Raúl Barrio

The Effect of Impeller Cutback on the Fluid-Dynamic Pulsations and Load at the Blade-Passing Frequency in a Centrifugal Pump

A study is presented on the fluid-dynamic pulsations and the corresponding dynamic forces generated in a centrifugal pump with single suction and vaneless volute due to blade-volute interaction. Four impellers with different outlet diameters, obtained from progressive cutbacks (trimmings) of the greatest one, were successively considered in the test pump, so that the radial gap between the impeller and the volute ranged from 8.8% to 23.2% of the impeller radius. The study was based on the numerical computation of the unsteady flow through the machine for a number of flow rates by means of the FLUENT code, solving the 3D unsteady Reynolds-averaged Navier-Stokes equations. Additionally, an experimental series of tests was conducted for the pump with one of the impellers, in order to obtain pressure fluctuation data along the volute front wall that allowed contrasting the numerical predictions. The data collected from the numerical computations were used to estimate the dynamic radial forces and torque at the blade-passing frequency, as a function of flow rate and blade-tongue radial gap. As expected, for a given impeller diameter, the dynamic load increases for off-design conditions, especially for the low range of flow rates, whereas the progressive reduction of the impeller-tongue gap brings about corresponding increments in dynamic load. In particular, varying the blade-tongue gap within the limits of this study resulted in multiplying the maximum magnitude of the blade-passing frequency radial force by a factor of about 4 for low flow rates (i.e., below the nominal flow rate) and 3 for high flow rates.

Performance characteristics and internal flow patterns in a reverse-running pump–turbine

Vaneless centrifugal pumps are reversible turbomachines that can operate also as centripetal turbines in low and very low-head power plants. However, the general performance in reverse mode is difficult to predict since the internal flow patterns are different from pump mode and the performance characteristics are not usually provided by manufacturers. This article presents numerical and experimental investigations on the operation of a reverse-running pump–turbine. The numerical calculations were carried out by solving the full unsteady Reynolds-averaged Navier–Stokes equations with the commercial code Fluent for several flowrates between 20 per cent and 160 per cent of rated conditions and both modes of operation. A complementary series of experimental measurements were performed in a test rig in order to obtain the general characteristics of the machine in pump and turbine modes, with the purpose of validating the numerical predictions. Once validated, the numerical model was used to investigate the flow patterns at some significant locations by means of pressure and velocity contours, and also by vector maps. Additionally, the model allowed the estimation of the steady load on the impeller as a function of flowrate in both modes of operation. It was concluded that, while the radial load in reverse mode is three times smaller than in pump mode, the axial load can be up to 1.6 times larger.

Numerical investigation of a centrifugal pump running in reverse mode

Abstract This article reports a work on the three-dimensional flow simulation in a centrifugal pump operating in reverse mode. The simulations were carried out with the commercial code Fluent using unsteady flow calculations together with a sliding mesh technique. Hence, it was possible to account for the effect of blade—tongue interactions on the local flow. The numerical predictions were compared with the experimentally determined performance characteristics and also with the static pressure distribution obtained around the periphery of the impeller. Once validated, the numerical model was used to investigate the global flow. Additionally, the total radial force (steady and unsteady components) on the impeller for a number of flowrates was estimated. It was found that the unsteady radial force (peak to peak) varied between 24 and 54.3 per cent of the steady value within the considered flow interval. The maximum force amplitude was reached when the trailing edge of one blade (pressure side) was located 3° downstream of the tongue tip.

Studies of the flow of air in a mixed-flow pump using numerical simulations

This article presents an investigation of the three-dimensional (3D) turbulent flow through the impeller passages and surroundings of a mixed-flow pump. Rotating passages of turbomachinery contain some very interesting and complex fluid flow phenomena. The model tested has five impeller blades mounted on a conical hub and nine stator blades in a diffuser which brings the diagonally outward flow back to the axial direction. Numerical calculations of the unsteady flow were carried out with the code Fluent, using air as the working fluid. Three models have been applied for turbulence closure: standard k-epsilon, renormalization group k-epsilon, and Reynolds stress model, using conventional wall functions near solid surfaces. For this transient 3D computation, the numerical grid has been decomposed into eight separate regions in order to process these in a parallel cluster of desktop computers. The results obtained show entirely reasonable correlations with previously published experimental data, as detailed in the performance curve comparisons and also in the numerical and experimental flow fields. These outcomes confirm that such a complex transient phenomenon may be reasonably captured by means of a commercial computational fluid dynamics code.

The use of computational fluid dynamics to estimate fluid residence time and flow hydrodynamics in open digesters of wastewater treatment plants: a case study

AbstractResidence time is an important parameter in mixing systems. Particularly, the performance of the processes in the digesters of wastewater treatment plants is influenced by the residence time distribution. This can be obtained experimentally by injecting inert chemicals to measure their concentration at the outlet. An alternative approach is to simulate the flow field with computational fluid dynamics (CFD) software. This paper explores a methodology to compute fluid residence time and investigate flow hydrodynamics in open digesters of full-scale wastewater treatment plants by CFD calculations through a case study. The methodology is based on the resolution of an additional transport equation for a separate species. The concentration of this species at the outlet of the digesters is monitored and can be related with the residence time of the fluid. Furthermore, the resolution of the unsteady flow field provides a complete set of data that was used to detect stagnation or bypass regions.

Prediction of pump–circuit interactions by computational fluid dynamics calculations coupled with a one-dimensional acoustic model

Centrifugal pumps generate perturbations due to the intermittent interaction between the blades and the volute that represent a dynamic load which limit the pump’s performance. Their magnitude depends on the acoustic coupling between pump and piping, and so there is interest in reducing the blade-passing frequency excitation by modification of the acoustic impedance of the piping. The objective of this work is to investigate the pressure pulsations predicted in a three-dimensional numerical model of a commercial pump under different coupling conditions. For this purpose, an expression for the acoustic impedance in a basic arrangement is deduced first. This impedance is imposed at the exit boundary of the model by means of an external user-defined function to characterize the impulse pipeline. Simulations are carried out from part flow to overflow and a range of impedances. The predictions for a specific flow rate are compared with experimental measurements of the blade-passing frequency amplitude at the exit of the pump. It is observed that the largest amplitudes among the coupling conditions tested reach relative values of about 11%.

A Methodology for Geometry Generation of the Lower Conductive Zone of the Lung Airways and Simulation by Intermediate Boundary Conditions

This paper presents a general methodology for the development and simulation of a human lung between scales 0–16. The methodology is based on the simulation of only one of the two possible branches at each bronchiole. The operation of the truncated branches is included by means of a user-defined function. This function prescribes the velocity profile calculated for the active branches in the truncated ones in order to make the hydraulic losses equal between them. This procedure was tested between 0 and 7th generation by imposing the time profile of a real forced spirometry test in the trachea as boundary condition. The test showed a very good agreement between the numerical predictions and the spirometry data.Copyright

Experimental and Numerical Investigation of a Centrifugal Pump Working as a Turbine

An experimental and numerical investigation of a conventional centrifugal pump working as a turbine is presented. The numerical simulations were performed with the code Fluent by means of unsteady flow calculations and a sliding mesh technique to account for the impeller-volute interactions. Thus, it was possible to properly simulate the effect on the local flow of the passage of the impeller blades in front of the volute tongue. The numerical results were compared with the experimentally determined performance curves and additionally with the static pressure distribution measured around the impeller periphery. Once validated, the model was used to estimate the steady and unsteady radial forces on the impeller for a number of flow rates. The steady radial force was also experimentally estimated from the static pressure measurements around the periphery of the impeller. The numerical predictions showed that, for the flow interval considered in the present investigation, the unsteady radial force varied between 24% and 54.3% of the steady magnitude, and that its maximum amplitude was reached when the trailing edge of one of the blades was located 3 deg downstream the tip of the tongue.Copyright

Fluid-Dynamic Pulsations and Radial Forces in a Centrifugal Pump With Different Impeller Diameters

A study is presented on the numerical computation of the unsteady flow through a single suction and single volute centrifugal pump equipped with three impellers of different outlet diameter. Computations were performed by means of the Fluent code, solving the 3D URANS equations. The study was focused on the effect of varying the impeller-volute radial gap on the flow perturbations associated to the fluid-dynamic blade-tongue interaction. In order to contrast the numerical predictions, an experimental series of tests was conducted for the pump with the bigger impeller, to obtain pressure fluctuation data along the volute front wall. Finally, the results from the numerical simulations were used to compute the radial forces at the blade passing frequency, as a function of flow-rate and blade-tongue radial gap.© 2005 ASME

Equivalent Electrical Model and Software Tool for SPICE-Compatible Thermal Simulations of High-Power Resistors

Optimized design of high-power braking resistors for traction applications is a complex task. The value of the power dissipated can be very high (up to the order of megawatts) and normally pulsating. In addition, these resistors should stay in predefined, usually small areas on the train, which makes them have to meet some tough specifications: minimum weight, high resistance to vibration, and high reliability and durability. Since it is necessary to anticipate and establish the distribution and evolution of the temperatures reached in different internal areas, it would be necessary to develop a complete numerical simulation using a software tool based on computational fluid dynamics (CFD) for each design, which implies a very high-computational cost. In this paper, a new equivalent electrical circuit is proposed to model the thermal behavior of this kind of resistors. The electrical equivalent circuit parameters are obtained by a newly developed software tool that collects results and trends obtained from a large number of previous CFD simulations. After that, SPICE can be employed to obtain thermal simulations and predict temperature evolutions.