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Research Papers

Feasibility of Metallic Structural Heat Pipes as Sharp Leading Edges for Hypersonic Vehicles

[+] Author and Article Information
Craig A. Steeves, Ming Y. He, Anthony G. Evans

Department of Materials, University of California, Santa Barbara, Santa Barbara, CA 93106

Scott D. Kasen, Haydn N. G. Wadley

Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904

Lorenzo Valdevit

Department of Mechanical and Aerospace Engineering, University of California, Irvine, Irvine, CA 92697

J. Appl. Mech 76(3), 031014 (Mar 13, 2009) (9 pages) doi:10.1115/1.3086440 History: Received January 25, 2008; Revised May 07, 2008; Published March 13, 2009

Hypersonic flight with hydrocarbon-fueled airbreathing propulsion requires sharp leading edges. This generates high temperatures at the leading edge surface, which cannot be sustained by most materials. By integrating a planar heat pipe into the structure of the leading edge, the heat can be conducted to large flat surfaces from which it can be radiated out to the environment, significantly reducing the temperatures at the leading edge and making metals feasible materials. This paper describes a method by which the leading edge thermal boundary conditions can be ascertained from standard hypersonic correlations, and then uses these boundary conditions along with a set of analytical approximations to predict the behavior of a planar leading edge heat pipe. The analytical predictions of the thermostructural performance are verified by finite element calculations. Given the results of the analysis, possible heat pipe fluid systems are assessed, and their applicability to the relevant conditions determined. The results indicate that the niobium alloy Cb-752, with lithium as the working fluid, is a feasible combination for Mach 6–8 flight with a 3 mm leading edge radius.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

The local geometry and flow conditions near the leading edge

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Figure 2

A structural heat pipe for the leading edge of a hypersonic vehicle showing (a) a cutaway view with the metallic wick material removed to show the cruciform structural members, and (b) the assembly with both metallic faces and the wick installed

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Figure 3

The temperature dependence of the yield strength for the two refractory alloys, Inconel 625 (red) and Cb-752 (blue), used in the analysis, shown as solid lines. Analytical cross-plots of maximum stress as a function of temperature are presented as dashed lines for Mach 6–8. The corresponding finite element results are plotted as circles.

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Figure 4

The assumed geometry for the calculation of the heat transfer coefficient along the flat radiating surface. The actual geometry is depicted with solid lines, while the dashed lines indicate the geometry for a perfectly sharp leading edge.

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Figure 5

The stagnation point heat transfer coefficient as a function of the leading edge radius for a range of Mach numbers

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Figure 6

The leading edge temperatures calculated analytically (lines) over the relevant range of parameters, and by finite elements (circles and crosses) at Mach 6 and 8 for a vehicle with Rle=3 mm, ϵ=0.9, and θ=6 deg. (a) The isothermal temperature approximation as a function of the length of the radiating surface. Note that the analytical predictions are identical for the two materials, and that the finite element calculations at Mach 6 for Inconel 625 and Cb-752 fall on top of each other. (b) The maximum temperature at the stagnation point on the surface of the leading edge for Inconel 625 (dots and crosses) and Cb-752 (dashes and circles).

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Figure 7

A sketch of the region along the stagnation line for which the temperatures are solved by a cylindrical finite difference scheme

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Figure 8

The maximum thermally induced Mises stress at the exterior surface along the stagnation plane for a vehicle with Rle=3 mm, ϵ=0.9, and θ=6 deg. The analytical results are lines, while the finite element results are circles and crosses.

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Figure 9

The behavior of the leading edge in the absence of a heat pipe: The temperature distribution along the length of an Cb-752 (k=48 W/m K) leading edge at Mach 7 for a range of design lengths L, assuming that the material is isothermal through the thickness and that ϵ=0.9, Rle=3 mm, and t=1 mm

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Figure 10

A typical finite element mesh used in the simulations. The solid metallic component is represented in black, while the heat pipe is in gray.

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Figure 11

(a) Contours of the temperatures induced in the nickel alloy at Mach 6 when the heat pipe is functioning. (b) The temperatures along the external surface with analytic results for Tiso and Tmax for ϵ=0.9 are superimposed.

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Figure 12

(a) Contours of the temperatures induced in the niobium alloy at Mach 8 when the heat pipe is functioning. (b) The temperatures along the external surface with analytic results for Tiso and Tmax for ϵ=0.9 are superimposed.

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Figure 13

Heat flows at Mach 6 for an Inconel 625 leading edge. (a) Contours of heat flux at the design length L=0.15 m and ϵ=0.9. (b) Local heat flux into the exterior surface. (c) Integrated heat input from the leading edge. At the design length the net total heat input is zero.

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Figure 14

Heat flows at Mach 8 for a Cb-752 leading edge. (a) Contours of heat flux at the design length L=0.15 m and ϵ=0.9. (b) Local heat flux into the exterior surface. (c) Integrated heat input from the leading edge. At the design length the net total heat input is zero.

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Figure 15

Mises stress contours when the heat pipe is functioning: (a) Mises stresses for Mach 6 with nickel-based superalloy Inconel 625; and (b) Mises stresses for Mach 8 with niobium alloy Cb-752

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Figure 16

The temperature distribution through a niobium leading edge with a 100 μm anti-oxidation environmental barrier coating for a Mach 7 vehicle

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Figure 17

The idealized behavior of the leading edge during the startup phase for a Cb-752 leading edge at Mach 7. (a) The temperature at the external surface Tmax, at the back surface Thp, and the difference between the two temperatures Tdiff. (b) The resulting Mises stress at the stagnation point.

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Figure 18

A cross-sectional schematic showing the operating principles and relevant geometry of a heat plate leading edge

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Figure 19

The predicted operational limits of a sodium-Inconel 625 heat plate leading edge with thermal flux leading edge input corresponding to Mach 6 (26.93 km, q̇max≈1.2 MW/m2)

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Figure 20

The predicted operating limits of a lithium-Cb-752 heat plate leading edge with thermal flux inputs corresponding to Mach 8 (30.76 km, q̇max≈3.2 MW/m2)

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