Study On Evaporation Heat Transfer Mechanism And Performance Of Refrigerant Atomizing Jet

In recent years,falling film evaporators have been used in large and medium-sized chillers in the central air-conditioning industry because of their advantages such as energy saving,environmental protection,and heat transfer.However,there is a problem of uneven distribution of the refrigerant,resulting in insufficient performance.The spray cooling technology has the characteristics of large heat transfer coefficient,good temperature uniformity,low superheat,high critical heat flux density and low circulating flow rate.At the same time,it meets various requirements for the optimal design of falling film evaporators.But the current application research is mainly focus on electronic cooling and chemical direction.In this paper,the research of the heat transfer mechanism in spray cooling and falling film evaporation of electronic components is introduced.Innovatively,the spray jet evaporation is introduced into the field of refrigeration and air conditioning.The function of the liquid distributor in the falling film evaporator is replaced by the nozzle spray jet method.The condensing throttling high-pressure refrigerant liquid passes through the nozzle and is atomized into a fine liquid in the evaporator.And it is sprayed on the horizontal heat exchange tube bundle for evaporation heat exchange.It not only has the advantages of more uniform distribution and less refrigerant usage,but also has extremely high heat transfer potential.After extensive literature review and experimental simulation analysis,a visualized horizontal tube atomization evaporation test bench was set up and a corresponding mathematical model was established.Through the experiments and numerical simulation,the evaporation heat transfer of the spray jet on the horizontal tube was comprehensively analyzed.The main content of the paper is as follows:1.Set up a visualized horizontal tube atomization jet evaporation test bench which is using R22 as a refrigerant.In the pressure difference of the conventional refrigeration cycle,the refrigerant through the 0.3mm aperture nozzle in the evaporation tube to complete the process of atomization jet evaporation,through the visible evaporating tube can clearly judge refrigerant atomization state is good.The experimental results show that when the inlet water temperature of the chilled water is constant,the pre-nozzle pressure increases from 0.8 to 1.2 MPa,the refrigerant charge and the suction port pressure increase.And at the same time,due to the liquid filling amount,the cooling capacity of copper pipes also increased.The experiment proved the feasibility of the spray jet evaporation technology in the refrigeration cycle system.2.The physical model and mathematical model of the heat transfer process of nozzle atomized jet tube bundles were established.The CFD software was used to simulate the evaporation process of the atomized jet in the evaporation tube in the experiment.The reliability of the model is verified by comparing the experimental values with the simulated values under different operating conditions.In the operating conditions of spraying pressure 0.8MPa-1.2MPa and evaporating pressure 0.42MPa-0.584MPa,the mass flow of the liquid refrigerant passing through the nozzle increases with the increase of the pressure before the nozzle,resulting in increased cooling capacity of all tube bundle surfaces.However,the greater the concentration of droplets closest to the spray center,the greater the heterogeneity of heat transfer between the pipes.3.A numerical Simulation of nozzle atomized jet single-pipe was established.Using a 0.6mm-diameter cyclone atomization nozzle,variable conditions affecting the heat transfer performance were calculated.The experimental results show that the liquid film is divided into two parts,a liquid droplet striking area and a liquid film diffusion area on the surface of the tube.The film thickness of the liquid droplet striking area is thin,the liquid film flow velocity is faster,and the heat flux density and heat transfer uniformity are stronger than the liquid film diffusion area.When the spray pressure is 1.2MPa?1.7MPa and the evaporation pressure is constant,the larger the nozzle pressure difference,the smaller the diameter of the atomized droplet and the faster the droplet speed.Raising the spray height in the range of 50mm to 100mm will increase the liquid area of the cloth,and the thickness of the liquid film will become thinner and the heat transfer will increase.The effect of wall temperature on the heat transfer performance was evident when the wall temperature was raised from 9.5? to 14.5? while keeping the evaporating temperature constant at 5?,and the average heat flux density in the impact area of the droplets increased from 3 5kW/m2 to 90kW/m2.The uniformity of heat transfer across the tube surface can be improved by two-nozzle or four-nozzle interference sprays.