| Microchannel boiling cooling system because of its high heat transfer coefficient,good temperature uniformity and less demand for working fluid,making it a promising development of one of the cooling methods.A large number of studies have shown that there are some obvious characteristics of microchannel boiling cooling system,such as microchannel size and solid wall surface characteristics of the boiling heat transfer performance of a great impact,easy to cause flow instability,pressure and flow pulsation phenomenon.In this paper,we focus on porous microchannels,and study the structure parameters of porous microchannels,the undercooling degree and the flow rate on the boiling heat transfer performance and flow instability.The microchannels Conducted a comparative study.At the same time,the single-phase and boiling heat transfer characteristics of sintered ultra-thin porous metal(porosity 68 %-85 %)were studied.And the porous microchannels were visualized.The porous microchannels were designed and fabricated.The porous microchannels were designed with solid-phase sintering technology.The sintered particle size of the seven groups of microchannels was 30 μm,50 μm,90 μm and the mixed particle size,the bottom thickness was 200 μm and 400 μm,The mass flow rate range is 71-213 kg/m2·s,working fluid inlet temperature of 60 ℃ and 80 ℃.The specific effects of porous microchannel structure parameters(particle size,bottom thickness,sintering temperature,blending powder diameter)are explored in detail.Based on the analysis of the heat transfer and flow characteristics of the copper-based microchannels,it is known that the change of the flow rate has little effect on the wall superheat degree,only the critical heat flux is changed.The pressure drop of the two-phase flow increases linearly with the increase of the heat flux density,The pressure fluctuation is gradually increased with the increase of the heat flux density and the low mass flow rate.High-quality flow rate,the pressure drop in the late gradually slow down,the overall shock is more uniform..It is found that the mass flow rate of copper-based microchannels has little effect on the superheat of the walls,and the boiling heat transfer coefficients are similar in the five groups at the mass flow rate,and the increase of the dryness is basically the same.The pressure drop of the two-phase flow increases linearly with the increase of the heat flux,which is influenced by the mass flow rate.With the increase of the heat flux density,the pressure drop is gradually exacerbated at low mass flow rate.High-quality flow rate,the pressure drop pulsation in the high heat flux density gradually slowed down,the overall pulse is more uniform.The experimental results show that the sample with the highest porosity of 85 % has the best heat transfer performance and the inlet temperature.The heat transfer performance of all the samples is also improved.It was found that the porous microchannel had the best thickness ratio of 2-5,particle size 90 um,bottom thickness 200 μm,400 μm and particle size 50 μm,and the thickness of 200 μm was the most significant,and the heat transfer coefficient was the highest 5 times the copper-based microchannel.The superheat of the porous microchannels is smaller than that of the copper-based microchannels.The heat transfer coefficient increases with the increase of the mass flow rate at medium and high heat flux.Improve the inlet temperature,particle size 90μm thick 400 μm porous microchannel boiling heat transfer coefficient increased by about 40 %.Porous microchannels can effectively suppress the pressure pulsation.The large particle size and small sintering bottom thickness can slow down the pressure pulsation and improve the pressure drop of the inlet temperature.The results show that the number of bubbles in the mixing channel is much more and the flow pattern is faster than that of the single aperture channel.At high heat flux,the convective boiling heat is mainly driven by thin film evaporation,and the two-phase flow pattern is dominated by annular flow.The results of single phase experiments on porous surface show that the higher the porosity,the larger the pressure drop and the smaller the thermal resistance.The largest sample of Nu is almost 15 times the empty passage.This finding suggests that foamed copper can enhance heat transfer better at high flow rates.Two-phase flow boiling experiments show that the higher the porosity,the better the heat transfer performance.Improve the inlet temperature of porous metal two-phase heat transfer coefficient increased by about 30 %.In this paper,the heat transfer performance of porous microchannel and ultra-thin porous structure is improved,and the porous microchannel can effectively suppress the pressure drop instability of the two-phase flow and has great potential for industrial application. |
