MOISTURE FRACTION MEASUREMENTS FOR 2-PHASE MIST FLOW USING HIGH-SENSITIVITY CAPACITANCE SENSORS

A capacitance-type sensor whose sensitivity is sufficiently high to de tect liquid water droplets in a stream of gas with very low moisture c ontent is described. Instantaneous capacitance is measured for a probe consisting of two closely spaced parallel plates through which a two- phase mixture is flowing; the presence of liquid within the sensor's a ctive volume generates a measurable capacitance increase due to the mu ch higher dielectric constant of liquid water compared with gas or ste am. Proof-of-concept experiments were carried out to determine the rel ative effects of droplet size, velocity, and position on the output of the sensor. The probe detected individual water droplets in the range of 7.5 to 20.0 mu l and had a voltage output that was linear with dro plet volume in this range. In addition, the signal pulse width was fou nd to be a sensitive indicator of droplet velocity. The data indicate that the probe can be used to measure the moisture fraction of a high- quality gas flow in the range from 0 < beta < 0.013 (liquid volume/tot al volume) with a maximum standard deviation of similar to 0.001.