Gas-liquid Two-phase Flow And Boiling Heat Transfer Characteristics Of Porous Microchannel Flat Tube

Microchannel flow boiling heat transfer has the advantages of high heat transfercoefficient,high temperature uniformity and low refrigerantcharge,which canreduce the consumption of resources and energy.So microchannel technologyused in various fieldswidely,and become one of the most promising heatexchange technology.Due to the miniaturization of the scale,the flow boiling heat transfer in the microchannel is very complicated,which needs further exploration and research.In this paper,the gas-liquid two-phase flow and boiling heat transfercharacteristics of the refrigerant in the microchannel flat tube were studied by numericalsimulation,experimental test and theoretical analysis.Firstly,the mathematical modelof the microchannel flat tube was established.The gas-liquid two-phase simulation ofthe refrigerant was carried out by using the simulation software Fluent and the VOF multiphase flow model was employed.The effects of type of refrigerants,mass flow rate,heat flux,number offlat tubes and flat tube width on the phase transition of the working fluidwere analyzed.Thenexperimental test system was set up,and the parameters such as the flat tube inlet and outlet temperature,wall temperature,inlet and outlet pressure were researched by changing the heating power,therefrigerant charge,and the flat tube size of the experimental section.In the end,the flow and heat transfer performance of the refrigerant inthe microchannel flat tube were analyzed combined with the simulation results.(1)During the simulation,bubbles are generated on the inner surface of the microchannel,then detached from the wall surface,and finally aggregated into block flow,slug flow,annular flow and fog flow;In the process of refrigerant changing from liquid phase to gasphase,bubbles are generated,grown,merged and separated.As the shape of the bubble changes,the velocity vector diagram,cloud map and temperature field in the flat tube also change accordingly.(2)Under the same conditions,the length of the phase change interface increased slightlywith the increase of the number of microchannel;as the width of the flat tube increases,the boilingpoint position move toward the entrance and the length of the phase interface decreases.At the same position of different flat tubes,the volume fraction of liquid phaseof the refrigerant increases with the increase of the number of holes.For N_k=18,the volume fraction of liquid phase at outletis increased by 24.5%compared with N_k=10.For the same number of holes of three flat tube width refrigerants(A_l=11mm?16mm?21mm),whenA_l=16 mm,the liquid volume fraction isthe smaller,the value is 0.76.(3)As the mass flow rate increases,the boiling point position is delayedand the length ofphase interface becomes longer.However,with the heat flux increases,the boiling pointposition move toward the entrance and the length of phase interface become shorter.When the mass flow rate is G_m=200kg/(m~2·s),the liquid volume fraction of high heat flux is reduced by 19.7%compared with the low heat flux.The liquid volume fraction decreases with the increase of heatflux at low mass flow rate,and decreases first and then increases with theincrease of heat flux density at high mass flow rate.Under the same conditions,different refrigerants have different phasetransitions,the liquid phase volume of the refrigerant R410a is reduced by 53.06%compared with R134a.(4)The experimental results show that the temperature of the flat tube gradually increases with the increase of heating power.Whenthe heating power is Q_w=1200W and 1500W,the temperature is basic same,and it is 32.5%higher than the average temperature of Q_w=600W.As the refrigerant charge increases,the overall temperature on the flat tube first increases and remains stable,and the temperature change is small compared with the influence of the heating power.(5)Through experiment comparison,the temperature of each measuring point of2'#,3'#flat tube is increased by 1.86%and 3.94%compared with 1'#flattube,respectively.For different width of flat tube,the temperature of 4'#flat tube is about3°Chigher than that of 1'#flat tube at adistance after 240mm.Comparing the refrigerants R134a and R600a,thetemperature turning point of the refrigerant R600a isbetween 120mm and 480mm,and thetemperature turning point of R134a is between210mm and 510mm,and the averagetemperature is also lower than R134a.(6)At the entrance,the flat tube has high heat transfer coefficient,and then decrease rapidly along the flow direction.After 90 mm from the inlet of the flat pipe,the heat transfer coefficient grandually goes to smooth and slow.Analysis of the results of each working condition shows that when the heating power is 1200W andthe refrigerant charge is 0.8kg/cm~2 that is the optimal operating conditions for thisexperimental system.