Investigations on cavitation suppression of bionic water-jet impeller

Bionic impeller blades can improve cavitation performance of waterjets. A Special kind of bionic impeller with non-smooth leading edge is designed to improve the NPSH (net positive suction head) performance of waterjets. Based on the cavitation experiment of the waterjet with original impeller, a reliable CFD method using the commercial software StarCCM+ is verified. The detailed flow field of the bionic blades is obtained by simulating the viscous flow field. The stream line, pressure distribution and vortex reveal the mechanism of bionic blades flow adjustment. The influence of bionic impeller factors, which are the amplitude, the interval and the phase, on cavitation performance is systematically studied. The area and volume of the blade cavitation pattern is used to characterize the blade cavitation performance. An efficient method of cavitation pattern recognition is proposed to determine the information of the blade cavitation pattern accurately. A feasible cavitation-adjustment method is proposed to improve the hydraulic performance by optimizing the cavitation distribution and strength.


Introduction
The ability of the humpback whale to swim and steer flexibly and quickly at high speeds is attributed to the irregular bumps on the leading edge of its flippers, which can create vortices to regulate flow.In addition to delaying the wing stall and widening the wing's operating range, these bumps can also improve the air bubble performance and enhance the air bubble performance.Inspired by the humpback whales' flipper leading edge, many scholars have carried out research on the bionic wave leading edge of hydrofoils and impellers.
At the end of the last century, Fish F E and other scholars [1], inspired by the humpback whale flipper leading edge convex knot, carried out experimental and computational studies on the lift drag performance of hydrofoils with a convex leading edge.It was hypothesized that the convex junction could control the flow field around the flipper limb and may excite a series of vortices that inhibit boundary layer separation to ensure the hydrodynamic performance of the wing.Subsequently, Watts P and Fish F E [2] used a three-dimensional surface element method to explore the enhancement of hydrofoil performance by convex junctions, which could improve the airfoil performance and extend the operating range with a lift-to-drag ratio increase of 17.6% compared to the prototype hydrofoil at the appropriate angle of attack.On the other hand, Miklosovic D S [3], Johari [4] and other scholars have investigated the effect of different bionic leading edge parameters (wavelength, amplitude, etc.) on the performance of hydrofoils by using experimental facilities such as wind tunnels and water tunnels.Weber P W [5] used the bionic leading edge in rudders for regulating the position of the cavitation emergence and adjusting the position of the cavitation primordium to the trough region.Wei et al. [6] investigated the control of flow separation by bionic leading edge at low Reynolds number by experimental methods.Many scholars [7~9] attribute the bionic leading edge blade flow mechanics to the tubercles as a vortex generator, which can generate high intensity flow direction vortices.These vortices can suppressed flow diversion thus inhibit the cavitation inception and delay the thrust breakdown.Guo Chunyu et al. [10] carried out experimental and numerical computational studies on seven types of hydrofoils with different bionic leading edge parameters based on NACA 0020 hydrofoils, and obtained the laws of lift drag performance and hydrofoil parameters.Chen Liu et al [11] investigated the trend of the evolution of the convex-knot vortex series based on NACA 63(4)-021 hydrofoils by numerical computational methods, and clarified the effect of the convex-knot flow results on the chordal and spreading development of the hydrofoil cavitation.Stark et al. [12] investigate the leading-edge tubercle application on ducted marine propeller blades and find that the tubercles can successfully reduce the sheet cavitation development by a maximum of 50%, while enhancing the total thrust coefficient in all operating conditions by a maximum of 10%.Sun S [13] applied the bionic leading edge blade to propeller to investigate the cavitating wake dynamic based on DES numerical method.The results indicated that the effect of bionic tubercle comes from the formation of the counter-rotating vortex pairs which change the distribution of streamlines on leading edge.
In this paper, a study of a bionic leading edge impeller applied on the axial flow pump was carried out for cavitation suppression and improvement of hydraulic performance under heavy loads.

Research object
The object of this study is a type of axial-flow water jet propeller with a 5-blade impeller and a 9blade guide vane.The impeller outlet conduit diameter D=300mm, radius R=150mm, and the tip clearance is 0.3mm.

Figure 1. Origin waterjet propulsion model.
At the impeller blade leading edge, the bionic leading edge treatment is carried out to superimpose the sine waves with different amplitudes and wavelengths to the original impeller leading edge position to ensure that the centreline of the sine wave overlaps with the impeller leading edge line.The expression of the sine wave is shown in Equation 1, where A is the amplitude and  0 is the phase, andω is the angular frequency, andλ is the wavelength.This ensures that the blade area of the impeller is basically unchanged, and the disk ratio is unchanged, which facilitates the study of the impeller hydrodynamic and cavitation performance under the same load.y = A sin(ω +  0 )  and the rotational speed is 1450 revolutions per minute.The upstream distance from the pump is 9D, and the downstream distance from the pump is 9D.The numerical calculations are performed using a polyhedral mesh with a total mesh count of 13.22 million.Targeted encryption is carried out in the bionic leading edge region of the blade, than to ensure that complex line patterns can be captured, the origin pump rotor region mesh 11.02 million, and the A6λ0.1rbionic leading edge blade rotor region mesh is 11.12 million.Y+ of rotor blades is 30~100.In the numerical calculations, a cavitation model is incorporated by considering the phase change process.Numerical simulation is carried out at the design flow coefficient KQ=0.7663.The rotational speed is kept at 1450 rpm.The cavitation and hydraulic performance at 3% reduction in head is calculated and the results are shown in Table 2.It can be seen that the deviation of numerical calculations from the model test is small and can be used for the subsequent study of the performance of the bionic leading edge blade.

Investigations on origin pump performance
Numerical calculation simulation is carried out for the five designed bionic leading edge impeller.The hydraulic performances are analysed under the same wet flow conditions of origin pump.From Table 3, except the A3λ0.2rblade, the efficient of other bionic blades shows little drop comparing with the origin blade, the decreasing magnitude is less than 0.6%.The results of full wet condition show that bionic leading edge impellers basically keep the hydraulic performance.The cavitation and hydraulic performances are analysed under the same flow conditions of origin pump NPSH3.The cavitation pattern of origin pump in the NPSH3 conditions is a large area of uniform continuous distribution.Because of its large cavitation area, the flow is in a poor state of turbulence, resulting in a certain degree of instability of the cavitation, reflected in the form of the individual blade differences.From the vortex distribution of the origin pump, it can be seen that the vortex distribution in the region of the suction surface leading edge is distributed along the radial direction, and there is a discontinuity along the chord direction.
From Table 4, it can be seen that A3λ0.2r, as a large-wavelength mimetic leading edge, has a larger cavitation volume in the impeller region than the origin pump and a lower efficiency than the origin pump.As can be seen from the cavitation region, it fails to optimize the cavitation performance.It can also be seen from the vortex map that it fails to change the characteristics of vortex distribution along the radial direction.

Summary
In this paper, the hydraulic performance of an axial flow pump is investigated.NPSH and the cavitation performance are studied by numerical calculations and model test methods.The effects of five different types of bionic leading edge impellers on NPSH and the cavitation performance are investigated based on validated numerical calculation methods.The following conclusions can be obtained: (1) There is no obvious reduction of hydraulic performance comparing five bionic leading edge blades with origin blades.
(2) Bionic leading edge blades with a wavelength of 0.2R worsened NPSH and cavitation performance.
(3) Both A3λ0.1r and A3λ0.05r can improve NPSH and cavitation performance, and the effect of small wavelength bionic leading edge blade is better.The reason for this effect is that the small wavelength can better regulate the radial vortex series in the leading edge region, thus delaying the generation of the original leading edge vortex series.
(4) The A6λ0.1r has the best NPSH and cavitation performance, and its efficiency is improved by 1.2% compared to the origin blade.Within a certain range, the large amplitude has a neater and more stable leading edge vortex series structure and postpones the original leading edge vortex farther.

Table 1 .
Bionic leading edge blade model.
Figure 2. Definition of bionic leading edge parameters.

Table 2 .
CFD and model test results for the NPSH3 condition of the origin pump.

Table 3 .
Bionic leading edge impeller hydraulic performance of wet condition.

Table 4 .
Bionic leading edge impeller cavitation and hydraulic performance.