John Lawler

Novel Method for Measurement of Total Hemispherical Emissivity

Abstract : This report was developed under a SBIR contract. This paper describes a heat flux-based method for measuring emissivity of a surface. In this method the emissivity of a surface is calculated using direct measurement of the heat flux passing through the surface. Unlike storage-based calorimetric methods, this method does not require application of known amounts of heat to the surface or the temperature history of a known amount of thermal mass to calculate the surface emissivity. Application and operation of this method is much simpler than calorimetric methods as it does not require careful thermal insulation of the heat radiating body from the surroundings. This technique allows emissivity measurements of the newly developed variable emissivity surfaces with significantly lighter and energy efficient measurement equipment that can operate for long term space missions. In this study, a commercially available thermopile heat flux sensor was used to measure the emissivity of a black paint and a variable emissivity surface, Electrostatic Switched Radiator (ESR). This paper details the concept, experimental setup, and the experiment results.

Performance Analysis of an Electrostatic Switched Radiator Using Heat-Flux-Based Emissivity Measurement

The heat-flux-based emissivity measurement technique developed in our earlier work has been used to study the performance of an electrostatic switched radiator. The capability of fast and accurate measurement of the real-time changes in emissivity enabled by this technique allowed understanding of the transient behavior during activation, as well as identification of a major failure mode of the second-generation electrostatic radiator. A solution for resolution of this failure mode was then proposed and successfully tested, producing accurate and repeatable results over many cycles. A change in emissivity of 0.52 was achieved with 280 V applied, among the best consistent results achieved through electrostatic technology. The current work offers further understanding of electrostatic radiator performance and its application to space vehicles.

Design, testing and optimization of a micropump for cryogenic spot cooling applications

The performance of two micropumps with different electrode designs in liquid N/sub 2/ was investigated in this study. One of the experimental challenges was developing a method for measuring the flow rate of the liquid N/sub 2/. By combining a numerical model of the non-isothermal flow of the liquid N/sub 2/ around the loop and the experimental measurements of the temperatures around the flow loop, the liquid N/sub 2/ flow rates could be determined accurately enough to compare the pumping performance of these micropumps. The first tested micropump had an electrode spacing of 20 /spl mu/m and an electrode pair spacing of 80 /spl mu/m, while the second pump had an electrode spacing of 50 /spl mu/m and electrode-pair spacing of 200 /spl mu/m. The results showed that both micropumps pumped sufficient liquid N/sub 2/ to cool a typical superconductive sensor or other low power device. For both pumps, the pumping capacities increased with increasing EHD voltage. The pump with the smaller electrode spacing generated a higher flow rate at a lower applied voltage. This pump generated flow rates as high as 10 mL/min at an applied voltage of 500 V.

Heat Flux‐Based Emissivity Measurement

This paper describes a heat flux‐based method for measuring emissivity of a surface. In this method the emissivity of a surface is calculated using direct measurement of the heat flux passing through the surface. Unlike storage‐based calorimetric methods, this method does not require application of known amounts of heat to the surface or the temperature history of a known amount of thermal mass to calculate the surface emissivity. Application and operation of this method is much simpler than calorimetric methods as it does not require careful thermal insulation of the heat radiating body from the surroundings. The low thermal capacitance and scalability of the heat flux sensors can provide good temporal and spatial resolution of the heat flux and, therefore, the emissivity. This technique allows emissivity measurements of the newly developed variable emissivity surfaces with significantly lighter and energy efficient measurement equipment that can operator for long term space missions. In this study, a co...

Development of Electrohydrodynamic (EHD) Micropumps for Cryogenic Applications

This paper presents the development of an innovative electrohydrodynamic (EHD) ion‐drag micropump for circulating liquid nitrogen in a cryogenic cooling loop. Two micropumps with different electrode designs were tested in this study. Their electrode arrays are composed of multi‐stages of saw‐tooth emitters and planar collectors electroplated on alumina substrates. The pumps had electrode spacings of 20 and 50 μm, respectively, for the distance between electrodes in a pair and 80 and 200 μm, respectively, for the distance between electrode pairs. The pump with closer electrode spacing produced a mass flow rate of twice the other pump at half the applied voltage. This improvement in performance is due to the closer electrode spacing, which allows the incorporation of more electrode pairs in a given pump size. The required voltage is also lowered, since the electric field is inversely proportional to the electrode spacing.

Thermal simulations of packaged IR LED arrays

The steady-state and transient behaviors of packaged IR LED arrays have been studied via numerical simulations. The waste heat generated by LEDs must be removed through a cold plate or a cryogenic cold finger attached to the backside of the driver array. Therefore, this heat must travel across the LED array-driver interface and through the driver array. The modeling results demonstrate that the thermal resistance of these components can be significant. The steady-state temperature profiles across several configurations are used to identify the thermal bottlenecks. Transient simulations are used to quantify the rise and fall times of the IR LEDs, and the fall times can be significantly reduced by changes in the LED layout. These proposed guidelines to minimize thermal issues in LED arrays should result in better performing and more reliable IR LED arrays.

A Space‐Based Experiment to Evaluate Performance of Electrostatic Switched Radiator (ESR)

This paper describes the design and ground testing of a test module prepared for evaluating the performance of an active emissivity surface in space. The test module will be incorporated into the MISSE‐6 (Materials International Space Station Experiment) package, which will be deployed on the exterior of the International Space Station (ISS). The test module combines an Electrostatic Switched Radiator (ESR), developed by Sensortex, Inc., along with a new emissivity measurement technique developed by ATEC, Inc. The ESR is an advanced spacecraft thermal control device that uses electrostatic hold‐down of a high emissivity composite film to control radiator heat dissipation. The electrostatic hold‐down switches the heat transfer mode of the radiator from conduction to radiation resulting in a large change in the radiator’s effective emissivity. In order to measure the variable emissivity of the ESR, a heat flux sensor was incorporated into the ESR structure, which measures directly the heat flux passing thro...