Mechanism Of Ultrasonic Heat Transfer Enhancement On Finned Tubes And Its Application In The Design Of Solar LiBr Refrigerator

The world is facing the dual threat of energy scarcity and environmental pollution,hence solar absorption refrigeration system gets more extensive research due to the fact that it is environmental friendly and energy saving.However,LiBr absorption chillers driven by low-grade solar energy has the problem of low efficiency,as heat exchange of the copper in separation is difficult to work effectively under low-grade solar energy.In order to improve the cooling efficiency of the unit,this paper puts forward the integrated strengthening technology of finned tube and ultrasound to enhance tube heat transfer performance in working fluid.Firstly,the main mechanisms of ultrasonic heat transfer enhancement and cavitation effect were studied.The dynamic characteristic equation of cavitation bubble was deduced for simulation and analysis of the bubble dynamics.This work studied the effect of ultrasonic parameters on cavitation bubble and found that the lower the frequency and the greater the power,the maximum radius expansion ratio of bubble in the sound pressure is higher,so ultrasonic enhancement is better.The influence of liquid properties on dynamic characteristics of bubble was discussed.It was found that the degree of cavitation is different for different liquids.The lower the viscosity and surface tension,the stronger cavitation effect of the bubbles and the higher heat transfer efficiency.The ultrasonic enhanced heat transfer mechanism of the finned tube was further studied.Ultrasonic wave has a significant role in promoting the heat transfer process of copper tube in subcooled boiling deionized water,especially for finned tube.In low heat flux,the enhanced effect of ultrasonic is better.Howevr,the ultrasonic wave shows a hindering effect on the tube-saturated boiling heat transfer and the heat transfer efficiency decreases.Ultrasonic parameters greatly affect on the boiling heat transfer process of finned tubes and effects of ultrasound with different parameters are also different.By selecting the appropriate parameters,ultrasonic wave improves the heat transfer efficiency of finned tube considerably.The ultrasonic heat transfer enhancement of finned tube in LiBr solution was studied for the purpose of unit application.The effect of ultrasonic wave on the heat transfer of copper tube in LiBr solution is similar to that of water.It shows enhancement in subcooled boiling condition while heat transfer is inhibited under saturated boiling condition.The boiling heat transfer performance of LiBr solution is poor and ultrasonic enhancement in the LiBr solution is not as efficient as that in water.Tube surface structures can improve the heat transfer performance in the Li Br solution and the three-dimensional finned tube has the best strengthening effect.Fin structure and the ultrasonic field can promote each other to provide better subcooled boiling heat transfer performance,while the strengthening effect caused by the structure itself is more excellent under saturated boiling condition.Based on the ultrasonic heat transfer enhancement of finned tube,the integrated structural optimization design was carried out for the high-pressure chamber of solar LiBr absorption chiller.In this work,ultrasonic technology and three-dimensional finned tube synergistic effect was applied to improve the heat transfer performance of tubes in the refrigerant.And the whole unit is optimized to improve the working efficiency of the unit.