Aerodynamic heating

Aerodynamic heating is the heating of a solid body produced by the passage of fluid (such as air) over a body such as a meteor, missile, or airplane. It is a form of forced convection in that the flow field is created by forces beyond those associated with the thermal processes. The heat transfer essentially occurs at vehicle surface where aerodynamic viscous forces ensures that the flow is at zero speed relative to the body for a very small layer of molecules at the surface. Because the flow has slowed to zero speed at this point a significant amount of its kinetic energy from the free-field is converted to heat. In high speed flows, tremendous energy is represented by the mean motion of the flow. As the flow is slowed to the zero speed, its temperature is increased. But the gradient in the speed in the direction normal to the surface, allows to small scale mass transport effects to dissipate the temperature in the outward direction and thus the termperature at the surface is less than the stagnation temperature. The actual temperature is referred to as the recovery temperature. These viscous dissipative effects to neighboring sub-layers make the boundary layer slow down a non-isentropic process. Heat then conducts into the surface material from the higher temperature air. The result is an increase in the temperature of the material and a loss of energy from the flow. The forced convection ensures that other material replenishes the gases that have cooled to continue the process. The stagnation and the recovery temperature of a flow increases with the speed of the flow and are greater at high speeds. The total thermal loading of the structure is a function of both the recovery temperature and the mass flow rate of the flow. Aerodynamic heating is greatest at high speed and in the lower atmosphere where the density is greater. In addition to the convective process described above, there is also radiative heat transfer from the flow to the body and vice versa with the net direction set by the relative temperature of each.

Aerodynamic heating increases with the speed of the vehicle and is continuous from zero speed. It produces much less heating at subsonic speeds but becomes more important at supersonic speeds. At these speeds it can induce temperatures that begin to weaken the materials that compose the object. The heating effects are greatest at leading edges. Aerodynamic heating is dealt with by the use of high temperature alloys for metals, the addition of insulation of the exterior of the vehicle, or the use of ablative material.

Aircraft

Aerodynamic heating is a concern for supersonic and hypersonic aircraft. The Concorde dealt with the increased heat loads at its leading edges by the use of high temperature materials and the design of heat sinks into the aircraft structure at the leading edges. Higher speed aircraft such as the SR-71 deal with the issue by the use of insulating material and material selection on the exterior of the vehicles. Some designs for hypersonic missiles would employ liquid cooling of the leading edges (usually the fuel en route to the engine).

Reentry Vehicles

Aerodynamic heating is topic of great concern in atmospheric reentry. The heating induced by the very high speeds of reentry of greater than Mach 20 is sufficient to destroy the structure of the vehicle. The early space capsules such as Mercury, Gemini, and Apollo were given blunt shapes to produce a stand-off bow shock. As a result most of the heat is dissipated to surrounding air. Additionally, these vehicles had abalative material that sublimates into a gas at high temperature. The act of sublimation absorbs the thermal energy from the aerodynamic heating and erodes the material away. The Space Shuttle uses an insulating tile on its lower surface to absorb and radiate heat while preventing conduction to the aluminum airframe.

References

• Moore, F.G., Approximate Methods for Weapon Aerodynamics, AIAA Progress in Astronautics and Aeronautics, Volume 186
• Chapman, A.J., Heat Transfer, Third Edition, Macmillan Puslishing Company, 1974