Effects of the blade number and solidity on the performance and cavitation instabilities of a turbopump inducer

Abstract

The turbopump used in a liquid rocket propulsion system generates cavitation at the inlet because of the high rotating speed. To improve the suction performance, an inducer was installed at the front impeller. Cavitation instabilities, such as cavitation surge, rotating cavitation, and asymmetric cavitation, occur on the shaft vibration and noise. These phenomena can lead to the destruction of the impeller. Therefore, the proper suction performance and instabilities control are required when an inducer designing. Recently, many studies have been reported in which an experiment test and numerical analysis for the turbopump inducer were performed. Researches confirmed the differences in the performance and the efficiency for shape modifications and scales. Studies have also been carried out on the blade angle, tip clearance, and solidity of design parameters. The number of blade, which is one of the important parameters, has a very significant impact on the head, the suction performance, and instabilities of the turbopump. However, in the previous studies, the number of blade is simply changed with respect to the same axial length. In other words, as the number of the blade increases, the solidity also changes at the same time. Thus, as reported in these studies, this effect makes it difficult to determine whether the influence is due to the blade number or the solidity. For this reason, in this study, the performance, the efficiency, and cavitation instabilities were evaluated in which the different blade number for the inducers with the same solidity. To analyze the solidity effect, the different blade number for an inducer with respect to the same axial length was included together. Flow characteristics were evaluated using the numerical calculation, and the verification of the validity and the reliability of results were obtained by performing the experiment test. The total pressure rise of Z3s3 of the experiment test was 12.75 % smaller than that of Z2s2 at design mass flow rate, and the total pressure rise of Z3s2 decreased by 18.74 % less than that of Z2s2. The efficiency of Z2s2 was the highest and then, the values of Z3s2 and Z3s3 were followed. The mass averaged deviation angle of Z3s2 was smaller than that of Z2s2 which represents comparison of the blade number effect. The mass averaged deviation angle of Z3s2 was larger than that of Z3s3. This trend was satisfied with the empirical rule. The relative total pressure loss for all three cases increased while passing the inducer. Especially, the losses of Z3s3 and Z3s2 were bigger than that of Z3s2, and continue to the impeller inlet. Those loss characteristics influenced on the efficiency. Therefore, the efficiency of Z3s3 was the lowest compared with other cases, even though the pressure rise performance of Z3s3 was higher than that of Z3s2. The existing efficiency empirical equation did not reflect to the blade number. So the new empirical equation was suggested by appling the blade number effect. To get the reliability, two more models were added. The efficiencies of Z3s4 and Z4s2 were reflected the correction between the numerical calculation and the experiment test. The suction performance curve of the inducer used for this study at design mass flow rate. The NPSHr of Z3s3 and Z3s2 increased 25.8% and 14.1%, respectively. The biggest cause for the difference between the experiment test and the numerical calculation was the cavitation model. The empirical equation of NPSHr was included with valuables of the blade number and the solidity. However, the result had a big difference. As the inlet pressure decreased from non cavitation condition, cavitation occurred at blade tip that called the tip vortex cavitation. And then, the tip vortex cavitation was getting longer and connected to the bubble cavitation increasingly. Cavitation inception was not related to the solidity, and only depended on the blade number. The pressure of the fluid coming to the leading edge reduced due to the interference with the next blade. Because of this phenomenon, the head drop started once the cavity occurred on the suction surface. As the inlet pressure decreased, a symmetry cavitation corresponding to 2 or 3 times the rotational speed appeared. An asymmetry cavitation corresponding to the rotational speed also occurred. Cavitation surge occurred at low frequency when the inlet pressure decreases further. In case of the three bladed inducers, rotating cavitation was strongly observed. In case of the Z3s2, it was found that the intensity of symmetry cavitation and rotating cavitation significantly weak compared with case of the Z3s3. Especially, higher order rotating cavitation did not occur.