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.