A CAPACITANCE SENSOR FOR 2-PHASE LIQUID-FILM THICKNESS MEASUREMENTS IN A SQUARE DUCT

The use of capacitance sensors for measuring liquid film thickness or phase concentration in two-phase flow has gained popularity in recent years. In designing such sensors, there are many issues which must be considered in order to optimize performance: desired temporal and spat ial resolution, two-phase flow regime, permittivity of the phases, duc t geometry, electrical shielding, and temperature variation in the flo w field. These issues are discussed, and the design of a 12.7 mm squar e cross section capacitance sensor which measures liquid film thicknes s in either stratified or annular two-phase flow is presented. Using a composite material analysis and an effective permittivity ratio, pred ictive relations for capacitance as a functional of liquid film thickn ess have been derived for stratified and annular film patterns. The an alysis eliminates the need for calibrating the sensor for stratified a nd annular flow regimes. Optical measurements of liquid film thickness using a high resolution CCD camera are compared against those using t he capacitance sensor in conjunction with the predictive relations. Th e sensor was rested on a bench top for a stratified film pattern with Ilo flow and two different electrode configurations (upward and side c onfigurations) using FC-87, a low-permittivity (epsilon(r) = 1.72) die lectric fluid. The standard deviations between the film thicknesses me asured optically and those predicted using the capacitance sensor and analysis are 0.014 and 0.019 mm for the respective upward and side ele ctrode configurations The sensor was also implemented in a vertical fl ow boiling facility, which uses FC-72 (epsilon(r) = 1.75) as the worki ng fluid. Time-averaged film thicknesses measured using the capacitanc e sensor are compared against ensemble-averaged measurements using the CCD camera for annular vertical upflow and downflow. The upflow and d ownflow standard deviations are 0.17 and 0.093 mm, respectively. As ex pected, the agreement for vertical flow is not as good as that for the horizontal no-flow case, because large fluctuations in film thickness are characteristic of annular two-phase flow, and the uncertainty of the photographic measurement is increased.