Turbulent Flow and Heat Transfer Characteristics of a Micro Combustor

Currently, a micro gas turbine (MGT) has been widely drawing attention as a distributed energy generation system for an individual household or a small community. In parallel to the progress of MGT technology, a fuel cell has been highlighted for its high efficiency and environmental advantages. For MGT, its efficiency can reach to 40% (Mcdonald, 2000), but it seems to be difficult to achieve higher efficiency than 40% (Kee et al., 2005; Suzuki et al., 2000). However, the efficiency of solid oxide fuel cell (SOFC) for electricity generation recently becomes 50% or higher. Therefore, a hybrid system with MGT and SOFC is promising because the MGT/SOFC hybrid system can provide higher efficiency over 70% (Massardo & Lubelli, 2000). Recently, (Suzuki et al., 2000) proposed the MGT/SOFC hybrid system having a micro combustor. In their system, the micro combustor is an important component to realize high system efficiency and low toxic exhausted gas. In a combustor of that kind, combustion efficiency may decrease by two critical issues, because combustion in a very small chamber may not simply resemble a scaled-down version of its large-scale counterpart (Choi & Park, 2009). One is the incomplete mixing between oxidant and fuel, which is highly related to reliable ignition and flammable limits. As a combustor size decreases, Reynolds number becomes smaller. Such a small combustor may be suffering from incomplete mixing between fuel and oxidant, insufficient fuel residence time for complete combustion and high heat transfer rate to combustor outside because of high surface to volume ratio. In that situation, flow field is significantly stabilized by viscous effect and this may restrain active turbulent mixing. This mechanism is not favorable for small combustor. Therefore, mixing enhancement is a critical consideration to develop such a micro combustor. (Suzuki et al., 2000) suggested a baffle plate to enhance the slow mixing in low Reynolds number condition. Its effectiveness is evaluated by the succeeding studies by (Choi et al., 2001; Choi et al., 2005; Choi et al., 2006a; Choi et al., 2008) for turbulent mixing fields downstream of the baffle plate. Micro combustor of this type is expected to secure zero emission of toxic gases and to maintain a stable flame for burning the effluent of SOFC in an extraordinary fuel lean condition. The other issue is the effect of heat loss to wall. This may be ignored in a large-scale combustor, but it is an important factor to a micro combustor. Heat generation depends on the combustor volume and mixing characteristics, while the heat loss is proportional to the surface area. In general, the surface-to-volume ratio for a micro combustor is larger than that