冲击式速冻设备上下送风速度对虾仁冻结过程的影响

随着人们对速冻食品品质的要求不断提高,商家对食品速冻技术的要求也在不断提高,冲击式速冻技术作为目前先进的速冻技术之一成为研究热点。但由于其高速射流冲击导致的内部换热区流场的不均匀会造成设备换热效率差,能效比低等问题。为找到使得设备换热区流场最优的送风速度,该文以明虾虾仁为研究对象,利用数值模拟结合试验验证的方法研究了冲击式速冻设备中上下送风速度对虾仁冻结过程的影响,分为上下两侧风速保持一致且同时改变;上侧送风速度为15 m/s、下侧为0~15 m/s;以及下侧送风速度为15 m/s、上侧为0~15 m/s 3个试验组进行研究。研究结果为:当冲击式速冻设备两侧送风速度保持一致时,随着风速的增大,虾仁冻结时长缩短但减小幅度也会不断减小;当上下两侧送风速度大小相差悬殊时,两股冲击射流相对冲击会在低速侧形成促进虾仁表面流场流动的涡流,提高换热效率,减小虾仁冻结时长;当上下两侧送风速度大小相差不大时,两股冲击射流相对冲击会在虾仁表面形成流速较低的射流"真空区",降低虾仁换热效率,增大虾仁冻结时长;在试验的两侧送风速度范围内,当上侧送风速度为15 m/s,下侧送风速度为2 m/s时,虾仁对流换热强度最大,冻结时长最短。

Effects of upper and down air supply velocities in impingement quick freezing equipment on freezing process of shrimp

Demanding for quick-frozen food and its quality in modern life, the requirements for food quick-freezing technology become challenging.The impingement quick-freezing equipment has drawn much attention in the most advanced quick-freezing technologies. The appropriate low-temperature flow field in the heat exchange zone of impingement quickfreezing equipment can reduce the time of food quick-freezing, while improve the quality of quick-frozen food. This paper aims to investigate the effect of air supply velocity on the freezing process of a single shrimp at the up and down within the chamber of impingement quick freezing equipment, using the numerical simulation and experimental verification. A single shrimp was selected as the research object. The main purpose of this research wasto find the air supply conditions, which can shorten the freezing time of shrimp, and achieve the optimal flow field in the heat transfer area of the equipment. Three experimental groups were divided into: 1) the air supply velocity at the upper and downsides were kept the same and changed at the same time, 2) the air supply velocity of the upside was 15 m/s, while the downside was from zero to15 m/s, and 3) the air supply velocityon the downside was 15 m/s, while that on the upside was from zero to15 m/s. The results showed that the freezing time of the shrimp shortened, but the gradient of decrease also reduced, as the increase of air supply velocity, when the air supply velocity was the same on both sides of the up and down impingement quick-freezing equipment. When the air supply velocity on the upside was 15 m/s and the air supply velocity on the downside was from zero to four m/s, the relative impact of the two impinging jets can form an eddy current on the low velocity side, to promote the flow field on the surface of shrimp, and improve the heat transfer efficiency of shrimp. When the air supply velocity of the downside exceeds 4 m/s, the relative impact of the two impinging jets can form a jet vacuum zone with down velocity on the surface of shrimp, reduced the heat transfer efficiency of shrimp. With the continuous increase of the downside air supply velocity, this adverse effect first increased and then decreased, the maximum appearedwhen the upside air supply velocity was 15 m/s and the downside air supply velocity was eight m/s, and the frozen time of shrimp was the longest, which was 909 seconds. By the same token, when the downside air supply velocity was 15 m/s and the upside air supply velocity was from zero to two m/s, the relative impact of the two impinging jets can form an eddy current on the low velocity side, improve the heat transfer efficiency of shrimp. When the air supply velocity of the upside exceeded two m/s, the relative impact of the two impinging jets can form a jet vacuum zone with down velocity on the surface of shrimp, reduced the heat transfer efficiency of shrimp. With the continuous increase of the upside air supply velocity, this adverse effect first increasedand then decreased, it had a maximum when the downside air supply velocity was 15 m/s and the upside air supply velocity was 5 m/s, and the frozen time of shrimp was the longest, which was 920 seconds. In the experimental range of impinging jet velocity, when the air supply velocity at the upside was 15 m/s and the air velocity of the downside was 2 m/s, the freezing time of shrimp was the shortest, which was 617 seconds.It inferedthat the influence of the upper impinging jet on the freezing process was greater than that of the down impinging jet on the freezing process in the upper and down impingement quick-freezing equipment. Therefore, the upper impinging jet can play a leading role in the impingement quick-freezing equipment. The finding can provide an insightful reference to design the promising operating conditions for the up and down impingement quick-freezing equipment.