The measurement of the mass flow rate and quality of a two-phase gas-liquid flow is of interest in many fields of engineering such as refrigeration, power cycles, chemical processes, nuclear plants and petroleum production. This paper presents the use of a venturi and a void fraction meter for the measurement of refrigerant R-134a liquid-vapor mass flow rate and quality in vertical pipes.
Measuring a single-phase mass flow is comparatively easy. The medium is a continuum with either a laminar or a turbulent flow. Measurements of the pressure drop produced by a venturi meter allow the instantaneous mass flow rate to be determined unequivocally and with great accuracy. In two-phase flow, the phases occurring in the flow channel are discontinuous. There may be such flow regimes as bubbly, slug, churn, annular and dispersed flows, which are difficult and expensive to identify. Even pressure drop measurements do not allow the mass flow to be derived unequivocally and precisely because there is a strong dependency on the flow regime and the vapor quality (vapor mass fraction). Since these quantities are not known in practical measurements, the measuring errors of such techniques frequently range between 5 and 20%.
Fundamental improvements in the accuracy of measuring two-phase flows can only be attained with the measurement of at least two independent parameters. These are typically a measure of velocity or volumetric flow rate and a measure of the mixture density or void fraction.
Differential pressure instruments, such as orifice, venturi, and nozzle meter, were the earliest and simplest forms of metering devices used in two-phase mass flow rate and quality measurements. The venturi meter induces the lowest pressure loss relative to the other differential pressure instruments.
In this work, the measurement of the pressure drop in a venturi meter is complemented with the measurement of the void fraction by a electrical impedance sensor. Then, the two-phase mass flow rate and quality were determined from a theoretical model. It was found that proposed theoretical model provides a good accuracy for all experimental conditions.