EFFECT OF UNCONSTANT OVERALL HEAT TRANSFER COEFFICIENT ON THERMAL PERFORMANCES OF MULTIPLE ASSEMBLIES OF AUTOMOBILE RADIATORS S. Vithayasai T.Kiatsiriroat Department of Mechanical Engineering, Chiang Mai University, Chiang Mai 52 ABSTRACT In this study, a set of automobile radiators are used as heat recovery system of a hot gas stream. Its thermal performance for preheating water with series flow and parallel flow of water stream is carried out when the heat exchanger effectiveness of each is assumed to be unconstant and constant. It could be seen that the results from both techniques are nearly the same for the series flow and for the parallel flow the results of the conventional method are different from those of the unconstant effectiveness method. In this study the effects of heat capacity ratio, and the inlet temperatures of the fluids on the total thermal characteristics have been considered. INTRODUCTION Automobile radiator is one type of cross flow heat exchanger and normally, it is used as a cooling unit of vehicle engine with water as a heat transfer medium. The heat exchanger configuration is louver fin and flat tube. The unit is also modified to be a heat recovery device or a heat exchanger in other thermal processes because of its cheap price compared to other heat exchanger types and its high thermal performance[1]. Nuntaphan and Kiatsiriroat [2,3] modified automobile radiators to be thermosyphon heat pipes and they were used as waste heat and coolness recovery devices. As heat recovery unit, to increase thermal load, or to enhance heat exchanger effectiveness, connecting of multiple exchangers could be performed. Multipassing is also employed to utilize the allowable pressure drop effectively. Two or more heat exchanger units can be coupled either in series or parallel connections. Fig. 1 shows the water stream in series and in parallel for the assemblies of automobile radiators. Water Out Water Out Hot Air In Hot Air Out Hot Air In Hot Air Out Water In Water In a. Series assembly. b. Parallel assembly. Fig. 1. Assemblies of the heat exchangers. In general, the overall heat transfer coefficient or the heat exchanger effectiveness of each unit is assumed to be the same value for simplified calculation. In practice, the value of the overall heat transfer coefficient or the effectiveness depends on the temperatures of the heat exchanger fluids. For the automobile radiators in multiple connections, the temperature of the hot fluid when it passes to the next heat exchanger unit drops quickly therefore the units downstream could not transfer heat effectively thus both thermal parameters will change along the unit assemblies. Hewitt et al[4] gave a concept for calculating total heat transfer area of heat exchanger by splitting thermal load into small values and finding its heat transfer area. In each calculating zone the overall heat transfer coefficient is varied with the temperatures of the working fluids. With this technique, it could be found that the incremental method could give smaller size of the total heat transfer area than the general procedure.
Nuntaphan et al[5] calculated the performances of a thermosyphon heat exchanger both counterflow and cocurrent flow by varying the overall heat transfer coefficient along the heat transfer unit. With this technique the heat transfer area is 4 % less than that with the conventional method. In this section, thermal performance analyses of the automobile radiator assemblies in series and parallel connections will be considered. The effects of the variable overall heat transfer coefficient or the heat exchanger effectiveness in each heat exchanger unit on the total heat transfer performances are discussed and the results are compared with those from the general method. The automobile radiator is shown in Fig. 2 and the specifications of the unit are shown in Table 1. Fig. 2. The automobile radiator. Table 1 Dimensions of the tested automobile radiator. big size medium size small size Cross section.7 m x.69 m.6 m x.45 m.22 m x.21 m (length x height) No.rows of water tube 5 3 2 Fin material copper Fin shape Louver fin with triangular channel Fin dimensions Lp (Louver Pitch) Ll(Louver length) θ (Louver Angle) T p (Tube pitch) F p (Fin Pitch) F d (Fin depth) F l (Fin Length) F t (Fin thickness) Dm (Major tube diameter) =.115 m =.86 m = 45 Degree =.124 m =.22 m =. m =.1 m =. m =.26 m Data Reduction The heat transfer rate of heat exchanger is normally calculated from Q = ε C ( T h T ), (1) min i c i Q is heat rate, C min is lower total heat capacity from heat exchanging fluids, T hi and T ci are the inlet temperatures of hot and cool fluids and ε is heat exchanger effectiveness. For the automobile radiators in this study, the heat exchanger effectiveness of each unit have been tested[1] and its heat transfer correlation could be fitted by the crossflow equation as Multiple Assemblies of Heat Exchangers with Constant ectiveness Series Coupling of Heat Exchangers For multiple assemblies of crossflow heat exchanger units, when the cold fluid is in series, there is general formula[6] for calculating the total heat exchanger effectiveness ε N and the NTU of each unit which are assumed to be the same values. The correlation is as follows:.78 [ [ ] ] 1.22 ε = 1 exp ( NTU ) exp C ( NTU ) 1 (2) C C* is C min /C max and NTU is the transfer unit of the heat exchanger. (3) Parallel Coupling of Heat Exchangers
For the connection that the cool stream is in parallel flow, a general formula for the heat calculation[6] when the thermal parameters are the same in each heat exchanger unit as (4a) when the minimum total heat capacity fluid is in parallel, and ect of C* on Heat Exchanger ectiveness and Fig. 3 shows the effect of C* on the effectiveness of two automomile radiators in series connection of cold fluid on the total heat exchanger effectiveness and the oulet temperature of cold fluid. The total effectiveness slightly drops as the C* increases since the outlet temperature of the hot stream increases and makes more temperature difference between the hot and cold streams. The phenomenon is similar to the counterflow heat exchnager. Consider T co, the value could be estimated from T co = T ci + εc*( T hi - T ci ). (5) As C* increases but the total effectiveness, ε, is nearly constant in this case then the outlet temperature, T co, is also increased. The results of the coventional method are seemed to be slightly lower than those of the unconstant effectiveness method. (4b) when the maxmum total heat capacity fluid is in parallel. For Unconstant ectiveness 1..8.6.4.2. Eall (Constant UA) Eall (Unconstant UA).2..3.35.4.45.5.55 In practice, the automobile radiator is an effective heat transfer device, the hot stream after passing the first unit, its temperature will drop quickly before entering the second unit and so on, thus, the temperature of the hot stream varies along each heat exchanger, therefore the overall heat transfer coefficient and the heat exchanger effectiveness of each unit are not constant. In this part, the calculation will be carried out unit by unit for both series and parallel connections. The water stream is the total maximum total heat capacity fluid and absorbing heat from the hot gas stream. The value of the heat exchanger effectiveness of each unit is taken from eqn(2). In the calculation, the reference values of T hi, T ci, C*(C h =.15kg/s) are at 2 o C, 3 o C and., respectively. The simulated results of the thermal performances of the coupling automobile radiators from the non-uniform effectiveness compared with those of the uniform effective method and with the experimental data are presented. Results 1 1 5 (Constant UA) (Unconstant UA).2..3.35.4.45.5.55 Fig. 3. ect of C* on the assemblies thermal b. For parallel Coupling Fig. 4 gives the results of the two automobile radiators in parallel connection. For the unconstant effectiveness method, the first unit of the hot stream gives a high thermal performance and dominates the total heat exchanger effectiveness. As C* increases, the unconstant effectiveness method gives a big drop of the total heat exchanger effectiveness for the consecutive unit. For the constant effectiveness method, the outlet temperature of the hot stream after leaving the first unit will be higher than the previous method to maintain the effectiveness in the second unit. Then the total effectiveness from the conventional method is higher
than that from the unconstant effectiveness method. As C* increases, the effectivenesses from both method decrease. From eqn(5), T co depends on ε and C*. For C* less than around.38, when the value increases, the temperaure also increases but for C* greater than.38, even C* increases but the temperature decreases due to a big drop of the effectiveness in each method. 1..8.6.4.2. 1 1 5.2..3.35.4.45.5.55 (Constant ε (Unconstant ε).2..3.35.4.45.5.55 Fig. 4. ect of C* on the assemblies thermal characteristics for parallel coupling. 1 1 5 (Constant ε) (Unconstant ε) 1 1 15 1 2 2 Fig. 5. ect of T hi on the assemblies thermal b. For Parallel Coupling The results are nearly the same as those of the series coupling but now the conventional method gives a little higher value of the effectivenesss. The total effectivevesses in this case is slight lower than the previous case. 1..8.6.4.2. 1 1 15 1 2 2 ect of T hi on Heat Exchanger ectiveness and Fig. 5 shows the effect of the temperature inlet of the hot fluid on the thermal performance for series flow of the cold stream. The effectivenesses from both methods are nearly the same even the temperaure is changed. The conventional method gives slight lower effectiveness compared with the unconstant effectiveness method. As T hi increases more heat can be transferred to the cold stream then the outlet temperature of the cold stream increases. The results from both methods are nearly the same. 1 1 5 (Constant ε) (Unconstant ε) 1 1 15 1 2 2 Fig. 6. ect of T hi on the assemblies thermal ect of on Heat Exchanger ectiveness and 1..8.6.4.2. 1 1 15 1 2 2 Fig. 7 gives the effect of the inlet temperature of the cold stream on the thermal performance. The effectivenesses from both method are nearly the same. The conventional method gives a little lower value compared with the unconstant effectiveness method. Similar to the previous figure as the inlet temperature increases the oulet temperature also increases but the temperaure difference is less. Both methods give similar results on the temperature.
1 1 5 1..8.6.4.2. (Constant ε) (Unconstant ε) 3 35 4 45 5 55 3 35 4 45 5 55 Fig. 7. ect of T ci on the assemblies thermal b. For parallel Coupling The results on the effectiveness are similar to the series coupling but in this case, the conventional result gives a little higher. For the outlet temperature of the cold stream, the results are nearly the same for both methods. 1..8.6.4.2. 1 1 5 3 35 4 45 5 55 (Constant ε) (Unconstant ε) 3 35 4 45 5 55 Fig. 8. ect of T ci on the assemblies thermal characteristics for parallel coupling. CONCLUSIONS 1) The constant effectiveness method gives different values on the thermal performances of multiple assemblies of automobile radiator as heat exchanger. 2) The total heat capacity ratio gives a strong effect on the heat exchanger effectiveness in case of the parallel connection of the cold stream. For series coupling, the conventional method gives slight lower value compared with the unsconstant effectiveness method. 3) The inlet temperatures of the heat exchanging fluids do not give strong effect on the total thermal performances. ACKNOWLEDGEMENT s research study is supported by Thailand Research Fund and National Planning and Policy office. REFERENCES [1]. Kiatsiriroat,T., The applications of automobile radiator in waste heat recovery processes, Final Report, EPPO, Thailand, 24. [2]. Nuntaphan, A. and Kiatsiriroat, T., Performance of thermosyphon heat exchanger modified from automobile radiator, 18 th Conf. of Mechanical Engineering Network of Thailand, Thailand, 24. [3]. Nuntaphan, A. and Kiatsiriroat, T., Application of thermosyphon heat exchanger in coolness recovery process of air-conditioning system, 3 rd Conf. of Energy, Heat and Mass Transfer in Thermal Equipments, Thailand, 24 [4]. Hewitt, G. F., Shires, G. L. and Bott, T. R., Process Heat Transfer, CRC Press Inc. 1994. [5]. Nuntaphan, A., Tiansuwan, J., Kiatsiriroat, T. and Wang, C.C., Performance analysis of thermosyphon heat exchanger using various kinds of working fluids, Heat Transfer Engineering, vol.22, no. 4, 21, pp. 28-4. [6]. Kuppan, T., Heat Exchanger Design Handbook, Macel Dekker, 2 Nomenclatures C max = higher total heat capacity(kj/k) C min = lower total heat capacity(kj/k) C* = heat capacity ratio, C min /C max Q = heat rate(kw) T ci, T co = inlet and outlet temperatures of cold fluid T hi, T hi = inlet and outlet temperatures of hot fluid ε = heat exchanger effectiveness NTU = number of transfer unit