Experimental investigation of two-phase flashing flows of a binary mixture of infinite relative volatility in a Venturi tube

• Combination of HSV, PIV and static pressure measurements in R-134a/oil mixture flow.
• Cavitation and flow structure very sensitive to total mass flux and concentration.
• Flow develops into a homogeneous frothy mixture downstream of the throat.
• Liquid viscosity is the most important parameter determining the pressure behavior.

An experimental facility was constructed to study two-phase flashing flows of binary mixtures in a converging–diverging nozzle (Venturi tube). The effects of mass flow rate, concentration of the volatile component in the liquid phase and inlet sub-saturation (overpressure) on the axial distribution of static pressure are discussed. A transparent test section enabled visual observation of the two-phase flow in the Venturi by means of high-speed visualization (HSV) and a quantitative assessment of the velocity field was carried out using particle image velocimetry (PIV). 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 was quite sensitive to changes in the conditions of the flow, which quickly evolved into a homogeneous frothy mixture when the drop in pressure at the throat exceeded a certain limit.