A Dimensionless Scaling Parameter for Thermal Effects on Cavitation in Turbopump Inducers

Abstract

To characterize cryogenic pump performance, at least one parameter in addition to flow coefficient and cavitation number is required. This parameter arises because the heat of vaporization and other physical parameters change along the saturation line of liquids and results in a thermal effect on cavitation that has been observed and studied by previous researchers over a range of operating conditions and working fluids. These previous efforts have defined both dimensionless and dimensional parameters governing thermal effects in pumps. In the present work, a dimensionless parameter (DB) scaling thermal effects in a cavitating pump across different tip diameters, rotational speeds, and working fluids is derived using a model of bubble growth in a time-varying pressure field. Although the derivation is somewhat different than others have used, the result is similar and in some cases identical to that of others. Careful testing is carried out to experimentally validate this parameter with (deaerated) variable temperature water and with variable pump speed. The results show that within the accuracy of the data, the same head fall-off curve is obtained when either the water temperature or the pump speed is used to set the DBs. This suggests the proposed parameter can thermally scale cryogenic pumping conditions for suction performance when testing in hot water. Also examined is the effect of thermodynamics on inducer cavitation instabilities. The quasi-steady, dynamic environments at the pump inlet are compared in cold and superheated water. The cavitation instabilities of the test inducer are found to be dramatically changed by thermal effects. These findings emphasize the importance of considering both fluid mechanical and thermal scaling when designing a test program to evaluate the suction performance of a cryogenic pump.

Experimental Visualization of Cryogenic Backflow Vortex Cavitation with Thermodynamic Effects

“…In general, the thermodynamic effects in a certain fluid increase as the temperature increases. If much higher temperatures had been chosen, some differences would have been detected, as many researchers have reported [4][5][6]8,9,19]. In this study, the liquid nitrogen results at 77.9 0.5 K, which are considered representative of cryogens with high thermodynamic effects, were compared with the water results at 333.5 4.0 K, which are considered representative of fluids with low thermodynamic effects.…”

“…Recently, Ehrlich and Murdock reported cavitation on a rotating inducer in water at 23.3°C (296.5 K) in Fig. 6 of [19] in 2015. The water cavitation at 23.3°C contained bubbles with almost the same size as the water bubbles at 337 K in our experiments, as shown in Figs.…”

“…However, each data point was spread over a wide range for each flow coefficient. Recently, in 2015, Ehrlich and Murdock [19] compared cavitation on a rotating inducer in water at various temperatures in the range of 23.3-127°C (296.5-400.2 K). Backflow vortex cavitation was recorded in some of their photographs, but they did not investigate the phenomenon in detail.…”

Numerical study of cavitating flows in a wide range of water temperatures with special emphasis on two typical cavitation dynamics

“…As it is known, the thermodynamic effects significantly affect the thermo-fluids cavitation dynamics [1,[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]23,33,34,36,44,50,55,58,64], and hence a heat transfer process is taken into consideration in the transport equation cavitation model. In the case of a single bubble, an estimation of the heat flux transferred from the liquid to the vapor can be obtained from the Fourier's law in the thermal boundary layer:…”

“…systems for liquid rocket engines, and a certain number of specialized fluid machineries for high temperature water [11][12][13]. The thermodynamic effects on developed cavitation were firstly investigated by Stahl and Stepanoff [14] as early as 1956.…”