Study On Continuous Flow Microwave System With Helical Tubes For Liquid Food Heating

In recent years,continuous microwave heating technology has been fully proved to have great potential in the development of liquid food processing,especially in solving the uneven distribution of electromagnetic field and temperature in this process.However,compared with the traditional UHT process,continuous microwave heating of liquid food still has the problem of a slower initial heating rate.In order to optimize the above problems,this study tried to make the liquid material absorb electromagnetic energy to the greatest extent through the change of its structure parameters.By constructing a numerical simulation model,a continuous flow microwave method with helical tubes for liquid food heating was proposed.The purpose of this study was to provide a theoretical basis for the industrial application of continuous microwave heating of liquid food.The main contents and results were as follows:(1)The method and computer simulation study of continuous microwave heating with helical tubes for liquid food heatingBased on the computer simulation,a numerical model of continuous microwave heating system with helical tubes for liquid food heating was established.When the distance between the center of the helical tube coil and the top of the cavity was 0.35 m,the inside of the cavity and the helical tube had the best electric field distribution effect.The simulation results showed that the continuous microwave heating system with helical tubes could achieve rapid and uniform heating due to the secondary flow effect.In the laminar flow state,the De value could reach more than 230.14,the De value exceeded 3929.22 in the turbulent flow state,and the character of liquid heat transfer was further enhanced by the generation of vortices.For different fluid types(Newtonian fluid food and non-Newtonian fluid food),the system still presented the same efficient heating effect.Due to the influence of non-Newtonian fluid food viscosity,the De value was about 2.129,the secondary flow strength was weak,and the torque effect and vortices were the keys to improving the heating uniformity of non-Newtonian fluid food.(2)Study on the relationship between structure parameters and heating efficiency of helical tubes under the microwave fieldAccording to compare the experimental and the simulated results,the accuracy of this model was verified.As the volume flow was 0.50 L/min,0.75 L/min and 1.00 L/min,the RMSE values were 2.59,1.84 and 1.44,respectively.At the same time,the temperature values of 6points on the surface of the helical tube were obtained through an infrared thermal imaging camera,which also confirmed the accuracy of this model.Distilled water,as the representative of Newtonian fluid food,was used to be the test fluid.It was found that as the PCD decreased,the De value increased from 214.249 to 230.140,and the heat transfer performance in the helical tube was enhanced.When PCD was 0.130 m,the average outlet temperature had a maximum value of 346.50 K.Meanwhile,high electromagnetic hotspots were found between adjacent helical tubes walls.Changes in diameter and pitch would influence the distribution of electric field and high electromagnetic hotspots in the cavity.When the tube diameter was 0.016 m,there was a maximum average outlet temperature value of 346.50 K.As the pitch was 0.024 m,the average electromagnetic power density in the tube had a maximum value of 3.92×106 W/m~3,and the maximum average outlet temperature value was 348.91 K.Therefore,when selecting the tube diameter,the wavelength and penetration depth of the electromagnetic wave should be fully considered and proved the appropriate screw pitch.The utilization rate of microwave energy could be improved by microwave resonance effect.(3)Study on the continuous heating system with helical tubes for liquid food under the microwave fieldIn actual food production,most of liquid foods are non-Newtonian fluid foods.At the same time,CMC-Na,as a food additive,can improve and stabilize the structure of liquid food.Through the analysis of particle size distribution,infrared spectroscopy,rheological characteristics and dielectric characteristics,the CMC-Na solution with a degree of substitution of 1.2,as the representative of non-Newtonian fluid food,was used to be the test fluid.It was found that there were still high electromagnetic hotspots between adjacent helical tubes walls.Due to the difference in dielectric properties,the electromagnetic power density distribution pattern was different from that of distilled water,and the increasing in energy absorption was obvious.When the tube diameter was 0.018 m,there was a maximum average outlet temperature value of 378.48 K.Due to the viscous effect,the main influence of the heating efficiency of non-Newtonian fluids was affected by the strength of the torque effect.The microwave resonance effect was still the key to improving the utilization of microwave energy.