The main objective of this work was to evaluate the behavior of an oval heated closed enclosure, when variable radiant panels were introduced. The experimental investigation showed that their efficiency was depending on their position. An experimental investigation, as well as numerical simulation was carried out. Totally, 24 test runs were performed from which the maximal heating temperature was measured. The experimental findings were also compared to the simulation results and a reasonable agreement was observed. Finally, based on the results of this study, a correlation was developed to predict the inner configuration for heat transfer enhancement of an oval furnace.Glavni cilj ovog rada bila je procijena ponašanja ovalno zagrijavanog zatvorenog prostora, podijeljenog u različita polja. Experimentalna istraživanja ukazuju da iskoristivost zavisi od položaja. Također je provedena i numerička simulacija. Ukupno, provedena su 24 mjerenja maximalne teperature zagrijavanja. Provedena istraživanja su također uspoređivani s rezultatima simulacije i ustanovljena su dobra slaganja. Završno, na temelju rezultata ove studije razvijena je korelacija predkazivanja unutarnje konfiguracije za povećanje prijenosa topline u ovalnoj peći

A. A. Minea

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199METALURGIJA 51 (2012) 2, 199-202A. A. MINEAEXPERIMENTAL AND NUMERICAL SIMULATION ANALYSIS OF HEAT TRANSFER ON A CLOSED ENCLOSUREReceived – Prispjelo: 2011-07-15Accepted – Prihvaćeno: 2011-08-31Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 51(2) 199-202 (2012)UDC – UDK 669.041:621.3.036 =111The main objective of this work was to evaluate the behavior of an oval heated closed enclosure, when variable radi-ant panels were introduced. The experimental investigation showed that their effi  ciency was depending on their position. An experimental investigation, as well as numerical simulation was carried out. Totally, 24 test runs were performed from which the maximal heating temperature was measured. The experimental fi ndings were also com-pared to the simulation results and a reasonable agreement was observed. Finally, based on the results of this study, a correlation was developed to predict the inner confi guration for heat transfer enhancement of an oval furnace.Key words: furnace, heat transfer, numerical simulation, experimentalEksperimentalna i numerička simulacija analize prijenosa topline u zatvorenom prostoru. Glavni cilj ovog rada bila je procijena ponašanja ovalno zagrijavanog zatvorenog prostora, podijeljenog u različita polja. Experimen-talna istraživanja ukazuju da iskoristivost zavisi od položaja. Također je provedena i numerička simulacija. Ukupno, provedena su 24 mjerenja maximalne teperature zagrijavanja. Provedena istraživanja su također uspoređivani sa rezultatima simulacije i ustanovljena su dobra slaganja. Završno, na temelju rezultata ove studije razvijena je korel-acija predkazivanja unutarnje konfi guracije za povećanje prijenosa topline u ovalnoj peći.Ključne riječi: peć, prijenos topline, numerička simulacija, eksperimentINTRODUCTIONFurnaces are used in a wide variety of applications in-cluding power plants, nuclear reactors, refrigeration and other heating systems, automotive industries, heat recov-ery systems, chemical processing, food industries, metal-lurgy etc. [1,2]. Besides the performance of the furnace being improved, the heat transfer enhancement enables the exterior size of the furnace to be considerably de-creased. Due to their compact structure and high heat transfer coeffi cient, oval furnaces have been introduced as one of the passive heat transfer enhancement techniques and are widely used in various industrial applications [3].In this paper simplifi ed models based on numerical simulation and experimental methods are proposed to predict the radiation and convection heat transfer in heating processes. The simulation investigations were done to understand forced laminar fl uid fl ow in the fur-nace walls boundary layer when panels are introduced. These panels, showed in Figure 1, were used in simula-tion, and an experiment was carried out for the valida-tion of the system.NUMERICAL SIMULATION SET-UPFor solving the heat transfer problem it was chosed numerical simulation of radiation and convection in a closed domain.A. A. Minea, Technical University gh. Asachi Iasi, RomaniaAn important fi rst step for establishing the model ac-curacy was to identify the boundary conditions and the properties of materials involved in the process. All the fl uid properties were assessed at the mean temperature of the fl uids (average of inlet and outlet temperatures) [4].The models used in the simulation are collected in Table 1. The above formulation was used to build a model using FLUENT commercial CFD (Computer Fluid Dynamics) code.The entire study has as starting point the geometry of a Barnstead manufactured oval furnace, so the oval geometry (0,11m x 0,19m) was simulated to study the effect of heat chamber geometry on heat transfer rate. The fi nal default interior temperature was also com-Figure 1 Panels geometry200  METALURGIJA 51 (2012) 2, 199-202A. A. MINEA: EXPERIMENTAL AND NUMERICAL SIMULATION ANALYSIS OF HEAT TRANSFER ON A CLOSED ENCLOSUREpared and a good agreement was observed. The geom-etry models were completed on Gambit pre-processor for FLUENT. First of all it was made a base model, grid independent, and after this, with the help of the journal fi les from Gambit was obtained all the study geometries. Working with journal fi les in Gambit is a good tech-nique applied in cases when modifying geometry is needed and it offers a good accuracy of experiment re-peatability. So, the simulation conditions were kept the same for all the cases and 1200 iteration steps were per-formed.In the Figure 2 is shown the grid for the oval fur-nace. The fi nal heating temperature can be estimated by monitoring the iteration time. From the time - tempera-ture history, heating rates at different locations could be easily calculated. The fi nal simulation results are pre-sented in Table 4, along with the experimental results.EXPERIMENTAL SET-UPAs was stated before, the investigations were started from the geometry of an oval manufactured furnace. The aim was to establish a more convenient heating space [5]. Experiments were carried out in the same conditions for every case, and for the same initial heat-ing conditions.The experimental set-up is shown in Figure 3. The set-up is a well instrumented single-phase heat provided to measure the maximum heating temperature of both furnace and study charge. The heated enclosure includes two metallic radiant panels. The dimensions of the radi-ant panels were illustrated in Figure 1 and are the same with those used for simulation.The active experiment has started with a preliminary experiment performed in order to establish the effi cien-cy of using the radiant panels. After this, the radiant panels have been introduced in the furnace and dramat-ic decrease of the heating time has been noticed. The experiments have been performed in different days, thus maintaining the initial heating conditions for equipment as well as for charges. The data collection has been per-formed with the help of the computer by a Nomadics Thermocouple acquisition system. The temperatures were recorded automatically using two K type thermo-couples inserted in the furnace and sealed to prevent any leakage. Their accuracy is of ± 1,6 K at 773 K and ± 1 K at 673 K according to USA National Institute of Standards and Technology (NIST) as it is specifi ed in probes technical data’s. Respectively, furnace tempera-ture control accuracy is ± 0,25 K, as it is mentioned in furnace operator’s manual and, further, experimental system accuracy goes to ± 1,25 - ± 1,85 K, according to USA NIST.Kang and Rong [6] suggested that in order to ac-complish the desired interpretation, the experiment has to be programmed and this implies the following:• Set objectives,• Select process variables and levels, and• Select experimental design.This method is a common one in choosing an ex-perimental design and in this context, for each variable, there have been established the basic levels as well as the variation intervals. Choosing the variation interval must offer the most accurate values from the functional point of view. A fi rst step is to establish the basic levels and the variation intervals. In Table 2 the variation in-terval and the basic level for programming the experi-ment are presented. Moreover, Table 3 represents the furnace heating regime for all the experimental cases. The experiment has been rigorously conducted in order to assure its repeatability. As a testing charge, an AlMgSi cylindrical part of Φ 0,024 x 0,1 m has been used.Table 1 Simulation ModelsModel SettingsSpace 2DTime Unsteady, 1st-Order ImplicitViscous LaminarHeat Transfer EnabledRadiation Discrete Ordinate ModelFigure 2 Confi guration of the studied gridFigure 3 Experimental set-upTable 2 Experiment programmingFactor Panels’ adjustment distance /mBasic Level 0,12Variation interval 0,02 Superior Level (+1) 0,20Inferior Level (-1) 0,08201METALURGIJA 51 (2012) 2, 199-202A. A. MINEA: EXPERIMENTAL AND NUMERICAL SIMULATION ANALYSIS OF HEAT TRANSFER ON A CLOSED ENCLOSUREFrom the data collected in Table 4 it will consider the experimental curve:T = 1009,3 – 8,8743·x + 0,11064·x2 –– 0,00057802·x3 + 1,0807·10-6 ·x4 (1)with the notations:T – maximum heating temperature, Kx – panel position, m.This equation is very important because it gets a cor-relation between the maximum heating temperatures obtained depending on panels position for a maximum constructive heating rate for the considered oval fur-nace.This equation can be used according to technologi-cal needs refl ected in maximum heating rate and dimen-sions of the working parts.CONCLUSIONSAn experimental investigation, as well as simulation was carried out to study heat transfer in a modifi ed closed enclosure of an oval furnace.Furnaces with different heated enclosures were test-ed for counter-fl ow confi guration. It was revealed that the empirical correlation for constant temperature boundary condition is quite in agreement with the present data. From the results of the present study, it was found out that the total heating temperature is de-pending on the position of the radiant panels.If it compares the heating temperature obtained through introducing the panels with the heating tempera-ture after 1 200 seconds heating of the furnace without panels it can remark an increase of minimal 17,25 %.Also, the results obtained from experimental are similar with that ones obtained through simulation.As a fi nal conclusion, based on discussed results, a correlation was developed to predict the optimum cham-ber design for heat transfer enhancement of an oval fur-nace.DATA ANALYSISThe fi rst experiment was conducted for the basic furnace, without panels. A maximal heating temperature of 758,77 K was registered.In Table 3 the results of the centralized simulation and experiments are presented.The interpretation of the results has as purpose fi nd-ing a mathematical model that can describe as precisely as possible the physical processes that take place in that situation. Thus, for fi tting the experimental and simula-tion data’s a IGOR PRO commercial code was used. This program has wide capabilities to fi t the experimen-tal data with different equations and to calculate all the numerical data’s needed in order to establish its’ accu-racy. Data analysis starts with the most trustable equa-tions [7, 8 ].Figure 4 shows the 2D representation of the experi-mental data, which stood at the bases of this work. Thus, in Figure 4 are the experimental and simulation points along with the polynomial fi tted curves are presented for increasing heating temperature study.The fi tting curves were obtained by computer, by polynomial fi tting method and represent a correlation between panel position and heating temperature. More-over, in Table 4 are the statistical results for this inter-pretation.From Figure 4 it can conclude that the differences between the simulation and the experimental curve are very low. If it consider last column from Table 3 it can see that the differences are in a range of about 0,5-2 %, which is acceptable. Further a correlation between heat-ing time (at a maximal heating rate) and fi nal tempera-ture is proposed.Table 3 Centralized resultspanel posi-tion/ msimulation fi nal temperature / Kexperimental fi nal temperature / Kdiff erence / K0,200 756,74 765,10 8,360,180 758,77 759,31 0,540,160 760,31 764,79 4,480,140 758,22 760,95 2,730,120 751,04 765,86 14,820,100 751,07 756,98 5,910,080 751,77 756,01 4,24Figure 4 Data fi ttingTable 4 Statistical data interpretation for the fi tting curvesStatisticals Experimental Simulationfi t coeffi  cients 1009,3;-8,87;0,11;-5,7∙10-4;1,08∙10-6957,89;-6,12;0,06;-2,7∙10-4;4,1∙10-7measure of the goodness of fi t29,5215 7,17number of points that were fi tted7 7last wave point 6 6standard deviation of the fi t coeffi  cients360;11,3;0,12;0,001;1,13∙10-6177;5,59;0,06;0,0003;5,56∙10-7coeffi  cient values ± one standard devia-tionK0=1009,3 ± 360K1=-8,87 ± 11,3K2=0,11 ± 0,129K3=-0,0006 ± 0,0006K4=1,08∙10-6± 1,13∙10-6K0=957,89 ± 177K1=-6,12 ± 5,59K2=0,063 ± 0,06K3=-0,0003 ± 0,0003K4=4,1∙10-7 ± 5,56∙10-7202  METALURGIJA 51 (2012) 2, 199-202A. A. MINEA: EXPERIMENTAL AND NUMERICAL SIMULATION ANALYSIS OF HEAT TRANSFER ON A CLOSED ENCLOSUREREFERENCES[1] W.H. Fleming, J. A. Khan, C.A. Rhodes, Int. J. Hydrogen Energ. 26 (2001) 711–724.[2] P.K. Sahoo, M.I.A. Ansari, A.K. Datta, J. Food Eng. 58 (2003) 211–214.[3] I. Conte, X.F. Peng, Appl. Therm. Eng. 28 (2008) 423–432.[4] J. Yi, Z.H. Liu, J. Wang, Appl. Therm. Eng. 23 (2003) 89–99.[5] F.M. White, Heat Transfer, Addison-Wesley Publishing Company Inc., New York, 1994.[6] J. Kang, Y.K. Rong, Modeling and simulation of heat tran-sfer in loaded heat treatment furnaces. 1st ASM Internatio-nal Surface Engineering Conference and the 13th IFHTSE Congress; Columbus, OH; USA, 2002, 337-343[7] E. H. Chui, G.D. Raithby, P.M.J. Hughes, J. Thermophys. Heat Transfer, 1992.[8] J. Kang, Y.K. Rong, W. Wang, J. Phys. IV France (2004) 545-553Note:  Responsible translator the lecturer from Technical University Gh. Asachi Iasi, Romania

Eksperimentalna i numerička simulacija analize prijenosa topline u zatvorenom prostoru

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