NUMERICAL INVESTIGATION AND EXPERIMENTAL VALIDATION OF HEAT TRANSFER IN A SMALL SIZE SHELL AND TUBE HEAT EXCHANGER

Transcription

1 1 M. Kirinčić, A. rp, K. Lenić: Numerical investigation and NUMERICAL INVESIGAION AND EXPERIMENAL VALIDAION OF HEA RANSFER IN A SMALL SIZE SHELL AND UBE HEA EXCHANGER Mateo Kirinčić * Anica rp Kristian Lenić Department of hermodnamics and Energ Engineering, Facult of Engineering, Universit of Rijeka, Vukovarska 58, Rijeka, Croatia ARICLE INFO Article histor: Received: Received in revised form: Accepted: Keords: Heat transfer Small sie shell and tube heat echanger Segmental baffles Finite volume method Eperimental validation 1 Introduction Heat echanger tpes and designs var greatl depending on their use, but the all possess a common feature: their purpose is to transfer heat beteen to fluids so as to heat up or cool don one of them. Of all designs probabl the most common is the shell-and-tube one; the heat echanger consists of a tube bundle and an outer shell surrounding it. One fluid flos through the tubes, and the other around them. In order to support the tube bundle and increase heat transfer b increasing turbulence and retention of one fluid, flo-directing panels called baffles are used. Since eperimental research is often comple and financiall demanding, it is useful to design a mathematical model for a heat transfer problem in a heat echanger and easil determine all heat transfer- Abstract: Heat echangers are integrated in all process and energ plants. Shell and tube heat echanger designs are most commonl used. he efficienc and performance of the device can be determined both eperimentall and numericall. In this stud, a numerical model of heat transfer in a small sie shell and tube heat echanger is presented, and the results are compared ith eperimental data. he problem ith laminar flo and stead state heat transfer as solved using the finite volume method. hree eperiments ere performed, and all of them shoed a high match beteen outlet fluid temperatures. As additional validation, heat flu balance as set and calculated for both methods, hich also shoed a considerable match. It can be concluded that the model accuratel predicts phsical phenomena in analed heat echanger, and can be used in further studies. related data required, especiall hen it requires varing sets of inlet parameters. Previous research has dealt ith similar problems, but some as purel eperimental or purel numerical, and the research that included both methods has used heat echangers ith different geometr and different kinds of flo. For eample, Jadhav and Koli [1] performed some numerical research into pressure drops and heat transfer coefficient variations on the shell side depending on baffle number and height, as ell as shell diameter. A similar stud performed b Arjun and Gopu [] dealt ith optimiation of a numerical model of a heat echanger ith helical baffles, regarding the flo rate and heli angle. Wen et al. [3] proposed a ladder-tpe fold baffle to enhance the performance of a heat echanger ith helical baffles, hich resulted * Corresponding author. el address: mateo.kirincic@riteh.hr

2 Engineering Revie, Vol. 37, Issue, 1-133, in significant improvement in heat transfer coefficient and thermal performance in general. You et al. [4] presented a numerical validation of a heat echange problem for a turbulent flo and suggested heat transfer enhancements b structural and geometrical modifications. A research performed b Pal et al. [5] dealt ith turbulent heat transfer using the k-ε method in a small sie shell and tube heat echanger, both ith or ithout baffles, observing that the cross flo near the nole region has a much higher effect on the heat transfer, as opposed to the parallel region. he correlation beteen heat echanger sie and nole region influence as previousl observed b Kim and Aicher [6]. A combined research as performed b Vukić et al. [7] for a turbulent flo using the PHOENICS code. Yang and Liu [8] described a numerical model ith eperimental validation of a novel heat echanger ith ne plate baffles, as opposed to rod baffles, hich resulted in an improved performance. Yang et al. [9] designed four different numerical models (the unit model, periodic model, porous model, hole model) of heat transfer in a rod-baffle shell and tube heat echanger and compared it to eperimental data, resulting in fairl good results in all models ecept the unit one. he goal of this research is to investigate the validit of a 3D model of a heat transfer problem in a small sie shell-and-tube heat echanger ith segmental baffles b comparing the results of a numerical calculation to eperimental results acquired from the device. he accurac is to be determined b three separate eperiments, all ith different inlet parameters. Primar values that are to be compared are the outlet fluid temperatures and heat flues acquired and calculated b both methods. Eperiments ere performed on the educational D360c heat echanger, manufactured b ecquipment Ltd. Both fluids are single phase (liquid ater), the flo is laminar and countercurrent, and heat transfer is stead state. he numerical method used is the finite volume method, and the model as designed in ANSYS softare. Mathematical model Mathematical modeling is used to describe an actual phsical phenomenon using differential equations, and initial and boundar conditions. Differential equations describe the change of a variable ithin the domain, initial conditions define values of all variables in the initial moment, and boundar conditions define the variables at geometric boundaries of the domain..1 Phsical problem he device in hich the heat transfer problem takes place is a small sie heat echanger, consisting of: outer shell 7 4/6 mm tubes in the tube bundle 3 segmental baffles inlet and outer plena. Hot ater enters through the inlet plenum, flos through the tubes, and eits at the outer plenum. Cold ater flos on the shell side around the tubes, entering and eiting the device through the noles on the top. Fig. 1 shos dimensions of analed heat echanger. Since the heat transfer environment, i.e. the device, is longitudinall smmetrical (as shon in Fig. 1) and both fluids feature a single flo, it is sufficient to single out one half of the heat echanger for an adequate thermodnamic analsis; since the phsical changes on that side mirror themselves along the longitudinal smmetr plane, thus avoiding redundant calculation and saving time and memor. Because of that, onl the longitudinal half is used as the geometric model environment, and it is displaed in Fig... Governing equations he model domain includes three subdomains; hot ater, all, and cold ater, each of them represented in Fig. 3. It is assumed that there is no heat transfer beteen cold ater and shell. For each of the subdomains, the conservation equations ill be applied. hese include the continuit equation, hich presumes that the amount of matter entering a certain volume of space must be equal to the amount eiting it; momentum equations (Navier-Stokes equations), the three equations in three spatial directions defining the balance of forces in a volume; and the energ equation, also called the heat balance equation. It is also assumed that the phsical properties of the materials, such as densit, heat capacit, conductivit, and viscosit are constant. Since the all is a solid, onl the energ equation applies for this subdomain.

3 14 M. Kirinčić, A. rp, K. Lenić: Numerical investigation and Figure 1. Heat echanger cross-section ith displaed dimensions. Figure. CAD model of the heat transfer domain. he equations for laminar flo are as follos: 1. Hot ater 0 (1) h h p ()

4 Engineering Revie, Vol. 37, Issue, 1-133, Figure 3. Calculation domain, (a) hot ater subdomain, (b) all subdomain, (c) cold ater subdomain. h h p (3) h h p (4) h h h c (5). Wall 0 (6) 3. Cold ater 0 (7) c c p (8) c c p (9) c c p (10)

5 16 M. Kirinčić, A. rp, K. Lenić: Numerical investigation and c c c c.3 Boundar conditions (11) Variables distribution ithin the domain is defined b boundar and initial conditions. Since the problem is stead state, onl boundar conditions ill appl. he position of each boundar condition is highlighted green in Fig. 4. Inlet boundar conditions are set at the entr point of each fluid at the model boundar. In this case, the values of velocit components and temperature are set. hese values are based on the input parameters in each of the three eperiments on the D360c heat echanger. Outlet boundar conditions are set at the model outlet boundar perpendicular to the flo, here it is assumed that the flo is full developed and no changes occur in flo direction, so the gradients of all variables (ecept pressure) are ero. Figure 4. Boundar conditions marked in green, (a) inlet boundar condition, (b) outlet boundar condition, (c) smmetr boundar condition, (d) all-ater interface, (e) all boundar condition (insulated), (f) all boundar condition (insulated - rear).

6 Engineering Revie, Vol. 37, Issue, 1-133, For this problem, it means that gradients of fluid temperature and velocities are both ero: n 0 n 0 n (1) (13) Wall boundar conditions assume that the velocit components at the alls equal ero, = 0. It is also assumed that heat transfer ithin the boundar laer beteen the fluid and the all occurs onl b conduction: f (14) n n In this particular instance, the thin laer of hot ater in contact ith the all (inner tubes all) delivers heat to the all b conduction, the heat is conducted through the all, and the cold ater on the other side receives heat from the all b conduction also. his boundar condition also applies to those surfaces of the baffles that are in contact ith cold ater. Wall boundar conditions are also set at previousl undefined surfaces of model edges, including surfaces of the baffles in contact ith the shell. It is assumed that those surfaces are perfectl insulated and do not echange heat ith the environment. n 0 (15) Smmetr boundar condition is set along the longitudinal smmetr plane of the model. It states that the flo across the boundar equals ero, and that the scalar flo across the boundar is ero. For this problem, it means that the velocit component perpendicular to the smmetr line is equal to ero, as ell as the gradients of temperatures and all other velocit components ( being each of the variables in question): 0 n (16) 0 n (17) 3 Numerical solution he problem as solved using the finite volume method, a numerical method based on the general conservation equation, described b Patankar [10]. Since velocit distribution throughout the model as unknon, an algorithm for its calculation as used. For pressure and velocit coupling, the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm as used. he numerical calculations ere performed in Anss (Anss Geometr, Anss Mesh, Anss Fluent). A grid of 5899 cells as used. Mesh independenc analsis has been performed. he used mesh has been selected as the most suitable ith respect to accurac of the solution and calculation duration. Fig. 5 shos the final mesh. Convergence criteria ere set for each of the conservation equations as follos: continuit: 10-3, momentum: 10-6, energ: Eperimental setup and validation he eperimental part of the research as done on the D360c, an educational single flo shell-andtube heat echanger. Its parts, and the close-up of the device itself, are shon in Fig. 6. he entire sstem consists of the heat echanger, control panel, a storage tank ith heating capabilit, copper tubes hich suppl the fluids, and a pump hich enables the circulation of the armer fluid. On both ends of the echanger, there are temperature sensors and flo meters, the measured values of hich are displaed on the control panel for each fluid. emperature is measured ith thermocouples ith an accurac of ±0.5 C. he heat echanger is so designed that the higher temperature fluid flos through the tubes, and the loer temperature one around them. In this case, both fluids are fed from a ater suppl and are at the same temperature. Hoever, before reaching the heat echanger, the fluid that is to flo through the tubes is heated to a desired temperature in the aforementioned tank, and afterards pumped through the tubes. After passing the heat echanger, it is refunded back to the tank and reheated. he temperature in the tank is defined on the control panel, the maimum value being 60 C. he flos of

7 18 M. Kirinčić, A. rp, K. Lenić: Numerical investigation and both fluids are controlled ith the valves on the control panel, and, in this case, ith the faucets. he loer temperature fluid connections can be sitched so the heat echanger can be used both in concurrent flo and countercurrent flo. Since concurrent flo is not as efficient as countercurrent flo, the latter is used. All activities for this segment of the research ere performed in the Laborator for hermal Measurements at the Facult of Engineering, Universit of Rijeka. After filling the heat tank, desired temperature in the tank is set. Figure 5. Meshed calculation domain. Figure 6. he D360c heat echanger sstem (left) and close-up of the device (right).

8 Engineering Revie, Vol. 37, Issue, 1-133, Cold ater is fed from the ater suppl, so it enters the heat echanger at the temperature that is in the ater suppl grid. here is a constant flo of both fluids through the heat echanger, and it is regulated on the faucet or small valves at their entrance to the device. During the first eperiment, after about ten minutes of fluctuation, the heat echange got stead, i.e. there ere no changes in each of the temperature values. For additional validit check of numerical results, a heat flues balance beteen hot and cold ater has been calculated: V V h c h c c c h c t t h c t t h c (18) he second and third eperiments differ from the first one in terms of temperature (loer than the first) and volume flo (tice as big as the first), respectivel. able 1 shos inlet parameters (temperature and flo), outlet temperatures for both hot and cold ater for all three eperiments, and heat flues on both sides. 5 Numerical results Numerical calculations have been performed for three different cases in total, input parameters of hich are displaed in able 1. hese parameters match the conditions set in the three eperiments on the heat echanger D360c. Fig. 7 displas the temperature distribution ithin the heat echanger for the first set of input parameters. A decrease in temperature is clearl visible in the central tube section. Consequentl, an increase in cold ater temperature along the shell is shon, as ell as the influence of the baffles, hich increase heat transfer b prolonging the passage of cold ater, as ell as turbulence. Fig. 8 displas velocit vectors and Fig. 9 the closeups on the entr and eit points for each fluid. It is visible that both fluids enter the heat echanger ith a uniformed velocit profile, and once the flo has progressed, their profile becomes less uniformed, i.e. the flo gets more developed. It is also visible that due to the no-slip condition, velocit is highest at the tubes' center and loest as it approaches the tubes' all. In order to preserve continuit, there needs to be an increase at the center to compensate for the drop at the edge. Calculations results ill also serve in setting up heat flues balance beteen the hot and cold ater. It ill, along ith outlet fluid temperatures, serve as an additional validit check for the model. he heat flu given aa b hot ater must be equal to the heat flu received b cold ater: Q (19) 1 Q For each fluid, heat flues calculated from the data acquired in numerical investigation are represented in able. able shos variations in outlet temperatures for both fluids due to varing inlet conditions in each case. able 1. Eperimentall obtained outlet temperatures and heat flues for different ater inlet parameters Hot ater Cold ater Case V h [m 3 /s] t h t h Q 1 V c [m 3 /s] t c t c Q

9 130 M. Kirinčić, A. rp, K. Lenić: Numerical investigation and Figure 6. emperature distribution ithin the heat echanger for t h = 56.4 C, h = 0.1 m/s, t c = 6.9 C: c = m/s: (a) front vie, (b) isometric vie, (c) hot ater inlet section, (d) hot ater outlet section. Figure 7. Velocit vectors in the smmetr plane for t h = 56.4 C, h = 0.1 m/s, t c = 6.9 C: c = m/s in the smmetr plane (vector color denotes velocit intensit).

10 Engineering Revie, Vol. 37, Issue, 1-133, Figure 8. Close-ups of velocit vectors: (a) hot ater inlet, (b) hot ater outlet, (c) cold ater outlet, (d) cold ater inlet (vector color denotes velocit intensit). able. Numericall obtained outlet temperatures and heat flues on both sides for different ater inlet temperatures Case t Hot ater h Q 1 t c Cold ater Q In the case ; here the inlet temperature of hot ater is loer than the first one, outlet temperatures var less than in the first eperiment. hat is to be epected, considering the decrease in inlet temperature difference of both fluids, hich results in a less intense heat transfer. A direct result of smaller inlet temperature difference is a decrease in the heat flu values. In the case 3; here hot and cold ater volume flos are approimatel tice the value of the first, cold ater outlet temperature is loer in comparison ith the case 1. Providing the flos of both hot and cold ater have doubled, the hot ater does not cool, and the cold ater does not arm to the etent as the did in the case 1. he heat flu is the highest in this case, the reason being the flo increase ith respect to the first case. After having made three numerical investigations and three eperiments ith different inlet parameters, the results of both methods need to be compared in order to assess the model's validit. his comparison is presented in able 3, hich shos outlet temperatures of hot and cold ater, as ell as both heat flues. he table shos a ver good match beteen outlet temperatures of both fluids and beteen heat flues acquired b numerical calculation and eperimental data in all three cases considered. It can be concluded that this model faithfull represents the phenomenon of heat transfer in the small sie D360c shell-and-tube heat echanger.

11 13 M. Kirinčić, A. rp, K. Lenić: Numerical investigation and able 3. Comparison of outlet temperatures and heat flues on both sides for numerical and eperimental investigation Numerical model Eperimental results Case t h t c Q 1 Q t h t c Q 1 Q Conclusion he purpose of this research as both to design a numerical model of an educational shell-and-tube ater-ater heat echanger (the ecquipment's D360c) and, using the data acquired in eperimental research, to question its validit. It as done b performing three eperiments ith different inlet parameters, hich ere later used in the numerical calculations. he model as solved ith the finite volume method, using the SIMPLE algorithm, for laminar flo and stead state heat transfer. Numerical analsis as performed using Anss (Geometr, Mesh, Fluent) softare. B comparing the results acquired b both numerical modelling and eperimental investigation, a validit assessment of the model as determined. Outlet temperatures of both hot and cold ater, as ell as heat flues, calculated numericall and measured eperimentall in all three cases sho a satisfactor match, hich points to a conclusion that the model accuratel describes the heat transfer problem ithin the educational D360c heat echanger. he developed model can be used in further studies as a basis for creating other similar models of fluid flo and heat transfer in shell and tube heat echangers. Nomenclature c Specific heat capacit, J/kgK F Area of heat transfer, m n Normal p Pressure, Pa Q Heat flu, W q Heat flu densit, W/m emperature, K t Water inlet temperature, C t V Water outlet temperature, C Volume flo, m 3 /s Velocit, m/s,, Coordinates, m Greek smbols Subscripts c f h n References Dnamic viscosit, Pa s hermal conductivit, W/mK Densit, kg/m 3 Phsical propert, various Cold ater Fluid Hot ater Normal Wall [1] Jadhav, A.D., Koli,.A.: CFD Analsis of Shell and ube Heat Echanger to Stud the Effect of Baffle Cut on the Pressure Drop, International Journal of Research in Aeronautical and Mechanical Engineering, (014), 7, 1-7. Arjun K.S., Gopu K.B.: Design of Shell and ube Heat Echanger Using Computational Fluid Dnamics ools, Research Journal of Engineering Sciences, 3 (014), 7, Wen, J., Yang, H., Wang, S., Xue, Y., ong, X.: Eperimental investigation on performance comparison for shell-and-tube heat echangers

12 Engineering Revie, Vol. 37, Issue, 1-133, ith different baffles, International Journal of Heat and Mass ransfer 84 (015), You, Y., Chen, Y., Xie, M., Luo, X., Jiao, L., Huang, S.: Numerical simulation and performance improvement for a small sie shelland-tube heat echanger ith trefoil-hole baffles, Applied hermal Engineering, 89 (015), 0-8. [5] Pal, E., Kumar, I., Joshi, J.B., Maheshari, N.K.: CFD simulations of shell-side flo in a shell-and-tube tpe heat echanger ith and ithout baffles, Chemical Engineering Science, 143 (016), [6] Kim, W-K, Aicher,.: Eperimental investigation of heat transfer in shell-and-tube heat echangers ithout baffles, Korean J. of Chem. Eng., 14 (1997),, [7] Vukić, M. Vučković, G., Živković, P., Stevanović, Ž., omić, M.: 3D Numerical Simulations of the hermal Processes in the Shell and ube Heat Echanger, Facta Universitatis, Series: Mechanical Engineering, 11 (013),, [8] Yang, J., Liu, W.: Numerical investigation on a novel shell-and-tube heat echanger ith plate baffles and eperimental validation, Energ Conversion and Management 101 (015), [9] Yang, J., Ma, L. Bock, J., Jacobi, A.M., Liu, W.: A comparison of four numerical modeling approaches for enhanced shell-and-tube heat echangers ith eperimental validation, Applied hermal Engineering, 65 (014), 1-, [10] S.V. Patankar: Numerical Heat ransfer and Fluid Flo, Hemisphere Publishing Corporation, alor & Francis Group, Ne York, 1980.