Session: 7.2.2 - Cavitation II

Paper Number: 167625

167625 - Thermodynamic Self-Suppression Effect of Unsteady Cavitation in Hot Water With Scale Effect 

Abstract: 

  In high-temperature water and cryogenic fluids, cavitation generation accompanies a thermal transport due to latent heat of evaporation and a decrease in saturated vapor pressure due to a decrease in local temperature, which suppresses additional generation of cavitation and the cavity volume shrinks. This means that the more　 cavitation occurs, the less it is to occur. This is called the thermodynamic self-suppression effect of cavitation.

  Since the thermodynamic self-suppression effect of cavitation is attributed to the temperature drop due to the latent heat of evaporation, at the same time, disturbance of the flow field contributes to promotion of heat transfer. In the cavitating flow with that effect, heat transfer between mainstream and the cavity takes a role of temperature drop during the evaporation, which promotes the suppression of cavitation and temperature recovery after the evaporation, which inhibits the suppression effect. Thermodynamic parameters considering heat transfer including flow turbulence at the local cavity interface have not been considered so far. Additionally, it has been thought so far that when the mainstream velocity increases, the duration time for evaporation becomes shorter, and the suppression effect decreases. However, this does not take into account that the duration time for temperature recovery also becomes shorter at the same time. Furthermore, when it comes to unsteady cavitating flows, the duration time for evaporation and temperature recovery becomes even more complicated.

  When the working fluid is water, it is well known that a scale effect appears in cavitation. This is a promotion effect of cavitation, in which the higher the flow velocity, the easier it is for dissolved gas to separate, and cavities become larger even at the same cavitation number. Our research group has previously shown that the scale effect is also influenced by the unsteadiness of the cavitating flow. Therefore, when the mainstream velocity changes in high-temperature water, multiple factors affect cavity volume in a complex way in connection with thermodynamic suppression effect and scale effect, which are the strength and duration time of heat transfer in the cavitating flow, and thus the unsteadiness of cavitating flow affects the cavity volume.

  In this study, we investigate the influence of unsteadiness of cavitating flow on the thermodynamic self-suppression effect by comparing the cavitation suppression on a hydrofoil under quasi-steady and unsteady cavitating flows in high-temperature water. The experiment was conducted using a high-temperature water cavitation tunnel. In the comparison of cavity lengths, the thermodynamic suppression effect and scale promotion effect are superimposed, so it is not possible to separate the influence of unsteadiness on each effect. Therefore, by measuring the temperature drop inside the cavity, the amount of suppression due to the thermodynamic effect is extracted and compared.

Presenting Author: Gen Nakamura Department of Mechanical Systems Engineering, Graduate School of Engineering, Tohoku University

Presenting Author Biography: