Power plants – Combustion products used as motive fluid
Patent
1997-03-17
1998-12-08
Freay, Charles G.
Power plants
Combustion products used as motive fluid
602263, F02C 618
Patent
active
058454829
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention.
The present invention relates to gas turbines, especially gas turbine aeroengines adapted for terrestrial uses such as generation of electricity.
2. Background Art.
Gas turbine aeroengines are sometimes adapted for terrestrial uses in which a constant speed is required, for example a speed of 3,000 or 3,600 rpm when driving electrical generators to produce electricity at 50 or 60 Hz. A problem arises with such engines when run at partial load, since the rotational speed of the low pressure compressor (LPC) which is coupled to the generator is fixed and thus the amount of compressed air delivered to the next compressor stage, which is usually the intermediate pressure compressor (IPC), would normally be constant and would be more than that required by the engine core. In order to alleviate this problem it is usual to bleed air from the inter compressor duct (ICD) to atmosphere. The ICD is typically a converging annular duct, "swan necked" in cross section, which joins the LPC to the IPC. This bleeding of air is problematic for two reasons. Firstly, it affects the flow of air within the ICD and increases its total pressure loss. Secondly, it results in poor distribution of flow at the inlet to the IPC which causes off-design operation of the compressor and a reduction in efficiency.
The ICD bleed-off can be seen as a flow bifurcation with two significant features: the bleed air flow experiences a sudden expansion exterior to the swan-neck air duct, while the main flow remaining within the duct effectively experiences an increase in area, an adverse pressure gradient and poor diffusion which can cause boundary layer separation, especially at the inner ICD wall. The latter flow is governed by the principles of subsonic diffusion for which the area ratio (AR) and non-dimensional wall length (L/D) are the principle determinants of optimum geometry for straight walled diffusers.
Up until now, the excess air has been bled off in one of two ways. In one method excess air is bled off by an assembly of discrete, circumferentially spaced bleed flaps located on the outer wall of the ICD. These bleed flaps open outwards, being hinged at their upstream ends. U.S. Pat. No. 5,261,228 to Shuba entitled "Apparatus for Bleeding Air" describes an apparatus that is typical of this arrangement. Shuba uses ten bleed valves 44 which are circumferentially spaced apart from each other around the axis of the engine. The other method is described in U.S. Pat. No. 5,279,109 of Liu et al. entitled "Gas turbine engine variable bleed pivotal flow splitter". Their method employs a circumferentially disposed plurality of pivotal flow splitters which are pivotally mounted at their downstream ends and which have means to pivot the leading edges of the splitters into the compressor flowpath. In both cases the bleed valves or splitters have their adjacent edges spaced apart and provide a discontinuous surface for diverting the flow outwardly.
Both concepts result in air being bled off from the outer wall of the ICD and can be categorised as wide angled, non-continuous diffusers. The method by which they bleed off air is problematic for two reasons. First, it affects the entire flow field within the ICD; as far as the flow remaining in the ICD is concerned, there is now a sudden area increase (low ratio of L/D) in the vicinity of the open bleed valves, and this represents a major problem with the local diffusion. This geometric configuration can cause flow separation on the inside wall of the ICD especially at large bleed flow rates when the effective area ratio of the continuing main flow is large. Because of the local diffusion and possible separated flow, pressure losses occur and result in poor distribution of velocity and total pressure at the inlet to the IPC. Secondly, the three-dimensional nature of this poor flow distribution at the inlet to the IPC results in off-design operation of the compressor stage and a reduction in efficiency.
DISCLOSURE OF THE INVENTION
The pr
REFERENCES:
patent: 5048280 (1991-09-01), Stransky
patent: 5119625 (1992-06-01), Glowacki
patent: 5261228 (1993-11-01), Shuba
patent: 5279109 (1994-01-01), Liu
patent: 5515673 (1996-05-01), Leclerrco et al.
patent: 5694768 (1997-12-01), Johnson et al.
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