Date on Master's Thesis/Doctoral Dissertation

8-2014

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Committee Chair

McNamara, Shamus

Committee Co-Chair (if applicable)

Brehob, Ellen G.

Committee Member

Williams, Stuart J.

Committee Member

Sumanasekera, Gamini U.

Committee Member

Lian, Yongsheng

Subject

Fuel pumps

Abstract

This dissertation reports on the fabrication of a bi-directional Knudsen gas pump using a multifunctional nanoporous thermoelectric material. The integration of the channels into the thermoelectric material has the advantages of reducing heat losses and power consumption, improving the pump efficiency, and reducing the overall pump size. The Knudsen pump operates on the thermal transpiration principle, the phenomenon that gas molecules drift from the cold end to the hot end of a narrow channel. The gas pumping is realized in the transition or free molecular regime. The Knudsen pump is needed in a great number of applications such as gas chromatography, spectroscopy, gas manipulation, and cooling in microelectronics. Pressure and pressureless sintering techniques are used to fabricate the nanoporous thermoelectric material. The thermoelectric material was characterized by measuring the pore size distribution, electrical conductivity, and thermal conductivity, thermopower, and temperature distribution. Two Knudsen pumps are then fabricated. In the first pump, the circular thermoelectric sample is placed on a copper heat sink. When the power is turned on, one side of the thermoelectric sample gets hot, and the other side gets cold providing the necessary temperature gradient for thermal transpiration to take place. Reversing the voltage polarity reverses the hot and cold sides and therefore reverses the pumping direction. The pump generated a pressure of 300 Pa for an input power of 0.96 W. In the second design, the fabricated square-shaped thermoelectric sample is placed in a machined plastic channel. 520 Pa pressure difference and 1 µl/min flow rate are obtained for an input power of 1.33 W. Using a combination of the Sharipov and thermoelectric equations, expression for the maximum pressure difference, max P D , and maximum mass flow rate, max M & , are derived and analyzed. We conclude the following. As the thermoelectric figure of merit Z increases, max M & and max P D increases. As the pore size increases, the max P D decreases, and max M & increases. As the molecular mass of the working gas increases, max P Ddecreases, and max M & increases. As the tangential momentum accommodation coefficient increases, max P D increases, and max M &decreases. The last section of this dissertation is concerned with gas flow through track-etch Whatman nanoporous membranes. We found that the tangential momentum accommodation coefficient fails to solve the discrepancy between experimental and theoretical flow rate. A simple measurement of pressure versus the flow rate through the membranes was conducted next. Among the analyzed models from the literature, the surface flow diffusion model provides the best fit to the experimental data.

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