Numerical And Experimental Investigation Of Liquid-vapor Two-phase Flow Boiling In Manifold Microchannel

In recent decades,with the vigorous development of micro/nanofabrication technology and micro-electronic devices,high power density integrated circuits and light-emitting diodes has also made great strides forward.The rapidly increasing power consumption of high-power electronic equipment puts forward higher requirements for thermal management solutions,so the development of suitable and efficient heat dissipation technology for high-power equipment has far-reaching benefits.Compared with conventional size heat exchangers,the micro-channel heat exchanger is favored by industry and academia because of its ultra-compact structure and high heat transfer coefficients.The Manifold Microchannel(MMC)heat sink is a specific structural design in the microchannel heat sink.In recent years,it has attracted the attention of researchers due to its excellent heat transfer performance and low pressure loss.Although the current research on conventional microchannel heat dissipation technology is extensive,the research on the flow boiling heat transfer characteristics in the MMC is still very insufficient.The mechanism of internal flow and heat transfer is not clear,as well as research methods and manufacturing technology need to be further improved.In this paper,we first focus the current research status on single-phase flow and two-phase flow boiling heat transfer in MMC.The self-programming numerical codes are validated,and the heat transfer characteristics of flow boiling in rectangular microchannels are numerically analyzed.Then,effects of microchannel geometrical parameters and manifold types are discussed.Finally,supported by the microchannel visualization experimental test systems,two-phase flow pattern evolutions during the boiling of the MMC heat sink is investigated.We found it hard to get sharp liquid-vapor interface using the original VOF(Volume of Fluid)method,while the VOF smoothing method and the S-CLSVOF(Simple Coupled Level Set and Volume of Fluid)interface reconstruction method can obtain a sharp phase interface to effectively avoid parasitic currents.The numerical results of the one-dimensional Stefan problem verify the energy and momentum transfer in our phase change models.The simulation data of single bubble growth on the heating surface is also in good agreement with experimental data.As for the saturated flow boiling process in the rectangular microchannel,this paper not only analyzes the effects of working condition variables during the single bubble growth,but also study the influence of varying width-to-height ratios of microchannel for the annular flow boiling.The results show that heat transfer coefficients will be magnified when bubbles grow on the heating surface of the rectangular microchannel.Increasing the inlet Re number can significantly improve the heat transfer performance of the microchannel during bubble growth;improving the hydrophilicity of the heating surface helps increase the surrounding area of the bubbles,which enhances heat transfer;the change in surface tension coefficients has no obvious effect on heat transfer during the bubble growth process;the merging of two growing bubbles will produce instantaneous high heat transfer.There is an optimal width-to-height ratio for rectangular microchannel during annular flow boiling process,which makes the thickness of the liquid film thin enough to achieve extreme high heat transfer performance.However,it is also prone to partial dryout and heat transfer deterioration.It is found that the width of the microchannel part of the heat sink has a significant effect on the flow and heat transfer performance.When w_c=w_f,decreasing w_c+w_freduces the thermal resistance of the MMC heat sink;when w_f stays unchanged,pressure drop will be decreased when w_c get reduced;When the inlet-to-outlet width ratio?is less than 1,the thermal resistance of the heat sink rises significantly with the decrease of?,and the inlet and outlet pressure drop show lower values under various heat flux conditions;when?>1,the pressure loss will increase dramatically when?increases.There will be an optimal value for the MMC heat sink during flow boiling to achieve low thermal resistance and pressure drop.The effects of manifold types on flow characteristics and heat transfer performance are investigated.It is found that Z-type and C-type manifold channels show great flow maldistribution in the single-phase flow analysis.On the contrary,H-type and U-type manifold channels lead to a more uniform flow distribution in the microchannel heat sink.Flow patterns in MMC roughly evolve according to the basic microchannel flow patterns of bubbly flow,slug flow,intermittent flow and annular flow as the heat flux increases.Due to the restriction of the manifold structure,Z-type and C-type are easy to form intermittent flow and annular flow with higher cavitation rate in the outlet manifold channel under high heat flow conditions;when the control operation conditions are exactly the same,U-type manifold provides the best heat transfer performance while also having the smallest pressure loss.After using the test facility and experimental equipment to visualize the MMC heat sink test module,it is found that flow boiling improves the heat transfer performance of the MMC.When increasing the inlet mass flow rate or inlet subcooling,it helps to delay the occurrence of the onset of nucleate boiling point;increasing the flow rate will increase the pressure loss of the MMC heat sink.In the process of gradually increasing the heat flux density,it is found that the two-phase flow pattern in the heat sink can basically be divided into two types:Bubbly Flow and Bubble Cross Flow.