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 the vehicle surface where aerodynamic viscosity ensures that the flow is at zero speed relative to the body for a very thin layer of molecules at the surface.

When fluid flow slows down its kinetic energy 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 near zero speed, its temperature increases, the gradient in the speed in a direction normal to the surface allows small scale mass transport effects to dissipate the temperature in the outward direction and thus the temperature 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 via 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 Thermal radiation 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). The Sprint missile's heat shield, in addition to speeds approaching Mach 10, had to protect against (possibly multiple) Nuclear Electromagnetic Pulses. The Sprint's heat shield needed several design iterations to handle expansion caused by the extreme heat.[1]

Reentry vehicles

Aerodynamic heating is also a topic of concern in reentry vehicles. 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 those on 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 without transferring through the vehicle structure. Additionally, these vehicles had ablative 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 as opposed to heating the capsule. The surface of the heat shield for the Mercury spacecraft had a coating of aluminum with glassfiber in many layers. As the temperature rose to 2,000 °F (1,100 °C) the layers would evaporate and take the heat with it. The spacecraft would become hot but not harmfully so.[2] The Space Shuttle used insulating tiles on its lower surface to absorb and radiate heat while preventing conduction to the aluminum airframe. The compromise of the heat shield during liftoff of Space Shuttle Columbia contributed to its destruction upon reentry.

References

  1. ↑ Bell Labs 1974, 9-17
  2. ↑ "How Project Mercury Worked". How Stuff Works. http://science.howstuffworks.com/project-mercury2.htm. Retrieved 2011-10-04.