Influence of vaned diffuser on the potential effect of volute and stall inception of centrifugal compressor

In the automotive turbochargers centrifugal compressor, the volute has a strong potential effect, leading to circumferential nonuniformity of the impeller flow field and compressor stall. In this study, full-annulus unsteady simulations for centrifugal compressors with vaned/vaneless diffusers are carried out. The influence of the diffuser vane on the potential effect of the volute and stall behavior of a centrifugal compressor is studied in detail. Based on the distribution of the casing static pressure, the formation mechanism of the circumferential distribution of static pressure and the reverse propagation process of pressure waves caused by pressure distortion are revealed. The results of this study show that the diffuser vanes can weaken the potential effect of the volute on the impeller flow field by reducing the degree of static pressure distortion. The number of static pressure peaks in the circumferential direction is related to the number of vanes/blades. The diffuser vanes can change the circumferential position of the stall inception, but cannot eliminate the “locking effect” of the volute tongue on the circumferential position of the stall. In other words, the circumferential position of the stall inception is still determined by the volute tongue for a centrifugal compressor with a vaned diffuser. Although the degree of circumferential static pressure distortion inside the impeller is reduced, the stable operating range of the compressor cannot be significantly widened by adjusting the stagger angle of the diffuser vane when the stall first occurs at the impeller inlet.

Nonaxisymmetric Study of Tip Leakage Flow in a Centrifugal Compressor With a Volute During Stall Process

Tip leakage flow (TLF) patterns, which affect compressor performance, are closely related to compressor stability. To date, minimal attention has been given to circumferential nonuniformity of the TLF in a centrifugal compressor with a nonaxisymmetric volute structure. In this study, the circumferential difference of the TLF in a centrifugal compressor with a volute during the stall process is analyzed. The circumferential nonuniformity of tip leakage vortex (TLV) trajectories, loading distribution near the tip, and distance between the TLV core and the leading edge (LE) of splitter blades were also investigated. It is shown that in the circumferential direction, there are two peaks associated with the angle (α) between the TLV trajectory of the seven main blades and the axial direction. As the stall process progresses, the blade whose LE is affected by the high static pressure band (PP) induced by the volute tongue (VT) loses its work capacity first and the α difference between this blade and the other blades increases. In addition, the tip loading and TLF velocity of the blade whose LE is affected by the high static pressure band induced by the VT are at a minimum, and the flow loss in the tip clearance is higher. There is a phenomenon of the TLV breakdown. When the blade trailing edge (TE) is located in the low static pressure region, TLV streamlines appear as a significant turn at the breakdown point. However, the TLV streamlines at other circumferential positions do not exhibit this phenomenon.

Explicit Analytical Expression for Solar Flux Distribution on an Undeflected Absorber Tube of Parabolic Trough Concentrator Considering Sun-Shape and Optical Errors

The absorber tube of the parabolic trough receives the concentrated sun-rays only on the portion facing the reflector. It leads to nonuniformity in the temperature of absorber tube. Thus, the material of tube expands differentially and the tube experiences compression and tension in its different parts. It leads to bending of the tube and the glass cover can be broken. The bending can be reduced by (i) reducing the circumferential nonuniformity in absorber's temperature (using material of high thermal conductivity) and (ii) reducing the nonuniformity in solar flux distribution (using appropriate rim angle of trough). In most of the available studies, Monte Carlo Ray Tracing software has been used to calculate the distribution of solar flux and few studies have used analytical approach. In the present work, an explicit analytical expression is derived for finding the distribution of solar flux accounting for the sun-shape and optical errors. Using it, the design calculations can be carried out in significantly lesser time and lesser computational effort. The explicit expression is also useful in validating the results computed by softwares. The methodology has been verified with the already reported results. The effects of optical errors, rim angle, and aperture width of trough on the solar flux distribution and total flux availability for absorber tube have also been studied. From the calculations, it is found that for Schott 2008 PTR70 receiver (absorber tube with 70 mm outer diameter), 126 deg, 135 deg, and 139 deg, respectively, are the appropriate rim angles corresponding to minimum circumferential nonuniformity in solar flux distribution for 3 m, 6 m, and 9 m aperture width of trough. However, 72 deg, 100 deg, and 112 deg, respectively, are the appropriate rim angles corresponding to the maximum solar flux at absorber tube for 3 m, 6 m, and 9 m aperture width of trough. Considering both the circumferential nonuniformity and the total solar flux availability, 100 deg, 120 deg, and 130 deg, respectively, are the best rim angles.

Methods of Improving the Axial Compressor Flow Passage to Reduce the Flow Circumferential Nonuniformity

Circumferential nonuniformity of gas flow is one of the main problems that can occur in the gas turbine engine. Usually, the flow circumferential nonuniformity appears near the support, located in the flow passage of the engine. The presence of circumferential nonuniformity leads to the increased dynamic stresses in the blade rows and the blade damage. The goal of this research was to find the ways of the flow non-uniformity reduction, which would not require a fundamental changing of engine design. A new method for reducing the circumferential nonuniformity of gas flow was proposed. It has been suggested to increase the gap of the leading edges of support racks from the trailing edge of the upstream guide vane blades which will result in achieving the desired results. An important advantage of this method is that the internal cavities of racks remain unchanged for the placement of engine systems. Moreover, the proposed method allows the prediction of the pressure peak values after the rotor blades without .

Method to Control Unsteady Force of Submarine Propeller Based on the Control of Horseshoe Vortex

The submarine propeller works in the submarine wake with severe circumferential nonuniformity, which causes the hydrodynamic force to act on the blade. This results in severe oscillation with the rotation of the propeller and impairs the hydrodynamics and noise performance of the submarine propeller. The horseshoe vortex generated at the hull-appendages junctions of the submarine has important influence on wake uniformity. In the present study, the state of the submarine horseshoe vortex is analyzed and a new method of vortex control baffler is presented. The aim is to weaken the horseshoe vortex. Based on the wind tunnel experiment and numerical simulation on the submarine model, the vortex control baffler can decrease the strength of the submarine horseshoe vortex and improve the uniformity of the submarine wake. Vortex control bafflers adapted for the fully appended SUBOFF model are designed, and the unsteady force of three kinds of propellers functioned after the SUBOFF model is calculated numerically. The results show that although the skew angle and blade number are different, the amplitudes of the unsteady force acting on the blades of all three propellers decreased by 50% to 80% due to the effects of the vortex control baffler.

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.

Behavior of Three-Dimensional Boundary Layers in a Radial Inflow Turbine Scroll

A detailed experimental investigation was carried out to examine the three-dimensional boundary layer characteristics in a radial inflow turbine scroll. Some basic flow phenomena and growth of secondary flow were also investigated. In the inlet region of the scroll, the incoming boundary layer begins to have a skewed nature, namely the radially inward secondary flow caused by the radial pressure gradient. From the inlet region to one third of the scroll circumference, the secondary flow grows so strongly that most of the low-momentum fluid in the incoming boundary layer is transported to the nozzle region. The succeeding elimination of the low-momentum fluid in the boundary layer suppresses growth of the boundary layer farther downstream, where the boundary layer shows a similar velocity profile. The distributions of the boundary layer properties in the scroll correspond well to those of the flow properties at the nozzle. The behavior of the boundary layer in the scroll is found to affect the circumferential nonuniformity of the nozzle flow field.