Alfonso Marcos

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.

Numerical and experimental analysis of the transitional flow across a real stenosis

In this paper, we present a numerical study of the pulsatile transitional flow crossing a severe real stenosis located right in front of the bifurcation between the right subclavian and right common carotid arteries. The simulation allows one to determine relevant features of this subject-specific flow, such as the pressure waves in the right subclavian and right common carotid arteries. We explain the subclavian steal syndrome suffered by the patient in terms of the drastic pressure drop in the right subclavian artery. This pressure drop is caused by both the diverging part of the analyzed stenosis and the reverse flow in the bifurcation induced by another stenosis in the right internal carotid artery.

A new adaptive time step method for unsteady flow simulations in a human lung

Abstract The innovation presented is a method for adaptive time-stepping that allows clustering of time steps in portions of the cycle for which flow variables are rapidly changing, based on the concept of using a uniform step in a relevant dependent variable rather than a uniform step in the independent variable time. A user-defined function was developed to adapt the magnitude of the time step (adaptive time step) to a defined rate of change in inlet velocity. Quantitative comparison indicates that the new adaptive time stepping method significantly improves accuracy for simulations using an equivalent number of time steps per cycle.

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

Computational simulation of aqueous humour dynamics in the presence of a posterior-chamber versus iris-fixed phakic intraocular lens

Purpose To compare aqueous humour (AH) dynamics in the presence of a precrystalline (Implantable Collamer Lens®; ICL) or iris-fixed (Artiflex®) phakic intraocular lens (PIOL). Methods By computational fluid dynamics simulation, AH flow was modelled through a peripheral iridotomy (PI) or central lens hole (both 360 μm) in the presence of an Artiflex or ICL lens, respectively. The impacts of AH flow were then determined in terms of wall shear stress (WSS) produced on the endothelium or crystalline lens. Effects were also modelled for different scenarios of pupil diameter (PD 3.5 or 5.5 mm), ICL vault (100, 350, 800 μm) and number of Artiflex iridotomies (1 or 2) and location (12 or 6 o’clock). Results For a PD of 3.5 mm, AH volumes flowing from the posterior to the anterior chamber were 37.6% of total flow through the lens hole (ICL) and 84.2% through PI (Artiflex). For an enlarged PD (5.5 mm), corresponding values were 10.3% and 81.9% respectively, so PI constitutes a very efficient way of evacuating AH. Central endothelial WSS in Pa was lower for the large vault ICL and the Artiflex (1−03 and 1.1−03 respectively) compared to the PIOL-free eye (1.6−03). Crystalline lens WSS was highest for the lowest vault ICL (1−04). Conclusions AH flow varied according to the presence of a precrystalline or iris-fixed intraocular lens. Endothelial WSS was lower for an implanted ICL with large vault and Artiflex than in the PIOL-free eye, while highest crystalline WSS was recorded for the lowest vault ICL.

Simulation of the human airways using virtual topology tools and meshing optimization

A method is proposed to improve the quality of the three-dimensional airway geometric models using a commercial software, checking the number of elements, meshing time, and aspect ratio and skewness parameters. The use of real and virtual topologies combined with patch-conforming and patch-independent meshing algorithms results in four different models being the best solution the combination of virtual topology and patch-independent algorithm, due to an excellent aspect ratio and skewness of the elements, and minimum meshing time. The result is a reduction in the computational time required for both meshing and simulation due to a smaller number of cells. The use of virtual topologies combined with patch-independent meshing algorithms could be extended in bioengineering because the geometries handling is similar to this case. The method is applied to a healthy person using their computed tomography images. The resulting numerical models are able to simulate correctly a forced spirometry.

Numerical Flow Simulation in a Reciprocating Valveless Micropump With Parallel Geometry Nozzle-Diffuser

This work presents a numerical study of a nozzle/diffuser micropump under several frequencies of operation. The investigation focuses on the effects caused by two principal changes in the geometry: 1) a small duct portion included before the nozzle/diffuser element, with the purpose of inducing a fully developed flow, and 2) the size of the fluctuating chamber. As expected, the net flow rate of a nozzle/diffuser micropump increases with pumping frequency and also with the size of the fluctuating chamber. However, the duct before the nozzle/diffuser element causes more losses and a decrement in the net flow, though inducing a better flowing of the fluid into the nozzle.Copyright

Air Flow Simulation in a Diesel Engine

The intake manifold of a Diesel engine is of significant importance. Several modifications are usually carried out in this system to increase the power generated without modifying the general design of the engine. This work presents a numerical and experimental investigation on the air flow in a commercial Diesel engine. The air average mass flow through the intake manifold was obtained experimentally for the D909 Deutz-Diter Diesel engine. Additionally, a numerical model was created for the test engine. The model reproduces the operating and boundary conditions of the flow and integrates the equations that describe the motion of the fluid. The average mass flow through the engine was obtained from the model and compared with the experimental measurements. The model was used subsequently to investigate the air flow within the engine.Copyright