This article reviews heat pipes that address thermal management problems inside high-performance aircraft engines. Higher performance engines demand that compressors develop higher pressure ratios which, in turn, result in higher temperatures at the entrance to the combustor. CCS Associates of Bethel Park, PA, proposes tackling the problem by using pipes to distribute heat more effectively throughout the combustor. The heat pipe liner must handle both acceleration and vibration. The heat pipe arrays, including half-thickness webs, can be fabricated into gas-side and air-side halves by extrusion, forging, stamping, chemical milling, or some combination of methods. The liquid flow channel would be formed as an integral part of the gas-side valves.
In their efforts to boost aircraft engine performance, engineers are running into material limits. Higher-performance engines demand that compressors develop higher-pressure ratios which, in turn, result in higher temperatures at the entrance to the combustor. Inside the combustion chamber the problem is exacerbated; high-performance engine cycles occur at elevated temperatures, adding further to the heat load.
The increased heat load cannot be handled by the conventional superalloys currently used to line aircraft combustors. High-temperature-resistant, nonmetallic liners under development may eventually solve this problem, but these ceramic liners will still be thicker, heavier, and more difficult to fabricate than the conventional metal ones.
CCS Associates of Bethel Park, Pa., proposes tackling the problem by using pipes to distribute heat more effectively throughout the combustor. Based on an analysis performed under contract to the Aviation Applied Technology Directorate in Fort Eustis, Va., CCS Associates found that if heat pipes were built into combustor liners, they could reduce combustor exit temperatures sufficiently to allow the continued use of conventional superalloy liners even in new, high-performance engines.
In one high-performance helicopter engine study, the compressor discharge air was delivered to the combustor at the rate of 15 lbs./sec., a pressure ratio of 40, and a temperature of 1,200°F. The gases exiting the combustor reached almost 3,500°F, yet heat pipes kept the metallic liner temperature below the design limit of 1,700°F.
Currently, a portion of the incoming compressed air not used for combustion is directed through slots in the liner to form a cooling film along the inside (gas side) of the liner. The remaining excess air is divided into two parts; most is injected through separate openings in the liner to mix with the hot combustion gas, while the remainder cools engine components downstream of the combustor.
This method is quite effective in existing engines, but as compressor ratios increase, the incoming air temperature also increases, making the air less effective as a cooling medium. At the same time, high-performance aircraft engine cycles demand more air for fuel combustion, leaving less for cooling purposes.
A heat pipe removes heat through simple vaporization and condensation of an appropriate heat transport fluid. The fluid is held in a thin porous layer called a capillary wick. Heat at one end of the pipe vaporizes the fluid, and the vapor travels to the other end of the heat pipe, where it condenses and returns by the capillary wick to the heat input end. Capillary pressure in the wick maintains continuous circulation between the liquid and vapor phases.
Heat pipes can move heat at high rates over appreciable distances isothermally and without any external pumping. The effective thermal conductivity in the direction of heat transport is tens of thousands of times that of copper.
In a proposed design, the annular combustor is lined inside and out with axially aligned heat pipe arrays that are spaced equally around the liner and held in position by metallic webs. Heat carried upstream by the heat pipes is extracted by the incoming air in the heat exchange zone. The air then passes through openings in the webs and enters the combustion chamber, where it mixes with the combustion gas. Unlike conventional designs, all excess air is used for the initial liner cooling and then for mixing and dilution of the combustion gas.
The proposed heat pipes are cylindrical to provide optimum resistance to the pressure differential between the internal heat pipe vapor and the higher- pressure, external casing air or combustion gas. Airflow channels, existing in the heat exchange zone, funnel the cooling air over the heat pipes. A thermal barrier coating, applied to heat pipe walls that are exposed to combustion chamber gases, reduces heat flux by 40 percent and isolates the liner from rapid temperature changes during power transients, making it possible to reduce the length and weight of the heat exchange zone.
A wall thickness of 0.02 inch is adequate to withstand stresses within the heat pipe wall. For the outer liner, stiffening rings may be needed at the combustor inlet and exit to prevent buckling. Buckling is not a problem tor the inner liner because the higher casing air pressure is on the convex side.
Sodium would be the mostly likely fluid candidate. Sodium heat pipes will operate at temperatures as high as 1,400 to 1,800°F, depending on the heat pipe design. They can handle surface heat fluxes of 4,000 W/cm2 and axial heat fluxes (heat transport rate per unit of vapor space flow area) on the order of 8,000 W/cm2. These levels are more than adequate for high-performance gas turbine combustors.
Because only a small amount of sodium—about two ounces—would be used in both liners, there would be no concern about leakage creating a fire hazard. Freezing of the liquid sodium also should not be a problem because, during startup, the compressed air temperature rises well above 208°F, the melting point of sodium.
Care must be taken to ensure that the liner materials do not pose a problem for heat pipe operation. If the combustor liner material is capable of reducing water vapor to tree hydrogen, the hydrogen could diffuse into the heat pipe vapor space and create a hydrogen bubble that could limit heat transport.
There are several ways to attack this problem. First, the exterior heat pipe surfaces could be pre-oxidized to prevent the reduction of hydrogen compounds to free hydrogen. Second, the heat pipe wall could be fabricated from materials with a high affinity for hydrogen—that is, hydrogen getters—that would intercept the hydrogen before it reached the heat pipe interior. Or small inserts °f getter materials, such as titanium or zirconium, could be placed inside the heat pipe to remove any incoming hydrogen. Still another method would be to apply a thin coating of a carbon compound or other low-hydrogen permeability material to the liner wall.
The heat pipe liner must handle both acceleration and vibration. Acceleration would create a static head in the liquid sodium that could be compensated for by reducing the wick pore size and thereby increasing the capillary pressure. Intense vibration does not appear to affect heat pipes adversely, but further testing is needed. One test on a water-based heat pipe exposed to typical vibration levels from a missile launch actually showed increased performance.
The heat pipe arrays, including half-thickness webs, can be fabricated into gas-side and air-side halves by extrusion, forging, stamping, chemical milling, or some combination of methods. The liquid flow channel would be formed as an integral part of the gas-side valves.
A continuous capillary (metallic screen wick) layer would be added to each half over the liquid flow channel on the gas side and spot welded to the heat pipe walls. The two halves would be welded, brazed, or sintered together at the common cell walls. The airflow channels in the heat exchange zone would then be brazed or sintered onto the liners.
After adding end caps and evacuating the heat pipes, sodium would be added and the assemblies sealed. Individual arrays would be formed into a complete liner by brazing together half-thickness webs on adjacent arrays.
The simplicity of heat pipe design lends itself readily to integration into gas turbine engines with minimum volume and low weight. Transient temperatures occurring during startup and shutdown present no problem; heat pipes move heat rapidly so there is no danger of overheating from sudden temperature changes. In short, heat pipes can effectively distribute heat within the combustor liners, eliminate hot spots, and make it possible to continue using conventional superalloys in liners for high-performance aircraft.