Effects of Alternate Leading Edge Cutback on Unsteady Cavitation in 4-Bladed Inducers
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A set of 4-bladed inducers with various amounts of cutback was tested with the aim of suppressing the rotating cavitation by applying alternate leading edge cutback. Unsteady cavitation patterns were observed by means of inlet pressure measurements and high-speed video pictures. It was found that the region with the alternate blade cavitation and asymmetric cavitation were enlarged with the increase of the amount of the cutback. As a result, the region with the rotating cavitation was diminished. At low flow rate, two types of alternate blade cavitation were found as predicted theoretically on 4-bladed inducer with smaller uneven blade length. One of them is with longer cavities on longer blades, and the other is with longer cavities on shorter blades. Switch was observed in these alternate blade cavitation patterns depending whether the cavitation number was increased or decreased. For an inducer with larger amount of cutback, the rotating cavitation and cavitation surge were almost suppressed as expected for a wide range of flow rate and cavitation number, although the cavitation performance was deteriorated. However, we should note that an asymmetric cavitation pattern occurs more easily in inducers with alternate leading edge cutback, and that the unevenness due to the cutback causes uneven blade stress.Keywords:
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Alternate blade cavitation, rotating cavitation and cavitation surge in rocket turbopump inducers were simulated by a three dimensional commercial CFD code. In order to clarify the cause of cavitation instabilities, the velocity disturbance caused by cavitation was obtained by subtracting the velocity vector under non-cavitating condition from that under cavitating condition. It was found that there exists a disturbance flow towards the trailing edge of the tip cavity. This flow has an axial flow component towards downstream which reduces the incidence angle to the next blade. It was found that all of the cavitation instabilities start to occur when this flow starts to interact with the leading edge of the next blade. The existence of the disturbance flow was validated by experiments.
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Rocket (weapon)
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A set of 4-bladed inducers with various amounts of cutback was tested with the aim of suppressing the rotating cavitation by applying alternate leading edge cutback. Unsteady cavitation patterns were observed by means of inlet pressure measurements and high-speed video pictures. It was found that the region with the alternate blade cavitation and asymmetric cavitation were enlarged with the increase of the amount of the cutback. As a result, the region with the rotating cavitation was diminished. At low flow rate, two types of alternate blade cavitation were found as predicted theoretically on 4-bladed inducer with smaller uneven blade length. One of them is with longer cavities on longer blades, and the other is with longer cavities on shorter blades. Switch was observed in these alternate blade cavitation patterns depending whether the cavitation number was increased or decreased. For an inducer with larger amount of cutback, the rotating cavitation and cavitation surge were almost suppressed as expected for a wide range of flow rate and cavitation number, although the cavitation performance was deteriorated. However, we should note that an asymmetric cavitation pattern occurs more easily in inducers with alternate leading edge cutback, and that the unevenness due to the cutback causes uneven blade stress.
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Leading edge
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Cavitation caused by phases exchange between fluids of large density difference occurs in a region where the pressure of water falls below its vapor pressure. The density of water in a water-vapor contact area decreases dramatically. As a result, the flow in this region is compressible, which affects directly turbulent dissipation structures. Leading edge cavitation is naturally time dependent. Re-entrant jet generated by liquid flow over a cavity is a main actor of cavity shedding. Simulation of unsteady leading edge cavitation flows through a 4-blade runner bulb turbine was performed. Particular attention was given to the phenomena of re-entrant jet, cavity shedding, and cavitation vortices in the flow over turbine blade. The Reynolds-Average Navier-Stokes equations with finite volume discretization were used. The calculations were done with pressure-based algorithms since the flow possesses a wide range of density change and high complexity turbulence. The new formula for dilatation dissipation parameter in k- model was introduced and the turbulent Mach number was calculated from density of mixture instead. 2-D and 3-D hydrofoils based on both numerical and experimental results accomplished a validation. The results show that re-entrant jet, shedding of cavity, and cavitation vortices can be captured. In addition, this paper also calculates the cycle frequency of torque generated by the runner and vapor area evolution on the blade surface. The cycle frequency varies with cavitation number. At normal operation of this turbine ( = 1) it is found that both of them have a frequency of 46 Hertz.
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A set of 4-bladed inducers with various amounts of cutback was tested with the aim of suppressing the rotating cavitation by applying alternate leading edge cutback. Unsteady cavitation patterns were observed by means of inlet pressure measurements and highspeed video pictures. The region with the alternate blade cavitation and asymmetric cavitation were enlarged with the increase of the amount of the cutback. As a result, the region with the rotating cavitation was diminished. At low flow rate, two types of alternate blade cavitation were found as predicted theoretically on 4-bladed inducer with smaller uneven blade length. One of them is with longer cavities on longer blades, and the other is with longer cavities on shorter blades. Switch was observed in these alternate blade cavitation patterns depending whether the cavitation number was increased or decreased. For an inducer with larger amount of cutback, the rotating cavitation and cavitation surge were almost suppressed as expected for a wide range of flow rate and cavitation number, although the cavitation performance was deteriorated
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Cavitation is a serious problem in the development of high-speed turbopumps, and an inducer is often used to avoid cavitation in the main impeller. Thus, the inducer often operates under the worst conditions of cavitation. If it could be possible to control and suppress cavitation in the inducer by some new device, it would also be possible to suppress cavitation occurring in all types of pumps. The purpose of our present study is to develop a new, effective method of controlling and suppressing cavitation in an inducer using shallow grooves, called “J-Grooves.” J-Grooves are installed on the casing wall near the blade tip to use the high axial pressure gradient that exists between the region just downstream of the inducer leading edge and the region immediately upstream of the inducer. The results show that the proper combination of backward-swept inducer with J-Grooves improves the suction performance of the turbopump remarkably, at both partial flow rates and the design flow rate. The rotating backflow cavitation occurring at low flow rates and the cavitation surge which occurs near the best efficiency point can be almost fully suppressed by installing J-Grooves.
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A large-scale perpendicular cavitating vortices (PCVs), at the trailing edge of attached cavitation on the blade suction side near the tip region, has been found recently due to the great impact on performance breakdown in an axial waterjet pump. However, the trajectory and dynamics of this structure have been given scant attention. In this study, some visualized experiments were carried out to elucidate the PCVs for different conditions. The high-speed imaging coupled with numerical computations show that the vortical cloud cavitation is induced by the combination of tip leakage vortex (TLV) and radial re-entrant jets from the hub to blade tip. Moreover, the trajectory and intensity of PCVs depend on the operating conditions strongly, whether the other parameters, e.g. blade number and blade geometries, are modified. When taken the blade number into consideration, as a consequence of flow passage width and blade loading distributions, the dynamics and strength of PCVs vary considerably. Furthermore, an optimum clearance geometry is seen to eliminate corner vortex and clearance cavitation when the clearance edge is rounded on the pressure side. However, the more intensive tip leakage vortex cavitation is observed due to the increased amount of leakage flux. Additionally, in the original blade with sharp edges, the PCVs is relative weak and has a loose structure, resulting in the multiple interaction with the next blade. These phenomenon are responsible for the severe performance degradation and flow instabilities in the tip region of an axial-flow pump.
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Leakage (economics)
Tip clearance
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For the numerical simulation of cavitating flows a cavitation model was implemented in the CFD-Code NS3D developed at the Institute of Fluid Mechanics, Munich University of Technology. The 2D flow around a hydrofoil with a circular leading edge and the 3D flow around a hydrofoil with swept leading edge are simulated. The results are compared with experimental data, see part one. The unsteady cavitating flow is characterized by the frequent shedding of vapor clouds caused by the development of a re-entrant jet. The shape of the cavitation zone, the frequency of the bubble cloud shedding and the pressure distributions on the surface of the hydrofoil agree well with the experimental findings. For industrial applications also a steady cavitation model has been developed by modifying the void fraction transport equation of the original model. Steady state simulations with the modified model are performed for the cavitating flow through a centrifugal pump impeller of low specific speed. Simulated head-drop-curves are compared with the measured ones and show a good agreement. The NPSH3%-values for a head-drop of three percent coincide well with the experimental results.
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With the idea of reducing the region of rotating cavitation by cutting back the leading edge of blade alternately and enhancing the stability of alternate blade cavitation, an analysis of steady cavitation and its stability were made for unequal blade cascade. The cavities on longer uncut blades are generally longer but they become shorter in a certain range of inlet cavitation number. This peculiar behavior is explained by an interaction effect of a local flow near cavity closure with the leading edge of the opposing blades. It is also shown that the region with stable cavities can be extended and the onset region of rotating cavitation can be diminished by cutting back the leading edge of blade alternately. In Part II, the effect of the amount of cutback is discussed. [S0098-2202(00)00402-8]
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Closure (psychology)
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Large-Eddy Simulation
Free surface
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