| Boiling heat transfer is one of the most efficient heat transfer methods in high-power industrial applications and is widely used in refrigeration and air conditioning,nuclear power plants,electronic cooling and many other fields.In the pool boiling process,the nucleation boiling stage is the focus of boiling heat transfer research due to its low wall superheat and high heat transfer efficiency,and the formation and evolution of bubbles in the boiling liquid is one of the key factors to determine the surface heat transfer.On this basis,many optimization techniques are used to enhance boiling heat transfer by affecting bubble dynamics in an active or passive manner.In this thesis,the effect of ultrasonic waves on the boiling heat transfer of smooth copper surface and sintered porous coating surface is investigated by visualization experiments at atmospheric pressure using deionized water as the working medium,and the effect of ultrasonic waves on the flow field as well as bubble departure is analyzed by numerical simulation.The experimental results showed that ultrasonic waves significantly improved the boiling heat transfer performance on both porous and smooth surfaces.The porous coating made of sintered copper grains effectively improved the bubble dynamics and boiling heat transfer performance,and its average heat transfer coefficient(HTC)was94%higher than that of the smooth surface without ultrasonic due to the wicking capacity of the porous structure and the additional nucleation sites.After applying ultrasonic,the boiling heat transfer performance of both surfaces was improved to different degrees.In the experimental heat flux range,the average HTC of smooth and porous surfaces under ultrasonic action increased by 23.7%and 30.9%.The porous structure is more susceptible to ultrasonic waves because it facilitates the departure of bubbles and reduces the agglomeration of bubbles.Based on the visualized images,the bubble dynamic characteristics were quantitatively analyzed,and the results showed that ultrasonic waves reduced the bubble departure diameter by 31%and 38%and increased the bubble departure frequency by 53%and 64%for smooth and porous surfaces,respectively.In particular,the effect of ultrasonic on bubble departure characteristics was greater at lower heat flux,and the effect of ultrasonic on bubbles was suppressed as the heat flux increased,more bubble active nucleation sites on the surface were activated,bubble diameters became larger,and more adjacent bubbles became larger after horizontal agglomeration occurred.Ultrasonic also has a facilitating effect on bubble nucleation on both surfaces.In addition,at the moment of adding ultrasonic,the wall temperature has a certain magnitude of decrease due to the rapid departure of bubbles,and the temperature fluctuation is larger when the heat flux is low,and the decrease of porous surface is larger than that of smooth surface,when q>50W/cm~2,the effect of ultrasonic on bubbles and wall temperature is greatly weakened due to the large number of bubbles agglomeration and departure and the violent disturbance inside the fluid.The local flow field near the bubble was analyzed by numerical simulation,and the results showed that the fluid was affected by acoustic streaming to produce micro-flow,and the transverse fluid velocity was about 0.30 m/s,which produced shear force on the bubble and led to the lateral movement of the bubble and promoted the departure from the heated surface.After the addition of ultrasonic waves,the temperature of the wall surface was significantly reduced.Because the fluid micro-flow exerted a shear force on the bubbles and promoted their departure;on the other hand,the fluid perturbation caused by the acoustic streaming action made the thickness of the thermal boundary layer of the heated surface decrease and the temperature gradient was reduced.In addition,the nucleation of bubbles is more favorable under the effect of acoustic pressure. |
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