| Boiling heat transfer is widely used in industry because of its strong heat transfer performance.However,for a flow boiling system,high heat flux density and low flow resistance cannot be achieved in the evaporator at the same time.On the one hand,too much heat flow leads to the formation of a gas film or evaporation in the evaporator,and soon the critical heat flow will be reached.On the other hand,when the heat flow increases,the fluid dryness in the channel rises,and the two-phase pressure drop increases accordingly.Based on the idea of "process decomposition and scale synergy",this paper constructs a multi-scale structure of capillary evaporator.In order to increase the critical heat flux density and reduce the flow resistance of the evaporator,this experiment uses deionized water as the working fluid in the flow boiling experiment and employs a multi-scale double-layer capillary structure in the evaporator to adjust the gas-liquid phase distribution in the evaporator.The capillary core adopts a multi-scale structure,and the capillary force of small pores provides power for liquid infiltration.The large pores provide a channel for the smooth discharge of gas.The working fluid enters the upper capillary core from the compensation chamber of the evaporator,and then enters the first capillary core to undergo a phase change.Steam flows from the pores inside the capillary core to the steam channel,and is discharged from the steam holes on the side of the evaporator without returning to the small holes or mixing with the liquid through the upper capillary core.In this way,the effect of vapor-liquid phase separation can be achieved.In this experiment,copper powder particles with average particle diameters of 57 μm and 107 μm were respectively sintered on the copper surface in high temperature to form a double-layer capillary core structure.The capillary core is immersed in H2O2 by immersion oxidation,so that the surface of the nano-grass grows dense,and then becomes super-hydrophilic.During the flow boiling experiment,the flow in the evaporator circuit and the amount of heating to the evaporator are controlled.three mass flow rates of 5.9 g/min,16.6 g/min,and 29.4 g/min were selected in the experiment with each mass flow rate covering a heat load of 0-600 W/cm2.The experimental results show that the boiling curve is different from the traditional curve,forming an "S" shape with two inflection points,and the maximum heat flux density reaches 578.4 W/cm2.The large and small pores provide different dimensions for boiling nucleation so that large pores are preferentially nucleated at low heat fluxes while small pores are activated at high heat fluxes.As a result,the capillary core structure undergoes three heat exchange stages during the process of heat flux density increasing gradually,that is,"pool-like boiling","second boiling" and "steam convection".At the same superheat degree,the heat flow density of the evaporator after immersion oxidation treatment can reach 48%higher than the original,and the heat transfer coefficient can reach 102%higher than that.The modified pressure drop has both positive and negative effects on the two particles,increasing the capillary indenter as well as increasing friction resistance.This double-layer multi-scale capillary core has phase separation,which significantly reduces the flow resistance.The maximum pressure drop is only 10 kPa,and even close to 0 kPa under many operating conditions,which is only 10%of the pressure drop of the traditional evaporator structure.The self-driving effect is obvious.Finally,a three-zone heat transfer model is established and analyzed in the basis of the heat transfer curve’s characteristics.The mechanism of heat transfer and the reason of vapor-liquid phase separation drag reduction were studied.This research provides a new idea for the design of evaporators with high heat flux and low flow resistance,which can be used in high-power electronic coolers and other equipment. |
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