Handbook of Turbomachinery Second Edition Revised - Ventech!

CHARACTERISTICS OF TURBOMACHINERY FLOW FIELDS
In this section of the chapter, the basic characteristics of turbomachinery
flow fields will be examined. Although the discussion will primarily target
aero engine applications, many of the flow features are common to other
turbomachines as well. In addition to a brief review of general flow-field
characteristics, the nature of flows specific to various component types will
also be considered.
Basic Characteristics of the Flow
Turbomachinery flow fields are highly complex and three-dimensional in
nature. In most cases, flows are compressible and may range from subsonic,
through transonic, to supersonic. Shocks may be present in the flow path,
and shock–boundary-layer interactions may occur, leading to performance
losses. Substantial pressure gradients can exist in virtually any direction.
Rotation is also a major factor influencing the flow behavior.
Flows are predominantly viscous and turbulent, although laminar and
transitional regions may also be present. Turbulence within the flow field
can occur in boundary layers and the free stream, where the level of
turbulence will vary depending on upstream conditions. For example, the
flow downstream of a combustor or a multistage compressor could have a
much higher free-stream turbulence level than the flow entering a fan.
Complex stresses and performance losses can result from viscous flow
phenomena such as three-dimensional boundary layers, interactions
between blade and end-wall boundary layers, near-wall flow migration,
separated flow, tip clearance vortices, trailing-edge vortices, wakes, and
mixing. In addition, relative end-wall motion and the transition between
rotating and stationary end walls can impact boundary layer behavior.
Unsteady flow can occur as a result of time-varying conditions
upstream, vortex shedding from blade trailing edges, flow separations, or
the interactions between rotating and stationary blade rows, which can
impart unsteady loadings on the blades.
Thermal effects and heat transfer can be significant factors,
particularly in the hot section of the engine. Hot combustor gases flow
through the turbine, producing hot streaks, which are transported by the
turbine flow field. To protect the components subjected to the highest
temperatures, cooling flows are injected through holes in the turbine blades
into the hot primary gas path, providing film cooling for the blade surfaces.
Similarly, cooling flows may also be injected into the main flow stream along
the end walls.
Copyright © 2003 Marcel Dekker, Inc.