Diffuser

3 Key excerpts on "Diffuser"

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  eBook - PDF

  Gas Turbine Combustion

  Alternative Fuels and Emissions, Third Edition

  • Arthur H. Lefebvre, Dilip R. Ballal(Authors)
  • 2010(Publication Date)
  • CRC Press

    (Publisher)

  79 3 Diffusers 3.1 Introduction In axial-flow compressors, the stage pressure rise is very dependent on the axial flow velocity. To achieve the design pressure ratio in the mini-mum number of stages, a high axial velocity is essential; in many aircraft engines, compressor outlet velocities may reach 170 m/s or higher. It is, of course, impractical to attempt to burn fuels in air flowing at such high velocities. Quite apart from the formidable combustion problems involved, the fundamental pressure loss would be excessive. For example, for an air velocity of 170 m/s and a combustor temperature ratio of 2.5, the pressure loss incurred in combustion would be approximately 25% of the pressure rise achieved in the compressor. Thus, before combustion can proceed, the air velocity must be greatly reduced, usually to about one-fifth of the compressor outlet velocity. This reduction in velocity is accomplished by fitting a Diffuser between the compressor outlet and the upstream end of the liner. In its simplest form, a Diffuser is merely a diverging passage in which the flow is decelerated and the reduction in velocity head is converted to a rise in static pressure. The efficiency of this conversion process is of considerable importance because any losses that occur are manifested as a fall in total pressure across the Diffuser. In long Diffusers of low diver-gence angle, the pressure loss is high due to skin friction along the walls, as shown in Figure 3.1. Such Diffusers are, in any case, impractical because of their extreme length. On all aircraft engines, and also on many indus-trial engines, length is crucial, and it is essential, therefore, that diffusion is accomplished in the shortest possible distance. With an increase in diver-gence angle, both Diffuser length and friction losses are reduced, but stall losses arising from boundary-layer separation become more significant.

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  eBook - PDF

  Radial Flow Turbocompressors

  Design, Analysis, and Applications

  • Michael Casey, Chris Robinson(Authors)
  • 2021(Publication Date)
  • Cambridge University Press

    (Publisher)

  12 Diffuser Design 12.1 Overview 12.1.1 Introduction This chapter considers the design parameters for the Diffuser system immediately downstream of the impeller. The components of the stator system downstream of the Diffuser are considered in Chapter 13, and Chapter 11 considers the design of the impeller. The function of the Diffuser is to transform the kinetic energy at its inlet into a rise in the static pressure. In a typical centrifugal compressor, around 35–40% of the energy input to the impeller is available as kinetic energy at the Diffuser inlet, so the effectiveness of the diffusion system is critically important to the performance of the whole stage. Centrifugal compressors are usually fitted with either a vaned or a vaneless Diffuser leading to a collector or volute. The Diffuser meridional channel comprises an annular channel extending radially outwards from the impeller outlet, usually of the same axial width as the impeller outlet but sometimes with a decreasing width as the radius increases, known as a pinched Diffuser. The axial width may occasionally increase with radius in a vaned Diffuser. In some low-speed ventilator and pump applications, a collector system may be fitted directly around the impeller, thus obviating the need for a Diffuser section. The simplest Diffuser system is a radial vaneless annular channel. In a vaneless Diffuser, the flow is decelerated in two ways. The meridional component of the velocity is reduced by increasing the area of the channel with radius (conservation of mass). The circumferential component is reduced by the increasing radius in the Diffuser (conservation of angular momentum). In a vaned Diffuser, of which there are several types, there is a small vaneless region upstream of the Diffuser vanes. The vanes themselves form flow channels designed to decelerate the flow more than is possible in a vaneless Diffuser by turning the flow to a more radial direction.

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  eBook - ePub

  Fluid Mechanics and Turbomachinery

  Problems and Solutions

  • Bijay K Sultanian(Author)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  13    Diffusers

  Review of Key Concepts

  We concisely present here some key concepts Diffuser flow and performance. More details on each topic are given, for example, in Sultanian (2015, 2019). Using results from computational fluid dynamics (CFD), a designer can generate an entropy map, discussed in Sultanian (2015), to delineate Diffuser flow zones of excess entropy production for design improvement.

  Diffusion is the conversion of dynamic pressure into the stream static pressure. For both incompressible (liquid) flow and subsonic gaseous flow, diffusion is typically achieved through increase in the downstream flow area. This is consistent with the main tenet of the Bernoulli equation, namely “low velocity, high pressure.” Sultanian (2015) discusses interesting features of a flow with and without swirl in a sudden pipe expansion. In gas turbines used for power generation, exhaust Diffusers play an important role in turbine power output by increasing the pressure ratio across the last stage turbine by making the turbine exit static pressure subambient.

  Ideal Diffusers are characterized by uniform axial velocities with no swirl at both inlet and exit and with no loss in total pressure across the Diffuser. Actual Diffusers, however, often have nonuniform profiles of all three velocity components, pressure, and temperature at both inlet and outlet. We discuss here how to handle these nonuniformities in calculating the performance of a real Diffuser.

  Isentropic Efficiency

  (

  η D

  )

  Figure 13.1 shows both isentropic and nonisentropic variations of flow properties in a Diffuser from its inlet to outlet. For an isentropic Diffuser operating along 1–2′, we have no loss in total pressure. For the nonisentropic process along 1–2, we have

  p 02

  <

  p 01

  . However, the total enthalpy, or the total temperature for a constant

  c p

  , in an adiabatic Diffuser remains constant (

  h 02

  =

  h 01

  or

  T 02

  =

  T 01

  ) both along 1–2 and 1–2′. This figure further shows that, compared to a nonisentropic Diffuser, the isentropic Diffuser will have lower flow area and higher dynamic pressure at its exit when the pressure recovery

  p 2

  −

  p 1

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