The role of thermophoresis in trapping of diesel and gasoline particulate matter

Thermophoresis is the phenomenon by which sub-micron particles suspended in a gas move in the directionof decreasing temperature. It occurs naturally in automotive catalysts and is responsible for improvedparticle deposition during transient operation when a hot, particle-laden exhaust gas flows through acool monolith. Although the time-averaged effects of thermophoresis are expected to be relatively small,it is possible that the effect can be enhanced by a properly designed aftertreatment system.In the current work, thermophoretic effects on particle deposition in automotive catalysts are studiedby means of numerical simulations. The potential gain in deposition efficiency obtainable by employingthermophoresis is investigated for both laminar and turbulent flow in a standard monolith channel.The monolith is also compared to a plate-to-plate thermal precipitator. We show here that a substantialenhancement of especially the intermediate-sized particulate matter may be obtained by taking advantageof thermophoresis. In addition, we investigate a number of design ideas for thermophoretic particletrapping devices.We also discuss the difficulties involved in using thermophoresis for particle capture, for example thatthe collecting surface must be cooler than the exhaust gas whereas catalytic activity for the oxidation ofsoot is still a challenge at low temperatures