Effects of fan cavities on fluid flow and heat transfer in helical and straight mini-channels of heat exchanger

With the rapid development of microminiaturization technology and the urgent requirements of industrial field, the sizes of many devices are reduced continually. This causes the thermal load to increase sharply when the device is working, and further leads to the decreasing of the working stability gradually. So the conventional heat exchanger can't meet the heat transfer requirement for these micro-devices. Thus, the micro/mini-channel heat exchanger emerges as the times require. This exchanger has many advantages, such as compact structure, high efficiency for heat dissipation, low power consumption and few coolant requirements. These advantages motivate many researchers to conduct numerous studies on the micro/mini-channel heat exchanger continuously to further enhance heat transfer performance. Adding cavities, fins or ribs on the wall of the channel is a solution for enhancing heat transfer performance in the micro/mini-channel heat exchanger. As we know, the helical and straight channels are widely applied to the heat exchanger. The difference between both channels is the existence of secondary flow in former, inducing heat transfer enhancement. Therefore, it is important to understand the effects of cavities or fins on the fluid flow and heat transfer enhancement in the helical and straight micro/mini-channel heat exchangers. As a consequence, the cavities are added on the both sidewalls of helical and straight mini-channels in this work, and the effects of the cavities on the fluid flow and heat transfer in the helical and straight mini-channels are studied using numerical simulation method. Specifically, the comparative analysis in the effects of cavities on the flow, heat transfer, entropy generation and overall performance in the helical and straight mini-channels is performed based on the first and second thermodynamics law. This aims to analyze the different effects of cavities on both mini-channels. The cross-sections for both mini-channels are the same, and the width and height of this cross-section are 3 and 3 mm, respectively. The working conditions include the Reynolds number of 168-2017, the heat flux density of 1.145 98×105 and 1×105 W/m2 for helical and straight mini-channels, respectively, based on the condition of different heating area and same power input. The numerical results show that for helical mini-channel, the cavities can increase flow resistance, and the maximum increasement reaches up to 23%. But they have no obvious influence on heat transfer performance. For straight mini-channel, the cavities can slightly reduce flow resistance and heat transfer performance when the Reynolds number is less than 1 008. However, they rapidly increase flow resistance and heat transfer performance when the Reynolds number is greater than 1 008, and the friction factor and Nusselt number grow to 50% and 45% respectively. In the whole Reynolds number range, the cavities can't improve the overall performance in the helical mini-channel. Although the cavities slightly weaken the overall performance in the straight mini-channel at low Reynolds number, they obviously enhance the overall performance at high Reynolds number, and the maximum heat transfer augmentation factor grows to 1.27. The results of entropy generation analysis indicate that the cavities can reduce irreversible loss in the flow and heat transfer process for helical and straight mini-channels, thereby improving effective utilization of thermal energy. However, the decrement rate of irreversible loss for straight mini-channel with the cavities is greater than that for helical mini-channel, for the former's decrement rate is about 2 times that of the latter. This work provides a reference for improving the performance of heat exchanger with mini/micro-channels. © 2017, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.