Ways to Prevent the Effects of Cavitation on the Centrifugal Pump Wheel of Liquid Rocket Engines

The pumps used in rocket and space systems and complexes have always been subject to high requirements for reliability and uninterrupted operation, because they are sources of energy, on the stability of which the operation of the entire spacecraft as a whole depends. When designing and manufacturing liquid-propellant rocket engines, much attention is paid, in particular, to the main pumping unit for propellant components, which is a turbopump unit. Specialists need to take into account many factors that affect the efficiency and operational reliability of a centrifugal pump, one of which will be an increase in the anti-cavitation qualities of the product.
At high flow rates, the saturated vapor pressure can exceed the static pressure and then the liquid boils, that is, cavitation. Cavitation, or cold boiling, is the occurrence of voids or gaps filled with liquid vapor in the liquid flow in the zone of minimum pressure. [1]
Most of all, a centrifugal pump is subject to an unsafe phenomenon in the section of the working fluid inlet to the impeller blades, where the total fluid pressure is minimal (the pump has not yet transferred the energy to the fluid), and the relative and absolute flow rates are high. When in contact with the blade, increased relative velocities contribute to the appearance of areas of low pressure from the reverse side of the blade, resulting in cavitation. In addition, the uneven field of absolute velocities when approaching the blade forms an additional pressure drop in the jets, where the speed will be above average.[2-3]
Cavitation is detrimental to the normal operation of the pump for several reasons:
1. One of the portions of the volume supplied by the pump is filled with liquid vapor, as a result of which a pressure drop occurs and the flow rate of the supplied working fluid decreases.
2. When the liquid enters the working area, which has steam bags in its mass, the steam condenses into the zone of higher pressures and the contents of the steam bags are filled with liquid at a higher speed (up to 1600-1900 m/s), which contributes to the appearance of hydraulic shock at the moment volume filling. Some part of such blows directed to the focus of the hemisphere of steam volumes located on the surface of the blades also contributes to the erosion destruction of the product. [4]
For the calculation of pumping units, one of the significant tasks is to establish the maximum allowable number of revolutions of the pump with the available inlet pressure and a known flow rate. Based on the condition of cavitation-free operation, it is necessary to find the maximum number of revolutions of the pump, for this we use the Rudnev S.S. formula:
nmax=Cr*(1/√Q)*((Hin-Hs)/10)^(3/4)
where, ccr is the critical cavitation coefficient, which is determined empirically and characterizes the cavitation qualities of the pump, or in other words, the degree of the pump's predisposition to cavitation with a decrease in. Hin — pressure at the inlet to the pump impeller;
Hs — head corresponding to the conditions for the formation of saturated vapors of the liquid.[2]
 
For conventional pumps, skr = 820-1130, but if axial or screw pre-pumps are used in the design, then we will increase the coefficient to 3000-3200, which is one of the main measures to prevent hydrodynamic cavitation. The screw pre-pump is capable of not only increasing the pressure of the liquid, but also, due to its shape, creates a swirl of the flow, which leads to a decrease in the relative velocity of the liquid at the inlet. Another way to eliminate cavitation is possible by pressurizing the tanks with a pressure of 0.2 — 0.6 MPa, so thereby increasing the pressure at the inlet to the pump. In addition, to improve the anti-cavitation properties of pumps, it is necessary to take into account the design features of the shapes (length and number of blades, the use of a double-sided inlet, the use of oversized wheels), as well as take into account the thermodynamic properties of the components used. [5]
Thus, the main causes of the impact of cavitation on the wheel of a centrifugal pump, and the factors of their occurrence in liquid rocket engines are considered. The critical cavitation coefficient for various pumps and its influence on the conditions of cavitation-free operation, as well as methods and devices that increase the cavitation resistance of pumps are analyzed.