Diffuser (thermodynamics)

About: Diffuser (thermodynamics) is a research topic. Over the lifetime, 6731 publications have been published within this topic receiving 54738 citations.

One-Dimensional Analysis of Full Load Draft Tube Surge

Abstract: One-dimensional stability analysis of a hydraulic system composed of a penstock, a runner and a draft tube was carried out to determine the cause of the full load draft tube surge. It is assumed that the cavity volume at the runner exit is a function of the pressure at the vortex core evaluated from the instantaneous local pressure at the runner exit and an additional pressure decrease due to the centrifugal force on the swirling flow. It was found that the diffuser effect of the draft tube has a destabilizing effect over all flow rates while the swirl effects stabilize/destabilize the system at larger/smaller flow rates than the swirl free flow rate. Explanations of the destabilizing mechanism are given for the diffuser and swirl flow effects. The effect of finiteness of sound velocity in the penstock is also discussed

Subsonic diffusers designed integrally with vortex generators.

Abstract: A short subsonic diffuser has been developed incorporating vortex generators as an integral design feature. The principle of operation is that line vortices, when suitably arranged, mutually interact to drive each other towards an adjacent plane wall. The wall may then be pulled away from the vortices at such a rate that the vortices remain a constant distance from it. The ideal arrangement, having the vortex lines running along the edge of the boundary layer, can thus be obtained by design. A simple, two-dimensional diffuser was developed at Lockheed using these principles. It was compared experimentally with a conventional high-performance trumpet-shaped diffuser. Two significant results were observed in this test series. One was that both pressure recovery losses and distortion were reduced by about 40% by the new design. The other was that vortex generator design mismatches carry larger penalties with the integrally designed diffuser than with the conventional type. The subsonic diffuser of an inlet for a Mach 2.7 supersonic transport airplane was shortened and redesigned in two stages, using the integrated vortex generator approach. First, vortex generators were tailored to a short subsonic diffuser of conventional design, and then a new inlet was tested which included a subsonic diffuser designed on the integral basis. The successive changes improved both pressure recovery and flow uniformity. Thus, the basic concept was validated in a practical application.

Unsteady Flow Visualization at Part-Load Conditions of a Radial Diffuser Pump: by PIV and CFD

Abstract: The present study provides flow visualization on complex internal flows in a radial diffuser pump under part-load conditions by using the three-dimensional Navier-Stokes code CFX-10 with Detached Eddy Simulation (DES) turbulence model. Particle Image Velocimetry (PIV) measurements have been conducted to validate numerical results. The CFD results show good agreements with experimental ones on both the phase-averaged velocity fields and turbulence field. The detailed flow analysis shows that no separation occurs at 0.75Qdes although a low-velocity zone develops on the rear impeller suction side. Steady flow separations are observed on the impeller suction sides at 0.5Qdes but with different onsets and amounts. When reducing the flow rate to 0.25Qdes, CFD predicts different types of back flows in the impeller region, including steady leading edge separations, rotating vortex in the impeller wake region, and back flow on the impeller pressure side.

Experimental Investigation of Centrifugal Compressor Stabilization Techniques

Abstract: Results from a series of experiments to investigate techniques for extending the stable flow range of a centrifugal compressor are reported. The research was conducted in a high-speed centrifugal compressor at the NASA Glenn Research Center. The stabilizing effect of steadily flowing air-streams injected into the vaneless region of a vane-island diffuser through the shroud surface is described. Parametric variations of injection angle, injection flow rate, number of injectors, injector spacing, and injection versus bleed were investigated for a range of impeller speeds and tip clearances. Both the compressor discharge and an external source were used for the injection air supply. The stabilizing effect of flow obstructions created by tubes that were inserted into the diffuser vaneless space through the shroud was also investigated. Tube immersion into the vaneless space was varied in the flow obstruction experiments. Results from testing done at impeller design speed and tip clearance are presented. Surge margin improved by 1.7 points using injection air that was supplied from within the compressor. Externally supplied injection air was used to return the compressor to stable operation after being throttled into surge. The tubes, which were capped to prevent mass flux, provided 9.3 points of additional surge margin over the baseline surge margin of 11.7 points.

Experimental investigation of the flow through axisymmetric expansions

Abstract: This study examines the flow field in three axisymmetrtic expansions having diffuser halft-angles of 14, 18, and 90 deg, respectively Velocity measurements were performed at a Reynolds number of Re=1,56×10 4 using a single component LDA operated in forward scatter The test facility was refractive index matched, allowing measurement of the velocities U, V, W, u 2 , v 2 , w 2 , uv and uw upstream of, and throughout the entire recirculation region The results indicate that the diffuser geometry influences the separated shear layer appreciably over the entire length of the diffuser section