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Numerical Analysis And Experimental Research On The Optimization Of Forced-air Precooling Technology For Fruit And Vegetable

Posted on:2019-07-18Degree:MasterType:Thesis
Country:ChinaCandidate:Y LiFull Text:PDF
GTID:2381330566474583Subject:Power Engineering
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Cold chain logistic is the only way to guarantee the quality of agricultural products which has received great attention from the country in recent years.In 2017,the General Office of the State Council issued opinions on accelerating the development of cold chain logistics to ensure food safety and promote consumption upgrades.It was clear that the key issues for the existence of cold chain logistics were the first mile and last mile in China.And pre-cooling was considered the crucial part.Among them,the forced-air pre-cooling technology was particularly important because of the advantages such as fast pre-cooling and wide application of fruits and vegetables.Most of the existing resources were invested in the construction of forced-air precooling,and the details of the precooling technology were weak.This article had done a more in-depth study of this phenomenon.It was taken lettuce as the example,which was the spherical fruit and vegetable.And it was established a physical model numerical simulation model for the forced-air precooling of lettuce.The CFD simulation software was used to simulate the pre-cooling process.It was simulated the distribution of the flow field in the return passage under different parameters such as different fan pressures and different spacing in the return air passage.And the precooling process of lettuce was simulated under different supply air temperature strategies and different air supply speeds.The theoretical model was verified by the high-humidity variable forced-air precooling room designed and constructed by the research team of Liu Sheng,vegetable research center of Beijing Academy of Agriculture and Forestry Sciences.The experimental results were consistent with the simulation,and it was verified the reliability of the model.It was built a system of the precooling room optimization design and lettuce pre-cooling effect evaluation based on pre-cooling time,pre-cooling energy consumption,weight loss rate and pre-cooling uniformity.In the following,the optimum design of the forced-air precooling room was categorized the forced-air precooling technology.In optimizing forced-air precooling design,it was studied the effects of different fan pressures on the flow field distribution of the return air passage.Fan pressure parameters were selected according to different models of T35-11 axial fan.The results showed that when selecting a T35-11-4 fan with a full pressure of 483 Pa,the distribution of the return air passage flow field was more dense and uniform.When selected a T35-11-2.8 fan with a total pressure of 237 Pa,the flow field was not uniformly distributed and there was significant attenuation.It was concluded that if the fan pressure was smaller,the flow field was disordered,the heat transfer was lower efficiency and the pre-cooling was slower.It was studied the influence of the distance between the return air passages on the flow field distribution of the return air passage in the case of where the wind speed and other parameters were consistent and the palletizing mode was vertical.The original spacing design was 600 mm,and it was divided into four kinds of setting programs of 600 mm,500mm,400 mm and 300 mm.The results showed that in the case of a fixed-size differential pressure precooling system modified with a standard container,the other parameters were the same,the narrower the distance between the return air passages(the wider the distance between the air intake passages),the more uniform and uniform the flow field distribution.This meant that the greater the heat transfer coefficient,the higher the heat transfer efficiency,and the faster the pre-cooling rate,the higher the pre-cooling efficiency.In optimizing forced-air pre-cooling technology,the pre-cooling process was divided into two pre-cooling sections with the limit of the semi-cooling temperature.The goal was to precool the lettuce from the initial temperature of 20C to 5C.Each section adopted air supply temperature of 4C,2C,and 0C,respectively,for a total of nine precooling strategies.The results showed that before the semi-cooling temperature,the difference of air supply temperature had little effect on the pre-cooling time of lettuce.The pre-cooling time was only extended by 30 minutes when the air temperature at 4C compared with the air temperature at 0C.Therefore,the air temperature at 4C was selected before the half-cooling temperature.Since the pre-cooling final temperature was 5C,if 4C air supply temperature was selected after the semi-cooling temperature,the precooling time of lettuce would be greatly extended,and the energy consumption would be too large.This strategy was not considered.Comprehensive analysis showed that the air supply strategies of selecting 4C and 2C before and after the semi-cooling temperature were compared with the traditional strategy with the air temperature of 0C.The pre-cooling time spent was suitable,and the energy consumption was the lowest.The practical significance of green environmental protection,energy saving and emission reduction was in actual production,so it was selected as the best strategy.It was studied the effect of different air supply speeds on the precooling effect of lettuce.1.5 m/s,1 m/s,and 0.76 m/s of the inlet speed of the plastic baskets for precooling of lettuce were corresponding to the 50 Hz,40Hz,30 Hz fan frequency and 16m/s,12m/ s,7.5m/s axial fan outlet wind speed.The results showed that the greater the air supply speed,the shorter the precooling time and the better the precooling uniformity.And there was no obvious difference in precooling energy consumption.Larger air supply speeds did not increase the energy consumption of a single precooling.There was also a certain guiding significance for the actual production.In summary,in the fixed-size forced-air pre-cooling room remodeled with a standard container,bulbous fruits and vegetables were precooled represented by lettuce,when the palletizing method is the longitudinal arrangement of pre-cooled plastic baskets.In summary,the bulbous fruits and vegetables which represented by lettuce were precooled in the fixed-size forced-air pre-cooling room remodeled with a standard container.The palletizing method was the longitudinal arrangement of precooled plastic baskets.The pre-cooling final temperature was 5C.The strategy of the return air channel spacing was 300 mm,the speed and pressure of the air supply were large,the air supply temperature method was 4C to 2C before and after the semi-cooling temperature was the best.It had higher pre-cooling efficiency and lowest energy consumption.This model provided an advanced thinking concept for actual production.It had certain guiding significance in energy conservation,emission reduction and environmental protection.
Keywords/Search Tags:Lettuce, differential precooling, numerical analysis, supply air temperature, supply air speed
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