| Miniaturization of the electronics industry,miniaturized components in various fields are facing serious heat dissipation problems.When the common surface of traditional microchannels is in gas-liquid two-phase flow,the heat dissipation effect cannot meet the heat dissipation requirements of current micro devices.Therefore,how to study the new surface structure to enhance the boiling heat transfer performance is an urgent scientific and technical problem.In this paper,deionized water was used as the flow medium to study the flow boiling heat transfer of porous microchannels with different single particle size and mixed particle size.The effects of particle size,mixed particle size parameters,sintered bottom thickness,inlet water temperature and mass flow rate on the heat transfer performance and flow instability of single particle size/mixed particle size porous microchannels were studied.The heat transfer characteristics of particle size/mixed particle size porous microchannel single-phase flow and gas-liquid two-phase flow,and the gas-liquid two-phase flow during flow boiling were visualized to study the bubble generation mechanism.The experimental results show that:(1)The boiling initial superheat OBN of a porous microchannel with a single particle size of 50?m is similar to the boiling initiation superheat OBN of a 30?m,90?m porous microchannel,but its critical heat flux(CHF)is higher than that.The other two samples were 107.8W/cm~2.In the high heat flux state,the porous microchannels with a particle size of 50?m have a superheat of less than 30 and 90?m corresponding to the same heat flux density,and the heat transfer performance at high heat flux is better than the other two microchannels.Under certain conditions of fluid inlet temperature,the increase of mass flow rate also increases the critical heat flux density(CHF)point of the wall.The CHF point of the mass flow rate of 213 kg/(m~2·s)was increased by1.2 times with respect to the CHF point of 71 kg/(m~2·s).When the mass flow rate is low,the porous microchannels have a strong limiting effect,inhibiting the nucleation process,thereby reducing the porous wall strengthening ability.(2)Increasing the thickness-to-particle ratio of the porous microchannel can increase the critical heat flux(CHF)of the porous microchannel,which is because the increase in the bottom thickness also increases the heat transfer resistance,and the critical heat flux requires higher heating power.At high heat flux density,the wall thickness of the porous microchannels with a particle size of 30?m and a sintered base thickness of 400?m is lower than the sintered bottom thickness of 400?m.The 90?m porous microchannels exhibit opposite heat transfer mechanisms,which means that increasing the thickness-to-particle ratio for 90?m porous microchannels can improve the heat transfer performance.The increase of the thickness of the 30?m porous microchannels is lower than that of the heat transfer.(3)At low heat flux densities,the pressure drop curves at the three mass flow rates are similar.This is due to the same porous microchannel,the pressure drop in the flow channel is mainly similar to the frictional pressure drop and acceleration pressure drop.At high heat flux densities,the superheat of the porous wall surface is increased,accelerating the growth of the bubble on the porous wall surface.With respect to the low heat flux density,the bubble diameter becomes large,and the bubble merges to form an annular flow inside the microchannel,so that the pressure drop of the two-phase flow inside the microchannel is increased.(4)Low heat flux density,single-phase flow,single-mixed particle size porous microchannels have similar heat transfer performance,and hybrid particle size porous microchannels have no obvious performance.At high heat flux density,the mixed particle size porous microchannel exhibits high heat exchange performance with its advantages of more vaporized cores.The heat transfer coefficient is nearly one time higher than that of a single microchannel.For the critical CHF value,the mixed particle size microchannel is also significantly higher than the single particle size microchannel.(5)Through visual research,it is observed that the mixed particle size is larger than the number of bubbles in the single particle flow channel,and at the same mass flow rate,the bubble diameter of the mixed particle size porous surface is smaller than the single particle size because of the mixed particles.Due to the large number of porous wall face holes,the porous microchannels are not able to grow and are carried away by the bubbles.Under the experimental conditions,at low heat flux density,the bubble detachment frequency of the porous surface of the mixed particle size porous microchannel is higher than that of the single particle size porous microchannel,which is beneficial to strengthen the nucleate boiling.(6)Through the visual study,the porous microchannels with particle size are mixed.Under the low heat flux density,the bubble growth rate of the mixed particle size porous microchannels is lower than that of the single particle size.At high heat flux density,the mixed particle size porous microchannels are beneficial to reduce wall superheat,increase the critical heat flux density of porous microchannels,and improve the heat transfer performance of the whole body.The research in this paper shows that the heat transfer performance of the mixed particle size porous microchannel is obviously improved compared with the single particle size porous microchannel.Moreover,the mixed porous microchannel can effectively suppress the pressure drop instability of the two-phase flow,and has an excellent application prospect in the micro heat dissipation field compared to the single particle size porous microchannel. |
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