The primary focus of this paper is convective heat transfer in axial flow turbines. Research activity involving heat transfer generally separates into two related areas: predictions and measurements. The problems associated with predicting heat transfer are coupled with turbine aerodynamics because proper prediction of vane and blade surface-pressure distribution is essential for predicting the corresponding heat transfer distribution. The experimental community has advanced to the point where time-averaged and time-resolved three-dimensional heat transfer data for the vanes and blades are obtained routinely by those operating full-stage rotating turbines. However, there are relatively few CFD codes capable of generating three-dimensional predictions of the heat transfer distribution, and where these codes have been applied the results suggest that additional work is required. This paper outlines the progression of work done by the heat transfer community over the last several decades as both the measurements and the predictions have improved to current levels. To frame the problem properly, the paper reviews the influence of turbine aerodynamics on heat transfer predictions. This includes a discussion of time-resolved surface-pressure measurements with predictions and the data involved in forcing function measurements. The ability of existing two-dimensional and three-dimensional Navier-Stokes codes to predict the proper trends of the time-averaged and unsteady pressure field for full-stage rotating turbines is demonstrated. Most of the codes do a reasonably good job of predicting the surface-pressure data at vane and blade midspan, but not as well near the hub or the tip region for the blade. In addition, the ability of the codes to predict surface-pressure distribution is significantly better than the corresponding heat transfer distributions. Heat transfer codes are validated against measurements of one type or another. Sometimes the measurements are performed using full rotating rigs, and other times a much simpler geometry is used. In either case, it is important to review the measurement techniques currently used. Heat transfer predictions for engine turbines are very difficult because the boundary conditions are not well known. The conditions at the exit of the combustor are generally not well known and a section of this paper discusses that problem. The majority of the discussion is devoted to external heat transfer with and without cooling, turbulence effects, and internal cooling. As the design community increases the thrust-to-weight ratio and the turbine inlet temperature, there remain many turbine-related heat transfer issues. Included are film cooling modeling, definition of combustor exit conditions, understanding of blade tip distress, definition of hot streak migration, component fatigue, loss mechanisms in the low turbine, and many others. Several suggestions are given herein for research and development areas for which there is potentially high payoff to the industry with relatively small risk.

Convective Heat Transfer and Aerodynamics in Axial Flow Turbines

Turbine blade (10) has a plurality of trailing edge discharge openings (28) discharging cooling air. The blade trailing edge has an increasing thickness 'E' toward the tip end (16). Discharge openings with the shortened pressure wall 'L' have lesser distances 'L' toward the tip end.

Cooled turbine blade

A gas turbine engine airfoil includes a serpentine cooling circuit with a branch cooling circuit disposed in parallel therewith for independently controlling discharge of cooling air therefrom through respective discharge holes in pressure and suction sides of the airfoil. Metering orifices are provided between the serpentine circuit and the branch circuit for controlling flow of cooling air into the branch circuit from the serpentine circuit, and therefore controlling discharge of the cooling air from the branch discharge holes relative to the serpentine discharge holes.

Turbine blade composite cooling circuit

A gas turbine engine rotor blade includes an airfoil having first and second sidewalls joined together at leading and trailing edges. First and second tip walls extend from adjacent the leading edge along the respective first and second sidewalls to adjacent the trailing edge and are spaced apart to define a tip cavity therebetween. A first notch is disposed in the first tip wall adjacent to the leading edge for channeling into the tip cavity a portion of combustion gases flowable over the airfoil for reducing the heating effect of the gases on the blade tip. In a preferred embodiment, a second notch is disposed adjacent to the trailing edge for promoting flow through the tip cavity from the first notch.

Turbine blade having tip slot

An improved cooling passage configuration for a gas turbine engine airfoil is disclosed for enhancing convective cooling over that afforded by conventional wall turbulator ribs. An internal cooling passage bounded by pressure and suction side walls and at least one partition has raised turbulator ribs disposed on at least one side wall and the partition. A side wall turbulator rib extends over at least a portion of the side wall and abuts the partition. A turbulator rib disposed on the partition extends from a point spaced from the wall rib to the opposed side wall. The gap formed between the partition rib and wall rib accelerates a coolant flow passing therethrough, locally enhancing convective heat transfer. Partition ribs may be aligned with or offset from the wall ribs in the spanwise direction and may be normal to the primary direction of cooling flow or angled thereto. Convective heat transfer is particularly enhanced in flow passages having cavity height blockage due to wall ribs greater than about 10%.

Enhanced cooling apparatus for gas turbine engine airfoils

A hollowed, internally cooled airfoil per gas turbine engines with an improved internal configuration for pronounced impingement cooling. Internal ribs extend across the hollowed interior and cooling passages through these ribs cause impingement cooling of the next-adjacent rib as well as the internal surfaces of both the pressure side and suction side of the airfoil.

Internally cooled airfoil

Reduction in turbine engine cooling flows is required to meet the IHPTET Phase II engine performance levels. Heat pipes, which are devices with very high thermal conductance, can help reduce the required cooling air. A survey was conducted to identify potential applications for heat pipes in turboshaft engines. The applications for heat pipe cooling of turbine engine components included the power turbine first stage vanes, shroud, and case, the HP turbine vanes and shroud, and the T5 temperature probe. Other potential applications for heat pipe cooling include regenerative cycle and intercooling, bearing cooling, IR signature reduction, and active clearance control. Calculated performance benefits included an increase in specific shaft horsepower, and a decrease in specific fuel consumption, as determined with an IHPTET Phase II turboshaft engine performance model. For example, using heat pipes to cool the power turbine vanes, shroud, and case would increase the specific shaft horsepower by 6 percent, while decreasing the specific fuel consumption by 2.2 percent. While this study examined turboshaft engines, most of the applications are also applicable to turbofan engines.Copyright © 1993 by ASME

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John K. Phillips

Papers

Internally cooled airfoil

Heat Pipe Cooling of Turboshaft Engines