D232 マイクロ・ベアチューブ熱交換器の電子機器冷却への適用性評価

Abstract

In the present study, a micro bare-tube heat exchanger without conventional fins is proposed and evaluated for electronic equipment cooling application. A micro bare-tube heat exchanger composed of 0.5mm outer diameter copper tubes is manufactured and tested experimentally. The optimal dimensionless transverse and longitudinal tube pitches were PT = 2.28 and PL = 1.31, respectively. It is shown that the resultant micro bare-tube heat exchanger can drastically reduce the core volume compared to the conventional plate fin and tube heat exchanger. INTRODUCTION Great efforts have been made for heat transfer augmentation, and a number of design concepts for compact heat exchangers have been proposed (e.g., see Kays and London [1]). Most of the gas-liquid heat exchangers adopt fins in order to compensate a lower heat transfer rate on the gas side. Paitoonsurikarn et al. [2] proposed a micro bare-tube heat exchanger, which was composed of a bundle of small diameter tubes without conventional fins. It was shown that the micro bare-tube heat exchanger had a possibility of improving heat exchanging performance and compactness with its high over-all heat transfer rate and large heat transfer area density. Conventional liquid-air heat exchangers such that used for air conditioners and electronic equipment coolers generally require large heat transfer area and air flow rate because of the small thermal conductivity of air. This often increases the volume, noise and also initial and running costs of the total system. On the other hand, the micro bare-tube heat exchanger has a potential for improving air-side performance drastically, because both the heat conduction resistance of fins and the inner-tube heat resistance can be ignored compared to those of conventional heat exchangers. Moreover, it is possible to achieve a large heat transfer area density. This feature is really desirable for designing compact cooling systems. Paitoonsurikarn et al. [2] utilized the heat transfer and pressure drop correlations proposed by Zukauskas [3]. However, those correlations are not verified for predicting a wide range of tube arrangements and also at low Reynolds numbers, e.g., Re < 500, which is a characteristic Re range for compact heat exchangers. In the present study, optimization method based on simulated annealing (SA) with a trained neural network representing the heat transfer and pressure drop characteristics of a prescribed tube bank is introduced. A commercial CFD code, FLUENT5, was employed to train the neural network of this flow and thermal fields around various in-line tube bundle arrangements at low Reynolds numbers. Then an optimal micro bare-tube heat exchanger composed of 0.5 (mm) outer diameter copper tubes was manufactured and tested experimentally. The optimal dimensionless transverse and longitudinal tube pitches were PT = 2.28 and PL = 1.31, respectively. The experimental data were then directly utilized for the assessment of CPU cooling system. It is shown that the resultant micro bare-tube heat exchanger can drastically reduce the core volume compared to the conventional heat exchangers. NUMERICAL SIMULATION OF FLOW AROUND IN-LINE TUBE BUNDLES A commercial CFD code, FLUENT5, is employed to calculate the flow and thermal field around in-line tube bundles with three rows in the transverse direction and 10 columns in the longitudinal direction as shown in Fig. 1. The tube surface is assumed to be isothermal. Periodic boundary condition is employed in the transverse direction, and uniform velocity and free outflow conditions are given at inlet and outlet boundaries, respectively. The temperature dependence of the physical properties of working fluids is neglected. The Reynolds number based on the tube diameter and the maximum velocity at the minimum cross section, Remax, is varied in the range of Remax=10 to 300. The dimensionless transverse and longitudinal tube pitches, PT and PL, are varied from 1.25 to 4.5. In total, 46 cases are calculated. It is known that three dimensionality of the flow is observed in the case of a single cylinder around Remax =100. In the present study, however, two-dimensional unsteady calculation is performed because it is considered that the effect of three dimensionality of the flow on the mean heat transfer and pressure drop is negligibly small. Finally, the Nusselt number Nui and the pressure coefficient Cp at i-th column are obtained by time 10d 40d d PL・d PT・d Outlet Inlet Periodic Boundary Fig. 1. Computational Domain.