Summary

Flows through singularities (e.g., valves, orifices or Venturi tubes) are encountered in several engineering applications. Industrial examples of two-phase flashing flows in expansion devices can be found in geothermal power plants (nozzles), emergency systems (safety relief valves), refrigeration systems (valves, short tubes and capillary tubes) and flow metering devices (Venturi tubes). An experimental facility was designed and constructed in order to study two-phase flashing flows of binary mixtures in a converging-diverging nozzle (Venturi tube). The effects of mass flux, concentration of the volatile component in the liquid phase and inlet sub-saturation (over pressure) on the axial distributions of static pressure and temperature is discussed. A transparent test section enabled visual observation of the two-phase flow in the Venturi by means of high-speed visualization (HSV) and quantitative assessments of the velocity field and the viscosity dissipation rates by means of a particle image velocimetry (PIV) method. Experimental results generated for mixtures of R-134a (volatile component) and POE ISO 10 lubricating oil (non-volatile component) showed that the liquid phase viscosity has a significant effect on both the throat pressure drop and pressure recovery in the diverging section, with the latter being directly proportional to the refrigerant concentration in the liquid phase. Visual analysis of bubble cavitation in the converging-diverging nozzle revealed that the occurrence of two-phase flow in the throat and downstream of it is quite sensitive to changes in the conditions of the flow. R-134a vapor bubbles and rhodamine B acted as PIV tracking particles. PIV analysis showed that as refrigerant concentration increases, Venturi throat velocity levels decreases. Viscosity dissipation rates are mainly altered by the mixture viscosity.

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