Hideaki Nanri

Thermodynamic Effect on Subsynchronous Rotating Cavitation and Surge Mode Oscillation in a Space Inducer

The relationship between the thermodynamic effect and subsynchronous rotating cavitation was investigated with a focus on cavity fluctuations. Experiments on a three-bladed inducer were conducted with liquid nitrogen at different temperatures (74, 78, and 83 K) to confirm the dependence of the thermodynamic effects. Subsynchronous rotating cavitation appeared at lower cavitation numbers in liquid nitrogen at 74 K, the same as in cold water. In contrast, in liquid nitrogen at 83 K the occurrence of subsynchronous rotating cavitation was suppressed because of the increase of the thermodynamic effect due to the rising temperature. Furthermore, unevenness of cavity length under synchronous rotating cavitation at 83 K was also decreased by the thermodynamic effect. However, surge mode oscillation occurred simultaneously under this weakened synchronous rotating cavitation. Cavity lengths on the blades oscillated with the same phase and maintained the uneven cavity pattern. It was inferred that the thermodynamic effect weakened peripheral cavitation instability, i.e., synchronous rotating cavitation, and thus axial cavitation instability, i.e., surge mode oscillation, was easily induced due to the synchronization of the cavity fluctuation with an acoustic resonance in the present experimental inlet-pipe system.

Influence of Thermodynamic Effect on Blade Load in a Cavitating Inducer

Distribution of the blade load is one of the design parameters for a cavitating inducer. For experimental investigation of the thermodynamic effect on the blade load, we conducted experiments in both cold water and liquid nitrogen. The thermodynamic effect on cavitation notably appears in this cryogenic fluid although it can be disregarded in cold water. In these experiments, the pressure rise along the blade tip was measured. In water, the pressure increased almost linearly from the leading edge to the trailing edge at higher cavitation number. After that, with a decrease of cavitation number, pressure rise occurred only near the trailing edge. On the other hand, in liquid nitrogen, the pressure distribution was similar to that in water at a higher cavitation number, even if the cavitation number as a cavitation parameter decreased. Because the cavitation growth is suppressed by the thermodynamic effect, the distribution of the blade load does not change even at lower cavitation number. By contrast, the pressure distribution in liquid nitrogen has the same tendency as that in water if the cavity length at the blade tip is taken as a cavitation indication. From these results, it was found that the shift of the blade load to the trailing edge depended on the increase of cavity length, and that the distribution of blade load was indicated only by the cavity length independent of the thermodynamic effect.

Influence of Rotational Speed on Thermodynamic Effect in a Cavitating Inducer

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

The Status of the Research and Development of LNG Rocket Engines in Japan

This chapter reports the status of research and development which have been carried out to realize a liquefied natural gas (LNG) rocket engine with higher performance in Japan. As a fuel of rocket engine, LNG has better characteristics, i.e., longer storage, lower cost, and nontoxic; hence, LNG rocket engines have been investigated among many countries. However, LNG rocket engines have never been used for actual flight application in Japan, even in the world. The reason is that the performance and characteristics of the current LNG rocket engines do not have enough advantages compared with other liquid rocket engines. For example, if it would apply to a future reusable liquid rocket booster, the specific impulse (Is) of LNG rocket engine should be higher than 360 s in order to get more advantages than other liquid rocket engines. The Is of the current LNG rocket engines in Japan is between 310 and 350 s, falling short of the target level of 360 s. Therefore, continuous research and development have been conducted for the purpose of extending the advantages and promoting the practical use of LNG rocket engines in Japan.

Analysis of Acoustic Cavitation Surge in a Rocket Engine Turbopump

In a liquid rocket engine, cavitation in an inducer of a turbopump sometimes causes instability phenomena when the inducer is operated at low inlet pressure. Cavitation surge (auto-oscillation), one such instability phenomenon, has been discussed mainly based on an inertia model assuming incompressible flow. When this model is used, the frequency of the cavitation surge decreases continuously as the inlet pressure of the turbopump decreases. However, we obtained an interesting experimental result in which the frequency of cavitation surge varied discontinuously. Therefore, we employed one-dimensional analysis based on an acoustic model in which the fluid is assumed to be compressible. The analytical result qualitatively corresponded with the experimental result.

Analysis of Cavitation Surge Considering the Acoustic Effect of the Inlet Line in a Rocket Engine Turbopump

In a liquid rocket engine, cavitation in an inducer of a turbopump sometimes causes instability phenomena when the inducer is operated at low inlet pressure. Cavitation surge (auto-oscillation), one such instability phenomenon, has been discussed mainly based on an inertia model assuming incompressible flow. By using this model, the frequency of the cavitation surge decreases as the inlet pressure decreases. However, we obtained interesting experimental results in which the cavitation surge frequency varied disconnectedly. Therefore, we considered the factor of fluid compression employed one-dimensional analysis applying an acoustic model, combining the inlet pipe with the sonic velocity of liquid oxygen. Consequently, the analytical results qualitatively corresponded with the experimental results. In addition, an actual liquid rocket propulsion system is usually equipped with a Pogo suppression device (PSD), which is a kind of accumulator with a hydraulic compliance, upstream of a liquid oxidizer turbopump. We modified the analytical model to include the effect of this PSD and compared the analytical results with the experimental results. It was found that the frequency of cavitation surge basically became the Helmholtz frequency, defined by the cavitation compliance and the length of pipe between the PSD and the turbopump. And when the frequency of cavitation surge coincided with one of the acoustic resonance frequencies of the inlet pipe, the cavitation surge was strongly excited.Copyright

Thermodynamic Effect on Sub-Synchronous Rotating Cavitation and Surge Mode Oscillation in a Space Inducer

The relationship between the thermodynamic effect and sub-synchronous rotating cavitation was investigated with a focus on cavity fluctuations. Experiments on a three-bladed inducer were conducted with liquid nitrogen at different temperatures (74 K, 78K and 83 K) to confirm the dependence of the thermodynamic effects. Sub-synchronous rotating cavitation appeared at lower cavitation numbers in liquid nitrogen at 74 K, the same as in cold water. In contrast, in liquid nitrogen at 83 K, the occurrence of sub-synchronous rotating cavitation was suppressed because of the increase of the thermodynamic effect due to the rising temperature. Furthermore, unevenness of cavity length under synchronous rotating cavitation at 83 K was also decreased by the thermodynamic effect. However, surge mode oscillation occurred simultaneously under this weakened synchronous rotating cavitation. Cavity lengths on the blades oscillated with the same phase and maintained the uneven cavity pattern. It was inferred that the thermodynamic effect weakened the peripheral cavitation instability, i.e., synchronous rotating cavitation, and thus axial cavitation instability, i.e., surge mode oscillation, was easily induced due to the synchronization of the cavity fluctuation with an acoustic resonance in the present experimental inlet-pipe system.Copyright