Rebecca N. Webb

Numerical Analysis of a Single Microchannel Within a High-Temperature Hydrogen Heat Exchanger for Beamed Energy Propulsion Applications

The requirement that the propellants used in launch vehicle systems must also provide the thermal energy to be converted to kinetic energy in the rocket nozzle has plagued rocket designers since the dawn of the space age. Beamed propulsion systems, however, avoid this constraint by placing the energy source on the ground and transmitting the energy to the spacecraft via microwaves. This computational work evaluates three different heat exchanger channel designs for use in a beam propulsion spacecraft. The working fluid was hydrogen and the input energy was 1.3 kW. The increase in axial temperature along the 0.1 m long channel was as high as 2000 K. In addition, it was found that despite the very small diameter of the minichannels, 1 mm, each design produced extreme temperature gradients across the channel cross section. These temperature gradients affected the velocity profile; the maximum velocity was not located at the channel center.© 2013 ASME

Experimental Investigation of Latent Heat Thermal Energy Storage for Bi-Modal Solar Thermal Propulsion

Abstract : A bi-modal solar thermal system capable of providing propulsive and electric power to a spacecraft has been identified as a promising architecture for microsatellites requiring a substantial _V . The use of a high performance thermal energy storage medium is the enabling technology for such a configuration and previous solar thermal studies have suggested the use of high temperature phase change materials (PCMs) such as silicon and boron. To date, developmental constraints and a lack of knowledge have prevented the inclusion of these materials in solar thermal designs and analysis has remained at the conceptual stage. It is the focus of this ongoing research effort to experimentally investigate using both silicon and boron as high temperature PCMs and enable a bi-modal system design which can dramatically increase the operating envelope for microsatellites. This paper discusses the current progress of a continued experimental investigation into a molten silicon based thermal energy storage system. Using a newly operational solar furnace facility, silicon samples have been melted and results indicate that volumetric expansion during freezing will be the primary difficulty in using silicon as a PCM. Further experimental studies using different materials and test section fill factors have identified potentially reliable experimental conditions at the expense of energy storage density. In addition to conducting experiments, a concurrent computational effort is underway to produce representative models of the experimental system. The current models generally follow experimental results; however, difficulties still remain in determining high temperature material properties and material interactions. This paper also discusses the future direction of this research effort including modeling improvements, analysis of convective coupling with phase change energy storage and potential facility improvements.

Preheating Cold Gas Thruster Flow Through a Thermal Energy Storage Conversion System

Abstract : A thermal energy storage system capable of receiving, absorbing, and collecting solar energy, and storing it within a phase change material, has been designed as part of a power and propulsion system for use in low Earth orbit. The design includes thermophotovoltaic cells for the conversion of stored heat to electrical energy for various satellite systems, as well as a heat exchanger imbedded in the phase change material for propellant heating during thrust maneuvers. Through computational analysis, the likely thrust was evaluated as well as the impact of the propellant flow on the thermal to electric conversion. For the scenario analyzed, a propellant exit temperature of 1500 K, translating to an Isp of 300 seconds, was achieved. Passing the propellant through the phase change material decreases the temperature of the emitter used to power the thermophotovoltaics and resulted in a 20% decrease in electric power output.

Computational Evaluation of Latent Heat Energy Storage Using a High Temperature Phase Change Material

Latent heat energy storage systems have higher energy density than their sensible heat counterparts and have the added benefit of constant temperature operation. This work computationally evaluates a thermal energy storage system using molten silicon as a phase change material. A cylindrical receiver, absorber, converter system was evaluated using the heat transfer in solids with surface-to surface radiation physics module of the commercially available COMSOL Multiphysics simulation software. The progression of the solidification and melting fronts through the phase change material was modeled for two different methods of concentrated solar irradiation delivery. Heating the core of the PCM rather than the top of the PCM decreased the required solar input by 17%, decreasing the solar collector area required as well as lowering overall system weight.Copyright

Thermal Modeling and Performance Measurements of Radiometric Arrays for Near Space Propulsion

Abstract : A propulsion system based on an array of radiometer vanes has been proposed for wind disturbance correction on near space vehicles. The radiometric force is produced by a vane due to a temperature gradient between a surface heated by insolation and a convectively cooled surface. Previous research has identified several key parameters that might affect the thrust produced by a radiometer array. These parameters include the heat transfer mechanisms on the individual vanes and between nearby vanes in an array, the proximity or spacing of radiometer vanes in an operational array, the effects of the vanes attached to a near space vehicle operating in the boundary layer near the surface of the vehicle, and the effects of attaching the vanes to a substrate on the vehicle itself. A combined numerical and experimental approach was used to investigate these effects. Numerical codes were used to assess the heat transfer mechanisms and the boundary layer flow. Experiments were designed and carried out to measure the force produced by a small array of radiometers. These results were combined to assess the performance of a notional array of radiometer vanes for the purpose of counteracting drag on a near space vehicle operating at an altitude of 60 km. The array performance was assessed based on the maximum force that can be produced as a function of array mass and area.

Repulsion and attraction caused by radiometric forces

Thermophoretic force on a sphere in rarefied gas is studied experimentally and computationally for Knudsen numbers on the order of 0.1. Both experiment and numerical modeling have shown the force maximum for a Knudsen number of about 0.3 based on the sphere diameter. The measured and computed maxima agree within the error bars of the experiment and simulation. The phenomenon of negative thermophoresis, where the force on the sphere acts in the direction of temperature gradient (cold to hot) is established numerically and, for the first time, experimentally.

Enhanced Heat Collection Element Performance Through Surface Geometry

Concentrated solar power produces electricity by using a mirror to focus sunlight on a fluid filled tube known as a heat collection element. The fluid inside the element is then used as a heat source for steam generation in a conventional steam turbine power plant. It is possible that adding microstructures to the surface of the conventionally smooth heat collection element could improve system efficiency, motivating the need for an improved understanding of the radiation characteristics of a microstructured surface. The goal of this work was to experimentally determine the impact of different microscale geometries on net radiative heat gain when angle of incidence was varied. Five test pieces, one with a smooth surface and four with microstructured surfaces were compared experimentally for a given infrared energy input and flow rate and five different angles of incidence. Over the entire range of angles the microstructured test pieces absorbed more energy than the smooth test piece.© 2011 ASME

Numerical Radiation Exchange in Rectangular Micro- and Mini-Channels Using the Monte Carlo Method

The addition of appropriately shaped macroscale structures to a surface results in a directionally selective surface capable of high absorption of direct solar radiation and low hemispherical emission. This work investigates the effect of adding sub-macroscale structures to a smooth surface on net radiative heat transfer. The Monte Carlo method was used to characterize the net radiative heat transfer of rectangular micro- and mini-channels. The effects of varying the aspect ratio, surface absorptivity, and incident angle were determined. The effect of diffuse and specular reflections was also examined. For a diffuse surface, as the absorptivity increases so does the net heat transfer however, higher incident angles result in lower net heat transfer. For a specular surface, net heat transfer increases with both incidence angle and aspect ratio. In general, deeper channels increase net heat transfer. The effect of channel periodicity was also examined. In general, shorter periods increase net heat transfer when normalized by system length.Copyright

Computational Evaluation of the Effects of Voids on a Thermal Energy Storage System Using Molten Silicon as the Phase Change Material

Latent heat energy storage is one of the most efficient ways to store solar thermal energy. A system capable of receiving, absorbing, and collecting solar energy and storing it within a high temperature phase change material has been designed as part of a power system to be used on a low Earth orbit satellite. The system employs silicon as the phase change material and thermophotovoltaic cells for the conversion of stored heat energy into electrical energy. The effect of a void, in the phase change material, on system temperature and the associated thermophotovoltaic power production is determined through computational evaluation.Copyright