The goal of this thesis is to develop a set of standards for use in calibrating heat flux gages at elevated temperatures in a radiant heat transfer environment by comparing several "trial" standards. Ideally, the same incident heat flux is derived from each standard when exposed to the same heat source. Three heat flux standards are proposed and evaluated. The standards are based on temperature measurements, material properties, and electrical measurements. The theory and design of each standard are described, as are the calibration procedures used. For experimental comparisons, two standards are simultaneously exposed to heat fluxes of up to 220 W/cm² by placing one standard on each side of a graphite flat plate heater. The temperature measurement based standard derives incident heat flux from the temperature of a blackbody heat source and the Stefan-Boltzmann law. The heat flux gage employed in this standard is a water-cooled Gardon gage. This standard does not operate at high temperatures. The calibration of this standard produced highly repeatable results. / Master of Science

The concept, design, fabrication, and calibration of a new type of layered heat flux gage is described. Using thin-film microfabrication techniques the gage design is able to combine many of the desirable characteristics of other previous gage designs to produce a gage that has a very small size, high frequency response, and the ability to measure very high heat flux rates. The heat flux microsensor incorporates a differential thermopile, a set of 100 differential thermocouple pairs connected in series across a thin thermal resistance layer. The gage is fabricated on a model surface without the need for additional adhesives. The design configuration allows the thermal insulating layer to remain very thin, while still retaining the high output typical of much thicker gages. The small overall thickness of the gage, less than 2 µm, gives it a fast time response with the capability of measuring heat transfer in transient flow conditions. The combination of small thickness and small surface dimensions, 1 mm by 1 cm, gives the microsensor negligible flow and thermal disruption. The performance of a prototype microsensor deposited on Corning 7059 glass was measured. Steady-state calibrations were done using a convection calibration apparatus. The measured sensitivity (before amplification) was 0.164 mV per watt/cm². The output was linear over the range tested from 800 to 9000 watts/m². The unsteady response was tested using a continuous laser beam directed through a variable speed chopper wheel onto the gage surface. Results showed a frequency response of at least 1 kHz. / Master of Science

M. S.

M. S.

This work reports on the design, fabrication and testing of a thermal switch wherein the open and closed states are actuated by shape memory alloy elements while heat is transferred by a heat-pipe. The motivation for such a switch comes from NASA's need for thermal management in advanced spaceport applications associated with future lunar and Mars missions. For example, as the temperature can approximately vary between 40 K to 400 K during lunar day/night cycles, such a switch can reject heat from a cryogen tank in to space during the night cycle while providing thermal isolation during the day cycle. By utilizing shape memory alloy elements in the thermal switch, the need for complicated sensors and active control systems are eliminated while offering superior thermal isolation in the open state. Nickel-Titanium-Iron (Ni-Ti-Fe) shape memory springs are used as the sensing and actuating elements. Iron (Fe) lowers the phase transformation temperatures to cryogenic regimes of operation while introducing an intermediate, low hysteretic, trigonal R-phase in addition to the usual cubic and monoclinic phases typically observed in binary NiTi. The R-phase to cubic phase transformation is used in this application. The methodology of shape memory spring design and fabrication from wire including shape setting is described. Heat transfer is accomplished via heat acquisition, transport and rejection in a variable length heat pipe with pentane and R-134a as working fluids. The approach used to design the shape memory elements, quantify the heat transfer at both ends of the heat pipe and the pressures and stresses associated with the actuation are outlined. Testing of the switch is accomplished in a vacuum bell jar with instrumentation feedthroughs using valves to control the flow of liquid nitrogen and heaters to simulate the temperature changes. Various iv performance parameters are measured and reported under both transient and steady-state conditions. Funding from NASA Kennedy Space Center for this work is gratefully acknowledged.

Master of Science

The concept of using heat flux measurements to provide the input for a feedforward temperature control loop is investigated. The feedforward loop is added to proportional and integral feedback control to increase the speed of the response to a disturbance. Comparison is made between the feedback and the feedback plus feedforward control laws. The control law with the feedforward control loop is also compared to the conventional approach of adding derivative control to speed up the system response to a disturbance. The concept was tested using a simple flat plate heated on one side and exposed to a step change in the convective heat loss on the other side. A controller was constructed using an analog computer to compare the feedforward and feedback approaches. The conventional control approach was tested using a commercial temperature controller. The feedback and feedforward approaches were also simulated. The results showed that the feedforward control approach produced significant improvements in the response to the disturbance. The integral of the squared error between the setpoint and actual temperature was reduced by approximately 90 percent by the addition of feedforward control to the feedback control. The maximum temperature deviation from the setpoint was also reduced by 70 percent with the addition of feedforward control. Qualitative agreement was obtained between the experimental results and the computer simulations. The conventional approach of adding derivative control to the proportional and integral control showed an increase of 20 percent in the integral of the squared error, but offered no significant improvement in the maximum temperature deviation. The addition of derivative control also caused the stability of the system to decrease, while the addition of feedforward had no adverse effects on the system stability. The concept of using heat flux measurements for feedforward control was successfully demonstrated by both simulations and experiments. / Master of Science