Study On The Direct Contact Phase Change Heat Transfer Of Steam-Subcooled Water Spray

The direct contact phase change heat transfer of saturated steam to subcooled liquid is a vapor-liquid direct contact heat transfer technology without solid walls.Compared with the traditional heat transfer with solid wall,the direct contact phase change heat transfer process of steam-subcooled liquid has many advantages,such as simple structure of heat transfer equipment,unrestricted minimum heat transfer temperature difference between cold and hot streams,low pressure drop and high heat transfer coefficient.In industrial application,it can be used to replace the packing in the distillation column section of the large refining unit,effectively reduce the pressure drop of the devices,eliminate the packing blockage and corrosion,thus greatly reduce the energy consumption and equipment cost.Therefore,steam-subcooled liquid direct contact phase change heat transfer technology has a wide application prospect in the refining industry.The heat-transfer characteristic of the direct contact condensation heat transfer process of steam-subcooled liquid is very complex,affected by the flow form of liquid dispersed phase,the distribution of liquid sheet/droplet,and different liquid phase composition.There are few experimental and theoretical studies on this process,and systematic theoretical methods is still lack.Therefore,in this paper,the direct contact condensation heat transfer process of steam-subcooled liquid were studied by experiments and numerical simulation.The flow field distribution of liquid phase,the morphology changes of the two-phase interface,and heat transfer law were obtained in direct contact phase change heat transfer.In this study,the direct contact condensation heat transfer process of steam-subcooled liquid was achieved by spraying liquid into saturated vapor phase in the form of liquid sheet spray.In the experimental study,the changes of liquid sheet thickness and breakup length as well as the temperature distribution of liquid sheet axial and radial at different inlet liquid temperatures were investigated,and the local heat transfer coefficient and total heat transfer coefficient of liquid sheet were calculated based on the experimental data.In the numerical simulation study,the three-dimensional flow was simulated by using a two-dimensional axisymmetric model by reasonably simplifying the internal structure of a pressure swirling nozzle.The Volume of Fluid（VOF）model was used to capture the vapor-liquid interface,and the Hertz-Knudsen-Schrage model was used to calculate the mass and heat transfer at the phase interface.In addition,the reliability of the numerical model was verified by the consistency between the simulation and experimental data of the liquid sheet temperature along the axial direction under the same conditions.The results show that the liquid sheet thickness and breakup length at lower inlet liquid temperature are larger in the direct contact condensation heat transfer process of steam-subcooled water.With the increase of liquid mass flow rate,the overall thickness of liquid sheet decreases slightly,and the thickness of liquid sheet decreases with the increase of axial distance.The liquid sheet thickness ranges from 100 to 700μm.The thickness of liquid sheet during condensation heat transfer is greater than that without heat transfer.The spray cone angle of liquid increases obviously with the increase of liquid mass flow rate.However,with the inlet liquid temperature increases,the spray cone angle does not change significantly but only increases slightly,which is slightly smaller than that without heat transfer.The liquid sheet breakup length decreases with the increase of inlet liquid temperature and liquid mass flow rate,and it is smaller with condensation heat transfer than that without heat transfer under the same conditions.In heat transfer process,there is a temperature gradient in the radial direction of the liquid sheet,the temperature of the inner and outer surface of the liquid sheet is higher,and the lowest temperature could be found in the center.With the increase of the axial distance of the liquid sheet,the temperature of the liquid sheet increases gradually and increases fastest at the outlet of the nozzle.In the process of condensation heat transfer of the whole spray,the temperature rise of the liquid sheet accounts for 80 to 85 percent of the total temperature rise of the spray heat transfer.In addition,compared with the droplet region,the liquid sheet region plays a major role in heat transfer.The local heat transfer coefficient at the nozzle exit is the largest and decreases with the increase of axial distance.The local heat transfer coefficient increases with the increase of inlet liquid temperature and increases slightly with the increase of liquid mass flow rate.Under the experimental conditions,the total heat transfer coefficient of the process is about 400 k W·（m²·K）^-1.To sum up,the direct contact condensation heat transfer process of steam-subcooled liquid was systematically studied by combining experimental and numerical simulation methods.In this paper,the heat transfer characteristics and laws obtained in the direct contact condensation heat transfer process of steam-subcooled liquid are of great significance to further study the law of fluid distribution and heat transfer matching,understand the coupling mechanism and control method of mass and energy transfer process,guide the engineering design and optimization of large industrial equipment,improve the efficiency of industrial equipment,and develop efficient equipment.