Several designs can be used to achieve the goal of transferring heat of combustion to a working fluid. One example of such a device is combustion of hydrogen in microchannels and transferring heat to oil streams flowing in channels parallel to the combustion channels. All channels can be integrated in one single unit to have better heat transfer and prevent heat losses. For more information on this example see the study done by Ghazvini and Narayanan at Oregon State University:http://dx.doi.org/10.1115/AJTEC2011-44633
Because of its ability to produce and transfer a large amount of heat in a very small area, combustion in microchannels has become an area of interest in fields such as aerospace and energy. The small size and high performance of microscale combustion make it a method to increase the efficiency and to reduce the size of power generation devices.
The reaction in the microreactor side can be homogeneous reaction, heterogeneous reaction or both. Reducing the size of the reactor may impose some limitations on the reaction phenomena.
Having the reactor and heat exchanger part in single unit have advantages of both microreactors (described here) and microHX’s (described here), however, the integration of these systems gives the following benefits.
The rate of heat generation and temperature can be controlled to some extent by tailoring heterogeneous reactions to occur at specific locations within the channel walls. Thus, high temperatures that cause the formation of NOX can be mitigated.
For channels smaller than 500 µm, the dominant reaction phenomenon would be heterogeneous reaction and for dimensions smaller than that, no homogeneous reaction happens. Therefore wherever a catalyst exists, the reaction occurs. That is how the location of the reaction can be defined. The rate can be adjusted by some of the catalyst characteristics.
The ability to determine the location of reaction provides the ability to have better heat transfer by distributing hot (or cold) area along the channel and at different locations. That was temperature difference remains more uniform and the heat transfer rate increases.
Having the two units of operations integrated in one device instead of having two different sections (i.e. microreactor and micro heat exchanger) reduces the size of the system and also fabrication costs. And since both heat production and heat consumption happen at the same location, heat losses reduces significantly. As we know, heat losses can be a major concern in microscale devices.
By adding a preheat section to the reaction and using recuperation combustion of fuels can happen at any air temperatures. The innovative way of having recuperator channels to preheat the incoming gas mixture was presented by Ghazvini and Narayanan. The following picture shows their designs, more information can be found at http://dx.doi.org/10.1115/AJTEC2011-44633.