Stability analyses of 1-3 dimensional cavitating flow through turbopump inducers are reviewed with a special focus on the cause of cavitation instabilities. In one-dimensional analysis, cavitation is modeled with the cavitation compliance, defined as the decrease of cavity volume due to the increase of inlet pressure, and the mass flow gain factor, defined as the decrease of cavity volume due to the increase of flow rate. It was shown that the positive mass flow gain factor is the cause of cavitation surge and rotating cavitation. In two-dimensional stability analysis, the blade surface cavity is modeled by a free streamline with a constant pressure. It is shown that various modes of cavitation instabilities start to occur when the cavity length becomes about 65% of the blade spacing. It was found that there is a region near the cavity trailing edge in which the incidence angle to the next blade is decreased. This flow occurs to satisfy the continuity equation near the cavity closure. The cavitation instabilities start to occur when this region starts to interact with the leading edge of the next blade. In three-dimensional real flows, cavitation occurs mostly near the tip. Cavitation instabilities are simulated by three dimensional unsteady cavitating CFD. By separating out the disturbance caused by cavitation, it was found that there exists a flow component towards the trailing edge of tip cavities to fill up the volume of collapsing bubbles. This disturbance flow has an effect to reduce the incidence angle to the next blade. It was found that cavitation instabilities start to occur when this disturbance flow starts to interact with the leading edge of the next blade. So, it was found that the steady cavity length at the tip is the most important parameter in three dimensional real flow. Thus, it was found that the continuity equation plays the most important role in the mechanism of cavitation instabilities in 1-3 dimensional flows.
Artificial heart pumps have attracted the attention of researchers around the world as an alternative to the organ used in cardiac transplantation. Conventional centrifugal pumps are no longer considered suitable for long-term application because of the possibility of occurrence of blood leakage and thrombus formation around the shaft seal. To overcome this problem posed by the shaft seal in conventional centrifugal pumps, the magnetically suspended centrifugal pump has been developed; this is a sealless rotor pump, which can provide contact-free rotation of the impeller without leading to material wear. In Europe, clinical trials of this pump have been successfully performed, and these pumps are commercially available. One of the aims of our study is to numerically examine the internal flow and the effect of leakage flow through the gap between the impeller and the pump casing on the performance of the pump. The results show that the pressure head increases compared with the pump without a gap for all flow rates because of the leakage of the fluid through the gap. It was observed that the leakage flow rate in the pump is sufficiently large; further, no stagnant fluid or dead flow regions were observed in the pump. Therefore, the present pump can efficiently enhance the washout effect.
LES(Large Eddy Simulation) with a cavitation model was performed to calculate an unsteady flow for a mixed flow pump with a closed type impeller. First, the comparison between the numerical and experimental results was done to evaluate a computational accuracy. Second, the torque acting on the blade was calculated by simulation to investigate how the cavitation caused the fluctuation of torque. The absolute pressure around the leading edge on the suction side of blade surface had positive impulsive peaks in both the numerical and experimental results. The simulation showed that those peaks were caused by the cavitaion which contracted and vanished around the leading edge. The absolute pressure was predicted by simulation with -10% error. The absolute pressure around the trailing edge on the suction side of blade surface had no impulsive peaks in both the numerical and experimental results, because the absolute pressure was 100 times higher than the saturated vapor pressure. The simulation results showed that the cavitation was generated around the throat, then contracted and finally vanished. The simulated pump had five throats and cavitation behaviors such as contraction and vanishing around five throats were different from each other. For instance, the cavitations around those five throats were not vanished at the same time. When the cavitation was contracted and finally vanished, the absolute pressure on the blade surface was increased. When the cavitation was contracted around the throat located on the pressure side of blade surface, the pressure became high on the pressure side of blade surface. It caused the 1.4 times higher impulsive peak in the torque than the averaged value. On the other hand, when the cavitation was contracted around the throat located on the suction side of blade surface, the pressure became high on the suction side of blade surface. It caused the 0.4 times lower impulsive peak in the torque than the averaged value. The cavitation around the throat caused the large fluctuation in torque acting on the blade.
The attachment of inducer in front of main impeller is a powerful method to improve cavitation performance; however, cavitation surge oscillation with low frequency occurs with blade cavity growing to each throat section of blade passage simultaneously. Then, one conceptual method of installing suction axi-asymmetrical plate has been proposed so as to keep every throat passage away from being unstable at once, and the effect on suppression of the oscillation were investigated. In the present study, cavitation behaviors in the inducer is observed with distributing multi-cameras circumferentially, recording simultaneously and reconstructing multi-photos on one plane field as moving a linear cascade. Observed results are utilized for discussion with other measuring results as casing wall pressure distribution. Then the suppression mechanism of oscillation by installing axi-asymmetrical inlet plate will be clarified in more details.
The attachment of inducer upstream of main impeller is an effective method to improve the suction performance of turbopump. However, various types of cavitation instabilities are known to occur even at the designed flow rate as well as in the partial flow rate region. The cavitation surge occurring at partial flow rates is known to be strongly associated with the inlet back flow. In the present study, in order to understand the detailed structure of internal flow of inducer, we firstly carried out the experimental and numerical studies of non-cavitating flow, focusing on the flow field near the inlet throat section and inside the blade passage of a two bladed inducer at a partial flow rate. The steady flow simulation with cavitation model was also made to investigate the difference of flow field between in the cavitating and noncavitating conditions.
The purpose of the present research is to suppress cavitation instabilities by using a circumferential groove. The circumferential groove was designed based on CFD so that the tip leakage vortex is trapped by the groove and does not interact with the next blade. Experimental results show that the groove can suppress rotating cavitation, asymmetric cavitation and cavitation surge. However, weak instabilities with higher frequency could not be suppressed by the groove. From the analysis of pressure pattern similar to that for rotor-stator interaction, it was found that the higher frequency components are caused by the interaction of backflow vortices with the inducer blades.
Laidback fan shaped film-cooling hole is formulated numerically and optimized with the help of three-dimensional numerical analysis, surrogate methods, and the multi-objective evolutionary algorithm. As Pareto optimal front produces a set of optimal solutions, the trends of objective functions with design variables are predicted by hybrid multi-objective evolutionary algorithm. The problem is defined by four geometric design variables, the injection angle of the hole, the lateral expansion angle of the diffuser, the forward expansion angle of the hole, and the ratio of the length to the diameter of the hole, to maximize the film-cooling effectiveness compromising with the aerodynamic loss. The objective function values are numerically evaluated through Reynolds- averaged Navier-Stokes analysis at the designs that are selected through the Latin hypercube sampling method. Using these numerical simulation results, the Response Surface Approximation model are constructed for each objective function and a hybrid multi-objective evolutionary algorithm is applied to obtain the Pareto optimal front. The clustered points from Pareto optimal front were evaluated by flow analysis. These designs give enhanced objective function values in comparison with the experimental designs.
It is important in pump design that the axial thrust of mixed-flow pump is predicted with high accuracy. In this paper, predictions of the axial thrust were carried out with CFD for mixed-flow pumps of three specific speeds. The region concerning the axial thrust prediction was picked out, and was divided into two parts. One of them was hydraulic part, which included the impeller and the vaned diffuser. The other was the rear part of impeller. These parts were calculated and evaluated individually. The CFD results were compared with experimental ones. They showed good agreements. It is shown that the axial thrust for a mixed-flow pump can be predicted by using CFD with practical accuracy.
Performance prediction methodology for centrifugal submersible slurry pump has been presented in this paper. An in-depth study on various energy-head losses occurring through the pump flow in rotating reference frame has been carried out in this research work. Head-flow characteristics of the centrifugal pump have been accomplished in two stages. First performance of the centrifugal pump with clear water has been predicted by analyzing and deducting head losses from the theoretical head. Effects of solid particles size, specific gravity and concentration on pump slurry flow have been investigated. Additional head losses due to solid particles in the slurry have been predicted, analyzed and then deducted from clear water head to establish the performance of centrifugal slurry pump. The performance of centrifugal slurry pump has been predicted at with accuracy of 88 to 90 % for solid concentration of 18 % to 5 % by volume.
The present study investigated temperature, thermal stress, and the lifetime in film cooling systems with and without thermal barrier coating. 3D-numerical simulations using a FEM commercial code were conducted to calculate distributions of temperature and thermal stresses. In the simulations, the surface boundary conditions used the surface heat transfer coefficients and adiabatic wall temperature which were converted from the Sherwood numbers and impermeable wall effectiveness obtained from previous mass transfer experiments. Then, the lifetime of the film cooling systems is predicted using thermal analysis data and the material creep data. The minimum lifetime is approximately 1,100 hours on the sides of the hot side surface in case without TBC.
Edited and published by : Turbomachinery Society of Japan, Korean Fluid Machinery Association, Chinese Society of Engineering Thermophysics, IAHR Produced and listed by : Turbomachinery Society of Japan, Korean Fluid Machinery Association, Chinese Society of Engineering Thermophysics, IAHR