Thermally driven Knudsen gas pump enhanced with a thermoelectric material.

摘要

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.;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-cm 2 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-cm 2. It also recorded a maximum normalized flowrate of 1.5 mL/min-cm 2 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.;Finally, initial progress has been made for a Knudsen pump utilizing a porous thermoelectric. The nanoporous thermoelectric material would reduce the pump's size and lower the power consumption, which could help increase the efficiency of the pump. (Abstract shortened by UMI.).;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 microfluidic 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.