Thermal Analysis of an Industrial Furnace

energies

Article Thermal Analysis of an Industrial Furnace

Mirko Filipponi 1,*, Federico Rossi 1, Andrea Presciutti 1, Stefania De Ciantis 1, Beatrice Castellani 1 and Ambro Carpinelli 2 1 CIRIAF (Centro Interuniversitario di Ricerca sull’Inquinamento e sull’Ambiente), Università degli Studi di Perugia, Via G. Duranti 67, 06125 Perugia, Italy; [email protected] (F.R.); [email protected] (A.P.); [email protected] (S.D.C.); [email protected] (B.C.) 2 Divisione Fucine di Acciai Speciali Terni, V.le B.Brin 218, 05100 Terni, Italy; [email protected] * Correspondence: mirko.ﬁ[email protected]; Tel.: +39-0744-492-969

Academic Editor: Francesco Asdrubali Received: 17 May 2016; Accepted: 10 October 2016; Published: 18 October 2016 Abstract: Industries, which are mainly responsible for high energy consumption, need to invest in research projects in order to develop new managing systems for rational energy use, and to tackle the devastating effects of climate change caused by human behavior. The study described in this paper concerns the forging industry, where the production processes generally start with the heating of steel in furnaces, and continue with other processes, such as heat treatments and different forms of machining. One of the most critical operations, in terms of energy loss, is the opening of the furnace doors for insertion and extraction operations. During this time, the temperature of the furnaces decreases by hundreds of degrees in a few minutes. Because the dispersed heat needs to be supplied again through the combustion of fuel, increasing the consumption of energy and the pollutant emissions, the evaluation of the amount of lost energy is crucial for the development of systems which can contain this loss. To perform this study, CFD simulation software was used. Results show that when the door opens, because of temperature and pressure differences between the furnace and the ambient air, turbulence is created. Results also show that the amount of energy lost for an opening of 10 min for radiation, convection and conduction is equal to 5606 MJ where convection is the main contributor, with 5020 MJ. The model created, after being validated, has been applied to perform other simulations, in order to improve the energy performance of the furnace. Results show that reducing the opening time of the door saves energy and limits pollutant emissions.

Keywords: CFD simulation; industrial furnace; heat ﬂux; forging industry; thermal analysis

1. Introduction Global carbon dioxide emissions reduction is becoming a key target for government policies, which is resulting in a reduction of fossil fuel usage [1]. Global greenhouse gases are produced by several economic activities, mainly electricity generation, heat production and industry activities [2], as well as agriculture and breeding [3], transportation [4], buildings [5] and fuel extraction, processing and transportation [6]. Greenhouse gas emissions from industry primarily involve fossil fuels burned on-site at facilities for producing energy, and the iron and steel industry is among the top ﬁve most energy-intensive industry sectors, as reported in the Energy Technology Perspectives 2014 [7]. The main challenge for this sector is lowering energy consumption, greenhouse gas emissions and pollutant emissions [8]. Iron and steel industries need to invest resources to limit their environmental footprint. The most used methods to determine energy consumption and efﬁciency of the production processes involved, are numerical thermal simulations and Life Cycle Analysis (LCA) [9,10]. Most of the works reported in the literature refer to walking beam furnaces and have the objective of predicting the temperature distribution in the slabs, which plays a major role in the quality of the

Energies 2016, 9, 833; doi:10.3390/en9100833 www.mdpi.com/journal/energies Energies 2016, 9, 833 2 of 13

final product, and the heating efficiency of the furnace [11–17]. Most of these works have relied on mathematical models, since experiments are quite difficult to perform due to the large size of real furnaces, limited physical access and the harsh environmental conditions of the furnace [18]. A recent and interesting work about the modeling of a conjugate heat transfer and fluid flows inside an industrial furnace with a CFD software, has been done [19]. The study presents a model able to simulate the heat transfer flow of convection, conduction and radiation in a 3D furnace with the presence of six conducting steel solids [20]. Instead, this paper focuses on the thermal analysis of a forging furnace, and discusses the results of joint research between the University of Perugia and Divisione Fucine di Acciai Speciali Terni (SdF), a worldwide leader in the steel forging industry. At SdF the forged products are obtained through a production process, which involves furnaces and presses. Generally several cycles of heating and machinery are needed to achieve the right geometry. However these processes generate internal stresses and defects that compromise the products’ quality. For this reason, the forged steel is subjected to other thermal and finishing treatments to release stresses, modify the molecular structure and improve the quality and performance of the final materials. From several analyses made in SdF, it has emerged that the overuse of the furnaces throughout the production processes is one of the main factors responsible for the high energy consumption. An improvement of their energy efficiency would allow the company to reduce pollutant emissions, save energy and decrease the cost of the final product. All the furnaces in SdF are fueled with natural gas but differ from each other in their dimensions, number of burners, refractory properties and so on. It is, however, possible to classify them into two main categories. The heat treatment furnaces that are generally turned off when the processing is finished, and the forging furnaces which are always turned on at maximum temperature, and are used for the heating of the ingots. The continuous operation of the forging furnaces is the main contributory reason for the high energy consumption, since a continuous energy supply is required to compensate the energy loss during the furnace opening and maintain the operating temperature conditions. A typical insertion/extraction operation starts with the opening of the furnace door, continues with the movement of an extractable bogie hearth, then the positioning or the picking up of the products by means of an overhead travelling crane, and finally finishing with the insertion of the bogie hearth and the door closing. The time necessary to perform these operations depends on many factors, such as the dimensions of the ingots, the maneuvering time, the presence of faults and so on. From the analysis of the data available to SdF, it emerged that the time varies from about 10 min up to 1 h. In this time interval the furnace temperature decreases by hundreds of degrees, from 300 ◦C for brief openings, up to 700 ◦C for longer openings. To the best of our knowledge, in the literature there are no other studies on the assessment of the thermal behavior of forging furnaces during transient insertion and extraction periods, even though the evaluation of energy losses and other parameters, such as the velocity and the directions of the gases, represents a valid decisional instrument to reduce energy consumption. The complexity of the phenomena that occur in real conditions makes difficult to analyse the study where conduction, convection and radiation occur concurrently [21]. For this reason, the present paper discusses the results of three dimensional transient simulations, done with CFD Software, of a forging furnace, considering a total opening time of 10 min. The first simulation was done with a lifting time of the door of 157 s, which represents the real operating condition of the furnace in SdF. The model created was then validated through an experimental test carried out in SdF with the same operating condition applied to the model. Successively, in order to improve the energy efficiency of the furnace, two other simulations with a different door opening speed were conducted and in this paper discussed. In particular, an opening time of 40 s and 60 s were considered. The results obtained were also used to determine the carbon footprint of the process. Energies 2016, 9, 833 3 of 13

2. ModelEnergies Settings2016, 9, 833 3 of 13 The2. Model model Settings creation started with the drawing of the geometry and its discretization through a meshingThe process. model creation It continued started with the the drawing definition of the of geometry the simulation and its setup,discretization and ended through with a the analysesmeshing of the process. results It [ 22 continued]. with the definition of the simulation setup, and ended with the analyses of the results [22]. 2.1. Geometry Creation 2.1. Geometry Creation The geometry of this case of study is composed only by the fluid domains involved in the heat fluxes andThe by geometry the furnace of this door. case of Since study the is composed furnace is only under by the operating fluid domains conditions involved when in the the heat door is liftedfluxes up, and and allby thethe furnace burners door. are turnedSince the off, furnace the model is under can operating be created condition with as verywhen cleanthe door geometry, is lifted up, and all the burners are turned off, the model can be created with a very clean geometry, without drawing components that do not influence the motion of the fluid and the heat fluxes. without drawing components that do not influence the motion of the fluid and the heat fluxes. In In FigureFigure1 two 1 two particulars particulars of of the the geometry geometry created are are shown, shown, specifically specifically the the position position of the of furnace the furnace geometrygeometry is considered is considered from from one one corner corner ofof the entire entire domain domain to toenable enable the complete the complete view of view the model. of the model.

(a) (b)

FigureFigure 1. Geometry 1. Geometry used used for for the the simulation: simulation: ( (aa) ) The The entire entire domain domain considered considered for the for analysis, the analysis, consisting of a fluid domain of the furnace enclosed in another fluid domain which represents the consisting of a fluid domain of the furnace enclosed in another fluid domain which represents the ambient air; (b) Particular of the geometry where the fluid domain of the furnace is shown in grey, ambient air; (b) Particular of the geometry where the fluid domain of the furnace is shown in grey, while the door is showed in blue. while the door is showed in blue. The furnace is 4.7 m height × 6.9 m width × 18.55 m depth. These dimensions are relative to the Theinternal furnace spaceis of 4.7 the m furnace, height which× 6.9 is m the width space× occupied18.55 m by depth. the fluid. These The dimensions door has been are drawn relative at a to the internaldistance space of of10 thecm furnace,from the furnace which isborder, the space since occupiedat the moment by the of fluid.opening The it slides door hasin a beenhorizontal drawn at a distancedirection of 10before cm frombeing thelifted furnace up. This border, movement since reduces at the momentfriction between of opening the door it slides and the in afurnace horizontal borders. The width and the height of the door are the same as the furnace, while the depth is 50 cm. direction before being lifted up. This movement reduces friction between the door and the furnace Finally the enclosure, which represents the ambient air outside the furnace, has been drawn with a borders.distance The of width 20 m andfrom thethe heightupper surface of the of door the arefurnace, the same1 m from as the the furnace,lower and while back surfaces, the depth and is 5 50 cm. Finallym from the enclosure,the lateral surfaces. which representsThe following the Table ambient 1 shows air the outside dimensions the furnace, of the model has components. been drawn with a distance of 20 m from the upper surface of the furnace, 1 m from the lower and back surfaces, and 5 m from the lateral surfaces. The followingTable 1. Table Model1 components shows the dimensions. dimensions of the model components. Component Height (m) Width (m) Depth (m) Table 1. Model components dimensions. Furnace 6.9 4.7 18.55 Door 6.9 4.7 0.5 EnclosureComponent Height25.7 (m) Width16.9 (m) Depth30.06 (m) Furnace 6.9 4.7 18.55 The representation ofDoor the furnace contained 6.9 in an 4.7 enclosure, allows 0.5 us to characterize the motion of the fluids betweenEnclosure the furnace and 25.7 the environment. 16.9 30.06

2.2. Meshing Process The representation of the furnace contained in an enclosure, allows us to characterize the motion of the ﬂuidsTo betweenguarantee the the furnace continuity and between the environment. the two domains: furnace and the environment, a conformal mesh has been used (Figure 2a). Because the door does not influence significantly the heat

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2.2. Meshing Process To guarantee the continuity between the two domains: furnace and the environment, a conformal mesh hasEnergies been 2016 used, 9, 833 (Figure 2a). Because the door does not influence significantly the heat fluxes,4 of 13 it has not been subject to the meshing process, as it demonstrated by Figure2b. In addition, being a transient fluxes, it has not been subject to the meshing process, as it demonstrated by Figure 2b. In addition, simulation with a dynamic mesh, the entire domain has been meshed with a tetrahedral mesh that being a transient simulation with a dynamic mesh, the entire domain has been meshed with a supportstetrahedral the operation mesh that of remeshingsupports the atoperation every timeof remeshing step. at every time step.

(a) (b)

Figure 2. Drawings of a meshed domain: (a) Representation of the entire domain meshed with a Figure 2. Drawings of a meshed domain: (a) Representation of the entire domain meshed with conformal method; (b) Particular of the meshed domain near the door which is not included in the b a conformalmeshingmethod; process. ( ) Particular of the meshed domain near the door which is not included in the meshing process. 2.3. Models for Convection and Radiation 2.3. Models forTo Convectionmodel the turbulent and Radiation flows which occur during the door opening, the Reynolds-Averaged Navier-Stokes (RANS) approach was used. Recent studies demonstrate that it represents the most To model the turbulent flows which occur during the door opening, the Reynolds-Averaged commonly accepted model for turbulence modeling, especially for industrial applications [18]. This Navier-Stokesmodel is (RANS) derived approach from the formulation was used. of Recent the standard studies k- demonstrateε model proposed that by it represents Launder and the most commonlySpalding accepted which model is based for on turbulence two transport modeling, equations which especially includes for the industrial two variables applications of the [18]. This modelturbulent is derived kinetic energy from (k), the and formulation the turbulent of dissipation the standard rate (ε) k- [ε23model]. With the proposed standard byk-ε Laundermodel and Spaldingit which is possible is based to predict on two the transport behavior of equations a flow in which presence includes of turbulences the two with variables high accuracy.of the turbulent kinetic energyHowever, (k), the and realizable the turbulent k-ε model, dissipation developed rate after (ε the)[23 standard]. With thek-ε model,standard proposes k-ε model a different it is possible formulation giving more accurate results than the first one, for turbulent flows [24]. With the use of to predict the behavior of a flow in presence of turbulences with high accuracy. However, the realizable the “enhanced wall treatments” option, simplified formulas for turbulence quantities in the cells k-ε model,near developedthe wall are used. after the standard k-ε model, proposes a different formulation giving more accurate resultsFor the than modeling the firstof the one,radiative for heat turbulent flux a preliminary flows [24 analysis]. With about the the use concentration of the “enhanced of the wall treatments”flue gases option, inside simplified the chamber formulas of the furnace for turbulence at the door opening quantities has inbeen the done. cells Results near the showed wall that are used. Forthe the burners modeling work with of the anradiative excess of air heat avoiding flux asoot preliminary formation. For analysis this reason, about the the assumption concentration of an of the flue gasesoptical inside thickness the chamber equal to zero of the was furnace done and at the the model door Surface opening to Surface has been has done.been used Results [25]. In showed this that model, the energy exchange between surfaces depends on their size and position with respect to the the burnersothers. work The model with considers an excess these of geometric air avoiding factors soot through formation. the computation For this of reason,the view thefactors assumption [26]. of an optical thicknessThe S2S model equal also to zero assumes was that done the and surfaces the model are gray Surface and diffuse, to Surface and the has emissivity been used and [ 25the]. In this model,absorptivity the energy do exchange not depend between on the surfaceswavelength. depends It is well on known their that size for and the position gray bodies, with the respect total to the others. Theenergy model incident considers (E) on a surface these geometric is partly reflected factors (ρE), through partly theabsorbed computation (αE) and partly of the transmitted view factors [26]. The(τE). S2S Since model this also model assumes is often that used the with surfaces opaqueare surfaces gray andto thermal diffuse, radiation and the in emissivity the infrared and the spectrum, the fraction of energy transmitted can be not considered. With these assumptions, from absorptivity do not depend on the wavelength. It is well known that for the gray bodies, the total the Kirchoff’s law [27] results that α + ρ + ε = 1, but α = ε, therefore ρ = 1 − ε [26]. energy incident (E) on a surface is partly reflected (ρE), partly absorbed (αE) and partly transmitted (τE). Since this2.4. modelBoundary is Conditions often used with opaque surfaces to thermal radiation in the infrared spectrum, the fractionThe of energy boundary transmitted condition can at bewall not boundaries considered. was With given these using assumptions, Fluent’s “enhanced from the wall Kirchoff’s law [27]treatment” results that whichα + allowsρ + ε us= to 1, predict but α =theε ,flows therefore in the ρnear= 1-wall− ε region[26]. where laminar and turbulent flows are blended [28]. The boundary and cell conditions given for the initial time were provided by 2.4. BoundarySdF, in Conditionsparticular the temperature of the furnace is at the operating condition of 1500 K, while the ambient temperature is at 300 K. The pressure is set at 101,325 Pa for the environment and an over Thepressure boundary of 40 conditionPa respect to at the wall ambience, boundaries for the was furnace. given using Fluent’s “enhanced wall treatment” which allows us to predict the flows in the near-wall region where laminar and turbulent flows are blended [28]. The boundary and cell conditions given for the initial time were provided by SdF, Energies 2016, 9, 833 5 of 13 in particular the temperature of the furnace is at the operating condition of 1500 K, while the ambient temperature is at 300 K. The pressure is set at 101,325 Pa for the environment and an over pressure of 40 Pa respect to the ambience, for the furnace. Energies 2016, 9, 833 5 of 13 2.5. Dynamic Mesh 2.5. Dynamic Mesh The use of a dynamic mesh allowed us to do a transient simulation with the movement of the door. ThroughThe use of the a usedynamic of the mesh first-order allowed backward us to do differencea transient formulasimulation it waswith possible the movement to calculate of the the door. Through the use of the first-order backward difference formula it was possible to calculate the conservative energy equation for each time step, considering two steps concurrently [29]. For the conservative energy equation for each time step, considering two steps concurrently [29]. For the motion of the door, a profile has been defined and applied. In particular, the door has been set as motion of the door, a profile has been defined and applied. In particular, the door has been set as a a “rigid body” using the profile for motion, while the adjacent cells have been set as “deforming” with “rigid body” using the profile for motion, while the adjacent cells have been set as “deforming” with the application of remeshing and smoothing methods. the application of remeshing and smoothing methods. 2.6. Control Surfaces 2.6. Control Surfaces In order to monitor and evaluate the fluid motion, and the heat fluxes that occur during the In order to monitor and evaluate the fluid motion, and the heat fluxes that occur during the openingopening of of the the door, door, three three control control surfacessurfaces werewere created. As As shown shown in in Figure Figure 3a3 athe the first first surface surface A, A, havinghaving dimensions dimensions equal equal to to 6.9 6.9m m× × 4.7 m, has been been positioned positioned in in correspondence correspondence of ofthe the exit exit of ofthe the gasesgases from from the the furnace. furnace. This This surface surface allows allows us us to to quantify quantify the the amount amount of of the the total total convective convective flow flow that leavesthat leaves the furnace the furnace during during all the all transient the transient time time considered. considered. As As it isit intuitiveis intuitive to to expect expect that that the the hot gaseshot leavinggases leaving the furnace the furnace tend totend move to move upwards, upwards, passing passing through through the space the space between between the door the anddoor the edgeand of the the edge furnace of the formed furnace because formed the because scrolling the of scrolling the door, of athe new door, surface a new B wassurface created. B was This created. surface withThis dimensions surface with of dimensions 6.9 m × 0.1 of m 6.9 is shownm × 0.1 inm is Figure shown3b. in To Figu capturere 3b. theTo capture flows deflected the flowsin deflected the other directionsin the other the last directions control the surface last C control was positioned surface C behind was positioned the door behind with a larger the door dimension with a largerthan the othersdimension of 6.3 mthan× the9.2 mothers (Figure of 6.33c). m The× 9.2 orientation m, (Figure 3c of). the The geometry orientation in of the the figures geometry has beenin thechosen figures to showhas abeen clear chosen view to of show the surfaces a clear view created. of the surfaces created.

(a) (b) (c)

FigureFigure 3. 3.TheThe three three control control surfaces surfaces created created for for the the evaluation evaluation of the of the convective convective flow flow in different in different zones: (a)zones: Control (a) surface Control A surface positioned A positioned in correspondence in correspondence of the exit of of the the exit gases of the from gases the from furnace; the (furnaceb) Control; (b) Control surface B created in the upper section between the door and the furnace edge; (c) Control surface B created in the upper section between the door and the furnace edge; (c) Control surface C surface C created outside the door. created outside the door. 3. Results 3. Results The period of 10 min from the opening of the door with a time step of 0.1 s was considered. In particular,The period the first of 10 simulation, min from related the opening to the real of operating the door withcondition a time of the step furnac of 0.1e in s wasthe plant considered. with Ina particular, complete opening the first in simulation, 157 s, was conducted related to theand realvalidated operating through condition an experimental of the furnace test done in the in the plant withreal a furnace complete in openingSdF. in 157 s, was conducted and validated through an experimental test done in the real furnace in SdF. 3.1. Fluids Behavior 3.1. Fluids Behavior The first parameter monitored with the simulation was the maximum temperature reached on theThe two first control parameter surfaces monitored B and C withduring the the simulation opening. wasThis the trend maximum is shown temperature in Figure 4 reached where it on is the twopossible control to surfaces see that Bthe and highest C during temperature the opening. is registered This trend on surface is shown B, which in Figure reaches4 where values it isof possibleabout to1200 see that K after the a highest few seconds temperature from opening is registered and then on decreases, surface B, reaching which reaches values of values 800 K. of Instead, about 1200 the K aftermaximum a few seconds temperature from openingregistered and on thensurface decreases, C starts from reaching the ambient values of value, 800 K.up Instead, to values the of maximumaround temperature700 K. registered on surface C starts from the ambient value, up to values of around 700 K.

Energies 2016, 9, 833 6 of 13 Energies 2016, 9, 833 6 of 13 Energies 2016, 9, 833 6 of 13

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FigureFigure 4. 4.The The trend trend of of the the maximum maximum temperaturetemperature reache reachedd in in the the surface surface B Band and C Cduring during the the 157 157 s of s of Figure 4. The trend of the maximum temperature reached in the surface B and C during the 157 s of thethe door door opening. opening. the door opening. The values of the max temperature registered in the two surfaces depend on the motion of the fluid The values of the max temperature registered in the two surfaces depend on the motion of the ﬂuid leavingThe the values furnace. of the For max example temperature Figure registered 5 shows a in sequ theence two ofsurfaces velocity depend vectors on from the motion the initial of thetime, fluid up leaving theFigure furnace. 4. The For trend example of the maximum Figure 5temperature shows a reached sequence in the of surface velocity B and vectors C during fromthe 157 the s of initial time, leavingto 1 s. This thethe furnace.sequence door opening. For of framesexample demonstrates Figure 5 shows how a thesequence hot gases of velocity inside thevectors chamber from move the initial upwards time, andup uptoleave to 1 1s. s.theThis This furnace sequence sequence through of frames of the frames upper demonstrates demonstrates section with how a howthe velo hot thecity gases hotthat gases insidereaches inside the 12 chamber m/s. the Meanwhile, chamber move upwards move the external upwards and andleaveair, leave at the ambient thefurnaceThe furnace values temperature, through of throughthe maxthe moves upper temperature the toward section upper registereds with sectionthe furnace a velocity in with the through twoa that velocitysurfaces reaches the bottomdepend that 12 m/s. reaches onsection. the Meanwhile, motion 12 m/s. of the the Meanwhile,fluid external theair, external at leavingambient air, the temperature, at furnace. ambient For temperature, examplemoves towards Figure moves5 showsthe furnace towardsa sequence through the of velocity furnace the bottom vectors through section. from the the bottominitial time, section. up to 1 s. This sequence of frames demonstrates how the hot gases inside the chamber move upwards and leave the furnace through the upper section with a velocity that reaches 12 m/s. Meanwhile, the external air, at ambient temperature, moves towards the furnace through the bottom section.

(a) (b) (c) (a) (b) (c) Figure 5. Sequence of the velocity vectors at different time at the exit of the furnace: (a) Velocity FigureFigurevectors 5. 5.Sequenceat Sequence0 s; (b(a) )Velocity of of the the velocity vectors velocity vectors at vectors 0.5 s; at (c different at) Velocity different(b) time vectors time at the atat the1exit s. exit of the of furnace: the furnace:(c (a) ) Velocity (a) Velocity vectors vectors at 0 s; (b) Velocity vectors at 0.5 s; (c) Velocity vectors at 1 s. at 0 s; (bFigure) Velocity 5. Sequence vectors of at the 0.5 velocity s; (c) Velocity vectors at vectors different at time1 s. at the exit of the furnace: (a) Velocity Continuingvectors at the 0 s; anlyses (b) Velocity of thevectors fluid at 0 dynamics.5 s; (c) Velocity of the vectors model, at 1 s. it can be observed from Figure 6 that the hotContinuing gases which the anlyses cannot ofbe the released fluid dynamics from the upperof the model,section, it are can recalled be observed downwards, from Figure forming 6 that a Continuing the anlyses of the ﬂuid dynamics of the model, it can be observed from Figure6 theconvective hot gasesContinuing motion which when cannotthe anlyses they be meet releasedof the the fluid coldfrom dynamics air the coming upper of the frommodel,section outside. it, arecan berecalled observed downwards, from Figure forming6 that a thatconvective thethe hot hot motion gasesgases which when which cannot they cannot meet be released bethe releasedcold from air the coming from upper thefrom section upper outside., are section,recalled downwards, are recalled forming downwards, a formingconvective a convective motion motion when they when meet they the meetcold air the coming cold airfrom coming outside. from outside.

(a) (a) (b)( b) (a) (b) FigureFigure 6. Velocity 6. Velocity vectors vectors of theof the gases gases for for an an opening opening of 33 cm: cm: ( a()a Velocity) Velocity vectors vectors and and formation formation of of Figure 6. Velocity vectors of the gases for an opening of 3 cm: (a) Velocity vectors and formation of Figureconvectiveconvective 6. Velocity motion; motion; vectors (b) (Particularb) ofParticular the gases of of convective convective for an opening motion motion of atat 3 thethe cm: bottom bottom (a) Velocity of ofthe the furnace. vectorsfurnace. and formation of convective motion; (b) Particular of convective motion at the bottom of the furnace. convective motion; (b) Particular of convective motion at the bottom of the furnace.

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As the door continues to open, the vortexes disappear, the velocity vectors decrease, and the hot gasesAs leaving the door the continues furnace to start open, to the pass vortexes under disappear, the door (Figure the velocity 7). The vectors complete decrease, motion and the of hotthe convectivegases leaving fluxes the furnace for the start transient to pass time under of the 157 door s, relative (Figure 7 to). Thethe furnacecomplete domain, motion of is the sho convectivewn in the attachedﬂuxes for video. the transient time of 157 s, relative to the furnace domain, is shown in the attached video.

Figure 7. Fluid motion during the transient time from the opening of the door, up to the complete Figure 7. Fluid motion during the transient time from the opening of the door, up to the complete opening of 4.7 m at 157 s. opening of 4.7 m at 157 s.

It is important to underline that the vectors reported in the previous images refer to the gases that moveIt is important in the furnace to underline domain. thatFor example, the vectors observing reported the in picture the previous relating images to the refermotion to theof 50 gases s, it mightthat move seem in that the the furnace vectors domain. pass through For example, the door. observing In reality the this picture does relating not occur, to the in so motion far as of when 50 s, theit might gases seem pass thatthrough the vectors the air passdomain, through the presence the door. of In th realitye door this deflects does the not motion occur, in of so the far vectors as when in otherthe gases directions, pass through as it is thepossible air domain, to see from the presence Figure 8. of the door deﬂects the motion of the vectors in other directions, as it is possible to see from Figure8.

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Figure 8. The velocity vectors’ directions in the air domain that move vertically upwards between the Figure 8. The velocity vectors’ directions in the air domain that move vertically upwards between the Figure door8. The and velocity the furnace, vectors and ’immediately directions outsidein the air the domain door. that move vertically upwards between the doordoor andand thethe furnace,furnace, andand immediatelyimmediately outsideoutside thethe door.door. 3.2. Heat Flux Evaluation 3.2.3.2. Heat FluxIn Evaluationorder to evaluate the energy fluxes, the contributions of the three forms of heat exchange: convection, radiation and conduction, were identified. With the aid of the CFD Software, the amount InIn orderorder toto evaluateevaluate thethe energyenergy fluxes,fluxes, thethe contributionscontributions of the three forms of heat exchange: of the heat flux during the opening for convection and radiation has been evaluated, while a convection,convection,spreadshe radiationradiationet for the and and conduction conduction, conduction, waswere used.were identified. identified. With With the the aid aid of of the the CFD CFD Software, Software, the the amount amount of theof the heat heat flux duringflux during the opening the opening for convection for convection and radiation and radiation has been evaluated, has been while evaluated, a spreadsheet while a forspreadshe the conduction3.2.1.et Radiative for the was conduction Heat used. Flux was used. The values of the radiative energy, which passes through the opening of the door evaluated for 3.2.1.3.2.1. Radiativethe first 157 Heat s and Flux related to the complete opening of the door of 4.7 m, are reported in Figure 9. TheThe valuesvalues of the radiative energy, whichwhich passespasses throughthrough thethe openingopening ofof thethe doordoor evaluatedevaluated for thethe firstfirst 157157 ss andand relatedrelated toto thethe completecomplete openingopening ofof thethe doordoor ofof 4.74.7 m,m, areare reportedreported in in Figure Figure9 .9.

Figure 9. The graph reports the trend of the radiative heat flux through the door during the 157 s.

The values obtained show that the flux grows linearly with time and so with the opening. Even if during the opening the walls start to cool down, the radiative heat flux per unit of area remains about the same because the difference of temperature changes slightly. These values have been verified through the analytical calculation for several time steps, using the view factors given by Fluent. Figure 9. The graph reports the trend of the radiative heat ﬂuxflux throughthrough thethe doordoor duringduring thethe 157157 s.s.

The values obtained show that the flux grows linearly with time and so with the opening. Even if The values obtained show that the flux grows linearly with time and so with the opening. Even if during the opening the walls start to cool down, the radiative heat flux per unit of area remains about during the opening the walls start to cool down, the radiative heat flux per unit of area remains about the same because the difference of temperature changes slightly. These values have been verified the same because the difference of temperature changes slightly. These values have been verified through the analytical calculation for several time steps, using the view factors given by Fluent. through the analytical calculation for several time steps, using the view factors given by Fluent.

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3.2.2. Convective Heat Flux Energies 2016, 9, 833 9 of 13 TheEnergies evaluation 2016, 9, 833 of the convective heat flux is carried out through the calculation of the mass9 of 13 flow rate of3.2.2. enthalpy Convective as the Heat following Flux Equation (1): 3.2.2. Convective Heat Flux The evaluation of the convective heat flux is carried out through the calculation of the mass The evaluation of the convective heat fluxZ is carried→ → out through the calculation of the mass flow rate of enthalpy as the following EquationQ = (1H): ρv ·dA (1) flow rate of enthalpy as the following Equation (1): Q = ∫ Hρv⃗ ∙ dA⃗⃗ (1) → → ⃗⃗ where H is the Enthalpy, and ρv ·dA is the massQ = ∫ flow.Hρv⃗ ∙ dA (1) where H is the Enthalpy, and ρv⃗ · dA⃗⃗ is the mass flow. The knowledge of the values calculated⃗⃗ for every time step in the simulation allows us to obtain whereThe H knowledgeis the Enthalpy of the, and values ρv⃗ · calculateddA is the mass for every flow. time step in the simulation allows us to obtain the total convective heat flux. The trend of the fluxes for the complete opening of the door of 4.7 m in the totalThe convective knowledge heat of the flux. values The trendcalculated of the for fluxes every for time the stepcomplet in thee opening simulation of the allows door us of to 4 .obtain7 m in 157 s is shown in Figure 10. It is possible to see that the heat flux on the door is given from the sum 157the totals is shown convective in Figure heat 10. flux. It isThe possible trend ofto thesee fluxesthat the for heat the fluxcomplet on thee opening door is givenof the fromdoor theof 4 sum.7 m ofin of thethe157 others. others.s is shown By By comparing comparingin Figure 10. the It istwo possible types types toof of seeheat heat that flux fluxthe for heat for the theflux first firston 157 the 157 s, door it s, is it ispossible isgiven possible from to see the to that seesum the that of the convectiveconvectivethe others. heat heatBy flux comparing flux is higher is higher the than than two the thetypes radiative, radiative, of heat even evenflux iffor if the the the latter latter first depends 157 depends s, it onis onpossiblethe thefourth fourth to power see powerthat of the of the temperature.temperature.convective This heat This behavior flux behavior is higher is is due thandue to tothe the the radiative, fact fact thatthat even the turbulence turbulenceif the latter generateddepends generated on from the from thefourth theopening power opening creates of the creates a motionatemperature. motion of air of withair This with a behavior high a high Reynolds Reynolds is due to number. thenumber. fact that the turbulence generated from the opening creates a motion of air with a high Reynolds number.

FigureFigure 10. The 10. The convective convective heat heat flux flux evaluated evaluated in the three three control control surfaces surfaces created created for a for complete a complete Figure 10. The convective heat flux evaluated in the three control surfaces created for a complete openingopening of the of doorthe door of 4.7of 4.7 m. m. opening of the door of 4.7 m. 3.2.3. Conductive Heat Flux 3.2.3.3.2.3. Conductive Conductive Heat Heat Flux Flux It was possible to calculate the amount of the conductive heat flux through the walls of the Itfurnace wasIt possible was from possible data to re calculate lating to calculate to thethe amountstratification the amount of the of conductivethe the walls conductive as shown heat heat flux in fluxFigure through through 11, theand the walls from walls ofthe the oftrend the furnace fromoffurnace data the relatingtemperatur from data toe there obtainedlating stratification to from the stratification the ofsimulation. the walls of the as walls shown as shown in Figure in Figure 11, and 11, and from from the the trend trend of the temperatureof the temperatur obtainede fromobtained the from simulation. the simulation.

Figure 11. Drawing showing the stratigraphy of the walls of the furnace with the decrease of the FiguretemperatureFigure 11. Drawing 11. Drawing when showing the show furnaceing the theis stratigraphyunder stratigraphy operating of conditionthe walls wallss .of of the the furnace furnace with with the thedecre decreasease of the of the temperature when the furnace is under operating conditions. temperature when the furnace is under operating conditions.

Energies 2016, 9, 833 10 of 13

The application of the following gives the values of the heat ﬂux in function of the time:

Q = U·∆T·A (2) where U is the equivalent transmittance of the wall, ∆T is the difference of temperature and A is the area of the surface. However, this formulation does not consider the thermal bridges.

3.3. Total Energy Loss and Validation of the Model Energies 2016, 9, 833 10 of 13 The knowledge of the trends of the heat ﬂuxes in function of the time allowed the calculation of the energy loss.The In application particular, of the Table following2 gives gives the the energy values of for the the heat three flux in heat function ﬂuxes. of the time: Q = U · ∆T · A (2) Table 2. Amount of energy lost for an opening of 600 s for radiation, convection and conduction. where U is the equivalent transmittance of the wall, ΔT is the difference of temperature and A is the area of the surface. However, this formulation does not consider the thermal bridges. Type of Heat Flux Energy (MJ) 3.3. Total Energy Loss and Validation of the Model Radiation 313 The knowledge of the trends ofConvection the heat fluxes in function 5252 of the time allowed the calculation of the energy loss. In particular, TableConduction 2 gives the energy for the 41three heat fluxes.

Table 2. Amount of energy lost forTOTAL an opening of 600 s for radiation, 5606 convection and conduction.

Type of Heat Flux Energy (MJ) To evaluate the accuracy of theseRadiation values, an indirect313 analysis through the knowledge of the methane consumption of the furnace wasConvection made. Assuming5252 a lower calorific value of the natural gas 3 Conduction 3 41 equal to 34 MJ/m , the flow rate is equalTOTAL to 165 Nm of methane.5606 However the methane consumption calculated is relative to the model created with the ideal operating condition of the furnace. The real behavior insteadTo evaluateis conditioned the accuracy by of many these parameters values, an indirect such analysis as, for through example, the knowledge the presence of the of thermal bridges andmethane damaged consumption refractories, of the furnace which was increase made. Assuming the heat a lower flux throughcalorific value the of walls. the natural Another gas aspect is equal to 34 MJ/m3, the flow rate is equal to 165 Nm3 of methane. However the methane consumption the presencecalculated of an intersticeis relative to betweenthe model created the bogie with hearththe ideal andoperating the condition lateral wallsof the furnace. of the The furnace, real which is necessary tobehavior avoid instead friction is conditioned during movement,by many parameters but allows such as,part for example, of the the heat presence to exit of fromthermal the furnace increasingbridges the losses. and damaged These, refractories, and other which phenomena, increase the heat influence flux through the realthe walls. consumption Another aspect of is natural gas, the presence of an interstice between the bogie hearth and the lateral walls of the furnace, which is increasing thenecessary total toamount avoid friction of fuel during necessary movement, to maintain but allows the part furnace of the heat chamber to exit from at a the certain furnace temperature. The validationincreasing the of the losses. model These, was and othercarried phenomena, out through influence an the experimental real consumption test of made natural in gas, SdF. The test started withincreasing the opening the total amount of the of door fuel necessary of an empty to maintain forging the furnace furnace chamber at theat a certain operating temperature. temperature of 1230 ◦C. The furnaceThe validation remained of the model open was for carried 10 min out with through the an bogie experimental hearth test inside, made in and SdF. then The test closed again. started with the opening of the door of an empty forging furnace at the operating temperature of 1230 When the door°C. The was furnace completely remained open closed for 10 themin with burners the bogie started hearth toinside, call and up then to reachclosed again. again When the operating temperature.the Two door instant was completely photographs closed the of this burners experimental started to call test up are to reported reach again in the Figure operating 12 from which it is possibletemperature. to see the Two variation instant photographs of the intensity of this of experimental the furnace test color are reported which in corresponds Figure 12 from to a sudden which it is possible to see the variation of the intensity of the furnace color which corresponds to a fall in temperature.sudden fall in temperature.

(a) (b)

Figure 12. Pictures taken during the experimental test made in SdF: (a) The picture is related to a Figure 12. Picturespartial opening taken at during60 s; (b) The the picture experimental is related to test a complete made in opening SdF: (ata) 157 The s. picture is related to a partial opening at 60 s; (b) The picture is related to a complete opening at 157 s.

Energies 2016, 9, 833 11 of 13

Energies 2016, 9, 833 11 of 13 The consumption of methane was monitored between the closing time, after the 10 min, and the timeThe it takes consumption to return of at methane its operating was monitored conditions. between Taking the into closing account time, the after simpliﬁcation the 10 min, and of thethe model,time it the takes two to values return of at methane its operating consumption conditions. were Taking compared into and account resulted the that simplification the values of are the of themodel, same the order two ofvalues magnitude. of methane This consumption validation guarantees were compared the accuracy and resulted of the modelthat the and values allows are usof tothe use same it asorder a decisional of magnitude. instrument This validation for future guarantees developments the accuracy which will of the allow model us toand develop allows moreus to efﬁcientuse it as processes. a decisional instrument for future developments which will allow us to develop more efficient processes. In order to characterize the footprint of this phenomenon, the emission of CO2 caused by the combustionIn order of to the characterize 165 Nm3 of the methane footprint is evaluated. of this phenomenon, From the stoichiometric the emission equation of CO2 ofcaused the methane by the combustion of the 165 Nm3 of methane is evaluated. From the stoichiometric equation of the combustion results that 1 mol of CH4 produces 1 mol of CO2 (Equation (3)): methane combustion results that 1 mol of CH4 produces 1 mol of CO2 (Equation (3)):

CH4 + 2O2 → CO2 + 2H2O (3) CH4 + 2O2 → CO2 + 2H2O (3)

BeingBeing the molar the molar mass mass of the of methanethe methane equal equal to 16to g/mol16 g/mol and and the the molar molar mass mass of of the the CO CO22 equalequal toto 44 g/mol,g/mol, results results that that the combustion of 1 kg of CHCH44 producesproduces 2.752.75 kg ofof COCO22. For a densitydensity ofof thethe 3 3 3 3 methane equal to 0 0.73.73 kg/m kg/m resultsresults that that 1 1kg kg of ofCH CH4 corresponds4 corresponds to 1 to.4 1.4m mof CHof CH4. So4 .the So kg the of kg CO of2 3 3 COproduced2 produced by the by combustionthe combustion of 165 of 165 Nm Nm of methaneof methane are are 331 331 kg, kg, where where 309 309 kg kg are are due due to the the combustioncombustion ofof naturalnatural gasgas forfor thethe heatheat lostlost forfor convection.convection.

4. Application Application of of the the Model Model As shownshown by TableTable2 2,, the the major major contribution contribution to to the the total total heat heat loss loss is is due due to to convection. convection. ForFor thisthis reasonreason the the model model created created has has been been applied applied to evaluate to evaluate how an how increase an increase of the door of the speed door can speed influence can theinfluence convective the convective fluxes of the fluxes furnace of the and furnace the consequent and the consequent global energy global performance. energy performance. For this reason For twothis otherreason simulations two other simulations of a complete of a opening complete of opening 40 s and of 60 40 s were s and carried 60 s were out. carried Figure out. 13 shows Figure the 13 trendshows of the the trend convective of the convective heat flux on heat Surface flux on A for Surface the three A for different the three openings. different openings.

Figure 13. Convective heat heat flux ﬂux evaluated evaluated on on the the control control surface surface A Afor for the the three three different different opening opening of of40 40s, 60 s, 60s and s and 157 157 s. s.

As it is possible to see, a reduction of the opening time entails an increase of the maximum As it is possible to see, a reduction of the opening time entails an increase of the maximum value value of convective heat flux calculated, which exceeds 9 MW for a 40 s opening. From the of convective heat ﬂux calculated, which exceeds 9 MW for a 40 s opening. From the evaluation of the evaluation of the amount of the energy lost during the lifting time of the door and in the next 10 min amount of the energy lost during the lifting time of the door and in the next 10 min for the three different for the three different case studies, the results show that the opening of 40 s minimizes the heat loss, case studies, the results show that the opening of 40 s minimizes the heat loss, with a total of 4324 MJ with a total of 4324 MJ for a total time of 640 s. In the following Table 3 the values of the convective for a total time of 640 s. In the following Table3 the values of the convective heat ﬂux for the different heat flux for the different openings are shown. openings are shown.

Energies 2016, 9, 833 12 of 13

Table 3. Energy ﬂuxes evaluated on the control Surface A in the three different case studies for the time of the door opening and the total time.

Time (s) Energyﬂux (MJ) Time (s) Energyﬂux (MJ) 40 499 640 4586 60 607 660 4664 157 1304 757 5252

Thanks to this study the company made their production process more efﬁcient, saving fuel and limiting pollutant emissions, simply by reducing the opening time of the door, from 157 s to 40 s. An amount of about 60 kg of CO2 is saved simply with the implementation of a reduced door opening time. A further reduction of this time is not allowed due to the mechanical limits of the door lifting system. The data obtained from this study is also being used for other studies concerning the design of an air knife system to install close to the door.

5. Conclusions In this study the behavior of a real industrial furnace used in the forging industry was analyzed. The CFD simulation of a typical insertion/extraction operation done in a forging furnace showed that the amount of energy lost for a total time of 10 min is equal to 5606 MJ, where the main contributory factor is the convective heat ﬂux, with a value of 5252 MJ. In order to characterize the heat ﬂows during all the transient time, other variables were also monitored. In particular, results showed that at the opening, because of the differences in temperature and pressure internally and externally, turbulences were generated constituting the main reason for the heat loss. The validation of the model, carried out through an experimental test done in SdF, has allowed the use of the model as a decisional instrument. For this reason two other simulations were also conducted. Results showed that a reduction of the door opening time from 157 s to 40 s reduces the energy loss, limiting the fuel consumption and pollutant emissions. The importance of this study is due also to the fact that the model created is applicable to a very large variety of furnaces, as it depends only on the geometry and on the operating conditions. The model can be used for furnaces of every dimension and powered by any kind of energy.

Acknowledgments: This work was supported by the research funding of Divisione Fucine di Acciai Speciali Terni and CIRIAF. The authors are grateful for this support. Author Contributions: All the authors equally contributed to this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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