Thermo-chemical manifold reduction for tabulated chemistry modeling. Temperature and dilution constraints for smooth combustion reactors

New scenarios for energy systems pointed out the importance of designing innovative combustion systems. In this context, high levels of internal dilution and preheating show interesting features related to low emissions, smooth temperature gradients, absence of visible flame and large fuel and load flexibility. Those characteristics are very difficult to obtain simultaneously with conventional combustion processes in the same device. The large-scale utilization of such novel concepts relies on the developments of proper modeling tools that should consider the multiple physical phenomena involved under distributed ignition. A challenging modeling aspect is related to the strong coupling between fluid-dynamics and kinetic time scales that implies the use of detailed mechanisms. Moreover, the heat transfer mechanisms and the heat loss at walls play key roles. In this context, tabulated chemistry methods are viable solutions to represent the thermo-chemical pattern in combustion systems with internal recirculation. However, the identification of adequate controlling variables for these systems is not trivial. In fact, in addition to mixture fraction and progress variable, an internal dilution and a heat loss parameter must be considered, leading to a 4-dimensional thermo-chemical manifold, with an inherent increase of computational costs. In this work a novel tabulation procedure is proposed in order to represent such comprehensive manifold taking into account the primary role of the internal recirculation on system reactivity. Moreover, a reduction of the thermo-chemical manifold was carried out by exploiting active interconnections between experiments and computations and embedding physical and process constraints based on measurable quantities obtained from experiments. These constrains are related to minimum ignition and maximum attainable process temperatures, heat loss through the surroundings and recirculation rate. The reliability of the proposed approach was assessed by comparing the reduced manifolds to the measured data for a cyclonic burner operating under massive internal dilution levels.