In annular two-phase flow, part of the liquid flows as a liquid film on the channel wall, and part as droplets entrained in the gas core of the flow. The distribution between film and core is an important system variable and affects many other parameters (dryout, heat transfer coefficient, pressure gradient, etc). In vertical flows in round tubes, the film flow rate is constant around the periphery since the flow is axisymetric. For horizontal flows, the film flow rate may vary around the periphery of the tube and, in this case, there is considerable interest in measuring local flow rates as a function of circumferential position.
Average film flow rate can be measured by sucking off the liquid film through a porous section of tube as illustrated in Figure 1. The entrained droplets have sufficient momentum to carry them beyond the porous wall section; a small amount of the gas phase is collected in between waves on the film, but the liquid collection rate becomes independent of the gas collection rate over a wide range once all of the film has been captured. However, at high liquid flow rates, the results are not so clear cut.
To measure local film flow rate a strip of porous wall can be used (bounded by longitudinal fins), and the position of this can be rotated around the circumference to give distribution of film flow rate in, say, horizontal annular flow. An alternative method of measuring local film flow rate was described by Coney and Fisher (1976) and is illustrated in Figure 2.
Here, part of the liquid film is separated by guide fins and, in the separated region, potassium chloride solution is pumped in at a constant rate. The salt solution mixes with the film liquid and the concentration of the salt in the mixed film is determined using miniature conductivity probes as shown. The effectiveness of the mixing was checked by having two successive probes.
Reviews of film flow rate measurement are given by Hewitt (1978)(1982).
Coney, M. W. E and Fisher, S. A. (1976) Instrumentation for Two-phase Flow in Use or Under Development at the Central Electricity Research Laboratories, paper presented at the meeting of the European Two-Phase Plow Group, Erlangen, Germany.
Hewitt, G. F. (1978) Measurement of Two-phase Flow Parameters, Academic Press, London.
Hewitt, G. F. (1982) Measurement techniques. Chapter 10 of Handbook of Multiphase Systems, G. Hetsroni Ed., McGraw-Hill, New York.