Experimental investigation of the air flow behavior and heat transfer characteristics in microchannels with different channel lengths

This paper attempts to experimentally investigate the influence of channel lengths to the flow behavior and heat transfer characteristics in circular microchannels. The diameter of circular microchannels are 0.4mm and the lengths of them are 10mm and 20mm, respectively. All tests were performed with air. The experiments were completed with Reynolds number in the range of 300∼2700. Results of experiments show that the length of microchannels has remarkable effects on the performance of flow behavior and heat transfer characteristics. For the flow behavior, both the friction factor and Poiseuille number drop as the channel length increases, and all the experimental value are higher than the theoretical one. Moreover, the channel length doesn't influent the value of critical Reynolds number. For the heat transfer characteristics, Nusselt number decreases with the increase of the channel length. The channel length also has a huge influence on the thermal performance. A better thermal performance is obtained in a shorter channel. The results also indicated that in both cases, the friction factor decreases with the increase of the Reynolds number. At the same time, the Poiseuille number increases when the Reynolds number keeps rising. This phenomenon is different from traditional theory that Poiseuille number is widely considered as a constant in laminar regime. It is also observed that the value of critical Reynolds number is between 1500 and 1700 in this paper, this value is lower than the value of theoretical critical Reynolds number of 2300. Nusselt number increases as the increase of the Reynolds numbers, however, the traditional theory considered that it is a constant in laminar regime.