Flow Meas. Instrum. Vol 2 July 1991 193 On-line measurement of low void fraction and mass flowrate with a side-tube and an orifice in two-phase flow K. H. CHIEN*, N. M. CHUNG*, W. K. LIN*, B. S. PEI* and C. H. LEE* The development of practical and simple on-line measurement of two-phase mass flowrate is of prime interest to applied nuclear reactor safety research. Experiments were performed at a system pressure of 0.1 MPa. The highest value of the mass flowrate for air-water was 1233 kg m-2 s- i, while the void fraction range was 0. I ~0.75, which was equivalent to 0.0004~0.0023 for air quali~Y X. For steam-water mixtures, where the mass flowrate was up to 1662 kg m- s-1, the results showed good agreement, as did those for air-water two-phase flow. The measuring techniques are described briefly and a slip ratio associated with the drift-fux model is proposed to correlate the mass flowrate model. Keywords: mass flowrate, two-phase flow, orifices Nomenclature A Orifice flow area Co Distribution parameter for the drift flux equation C1 Liquid-phase discharge coefficient C2 Gas (vapour)-phase discharge coefficient CI Liquid-phase hypothetical discharge coeff- icient for two-phase flow C~ Gas (vapour)-phase hypothetical discharge coefficient for two-phase flow f Friction factor G Mass Velocity g Acceleration due to gravity j Superficial velocity L Length of the side-tube A P Pressure drop S Slip ratio ugj Weighted mean drift velocity V Specific volume W Mass flowrate X Quality Greek Symbols 0 P Void fraction Diameter ratio for the orifice diameter d to the pipe diameter D (/~ = d/D) Coefficient of modification Coefficient of viscosity Density Orifice thermal expansion factor * Department of Nuclear Engineering, National Tsing-Hua Univers- ity, Hsinchu, Taiwan, R.O.C. Thermohydraulic Laboratory of INER, Taiwan Lung-Tan P.O. Box No. 3-3 Subscripts accel Acceleration f Liquid phase fri Friction g Gas (vapour) phase gra Gravitation SP Single phase TP Two phase Introduction The measurement of two-phase flowrate is of funda- mental importance in many technical applications, such as for process control in chemical production plants, or for transport of oil-gas mixtures in pipe- lines, or for information about the loss of coolant in an accident. In the past, the mass flowrate was measured by separating, condensing and calculating the total mass accumulated in a given time interval. Recently, the development of two-phase mass flowrate in- strumentation has been strongly influenced by nuclear reactor safety research. The use of orifices for quality and mass flowrate metering has been described by a number of authors. 1-7 Fouda and Rhodes8 proposed a homogen- eous model to predict mass flowrate by using a venturi device. Their results show many correlation factors in the mass flux equation. Reimann et al. 9 suggested the following mass flowrate devices: the radionuclide technique; the combination of a venturi nozzle and a full flow turbine meter; the combination of a free gamma densitometer. Although the mass flux equation is generally valid, the models for the drag disk and turbine meter are unsatisfactory because the instru- ments measure a local value and the flow is generally 0955-5986/91/030193-07 (~) 1991 Butterworth-Heinemann Ltd