energies 2021, 14, 2224 process, an experiment was carried out in which a droplet of Jet-A1 fuel emulsion13 with of 15 water with a diameter of approx. 1 mm, suspended on a quartz needle, was quickly immersed in a stream of flowing hot exhaust gas at a temperature of approx. 1200 K. The course of the droplet combustion process is shown in Figure 13. It can be seen that the combustioncombustion process process of of emulsion emulsion droplets droplets ends ends with with its its violent violent explosion explosion and and intensification intensiﬁcation ofof the the combustion combustion process process of of the the emulsion emulsion micro-droplets micro-droplets formed formed after after the the explosion. explosion.

FigureFigure 13. 13. SubsequentSubsequent frames of the ﬁlmfilm presentingpresenting thethe process process of of burning burning a a drop drop of of fuel–water fuel–water emulsion emulsion in ain stream a stream of hotof hotexhaust exhaust gases. gases. Droplet Droplet diameter diameter approx. approx. 1 mm. 1 mm. Exhaust Exhaus gast gas temperature temperature approx. approx. 1200 1200 K. FilmingK. Filming speed speed 50 fps.50 fps.

ItIt is is worth worth noting noting that that the the use use of of FWE FWE (in (in terms terms of of the the tested tested water water concentrations) concentrations) alsoalso solves solves the the problem problem of of corrosion. corrosion. Since Since th thee water water is is supplied supplied to to the the engine engine inside inside the the fuelfuel droplets, droplets, there there is is no no direct direct contact contact between between the the metal metal and and the the liquid liquid water. water.

6.6. Conclusions Conclusions TheThe conducted conducted experimental experimental and and numerical numerical tests tests show show that that the the use use of of FWE FWE as as a a fuel fuel cancan significantly signiﬁcantly reduce reduce NOx NOx and and CO CO emissions, emissions, as as well well as as reduce reduce fuel fuel consumption consumption in in MGT.MGT. It was foundfound thatthat the the reduction reduction of of NOx NO emissionsx emissions is greater is greater when when there there is more is more water waterin the in FWE the (inFWE the (in range the range of 3% of÷ 12%3% ÷ of12% the of water the water in the in FWE). the FWE). The maximum The maximum reduction re- ductionin NOx in emission NOx emission was recorded was recorded for the measuringfor the measuring point corresponding point corresponding to 120 k rpmto 120 and k rpm12% and of the 12% water of the content water in content the FWE; in the it was FWE; 35.16%, it was compared 35.16%, compared to the case to in the which case thein whichMGT wasthe MGT powered was bypowered the DF. by the DF. TheThe use use of of FWE FWE in in MGT MGT also also leads leads to to a a re reductionduction in in CO CO emissions. emissions. It It was was observed observed forfor all all water water contents contents in in FWE FWE and and for for all all tested tested steady steady states states of of the the engine engine with with respect respect to to DF.DF. The maximum reductionreduction in in CO CO that that was wa recordeds recorded was was 12.32% 12.32% for thefor measuringthe measuring point pointcorresponding corresponding to 60 kto rpm 60 k and rpm 12% and of 12% the water of the content water incontent the FWE. in the At FWE. the operating At the oper- point atingof the point GTM-120 of the turbineGTM-120 (80 turbine k rpm), (80 a simultaneousk rpm), a simultaneous reduction reduction in NOx and in CONOx emissions and CO emissionswas achieved was byachieved 17.86% by and 17.86% 7.2%, and respectively. 7.2%, respectively. InIn addition, addition, it it was was found found that that the the use use of of FWE FWE in in the the tested tested range range of of water water content content in in fuelfuel leads leads to to a a reduction reduction in DFDF consumption.consumption. AtAt thethe engine engine operating operating point point corresponding correspond- ingto 80to k80 rpm, k rpm, a reduction a reduction in in DF DF consumption consumption by by 5.46% 5.46% was was recorded recorded for for the the 12% 12% water wa- content in the FWE. The FWE mass burned by the engine at all measuring points is greater ter content in the FWE. The FWE mass burned by the engine at all measuring points is than the fuel mass burned in the base case. greater than the fuel mass burned in the base case. Numerical calculations of the ﬂow and combustion process in the MGT showed a Numerical calculations of the flow and combustion process in the MGT showed a decrease in the maximum temperature in the combustion chamber as a result of the use of decrease in the maximum temperature in the combustion chamber as a result of the use FWE. In addition, the use of FWE contributes to the expansion of the primary combustion of FWE. In addition, the use of FWE contributes to the expansion of the primary com- zone and the reduction of temperature gradients inside the combustion chamber. This is bustion zone and the reduction of temperature gradients inside the combustion chamber. due to the formation of a more homogeneous, better mixed mixture in which the combus- This is due to the formation of a more homogeneous, better mixed mixture in which the tion process takes place more evenly. As a result of changes in the temperature distribution in the combustion chamber due to the use of FWE, the average temperature of the walls of the combustion chamber liner slightly increases.

Author Contributions: Conceptualization, M.G. and P.N.; methodology, M.G.; software, P.N.; vali- dation, P.N.; formal analysis, P.N.; investigation, P.N.; resources, M.G. and P.N.; data curation, P.N.; writing—original draft preparation, P.N. and M.G.; writing—review and editing, M.G.; visualization, P.N.; supervision, M.G.; project administration, M.G. and P.N.; funding acquisition, M.G. and P.N. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This research was supported by the Faculty of Power and Aeronautical Engi- neering, Warsaw University of Technology (equipment and materials used for experiments). Energies 2021, 14, 2224 14 of 15

Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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