Investigation of condensation in microchannels

Abstract

Advancements in the industry have raised the need for higher-capacity heat transfer. Two-phase flows have been primarily used for this purpose for quite a while. As manufacturing capabilities have improved, the fabrication of smaller passages has become possible. This brought the possibility of heat transfer from microchannel passages. Due to their high area-to-volume ratio, the heat transfer capability in microchannel flows increases significantly compared to macrochannels. Thus, interest in two-phase flows in microchannels has risen in the literature. In this study, R600a condensation in a horizontal multiport microchannel with a 0.399 mm hydraulic diameter is investigated experimentally. Local condensation heat transfer coefficients of R600a were investigated at saturation temperatures of 35°C, 40°C, and 45°C under mass fluxes of 50, 66, 82, and 98 kg/m2s. Additionally, tests were conducted for varying inlet vapour qualities to understand its effect on measurements for conditions where channel lengths are not sufficient to measure the entire range of vapour quality at once. Measured data of local heat transfer coefficients were then compared to the correlations stated in the literature. Moreover, the flow at 45°C saturation temperature for all mass fluxes was visualized with additional tests to understand the flow regimes. The outcomes of the experiments are presented on a map, and a comparison to maps stated in the literature is made. Furthermore, a new 3-sided condensation heat transfer coefficient correlation is proposed, and the MAEs were found to be between ±10.1-16.3%. For this purpose, an experimental test rig is built. This test rig includes an ATEX micro gear pump, a Coriolis mass flowmeter, an evaporator, an aluminium block assembly, a post-condenser, and a liquid reservoir. On the refrigerant loop, several pressure transducers, Class AA RTDs, and T-type thermocouples are mounted for measurement. Briefly, R600a refrigerant is circulated through the system with the help of the ATEX micro gear pump, and its flow rate is measured by the Coriolis mass flow meter. Afterwards, R600a is conditioned to the desired inlet conditions by the evaporator and enters the microchannel block assembly. While R600a passes through the upper channels, coolant water is circulated through the bottom of this block to perform condensation. Then, R600a leaves the microchannel block assembly and travels through the post-condenser to achieve a subcooled liquid phase. Finally, R600a accumulates in the reservoir, and the loop is completed. Each water line circulated through the evaporator, post-condenser, and coolant side of the aluminium block is conditioned in separate water baths and circulated by separate micro gear pumps. The fin analysis method is applied, assuming the fin tip is adiabatic and conduction heat transfer through the aluminium block is one-directional.