Visualization Of Boiling Heat Transfer Characteristics And Flow Instability In Sintered Microchannels

In recent years,microchannel boiling heat transfer has been initially developed in electronic products.In this thesis,the boiling heat transfer performance of three kinds of sintering channel structures were studied in detail,including microchannels sintered with copper powder,flat microchannels sintered with copper powder and microchannels sintered with copper powder and pore-forming agent.In the study,the visualization and pressure synchronous acquisition method was used to explore boiling heat trasnfer mode and CHF（critical heat flux）mechanism in the microchannel.On the basis of the previous work,the design of the heat sink chamber was improved,and the copper powder sintering microchannel process was also explored.Then,the structural parameters of the samples were characterized in detail,including SEM electron microscopy analysis,porosity measurement,and capillary rise characteristics.It was found that the capillary rise speed of the ordinary microchannel was generally faster than that of the flat channel.On this basis,three aspects of research work are carried out:For the sintered microchannels,it is found that the heat transfer effect of large-size samples show better heat transfer perfromance than small-size samples at high heat flux.When the mass flux is 142 kg/（m²·s）,the CHF of large particle size samples（90-200 PM and120-200 PM）reaches more than 180 W/cm²,while the CHF of small particle size samples（30-400 PM）only about 120 W/cm².The mass flux greatly affects the heat transfer coefficient curve of small particle samples,but less affect those of large particle samples.Pressure-sintering method can significantly improve the boiling heat transfer performance.Under the same bottom thickness,the average pressure drop of the sample was positively correlated with the particle size of the sample.Combined with visualization and pressure instability,it is found that the CHF for small particle size samples（30-400 PM）was triggered by film boiling,accompanied by frequent explosive boiling.It resulted in disorder pressure fluctuation curve;the CHF for large-size sample（120-400 PM）was mainly triggered by capillary limit due to insufficient liquid supply,and its pressure pulsation could still maintain quasi-periodic oscillations up to high heat flux,which was illustrated by its rapid capillary-rise speed.For flat channels with sintered layers,their CHF values were found much lower than that of the sintered microchannel.The ratio of the bottom thickness to the particle size（δ/d）has a great influence on the heat transfer performance of the flat channel.The optimal ratio of the bottom thickness to the particle size is between 1.7 and 4.0.Too large or smallδ/d will cause deterioration of heat transfer performance.Flat channel samples（90-200 PL and 120-400 PL）could reach about 110 W/cm² in CHF.HTCs of flat channel samples increases with the increase in mass flux.Average pressure drops of flat channel samples fluctuates with the increase in heat flux.Visual observation,along with pressure synchronous acquisition,found that the CHF of the small particle size sample was sourced from large-area explosion boiling phenomenon,which led to a sudden rise of inlet and outlet pressure,while the CHF of the large particle size sample resulted from large area dryout due to insufficient tail liquid supply.For porous microchannels,pore-forming agent significantly increases the sample porosity.For microchannels sintered with the 10μm spherical copper powder,the volume ratios of 20%and 30%pore-forming agent content presented the best heat transfer performance,up to 112.4 k W/（m²·K）.For the 30μm copper powder sintered sample,20%pore-forming agent content achieved the highest HTC,which wall superheat is only 6.8℃ in q_eff=179.8 W/cm².The average pressure drops were less affected by pore-forming agent content.It also indicated that there existed the optimum porosity.Visual results showed that sample porosities affected the pressure fluctuation curve.The pressure fluctuation curve of the sample with moderate porosity showed more orderly and periodic characteristics,and the explosion boiling phenomenon is also reduced.In summary,it is found that:（1）The performance of parallel sintered microchannels is improved by one level compared with that of flat sintered microchannels,which is also reflected in the heat transfer coefficient and CHF.The boiling heat transfer performance of the sintered structure is correlated with the rise rate of capillary suction;（2）Pressure-sintering method can significantly reduce the thermal resistance of the interface.There is a better range of bottom thickness particle size ratio and porosity,and too large or too small will cause heat transfer deterioration;（3）For the CHF mechanism,it is mainly caused by fluid deficiency drying.Small-sized samples are easy to transform into membrane boiling near CHF,and explosive boiling occurs frequently.Large-sized samples are mainly caused by fluid deficiency due to capillary liquid supply limit.