A project is underway in the Flight Loads Laboratory (FLL) at Dryden Flight Research Center to reduce the uncertainties in heat-flux measurements. The impetus for this project is provided, in part, by the observation that uncertainties in heat-flux measurements are large — often 10 to 20 percent or more. Further impetus is provided by the fact that heat-flux calibration facilities being developed at the National Institute of Standards and Technology (NIST) operate at heat fluxes well below the levels which can be achieved during high-speed flight. Thus, a heat-flux-gauge user interested in such high fluxes has only two options: (1) take the gauge manufacturer’s calibration on faith or (2) develop and understand his or her own calibration process.

Figure 1. This Heat-Flux-Gague Calibration System is to be characterized thoroughly, by a combination of experimental and theoretical means, as part of an effort to quantify and reuce calibration erros.
The broad objectives of the project are:

• Characterize a radiant-heat-flux gauge-calibration system in the FLL (see Figure 1) in order to quantify and, if possible, reduce calibration uncertainty and

• Be able to demonstrate to customers that the FLL personnel understand the heat-flux-gauge calibrations that they perform.

The first phase of the project involves a thorough characterization of the calibration system, which uses an electrical resistance heated graphite plate as a heat source, in order to reach the first stated objective. Future phases of the project will involve development of methods for using gauges calibrated in this system in radiant-heating tests performed in the FLL and in flight.

The following is a partial list of technical challenges that must be met in the project:

• Determine the effects of erosion of graphite plates,

• Determine the effect of convection on the heat flux gauge, and

• Determine the effect, upon the distribution of absorbed heat flux, of the distance between a graphite plate and heat-flux gauge.

Figure 2. The Isotherms in the Rectangles that represent a graphite plate and a nearby heat-flux gauge were computed for a test case, by use of a two-dimensional mathematical model.
Thus far, an initial two-dimensional mathematical model of heat flux and temperature distribution has been developed for a vertical cross section of the graphite plate and heat flux gauge (see Figure 2). The rectangles in Figure 2 representing the graphite plate and heat flux gauge are in their proper relative positions and orientations but are not shown to scale. The results of initial experiments include heat fluxes within 10 percent of those predicted by the model. An initial graphite plate erosion model has predicted plate erosion within 5 percent of the measured values.

Planned future efforts include the following:

• Refinements of the two-dimensional model, including an improved erosion model and incorporating internal details of a circular foil heat flux gauge;

• Refinement of experiments to collect such additional detailed information as rates of flow and temperatures of gauge-cooling water and distributions of temperature in graphite plates;

• Expansion of the model to three dimensions; and

• Determination of experimental heat fluxes by use of alternative physical principles.

This work was done by Tom Horn of Dryden Flight Research Center and Shanjuan Jiang and Vijay Dhir of the University of California. For further information, contact the Dryden Commercial Technology Office at 661-276-3689. DRC-98-77