Combustion associated noise in central heating equipment

M.L. Bondar

Project description:

In the field of small-scale combustion equipment for domestic appliances, such as central heating systems, the demand for clean and efficient burners has lead to the development and application of fully premixed burner systems. Fully premixed flames stabilised on porous and perforated burners are effectively cooled, resulting in very low NOx emmissions. Clean and highly efficient central heating systems equipped with such burners have rapidly been introduced in the last years. Since then, however, it has become clear that the interaction between the flames and the burner often leads to instable combustion phenomena and combustion noise. Interviews with burner and boiler manufacturers, carried out by the TUE for NOVEM, has shown that almost all companies have trouble to circumvent noise problems. Furthermore, for the new generation of equipment with a continuously variable working point, the problem is even harder to solve, since the acoustical properties of the system can depend strongly on the setting of the working point. Problems may become even more complex when faced with designing boilers for oter gas compositions like the ones found in other European countries or when H2 enriched combustion becomes a reality. The underlying problem is that the mechanism for sound generation and amplification by the burner-flame combination is not well understood. Consequently, sound in central heating systems is not under control. It is the aim of this project to develop a model for the interaction between the flame/burner and acoustic waves, with which it is possible to understand, predict and solve acoustic instabilities in small-scale combustion equipment. This project is a follow-up to a project started by TUE, TNO-TPD, Gasunie, EnergieNed, Gastec and VFK (Vereniging van Fabrikanten van Ketels, The Netherlands) in 1996. The project has been successful since then: a validated model has been developed by TUE for predicting the acoustic behavior of porous or perforated surface burners, with flames closely stabilized on top of them at a relatively low thermal load. Furthermore, TNO-TPD has used the developed model for the acoustic transfer of such burner-flame systems in their model to predict boiler system resonances for a number of boiler manufacturers. However, the coverage of the existing model is limited. The acoustic response of porous and perforated burners operated at high or intermediate loads (where so-called `Bunsen type' flames occur) has a different physical nature and for modeling other techniques are needed. Furthermore, noise problems are primarily observed at the cold start and during the modulating phase of the boiler. This asks for an acoustic response model which is valid for predicting noise during transient modulating phases of the system as well. Based on recent progress made in both research groups at mechanical engineering and mathematics, we expect to be able to develop the needed acoustic transfer model. To this end, state-of-the-art numerical and experimental techniques will be combined. The new models will be implemented in the acoustic boiler model of TNO, which will be used to assist boiler manufacturers in tackling acoustical problems as early as in the initial phase of the design process of new boilers. As a result of the project, the competitiveness of the Dutch manufacturers participating in this project will increase in the (European) field of high-efficiency domestic heating equipment. With a reliable acoustical prediction model, the time-to-market and the production costs can be significantly reduced. Also, the model will likely enable an extension of the range of operation of modulating boilers to lower pollutant emissions, more compact systems and a broader class of gas qualities. As a result manufacturers may be able to reduce the number of boiler models needed to cover the broad range of gas qualities present in the European market and to burn both natural and H2 enriched gas. Furthermore, noise free operation of heating equipment under a broader range of circumstances will add to a more robust public image of this technology in general.

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