Experimental Studies on Characteristics of Pressure Attenuation and Recovery to Saturation for Decompressive Disturbances in Low Pressurized Water

Abstract

Slightly pressurized water (p_sat=1.26.0bar) was flashed into steam void by step decom-pression to atmospheric pressure, using a shock tube type device with vertical test section, made of stainless steel pipe about 2m long, with an inner diameter of 40mm. The transient pressure depression below saturation and the time constant of pressure recovery to the satu-ration were measured, based on the observed transient pressure response signals. The transient pressure curve could be approximated by the expression p(t)=p_min+(p_sat-p_min)(1-e^-t/τ) for the period immediately following step decompression. This time constant τ represents the effective relaxation time of thermodynamic nonequilibrium.
Defining the term decompression ratio ε as the ratio of the excess transient depression below saturation pressure to the difference between saturation and atmospheric pressure, i.e., ε=(p_sat-p_min)/(p_sat-p_fin), the dependence of ε on the vertical position along the test column z and p_sat was found to follow approximately the empirical formula ε=exp[-β'{(p_sat/p_fin) -1}(z+z₀)], where β' was evaluated to be 0.43m^-1. Correspondingly, the attenuation constant β=-(-1/ε)•dε/dz=β'{(p_sat/p_fin)-1}. The time constant τ was found to range between 10 300msec, with a tendency to increase with decreasing saturation pressure determined by the measured water temperature. The value of τ showed little dependence on the position of observation along the test section. The expression 1/τ=βV (V being the propagation velocity of pressure waves) indicates a hyperbolic relation between τ and p_sat. This relation roughly agreed with experimental data.