Superadiabatic combustion in conducting tubes and heat exchangers of finite length

Classic theories of combustion rely upon the principle that the maximum temperature attainable in a reaction is predicted by the adiabatic equilibrium temperature. In certain burner configurations, however, the maximum temperature may locally exceed this value by a significant amount due to heat recirculation, which has led to the concept of superadiabatic combustion. A striking aspect of this type of combustion is a broadening of the limits of flammability due to the accelerating effect of temperature increases on chemical reaction rates. In this paper, a simple analytical model is developed to study the superadiabatic performance of a combustor consisting of two parallel channels of finite length that are divided by a conducting wall. Assuming equal flow rates in the individual channels, the co-flow configuration is equivalent to combustion in conducting tubes, whereas the counterflow configuration is conceptually similar to a Swiss-roll burner. In both cases, the characteristics of superadiabatic combustion of a fuel-rich premixed fuel/air mixture are studied in terms of wall conductivity, heat transfer, and geometry of the combustor.