This invention relates generally to the field of flow diffusers, and more particularly to a flow diffuser such as may be used to expand and to slow the velocity of a gas flow between a gas turbine and a heat recovery steam generator in a combined cycle power plant.
Diffusers are devices used to slow the velocity of a fluid flow by directing the fluid through a flow path of increasing cross-sectional area in the direction of the flow. As the flow area expands and the flow velocity decreases, the dynamic head of the fluid decreases and the static head of the fluid increases.
In a combined cycle power plant, the hot exhaust gas from a gas turbine engine is directed into a heat recovery steam generator (HRSG) in order to transfer heat from the hot gas, thereby cooling the gas before it is exhausted into the atmosphere. The recovered heat warms water passing through tubes of the HRSG and produces steam, which is then used to drive a steam turbine. It is known to install a diffuser between the exit of the gas turbine and the entrance of the HRSG in order to protect the tubes from excessively high velocity gas and to improve the heat transfer performance of the HRSG. U.S. Pat. No. 7,272,930 describes one such combined cycle power plant diffuser application.
A typical diffuser used upstream of a HRSG in a combined cycle power plant includes-an outer wall having a generally conical shape which expands in diameter in the downstream direction. Two parameters are used to describe such a diffuser: the area expansion ratio (outlet cross-sectional area divided by inlet cross-sectional area) and the expansion angle (or half-angle, expressed as the angle defined between one side of the wall and a flow direction centerline as viewed in cross-section). These two parameters control the overall length of the diffuser necessary to obtain a desired degree of flow slowing. If the expansion angle is too small, the diffuser is excessively long, which is undesirable in a power plant for space and cost reasons. If the expansion angle is too large, the flow separates from the wall and generates a reverse flow region along the wall, thereby reducing the functionality of the diffuser. The separated flow is unsteady and the separation bubble can move around in the diffuser, adversely affecting the downstream HRSG. Thus, diffusers for combined cycle power plants are generally designed to be conservatively long in order to avoid flow separation over an entire range of power plant operating parameters.
Studies have shown that it is possible to actively control flow separation in a diffuser by exciting vortex interactions in the separated shear layer, such as with acoustic energy, resulting in a reduction of the reattachment length. An active solution for a combined cycle power plant application is difficult because the shear layer can move within the diffuser, and because acoustic excitation requires knowledge of the optimal forcing frequency and amplitude in order to avoid potentially causing the reattachment length to grow. Active solutions also have the disadvantage of consuming power, and the imposed energy may have an adverse impact on the mechanical components of the system.
Studies have also shown that flow trip tabs can reduce flow separation reattachment length of a shear layer by generating longitudinal vortex pairs which increase mixing. A flow tab solution for a combined cycle power plant application is also difficult due to the uncertain location of the shear layer, and such tabs would create a relatively high energy loss due to the abrupt flow disturbances caused by the tabs.
The invention is explained in the following description in view of the drawings that show:
The present inventors have developed an innovative solution for flow separation control in a conical diffuser, such as may be used upstream of a heat recovery steam generator (HRSG) in a combined cycle power plant. Rather than trying to anticipate the location of a flow separation region under the many varying operating conditions of the diffuser, the solution of the present invention incorporates a backward facing step into the diffuser wall. The step is effective to stimulate the formation and reattachment of a downstream flow separation bubble under conditions conducive to flow separation in order to fix the location of the separation within the diffuser, and to do so with a minimal flow energy loss and with a minimum diffuser length. Moreover, the height of the step is varied around the circumference of the diffuser wall in a peak/valley pattern such that the resulting separation bubble is segmented into a series of smaller cells, with one cell being located behind each peak in the step.
Embodiments of the present invention are described below using the following terminology. Flow at any given cross section is generally considered to be separated from the wall of the diffuser when the total reverse flow area is 1% or more of the total flow area. A backward facing step is understood to be an abrupt increase in flow area in a downstream direction causing downstream recirculation. A smoothly lofted wavy backward facing step is one with a non-circular perturbation section leading to the step edge, where the perturbation section transitions from a circular to a non-circular cross-sectional profile without creating any appreciable upstream recirculation region. The thickness of a boundary layer is considered to be the distance from the wall at which the viscous flow velocity is 99% of the free stream velocity. The term “generally conical shaped” means a cone shape having circular or annular cross sections but allowing for some local areas to have variations in the cone shape, such as constant diameter regions, so long as the overall shape expands the cross section from inlet to outlet.
A prior art diffuser 10 is illustrated in cross section in
An embodiment of the invention is illustrated in
The shape of the smoothly lofted backward facing step 46 of the embodiment of
In the embodiment of
The periodically varying step heights of the embodiments of
Advantageously, a diffuser designed in accordance with embodiments of the present invention may be shorter than a comparable prior art design due to a reduction of the bubble reattachment length. The wavy height backward facing step of the present invention has been shown experimentally to function similarly when used in a conical diffuser with or without Coanda blowing from a center body at the diffuser inlet. When a wavy step was modeled to have a height varying symmetrically about the circumference from Hpeak to Hvalley, the step bubble reattachment length (L) reduced by almost half when compared to a similar device with a constant step height of Hpeak.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
This application claims benefit of the 22 Jul. 2011 filing date of United States provisional patent application No. 61/510,551.
Number | Date | Country | |
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61510551 | Jul 2011 | US |