Performance and image analysis of a cavitating process in a small type venturi

• Precise control of mass flow rate can be achieved by using a cavitating venturi operated at cavitation conditions.
• Mass flow rate through a specific venturi is chocked at a certain pressure ratio depending on the operating temperature.
• Decreasing the pressure ratio beyond the critical value results in more vapor formation in the venturi divergent part.
• Average vapor void fraction in a cavitating venturi is mainly dependent on the pressure ratio.
• Image analysis is introduced as a tool for studying the cavitating process in venturis.

A cavitating venturi is a flow control device usually used to deliver constant mass flow rate when operated at cavitation conditions. Experiments were conducted on a small type venturi test rig under varying upstream, throat and downstream conditions. A model for water vapor void fraction is proposed and validated against detailed image analysis of the cavitating process. Both experimental and model results showed that cavitation occurs at a certain critical pressure ratio (downstream pressure/upstream pressure) where the liquid starts to evaporate at the throat of the venturi. As the venturi pressure ratio is decreased than the critical pressure ratio, the mass flow rate is choked resulting in an increased vapor formation in the divergent part of the venturi. This can be predicted by the proposed model as well as the image analysis. Through all the conducted venturi experiments, the critical pressure ratio ranges from 0.70–0.72 corresponding to an upstream temperature of 21 °C. At a temperature of 42 °C the critical pressure ratio was increased to 0.75. Image analysis clearly shows the vapor formation from the throat up to the middle of the divergent part of the venturi. Traces of vapor are observed at the exit of the venturi where the thermodynamic conditions cannot maintain the existence of vapor bubbles.