Experimental study of appearance of thermodynamic effect on cavitation in hot water

Yuka IGA Tohoku University， Teppei FURUSAWA Tohoku University

Abstract

In this study, in order to clarify a relation between thermodynamic effect, which suppresses cavitation, and scale effect, which promotes cavitation, tunnel experiment was done with NACA16012 single hydrofoil in hot water.

In the scale effect, cavitation is easier to occur and develop when Reynolds number Re is higher even though in the condition of constant cavitation number. The effect is affected by nuclei; residual bubble, contaminant, dissolved gas, roughness of a material surface and so on. When the mainstream temperature increases, the viscosity decreases, Re increases and then the scale effect appears. On the other hand, in the thermodynamic effect, when a cavity occurs, the local temperature decreases around the cavity due to the latent heat of evaporation of the cavity. The local saturated vapor pressure decreases due to the temperature depression, and then, evaporation is suppressed. The thermodynamic effect intensifies in a condition of higher mainstream temperature on the basis of a change of the thermosphysical properties when the temperature increases in each fluid. The effect becomes obvious in cryogenic fluids such as liquid hydrogen and liquefied natural gas as well as in hot water and refrigerant. Therefore, when the mainstream temperature increases, the contravariant two effect exist at once, one is scale promotion effect the other is thermodynamic suppression effect.

Appearance of the effects was estimated by occurrence region of cavitation for angle of attack vs cavitation number and variation of cavitation patterns. Main flow temperature was 30 ℃ and 70 ℃. By increasing main flow temperature under constant Reynolds number condition, scale effect was took away to some extent and thermodynamic effect was picked out. From the result, occurrence region of cavitation was suppressed in 70 ℃ hot water, where it is empirically known that thermodynamic effect does not appear in such a low temperature. The result indicates that thermodynamic effect and scale effect, which are conflicting effects, cancelled each other in water at 70 ℃. Additionally, it was shown that occurrence of cavitation was drastically suppressed in a condition of specific angle of attack.

Keywords

Cavitation, Hot water, Tunnel experiment, Thermodynamic effect, Scale effect, NACA16012

Figures

	(a) Attached sheet cavitation (α=8°,σ=2.1)
	(b) Sheet-cloud cavitation (α=8°,σ=1.0)
	(c) Supercavitation (α=8°,σ=0.6)
	(d) Bubble cavitation (α=4°,σ=0.4)
	(e) Back detachment cavitation (α=0°,σ=0.3)

Fig.2 Influence of increase of main flow temperature on occurrence map of cavitation in a condition of constant main flow velocity. The limiting cavitation number was slightly decrease according to increase of main flow temperature to 70 ℃.

Fig. 3 Influence of increase of main flow temperature on occurrence map of cavitation in a condition of constant Reynolds number. The occurrence region of cavitation was drastically suppressed between angle of attack 4 deg to 7 deg according to increase of main flow temperature to 70 ℃.