Gauges made of slugs of pyrolytic graphite with thermocouples em- bedded in them have been invented for use in measuring large, short-duration heat fluxes in hot, highly corrosive environments. These gauges were originally intended for use in combustion chambers of rocket engines; they might also be useful in terrestrial combustion chambers (e.g., in furnaces) and metal-processing equipment.

This Slug-Type Heat-Flux Gauge exploits the anisotropic thermal conductivity of pyrolytic graphite. Here, the high-conductivity dimensions are H and L, and the low-conductivity dimension is W.
A gauge of this type is basically a calorimeter with a thermal mass large enough that its thermal-transient-decay time is significantly greater than the duration of the heat flux that one seeks to measure. One surface of the slug is placed in contact with the surface across which one seeks to measure the heat-flux density; the opposite surface of the slug is kept insulated. Then given the mass and specific heat of the slug and assuming that the slug is approximately isothermal at any given instant, the net flux of heat into the slug can be estimated as the product of the mass of the slug, the specific heat of the slug, and the rate of change of temperature as measured by the thermocouple in the slug. Then given the area through which heat flows into or out of the slug on the surface of interest, the heat-flux density is given simply by the estimated flux divided by this area.

As described thus far, the slug could be made of any of a variety of thermally conductive materials. The reasons (other than high-temperature endurance and resistance to corrosion) for making the slug out of pyrolytic graphite are best explained by the example of the figure. Pyrolytic graphite exhibits anisotropic thermal conductivity: its conductivity is high (comparable to that of copper) in two perpendicular directions and low (about 10–2 × as much) in the third perpendicular direction. In this case, the material in the slug is oriented so that the high conductivity is along the H and L axes and the low conductivity along the W axis. The high conductivity along the H and L axes helps to keep thermal gradients within the slug small, thereby making the response of the thermocouple fairly insensitive to its depth within the slug and justifying the assumption of isothermality. The low thermal conductivity along the W axis makes it possible to fabricate the slug as a plate that is thin along the W axis while still avoiding thermal gradients that would otherwise be caused by edge effects.

This work was done by Robert C. Bunker, Mark E. Ewing, and John L. Shipley of Cordant Technologies for Marshall Space Flight Center.

Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457(f)} to Thiokol Propulsion. Inquiries concerning licenses for its commercial development should be addressed to

Thiokol Propulsion
P.O. Box 707
M/S A11
Brigham City, UT 84302-0707

Refer to MFS-31572, volume and number of this NASA Tech Briefs issue, and the page number.

NASA Tech Briefs Magazine

This article first appeared in the September, 2002 issue of NASA Tech Briefs Magazine.

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