Thermal Management at Hypersonic Leading Edges

Abstract

The intense heat flux incident upon the leading edges of hypersonic vehicles traveling through the low earth atmosphere at speeds of Mach 5 and above requires creative thermal management strategies to prevent damage to leading edge components. Conventional thermal protection systems (TPSs) include the ablative coatings of NASA's Mercury, Gemini, and Apollo vehicles and the reusable reinforced carbon-carbon (RCC) system of the Space Shuttle Orbiter. The ablative approach absorbs heat by endothermic transformation (phase and/or chemical change to the polymeric coating). The heat is dissipated from the vehicle as the single-use coating eventually vaporizes. The RCC approach manages the intense heat by operating at high temperatures and radiating heat to its surroundings. The effectiveness of both approaches is predicated on keeping the heat flux that impinges upon the susceptible aluminum airframe below a critical level.ii ________________________________________________________________________ This dissertation has explored an alternative metallic TPS concept which seeks to redistribute the heat from the leading edge, thereby eliminating local hot spots. It makes use of high thermal conductance heat pipes coupled to the leading edge so that the thermal load may be redistributed from a high heat flux location (at the stagnation point) to regions where it can be effectively radiated from the vehicle. The sealed system concept is based upon the evaporation of a fluid near the heat source that sets up a region of elevated vapor pressure inside the pipe. The latent heat is transported down the resulting pressure gradient by the vapor stream where it condenses at cooler regions, releasing the heat for removal.Replenishment of the condensed working fluid to the evaporator region is accomplished through the capillary pumping action of a porous wick which lines the interior surface of the pipe.A design methodology for a wedge-shaped heat pipe is presented which uses a coupled flow-wall temperature model to construct design maps which relate design parameters of the leading edge system (overall length, wall thickness, and alloy) to its operating conditions (isothermal temperature, maximum temperature, maximum thermal stress). Potential bounds on heat transport due to physical phenomena linked to the sound speed within a chamber (sonic limit), capillarity, and boiling nucleation are considered by extending models developed for tube designs to the wedge geometry. A new heat flux limit is proposed which, should it be exceeded, subjects the leading edge to thermally-induced plastic deformation of the TPS. iii ________________________________________________________________________To investigate the validity of the design approach and thermal spreading effectiveness of the proposed concept, a low temperature wedge-shaped leading edge was designed and constructed using stainless steel as the case material and water as the working fluid. Under localized tip heating, the maximum temperatures were significantly reduced compa...

Shape Optimization of a Blunt Body in Reacting Hypersonic flow in Thermal Non-Equilibrium for Reducing Both Drag and Heat Transfer

“…A former single objective shape optimization results for minimizing drag alone matched well with the results from Reference [14], and the multi-objective shape optimization results with 50% weight assigned to drag and 50% weight assigned to maximum heat flux on the body heat matched well with the results in Reference [14]. Heat flux values with dissociated air reported in this paper become higher, but are of similar magnitude to those reported in Reference [18]. Reference [15] shows experimentally that a blunted cone shape has lower drag than a sharp cone at supersonic speeds; in this paper the optimized shapes appear to be approaching a blunted cone shape with a bluntness ratio of nearly 0.4 as the importance of reducing the drag is increased in the MOGA optimization.…”

Hypersonic Blunt-Body Shape Optimization for Reducing Drag and Heat Transfer

“…A single objective optimization for minimizing only the drag matched well with the results from [12]. The multi-objective optimization results with 50% weight assigned to minimizing drag and 50% weight assigned to minimizing heat flux matched well with the results in [14]. Optimized heat transfer values obtained in this paper are slightly higher but are almost of similar magnitude to those reported in [14].…”

“…The multi-objective optimization results with 50% weight assigned to minimizing drag and 50% weight assigned to minimizing heat flux matched well with the results in [14]. Optimized heat transfer values obtained in this paper are slightly higher but are almost of similar magnitude to those reported in [14]. Eremenko et al [15] show experimentally that a blunted cone shape has lower drag than a sharp cone at supersonic speeds, and the optimized shapes appear to be approaching a blunted cone shape with a bluntness ratio of around 0.4 as the importance of reducing the drag is increased.…”

Two Phase Thermal Protection of the Hypersonic Leading Edge

“…A detailed derivation and discussion of these commonly cited heat transfer limits can be found in [17]. An additional operating limit recently identified by Kasen [21] of importance in hypersonics is the material thermomechanical yield limit, where excessive thermal gradient through the structural geometry results in mechanical stresses that exceed the temperature-dependent yield strength. Kasen summarizes this limit as a critical heat flux in the context of a cylindrically-blunted isothermal wedge leading edge heat pipe with constant wall thickness, , as…”