Gas-phase flow, heat transfer and chemical reaction in microchannels

icroreactors containing integrated fluid flow, heat transfer, separation and catalytic reactor units with millimeter or micrometer sized channels are fast becoming a reality. Microreactors may be used when size/weight is an issue, such as on-board reformers for fuel cells. They can achieve highly efficient heat transfer and temperature control through the increased surface area/volume ratio available in microchannels, for low-contact-time reactions (usually oxidations). There are also advantages in conducting reactions in geometries where gas-gas molecule collisions occur less frequently than gas-wall collisions. Research on micro systems has been classified (Hasebe, Computers and Chem. Eng. 29 (2004) 57-64) into four fields: efficient chemical laboratories ("lab-on a chip"), development of micro analytical devices, micro fabrication techniques (MEMS techniques) and development of micro production systems (micro-chemical plants). Our interest in microreactors falls into the last area.

The processing conditions at the millimeter scale let us use a continuum approach, for example the Navier-Stokes equations. As microchannels get smaller, gas flow in them stops following the differential equations, and we have to understand how individual gas molecules behave. Interesting new phenomena appear, for example the gas velocity no longer obeys the familiar "no-slip" condition at the tube wall, and if the tube wall has a temperature gradient along it, then fluid flow can occur from cold to hot ("thermal creep").

The objective of MQPs in this area is to understand how gas flow, heat transfer and chemical reaction interact in microchannels. The simulation tools available for this are computational fluid dynamics (CFD) and the Direct Simulation Monte Carlo (DSMC) method. CFD represents the physical and chemical phenomena in microchannels by the continuum approach, and discretizes the flow region into a large number of finite volumes, and solves the large set of equations that comes from applying conservation laws to each volume. The DSMC method is a statistical technique for the computer simulation of a real gas by thousands or millions of simulated molecules. The velocity components and position coordinates of these molecules are stored in the computer and are modified with time as the molecules are followed through representative collisions and boundary interactions in simulated physical space. Our group has access to a computer package that implements the CFD method (Fluent) and to computer programs that implement the DSMC method in 2D and in 3D (DS2V and DS3V). This allows students to investigate these interesting phenomena at the microscale without spending their time developing computer codes.

Results of a DSMC simulation of flow in a microchannel, the flow velocity in the channel (from left to right) is indicated by the color scale. The development of a boundary layer along the channel walls (at the top and bottom of the picture) can clearly be seen in blue, while the increased velocity near the exit that comes from flow acceleration due to compressibility shows up in red. The simulation was performed in a two dimensional domain, the results are comparable to a simulation of flow between two plates.