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Cavitation often occurs in inducer pumps of space rockets. Silver plated coating on the inducer liner faces the damage of cavitation. In this study, we carried out cavitation erosion tests using silver plated coatings with different thermodynamic parameter at a constant cavitation number. Then we carried out cavitation erosion tests using some liquid with the same thermodynamic parameter and cavitation number as liquid oxygen condition. Thermodynamic parameter Σ proposed by Brennen was used as a thermodynamic parameter. The fluids used for the cavitation erosion tests were water, ethanol and hexane. We discussed the relation between thermodynamic parameter and mass loss rate, and the relation between acoustic impedance and mass loss rate.

Cavitation often occurs in inducer pumps of space rockets. Silver plated coating on the inducer liner faces the damage of cavitation. In this study, we carried out cavitation erosion tests using silver plated coatings with different thermodynamic parameter at a constant cavitation number. Then we carried out cavitation erosion tests using some liquid with the same thermodynamic parameter and cavitation number as liquid oxygen condition. Thermodynamic parameter ∑ proposed by Brennen was used as a thermodynamic parameter. The fluids used for the cavitation erosion tests were water, ethanol and hexane. We discussed the relation between thermodynamic parameter and mass loss rate, and th e relation between acoustic impedance and mass loss rate.

For experimental investigations of the thermodynamic effect on a cavitating inducer, it is nesessary to observe the cavitation. However, visualizations of the cavitation are not so easy in cryogenic flow. For this reason, we estimated the cavity region in liquid nitrogen based on measurements of the pressure fluctuation near the blade tip. In the present study, we focused on the length of the tip cavitation as a cavitation indicator. Comparison of the tip cavity length in liquid nitrogen (80K) with that in cold water (296K) allowed us to estimate the strength of the thermodynamic effect. The degree of thermodynamic effect was found to increase with an increase of the cavity length. The temperature depression was estimated from the difference of the cavitation number of corresponding cavity condition (i.e., cavity length) between in liquid nitrogen and in cold water. The estimated temperature depression caused by vaporization increased rapidly when the cavity length extended over the throat. In addition, the estimated temperature inside the bubble nearly reached the temperature of the triple point when the pump performance deteriorated.

Cavitation often occurs in inducer pumps used for space rockets. Silver plated coating on the inducer liner faces the damage of cavitation. Therefore, it is important to study about the cavitation erosion resistance for silver plated coating at several operating conditions in the inducer pumps. In this study, the cavitation erosion tests were carried for silver plated coating in deionized water and ethanol at several liquid temperatures (273K–400K) and pressures (0.10MPa–0.48MPa). The mass loss rate is evaluated in terms of thermodynamic parameter Σ proposed by Brennen [9], suppression pressure p–pv (pv: saturated vapor pressure) and acoustic impedance ρc (ρ: density and c: sound speed). Cavitation bubble behaviors depending on the thermodynamic effect and the liquid type were observed by high speed video camera. The mass loss rate is formulated by thermodynamic parameter Σ, suppression pressure p–pv and acoustic impedance ρc.

Cavitation is physically “a vaporization of liquid” which needs latent heat for phase change. A cavity grows in the liquid, so the latent heat of vaporization can only be supplied by the liquid surrounding the cavity. Thus, the liquid close to the interface region of the cavity is cooled down. In general, cryogenic liquids are very thermosensitive. For liquid hydrogen and oxygen used in rocket propulsion, the temperature in the cavity, i.e., the vapor pressure in the cavity, is lower than those of the liquid bulk. Thanks to this thermal effect, cavitation in cryogenic liquids is less developed than that in water at room temperature. This thermal effect on cavitation is beneficial in that it improves cavitation performance and alleviates cavitation instability in space inducers. In previous works, we investigated the relationship between the thermodynamic effect and the cavitation instabilities, e.g., rotating cavitation and cavitation surge, with a focus on the cavity length as an indication of cavitation. In the present work, first, aspects of cavitation in the inducer were observed by direct optical visualization in liquid nitrogen. Second, joint experiments in liquid nitrogen and cold water were conducted on a cavitaing inducer. In nitrogen experiments, operating conditions, i.e., rotational speed and liquid temperature, were varied to determine the cavitation scaling law. Through these experimental results, characteristic times, namely, the transit time for bubble growth and the characteristic thermal time introduced from the thermal property, were investigated as a cavitation thermodynamic parameter. It was found out that the adjustment of cavitation number has a good correlation with the ratio of the transit time and the characteristic thermal time.

To estimate the influence of velocity on the thermodynamic effect, we conducted experiments in which the inducer rotational speed was changed in liquid nitrogen. The experiments in liquid nitrogen and in cold water allowed us to estimate the amplitude of the thermodynamic effect. In the experiment with lower rotational speed, suction performance was improved. The cavity length at lower rotational speed was shorter than that at higher speed. Thus, we confirmed that the degree of the thermodynamic effect depends on the rotational speed as lower rotational speed suppresses cavity length. Temperature depression was estimated based on a comparison of cavity length in liquid nitrogen and that in water. We found that the degree of temperature depression became smaller when the rotational speed was lower.Copyright © 2009 by ASME

Based on the length of the tip cavitation as an indication of cavitation, we focused on the effect of thermodynamics on cavitation performances and cavitation instabilities in an inducer. Comparison of the tip cavity length in liquid nitrogen (76K and 80K) as working fluid with that in cold water (296K) allowed us to estimate the strength of the thermodynamic effect on the cavitations. The degree of thermodynamic effect was found to increase with an increase of the cavity length, particularly when the cavity length extended over the throat of the blade passage. In addition, cavitation instabilities occurred both in liquid nitrogen and in cold water when the cavity length increased. Subsynchronous rotating cavitation appeared both in liquid nitrogen and in cold water. In the experiment using liquid nitrogen, the temperature difference between 76K and 80K affected the range in which the subsynchronous rotating cavitation occurred. In contrast, deep cavitation surge appeared only in cold water at lower cavitation numbers. From these experimental results, it was concluded that when the cavity length extends over the throat, the thermodynamic effect also affects the cavitation instabilities as a “thermal damping” through the unsteady cavitation characteristics.

We developed a new virtual reality (VR) system that enables direct interaction between a 3D image and a finger without wearing special 3D glasses and without attaching any marker or detector to the finger. Moreover, it frees the user from visual fatigue. The system consists of a 128-directional 3D display, a PC cluster, and a fingertip detection system. The 128-directional display provides a natural 3D image which does not have the accommodation-vergence conflict and has very smooth motion parallax. It differs from conventional multi-view displays in that it precisely reconstructs rays from a 3D object. It contains 128 LCD panels, and the 128 images displayed on them are projected in different horizontal directions with a horizontal angle pitch of 0.23°. The PC cluster consists of 16 PCs. Each PC generates eight video signals. The fingertip detection system employs a stereo infrared (IR) camera. The 3D position of the fingertip is estimated by triangulation. We also made three application programs. The first one enables the fingertip manipulation of a 3D image in VRML format. The second one is a 3D drawing program that allows users to draw lines in the air. The last one enables the rotation of a 3D image of a 360° directional image by the finger.

For experimental investigations of the thermodynamic effect on a cavitating inducer, it is necessary to observe the cavitation. However, visualizations of the cavitation are not so easy in cryogenic flow. For this reason, we estimated the cavity region in liquid nitrogen based on measurements of the pressure fluctuation near the blade tip. The degree of the thermodynamic effect was found to increase with the increase of the cavity length of the tip cavitation. The estimated temperature depression caused by vaporization increased rapidly when the cavity length extended over the throat. In addition, the estimated temperature inside the bubble nearly reached the temperature of the triple point when the pump performance deteriorated., 資料番号: AA0049479000, レポート番号: JAXA-RR-05-020