Film thickness measurements for annular flow in minichannels: Description of the optical technique and experimental results

An optical method is developed in order to measure the top and bottom liquid film thicknesses during evaporative annular flow in a horizontal minichannel. This method is based on the detection of liquid-vapor and liquid-solid interfaces. For each flow condition, about 4000 frames (corresponding to an observation time of 2 s) are recorded with a high-speed camera. Thus, these frames are processed with an image processing algorithm to obtain the average liquid film measurement. Three different configurations of the camera are tested. The chosen solution ensures a good reliability and accuracy of film measurements. The view section is a 2.95 mm glass circular channel and the working fluid is R-245fa. The tests are performed in annular flow, for different vapor qualities. A test facility allows reaching the desired conditions of the flow at the inlet of the test section. The examined saturation temperatures are 40, 60, 80 and 100 °C, whereas the mass velocities are 300 and 400 kg/m2 s. A parameter of eccentricity is introduced to characterize the stratification effect. The influence of the saturation temperature, mass velocity and vapor quality is physically discussed through the Froude and Bond numbers. Finally, by using the top and bottom liquid film measurements, the cross-sectional void fraction is estimated.