Microcombustor

Tonkovich, A. L. Y, Roberts, G. L, Fabrication of a stainless steel microchannel microcombustor using a lamination process, in Proceedings of the SPIE Gonference on Micromachined Devices and Gompo-nents IV, pp. 386-392 (September 1998),... 

Other processes that have increased importance at small length scales such as thermal creep (transpiration) and electrokinetic effects are also being considered for use in microcombustors. For example, transpiration effects are currently being investigated by Ochoa el al. [117] to supply fuel to the combustion chamber creating an in-situ thermally driven reactant flow at the front end of the combustor. 

Further, the development of miniaturized devices for the generation of power and/ or heat is discussed here as it represents an emerging field of application of catalytic combustion. Due to the presence of the catalytic phase, the microcombustors have the potential to operate at significantly lower temperatures and higher surface-to-volume ratios than non-catalytic microcombustors. This makes them a viable solution for the development of miniaturized power devices as an alternative to batteries. 

These microcombustors have demonstrated stable performance using propane as fuel [52,53] however, it must be observed that the presence of a central zone at very high temperature may be critical for the choice of materials and give rise to significant NOx emissions [54]. 

Early studies in this field [35, 36] indicated that a high surface-to-volume ratio, which represents a hurdle for gas-phase combustion, is instead an advantage for catalytic combustion. In fact the small scale enhances considerably the rate of gas-solid mass transfer, which favors the kinetics of the combustion process and compensates for the short residence time. Also, as is well established for large-scale systems, the presence of a catalytic phase allows for stable combustion at significantly lower temperature than traditional homogeneous burners [55, 56]. This makes the design and operation of microcombustors more fiexible. Several recent studies have explored the potential of catalytic microcombustors using H2 [37, 38, 50], methane [37], propane [52,53,57] and mixtures of H2 with propane [57], butane [38,47,52] and dimethyl ether [52]. 

Figure 12.7 Schematic drawing of microcombustors with internal waste heat recove. ...

Structured catalysts are used in GT and microcombustor applications because of the severe pressure drop constraints combined with the requirement for a fast rate of... 

On the other hand, following the development of hybrid combustor configurations that prevent operation of the catalyst module at temperatures above 900-1000 °C, the major drawback of metallic monoliths, namely the limited maximum operating temperature, has been overcome. Accordingly, honeycombs made of metal foils have been adopted in GT catalytic combustors in view of their excellent thermal shock resistance and thermal conductivity properties [9]. In addition, metallic substrates are a promising option for the fabrication of microcombustors. 

Finally, the development of microcombustors for energy conversion in small devices appears to be a stimulating field for catalytic combustion research, which once more will require the combined study of engineering solutions with advanced catalytic materials. 

Demonstrate high performance desulfurizer, catalyst, microreactor and microcombustor/ microvaporizer concepts that will enable production of compact fuel processors for proton exchange membrane (PEM) fuel cells ... 

While this project is relatively new, significant progress has been made regarding the component design and modeling, sorbent and catalyst development and microchannel system development tasks. We will continue to focus on these tasks and microcombustor/vaporizer development. In addition, methods will be demonstrated to integrate catalysts into the micro-reactors. 

EMM can also be effectively utihzed for fabrication of several of microfeatures for a wide range of microengineering applications such as fuel processing, aerospace, heat transfer, microfluidics, and biomedical applications. These microdevices have to often withstand high stresses at elevated temperatures during their service in different applications such as microcombustors, electrochemical reactions required at elevated temperatures in microreactors, and also in microthermal devices. For biomedical applications, microcomponents are to be made of biocompatible materials and... 

R. 1. Masel, M. Shannon, Microcombustor having submillimeter critical dimensions. The Board of Trustees of the University of Illinois, Urbana, IL, US Patent 6193501,27 February 2001. 

The heat produced by chemical reaction in a microcombustor (Qy) can be expressed as... 

The above-mentioned scaling law indicates rapid evaporation of droplets in microsystems because of smaller time constants in comparison to the larger droplets. This can be a disadvantage for LOC applications involving the transport of a small amount of liquids because of the loss of liquid by evaporation during transport. However, it can be an advantage in microcombustor applications, where the evaporation of droplets takes place rapidly. 

A detailed parametric study is undertaken in this chapter, using the full elliptic 2-D CFD code for both gas-phase and solid domains, in order to delineate the stable combustion regimes of propane-fueled catalytic microreactors at pressures 1 and 5 bar (pressures up to 5 bar are of interest to recuperated microturbine systems [1-3]), channel confinements 1.0 and 0.3 mm and wall thermal conductivities 2 and 16 W/mK. Methane simulations are also included, so as to exemplify the significant differences in both chemical and transport properties on microcombustor stability. The main objectives are to assess the effect of high pressure operation, molecular transport and gas-phase chemistry on the stability of propane-fueled catalytic microreactors and to study the impact of increased geometrical confinement and high wall thermal conductivity on the non-adiabatic reactor operation. Particular objectives were to quantify the differences between the two fuels in terms of reactor stability and performance. 

Recent studies on propane-fiieled catalytic microcombustors [25] at atmospheric pressure hinted towards the importance of moleeular transport effects in reactors of sub-millimeter scales. Since in lean methane/air combustion the fuel has Lewis number Lcch4 0.97, the methane transverse transport towards the catalytic channel surface will be considerably higher than that of propane, which is highly diffusionally imbalanced with LecjHs 1-82. It can thus be expected that when the residence time in the channel has the same order of magnitude as the transverse fuel diffusion times, fuel conversion and combustion stability will be impacted primarily by the transport rather than the chemical properties of the fuel. 

Dr. Karagiannidis doctoral thesis investigates combustion characteristics in channel-flow catalytic microcombustors/microreactors, with emphasis placed on microturbine concepts for portable power generation (an initiative within the Swiss Federal Institute of Technology Zurich) which employ reheat and have operational pressures up to 5 bar. 

The experimental investigation of microscale devices is hindered by many technical limitations, permitting only the acquisition of a rather narrow range of experimental data, unsuitable for an in-depth analysis of the aforementioned devices operational characteristics. In view of the absence of such sets of experimental data for catalytic microreactors/microcombustors, detailed numerical models prove to be invaluable in providing insight on the particular physics of hetero-Zhomogeneous combustion processes in the microscale. 

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