Hot-film sensors, consisting of a metallic film on an electrically nonconductive substrate, have been used to measure skin friction as far back as 1931. A hot film is maintained at an elevated temperature relative to the local flow by passing an electrical current through it. The power required to maintain the specified temperature depends on the rate at which heat is transferred to the flow. The heat-transfer rate correlates to the velocity gradient at the surface, and hence, with skin friction. The hot-film skin friction measurement method is most thoroughly developed for steady-state conditions, but additional issues arise under transient conditions.

Fabricating hot-film substrates using low-thermal-conductivity ceramics can offer advantages over traditional quartz or polyester-film substrates. First, a low conductivity substrate increases the fraction of heat convected away by the fluid, thus increasing sensitivity to changes in flow conditions. Furthermore, the two-part, composite nature of the substrate allows the installation of thermocouple junctions just below the hot film, which can provide an estimate of the conduction heat loss.

Figure 1. The Composite Ceramic Substrate of this hot-film sensor reduces conduction losses and increases sensitivity.
Figure 1 depicts a hot-film sensor of this type. The substrate is primarily composed of high- temperature reusable shuttle insulation (HRSI), a lightweight (density = 352 kg/m3), porous, ceramic material originally developed to protect the space shuttle from aerodynamic heating. A hard, non-porous coat of reaction-cured glass (RCG) extends over the face of the cylinder and about one-third of the way down the side providing a surface on which the metallic hot film and its leads can be deposited. Small-diameter [0.005 in. (0.127 mm)] thermocouple wires are routed through the HRSI. Small grooves in the end of the HRSI cylinder, form the lands of the thermocouples and are deep enough such that the wires lie flush with the HRSI surface prior to being coated with the RCG. The three thermocouple junctions are placed in a line. The substrates are placed in a machinable-ceramic sleeve that provides electrical isolation for the hot-film leads. Type R thermocouples must be used because the high firing temperature of the RCG coating precludes the use of the more-sensitive thermocouples of type K's.

Figure 2. Steady-State Temperature Contours, determined from conjugate heat-transfer analyses, illustrate the effect of the lower thermal conductivity of the composite ceramic substrate relative to a quartz substrate. Temperatures are indicated in °C.
The hot film itself is approximately 0.004 in. (≈0.102 mm) wide and 1/4 in. (6.35 mm) long. Fabrication of the hot film and its leads begins with hand painting the desired pattern using organo-metallic inks. The painted substrate is then heated in an oven, which removes the solvents from the ink leaving only a gold-alloy film (see Figure 1 photo). The sensor thermocouples provide feedback control to the oven. These techniques could be used for the fabrication of other temperature and heat-flux gauges on high-temperature ceramics.

Conjugate heat-transfer analyses were performed on different substrate materials in air at moderate velocity gradients (7,500 s–1). For the composite ceramic substrate, the ratio of heat leaving the sensor via convection to total heat produced is about 4 times higher than for a quartz substrate. Figure 2 depicts steady-state temperature contours for quartz and a composite ceramic substrate. Preliminary bench tests comparing hot films on composite ceramic and machinable-ceramic substrates indicate that, at overheat ratios of 1.2 and in horizontal orientations, the higher conductivity machinable-ceramic substrates require over 2.5 times the power.

This work was done by Greg Noffz of Dryden Flight Research Center, Daniel Leiser of Ames Research Center, Jim Bartlett of Langley Research Center, and Adrienne Lavine of UCLA. For further information, contact the Dryden Commercial Technology Office at (661) 276-3689. DRC-01-48.


NASA Tech Briefs Magazine

This article first appeared in the June, 2003 issue of NASA Tech Briefs Magazine.

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