GB2066372A - Coolable wall element - Google Patents

Abstract

The wall element, e.g. forming part of a turbine blade, has a plurality of cooling holes 30 extending thereacross which are contoured to provide lower rates of convective cooling in the wall material adjacent the cool side 32 than adjacent the hot side 34. This may be achieved by arranging the portion 36 of each hole adjacent cool side 32 as being perpendicular thereto whereas the remaining portion 38 is inclined relative to hot side 34 so as to increase the wetted surface area of the hole adjacent side 34. Thermal gradients and resulting stresses across the wall element are thus reduced. <IMAGE>

Description

SPECIFICATION Coolable wall element Technical Field This invention relates to coolable wall elements, and more particularly to wall elements in which a cooling medium is flowable thereacross to cool the wall material.

The concepts were developed in the gas turbine engine industry for protecting turbine components from hot working medium gases, but have wider wapplicability to industrial cooling in general.

Background Art In modern gas turbine engines, very high temperature, working medium gases are produced within a combustion chamber and are flowed in a downstream direction through the turbine section of the engine. At the upstream end of the turbine, the medium gases produced typically have characteristic temperatures on the order of twenty-five hundred degrees Fahrenheit (25000 F) or greater. The temperature of the medium gases in many cases exceeds the capabilities of the materials from which the components of the turbine are fabricated, and such components are cooled.

In the most commonly employed cooling techniques, high pressure air from the compression section of the engine is flowed to the turbine for air cooling of the components. The compressor air is sufficiently high in pressure to cause the air to flow into the local area of the turbine and across the structure to be cooled, and is sufficiently low in temperature to provide the required cooling capacity. One effective cooling technique is known in the industry as "film" cooling. The cooling air is flowed at a low velocity through a multiplicity of orifices in the wall of the component to be cooled. Air discharged from the orifices adheres to the surface of the wall to protect the wall, for example, from the hot working medium gases. Another effective cooling technique is known in the industry as "convective" cooling.By such technique the cooling air is flowed within the material to be cooled to remove heat from the material as a function of flow velocity and wetted surface area. Multiple cooling techniques are commonly applied in the same structure.

Cooled structures to which the concepts of the present invention apply include rotor blades, such as illustrated in U.S. patent 3,994,622 to Schultz et al. entitled "Coolable Turbine Blade"; blade tip shrouds, such as illustrated in U.S. patents 3,981,609 to Koenig entitled "Coolable Blade Tip Shroud" and 4,013,376 to Bisson et al. entitled "Coolable Blade Tip Shroud"; and stator vanes, such as illustrated in U.S. patents 4,025,226 to Hovan entitled "Air Cooled Turbine Vane" and 4,040,767 to Dierberger et al. entitled "Coolable Nozzle Guide Vane", of common assignee herewith.

Efficient cooling of such structures in a manner consonant with long component life has always been a prime concern of designers and manufacturers, and remains so today. It is against this background that the present concepts are developed.

Disclosure of the Invention According to the present invention the magnitude of the thermal gradient across a coolable wall structure in an operative environment is held to a relative low value by limiting the amount of convective cooling in the wall material adjacent the cool side of the wall in comparison to the amount of convective cooling in the wall material adjacent the hot side of the wall.

According to one detailed embodiment of the invention convective cooling in the material adjacent the cool wall is limited by providing cooling air holes having a first portion adjacent the cool side of the wall which is perpendicular thereto and a second portion adjacent the hot side of the wall which is sharply angled with respect thereto.

A primary feature of the present invention is the geometry of cooling air holes across the coolable wall structure. The holes in the region of the hot side of the coolable wall are sharply angled so as to be capable of laying a film of cooling air adjacent to the hot wall. The holes in the region adjacent the cool side of the coolable wall element are perpendicular to the cool side. Resultantly, the cooling hole density, that is the ratio of hole area to material area, at the hot side of the coolable wall is greater than the cooling hole density at the cold side of the wall. Additionally, the portion of each cooling air hole at the hot side of the collable wall element has a large length to diameter ratio when compared to the portion of the hole at the cool side of the coolable wall element.

A principal advantage of the present invention is improved life of the coolable wall structure when compared to conventionally structured walls. In the structure provided, the thermal gradients across the coolable wall are limited by convectively cooling at a lesser rate adjacent the cool side of the wall than adjacent the hot side of the wall. Reduced thermal stresses in the wall material and increased fatigue life result.

The foregoing, and other features and advantages of the present invention, will become more apparent in the light of the following description and accompanying drawing.

Brief Description of the Drawing Fig. 1 is a simplified cross section view of a portion of the turbine section of a gas turbine engine illustrating turbine components of the type to which the coolable wall structure of the present invention is applicable; Fig. 2 is a partial perspective view of a rotor blade including cut-away portions showing the coolable wall structure of the present invention; Fig. 3A is a sectional view taken along the line 3A-3A as shown in Fig. 2; and Fig. 3B is a sectional view taken along the line 3B-3B as shown in Fig. 2.

Best Mode for Carrying Out the Invention A portion of the turbine section of a gas turbine engine is illustrated in Fig. 1. A flowpath 10 for hot working medium gases extends axially through a turbine section 12 of the engine from a combustion chamber 14. Effluent from the combustion chamber is extremely hot, perhaps even exceeding twenty-five hundred degrees Fahrenheit (25000 F). The flowpath extends in a downstream direction through alternating rows of stator vanes and rotor blades. An upstream row of stator vanes is represented by the single vane 16 and a downstream row of stator vanes is represented by the single vane 18. A row of rotor blades disposed therebetween is represented by the single blade 20. Stator vanes and rotor blades in the initial stages of the turbine section are capable of employing the concepts of the present invention.An outer air seal 22 circumscribes each row of rotor blades and is also capable of employing the concepts of the present invention.

Other comparable components at which film cooling is required to protect the components from the hot working medium gases of the flowpath are equally suited to the structure and advantages of the coolable wall element taught herein.

A rotor blade incorporating the coolable wall concepts of the present invention is illustrated in Fig. 2. The rotor blade is formed of a suction side wall 24 and a pressure side wall 26 and includes a hollow cavity 28 extending in the spanwise direction therebetween. The hollow cavity is in gas communication with a supply of cooling air, as for example, the compression section of the engine.

Cooling air holes 30 penetrate the pressure and suction side walls of the blade over at least a portion of the airfoil surfaces to provide a layer of cooling air from the supply means over the airfoil surfaces in the operative mode. The layer of cooling air is preferably discharged from the blades at a shallow angle so as to be capable of providing in an adhering film layer adjacent to the walls of the airfoil. The layer of air prevents the incidence of hot working medium gases against the material from whch the blade is fabricated.

One particular geometry for cooling air holes 30 is illustrated in the Fig. 3A and Fig. 3B sectional views of a coolable wall element, which in this case is the airfoil wall of the rotor blade 20. Each hole 30 extends across the airfoil wall from a cool side 32 adjacent the hollow cavity 28 to a hot side 34 adjacent the working medium flowpath 10. It is particularly important to note that the cooling hole density adjacent the cool side of the wall is significantly less than the cooling hole density adjacent the hot side of the wall. Stated another way, the ratio of blade material cross sectional area to cooling hole cross sectional area is larger at the cool side than at the hot side.This distribution of material is achieved in the illustrated embodiment by providing a first hole portion 36 which is perpendicular to the cool side and extends approximately half-way across the coolable wall material. A second hole portion 38 extending from the first hole portion is canted to a shallow angle with respect to the hot side of the wall.

In an operative environment a substantial difference in temperature exists between the cooling air at the cool side and the hot working medium gases at the hot side and varies with the particular engine location and design. In modern gas turbine engines the temperature differential is typically on the order of two to three hundred degrees Fahrenheit (200--3000F) and perhaps more. The substantial temperature differential across the wall establishes a significant thermal gradient with resultantly large differences in thermal growth between the hot side and the cold side of the structure. The differences in thermal growth produce stresses in the wall material which are related through the modulus of elasticity of the material to the magnitude of the differential thermal growth.Materials having relatively high moduli of elasticity experience correspondingly high stresses which where excessive, may cause cracking of the coolable wall material. Stress problems are particularly severe in the high modulus turbine alloys in current use in gas turbine engines, and in those alloys being developed for future use. It is that by reducing the thermal gradient across the wall, excessively high stresses can be avoided.

Cooling air is flowable across the coolable wall element of the present invention to control the temperature of the material from which the wall is fabricated. Both convective cooling and film cooling concepts are employed. In the employ of the film cooling concepts a layer of cooling air is flowed adjacent to the hot side of the wall material to shield the wall material from the hot working medium gases of the flowpath. The film must be well adhering to the wall and of sufficient thickness to prevent the penetration thereof by the working medium gases. The cooling holes 30 through which the film of air is deposited are therefore closely spaced and canted at the hot side of the wall so as to be capable of flowing air adjacent the wall without undue penetration and mixing with the working medium gases.

Additionally, canting the holes increases the breakout hole area as is illustrated in Fig. 2 such that the air is discharged at relatively low velocity into the flowpath when compared to the axial velocity along the hole.

The degree of cant is selected to achieve the desired distribution of cooling air on the hot side.

Note particularly in the leading edge region 4() of the Fig. 2 blade that the degree of cant is less than along the pressure side of the airfoil. This enables the cooling air holes to be more closely packed in the leading edge region and provides a cooling air discharge direction at the leading edge which is more resistive to the direct approach of working medium gases. The greater number of holes in the leading edge region and the lesser degree of cant provide an increased volume of film air flow at the leading edge region when compared to the pressure and suction side walls. The degree of cant of the holes at the coolable wall in one effective embodiment ranged from approximately twenty degrees (200) at the leading edge to approximately six degrees (60) along the pressure side wall.

In combination with film cooling, convective cooling is employed within the wall material itself and it is again here that air velocity considerations are also important. Rates of convective cooling are a function flow velocity, temperature differential and wetted surface area.

Canting the holes has the effect of increasing the wetted surface area of the hole adjacent the hot side without a corresponding velocity decrease. Increased convective cooling adjacent the hot side results and the temperature of the wall material at the hot side is reduced.

Perpendicular holes at the cool side of the wall decrease the wetted surface area of the hole at the cool side when compared to the wetted surface area at the hot side. Lesser convective cooling results and the temperature of the wall material at the cool side is increased. While locally increasing the wall temperature does not readily appear desirable, the beneficial effect of reduced thermal gradients between the cool side and the hot side does make such an increase important to the reduction of stress levels in the wall material.

Additionally, increasing the diameter of the cooling hole in the perpendicular portion may decrease the air velocity and, therefore, decrease the convective cooling ratio at the cool side as long as the wetted surface area does not become excessively large. The effective employ of the above described concepts may result in approximately a one-third (3) reduction in the magnitude of the thermal gradient across the wall, which in typical gas turbine engine embodiments is about a one-hundred degree Fahrenheit (1 000F) reduction.

In summation, the canted portion of the holes 30 at the hot side of the wall provides the large discharge area and resultant low velocity flow into the medium flowpath that is cssential to effective film cooling, without sacrificing the high axial velocity along the wetted surface area of the holes that is essential to effective convective cooling.

Hot side cooling is maximized. Further, reduced convective cooling at the cool side, as provided by the perpendicular portion of the holes 30, allows the temperature at the cool side to rise to a level more closely matched to the temperature at the hot side. A reduced thermal gradient results.

Complex hole geometries required in the practice of the above embodiment are not easily achieved by conventional manufacturing techniques. Hole sizes and locations generally preclude full utilization of drilling and machining techniques. The concepts are, therefore, illustrated in conjunction with what is known in the industry as "wafer" technology. The turbine component, such as the blade illustrated in Fig. 2 is fabricated of a plurality of adjacently bonded plates or wafers 42. One half of the contour of each hole 30 is etched into each adjacent wafer, such that in the bonded condition the full holes 30 are formed.

Although the invention has been shown and described with respect to detailed embodiments thereof, it should be understood by those skilled in the art that various changes and omissions in form and detail may be made therein without departing from the spirit and the scope of the invention.

Claims (3)

1. A coolable wall element of the type disposed between a hot medium and a cool medium, and in which the cool medium is flowable thereacross to protect the coolable wall structure from the hot medium, comprising: a wall structure having a cool side and a hot side and a plurality of holes extending from the cool side across the structure to the hot side wherein the geometry of the holes is contoured to provide a rate of convective cooling in the wall at the cool side which is less than the rate of convective cooling in the wall at the hot side.

2. The coolable wall element according to claim 1 wherein each hole has a first portion which is substantially perpendicular to the cool side of the wall and extends from the cool side of the wall into the material from which the wall is fabricated and a second wall portion extending from said first portion across the portion of the wall material adjacent the hot side element which is canted to a shallow angle with respect to the hot side of the coolable wall element.

3. The coolable wall element according to claims 1 and 2 wherein said coolable wall element is fabricated from a plurality of adjacent wafer elements and wherein said cooling air holes are formed between adjacent wafer elements.