Study On Characteristics Of Heat Transfer Enhancement By Different-mode-interacting Boiling

Nucleate boiling has been used for cooling in thermal energy dissipation systems,such as high-power electronics,heat exchangers,and nuclear reactors.The advantage of pool boiling is that a high heat flux with high heat conductance can be removed passively while maintaining a low superheat compared with natural/forced convection without phase change.However,the heat removal capacity is limited by the upper limit of cooling,i.e.,the critical heat flux(CHF),where the heat transfer coefficient(HTC)decreases dramatically because the boiling regime is changed from nucleate boiling to film boiling.Therefore,the CHF enhancement is of great interest to researchers.Surface modifications are effective methods to enhance the CHF.However,the above methods have great difficulties in practical application due to problems such as insufficient strength and deterioration over time.Therefore,it is urgent to develop a novel method that does not rely on the surface characteristics to enhance the CHF.In present study,a novel method of different-mode-interacting boiling,in which the interactions between the different modes of nucleate and film boiling were expected,was proposed to improve the boiling heat transfer performance.Two types of thin materials with different thermal properties were alternately arranged inside the heat transfer plate near the surface(nonuniform conductance).There is no modification of the heat transfer surface,the CHF enhancement was achieved by changing the internal structure of the heat transfer plate.Experimental tests were conducted to systematically analyze the effect of factors such as the material width,material ratio,material arrangement pattern,and plate size,on characterisitics of heat transfer enhancement by different-mode-interacting boiling.It is very important to determine the material types and structural parameters of the nonuniform plate to achieve excellent performance of different-mode-interacting boiling.In this study,a numerical simulation method is used to obtain the material type and structural parameters of the nonuniform plate by creating a two-dimensional steady-state heat conduction calculation model.The results show that when the high and low conductance materials(high conductance material,HCM and low conductance material,LCM)are copper and polytetrafluoroethylene,respectively,and LCM with a thickness of 0.5 mm is arranged 0.3 mm away from the heating surface,the surface of the nonuniform plate appears the expected alternating distribution of high and low temperatures.Furthermore,the nonuniform plate(W_H=W_L)exhibits different surface temperature distributions and different material interface densities with the simultaneous variation of the width of high and low conductance materials.As the material width decreases,the amplitude of nonuniform temperature distribution on the surface decreases and the density of the interface between different materials increase.In order to verify the effect of different-mode-interacting boiling on heat transfer performance,a series of experimental investigations were carried out the nonuniform palte with equal intervals material distribution at different spatial scales.The experiment results show that the different-mode-interacting boiling on nonuniform plate with three different material widths improve the boiling heat transfer performance.The ratios of CHF enhancement were 11-68%for all test conditions.In particular,different liquid supply systems were used to explore boiling characteristics in micro gap.The result shows that the effect of different-mode-interacting boiling on heat transfer performance decreases as the decreasing ability of the surrounding liquid to enter the microgap.Furthermore,the material width corresponding to the maximum CHF changes as the gap size increases.The visualization experiments show that in the low and moderate heat flux regions,the bubble generation regions distributed in the high conductance region of the nonuniform plate.In the high heat flux near CHF,the bubble generation regions change to the low conductance region.In this study,in order to investigate the separate effect of surface temperature distribution on the performance of different-mode-interacting boiling,the nonuniform plate with three different material ratios are designed to obtaine different surface temperature distributions.The boiling experiments were performe to study the influence of the surface temperature distribution on the different-mode-interacting boiling at different spatial scales.The results show that the CHF for the nonunifrom plate was significantly enhanced compared with that for the uniform copper plate at same experimental conditions.Furthermore,the CHF increased with decreasing ratios of low-conductance material for constant density of interface between different materials.The nonuniform plate with large width of the low-conductance material shows the lower temperature in the center of material,which result in a decrease in heat flux.For the separate influence of the density of interface between different materials on the different-mode-interacting boiling,while keeping the constant material ratio,the nonuniform plate with material parallel and cross arrangement are designed to obtain different density of interface between different materials.The study found that the nonuniform plate with different material arrangement paterns improved the boiling heat transfer performance.When sufficient liquid is supplied to the heat transfer surface,the nonuniform plate with material cross arrangement provide more interfaces between different materials and exhibit better heat transfer characteristics.When the ability of the liquid to be supplied from the surroundings to the heat transfer surface is weakened,the effect of the nonuniform plate with material cross arrangement on different-mode-interacting boiling is weakened.The above results indicate that sufficient liquid supply in the low conductance region of the nonuniform plate is the prerequisite for achieving excellent different-mode-interacting boiling.In order to meet the heat removal of heating elements with different sizes in practical applications,this study design nonuniform plates with different size and explores its influence on the different-mode-interacting boiling.The experimental measurement shhown that a significant enhancement in CHF was achieved with nonuniform heating plates compared to uniform plate at same condition.Furthermore,the CHF showed a decreasing tendency with increasing plate size dur to the incraseing resistance of the rewetting liquid.In addition,the decreasing tendency of CHF with the increasing plate size is weaken by increasing material width.