Visualization Research On Liquid-liquid Phase Separation Characteristics Of LCST Solutions

Microchannel heat sink has the advantages of light weight,compact structure,high heat exchange efficiency and can be directly integrated on the heat sink chip.It is one of the effective methods to solve the heat dissipation problem of high heat flux micro-equipment in advanced engineering fields such as microelectronics technology,energy power and aerospace.However,with the rapid increase of chip thermal load,the heat dissipation problem is getting trickier.Although many solutions such as improving the microchannel structure,boiling heat transfer,and nanofluids have remarkably improved the heat dissipation performance,there are still some problems that need to be solved urgently.For example,improving the microchannel structure will significantly increase the pressure drop of the system.Moreover,flow boiling will induce flow instability,which will easily cause problems such as system vibration and premature evaporation of cooling fluid.Therefore,it is indispensable to find a new solution that takes into account the heat transfer performance of the microchannel heat sink,the pressure drop in the channel,and the stable operation of the system.Partially miscible solution with the lower critical solution temperature（LCST solution）will undergo liquid-liquid phase separation when the temperature is raised.It has the advantages of endothermic phase separation process,rapid separation,and reduced solution viscosity after separation,and has significant potential in improving the heat transfer performance of the microchannel heat sink,reducing the pressure drop of the channel,and ensuring the stable operation of the system.Nevertheless,liquid-liquid phase separation of the LCST solution is a multiphase energy transport process involving solute precipitation,growth and aggregation.It is complicated of the phase separation and flow heat transfer process of the solution in the microchannel.Consequently,in order to apply the LCST solution to the microchannel heat sink,it is necessary to first investigate the liquid-liquid phase separation characteristics at the static state.In this paper,a visual method is adopted to comprehensively study the liquid-liquid phase separation behavior under different conditions.Firstly,a visual microscopic observation platform was built to obtain the spinodal and nucleation separation process of the 2-butoxyethanol solution in a static state.Meanwhile,the effects of heating rate and mass fraction were discussed on the phase separation temperature range,separation time,growth rate of spherical separation domain and average separation rate.Secondly,improve the experimental device,and ascertained the law of the influence of natural convection on the phase separation process,clarified the weight of heat flux and mass fraction on the intensity of natural convection in the separation process.Finally,the experiment proved that the phase separation behavior is dominated by diffusion and explored the feasibility of the foamed copper to accelerate the LCST solution separation process.The main conclusions of this article are as follows:（1）LCST solution with critical mass fraction undergoes spinodal and nucleation separation processes in sequence during the heating process.The heating rate presents no noticeable effect on the duration of the spinodal separation process.Even so,the density of bicontinuous and dendritic morphology increases with an increase in the heating rate.In contrast,the diameter of the spherical separation domain（constituted by precipitated solute）increases linearly during the nucleation separation process.Moreover,both the growth rate of a single spherical separation domain in the solution with a mass fraction of 35%and the average separation rate of the solution are the largest.The phase separation temperature range of the critical mass fraction solution gradually increases with increasing the heating rate.When the heating rate exceeds 4°C·s^-1,its effect on the average separation rate and separation time can be ignored.（2）Phase separation process under the governance of natural convection is divided into three regions.From the heating wall to the top,there are phase-separated area,hazy area and mixed area.Moreover,the height of the phase-separated area for the solutions with a mass fraction above the critical value increases linearly with time.It should be noted that the maximum area percentage of the hazy area for the critical mass fraction solution is about 22%.Furthermore,the d T/dx is introduced to reflect the convection intensity between different positions（heights）inside the solution and the heating wall.It is found that the convection intensity gradually weakens along the height direction.The heat flux of the heated wall instead of mass fraction plays a dominant role in the natural convection intensity.For instance,the natural convection intensity can be promoted by 92.3%once the heat flux is appropriately increased.（3）In the diffusion-dominated phase separation process,the heat is transferred to the direction away from the heating wall（downward）through thermal conduction.Correspondingly,the solution can be divided into single-phase area（constituted by precipitated solute）,phase-separated area and un-separated area.The migration rate of the interface for single-phase area first increases and then decreases,but the maximum proportion of the phase-separated area is20%under different conditions.After 40 PPI foamed copper was inserted into the solution,the interface migration rate of the precipitated solute is 3.3 times larger than the original case.It also increases 70%,which compares with the natural convection condition.That means enhancing the solution thermal conductivity is a significant way to increase the phase separation rate.