Simulation of Steady-State and Dynamic Behaviour of a Plate Heat Exchanger

Transcription

1 Journal of Energy and Power Engneerng 10 (016) do: / / D DAVID PUBLISHING Smulaton of Steady-State and Dynamc Behavour of a Plate Heat Exchanger Mohammad Aqeel Sarareh Department of Mechancal Engneerng, Mutah Unversty, Kara 61710, Jordan Receved: June 14, 016 / Accepted: June 3, 016 / Publshed: September 30, 016. Abstract: The present paper deals wth both the steady-state and dynamc smulaton of a plate heat exchanger, n counter-flow arrangement. A CFD (computatonal flud dynamcs) program FLUENT has been used to predct the temperature dstrbuton n steady-state condtons n plate heat exchanger as well as flud temperatures at ext of flow channels n transent condton. The results are presented for the heat exchanger, whch s smulated accordng to the confguraton of the plate heat exchanger used n the experment. The smulated results obtaned by the CFD model have been compared wth the expermental data from the lterature, whch shows that the CFD model developed n ths study s capable of predctng the steady-state and transent performance of the plate heat exchangers satsfactorly. Key words: Heat exchanger, steady-state, CFD, smulaton. Nomenclature C p Specfc heat, J/g K E Energy, J eff Effectve thermal conductvty, W/m K f Turbulent thermal conductvty, W/m K Turbulent netc energy, m /s P Pressure, Pa S Modulus of the mean rate-of-stran tensor T Temperature, K t Tme, s u Velocty, m/s Gree symbols Densty, g/m 3 Turbulent energy dsspaton rate, m /s 3 Knematc vscosty, m /s Dynamc vscousty, g/m s t 1. Introducton Turbulent vscousty, g/m s Applcatons such as, power generaton, manufacturng and space ndustres, as well as chemcal reacton Correspondng author: Mohammad Aqeel Sarareh, Ph.D., assstant professor, research felds: computatonal heat and flud dynamcs, heat exchangers. processes use plate heat exchangers due to ther excellent heat transfer characterstc, easy fabrcaton and mantenance, low weght, excellent corroson resstance, and cost compettveness. Dynamc behavour of n-channel two-flud heat exchangers s studed n Refs. [1-4]. Zales and Tajszer [5] presented the smulaton of the dynamc performance for co-current plate heat exchangers. Khan et al. [6] presented the dynamc analyss of counter-current plate heat exchanger subjected to flow transent. Mehraban [7] and Mehraban and Poulter [8] developed analytcal solutons for temperature dstrbutons wthn a plate heat exchanger. The wor was extended to focus on expermental approach for local pressure and local temperature measurements to understand the hydrodynamc and thermal characterstcs of corrugated channels. Sharf et al. [9] numercally analysed a plate heat exchanger wth counter-flow arrangement n steady state and transent condtons. They used dfferent numercal methods to predct the temperature dstrbuton n steady state condton as well as flud temperatures at ext of flow channels n transent condton. They also valdated ther numercal

2 556 Smulaton of Steady-State and Dynamc Behavour of a Plate Heat Exchanger results by comparng them to expermental results n a test rg usng specally desgned flat plates. Srhar et al. [10] presented the dynamc behavour of the sngle pass plate heat exchangers. Galeazzeo et al. [11] studed heat transfer n plate heat exchanger expermentally and numercally. Computatonal flud dynamcs tool was used to buld the vrtual prototype of the plate heat exchanger wth four channels and flat plates. Temperature and velocty dstrbuton for the channels were presented and compared wth expermental data. Dwved and Das [1] nvestgated transent performance of the plate heat exchanger subjected to step flow dsturbances. In the study by Vas et al. [13], a transent model of fn-and-tube heat exchangers was derved, and the varaton of the temperatures of both flud wth tme and poston were obtaned for a step change n the nlet temperatures of the water and ar fluds. Yao et al. [14] developed transent response model for the water to ar heat exchanger by usng the state-space method. Dynamc responses of ext ar and water temperature subjected to the dsturbance of nlet water temperature were valdated by dynamc response experments. Sngh et al. [15] nvestgated numercally transent behavour of co-current parallel flow heat exchangers. In the present study, three-dmensonal numercal smulatons usng CFD (computatonal flud dynamcs) were carred out. Temperature dstrbuton n case of steady-state and transent response of the plate heat exchanger was obtaned.. Numercal Smulaton.1 Computatonal Doman The heat exchanger adopted n ths nvestgaton has the same dmensons and propertes as those nvestgated by Sharf et al. [9]. A general scheme of the counter flow, plate heat exchanger s shown n Fg. 1. The computatonal doman used n the smulaton s detaled by Fg. and Table 1.. Governng Equatons The governng equatons adopted by ANSYS FLUENT are as follows [16]: Contnuty equaton: Hot flud Cold flud t x u 0 (1) Fg. 1 Schematc drawng of the plate heat exchanger wth 7 flat plates and 6 flow channels. Fg. Computatonal doman of smulated heat exchanger flow passages. Table 1 Plate geometry and flow detals. Channel length L 0.33 m Channel wdth W 0.09 m Plate heat transfer area A m Channel gap B m Plate thcness m Hot water nlet temperature 65 C Cold water nlet temperature 10 C Hot water nlet mass flow rate g/s Cold water nlet mass flow rate 0.35 g/s W B L

3 Smulaton of Steady-State and Dynamc Behavour of a Plate Heat Exchanger 557 Momentum equaton: u P uu j t x x j u u u uu x x x x x where, j l ' ' j j j j 3 l j () ' ' u u j u uu j t t (3) j xj x 3 x Transport equatons adopted by the Realzable model: uj t x and where, x j S t t j xj u j t xj t C xj xj Energy equaton: t C, S SjSj, 1, 1., C 1.9 u E p eff x x (4) (5) T ( E) (6) t x where, E s the total energy and eff t s the effectve conductvty, ncludng the turbulent thermal conductvty t. Energy transport equaton n the sold regon:.3 Boundary Condtons T cpt s x x t (7) The velocty boundary condton s used to defne the velocty magntude and drecton of the water at the hot and cold nlets of the heat exchangers. The nlet temperature s also requred at ths boundary. The pressure outlet condton s used to model the flow condtons at the outlets of the heat exchanger. All sold surfaces of the heat exchanger are modelled usng the wall boundary condton. Because the wall zone here s a two-sde wall, whch s a wall that forms the nterface between two regons such as the flud/sold nterface, FLUENT enables the two sdes of the wall to be coupled, promptng the solver to calculate the heat transfer drectly from the soluton n the adjacent cells. All other sold surfaces are defned as an adabatc wall by settng a zero heat flux condton..4 Solver The governng equatons for the conservaton of mass, momentum, and energy equatons were solved usng commercal software FLUENT. The SIMPLE (sem-mplct method for pressure lned equaton) algorthm was used for the calculaton of the pressure and the velocty, whch were needed for the soluton of energy equaton. The smulaton was also solved usng the Realzable turbulence model wth enhanced wall treatment. Tests were conducted to verfy the grd ndependency of the results. Smulatons were performed for dfferent grds sze, untl the results were consstent. The numercal doman conssted of 00,000 elements and a second-order upwnd dfferencng scheme was used at all nteror grd ponts. 3. Results and Dscusson To valdate the model, smulated data were compared wth the expermental data reported by Sharf et al. [9]. The plate heat exchanger geometrc parameters and flow detals lsted n Table 1 were ncorporated n the present model for the valdatons. The smulated data from the present modellng n steady state condton were compared wth the

4 558 Smulaton of Steady-State and Dynamc Behavour of a Plate Heat Exchanger temperature dstrbuton of flow n the channels of plate heat exchanger reported by Sharf et al. [9]. The varatons of hot water temperature n channels 1, 3 and 5 wth respect to channel length are shown n Fgs As expected, the numercal smulatons ndcated a decreasng hot water temperature along the flow drecton due to heat transfer from hot to the cold water. From these fgures, t can be seen that, there s a good agreement between the smulaton results of presented model and the expermental results of Sharf et al. [9]. The dstrbutons of temperature for cold water along channels, 4 and 6 are shown n Fgs These fgures ndcate that, the temperature of cold water ncreased along the flow drecton. It can be concluded from Fgs. 6-8 that the smulaton results are reasonably n good agreement wth the expermental data. Smulatons have been carred out to predct the plate heat exchanger behavor, n terms of outlet Fg. 5 Comparson of expermental and smulated hot temperature dstrbuton n channel 5. Fg. 6 Comparson of expermental and smulated cold temperature dstrbuton n channel. Fg. 3 Comparson of expermental and smulated hot temperature dstrbuton n channel 1. Fg. 7 Comparson of expermental and smulated cold temperature dstrbuton n channel 4. Fg. 4 Comparson of expermental and smulated hot temperature dstrbuton n channel 3. Fg. 8 Comparson of expermental and smulated cold temperature dstrbuton n channel 6.

5 Smulaton of Steady-State and Dynamc Behavour of a Plate Heat Exchanger 559 temperatures of the two fluds, for transent operatng condtons. For example, t s useful to determne the response of the system to nputs changes of the nlet temperatures and the varatons of flow rates. Fgs. 9 and 10 show the response of temperatures when hot flud flow rate s 38 cm 3 /s, cold flud flow rate s 35 cm 3 /s, and hot flud step temperature s 56 C. To chec the valdty of the smulaton model the predcted results have been compared wth the expermental data. Fgs. 9 and 10 show the comparson between the smulaton and expermental results of Sharf et al. [9] run 1 for the outlet temperatures of hot and cold water. It can be observed from the fgures that there s a good agreement between model and expermental results. Fgs. 11 and 1 show the comparson of the model predctons and the expermental results of Sharf et al. [9] for run number when hot flud flow rate s 40 cm 3 /s, cold flud flow rate s 90 cm 3 /s, and hot flud step temperature s 56 C. It can be clearly observed from the fgures that represent the flow transent, both the outlet Fg. 11 Comparson of expermental and smulated outlet hot temperature for run number. Fg. 1 Comparson of expermental and smulated outlet cold temperature for run number. temperatures are ncreased. They go on ncreasng tll a new steady state s acheved. As depcted n Fgs. 11 and 1, the obtaned CFD results show a good agreement wth the expermental data. 4. Conclusons Fg. 9 Comparson of expermental and smulated outlet hot temperature for run number 1. Fg. 10 Comparson of expermental and smulated outlet cold temperature for run number 1. Steady-state and dynamc behavor of counter flow heat exchanger has been nvestgated usng CFD. Governng equatons are solved usng FLUENT software n order to get the steady state temperature dstrbuton along the channels of the plate heat exchanger. Tang ths temperature dstrbuton as ntal condton, governng equatons for the flow transent have been solved. To chec the valdty of the smulaton, the results of the CFD model have been compared to the experment from the lterature. The comparson ndcates a good agreement, whch valdates the suggested model for steady state and transent flow n plate heat exchangers. Results also

6 560 Smulaton of Steady-State and Dynamc Behavour of a Plate Heat Exchanger ndcate the allowable tme duraton requred for the control system to brng bac a plate heat exchanger to steady state. References [1] Correa, D. J., and Marchett, J. L Dynamc Smulaton of Shell-and-Tube Heat Exchangers. Heat Transfer Engneerng 8 (1): [] Lashmanan, C. C., and Potter, O. E Dynamc Smulaton of Plate Heat Exchangers. Internatonal Journal of Heat and Mass Transfer 33 (5): [3] Roetzel, W., and Xuan, Y Transent Behavour of Multpass Shell-and-Tube Heat Exchangers. Internatonal Journal of Heat and Mass Transfer 35 (3): [4] Roppo, M. N., and Ganl, E. N Tme-Dependent Heat Exchanger Modelng. Heat Transfer Engneerng 4 (): 4-6. [5] Zales, T., and Tajszers, J Dynamcs of the Plate Heat Exchangers. Computatons n the Desgn and Erecton of Chemcal Plants (September): 3-5. [6] Khan, A. R., Baer, N. S., and Wardle, A. P The Dynamc Characterstcs of a Countercurrent Plate Heat Exchanger. Internatonal Journal of Heat and Mass Transfer 31 (6): [7] Mehraban, M. A Influence of Overall Heat Transfer Coeffcent on Performance of Plate Heat Exchangers. Insttute of Mathematcs and Its Applcatons Conference Seres 66: [8] Mehraban, M. A., and Poulter, R Hydrodynamcs and Thermal Characterstcs of Corrugated Channels: Computatonal Approach. Appled Mathematcal Modellng 4 (5): [9] Sharf, F., Narandj, M. R. G., and Mehravaran, K Dynamc Smulaton of Plate Heat Exchangers. Internatonal Communcatons n Heat and Mass Transfer (): [10] Srhar, N., Rao, B. P., Sunden, B., and Das, S. K Transent Response of Plate Heat Exchangers Consderng Effect of Flow Maldstrbuton. Internatonal Journal of Heat and Mass Transfer 48 (15): [11] Galeazzo, F. C. C., Mura, R. Y., Gut, J. A. W., and Tadn, C. C Expermental and Numercal Heat Transfer n a Plate Heat Exchanger. Chemcal Engneerng Scence 61 (1): [1] Dwved, A. K., and Das, S. K Dynamcs of Plate Heat Exchangers Subject to Flow Varatons. Internatonal Journal of Heat and Mass Transfer 50 (13): [13] Vas, A., Taleb, S., and Esmaelpour, M Transent Behavor Smulaton of Fn-and-Tube Heat Exchangers for the Varaton of the Inlet Temperatures of Both Fluds. Internatonal Communcatons n Heat and Mass Transfer 38 (7): [14] Yao, Y., Huang, M., Mo, J., and Da, S State-Space Model for Transent Behavor of Water-to-Ar Surface Heat Exchanger. Internatonal Journal of Heat and Mass Transfer 64 (September): [15] Sngh, S. K., Mshra, M., and Jha, P. K Transent Behavor of Co-current Parallel Flow Three-Flud Heat Exchanger. Internatonal Communcatons n Heat and Mass Transfer 5 (March): [16] Ansys Fluent V14.0 Theory Gude, 011.