| Quick freezing is one of the important technologies for food preservation,and has gradually developed into an indispensable link in the cold chain industry.With the increasing demand for types and quantities of quick-frozen food,flat-panel quick-freezers are more and more widely used in aquaculture,cold chain logistics and other industries.When the food is quick-frozen on the shelf plate,the cooling capacity distribution of each layer of the shelf affects the quick-freezing effect of the food,and the different cooling capacity distribution of the shelf will cause the phenomenon of uneven freezing of the food.Therefore,whether the quick-freezing system can reasonably distribute the refrigerant to each shelf is of great significance to the quality of the quick-freezing effect of food.The optimization of the structure of the diverter can improve the uniformity of the liquid supply of each shelf plate,so that the heat exchange area of the evaporator can be fully utilized,thereby improving the refrigeration efficiency of the system and the quality of food freezing.This paper summarizes and analyzes the reasons for the uneven distribution of the two-phase flow,and finds that the phase separation of the two-phase flow is the main reason for the uneven liquid separation of the refrigeration system,and the factors that cause the phase separation are the principle of flow separation and the flow state of the upstream fluid.,Non-uniform heat exchange in the downstream branch.The research group has developed a new type of rectifying critical nozzle flow divider.The gas-liquid two-phase refrigerant reaches the speed of sound at the throat of the critical flow nozzle.After the fluid flow through the nozzle reaches a stable value,the fluid forms a critical flow and there is a blocking phenomenon.The blocking phenomenon of the critical flow can cut off the pressure disturbance downstream of the nozzle from propagating upstream,overcome the uneven liquid separation caused by the inconsistent downstream pressure between the split nozzles,and reduce the influence of non-uniform heat exchange.In this paper,a new type of rectifying critical nozzle diverter is applied to the quick-freezer system to solve the technical defect of the non-uniformity of liquid distribution in the shelf of the flat-plate quick-freezer.The main research contents and conclusions of this paper are as follows:1.This paper analyzes the research status of domestic and foreign scholars on the performance and freezing quality of quick-freezers,especially the problem of uneven liquid separation of quick-freezers.Among many flow splitting structures,the header pipe has a poor flow splitting effect,while the mixed-phase flow splitter and the split-phase flow splitter have better effects and are of various types.The various flow patterns that can be formed by gas-liquid two-phase flow are analyzed.Among them,the annular flow is a flow pattern in which the liquid phase wraps the gas phase,the annular liquid film flows close to the wall,and the gas phase and droplets flow along the central axis and show a symmetrical distribution.The flow pattern is the best gas-liquid two-phase distribution flow pattern.2.Apply a new type of rectifying critical nozzle diverter,and analyze its diversion principle and structure.The flow splitter is mainly composed of three parts: the swirl disturbance section,the annular flow setting stage and the critical flow split section.Through the test of the gas-liquid two-phase flow,it is found that the gas-liquid two-phase flow can reach the critical speed of sound at a lower speed.3.To obtain a preliminary model of the critical nozzle,deduce and calculate the critical speed of sound of R507,and obtain the throat diameter of critical nozzle.Change the design parameters such as the length of the constricted section,the length of the throat,the length of the expansion section and the diameter of the outlet,and establish the length of the constricted section(10?20mm),the length of the throat(0.5?1.5d),and the length of the expansion section(2.5?3.5mm)respectively.,outlet diameter(2.6?2.8d)four groups of critical flow nozzle models,to study the influence of different structure sizes on the critical injection characteristics of gas-liquid two-phase flow refrigerant.4.Carry out simulation calculation on the established mathematical model,and draw the following conclusions:(1)In the simulated pressure range(3.2?4.0MPa),the injection velocity of the two-phase flow refrigerant increases with the increase of the inlet pressure,and the change of the inlet pressure has basically no effect on the refrigerant gas content.(2)The change of the nozzle structure size has little effect on the refrigerant gas content,but has a significant effect on the injection velocity,and there are optimal structural parameters.The maximum injection velocity of the two-phase flow refrigerant increases with the decrease of the length of the constriction section,and the excessively long constriction section obviously limits the increase of the injection velocity.When the length of the contraction section is 12.5mm and the inlet pressure is 3.6MPa,the two-phase flow refrigerant reaches the critical velocity;the throat length of 1.5d is most conducive to the improvement of the injection velocity,and the refrigerant reaches the critical velocity when the inlet pressure is 3.3MPa;the expansion section The length of 2.75 mm is most conducive to the increase of the injection speed,and the critical speed is reached when the inlet pressure is 3.2MPa.(3)In the four groups of nozzle models with different structural sizes,the two-phase flow refrigerant forms an annular flow pattern in which the liquid phase surrounds the gas phase after passing through the critical flow nozzle.From the near wall surface to the central axis,the gas phase content increases first and then decreases,the wall surface is close to the state of full liquid,and it is symmetrically distributed on both sides of the radial direction.In the test range of dryness(0.2?0.32),the refrigerant at the nozzle outlet section maintains an annular flow pattern,and the effect of dryness on the gas content of the refrigerant near the central axis is greater than that near the wall. |
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