TRANSFER FUNCTION CALCULATIONS OF SEGREGATED ELEMENTS IN A SIMPLIFIED SLIT BURNER WITH HEAT EXCHANGER

Abstract

A simplified burner-heat exchanger system is numerically modelled in order to investigate the effects of different elements on the response of the whole system to velocity excitation. We model the system in a 2D CFD code, considering a linear array of multiple Bunsen-type flames with heat exchanger tubes downstream the flames. Thermoacoustic instability is one of the main issues in lean premixed combustion systems, especially domestic boilers. In compact condensing boilers the close distance between the burner surface and the heat exchanger has increased the importance of studying the interactions between the flames and the heat exchanger. The elements corresponding to the heat balance in the system are the flame as heat source and burner deck and heat exchanger as heat sinks. We use both transfer function and transfer matrix approaches to identify the response of these elements to a step function excitation of velocity at the inlet of the domain. Steady-state simulations show that the contribution of the burner deck to the heat balance of the whole system is negligible, leaving the flame and heat exchanger as main contributors to the response of the system. We separately investigate the behavior of these two elements by modeling cases with flame only and heat exchanger only. Then we calculate the behavior of the combined system and compare it to the results of modeling a case with flame and heat exchanger together. These results show that, assuming linear behavior of the elements, it is possible to predict the system behavior via its constructing elements. Further investigations of the effects of other parameters and the limits, within which the assumptions are valid, are currently in progress.