An experimental and analytical investigation of entrainment rates for downward annular mist two-phase flow in pipes and downstream of a localized contraction.

摘要

Measurements of entrainment parameters are presented for downward annular mist two-phase flow in a pipe and past an orifice plate contraction with a diameter ratio of 0.75. The experiments were conducted using air and water at atmospheric pressure in a 2.54 cm diameter pipe in a special flow channel capable of producing annular flow with variable liquid and vapor flow rates over ranges typically found in compact heat exchangers. The mass fluxes ranged from 28 to 70 kg/m{dollar}sp2cdot{dollar}s corresponding to qualities ranging from 0.3 to 0.8. The facility is modular allowing base line measurements to be made in a straight pipe configuration at axial locations of 50 and 70 diameters downstream of the inlet. For the contraction configuration the orifice plate was positioned 50 diameters downstream of the inlet. Data was taken at axial locations of 10, 15 and 20 diameters downstream of the restriction. Entrainment was determined from droplet size, number density and velocity measurements obtained using a Phase Doppler Particle Analyzer (PDPA) system. A hot-wire probe calibrated for use in two-phase flow was used to verify droplet number density measurements made with the PDPA.; For downwards annular mist two-phase flow in the straight pipe configuration 90% of the volume of the entrained liquid is carried by approximately 15% of the drops. Non-equilibrium conditions exist near the inlet where entrainment levels and droplet number densities rise significantly above measurements made further downstream. Values were found to increase by as much as a factor of 2 over values found under equilibrium conditions. Velocity profiles are fully turbulent with droplet slip ratios of about 0.8. For the orifice plate contraction, severe changes in entrainment were observed with levels rising from below 5% before to 100% after the restriction. The majority of the volume of the liquid entrained was shown to be contained in droplets larger than 580 microns in diameter. The velocity field remains turbulent and regains its equilibrium profile between axial locations of 5 and 15 diameters downstream of the restriction. Also during this range of axial locations, reattachment of the liquid film begins. Complete recovery of entrainment levels in the core region is slow and was estimated to occur between 30 and 40 diameters downstream of the restriction from an analysis that used measured changes in average vapor velocity and conservation of mass. These results provide a good qualitative and quantitative picture of changes in flow morphology for conditions near the onset of entrainment in pipes having localized restrictions.