Experimental and numerical investigation of the acoustic response of multi-slit Bunsen burners

The design and construction of combustion systems like central heating boilers is obstructed by acoustic problems because these are largely misunderstood, despite our increase in knowledge over the last decades. Current models for the phase of the transfer function of Bunsen-type flames, based on the kinematic behavior of the flame, completely miss the experimentally observed phase, unless the measured flow field is used in the model. In this paper we analyze numerical results of the steady flame form and flame transfer function obtained with detailed, two-dimensional numerical simulations of flames on multi-slit burners. The numerical model is validated with experiments of the flame shape (using chemiluminescence) and the flame transfer function (using OH luminescence for the heat release fluctuations and heated wire probe for the acoustic distortions). We subsequently study the influence of changes in mean flow velocity, slit width and distance between the slits on the transfer function, both numerically and experimentally. The experimentally observed effect of varying equivalence ratio on the flame transfer function is also analyzed. Good agreement is found which indicates the importance of predicting the influence of the flow on the flame and vice versa.