**5. Conclusion**

284 Heat Exchangers – Basics Design Applications

T 1-horizontal T 1-vertical T 3-horizontal T 3-vertical

100 140 180 220 260 300 **Re numbe r of cold side**

100 140 180 220 260 300 **Re number of cold side**

It was found that the pressure drop of T3 is 2 times higher than that of T1, while the heat transfer rate of T3 is 1.06 times higher than that of T1. As a result, the performance index (defined as the ratio of the heat transfer rate to the pressure drop in the heat exchanger) obtained from T1 is higher than that obtained from T3, as shown in Fig. 30. For heat exchanger T1, a performance index of 21.68 W/kPa was achieved for water from the hot side having an inlet temperature of 70 C and a mass flow rate of 0.2308 g/s and for water from the cold side having an inlet temperature of 22.5 C and mass flow rate of 0.2135 g/s. It is also observed that the change of performance between the two cases (horizontal channels and vertical channels) is negligibly small; the maximum change in performance is 5.5%, out

T1-horizontal T1-vertical T3-horizontal T3-vertical

0

1000

2000

3000

**Total pressure drop, Pa**

Fig. 29. Comparison of total pressure drops.

2

of a performance index from 13.69 to 21.68 W/kPa.

Fig. 30. Comparison of performance indices.

6

10

14

**Performance index, W/kPa**

18

22

4000

5000

6000

In this study, for the cases with both inlet temperature and mass flow rate constant at the cold side of the device, the trends for the results obtained from the actual effectiveness method and those obtained from the effectiveness (-NTU) method are in the opposite directions as the mass flow rate of the hot side increases. However, for the cases with constant inlet temperature and mass flow rate at the hot side of the device, the trends for the results obtained from both methods for evaluating effectiveness are in the same directions.

With all cases done in the study, the performance index obtained from the counterflow is always higher than that obtained from the parallel-flow. As a result, the microchannel heat exchanger with counter-flow should be selected to use for every case (except few special cases).

In the study, it indicates that the substrate thickness affects negligibly the parameters associated with the heat transfer process of the heat exchangers with the substrate thicknesses of 1.2 and 2 mm. The effect of the hydraulic diameter (cross-sectional area) on the performance index is more pronounced than that of the substrate thickness. In addition, it demonstrates that the lower the hydraulic diameter, the higher the heat flux and the pressure drop. Regarding the effects of inlet/outlet locations, for two types (I-type and Stype) of the microchannel heat exchangers, the heat flux and pressure drop obtained from the S-type are higher than those from the I-type, even though the performance indexes of both heat exchangers are essentially the same.

The impact of gravity on the fluid flowing through the microchannel heat exchanger was found to be small, with the maximum difference between the results of horizontal and vertical channels being less than 8%. In addition, in this study, good agreements were achieved between the results obtained from the present study and the results obtained from the literatures.

In the study, good agreements were achieved for the behaviors of heat transfer and fluid flow between the results obtained from numerical simulations and those obtained from experimental data for fluid flowing in the counter-flow microchannel heat exchanger used, with the maximum percentage difference between the two results of less than 9%.

This chapter summarized the work performed and the results obtained both in the fluid flow and heat transfer done by TFAG over the last several years. The authors would like to express their deep appreciation for the financial supports obtained from National Science Council, the Republic of China in Taiwan (Grant Nos. NSC93-2212-E-033-012, NSC94-2212- E-033-017, NSC95-2212-E-033-066, NSC96-2212-E-033-039, NSC97-2212-E-033-050, NSC99- 2212-E-033-025, and NSC 100-2221-E-033-065) and Chung Yuan Christian University (Grant No. CYCU-98-CR-ME).
