Research On Microwave Heating Uniformity And Temperature Control Based On Rectangular Waveguide

Microwave heating technology is an important field of microwave energy application.It is widely used in all walks of life due to its selective heating,efficient heating and easy control.However,in actual production,when the high-power microwave is applied to microwave heating,since the distribution of the electromagnetic field in the heating cavity is uneven and the ability of each material to absorb microwaves will change with temperature,hot spots and thermal runaway are prone to occur,which reduce the safety and effectiveness of microwave energy applications and hinder the development of microwave heating technology.Besides,in the fields of food,chemistry,biology,etc.,certain materials are sensitive to temperature conditions during the heating process.If the temperature is too low,the expected effect will not be achieved.If the temperature is too high,the heating material will be damaged and burnt.Therefore,when using microwaves for heating,it is necessary to improve the uniformity of microwave heating and consider the heated object’s actual temperature.The standard methods for improving the uniformity of microwave heating are to improve the uniformity of the electric field distribution in the reaction chamber and to change the heated object’s position during the heating process.Furthermore,the temperature control is mainly to change the output power of microwave source,frequency,and heating time during heating.However,these methods lack an effective combination of temperature uniformity and temperature control.To meet the temperature requirements of temperature-sensitive materials and improve the uniformity of microwave heating,a dynamic model for controlled uniform processing under microwave heating by using a heat exchanger is proposed,in which a double-layer annular test tube structure of the heat exchanger is used to improve the uniformity of microwave heating,then the simulation model is verified through experiments.When the heated object is placed in the inner tube of the heat exchanger and the fluid is passed through the gap between the inner and outer tube,the effects of the presence or absence of a heat exchanger,different fluid velocities,different reactor sizes,and different real and imaginary parts of the dielectric constant on heating uniformity and temperature rise are discussed.Finally,the changes in different parameters when reaching the same target temperature(take 60~oC as an example)are discussed.The results show that the temperature uniformity can be increased by up to 44%by using the heat exchanger,and the target temperature can be controlled by changing the fluid velocity,reactor size,and dielectric constant.In addition,when the heated object is located in the gap between the inner and outer tubes of the heat exchanger and fluid is passed through the inner tube of the heat exchanger,the influence of different fluid velocity and reactor sizes on temperature performance is discussed.On the other hand,for materials that require high temperature uniformity,a microwave heating uniformity improvement system based on ridge waveguides is proposed.The ridge waveguide is used as a heating cavity to improve microwave heating uniformity based on the uniform electric field distribution between the ridges.In the paper,the single-ridge waveguide and the double-ridge waveguide are discussed separately.When the reactor is a single-layer test tube and the heating material maintains a certain volume,the sizes of the single-ridge and double-ridge waveguide ridges are optimized.To verify the model’s correctness,the temperature distributions of the heated object’s upper surface in the experiment and simulation are compared when the object is heated for t=10s.Then the temperature uniformity of the optimized single-ridged waveguide,double-ridged waveguide and non-ridged waveguide is compared during microwave heating.The results show that the heating rate of the heated object is increased by 193.69%and the uniformity is increased by 80.71%,when the single-ridge waveguide is used for heating,compared with the ordinary rectangular waveguide.When the double-ridge waveguide is used for heating,the heated object’s heating rate is increased by 200%,and the uniformity is increased by 74.59%.When the reactor is a heat exchanger,the ridge waveguide is optimized according to the size of the heat exchanger.The results show that when the size of the heat exchanger is small,the presence of ridges may not necessarily improve the heating rate and uniformity of the heated object.If the size of the heat exchanger is increased,the heating rate of the heated object will be improved significantly,and the heating uniformity will also be improved.