An In-Situ Volume Fraction Sensor For Two-Phase Flows of Non-Electrolytes

Abstract

Abstract This paper provides a brief survey of methods which have been used to measure the in-situ volume fractions of two-phase mixtures flowing in pipes. A description is given of the development of an inexpensive device based on capacitance sensing which makes use of off-the-shelf electronics to yield a continuous monitoring of the in-situ volume fractions of flowing two phase mixtures of non-electrolytes. The details of construction for the sensors are given and possible applications are suggested. Introduction THE IN-SITU GAS (or liquid) volume fraction is a major parameter of interest in the study of the concurrent pipe flow of two-phase mixtures. Because the two phases do not generally flow at the same velocity, the in-situ volume fractions will almost invariably be different from those at the inlet of the pipe. In any case, the inlet conditions are often not known and a correlation based on inlet values has little use. Consequently, much effort has been devoted to the measurement of the phase volume fractions in the flowing system. The earliest measurement technique used in laboratory or pilot studies involves the use of quick-closing valves. These may be either mechanically or electrically activated (e.g., see Govier and Omer, 1962). Two valves are located a known distance apart on the test section. Under steady flow conditions the valves are simultaneously closed, trapping- a mixture of gas and liquid between them. The liquid i3 then drained from the trapping section, its volume recorded and the average in-situ liquid volume fraction is calculated knowing the total pipe volume between the valves. The advantages of this method are principally its simplicity and low cost, but there are some important and obvious disadvantages. It is time consuming to drain and measure the liquid for each determination, particularly for liquids of moderate to high viscosity which tend to cling to the pipe walls. Furthermore, this method is not suited for use with systems operated at elevated temperatures or pressures, or for single-component two-phase systems 'where the pressure change resultingfrom draining the liquid may have a significant effect on the phase behaviour of the material. To minimize equipment damage from the sudden flow stoppage, a test-section bypass is usually required. One is also limited to determining average volume fractions over a length of test selection and hence detailed information related to the transient nature of various flow patterns is unattainable. Finally, unless it is possible to use plug-type valves with the flow passage exactly conforming to the inside diameter of the pipe, flow pattern disruptions occur at both ends of this test section and a sufficient length must be provided to minimize these end effects. A number of methods which provide volume fraction measurements over a very short length of pipe have been successfully employed. These include the use of photographic and optical techniques (Hsu et al, 1969; Miller – and Mitchie, 1969), x-, beta- and gammaray attenuation devices (Schrade, 1969), hot film anemometry (Delhaye, 1969), and various other physical (Schraub, 1969) and electrical probes for Conductance and capacitance measurement (Bergles, 1969).