Prediction Of Combustion Driven Oscillation In A Residential Ultra-Low NOx Gas Furnace Product Using A Positive Feedback Stability Model

Combustion oscillation is a very common phenomena in many unsteady or transient HVAC applications (e.g., start-up operation). Combustion driven oscillations can occur once the sound (thermo-acoustically driven) is reflected from the combustion chamber back into the mixture supply region (and propagate even further upstream). The reflected sound causes a fluctuation in inlet mixture composition or mixture flow. In either cases, the equivalence ratio is changed and consequently the amount of heat release changes. This phenomenon could easily trap the combustion process in a loop that can easily result in combustion oscillations with higher amplitude. It can also produce unwanted transient noise tones and can raise NOx emission levels. The current work is aimed at providing a simulation model for diagnosing and preventing combustion driven oscillation using a modified positive feedback loop. This feedback loop is developed using upstream/downstream impedances (obtained from low-order acoustic simulations), and a flame transfer function. The value-add of this study (model-based approach) is that it can help design engineers to reduce the number of test iterations and optimize the design.