Predicting lean blow-off of bluffbody stabilized flames based on Damköhler number

Abstract

Lean Blow-Off (LBO) prediction is important to propulsion system design. In this paper, a hybrid method combining numerical simulation and Da (Damköhler) model is proposed based on bluffbody stabilized flames. In the simulated reacting flow, Practical Reaction Zone (PRZ) is built based on OH radical concentration, and it is considered to be the critical zone that controls LBO. Da number is obtained based on the volume-averaged parameters of PRZ. The flow time scale (τf) indicates the residence time of the fresh mixture flowing through the PRZ. It is obtained based on the characteristic length and volume-averaged axial velocity of the PRZ. The chemical time scale (τc) indicates the shortest time needed to trigger the reaction of the mixture. It is obtained by commercial software CHEMKIN through monitoring the transient variation of the reactor temperature. The result shows that the average Da number under different LBO conditions is 1.135 (the Da number under each LBO condition ranges from 0.673 to 1.351). This indicates that the flow time scale and chemical time scale are comparable. The combustion is in a critical state where LBO is easy to occur. With the increase of the fuel mass flow rate (the global fuel/air ratio increases from 0.004761 to 0.01095), τf increases from 0.001268 s to 0.007249 s, and τc decreases from 0.00124 s to 0.00089 s. Accordingly, Da number increases from 1.023 to 8.145, which shows that the combustion becomes more stable. The above results show that the method proposed in the present study can properly predict the LBO limits of combustors, which provides important technical supports for combustor design and optimization.