Electron transport in E x B devices

A Hall thruster, an E × B device used for in-space propulsion, utilizes an axial electric field to electrostatically accelerate plasma propellant from the spacecraft. The axial electric field is created by positively biasing the anode so that the positivelycharged ions may be accelerated (repelled) from the thruster, which produces thrust. However, plasma electrons are much smaller than ions and may be accelerated much more quickly toward the anode; if electrons were not impeded, a "short circuit" due to the electron flow would eliminate the thrust mechanism. Therefore, a magnetic field serves to "magnetize" plasma electrons internal to the thruster and confines them in gyro-orbits within the discharge channel. Without outside factors electrons would be confined indefinitely; however, electron-neutral collisions provide a mechanism to free electrons from their orbits allowing electrons to cross the magnetic field toward the anode, where this process is described by classical transport theory. To make matters worse, cross-field electron transport has been observed to be 100-1000 times that predicted by classical collisional theory, providing an efficiency loss mechanism and an obstacle for modeling and simulations in Hall thrusters. The main difficulty in studying electron transport in Hall thrusters is the coupling that exists between the plasma and the fields, where the plasma creates and yet is influenced by the electric field. A device has been constructed at MTU’s Isp Lab, the Hall Electron Mobility Gage, which was designed specifically to study electron transport in E × B devices, where the coupling between the plasma and electric field was virtually eliminated. In this device the two most cited contributors to electron transport in Hall thrusters, fluctuation-induced transport, and wall effects, were absent. Removing the dielectric walls and plasma fluctuations, while maintaining the field environment in vacuum, has allowed the study of electron dynamics in Hall thruster fields where the electrons behave as test particles in prescribed fields, greatly simplifying the environment. Therefore, it was possible to observe any effects on transport not linked to the cited mechanisms, and it was possible to observe trends of the enhanced mobility with control parameters of electric and magnetic fields and neutral density– parameters that are not independently variable in a Hall thruster. The result of the investigation was the observation of electron transport that was ~ 20-100 times the classical prediction. The cross-field electron transport in the Mobility Gage was generally lower than that found in a Hall thruster so these findings do not negate the possibility of fluctuations and/or wall collisions contributing to transport in a Hall thruster. However, this research led to the observation of enhanced cross-field transport that had not been previously isolated in Hall thruster fields, which is not reliant on momentum-transfer collisions, wall collisions or fluctuations.

Fluidic oscillator design for water removal enhancement in a PEM fuel cell

This research initiative was triggered by the problems of water management of Polymer Electrolyte Membrane Fuel Cell (PEMFC). In low temperature fuel cells such as PEMFC, some of the water produced after the chemical reaction remains in its liquid state. Excess water produced by the fuel cell must be removed from the system to avoid flooding of the gas diffusion layers (GDL). The GDL is responsible for the transport of reactant gas to the active sites and remove the water produced from the sites. If the GDL is flooded, the supply gas will not be able to reach the reactive sites and the fuel cell fails. The choice of water removal method in this research is to exert a variable asymmetrical force on a liquid droplet. As the drop of liquid is subjected to an external vibrational force in the form of periodic wave, it will begin to oscillate. A fluidic oscillator is capable to produce a pulsating flow using simple balance of momentum fluxes between three impinging jets. By connecting the outputs of the oscillator to the gas channels of a fuel cell, a flow pulsation can be imposed on a water droplet formed within the gas channel during fuel cell operation. The lowest frequency produced by this design is approximately 202 Hz when a 20 inches feed-back port length was used and a supply pressure of 5 psig was introduced. This information was found by setting up a fluidic network with appropriate data acquisition. The components include a fluidic amplifier, valves and fittings, flow meters, a pressure gage, NI-DAQ system, Siglab®, Matlab software and four PCB microphones. The operating environment of the water droplet was reviewed, speed of the sound pressure which travels down the square channel was precisely estimated, and measurement devices were carefully selected. Applicable alternative measurement devices and its application to pressure wave measurement was considered. Methods for experimental setup and possible approaches were recommended, with some discussion of potential problems with implementation of this technique. Some computational fluid dynamic was also performed as an approach to oscillator design.

Experimental investigation of regular fluids and nanofluids during flow boiling in a single microchannel at different heat fluxes and mass fluxes

The dissipation of high heat flux from integrated circuit chips and the maintenance of acceptable junction temperatures in high powered electronics require advanced cooling technologies. One such technology is two-phase cooling in microchannels under confined flow boiling conditions. In macroscale flow boiling bubbles will nucleate on the channel walls, grow, and depart from the surface. In microscale flow boiling bubbles can fill the channel diameter before the liquid drag force has a chance to sweep them off the channel wall. As a confined bubble elongates in a microchannel, it traps thin liquid films between the heated wall and the vapor core that are subject to large temperature gradients. The thin films evaporate rapidly, sometimes faster than the incoming mass flux can replenish bulk fluid in the microchannel. When the local vapor pressure spike exceeds the inlet pressure, it forces the upstream interface to travel back into the inlet plenum and create flow boiling instabilities. Flow boiling instabilities reduce the temperature at which critical heat flux occurs and create channel dryout. Dryout causes high surface temperatures that can destroy the electronic circuits that use two-phase micro heat exchangers for cooling. Flow boiling instability is characterized by periodic oscillation of flow regimes which induce oscillations in fluid temperature, wall temperatures, pressure drop, and mass flux. When nanofluids are used in flow boiling, the nanoparticles become deposited on the heated surface and change its thermal conductivity, roughness, capillarity, wettability, and nucleation site density. It also affects heat transfer by changing bubble departure diameter, bubble departure frequency, and the evaporation of the micro and macrolayer beneath the growing bubbles. Flow boiling was investigated in this study using degassed, deionized water, and 0.001 vol% aluminum oxide nanofluids in a single rectangular brass microchannel with a hydraulic diameter of 229 µm for one inlet fluid temperature of 63°C and two constant flow rates of 0.41 ml/min and 0.82 ml/min. The power input was adjusted for two average surface temperatures of 103°C and 119°C at each flow rate. High speed images were taken periodically for water and nanofluid flow boiling after durations of 25, 75, and 125 minutes from the start of flow. The change in regime timing revealed the effect of nanoparticle suspension and deposition on the Onset of Nucelate Boiling (ONB) and the Onset of Bubble Elongation (OBE). Cycle duration and bubble frequencies are reported for different nanofluid flow boiling durations. The addition of nanoparticles was found to stabilize bubble nucleation and growth and limit the recession rate of the upstream and downstream interfaces, mitigating the spreading of dry spots and elongating the thin film regions to increase thin film evaporation.