CFD Numerical Simulation And Experimental Study Of The Pumping Mechanism And Pumping Performance Of Labyrinth Screw Pump

A labyrinth screw pump (LSP) is a super-low specific speed rotational pump with small capacity, high head and a very simple structure, and a special pump urgently needed by oil, chemical, water treatment and other industries. However, its pumping mechanism has not been fully understood and a study of its pumping performance has still been in a primary experimental stage. The basis for the pump design and application is a lot of unanalyzed and untreated theoretically experimental data. So there exist many uncertainties in the design of LSP, which has greatly limited its wide application.For above reasons, the fluid quasi-steady flow in the pump is approximately modeled by the shear-stress transport k-ωmodel (SST k-ωmodel) in multiple reference frame (MRF) of CFD. Based on CFD numerical simulation, two-dimension turbulence equations and Prandtl's mixing length theory, the qualitative and quantitative analysis is carried out for the pumping mechanism and pumping performance, as well as the fluid flow in the pump. The triangular thread shape and its geometric dimensions are optimized, and the pump performances are converted similarly. At last, the changing tendency of the pump characteristics under the effects of different factors is verified by experiments.The flow field in the pump simulated by CFD shows that the static pressure distributes linearly along the pump axis. The intense centrifugal flow and centripetal flow coexist on the nominal clearance interface between the screw and the sleeve and increase with an increase in the pump pressure. The tangential velocity in the screw grooves is opposite to that in the sleeve grooves, both of which increase with the increasing pressure. As a result, an intense shear zone is formed in the screw-sleeve clearance. The leakage flow velocity in the sleeve grooves is also opposite to that of the pumping flow in the screw grooves. With an increase in pressure, the leakage flow velocity increases but the driven flow velocity in the screw grooves decrease. Taking into account of the above CFD simulation and the pump performance experimental results, it is proposed that the high pressure for LSP is established by the fluid elements obtaining energy from the cell again and again while flowing along the axial direction. The intense turbulent friction at a nominal clearance interface is an essential motive force to transfer momentum and energy of fluid between fluid elements in a cell. Based on two-dimension turbulence stress equations, an empirical equation is deduced to quantify the relation between the pumping performance of LSP and its several key parameters. Consequently, two innovative concepts, a cell head coefficient K_f reflecting the hydraulic performance of a cell and a pump total head coefficient K_b indicating the relative level of the pump head for pumps having the same main dimensions, are defined as criterions for LSP design and selection.Simplifying the fluid flow in the pump as a linear superposition of a Couette flow dragged by the rotor screw and a pressure flow driven by the differential pressure of LSP, and combining the two-dimension turbulence theory and CFD numerical simulation, a model for the flow velocity distribution in the pump is established and it's different from that of Bilgen. The tangential pressure flow velocity is an odd function of the parameter y, and the axial Couette flow velocity distribution along the radial direction in the screw grooves is a similar parabola distribution. Integrating the flow velocity along the depth of the thread grooves, a capacity-head equation for the pump is derived.Based on the fluid flow in the pump and the pumping mechanism, the pump power consumption is considered to be a sum of a power for the screw to drive fluid in thread grooves to flow, a wall friction loss, a cell turbulent friction loss for fluid separately in the screw thread grooves and the sleeve thread grooves to mix each other, fluid kinetic energy and a mechanical friction loss at shaft bearings and seals. So the pump efficiency is a product of a volumetric efficiency, a hydraulic efficiency and a mechanical efficiency. The experimental study for the pump performance indicates that the pump hydraulic efficiency decreases obviously while the volumetric efficiency keeps almost invariant when the screw-sleeve clearance becomes wide.Taking the maximum cell head coefficient K_f as an optimal target, and using the CFD numerical simulation and Golubiev's experimental data, some optimal parameters including the relative depth of thread groove, the thread tooth angle and lead are optimized for the triangular thread. According to the equation quantifying the relation between the pump performance and the thread geometric parameters, some empirical relations between K_f and the screw lead, the screw-sleeve relative diametrical clearance and the relative depth of thread groove are fitted respectively. Also, the effect of a centrifugal force on K_f is qualitatively analyzed. The results show that with an increase in screw lead, K_f increases while K_b decreases. K_f decreases with an increase in screw-sleeve clearance and the degree for K_f to decrease varies with different shape of thread. K_f decreases with an increase in the relative depth of the thread groove over a wide range. When the centrifugal force becomes large, K_f increases first and then decreases.On the basis of the fluid flow similarity principle and the pumping mechanism, some empirical equations are obtained respectively for similar conversion of LSP characteristics after little changes of screw-sleeve clearance, thread lead, screw diameter and screw rotational speed. Using the conversion equations to convert the simulated and experimental performance for the pump, the converted performance is in agreement with CFD simulated and experimental results. At last, the pump performance experiments are carried out and show that the capacity and head for LSP are far greater than those for visco-pump having the same thread parameters as LSP. With an decrease in the thread lead, the pump flow rate decrease whereas its turn off head increase. Consequently, the pump performance curves fall sharply. The smaller the screw -sleeve clearance is, the higher the pump pressure and efficiency are. The tooth tip of the screw thread groove faces toward the rotational direction of the rotor, and this will lead to decrease of the pump capacity and head.