The evaporation of refrigerant films is common in aviation and aerospace, refrigeration and air-conditioning, and other industries. Film thickness measurement helps understand the heat transfer mechanisms of liquid films during the formation, flow, and evaporation processes. Several researchers have worked on numerical simulations of refrigerant films. However, because numerical simulations are often based on simplified assumptions, the simulation results are less credible. Meanwhile, a large number of experiments on the measurement of liquid film thickness have been performed using various methods, including electrical, acoustic, and optical methods, but the work relevant to refrigerant films is relatively limited. The capacitive method, optical interferometry, and imaging method have been used to investigate the refrigerant liquid film. However, the above-mentioned methods have certain limitations. Measurement of refrigerant film thickness cannot be performed with high accuracy. Therefore, a novel refrigerant film thickness measurement system based on absorption spectroscopy is proposed. It demonstrates that high-accuracy thickness measurement is possible. This system can provide scientific guidance for the design and optimization of relevant industrial devices and processes.
In the study, R1233zd is used as the refrigerant. The absorption spectra in the near-infrared region (5800.0-6300.0 cm-1) of R1233zd at different temperatures (11.8, 13.5, 15.2, 16.5, 18.0 ℃) below the boiling point are measured using a self-designed temperature control device with high precision and a Fourier-transform infrared spectrometer with high resolution (Fig. 2). Based on the Beer-Lambert law, two wavenumber positions (υ1 and υ2) are chosen to eliminate the influence of light intensity attenuation caused by other factors. The absorption coefficient is significant at υ1 and close to zero at υ2. The absorption coefficients at these two wavenumber positions should be nearly temperature independent. An inversion model is developed, which is also applicable to other refrigerants.
The measurement system is developed based on absorption spectroscopy. The system’s measurement accuracy is validated using a calibration tool with known film thicknesses (0-800 μm). Eight different film thicknesses (100, 200, 300, 400, 500, 600, 700, 800 μm) are measured ten times. The largest standard deviation of film thickness for the ten repeated experiments is 0.5 μm when the known thickness is 800 μm. Moreover, the average relative deviation between the measured and known thickness is 1.0%. This demonstrates that the system can achieve high accuracy in measuring refrigerant film thickness. Furthermore, the processes of R1233zd film evaporation on a horizontal transparent quartz plate are investigated. In this case, the imaging method is used as a comparison method. To ensure clear images, the distance between the measuring point of the absorption spectroscopy and the camera is kept at 110 mm (i.e., the focal length of the camera). The obtained liquid film image is processed, and the liquid film thickness can be accurately determined. The initial film thicknesses obtained using absorption spectroscopy and imaging method are 410.8 μm and 409.7 μm, respectively. The relative deviations in thickness measured using the two methods are 0.26%. It is discovered that the film thickness variations measured using the two methods are similar during the evaporation processes, with average relative deviations of 1.1%. The absorption spectroscopy can also be used to track the changes in film thickness throughout the evaporation process. However, the imaging method is not available due to its contrast ratio limitation when the film thickness is less than 50 μm. The liquid film evaporates slowly at the beginning of the evaporation process and the film thickness changes little. The effect of liquid evaporation is more significant than that of liquid shrinkage at the measuring point in the middle of the evaporation process. The thickness of the liquid film decreases rapidly later in the evaporation process. There is no excess liquid converging to the liquid film’s central position.
In this work, a novel refrigerant film thickness measurement system is developed by absorption spectroscopy. An inversion model is established to determine film thicknesses. The measurement accuracy of the system is validated by a calibration tool. Furthermore, the R1233zd film evaporation processes are investigated. The system can measure liquid film thicknesses noninvasively and with high accuracy, and it is also suitable for other refrigerant types. It has the advantages of a compact structure and simple operation. Wide-band spectral information can be obtained using this system, which will be useful for determining temperature and other parameters in future research.
