Design And Process Research Of Continuous Microwave Heating Method For Liquid Foods

In the process of conductive heating of liquid foods,steam is often used as a heat source.In order to meet the requirements of sterilization,steam heating method often causes serious heat loss and tube fouling.Microwave has an intrinsic advantage to solve such problems fundamentally by its dielectric heating properties that does not require a heat transfer medium.However,existing microwave heating systems generally have some problems during continuous applications,such as high port reflection coefficient,unstable heating rate,and low energy conversion rate,etc.In this study,a coupling multi-physics model was established to develop a novel continuous microwave heating process to overcome above shortcomings.The purpose of this study is to provide a theoretical basis for the application of continuous microwave heating in liquid foods processing.The contents and results of this study are as follows:(1)Study on computer simulation and design of continuous microwave heating method for liquid foodsThe system optimization design was carried out based on the coupling multi-physics methods.When using a rectangular microwave resonator,the fluid can be heated to a higher temperature with unideal heating uniformity by microwave input from the E-plane of the resonator,and the effect of inputting from the H-plane is opposite to the E-plane.Therefore,a combined input method from both the E-plane and H-plane was chose to further optimization.The optimal center distance between the waveguides on the H-plane and E-plane is 0.117 m when using the WR-430 standard waveguide.In addition,when the connection angle of the waveguides is 45~o,the reflection loss of the incident port decreases significantly.Compared with the 90~o connection waveguides,the ports reflection loss of the waveguides on the E-plane and the H-plane decrease by 9.19%and 17.87%,respectively,and the scattering parameters of other ports did not increase at the same time.Finally,the impedance matching characteristics of the system can be further improved by adjusting the position and insertion depth of the single stub tuner in the waveguides.(2)Study on multi-physics coupling effect of continuous microwave heating method for liquid foodsAccording to the simulation results,a pilot equipment is manufactured and the accuracy of the model is verified.When the flow rate is 1.25 L/min,1.50 L/min and 1.75 L/min,the root-mean-square error of the averaged outlet temperature within 600 s between the experiment and simulation results are 3.46,3.66 and 2.39,respectively,which proves that the model has high forecast accuracy.The results of the thermal response of the fluid during the heating process found that the fluid heat absorption is closely related to the electric field distribution in the resonant cavity,and the area with high field strength is conducive to local heating speed of fluid.The various of flow rate,microwave input power and microwave frequency affect the heating effect from the aspects of heating time,electric field strength and dielectric properties of materials,respectively.The multi-port input method of microwave can make the electric field distribution more uniform,and the change of the waveguide connection angle helps to concentrate the microwave energy around the tube,thereby promoting the microwave absorption effect of the fluid.In addition,the smaller the initial flow rate of the fluid,the higher the velocity gradient inside the tube,and the maximum pressure of tube wall appears at the inlet position.(3)Energy efficiency analysis of continuous microwave heating method for liquid foodsAfter the system design and optimization,the reflection loss of the microwave input ports on the H-plane and E-plane was finally reduced to 11.54%and 17.29%,the averaged outlet temperature was increased to 369.07 K,and the microwave energy conversion rate was increased to 71.10%.The above indicators improved significantly compared to the traditional microwave system with single incident port.When compared to the direct and indirect steam heating methods,the energy consumption of microwave heating mainly from electrical power,which has higher energy utilization efficiency and avoid the consumption of steam and cooling water fundamentally.In addition,the microwave heating methods can avoid the problem of tube fouling,thereby reducing the consumption of clean-in-place and sewage treatment process,which provides an effective method for the composite cost optimization of the liquid food thermal processing.