Vaneless Volute Research Articles

Performance Investigation of Stator-Less and Blade-Less Radial Expander

Abstract Interests in small-scale turbomachinery are gaining momentum, particularly around waste heat recovery using Organic Rankine Cycle (ORC), energy harvesting, pico-hydro, refrigeration and heat pumps and small-scale power generation. These applications demand to have economical, simple construction, and reasonably efficient machines. The performance of bladed turbomachine at a small scale is poor mainly due to viscous losses and relatively large clearances. In some cases, like ORC, it requires a lubrication system, making it complex and costly. Bladeless or Tesla turbomachinery is seen as one of the solutions for these applications due to its simple construction and cost-effectiveness. However, the experimental efficiency of the bladeless turbines/compressors is found in the low region, <40%. In this article, the performance of a bladeless turbine is investigated using a vaneless volute configuration, making the turbine stator-less (vaneless volute) and bladeless (vaneless rotor). This study presents numerical and experimental performance investigation with a volute as a stator of the bladeless rotor. Three-dimensional (3D) Numerical results show very promising performance of the turbine with total to static efficiencies calculated above 65%. In the second part of the article, turbine prototype components, assembly, and test setup are discussed. Experimental maximum efficiency of 41.5 ± 0.88% at 3.5 kg/s@5000 rpm and power of 915 W is obtained. This is the highest recorded efficiency for the Tesla turbine in peer-reviewed research. The overall turbine performance from 3D numerical simulation with ventilation and mechanical losses is compared with experimental results. This work demonstrates that the proposed stator-less/volute configuration provides an efficient way for bladeless or Tesla turbines, particularly for low-head applications.

Study on aerodynamic excitation of radial turbine blades with vaneless volute at low excitation order

High cycle fatigue (HCF) of radial turbine blades is one of key issues of turbochargers and it determines the reliability of the device. Most of researches about HCF of radial turbine blades focus on vaned turbines, whereas few investigations have been carried out on vaneless turbines which are widely applied in automobile turbochargers. This paper studies the blade vibration mechanism of a vaneless radial turbine at low excitation order via one-way fluid–structure interaction simulation method validated by experiments. The results surprisingly demonstrate that blade vibration response does not increase consistently with turbine load. Instead, a non-monotonic correlation is observed between vibration response and turbine load. The energy analysis is employed to understand the phenomenon and the results manifest that the variation of energy imposed on blade suction surface is the direct reason for the non-monotonic correlation. Detailed flow field analysis further suggests that the non-monotonic trend of energy on suction surface is caused by the interaction of two flow structures at different turbine loads: separation vortex at blade leading edge and tip leakage vortex caused by pressure difference between pressure surface/suction surface. The study of blade excitation in this research can enlighten the new design of vaneless turbine with both good performance and high reliability.

Investigation on Interaction between Geometry and Performance and Design of S-CO2 Radial Inflow Turbine’ s Vaneless Inlet Volute

The inlet volute is one of the key components that guide the working fluid into the radial inflow turbine. It has a significant effect on the performance of the radial inflow turbine. The conventional design criteria for the vaneless inlet volute mainly focuses on the ideal gas, which does not apply to real gas cases, such as supercritical carbon dioxide(S-CO2). In this paper, an MW-class S-CO2 radial inflow turbine is studied. Firstly, the inlet volute design of the radial inflow turbine with a vaned stator is completed, and the flow in the whole machine is calculated using CFD simulations. Then, based on the inlet boundary conditions of the impellers, the vaneless inlet volute with different throat areas and cross-sectional area distributions are investigated. The results indicate that the size of the counter rotating vortexes in the vaneless inlet volute increases with the throat area increasing, and is also influenced apparently by the cross-sectional area distribution. The volute throat area has little effect on the circumferential distribution of the outlet airflow angle but changes the average value of the airflow angle. The cross-sectional area distribution of the volute influences the circumferential distribution of the outlet flow angle. The result indicates that volute with a small-scale convex profile has a uniform outlet flow field. Compared to the turbine with the vaned stator, the radial dimension of the vaneless volute turbine obtained in the final design is significantly reduced, and the total-total efficiency at the design point is 85.62%, which meets the design requirements. Moreover, it also has good performance at off-design operating conditions.

Geometry dependence of the acoustic transmission and reflection properties of centrifugal pumps based on an internal lumped parameter model

As a tool to characterize the acoustic properties of conventional centrifugal pumps, consisting in impeller and vaneless volute casing, a new acoustic lumped parameter model has been developed that simulates the acoustic field induced inside the pumps when subject to low-frequency sound sources. The model is based on a network of nodes distributed through the pump, with local transfer matrices connecting pairs of nodes at neighboring regions, and so it is referred to as an internal transfer matrix model. At the present stage the model has been implemented in a procedure to determine the pump passive acoustic properties, either in terms of a transmission matrix or a scattering matrix, though in a future step the model is to be applied for the characterization of the internal sound sources in centrifugal machinery. This paper presents the assumptions of the model and the resolution approach to calculate transmission or scattering matrices, as well as a summary of results for a reference pump that shows the sensitivity of predictions with respect to several geometrical pump parameters. Finally, a contrast is carried out between experimental data recently reported in the literature on the scattering matrices for five pumps of different geometry and the corresponding predictions obtained from (i) the new lumped parameter model based on internal transfer matrices and (ii) a classical pump model based on an electrical system analogy. The results show that the predictions of the new transfer matrix model are always in reasonable quantitative agreement with measurements, and that, as frequency is increased, its prediction capability clearly outperforms that of the electrical analogy model regardless the pump specific speed or the geometrical features.

Investigation of Flow Distortion Generated Forced Response of a Radial Turbine with Vaneless Volute

Abstract For a radial turbine with vaneless volute, the inflow of turbine rotor usually has a circumferential flow distortion due to the influence of the volute tongue. The rotating blades of the rotor are exposed to harmonic aerodynamic loads caused by the distortion, which may induce rotor resonance and lead to high cycle failures (HCF). To understand the forced response mechanism clearly, a numerical analysis was carried out based on a fluid structure interaction (FSI) method. The pressure functions were extracted from the results of a computational fluid dynamics (CFD) analysis by Fourier decomposition. The first three harmonic pressures were identified as the primary engine order (EO) excitations and imposed on the structural model for computational structural dynamics (CSD) simulation. The quantification and assessment of the rotor response were attained by mode superposition method. The simulation results are shown to be consistent with the predictions of Singh’s advanced frequency evaluation (SAFE) diagram.

Rotor-Alone Tones in the Outflow Noise of a Centrifugal Compressor

Abstract This article investigates the generation of rotor-alone tones and their contribution to the outflow noise of a transonic centrifugal compressor stage with vaneless diffuser and volute by means of unsteady full-annulus computational fluid dynamics (CFD) simulations. The aerodynamic field and the generation and propagation of sound were simulated simultaneously using the unsteady Reynolds-averaged Navier–Stokes (URANS) approach of the solver trace and a numerical grid consisting of 170 M cells. To assess the accuracy of the predicted fluctuations, the investigation compares the simulated diffuser flow field to measured flow angles and pressure fluctuations obtained from experiments conducted on a large-scale test rig. The analysis explains the different sound generation mechanisms responsible for tonal components in the acoustic spectrum at the compressor outlet based on the Fourier decomposition of the pressure fluctuations in diffuser and volute. Furthermore, this article analyzes the modal structure of the simulated sound field at the volute outlet by means of a radial mode analysis and discusses the influence of changing operating conditions on the sound power emitted. The analyses reveal that supersonic flow phenomena occurring at choked operating conditions cause a significant increase in noise emissions. Furthermore, the investigation shows that the sound field at the volute outlet is dominated by few low-order modes, a fact that justifies the analysis using methods based on the compressed sensing in future experimental investigations.

A/R Effects on Asymmetric Twin-Entry Radial Turbine Performances Under Steady and Pulsatil Flow Conditions

The design for better performances of the twin-entry spiral housing volute flow turbines is of prime importance as it affects the stage turbine performance at both design and off design conditions. Thus, attentions have to be paid to both inlet domain and spiral domain since the main roles of the spiral domain are to collect gases to distribute it properly all around the periphery of the rotor in the case of vaneless volute and around the stator in the case of vaned volute. The gases have to be delivering in an appropriate flow direction dictated by optimum turbine efficiency. This paper investigates numerically the effect of A/R ration on the flow characteristic in a twin-entry turbine. The flow in the spiral part of volute is characterized by the secondary flows in the form of counter-rotating vortices and the Dean vortices formed practically within the hub side of the volute. The swirling flow is convected to the rotor entry from the outer wall and leaves the volute before an azimuthal position of 180 deg from the tongue. For a turbine in which the global ratio A/R was keep similar to reference one, whereas the ratio A/R related to hub-entry side is chosen greater than the shroud side. The flow structure within this turbine highlights that the secondary flow disappears. In pulsating flow condition and at the rotor entry there is a typical dissymmetrical behavior characterized by high non-uniformities in the spanwise and circumferential directions. Also, at a position around 180° from tongue, the relative flow angle is highly influenced by the migration of the two vortices from the hub side of volute to the interspaces. The finding of this paper could lead to improvement of such volute type.

Design and numerical simulation on coupled flow field of radial turbine with air-inlet volute

As one of the core components of turbocharger or micro-turbine, radial turbine has the features of small size and high rotation speed. In order to explore the design method and flow mechanism of the turbine with a volute, a centimeter-scale radial turbine with a vaneless air-inlet volute was designed and simulated numerically to investigate the characteristics of the coupled flow field. The results show that the wheel efficiency of single passage computation without the volute is 80.1%. After accounting for the factors of the loss caused by the volute and the interaction between each passage, the performance is more accurate according to the whole flow passage computation with the volute. High load region gathers at the mid-span and the efficiency declines to 76.6%. The performance of the volute whose structure angle of the trapezoid section is equal to 70 degree is better. Unlike uniform inlet condition in single passage, more appropriate inlet flow for the impeller is provided by the rectification effect of the volute in full passage calculation. Flow parameters are distributed more evenly along the blade span and are generally consistent between each passage at the outlet of the turbine.

The flow field in a centrifugal pump with a large tongue gap and back blades

In this paper, the unsteady 3-D turbulent flow fields in an end-suction centrifugal pump with a vaneless volute are studied numerically at three different rotation speeds. The unshrouded impeller of the pump, as a reduced diameter version of the original full-size impeller, has five backswept blades and five pump-out blades. The results show that the flow rate and pressure vary with the rotational speed according to the similarity law, and the normalized internal flow fields at all speeds are very similar. Due to the reduced impeller diameter, the pump efficiency is relatively low. Flows discharged from the front and back blade passages interfere with each other and swirling motion exists in the volute. The pressure fluctuations on the casing wall near the impeller outlet are extracted, and their spectra show that the component at the blade passing frequency was prominent. This study sheds some light on the characteristics of the internal unsteady flow of a centrifugal pump at different rotational speeds, and forms a basis for future study of flow induced pump vibration and noise.

PIV measurements of the unsteady flow structures in a volute centrifugal pump at a high flow rate

An experimental investigation based on PIV measurements is presented on the unsteady flow in a centrifugal pump with vaneless volute. The study has focused on the fluid–dynamic interaction between impeller and volute associated with the blade passage period when the pump operates well above design conditions (150 % of nominal flow rate). The test pump, which is fully transparent, has a 2D-shaped geometry and the impeller has 6 arc-shaped backward-curved blades. The flow in the mid-plane perpendicular to the pump axis was captured using fluorescent seeding particles and two fields of view (FOV), each with different magnification: a large FOV (low magnification) that covers a large portion of the impeller, volute and discharge duct, and a small FOV (high magnification) to observe the details of flow evolution at the tongue region. Results are presented on the in-plane phase-averaged relative and absolute velocities, turbulent kinetic energy, its production and vorticity fields. Their evolution over time during one blade passage is described, including a frequency analysis based on 32 blade positions. The spatial distribution of the spectral components at the blade-passing frequency and some harmonics reveals that the second harmonic is dominant in the narrow region of the volute at the tongue tip. The data obtained show that the fluid–dynamic blade–tongue interaction is dominated by high-vorticity sheets (positive and negative) being shed from the impeller channels, especially from the blade trailing edges, and their impingement on the tongue tip with subsequent cutting and distortion. The turbulence production is seen to be mainly concentrated in the wake regions from the blade leading and trailing edges and from the tongue tip. In particular, it gets maximum behind the blade trailing edge when the blade aligns with the tongue tip.

The CFD Analysis of Internal Flow Field in Turbocharger Compressor

In this paper, the 3D flow analysis model of gasoline engine turbocharger compressor was built by using the software NUMECA. The flow fields of the vaneless diffuser and volute, such as airflow velocity field, temperature field, pressure field and the entropy field were simulated. The internal flow performance of the vaneless diffuser and volute were analyzed. The simulation results show that the field changes accord with the compressor characteristics, thus the vaneless diffuser and volute of the compressor design is reasonable. The approach of numerical simulation and flow field analysis by using CFD method can accurately predict the compressor performance. The research methods and conclusions provide theoretical and practical reference for the optimization design of the compressor.

Experimental and numerical investigations of radial flow compressor component losses

This research numerically and experimentally investigates a small turbocharger radial flow compressor with a vane-less diffuser and volute. The geometry of the compressor is obtained via component scanning, through which a 3D model is prepared. The flow inside this model is numerically analyzed by using a Navier-Stokes solver with a shear-stress transport turbulence model. The characteristic curves of the compressor and the contributions of its components to total pressure drop are acquired by measuring the static and total pressures at different cross sections of the compressor. Numerical results are verified with the experimental test results. The model results exhibit good agreement with the experimental results. In particular, the results show that the losses related to the impeller are higher than those related to the stationary components at different conditions, with the former causing a decline of at least 15% in compressor isentropic efficiency. The contribution of stationary components to efficiency decrease is approximately 4.8% at maximum efficiency mass flow rate and is limited to 7.1%. At low mass flow rates, the contribution of the diffuser to efficiency decline is higher than that of the volute. This finding is reversed at high mass flow rates. The performances of the diffuser and the volute are also studied by exploring total pressure and static pressure recovery coefficients, as well as the net radial force on the impeller shaft under a wide operating range.

Design and Numerical Simulation of Radial Inflow Turbine Volute

AbstractThe volute of a radial inflow turbine has to be designed to ensure that the desired rotor inlet conditions like absolute Mach number, flow angle etc. are attained. For the reasonable performance of vaneless volute turbine care has to be taken for reduction in losses at an appropriate flow angle at the rotor inlet, in the direction of volute, whose function is to convert gas energy into kinetic energy and direct the flow towards the rotor inlet at an appropriate flow angle with reduced losses. In literature it was found that the incompressible approaches failed to provide free vortex and uniform flow at rotor inlet for compressible flow regimes. So, this paper describes a non-dimensional design procedure for a vaneless turbine volute for compressible flow regime and investigates design parameters, such as the distribution of area ratio and radius ratio as a function of azimuth angle. The nondimensional design is converted in dimensional form for three different volute cross sections. A commercial computational fluid dynamics code is used to develop numerical models of three different volute cross sections. From the numerical models, losses generation in the different volutes are identified and compared. The maximum pressure loss coefficient for Trapezoidal cross section is 0.1075, for Bezier-trapezoidal cross section is 0.0677 and for circular cross section is 0.0438 near tongue region, which suggested that the circular cross section will give a better efficiency than other types of volute cross sections.

A Comparison of the Flow Structures and Losses Within Vaned and Vaneless Stators for Radial Turbines

This paper details the numerical analysis of different vaned and vaneless radial inflow turbine stators. Selected results are presented from a test program carried out to determine performance differences between the radial turbines with vaned stators and vaneless volutes under the same operating conditions. A commercial computational fluid dynamics code was used to develop numerical models of each of the turbine configurations, which were validated using the experimental results. From the numerical models, areas of loss generation in the different stators were identified and compared, and the stator losses were quantified. Predictions showed the vaneless turbine stators to incur lower losses than the corresponding vaned stator at matching operating conditions, in line with the trends in measured performance. Flow conditions at rotor inlet were studied and validated with internal static pressure measurements so as to judge the levels of circumferential nonuniformity for each stator design. In each case, the vaneless volutes were found to deliver a higher level of uniformity in the rotor inlet pressure field.

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.

A Direct Performance Comparison of Vaned and Vaneless Stators for Radial Turbines

An extensive performance investigation has been conducted on a radial turbine with three different vaneless volutes and three corresponding vaned stators. Previously published comparisons have been based on turbines with unmatched flow rates, meaning that the impact of stator losses was not isolated from rotor and exit losses. Each vaned stator configuration tested in this investigation matched the flow rate of the corresponding vaneless volute to within 1%. The volutes and the vaned stators were all machined in order to achieve high quality and comparable surface finishes. At all operating conditions, the vaneless volutes were shown to deliver a significant efficiency advantage over the vaned stators. However, the vaneless volute turbines did not demonstrate any greater tolerance for off-design operating conditions than the vaned stator configurations. Full performance data are presented for the six different turbine configurations tested and a one-dimensional turbine performance model is evaluated as a means of predicting and extrapolating turbine performance.

Aerodynamically Induced Radial Forces in a Centrifugal Gas Compressor: Part 1—Experimental Measurement

Net radial loading arising from asymmetric pressure fields in the volutes of centrifugal pumps during off-design operation is well known and has been studied extensively. In order to achieve a marked improvement in overall efficiency in centrifugal gas compressors, vaneless volute diffusers are matched to specific impellers to yield improved performance over a wide application envelope. As observed in centrifugal pumps, nonuniform pressure distributions that develop during operation above and below the design flow create static radial loads on the rotor. In order to characterize these radial forces, a novel experimental measurement and post-processing techniquesis employed that yields both the magnitude and direction of the load by measuring the shaft centerline locus in the tilt-pad bearings. The method is applicable to any turbomachinery operating on fluid film radial bearings equipped with proximity probes. The forces are found to be a maximum near surge and increase with higher pressures and speeds. The results are nondimensionalized, allowing the radial loading for different operating conditions to be predicted.

Design and Performance of Vaneless Volutes for Radial Inflow Turbines: Part 3: Experimental Investigation of the Internal Flow Structure

An experimental study is presented which considers the flow structure in the vaneless volute of a radial inflow turbine. The data show evidence of significant secondary flows around the volute. The volute tested generated a uniform ‘free vortex’ type structure and produced a uniform mass flow distribution around the periphery of the rotor. The flow within the volute was turbulent although this did not appear to have a detrimental effect on the overall performance of the volute. However, the turbine performance appears to be insensitive to variations in the inlet conditions in the axial direction.

Design and Performance of Vaneless Volutes for Radial Inflow Turbines: Part 2: Experimental Investigation of the Mean Line Performance—Assessment of Empirical Design Parameters

The non-dimensional design procedure described in Part 1 assumed a one-dimensional compressible flow and as such relied on the empirical specification of the dissipation of angular momentum, the dissipation of energy and the deviation of the swirling flow from that of a free vortex. This was based largely on data available in the published literature. In order to develop and substantiate the empirical procedures further an experimental study was carried out in parallel with the development of the design procedure. The experimental investigation described here had the specific objective of assessing and developing appropriate empirical models and coefficients for application to the design procedure. The study concentrated on the application of a five-hole probe located in the centre of the volute passage and at a series of azimuth angles around the discharge. The probe provided stagnation and static pressure measurements, from which the gas velocity was derived, together with the flow angle. When taking measurements at the volute discharge a stationary dummy rotor was used to carry the five-hole probe.

Design and Performance of Vaneless Volutes for Radial Inflow Turbines: Part 1: Non-Dimensional Conceptual Design Considerations

The requirements for the volute of a radial inflow turbine are that it should collect the working fluid, deliver it to the turbine rotor as efficiently as possible and provide the desired rotor inlet conditions. The design requirements of the turbine leads to the rotor design and the identification of the desired flow conditions at rotor inlet in terms of the magnitude and direction of the absolute Mach number. The volute must then be designed to ensure that the desired rotor inlet conditions are attained. A non-dimensional design procedure for a vaneless turbine volute is described. Based on a knowledge of the flow direction and magnitude of the absolute Mach number at rotor inlet the overall dimensions of the volute in terms of the radius ratio and flow area ratio are first established. The overall design is then developed to provide the variation of the volute centroid radius and area ratio with azimuth angle. A trapezoidal cross-sectional shape is then used to establish the outer dimensions of the volute. The non-dimensional procedure assumes a one-dimensional compressible flow and as such relies on the empirical specification of the dissipation of angular momentum, the dissipation of energy and the deviation of the swirling flow from that of a free vortex. The effect of the uncertainties associated with the empirical data on the volute design geometry is assessed. A complementary experimental investigation to develop and substantiate the required empiricism is presented in Part 2, which follows. As the design procedure is essentially one-dimensional it must be interpreted with a knowledge of the actual three-dimensional flow within a volute passage. Supportive experimental studies will be presented in Part 3 in the next issue.