Abstract
This paper reports on a bidirectional Knudsen pump (KP) with a 3D-printed thermal management platform; the pump is intended principally for microscale gas chromatography applications. Knudsen pumps utilize thermal transpiration, where non-viscous flow is created against a temperature gradient; no moving parts are necessary. Here, a specialized design leverages 3D direct metal laser sintering and provides thermal management that minimizes loss from a joule heater located on the outlet side of KP, while maintaining convective cooling on the inlet side. The 3D-KP design is integrative and compact, and is specifically intended to simplify assembly. The 3D-KP pumping area is ≈1.1 cm2; with the integrated heat sink, the structure has a footprint of 64.2 × 64.2 mm2. Using mixed cellulose ester (MCE) membranes with a 25 nm average pore diameter and 525 μm total membrane thickness as the pumping media, the 3D-KP achieves a maximum flow rate of 0.39 sccm and blocking pressure of 818.2 Pa at 2 W input power. The operating temperature is 72.2 °C at ambient room temperature. In addition to MCE membranes, anodic aluminum oxide (AAO) membranes are evaluated as the pumping media; these AAO membranes can accommodate higher operating temperatures than MCE membranes. The 3D-KP with AAO membranes with 0.2 μm average pore diameter and 531 μm total membrane thickness achieves a maximum flow rate of 0.75 sccm and blocking pressure of 496.1 Pa at 9.8 W at an operating temperature of 191.2 °C.
Highlights
Bidirectional gas flow is necessary in a number of application contexts
Knudsen pump (KP) utilize the principle of thermal transpiration, wherein the net flux of gas molecules is induced against the temperature gradient in the direction from the low temperature region to the high temperature region [14]
We present a 3D-printed bidirectional KP (3D-KP) that addresses these problems
Summary
Bidirectional gas flow is necessary in a number of application contexts. Certain analytic instruments such as gas chromatographs [1,2,3,4,5] and mass spectrometers [6,7] require gas flow in opposing directions for sampling and analysis steps. In typical unidirectional implementations of KP that are intended to operate at atmospheric pressure, arrays of nanoscale channels can be provided by either micromachining silicon [15,17,18,19] or by nanoporous media such as ceramics, zeolites, and polymer membranes [20,21,22,23]. Pressure-driven viscous flow is usually more significant in the KPs that use nanoporous materials because of the nature of these media (which have more tortuosity, non-uniformity, and random defects) Such viscous flow is in the direction opposite to thermal transpiration flow, and causes deviation from the equations. ∆T when it is implemented at the cool upstream side, but significantly decreases the ∆T when the flow is reversed To address these problems, a more deliberate design is needed for the bidirectional KP architecture.
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