Study On Pressure Drop And Instability Of Flow Boiling In Micro/Mini-Channel Under Ultrasound

As an efficient heat exchanger,micro-channel heat exchanger has wide application prospects in electronic industry,aerospace and other fields.The application of ultrasound heat transfer enhancement technology to heat exchanger can further improve the heat transfer performance of equipment.Due to the small scale of the microchannel,the flow resistance is significantly increased compared to the conventional channel,and in the boiling state,the bubble easily blocks the channel and causes instability of the system.Therefore,it is necessary to study the boiling pressure drop and instability of micro-channel flow under the action of ultrasound.Firstly,pure refrigerant R141b and different concentrations of nanorefrigerant TiO₂/R141b were used as experimental working fluid to study the pressure drop characteristics of microchannel under ultrasonic action.The results show that friction pressure drop accounts for the largest proportion of total pressure drop,with a percentage range being 60.27%⁷6.07%.The total pressure drop of two phases and unit length two-phase friction pressure drop are the largest when ultrasonic wave is applied at both the inlet and outlet.Pressure drop characteristics of three kinds of nanorefrigerants with different concentrations and pure refrigerants were compared.The results show that the total pressure drop of two phases and the friction pressure drop of two phases per unit length are reduced by nanorefrigerants.Comparing the pressure curves along the measuring points under different working conditions,it is found that the turning point of the curve moves towards the exit with the decrease of heat flux and the increase of mass flow rate,and the ultrasonic wave has no obvious influence on the curve turning position.Secondly,the numerical simulation of the distribution of ultrasound in the micro-channel shows that the ultrasonic wave applied to the inlet and outlet can increase the intensity of sound pressure distribution compared with the ultrasonic wave applied at the inlet.The higher the power,the greater the corresponding sound pressure value in the positive and negative phases.With the increase of the frequency,the corresponding period of the positive and negative phases decreases.Finally,the microchannel instability under the action of ultrasonic waves was studied by time domain and frequency domain analysis,and the high-speed camera was used for visual analysis.The results show that the system is most unstable when ultrasonic waves are applied to the inlet and outlet,and the corresponding maximum oscillation amplitude is increased,and the oscillation period becomes longer.The visualization results show that the flow pattern is mainly bubbly flow and slug flow when ultrasonic wave is applied at the inlet,while the elongated restricted bubble is dominant when ultrasonic wave is applied at the inlet and outlet,and the reflux phenomenon is obvious.The detachment diameter of the bubbles is reduced to some extent by adding nanoparticles.