Abstract:Microchannel heat exchanger is widely used in agriculture due to compact structure and high efficiency. However, the minification of size can lead to the increase in flow resistance of the system, pressure drop and energy consumption, and finally cause lower economic efficiency. Previous work has proved that ultrasound can be applied to microchannel heat exchanger to improve its heat transfer performance, but the effect of ultrasound on flow boiling pressure drop is not clear. To investigate the effect of ultrasound on flow boiling pressure drop characteristics in microchannels, a microchannel experiment section with an ultrasonic transducer was designed. Using the refrigerant R141b as the experimental working fluid, the flow boiling experiment was conducted in rectangular microchannels with the cross-section of each single channel being 2mm×2mm to study the pressure drop characteristics of refrigerant R141b in microchannels under the action of ultrasound and a high-speed digital video camera was used to visualize the flow boiling in the microchannels. The pressure of the system was set as 152kPa, the heat flux density ranged from 10.01kW/m2 to 23.30kW/m2, mass flow rate ranged from 29.67kg/(m2·s) to 177.96kg/(m2·s), the applied ultrasonic power ranged from 12.5W to 50W and frequency ranged from 23kHz to 40kHz. The results showed that the proportions of the pressure drop components of the microchannels with or without ultrasound were approximately the same and the friction pressure drop accounted for the largest proportion, followed by gravity pressure drop and acceleration pressure drop, with the proportion of inlet and outlet pressure drop being the smallest. The ultrasound applied at the inlet of microchannels had a slight reduction effect on the flow boiling pressure drop of the microchannels. When the mass flow rate was 118.64kg/(m2·s) and the heat flux density was 17.03kW/m2, the application of 50W and 40kHz ultrasound could reduce the two-phase frictional pressure drop per unit length by 12.70%. When the mass flow rate was 118.64kg/(m2·s) and the heat flow density was 18.56kW/m2, the frictional pressure drop per unit length of two phases was increased by 36.15% with 50W ultrasound compared with 12.5W ultrasound, and the frictional pressure drop per unit length was decreased by 23.85% with 40kHz ultrasound compared with 23kHz ultrasound. The influence of ultrasonic parameters on the friction pressure drop per unit length was weaker at higher heat flux. In order to reduce the flow boiling pressure drop in microchannel, ultrasonic wave with relatively lower power and higher frequency would yield better results. The ultrasound affected the pressure drop of two-phase region mainly by changing the bubble dynamics. The visualization results showed that ultrasonic wave can affect the number and size of bubbles and flow pattern in the channel. In the bubble flow stage, acoustic cavitation and acoustic streaming could promote the formation of bubbles and increase the frequency of bubble detachment. In the intense boiling stage, the increase of gas-liquid interface would hinder the propagation of ultrasound and weaken acoustic cavitation. The research result can provide a reference for the application of ultrasound in microchannel heat exchanger.
