A theoretical study on an excess enthalpy flame

A further theoretical study was performed on the excess enthalpy flame system proposed by Takeno and Sato to burn mixtures of low heat content. The previous analyses were extended to include effects of the finite length of the porous solid inserted for internal heat recirculation, so as to predict the flammability limit. In the analysis the temperature of the solid is an eigenvalue of the system, while mass flow rate remains a controllable parameter. Numerical calculations have revealed the existence of a critical mass flow rate above which combustion cannot be sustained. The critical flow rate is more than ten times the burning velocity for a thin porous disk of the order of the flame thickness. Below the critical flow rate, the system has two combustion states with the distinct solid temperatures. As the flow rate is decreased asymptotically to the burning velocity, the inserted disk comes to play the role of a downstream flame holder for the state with the higher solid temperature, whereas it plays the role of an upstream flame holder for the state with the lower solid temperature.