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However, design of high temperature microchemical systems, including microburners and hydrogen production systems for fuel cell applications, demands an understanding of fundamentals in these devices.

Mass fraction contours of ammonia from CFD simulation coupled with complex catalytic chemistry in a post, penny-size microreactor. Each diamond depicts one catalytic post. Back diffusion is clearly observed and has important ramifications for microreactor operation. For details and comparison to experimental data of Masel and coworkers see Deshmukh et al., Ind. Eng. Chem. Res. 43 , 2986-2999 (2004).

In our group, we study microburners and catalytic microreactors. The former provide heat to drive endothermic reactions, such as steam reforming and ammonia decomposition. The latter are of interest to us for (a) developing detailed reaction mechanisms on various catalysts and (b) hydrogen production for H2-based fuel cells. Our work entails microreactor fabrication and modeling with detailed gas and surface chemistries, multicomponent transport, and variable properties giving emphasis on interfacial phenomena, which we find to be critically important. Typical systems under study include natural gas microburners and catalytic short-contact time partial oxidation microreactors.

Examples of experimental microchannels fabricated in our lab. For details see Norton et al., Ind. Eng. Chem. Res., accepted.

The effects of microchemical system dimensions and operating conditions, such as, preheating of the reactants and flow velocity, are investigated, and the microscale physics are illustrated. For example, we have found that the choice of materials in terms of thermal management is important at these scales, and the classic continuum models should be appropriately modified to explain the aforementioned experimental results. Quantitative comparison to experiments is done and guidelines for high temperature microchemical systems design are being developed.

IR image of the top portion of the microcombustor with copper thermal spreaders. The operating conditions were a stoichiometric mixture of H2/air at a total flow rate of 1.4 SLPM. For more details see Federici et al., Journal of Power Sources, 2006. Accepted. (Click on image to view Quicktime movie.)

In a parallel effort, we study experimentally and theoretically non-oxidation reactions that lead to hydrogen production for fuel cells driven by mic-roburners described above. One of the reactions studied is ammonia decomposition. We develop detailed and reduced reaction mechanisms for this chemistry over various catalysts. Computational fluid dynamics (CFD) simulations in various shaped catalytic microreactors are carried out to create design guidelines. Furthermore, we develop guidelines for fast ignition and optimum operation of these microreactors.

Comparison of experimental data and CFD results from post microreactors for ammonia decomposition at 1 atm. For more details see Deshmukh et al., Ind. Eng. Chem. Res. 43 , 2986-2999 (2004) and Int. J. Multiscale Comp. Eng. (2004).