Volute Exit Research Articles

Numerical investigation of clocking effect on unsteady fluid flow in vicinity of tongue for centrifugal pump with vaned diffuser

In order to investigate the clocking effect on unsteady flow distortion in the vicinity of the tongue and hydraulic loss in the volute, experiments and numerical simulations were conducted with four different relative angular positions between the vane and tongue. Numerical results were validated against experimental data, which included measurements of pressure pulsation at the tongue and downstream of the tongue. The radial and circumferential velocities near the tongue at the diffuser-volute clearance were depicted by expanding the rotating surface, and comparisons were made under different diffuser positions to reveal the flow distortion affected by the clocking effect. Shear stress rates and total pressure distributions at the volute exit pipe were also depicted to study the flow loss in the volute affected by the flow pattern from the tongue. The findings show that the pressure fluctuation at the tongue and downstream of the tongue is significantly affected by the diffuser mounting position. The amplitude and intensity of pressure pulsation under θ d1 are clearly smaller than those at other diffuser positions. As the vane trailing edge approaches the tongue, the pressure gradient becomes very large, leading to the formation of a vortex at the tongue that promotes crowding effects and reverse flow occurring in the exit pipe. This, in turn, leads to deflection of the main flow in the volute, high velocity gradients, and subsequently large shear stress rates and hydraulic losses. When the diffuser is installed at θ d4, the flow distortion and flow loss are improved.

Secondary flows and losses in radial turbine volutes

Secondary flows had been found in radial turbine volutes in the past, but the flow mechanisms governing their existence are far from clear. This paper describes an analytic and numerical work on the mechanism of the secondary flows, and the effects of volute shape, A/R distribution, and turbine speed on the development and associated losses of the secondary flows in the turbine volutes. The results show that the centrifugal and viscous forces acting on the wall boundary-layer, as well as the diffusion of radial flow in volute’s cross-sections, are the main generating mechanism of secondary flows, but they are worked against by volute discharging. Compared with the rectangular and trapezoidal shape volutes, the circular volute generates stronger secondary flows, resulting in no hydrodynamic benefit over the other two volutes. By using the dissipation function to separate the secondary flow loss from the total loss in the three volutes, it shows for the first time that although the loss from secondary flows alone is small, the flows can push the main flows toward volute side walls thereby increase wall friction loss. Three twin-entry volutes with different aspect ratios of their cross-sections are also studied. The offset of the cross-section center to volute exit affects volute discharging and the aspect ratio impacts on the radial flow in sidewall boundary layers, they therefore influence the secondary flows in the volutes. The slope of A/R curves is found to correlate well to the strength of the secondary flows in the volutes, and the reason behind is explained. Turbine speed’s influence on the secondary flows is largely through flow compressibility, and if the speed change reduces turbine inlet Mach number, it will increase discharge flow angle at the volute exit, and so too the strength of the secondary flows, but the changes will be small.

Application of an extended two-dimensional inverse method in turbine volute design

Nozzleless housings for turbocharger turbines accelerate and guide flow into downstream rotors. Their design affects the aerodynamic performance and reliability of the turbines. Due to the three-dimensional nature of the volute of the housings, turbine housing volute design is largely based on extended 1D theories and trial-and-error method. In this paper, a detailed description of an extended two-dimensional theory for volute design is given, including its numerical implementation. The method is then applied to design a twin-entry turbine housing for a turbocharger turbine under both equal and unequal admissions, to replace a highly optimized, manually designed housing. The results show that the new volute achieves the same turbine aerodynamic performance as the manually optimized volute with greatly reduced design time, and it also generates more uniform rotor inlet condition with lower pressure excitation force. A breakdown of the stage loss shows that the loss in the new volute housing is larger than that in the manual housing due to its smaller overall dimensions, but the losses inside rotor and downstream diffuser are both reduced due to the more uniform volute exit flow. A discussion on the secondary flow in the two volutes is carried out to show how the volute geometry, through influencing the radial discharge from the volute exit, affects this flow. A further discussion on the flow angle jump around the tongue is placed at the end of the paper to show the mechanism of this jump, how to control it in volute design, and to offer a way forward to improve the volute design method.

Experimental Hemodynamics Within the Penn State Fontan Circulatory Assist Device.

For children born with a single functional ventricle, the Fontan operation bypasses the right ventricle by forming a four-way total cavopulmonary connection and adapts the existing ventricle for the systemic circulation. However, upon reaching adulthood, many Fontan patients exhibit low cardiac output and elevated venous pressure, eventually requiring a heart transplantation. Despite efforts in developing a new device or using an existing device for failing Fontan support, there is still no Food and Drug Administration-approved device for subpulmonary support. Penn State University is developing a hydrodynamically levitated Fontan circulatory assist device (FCAD) for bridge-to-transplant or destination therapy. The hemodynamics within the FCAD, at both steady and patient averaged pulsatile conditions for three physiological pump operating conditions, were quantified using particle image velocimetry (PIV) to determine the velocity magnitudes and Reynolds normal and shear stresses within the device. Data were acquired at three planes (0 mm and ±25% of the radius) for the inferior and superior vena cavae inlets and the pulmonary artery outlet. The inlets had a blunt velocity profile that became skewed toward the collecting volute as fluid approached the rotor. At the outlet, regardless of the flow condition, a high-velocity jet exited the volute and moved downstream in a helical pattern. Turbulent stresses observed at the volute exit were influenced by the rotor's rotation. Regardless of inlet conditions, the pump demonstrated advantageous behavior for clinical use with a predictable flow field and a low risk of platelet adhesion and hemolysis based on calculated wall shear rates and turbulent stresses, respectively.

CFD tool application in predicting the behavior of a centrifugal fan designed by one-dimensional theory

Computational fluid dynamics (CFD) is the most current technology in the fluid flow study. Experimental methods for predicting the turbomachinery performance involve greater time consumption and financial resources compared to the CFD approach. The purpose of this article is to present the analysis of CFD simulation results in a centrifugal fan. The impeller was calculated using the one-dimensional theory and the volute the principle of constant angular momentum. The ANSYS-CFX software was used for the simulation. The turbulence model adopted was the SST. The simulation provided the characteristic curves, the pressure and velocity distribution, and the static and total pressure values at impeller and volute exit. An analysis of the behavior of the pressure plots, and the loss and recovery of pressure in the volute was performed. The results indicated the characteristic curves, the pressure and velocity distribution were consistent with the turbomachinery theory. The pressure values showed the static pressure at volute exit was smaller than impeller exit for some flow rate. It caused the pressure recovery coefficient negative. This work indicated to be possible design a centrifugal fan applying the one-dimensional theory and optimize it with the CFD tool.

Volute characteristics of centrifugal fan based on dynamic moment correction method

The multi-blade centrifugal fan features an abundance of vanes (>48) and a spiral volute. The flowability and noise characteristics of the centrifugal fan are dependent on the type-line of volute and tongue geometries at the volute exit. The aim of this research was to find a better volute type-line for fan noise reduction without compromising on the performance. First, the viscosity factors of gas were used to modify the type-line shape of the volute by introducing a dynamic moment correction coefficient; a modified volute was then obtained to match the new fan system. Next, the performance of the original fan and retrofit fan were tested. The optimization scheme was verified and the feasibility of the proposed numerical calculation technique was confirmed.

Influence of impeller staggered arrangement on radial force and pressure fluctuation for a double-suction centrifugal pump

Influence of impeller arrangement on performance, radial force, and pressure fluctuation for a double-suction centrifugal pump is investigated. In comparison of the original impeller with staggered angle α of 0°, the maximum improvement of pump efficiency is 2.19% for impeller with α of 30°, and the maximum improvement of pump head is 1.76% for impeller with α of 20°. The radial force vector on impeller is related to the staggered angle, and the phase angle of radial force vector is equal to the staggered angle. The staggered arrangement can effectively reduce the amplitude of radial force fluctuation. In comparison of case 0, the maximum amplitude of radial force fluctuation for case 30 reduces by 15.8%. The dominant frequencies of the radial force fluctuation for α = 0°, 10°, and 20° are fBP, while the dominant frequency for α = 30° is 2 fBP. An empirical formula is proposed to predict the radial fluctuation amplitude at specific staggered angle. For a point of the volute far from the exit volute, in comparison of case 0, the maximum amplitudes of pressure fluctuation for case 10, case 20, and case 30 are reduced by 13.1%, 49.8%, and 93.3%.

Analyses of steady and unsteady flows in a turbocharger’s radial turbine

This study presents the aerodynamic behaviours of a twin-entry radial inflow turbine under steady and transient conditions. The influence of the volute tongue is depicted by a low momentum wake propagating toward the rotor entry, but its effect does not extend beyond a circumferential position of 60°, and more total pressure loss is revealed with respect to the hub side. The transient simulations carried out at different operating conditions and Fast Fourier Transform analysis of static pressure fluctuations induced by the components’ interactions have revealed a space-time periodic behaviour which has been described by a double Fourier decomposition. The flow simulations considering the two sides subject to both non-pulsatile and pulsatile flows conditions have revealed the existing rotor and tongue potential effects and interaction effects the rotor and volute, in addition to the circumferential and spanwise flow non-uniformities at the volute exit, which are more accentuated with a pulsatile flow at inlet. The results of Fast Fourier Transform analysis of temporal pressure fluctuations at the inter-space depict an unsteady behaviour related to the pulsatile frequencies which are characterised by high amplitudes. On the other hand, the spatial pressure fluctuations for the non-pulsatile and pulsatile flows conditions seem to have the same dominant modes since Fast Fourier Transform analysis was carried out at a fixed instant.

Effects of Acoustic Resonance and Volute Geometry on Phase Resonance in a Centrifugal Fan

The effects of acoustic resonance and volute geometry on phase resonance are studied theoretically and experimentally using a centrifugal fan. One dimensional theoretical model is developed taking account of the reflection from the discharge pipe end. It was found that the phase resonance occurs, even with the effects of acoustic resonance, when the rotational speed of rotor-stator interaction pattern agrees with the sound velocity. This was confirmed by experiments with and without a silencer at the discharge pipe exit. The pressure wave measurements showed that there are certain effects of the cross-sectional area change of the volute which is neglected in the one dimensional model. To clarify the effects of area change, experiments were carried out by using a ring volute with a constant area. It was demonstrated that the phase resonance occurs for both interaction modes travelling towards/away from the volute. The amplitude of travelling wave grows towards the volute exit for the modes rotating towards the volute exit, in the same direction as the impeller. However, a standing wave is developed in the volute for the modes rotating away from the volute exit in the opposite direction as the impeller, as a result of the interaction of a growing wave while travelling towards the tongue and a reflected wave away from the tongue.

Numerical investigation of solid-liquid two phase flow in a non-clogging centrifugal pump at off-design conditions

The solid-liquid two-phase flow fields in the non-clogging centrifugal pump with a double-channel impeller have been investigated numerically for the design condition and also off-design conditions, in order to study the solid-liquid two-phase flow pattern and non-clogging mechanism in non-clogging centrifugal pumps. The main conclusions include: The sand volume fraction distribution is extremely inhomogeneous in the whole flow channel of the pump at off-design conditions. In the impeller, particles mainly flow along the pressure surface and hub; In the volute, particles mainly accumulate in the region near to the exit of volute, the largest sand volume fraction is observed at the tongue, and a large number of particles collide with volute wall and exit the volute after circling around the volute for several times. When the particle diameter increases, particles tend to accumulate on the pressure side of the impeller, and more particles crash with the pressure side of the blade. And larger sand volume fraction gratitude is also observed in the whole flow channel of the pump. With the decrease of the inlet sand volume fraction, particles tend to accumulate on the suction side of the blade. Compared with the particle diameter, the inlet sand volume fraction has less influence on the sand volume fraction gratitude in the whole channel of the pump. At the large flow rate, the minimum and maximum sand volume fraction in the whole flow channel of the model pump tends to be smaller than that at the small flow rate. Thus, it is concluded that the water transportation capacity increases with the flow rate. This research will strengthen people's understanding of the multiphase flow pattern in non-clogging centrifugal pumps, thus provides a theoretical basis for the optimal design of non-clogging centrifugal pumps.

Performance Investigation of Bell Mouth of the Multi-Blade Centrifugal Fan in Floor Standing Air-Conditioner

Three types of bell mouth with the inward bell mouths whose exit diameter are larger than the impeller inner diameter of multi-blade centrifugal fan used in floor standing air-conditioner are investigated by using the numerical simulation method. Three bell mouths are as follows: The first type of bell mouth is the inward inlet used in the original air-conditioner. The second type is the inward inlet whose axial height is larger than the first type. The third type is the new mold line inward inlet with smaller slope angle than the first type. Calculation results show that the later two type bell mouths can help to improve air volume flow ratio, reduce the vortex in the back of volute exit and increase the air volume flow rate of 6.8 % and 4.2% respectively. The experimental verification was conducted for the simulation results and the experimental results shows that the air volume rates increase 4.7 % and 3% respectively, the level of noise is constant. The entire performances of the two types are both improved.

Impeller Design of a Centrifugal Fan with Blade Optimization

A method is presented for redesigning a centrifugal impeller and its inlet duct. The double-discharge volute casing is a structural constraint and is maintained for its shape. The redesign effort was geared towards meeting the design volute exit pressure while reducing the power required to operate the fan. Given the high performance of the baseline impeller, the redesign adopted a high-fidelity CFD-based computational approach capable of accounting for all aerodynamic losses. The present effort utilized a numerical optimization with experiential steering techniques to redesign the fan blades, inlet duct, and shroud of the impeller. The resulting flow path modifications not only met the pressure requirement, but also reduced the fan power by 8.8% over the baseline. A refined CFD assessment of the impeller/volute coupling and the gap between the stationary duct and the rotating shroud revealed a reduction in efficiency due to the volute and the gap. The calculations verified that the new impeller matches better with the original volute. Model-fan measured data was used to validate CFD predictions and impeller design goals. The CFD results further demonstrate a Reynolds-number effect between the model- and full-scale fans.

Influence of the volute design parameters on the performance of a centrifugal compressor of an aircraft turbocharger

Theoretical and experimental studies on the effect of the volute design parameters on the centrifugal compressor range of stable operation and pressure rise coefficient were investigated. In the theoretical study, the effect of the symmetric and tangent volute area ratios (AR) on the diffuser pressure recovery factor and flow stability was investigated. In the experimental study, different configurations of a symmetrical and tangent (overhung) volutes with different ARs from 0.4 to 0.8 were investigated. Comparisons between the two different types of volute design parameters on compressor stable flow range and pressure rise coefficient were made. Effect of the gab between the diffuser vanes and the volute casing on the compressor stability was investigated. This gap was changed from 0 to 0.4 of the impeller exit width in five tests. The pressure fluctuation leading to stall was observed at the vaneless region. Stall and surge were detected by analysing both the fluctuations of pressure signals and the power spectrum density. The effect of different volute configurations and clearance on stall initiation, and hence compressor stable operation, were discussed. The theoretical results show that the increase in the symmetric volute exit radius increases the pressure recovery factor in the diffuser. Experimental data show that the increase in the AR of symmetric and tangent volutes increases the compressor range of stable operation as well as pressure rise coefficient.

Experimental Investigations on Pressure Fluctuations and Vibration of the Impeller in a Centrifugal Pump with Vaned Diffusers

The pressure fluctuations acting on the impeller and the impeller vibration excited by the fluctuations were investigated in a centrifugal pump with several vaned diffusers. Firstly, the pressures both acting on the impeller and in the volute were measured simultaneously. It was demonstrated that both the volute static pressure and the pressure fluctuations on the impeller are uneven circumferentially (not uniform) under off-design operating conditions. At high flow rates, the pressure fluctuations are large in the front of the volute exit, where the static pressure is low. Secondly, the impeller vibration excited by the fluctuations was measured. It was found that resonance can be excited even when the resonance condition of rotor-stator interaction is not satisfied and sidebands occur in frequency spectra because of the circumferential unevenness of the fluctuations. Resonance can also be excited when a natural frequency of the impeller coincides with the frequencies of sidebands. Furthermore, the vibration wave is a traveling wave in the circumferential direction when resonance of a nodal diameter mode is excited, and the resonant frequency may depend on the traveling direction of the wave due to the inertia effect (added mass) of water.

PIV Measurements of Flow in a Centrifugal Blood Pump: Steady Flow

Magnetically suspended left ventricular assist devices have only one moving part, the impeller. The impeller has absolutely no contact with any of the fixed parts, thus greatly reducing the regions of stagnant or high shear stress that surround a mechanical or fluid bearing. Measurements of the mean flow patterns as well as viscous and turbulent stresses were made in a shaft-driven prototype of a magnetically suspended centrifugal blood pump at several constant flow rates (3-9 L/min) using particle image velocimetry (PIV). The chosen range of flow rates is representative of the range over which the pump may operate while implanted. Measurements on a three-dimensional measurement grid within several regions of the pump, including the inlet, blade passage, exit volute, and diffuser are reported. The measurements are used to identify regions of potential blood damage due to high shear stress and/or stagnation of the blood, both of which have been associated with blood damage within artificial heart valves and diaphragm-type pumps. Levels of turbulence intensity and Reynolds stresses that are comparable to those in artificial heart valves are reported. At the design flow rate (6 L/min), the flow is generally well behaved (no recirculation or stagnant flow) and stress levels are below levels that would be expected to contribute to hemolysis or thrombosis. The flow at both high (9 L/min) and low (3 L/min) flow rates introduces anomalies into the flow, such as recirculation, stagnation, and high stress regions. Levels of viscous and Reynolds shear stresses everywhere within the pump are below reported threshold values for damage to red cells over the entire range of flow rates investigated; however, at both high and low flow rate conditions, the flow field may promote activation of the clotting cascade due to regions of elevated shear stress adjacent to separated or stagnant flow.

PIV Measurements of Flow in a Centrifugal Blood Pump: Time-Varying Flow

Measurements of the time-varying flow in a centrifugal blood pump operating as a left ventricular assist device (LVAD) are presented. This includes changes in both the pump flow rate as a function of the left ventricle contraction and the interaction of the rotating impeller and fixed exit volute. When operating with a pulsing ventricle, the flow rate through the LVAD varies from 0-11 L/min during each cycle of the heartbeat. Phase-averaged measurements of mean velocity and some turbulence statistics within several regions of the pump, including the inlet, blade passage, exit volute, and diffuser, are reported at 20 phases of the cardiac cycle. The transient flow fields are compared to the constant flow rate condition that was reported previously in order to investigate the transient effects within the pump. It is shown that the quasi-steady assumption is a fair treatment of the time varying flow field in all regions of this representative pump, which greatly simplifies the comprehension and modeling of this flow field. The measurements are further interpreted to identify the effects that the transient nature of the flow field will have on blood damage. Although regions of recirculation and stagnant flow exist at some phases of the cardiac cycle, there is no location where flow is stagnant during the entire heartbeat.

A prototype HeartQuest ventricular assist device for particle image velocimetry measurements.

The objective of this study is to fully characterize the flow within the HeartQuest ventricular assist device (VAD), a magnetically levitated centrifugal VAD, using particle image velocimetry (PIV) to identify regions of potential high shear or stagnation and validate and refine computational models of the flow. An acrylic model of the pump was designed and constructed to allow optical access into all interior regions of the pump. The geometry of the exterior housing and the use of a novel working fluid make quantitative measurements of velocity within the exit volute, blade passage, cut-water, blade tip clearance, and pump inlet possible. Highly accurate velocity measurements using particle PIV have been made in one region (the inlet elbow), and measurements in the other critical regions of the pump will be made. These measurements are used for investigation of regions with potential for hemolysis resulting from high shear stress or with potential for thrombosis caused by recirculation or stagnation. Quantitative velocity data are also needed for comparison with computational fluid dynamics (CFD) models of the VAD. In this study, experiments have again proven to be an essential complement to CFD for thorough investigations of the flow inside the pump.

PARTICLE IMAGE VELOCIMETRY WITHIN HEARTQUESTTM LVAD

A study was conducted to investigate the flow field within the clinical version of HeartQuestTM, a magnetically levitated centrifugal LVAD. An acrylic model of the pump was designed and constructed to allow optical access into all interior regions of the pump. The geometry of the exterior housing and the use of a novel working fluid makes quantitative measurements of velocity within the exit volute, blade passage (including leading and trailing edges), cut-water, blade tip clearance and pump inlet possible. Highly accurate velocity measurements using Particle Image Velocimetry (PIV) have been made in several critical regions of the pump. Additionally, the use of stereoscopic PIV, which simultaneously measures all three velocity components, has been demonstrated. These measurements have been used for investigation of regions of high shear stress, areas of recirculation or stagnation and for comparison to and validation of computational fluid dynamics (CFD) models. Similar velocity measurements in the front clearance gap of a previous HeartQuestTM centrifugal pump demonstrated the effectiveness of PIV as a technique. In this study, the experimental model has again proven to be an essential complement to CFD for thorough investigations of the flow inside the pump.

Prediction of the steady and non-steady flow performance of a highly loaded mixed flow turbine

This paper describes a method for predicting the performance under both turbine inlet steady state and non-steady state flow conditions of a mixed flow turbine used for turbocharged internal combustion engines. The mixed flow turbine steady state performances computed with the steady state performance prediction method are in good agreement with the experimental results obtained in the Imperial College turbocompressor cold air test rig. The unsteady state performance is computed using a one-dimensional model based on the solution of the unsteady one-dimensional flow equations. These equations are solved in the volute by a finite difference method using a four-step explicit Runge—Kutta scheme. The instantaneous volute exit condition is provided by the steady state rotor performance prediction model with the assumption of a quasi-steady state flow in the rotor. The computed instantaneous performances are in reasonable agreement with published experimental data for the same mixed flow turbine. The unsteady flow model is also used to study the effects of the frequency and the amplitude of the pulse on the performances of the mixed flow turbine.