Measurement Method’s Study Of Liquid Film Thickness In Rectangular Channel Under Heating Condition

The critical heat flux is an important parameter in the safe operation of nuclear reactors.Because the critical heat flux is affected by both flow parameters and various potential interaction instability mechanisms in the boiling heat transfer process,there is no unified statement on the physical mechanism of the critical heat flux.Liquid film thickness is one of the most important parameters in the study of the mechanism of critical heat flux.In this paper,the direct measurement of liquid film thickness under heating conditions was carried out based on the electrical method.Through the direct and accurate measurement of liquid film thickness,effective data was provided for the exploration of the physical mechanism of critical heat flux.Due to its low cost,high sampling frequency and wide application range,electrical method is becoming the main research method in the experimental research of liquid film thickness measurement.It is necessary to reduce the crosstalk between electrodes and improve the resolution of liquid film thickness measurement by using electrical method.Multi-physical field simulation software COMSOL was used to simulate the form,size and other parameters of the electrode pair.The electrode was set as a concentric ring,and the diameter of the electrode emitter was 2 mm,the inner diameter of the receiving electrode was 2.4 mm,and the outer diameter of the receiving electrode was 3.2 mm.Through reasonable wiring,the liquid film thickness measurement sensor is arranged on the printed circuit board to form a 5×6 electrode pair rectangle.At the same time,in order to avoid electrochemical reaction between electrodes,high frequency square wave excitation with amplitude of ±5 V and frequency of 100 k Hz was selected for the emitter signal excitation.The liquid film thickness signal is collected by the self-programmed Lab VIEW program and the liquid film thickness acquisition board card designed based on FPGA.The liquid film signal is amplified,sampled and maintained,and transmitted to the upper computer through serial port after analog-to-digital conversion.Liquid film thickness calibration is based on self-programmed program,51 MCU and stepper motor,and the liquid film with known thickness is generated to calibrate.In addition,due to the production process and other limitations,the corresponding characteristic curves of each sensor are not consistent,so it is necessary to carry out the corresponding liquid film thickness calibration experiment for all sensors.As heating will cause the change of the physical property parameters of the experimental working medium,this paper uses the multi-physical field simulation software COMSOL to carry out numerical simulation of the parameters that have the greatest influence on the electrical conductivity.It is found that the electrical conductivity has the greatest influence on the signal within the range of experimental conditions.Finally,the influence of physical parameters such as conductivity on the calibration and measurement of liquid film thickness was reduced by using the conductivity temperature curve obtained from the physical property measurement experiment of the experimental working medium.Combined with electrical signals in the liquid film thickness measurement curve,the temperature conductivity property measurement conductivity curve and calibration experiment to obtain the liquid film thickness of liquid film thickness electrical signal curve,found that the liquid film thickness measuring sensor can measure the thickness of liquid film is 20～1000 μm.The minimum liquid film measuring thickness is 22 μm,the maximum measuring liquid film thickness is 998 μm.