Colloquia

Samenvatting

Residential boilers are found in many households and are responsible for providing warm heating and tap water. Its start-up phase contains some irregularities pertaining to the timing of combustion from spark ignition, which produces a different pressure wave each time. The resulting sound wave may cause unrest for inhabitants as it can reach values above 85 dB in cases for delayed ignition, leading to an audible bang. In severe cases, it may damage the material and reduce the overall efficiency and structural integrity of the boiler.

With support of Bosch Thermo Techniek in Deventer, this study investigates the phenomena pertaining to the unsteady phase of residential boilers. The relevant subjects like inconsistency in spark ignition and delayed ignition are explained as well as their direct relations to the resulting ignition pressure wave. Combustion kinetics has been explored by using a 2D model in Ansys Fluent, based on the geometry of a real heat cell. The simulation is set up to imitate reality as close as possible, and includes spark ignition to induce chemical kinetics using the detailed GRI-Mech 3.0 reaction mechanism. Due to the change in rate of combustion, a pressure wave is formed and its pressure values are measured over time in strategically placed positions in the combustion chamber and heat exchanger. The resulting pressure time series are used to explain the unsteady pressure behaviour. Three situations corresponding to ignition delay times are simulated: nominal combustion, a combustion chamber fully filled with a fuel-air mixture and a halfway filled combustion chamber. For each of these situations, multiple configurations with varying cross sectional area have been simulated to show possible increases in ignition pressure. The maximum pressure amplitudes are compared with test data at three different ignition delay times. Although the results are slightly optimistic for very clean combustion, slightly delayed ignition shows promisingly accurate results to predict maximum pressure values and its corresponding sound level. These simulations are a viable way to predict the ignition pressure, and further research using these methods is encouraged.