Optimization Of Flow Field And Heat Transfer Characteristics In Static Pressure Cavity Of Impact Quick-freezer

With the improvement of social and economic level,people are increasingly demanding for the quality and flavor of food.The rapid development of the frozen food industry puts forward higher requirements for the freezer.Jet impingement technology is a kind of enhancing heat exchange technology because of its unique hydrodynamic and heat transfer characteristics.At present,the technology has been used in food processing and preservation.And the up and down impact freezer which fully utilizes the jet impact technology had been praised by food manufacturers for its high heat transfer coefficient and fast freezing speed.However,the heat exchange intensity and heat exchange uniformity could still be further improved.This article used a solid quick-freezer to build a model.Based on this,experiments were used to verify the accuracy of the model.A series of indicators such as the outlet wind speed,non-uniformity of the upper and lower nozzles,cross-flow wind velocity and non-uniformity,and heat transfer strength and non-uniformity on the surface of the steel strip were used to measure the flow characteristics and heat transfer characteristics of the fluid inside the static pressure cavity.The model was continuously optimized through numerical simulation,including improving the size of the static pressure chamber,changing the inlet pressure of the static pressure chamber,and the nozzle structure.Different types,sizes,and Numbers of deflectors were installed in the pressure cavity.The specific content was as follows:1.Optimize the size of the static pressure chamber of the up and down impact freezer.Based on a solid freezer with a static pressure cavity size of 4000 mm×1500mm×2000 mm,the inlet pressure was guaranteed to be 190 Pa and the inlet flow rate was 2.64 m³/s.The computational fluid dynamics?CFD?was used and testified by the experiment.It was found that the error of the model did not exceed 15%,and the inlet flow error was only 1.5%.5 different static pressure cavity sizes such as 4000 mm×1500 mm×1500 mm,4000 mm×2000 mm×1500 mm,4000 mm×2000 mm×2000mm,4000 mm×2500 mm×1500 mm,4000 mm×2500 mm×2000 mm,etc.were proposed.The change of the pressure chamber size changed the internal flow field of the freezer.It was found that under the condition that the inlet flow of the static pressure cavity was the same and the pressure was constant,Although the wind speed had changed in several sizes,the change was not large.In addition,although the heat transfer strength of the static pressure cavity with a size of 4000 mm×2000 mm×2000mm was higher than the heat transfer strength of 4000 mm×1500 mm×2000 mm,its uniformity was poor and it was not a preferred design.The steel surface Nu number of4000 mm×2500 mm×1500 mm and 4000 mm×2500 mm×2000 mm reached 177.76and 177.39,which were about 6.81%and 6.59%higher than the original surface Nu number of the steel strip,and the uniformity was also the best.Combining the above factors,4000 mm×2500 mm×1500 mm and 4000 mm×2500 mm×2000 mm were the optimal designed in six sizes regardless of the outlet wind velocity,heat transfer strength and uniformity.2.Analysis and comparison of three nozzle sizes.Taking the static pressure cavity of the slot-nozzle type up-down impact quick-freezer with a size of 4000 mm×2500mm×2000 mm as the research object,using CFD numerical simulation and field tests,the three sizes nozzles were compared.Nozzle structure of three different sizes of nozzles under different pressures?190,170,160 Pa?,the difference between the internal flow field and heat transfer characteristics under the conditions were analyzed.The results showed that the exit wind velocity,Nusselt number,and uniformity of the slit nozzle T₀ under the condition of 190 Pa was better than those under the condition of 170Pa,and the crossflow wind speed was also lower.The slit nozzle T₁'s exit wind velocity,Nusselt number,and uniformity were ideal under the condition of 170 Pa,but the cross-flow wind velocity increased.Compared with the slit nozzle T₂ and T₀ at the same inlet pressure,only the cross-flow wind speed was lower.In terms of heat transfer strength,when the inlet pressure was 190 Pa,the local Nusselt number of the steel strip surface corresponding to the slit nozzle T₁ was the largest,and the average Nusselt number of T₀ and T₁ was not much different.In terms of heat exchange uniformity,the slit nozzle T₀ had the lowest unevenness at the inlet pressure of 190 Pa,and the heat exchange was the most uniform.Therefore,when the mass flow of the air was the same,the slit nozzle T₀ corresponding to the inlet pressure of 190 Pa can better improve the heat exchange efficiency and was beneficial to the freezing rate of the food.3.Study on the flow field in the static pressure chamber of the impact freezer by different types of guide plates.The static pressure chamber of the quick freezer was optimized to ensure that the inlet pressure of the static pressure chamber was 190 Pa,the size of the static pressure chamber was 4000 mm×2500 mm×1500 mm,and the slit nozzle was T₀.By adding different types of baffles?arc-shaped baffles and rectangular baffles?inside the static pressure chamber,the initial position of the baffles and the baffles were guaranteed.Under the premise of constant baffle length and thickness,the change of the flow field inside the static pressure cavity was compared by changing the width of the rectangular baffle and the curvature radius of the curved baffle.Except for rectangular baffle with a length of 600 mm and a width of 50 mm,the wind velocity difference between the outlets of the upper and lower nozzles of other types of baffles were less than that in the static pressure cavity by CFD technology.The wind velocity at the baffle was extremely poor,and the wind velocity at the rectangular baffle decreased with increasing width,and the wind velocity at the arc-shaped baffle increased with increasing curvature radius.The average Nusselt number on the surface of the steel strip in all curved baffles were higher than those in the rectangular baffles,which could increase up to about 1 time.At the same time,the uniformity of heat transfer on the surface of the steel strip was relatively good when the arc-shaped baffle with a length of 600 mm,a radius of curvature of 50 mm and 100 mm were added to the static pressure cavity.In summary,the arc baffle with a length of 600 mm and a radius of curvature of 50 mm was an excellent choice in terms of extremely poor wind velocity,heat transfer intensity,and heat uniformity.4.Optimization design of the number of baffles and the center angle in the hydrostatic chamber.Arranged the baffles with a radius of curvature of 50 mm and a center angle of?/6 in the cavity of the freezer with the above conditions were selected.The Number of baffle was increased from 2 to 3 and 4 with center angle.The effects of increasing from?/6 to?/4,?/3 on the flow field in the static pressure cavity were proposed.Through research on the heat transfer and cross-flow wind velocity of the steel strip directly below the nozzle,it was found that changing the number of baffle and the center angle was beneficial to optimize the flow field and heat exchange.At this time,the local Nusselt number of the steel strip surface directly below the nozzle was the largest,and the average Nusselt number was also the highest.Cross-flow wind speed was the smallest when there were four baffles and the center angle was?/6,and the wind velocity distribution was the best.However,when there were three deflectors,the difference between the cross-flow wind velocity and the cross-flow wind velocity was small.Taking into account the above factors,the three baffle had the best heat transfer in the static pressure cavity when the center angle was?/4,and the cross-flow wind velocity was low.5.The optimization design of the perforation rate of the baffle in the hydrostatic chamber.Three arc baffles with a center angle of?/4 and a radius of curvature of 50mm were installed in the static pressure chamber under the above conditions.After the inlet pressure of the static pressure chamber was set to 190 Pa,the flow field and heat transfer characteristics of the initial static pressure chamber without baffle were compared with those of the original static pressure chamber with an inlet pressure of170 Pa and a size of 4000 mm×1500 mm×2000 mm before optimization.It was found that the Nusselt number was increased by 152.36,but the heat transfer uniformity decreased.In order to improve the deficiencies of the equipment in terms of flow field and heat transfer characteristics,according to the characteristics of baffle perforation to help the fluid flow,the perforation rates of the arc-shaped baffle were set to 30%,40%,and 50%,respectively on baffles.In the static pressure cavity,compared with the static pressure cavity without a baffle in the initial state,it was found that the perforated baffle could improve the heat exchange uniformity and the heat transfer intensity.Compared with non-porous baffles,40%and 50%perforation baffles improvd heat transfer uniformity and heat transfer intensity.Among them,50%perforation rate in the optimization results were the best.