Modeling thermal regeneration in reciprocating, reacting streams with applications in thermoelectric power generation and in internal combustion engines

Presence of bounding solid surfaces and reciprocation of the stream direction, allow for heat storage/release (thermal regeneration) in fluid streams undergoing an exothermic reaction (combustion). Direct fuel injection also allows for the control of the flame location, and therefore, ideally unlimited excess (above the adiabatic) temperature (called the superadiabatic temperature). While improvements in the surface-convection heat transfer and the ability of the solid to store/release heat are desirable, the solid thermal conductivity hinders performance. Porous solids (foams, tube bundles etc.), however, provide a way to avoid the difficulties. This superadiabatic temperature can be used, among other applications, to increase the efficiencies of thermoelectric power generation and internal combustion engines. These two examples are modeled and analyzed here in detail and the possibilities and limitations of using different porous materials are discussed.; Using silicon-based, high-temperature thermoelectric materials (such as Si_0.7Ge_0.3), a combustion-thermoelectric tube bundle is designed. The air stream reciprocates through the tubes having a short, central adiabatic methane-gas injection region and two (one on each end) thermoelectric regions (with the p- and n-type materials placed co-axially). The hot-junction temperature is only limited by the melting temperature of the silicon-germanium alloy, and through optimization of the parameters, an efficiency of about 11 percent is obtained.; An existing design using an in-cylinder, reciprocating SiC-foam regenerator in the Diesel internal combustion engine is analyzed to take advantage of the superadiabatic temperature. The intake air is heated in the regenerator, before entering the droplet-fuel injection region and this results in an enhanced evaporation and a more uniform fuel distribution (due to deflection of the droplets by the air emanating from the regenerator). Through optimization of the regenerative cooling/heating strokes, it is confirmed that the thermal efficiency of the engine can be noticeably improved.