| With the continuous improvement of living standards,people pay more and more attention to food safety and quality.As a convenient,fast and long stored diet,quick-frozen food has been paid more attention by people.How to improve the performance of quick freezer to obtain better quality of quick frozen food has become a research hotspot.Impingement quick freezing technology uses high-speed and low-temperature jet to freeze food,which is one of the advanced quick-freezing technologies at present.However,the uneven flow field in the heat transfer area caused by the impact of high speed jet will lead to some problems,such as poor heat transfer efficiency of frozen products,low energy efficiency ratio of equipment and so on.Therefore,this paper took shelled fresh prawns as the research object,using computational fluid dynamics(CFD)numerical simulation technology to study the effects of nozzle structure,loading mode,air supply mode and air supply velocity on the freezing time and uniformity of shrimp in the impingement quick freezer to optimize the operating conditions of the impingement quickfreezing device.An up-and-down impingement food quick freezing test-bed was set up to verify the simulation results.In this paper,firstly,the effects of slit nozzles with different structures on the heat transfer characteristics of impinging jets in the impingement quick freezer was studied,and the structural parameters of slit nozzles were determined for follow-up research.Secondly,the effects of different upper and lower air supply velocity,different air supply modes(unilateral air supply and bilateral air supply)and different load modes(plate load and net load)on the freezing process of shrimp were analyzed,and the better operating conditions of the flow field environment in the heat transfer area of the device were obtained.Finally,based on the above research results,the effects of slit nozzles and slit orifice plates on the freezing process of multi-row shrimp at different location in the device were compared.The specific research contents and conclusions are as follows:1.The effects of three kinds of V-shaped slit nozzles with different structures on the heat transfer of impact quick-freezing equipment under 170 Pa and 190 Pa pressure conditions were compared by means of numerical simulation and experimental verification.The nozzle outlet wind speed,transverse flow direction wind speed and Nusselt number(Nu)were studied respectively.The results showed that the nozzle outlet with larger nozzle extension length K had higher wind speed and more uniform flow field.However,larger nozzle extension length K weakened the convective heat transfer intensity.The nozzle with smaller angle ? between the nozzle shrinking section and the extension section had smaller wind speed and higher uniformity in the transverse flow direction,and is less affected by the transverse flow,but it had no obvious effect on improving the convective heat transfer intensity.When the length of the extension section of the Vshaped slit nozzle was K = 10 mm and the angle ? between the nozzle contraction section and the extension section was 165 °,the average Nusselt number was the highest and the convective heat transfer intensity was the highest.2.Shelled fresh prawn was selected as the research object,the effect of air supply velocity on the freezing process of shrimp in impingement quick freezer was studied by means of numerical simulation and experimental verification.It was divided into three experimental groups: the air supply velocities of upper and lower sides were kept the same and changed at the same time,the air supply velocity of the upper side was 15 m/s,the lower side was 0?15 m/s,and the lower side was 15 m/s,the upper side was 0?15 m/s.It was found that when the air supply velocities on both sides of the impingement quick freezer were the same,with the increase of air supply velocity,the freezing time of shrimp was shorten but the extent of decrease also decreased.When there was a great difference in air supply velocities between the upper and lower sides,the relative impact of the two impinging jets formed an eddy current on the low speed side to promote the flow field on the surface of shrimp,improved the heat transfer efficiency and reduced the freezing time of shrimp.When there was smaller difference in air supply velocities between the upper and lower sides,the relative impact of the two impinging jets formed a jet "vacuum zone" with lower velocity on the surface of shrimp,reducing the heat transfer efficiency of shrimp and increasing the freezing time of shrimp.When the upper air supply velocity was 15 m/s and the lower air supply velocity was 2 m/s,the convective heat transfer intensity of shrimp was the highest and the freezing time was the shortest.3.Shelled fresh prawns was selected as the research object,using the method of numerical simulation combined with experimental verification.The effects of two air supply modes(unilateral air supply and bilateral air supply)and two load modes(plate load and net load)on the freezing process of shrimps were studied,and the air supply and load mode which made the shrimp freezing process shortest was found.It was found that for shrimp freezing,the flow field velocity on the surface of shrimp can be increased by using bilateral air supply and net belt load,which was beneficial to improve the heat transfer efficiency and reduce the freezing time of shrimp.Compared with other air supply and loading methods,the freezing time of shrimp can be shortened by 14% to 25%.The bilateral air supply was helpful to form an eddy current to increase the flow velocity on the lower surface of shrimp at the weak side of the air supply.The net load can avoid the formation of jet "vacuum area" at the junction between the lower side of the shrimp and the net belt and the head of the shrimp.Both of them are helpful to increase the surface wind speed of shrimp and shorten the freezing time of shrimp.The faster the freezing speed was,the worse the freezing uniformity of shrimp was.In this experiment,the maximum temperature difference between inside and outside shrimp appeared in experimental group D(bilateral air supply + net load),and the maximum temperature difference was 13.02 K.4.The numerical simulation and experimental verification were used to study the effects of slit nozzle and slit orifice plate on the freezing process of two rows of shrimp under and between the spray holes in the heat transfer area of the up and down impingement quick freezer.It found that in the up and down impingement quick-freezing equipment,the freezing time of shrimp was proportional to the size of the air outlet of the equipment,and the freezing non-uniformity ? of each row of shrimp was 1?5%.And the freezing time of shrimp under the spray hole was longer than that of the shrimp between the two spray holes,and the freezing non-uniformity ? was smaller than that between the two spray holes.At the same outlet velocity,the up and down inlet pressure of the slit nozzle model were 27.78% and 40.00%,which were shorter than those of the slit orifice plate model.The freezing time of shrimp was nearly 40.00% shorter than that of the slit orifice plate model,and the energy efficiency ratio of the slit nozzle model was higher.On the other hand,the air flow resistance loss caused by the slit orifice plate was smaller,and the shrimp freezing in the model was more uniform. |
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