Date on Master's Thesis/Doctoral Dissertation

5-2012

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Electrical and Computer Engineering

Committee Chair

McNamara, Shamus

Author's Keywords

Thermal transpiration; Molecular flow; Gas micropump; Peltier coder; Knudsen pump; Nanoporous thermoelectric

Subject

Fuel pumps

Abstract

The thesis focuses on improving the flowrate of the Knudsen gas pump. The Knudsen pump uses thermal transpiration as the driving mechanism to pump gas. It is a motionless gas pump as the pump does not require any moving actuators for pumping. The thermally driven gas flow is accomplished in the molecular or transitional gas flow regime. The advantage of this pump is that without any moving parts it avoids friction losses and stiction problems which devices in micro scale are prone to suffering due to scaling issues. Thus, this pump is highly robust and reliable. Knudsen pumps in the past have suffered from the drawback of low flowrates and inability to operate at atmospheric pressure. In the early days lack of micromachining technologies limited minimum channel size which had to be operated at lower than atmospheric pressure to achieve free molecular flow. Various designs have been implemented with an impetus on increasing the flowrate of the pump. The key to this pump is establishing a temperature difference along the length of the channel. A higher temperature difference over a shorter channel length makes the pump more efficient. Pump channels have been made out of various materials like silicon, glass and polymer. The silicon microfabricated single channel conventional design pump suffered from the high thermal conductivity of silicon, which limited the thermal gradient that could be achieved. Silicon was replaced by glass, which has a lower thermal conductivity. The glass micro fluidic pump could pump water in reservoirs but at a slow rate. Renewable forms of Knudsen pump were also made by using nanoporous silica colloidal crystals which are robust and could use solar energy and body heat to create a temperature difference and achieve pumping. The pump powered by body heat produced a maximum pressure differential of 1.5 kPa. However, the use of these pumps is restricted to certain applications due to slow pumping. The polymer material, made of mixed cellulose ester, has a very low thermal conductivity, which aids in maintaining a higher temperature difference between the ends of a channel to achieve a higher flowrate. The polymer material used is in the form of a nanoporous template which has numerous pores each of which acts as a pump and thus the pump's conductance to gas flow is also increased which makes it faster. The pore sizes range from 25 nm to 1200 nm. It has been proven that a smaller channel diameter pump is more efficient. Efficiency decreases as the channel size approaches viscous flow regime. The initial design used a resistive heater to actively heat one end of the channel and a heat sink was used to passively cool the other end of the channel. This design was ineffective in achieving a significant temperature difference for a decent flowrate with the materials like silicon and glass. The conventional Knudsen pump design using a porous polymer matrix as channel material attained a normalized maximum no load flowrate of 135 µL/min-cm2 at 3.81 Watts of input power. This number is low compared to other micropumps. This led to the use of a thermoelectric material, which could actively heat and cool the pump channel ends and provide a much higher temperature difference over the same channel length as compared to the conventional Knudsen pumps which used only active heating of the channel's hot side. The thermoelectric strategy also eliminates the need for a heat sink in the pump. This transforms the design to bi-directional modes of operation. The first design using thermoelectrics is a lateral design in which the pump channels closer to the thermoelectric element developed a higher temperature difference across them compared to the channels away from the thermoelectric element. Thus, the thermoelectric energy was underutilized. Changing to the radial design made the pump more efficient compared to the lateral design since the thermoelectric energy was uniformly distributed on all the pump channels. The radial design also reduced air gap resistances and minimized energy losses which enhanced the output for the same input power. At an input power of 4.18 Watts it achieved a normalized no load flowrate of 408 µL/min-cm2. It also recorded a maximum normalized flowrate of 1.5 mL/min-cm2 while moving a drop of water which to date is the maximum flowrate reported by any Knudsen pump. A theoretical model has been developed to compute the pump's efficiency based on the flowrate and pressure difference obtained by the pump. The efficiency of the radial design pump with the thermoelectric is higher when compared to a conventional pump using a resistive heater whose channels are also made from the same material as that of the thermoelectric pump.

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