2.3 Void fraction measurement

Void fraction is defined as the ratio of gas volume to the total volume of the system. In the current work, void fraction was calculated from the differential pressure (∆p ) along the column height during operation. A differential pressure transducer (PX2300-DI, OMEGA) was employed to obtain the hydrostatic pressure between two pressure taps with a separation of 8D along the column height (see Figure 1). A data acquisition card (National Instruments, USB-6218 BNC) was used to acquire the output signal from the pressure transducer and the signal was recorded on a desktop computer (via LabVIEW 15.0.1). Here the uncertainty associated with the pressure measurement was calculated to be under 2% of the measured value for all cases tested. Void fraction was calculated using Equation (4), where ∆H is the vertical distance between the pressure taps, ρL andρG are the density of liquid and gas, respectively, and g is gravitational acceleration.