| exchanger has many advantages: small size, high heat flux, high performance in mechanics, saving materials, and high flexibility. With the rapid development of MEMS and the trend of thoroughly implement of energy saving laws in recent years, it is getting more necessary for the development of new type heat transfer equipment. Therefore, the present research has both significances in technical application and academic theory.Heat transfer and fluid flow in microtube or microchannels is a very complex issue. Many international scholars have devoted to this problem by both theoretical and experimental investigations, however, there is little common accepted conclusion so far. The aim of the present study is to understand the single phase convection heat transfer and pressure drop perforamce of R142b in MTHE. A microtube bundle heat exchanger (MTHE) was fabricated and the experimental test system was established. This paper also tries to numerically simulate the heat transfer of fluid passing arround a micro-tube bundle using Lattice Boltzmann Method,in which the natural convection effect was considered.The Wilson Plot Method was employed to investigate the heat transfer performance on the tube side. In the Reynolds number range of 500-1600, the single-phase forced convection heat transfer correlation was proposed. This correlation enables to predict the experimental Nusselt numbers with a mean absolute error of 3.52%. Compared with the Nusselt number calculated by the correlations for the conventional tubes and for the micro channels in the previous literature, it was found that the Nusselt numbers of this study is much higher than those predicted by those conventional correlations, and is also different form those predicted by those micro channels correlations. Taking into account the entrance effect in the thermal developing region, the averaged Nusselt number is about 2-4 times larger than that of the conventional scale heat exchanger. However, in the fully developed region, the averaged Nusselt number is about 1.44 times larger. According to our results, the thermal entry length in microtube is longer than that of the conventional tubes for laminar flow.In the Reynolds number range of Re = 500-1600, the pressure drop and friction coefficients were found to be considerably higher than those predicted by the conventional correlations. In this paper, the total friction coefficient of micro-tube heat exchanger in the tube side is defined, and its correlation with the Reynolds number is proposed. The product of friction factor and Reynolds number is no longer a constant number, but as a function of Reynolds number. The reasons resulting in these physical phenomenon have been preliminary analyzed. The experimental uncertainties and their evaluation were given.After simulating the heat transfer of mixed convection and "pure" natural convection for fluid flowing across a micro-tube bundle, we can reach some conclusions: (1) the overall heat transfer for opposing flow is advantage than cross flow, and the assisting flow is the worst. In other words, for the liquid pipe of MTHE shell side, the vertically downward installation is the best, followed by the horizontal installation, and the vertically upward installation. (2)the averaged Nusselt number for each tube and the overall bundle increases with increasing of the Reynolds number . (3) the averaged Nusselt number for each tube and the overall bundle increases with increasing of the Rayleigh number.Finally, the improvement and some proposals for future study on this subject have been outlined. |
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