Diffuser (thermodynamics)

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

A Comparative Study of Turbulence Models Performance for Turbulent Flow in a Planar Asymmetric Diffuser

Abstract: This paper presents a computational study of the separated flow in a planer asymmetric diffuser. The steady RANS equations for turbulent incompressible fluid flow and six turbulence closures are used in the present study. The commercial software code, FLUENT 6.3.26, was used for solving the set of governing equations using various turbulence models. Five of the used turbulence models are available directly in the code while the v2-f turbulence model was implemented via User Defined Scalars (UDS) and User Defined Functions (UDF). A series of computational analysis is performed to assess the performance of turbulence models at different grid density. The results show that the standard k-ω, SST k-ω and v2-f models clearly performed better than other models when an adverse pressure gradient was present. The RSM model shows an acceptable agreement with the velocity and turbulent kinetic energy profiles but it failed to predict the location of separation and attachment points. The standard k-e and the low-Re ke delivered very poor results. Keywords—Turbulence models, turbulent flow, wall functions, separation, reattachment, diffuser.

Numerical Investigation of the Effect of Different Back Sweep Angle and Exducer Width on the Impeller Outlet Flow Pattern of a Centrifugal Compressor With Vaneless Diffuser

Abstract: This paper presents a numerical investigation of the effect of different back sweep angles and exducer widths on the steady-state impeller outlet flow pattern of a centrifugal compressor with a vaneless diffuser. The investigations have been performed with commercial computational fluid dynamics (CFD) and in-house programmed one-dimensional (ID) codes. CFD calculations aim to investigate how flow pattern from the impeller is quantitatively influenced by compressor geometry parameters; thereby, the location of wake and its magnitude (flow angle and relative velocity magnitude) are analyzed. Results show that the increased back sweep impeller provides a more uniform flow pattern in terms of velocity and flow deviation angle distribution, and offers better potential for the diffusion process inside a vaneless (or vaned) diffuser Secondary flux fraction and flow deviation angle from CFD simulation are implemented into the ID two-zone program to improve ID prediction results. (Less)

Combustor with turbulence producing device

Abstract: The present invention relates to a gas turbine combustor comprising an air passage to supply air to the inside; and a fuel nozzle which is provided with an injection port to inject fuel and disposed in the air passage, wherein a turbulence producing means adjacent to the injection port of the fuel nozzle is provided in the air passage. Further, the present invention relates to a gas turbine combustor comprising an air passage to supply air to the inside; and a fuel nozzle which is provided with an injection port to inject fuel and disposed in the air passage, wherein a diffuser portion is provided in the air passage, and the diffuser portion causes the cross-sectional area of a part of the air passage positioned in the vicinity of the injection port to be smaller than that of a downstream portion of the air passage positioned downstream from the injection port in the direction of the airflow. Thus, the mixing action of fuel and air can be enhanced, and combustion vibration can be prevented.

Flow facility design and experimental studies of wall-bounded turbulent shear-flows

Abstract: The presen present thesis spans a range of topics within thearea of turbulent flows, ranging from design of flow facilitiesto evaluation aluation of scaling laws and turbulence modelingdeling aspects through use of experimental data. A newwind-tunnel has been designed, constructed and evaluated at theDept. of Mechanics, KTH. Special attention was directed to thedesign of turning vanes that not only turn the flow but alsoallow for a large expansion without separation in the corners.The investigation of the flow quality confirmed that theconcept of expanding corners is feasible and may besuccessfully incorporated into low turbulence wind-tunnels. Theflow quality in the MTL wind-tunnel at the Dept. of Mechanics,KTH, was as also in investigated confirming that it still isvery good. The results are in general comparable to thosemeasured when the tunnel was as new, with the exception of thetemperature variation ariation that has decreased by a factorof 4 due to an improved cooling system.Experimental data from high Reynolds number zeropressure-gradient turbulent layers have been investigated.These studies have primarily focused on scaling laws withe.g.confirmation of an exponential velocity defect lawin a region, about half the size of the boundary layerthickness, located outside the logarithmic overlap region. Thestreamwise velocity probability density functions in theoverlap region was found to be self-similar when scaled withthe local rms value. Flow structures in the near-wall andbuffer regions were studied ande.g. the near-wall streak spacing was confirmed to beabout 100 viscous length units although the relative influenceof the near-wall streaks on the flow was as found to decreasewith increasing Reynolds number.The separated flow in an asymmetric plane diffuser wasdetermined using PIV and LDV. All three velocity componentswere measured in a plane along the centerline of the diffuser.Results for mean velocities, turbulence intensities andturbulence kinetic energy are presented, as well as forstreamlines and backflow coefficientcien describing theseparated region. Instantaneous velocity fields are alsopresented demonstrating the highly fluctuating flow. Resultsfor the above mentioned velocity quantities, together with theproduction of turbulence kinetic energy and the secondanisotropy inariant are also compared to data from simulationsbased on the k -wformulation with an EARSM model. The simulation datawere found to severely underestimate the size of the separationbubble.Keywords:Fluid mechanics, wind-tunnels, asymmetricdiffuser, turbulent boundary layer, flow structures, PDFs,modeling, symmetry methods.

Negatively buoyant flow in diverging channel. IV: Entrainment and dilution

Abstract: A negatively buoyant inflow to a lake or reservoir sinks to the bottom and forms an underflow. In this process, the “plunging flow” entrains ambient water and becomes diluted. The dilution alters the buoyancy and hence the flow behavior of the underflow. Since literature on entrainment by underflows does not include the “plunging process” itself, great uncertainty exists when the initial conditions of underflows have to be estimated. Therefore, the density driven plunging phenomenon is experimentally investigated in a horizontal diverging channel. Entrainment rates are determined from velocity and temperature measurements and related to channel diffuser angle, inflow densimetric Froude number, and inflow channel aspect ratio. The experiments are conducted at inflow channel Reynolds numbers larger than 10,000. Experimental diffuser angles are varied from 3° to 180°, inflow densimetric Froude numbers from 1.8 to 21 and channel aspect ratios from 0.5 to 1.8. The information is of use as input to one‐dimensio...