Heat Transfer Model of Casted Heat Exchanger in Summer Condition Yu Jie 1,2,a, Ni Weichen 1,2,b and You Shijun 3,c

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1 Internatonal Conference on Intellgent Systems Research and Mechatroncs Engneerng (ISRME 205) Heat Transfer Model of Casted Heat Exchanger n Summer Condton Yu Je,2,a, N Wechen,2,b and You Shjun 3,c School of Automaton, Tanjn Unversty of Technology, , Chna 2 Tanjn Key laboratory for Control Theory and Applcatons n Complcated System, , Chna 3 School of Envronment Scence and Technology, Tanjn Unversty, , Chna a yuje_89@39.com, b vson.n@63.com, c yousj@tju.edu.cn Keywords: Seawater-source heat pump; Casted heat exchanger; Heat conducton; Natural convecton. Abstract. To overcome the dsadvantages of the open loop seawater source heat pump (SWHP), a closed loop SWHP system wth casted heat exchangers (CHEs) s presented n ths paper. Consstng of closed-loop of ppes mmersed n seawater, CHEs are devsed for extracton or njecton of thermal energy from/nto the seawater. A mathematcal model has been establshed and solved analytcally to descrbe the heat transfer process of CHEs n summer. An expermental system has been bult to smulate the heat transfer process. The expermental results show that the relatve error of the temperature dstrbutons along the CHEs between the expermental data and numercal smulaton result s less than 5%. As a result, the mathematcal model can be appled to optmzaton analyss of the CHEs used for SWHP systems. Introducton Due to reduced energy consumpton and mantenance costs, seawater source heat pump (SWHP) systems, whch use seawater as a heat source/snk, have been ganng ncreasng popularty for space condtonng n buldngs for coastal areas [-2]. Tanjn s an mportant port n Chna and s rch n seawater. The geographcal factors and recent energy polcy provde convenent condtons for the applcaton of SWHP project [3]. Recently, many researchers have carred out a great deal of research about SWHP systems. For nstance, A. Attomäk studed the possblty of usng lake water as heat source for heat pump heatng n cold clmate [4]. O. Büyükalaca, F. Eknc and T. Ylmaz nvestgated the Seyhan Rver and dam lake as heat source snk for a heat pump [5]. Yu Je researched the heat transfer process of the CHEs used for SWHP n cng and no-cng condtons n wnter [6].SWHP s classfed nto open and closed loop SWHP systems accordng to the shape of the loop under the water [7-0]. Most researches about SWHP use the open loop system. But one of the bggest challenges of ths system s scalng. The water used by the system needs to be recharged to the seawater, so the chemcal method can t be appled. In addton, sea creatures n seawater would enter the SWHP system and the larvae and spores may grow n the ppelne so that seawater supply system can t be operated normally. So seawater must be treated before t entered the SWHP system. Thus the ntal nvestment of the system was ncreased. Therefore, for the purpose of overcomng the dsadvantages of the open loop SWHP, a closed loop SWHP system wth casted heat exchanger (CHE) s presented n ths paper. The unt of the closed loop system doesn t contact wth the seawater drectly, so there s no scalng and breedng n ths system. Moreover, because there s no need to overcome the resstance from the dstrbuton pont to the unt so that the energy consumpton n the closed loop system s less than the open loop. And n the closed loop system, the heat exchanger can get more temperature dfference between nlet and outlet flud to reduce the water flow rate. In the present study, the mathematcal model that descrbed heat transfer process of the CHE n summer was establshed. And an expermental rg whch smulated the heat transfer process of CHE was bult to valdate the mathematc model The authors - Publshed by Atlants Press 482

2 Nomenclature c p specfc heat[j/kgk] Greek symbols d dameter[m] λ thermal conductvty[w/mk] Gr Grashof number ν knetc vscosty coeffcent [m 2 /s] h convecton coeffcent[w/m 2 K] ρ densty[kg/m 3 ] Pr Prandtl number Subscrpts Q heat energy[w] f flud r o foulng coeffcent[m 2 K /W] -th control volume t temperature[k] n nner ppe u velocty[m/s] out outer ppe x control volume length[m] w ppe wall Mathematcal modelng The schematc dagrams of a CHE control volume for heat transfer n summer are llustrated n Fg.. The flow drecton of the secondary refrgerant s also gven n Fg..The CHE s dvded nto N control volumes equally along the flow drecton. And the length of every control volume s x. The analyss of every control volume based on the energy balance s performed subject to the followng assumptons []: () Thermal propertes of the ppe and seawater are constant. (2) The temperature and flow rate of flud n the ppe are unform at any cross- secton. (3) The effect of the curve radus s neglected. (4) The conductve heat transfer along the flud channels s very small and therefore neglected. (5) The model s n the steady state. Model n summer. The heat transfer between the secondary refrgerant flowng n the CHE and the seawater outsde the heat exchanger goes through three processes as follows: () Convectve heat transfer between the flud nsde the ppe and the nsde wall of the ppe. (2) Conductve heat transfer through the ppe wall. (3) Convectve heat transfer between the seawater and the outsde wall of the ppe. tout Q tw,n - + dn dout tw,out x Fg. Schematc dagram of heat transfer n summer The heat flux due can be wrtten usng the followng equatons: Q = ( h + π x( ). () out, w ) d out t out t w, out, o r where r o s the foulng coeffcent, t s equal to m 2 K/W. Nu = C Pr. (2) ( ) n Gr out out where C s equvalent to 0.48 and n s 0.25 accordng to the low flow rate of the seawater. t w, out, t w, n, Q = 2π w x. (3) l d out ln d n where λ w s equal to 0.49 W/m K. Q = hn π d n x ( t w, n, t f ). (4), 483

3 T T T 0. 8 n Ren n λ w hn = Pr. (5) d n where n = 0.3 for coolng. uν d ν Reν = ν ν. (6) p t t 2 f, + f, Q = d n n n ρ n c p,n 4 2. (7) Experment results and dscusson Experment system. In order to valdate the mathematc model establshed above, an expermental rg whch smulated the heat transfer process of CHE was bult, as shown n Fg.2. The maxmum flow rate of the crculatng pumps s 6m 3 /h. The CHE conssts of a hgh-densty polyethylene ppe(hdpe) whch has 200m length and 32mm outer dameter submersed n a water reservor at 3m.The model of the casted heat exchanger s shown n Fg Fg.2 Flow dagram of expermental system n summer () fan col, (2) heat pump unt, (3)expanson tank, (4,6) stem thermometer,(5) fan col ppe, (7) pressure meter, (8) casted heat exchanger, (9) automatc exhaust valve, (0) float-flow-meter,() pump Fg.3 The confguraton vew of casted heat exchanger Experment valdaton. The parameters correlated wth the model valdaton n the expermental system are lsted n Tables and 2. For the CHE model, the test may be performed by the comparson on the outlet temperature of CHE between the expermental value and the calculated value by the model under gven nlet temperature and flow rate wth and wthout submerged pump n the water reservor. In order to valdate the mathematcal model developed above, an expermental test s undertaken n the expermental system shown n Fg.2. The test system ncludes the testng of temperature, glycol soluton flow rate and nlet and outlet pressures of the CHE. The temperature test s performed automatcally by T-type thermocouple usng a Fluent Data Acquston devce. The flow rate s tested by a float-flow-meter. 484

4 Table Propertes of water-ethylene glycol mxture 0% by weght Unt Value Densty kg/m 3 08 Specfc heat kj/kg K Knematc vscosty 0-5 m 2 /s 2.04 Pr Table 2 Propertes of seawater 0 Unt Value Knematc vscosty 0-6 m 2 /s.5 β 0-5 K Pr -.6 Results and dscusson. Under dfferent ethylene glycol soluton flow, whle the ethylene glycol soluton nlet temperature s constant, the calculated ethylene glycol soluton outlet temperature and correspondng test date n summer wth and wthout submerged pump s compared. The results are shown n Fg.4. It can be seen from the pcture (a) that wthout the submerged pump, whle the flow rate at 00L/h, the relatve error s more than 5%, whch s not acceptable n engneerng applcaton. But from the pcture (b), the concluson can be drawn that the largest relatve error s less than 5%, whch s acceptable. Smulaton shows great agreement wth the expermental data. Ths means the mathematcal model n summer developed above can smulate effectvely the heat transfer process of CHE. Conclusons (a)wthout submerged pump (b)wth submerged pump Fg.4 Comparson of the ethylene glycol soluton outlet temperature between smulaton and test n summer Ths paper presented the mathematcal model of CHE n summer based on energy balance equaton. The model was valdated by an experment system. The followng conclusons are acheved: () The ppe length, flow rate and seawater temperature have been used n computer smulaton and the results were satsfactory. As seen n Fg. 4, the temperature dstrbutons along the CHE of the smulaton are very close to the temperatures measured from the expermental work. The relatve error s less than 5%, whch s acceptable n engneerng applcaton. (2) Study demonstrates that the use of ocean thermal energy n Tanjn shows a promsng prospect n the near future and has great potental for development n coastal ctes n Chna. References [] V.R. Tarnawsk, W.H. Leong, T. Momose, Y. Hamada, Analyss of ground source heat pumps wth horzontal ground heat exchangers for northern Japan, Renewable Energy 34 (2009)

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