Fluid reaction surfaces (i.e. – impellers) – With heating – cooling or thermal insulation means – Changing state mass within or fluid flow through working...
Reexamination Certificate
1999-11-18
2001-08-28
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With heating, cooling or thermal insulation means
Changing state mass within or fluid flow through working...
Reexamination Certificate
active
06280140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to gas turbine engines in general, and to methods and apparatus for cooling a substrate exposed to high temperature gas in particular.
2. Background Information
Efficiency is a primary concern in the design of any gas turbine engine. Historically, one of the principle techniques for increasing efficiency has been to increase the core gas path temperatures within the engine. Core gas refers to air worked within the compressor that is mixed with fuel and combusted within the combustor. The increased gas path temperatures have been accommodated by using internally cooled components made from high temperature capacity alloys. Turbine stator vanes and blades, for example, are typically cooled using compressor air worked to a higher pressure, but still at a lower temperature than that of the core gas flow passing by the blade or vane. The higher pressure provides the energy necessary to push the air through the component. A significant percentage of the work imparted to the air bled from the compressor, however, is lost during the cooling process. The lost work does not add to the thrust of the engine and therefore negatively effects the overall efficiency of the engine. A person of skill in the art will recognize, therefore, that there is a tension between the efficiency gained from higher core gas path temperatures and the concomitant need to cool turbine components and the efficiency lost from bleeding air to perform that cooling.
There is, accordingly, great value in maximizing the cooling effectiveness of whatever cooling air is used. Prior art coolable airfoils typically include a plurality of internal cavities, which are supplied with cooling air. The cooling air passes through the wall of the airfoil (or the platform) and transfers thermal energy away from the airfoil in the process. The manner in which the cooling air passes through the airfoil wall is critical to the efficiency of the process. In some instances, cooling air is passed through straight or diffused cooling apertures to convectively cool the wall and establish an external film of cooling air. A minimal pressure drop is typically required across these type cooling apertures to minimize the amount of cooling air that is immediately lost to the free-stream hot core gas passing by the airfoil. The minimal pressure drop is usually produced through a plurality of cavities within the airfoil connected by a plurality of metering holes. Too small a pressure drop across the airfoil wall can result in undesirable hot core gas in-flow. In all cases, the minimal dwell time in the cooling aperture as well as the size of the cooling aperture make this type of convective cooling relatively inefficient.
Some airfoils convectively cool by passing cooling air through passages disposed within a wall or platform. Typically, those passages extend a significant distance within the wall or platform along a substantially straight line. There are several potential problems with this type of cooling scheme. First, the heat transfer rate between the passage walls and the cooling air decreases markedly as a function of distance traveled within the passage. As a result, cooling air flow adequately cooling the beginning of the passage may not adequately cool the end of the passage. If the cooling air flow is increased to provide adequate cooling at the end of the passage, the beginning of the passage may be excessively cooled, consequently wasting cooling air. Second, the thermal profile of an airfoil is typically non-uniform and will contain regions exposed to a greater or lesser thermal load. The prior art internal cooling passages extending a significant distance within an airfoil wall or a platform typically span one or more regions having disparate thermal loads. Similar to the situation described above, providing a cooling flow adequate to cool the region with the greatest thermal load can result in other regions along the passage being excessively cooled.
What is needed, therefore, is a method and apparatus for cooling a substrate within gas turbine engine that adequately cools the substrate using a minimal amount of cooling air and one that provides heat transfer where it is needed.
DISCLOSURE OF THE INVENTION
It is, therefore, an object to provide a method and an apparatus for cooling a wall within a gas turbine engine for removing more cooling potential from cooling air passed through the wall than is possible using most conventional methods and apparatus.
It is another object to provide a method for cooling a wall within a gas turbine engine that can produce a cooling profile that substantially matches the thermal profile of the wall, and an apparatus that can be used for the same.
According to the present invention, an apparatus and a method for cooling a wall for use in a gas turbine engine is provided that includes a cooling microcircuit. The cooling microcircuit, which can be disposed within the wall of a component such as a stator vane or a rotor blade, includes passage having a plurality of segments connected in series by one or more chambers. An inlet aperture connects the passage to one side of the wall. An exit aperture connects the passage to the opposite side of the wall. Cooling air on the inlet aperture side of the wall enters the passage through the inlet aperture and exits through the exit aperture.
The present cooling apparatus and method for cooling a wall provides significantly increased cooling effectiveness over prior art cooling schemes. One of the ways the present apparatus and method provides increased cooling effectiveness is by increasing the heat transfer coefficient per unit flow within a cooling passage. The transfer of thermal energy between the wall containing the passage and the cooling air is directly related to the heat transfer coefficient within the passage for a given flow. A velocity profile of fluid flow adjacent each wall of a passage is characterized by an initial hydrodynamic entrance region and a subsequent fully developed region as can be seen in FIG.
7
. In the entrance region, a fluid flow boundary layer develops adjacent the walls of the passage, starting at zero thickness at the passage entrance and eventually becoming a constant thickness at some position downstream within the passage. The change to constant thickness marks the beginning of the fully developed flow region. The heat transfer coefficient is at a maximum when the boundary layer thickness is equal to zero, decays as the boundary layer thickness increases, and becomes constant when the boundary layer becomes constant. Hence, for a given flow the average heat transfer coefficient in the entrance region is higher than the heat transfer coefficient in the fully developed region. The present apparatus and method increases the percentage of flow in a passage characterized by entrance region effects by providing a plurality of short length segments connected by chambers. Fluid entering a chamber diffuses and decreases in velocity. Fluid exiting a chamber is characterized by entrance region effects and consequent increased local heat transfer coefficients. The average heat transfer coefficient per unit flow of the relatively short segments of the present apparatus and method is consequently higher than that available in all similar prior art cooling schemes of which we are aware.
Another way the present invention provides an increased cooling effectiveness also involves the short length segment between chambers. The relationship between the beat transfer rate and the heat transfer coefficient in given length of passage can be mathematically described as follows:
q=h
c
A
s
&Dgr;T
lm
(Eqn. 1)
where:
q=heat transfer rate between the passage and the fluid
h
c
=heat transfer coefficient of the passage
A
s
=passage surface area =P×L=Passage perimeter×length
&Dgr;T
lm
=log mean temperature difference
The above equation illustrates the direct relationship between the heat transfer rate a
Auxier Thomas A.
Calhoun William H.
Downs James P.
Hayes Douglas A.
Kvasnak William A.
Getz Richard D.
Look Edward K.
McAleenan James M
United Technologies Corporation
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