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

A Review of Loop Heat Pipes for Aircraft Anti-Icing Applications

A turbomachine includes a compressor having an inter-stage gap between adjacent rows of rotor blades and stator vanes. A combustor is connected to the compressor, and a turbine is connected to the combustor. An intercooler is operatively connected to the compressor, and includes a first plurality of heat pipes that extend into the inter-stage gap. The first plurality of heat pipes are operatively connected to a first manifold, and the heat pipes and the first manifold are configured to transfer heat from the compressed airflow from the compressor to heat exchangers. A cooling system is operatively connected to the turbine, and includes a second plurality of heat pipes located in the turbine nozzles. The second plurality of heat pipes are operatively connected to a second manifold, and the heat pipes and the second manifold are configured to transfer heat from the turbine nozzles to the heat exchangers.

Heat pipe temperature management system for a turbomachine

Fundamental heat transfer experiments were carried out for three kinds of heat pipes that may be applied to turbine cooling in future aero-engines. In the turbine cooling system with a heat pipe, heat transfer rate and start-up time of the heat pipe are the most important performance criteria to evaluate and compare with conventional cooling methods. Three heat pipes are considered, called heat pipe A, B, and C, respectively. All heat pipes have a stainless steel shell and nickel sintered powder metal wick. Sodium (Na) was the working fluid for heat pipes A and B; heat pipe C used eutectic sodium-potassium (NaK). Heat pipes B and C included noncondensible gas for rapid start-up. There were fins on the cooling section of heat pipes. In the experiments, an infrared image furnace supplied heat to the heat pipe simulating turbine blade surface conditions. In the results, heat pipe B demonstrated the highest heat flux of 17 to 20 W/cm 2 . The start-up time was about 6 minutes for heat pipe B and about 16 minutes for heat pipe A. Thus, adding noncondensible gas effectively reduced start-up time. Although NaK is a liquid phase at room temperature, the start-up time of heat pipe C (about 7 to 8 minutes) was not shorter than the heat pipe B. The effect of a gravitational force on heat pipe performance was also estimated by inclining the heat pipe at an angle of 90 deg. There was no significant gravitational dependence on heat transport for heat pipes including noncondensible gas.

Fundamental Heat Transfer Experiments of Heat Pipes for Turbine Cooling

A turbomachine includes a compressor configured to compress air received at an intake portion to form a compressed airflow that exits into an outlet portion. A combustor is operably connected with the compressor, and the combustor receives the compressed airflow. A turbine is operably connected with the combustor, and the turbine receives combustion gas flow from the combustor. The turbine has a turbine casing. A cooling system is operatively connected to the turbine casing. The cooling system includes a plurality of heat pipes attached to and in thermal communication with the turbine casing. The plurality of heat pipes are operatively connected to one or more manifolds. The plurality of heat pipes and the one or more manifolds are configured to transfer heat from the turbine casing to a plurality of heat exchangers.

Heat pipe cooled turbine casing system for clearance management

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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Heat Pipe Cooling of Turboshaft Engines

A simple analytical model is derived for the prediction of time-dependent deformation and damage response of metal matrix composites under fiber direction loading. The model can be used in conjunction with a number of viscoplastic constitutive models to describe the matrix material behavior. Damage in the form of progressive fiber fractures is incorporated using a mechanistic approach. The criterion for fiber fractures can be based on statistical information on fiber strength. When used in conjunction with a prescribed failure condition for a composite, the model provides a means for predicting composite life under general thermomechanical load conditions. Based on comparison of results with detailed finite element analyses and with laboratory test data, it appears that the simple model provides reasonably accurate predictions.

Sandra Hoff

Papers

Heat Pipe Cooling of Turboshaft Engines

A Metal Matrix Composite Damage and Life Prediction Model