The main goal of the present work is to analyse the numerical simulation of a centrifugal pump by solving Navier-Stokes equations, coupled with the ‘standard k-∊’ turbulence model. The pump consists of an impeller having five curved blades with nine diffuser vanes. The shaft rotates at 890r/min. Flow characteristics are assumed to be stalled in the appropriate region of flowrate levels of 1.31-2.861/s. Numerical analysis techniques are performed on a commercial FLUENT package program assuming steady, incompressible flow conditions with decreasing flowrate. Under stall conditions the flow in the diffuser passage alternates between outward jetting when the low-pass-filtered pressure is high to a reverse flow when the filtered pressure is low. Being below design conditions, there is a consistent high-speed leakage flow in the gap between the impeller and the diffuser from the exit side of the diffuser to the beginning of the volute. Separation of this leakage flow from the diffuser vane causes the onset of stall. As the flowrate decreases both the magnitude of the leakage within the vaneless part of the pump and reverse flow within a stalled diffuser passage increase. As this occurs, the stall-cell size extends from one to two diffuser passages. Comparisons are made with experimental data and show good agreement.
Abstract. Nowadays, single and multistage centrifugal pumps are widely used in industrial and mining enterprises. One of the most important components of a centrifugal pump is the impeller. The performance characteristics are related to the pump comprising the head and the overall efficiency rely a great deal on the impeller geometry. In this work, effects of blade exit angle change on hydraulic efficiency of a multi stage pump impeller are investigated via Ansys-Fluent computational fluid dynamics software for constant width impeller entrance and exit gates, blade numbers and blade thickness. Firstly, the flow volume of a centrifugal pump impeller is generated and then mesh structure is formed for the full impeller flow volume. Secondly, rotational periodic flow model are adopted in order to examine the effect of periodic flow assumption on the performance predictions. Corresponding to the available experimental data, inlet mass flow rate, outlet static pressure and rotation of impeller are taken as 0.02m
The turbulent flow inside a low-speed centrifugal compressor at design condition is investigated using large-eddy simulation (LES) comprising of up to 26×106 computational volume cells. Unlike in the past, the current study’s special emphasis is placed on the turbulence field evolution inside the impeller. LES predictions suggest that the Boussinesq hypothesis does not seem to be valid, especially near the exit of the impeller where the blade unloading takes place. Reynolds stress variations show a tendency toward an “axisymmetric expansion” type of turbulence after the impeller exit for which the subgrid-scale stress contribution shows a monotonic decrease. Probability density function analysis for the leakage flow show that instantaneous velocities in the wake region are less intermittent as compared with those in the jet. Time spectra analysis display also another feature that the energy cascade proceeds at a higher rate and lasts longer in the wake region than in the tip jet region.
SUMMARYThe results of a series of numerical simulations for the NASA low-speed centrifugal compressor are presented. Large-eddy simulation (LES) is carried out together with the most commonly used Reynoldsaveraged Navier-Stokes (RANS) models, including the mixing length, k-and k-models. Predictions are compared with the available experimental data. It is seen that quantitative predictions of LES are better than those of RANS models for the tip jet and wake regions. LES, unlike these RANS models and the past computational fluid dynamics predictions, shows separation near the casing starting around the halfway of the impeller and reveals a vortex bubble close the impeller exit/casing corner. The tip leakage flow, which is the main characteristic of an unshrouded compressor, is investigated using the probability density function (PDF) and the power spectral density (PSD) of the instantaneous velocity values. The skewness and flatness calculated from the PDF profiles show that the unsteady flow in the tip wake region is less-intermittent compared with the flow in the tip jet region. The PSD analysis shows that the turbulent energy transfer rate from the larger scales to the smaller scales and the frequency range where the inertial region is valid increase in the tip wake region.
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