Experimental Investigation of Pressure Development and Flame Characteristics in a Pre-Combustion Chamber

Abstract

This study contributes to research involving wave rotor combustors by studying the development of a hot jet issuing from a cylindrical pre-combustion chamber. The pre-chamber was developed to provide a hot fuel-air mixture as an ignition source to a rectangular combustion chamber, which models the properties of a wave rotor channel. The pre-combustion chamber in this study was rebuilt for study and placed in a new housing so that buoyancy effects could be studied in tandem with other characteristics. The effectiveness of this hot jet is estimated by using devices and instrumentation to measure properties inside the pre-chamber under many different conditions. The properties tracked in this study include maximum pressure, the pressure and time at which an aluminum diaphragm ruptures, and the moment a developed flame reaches a precise location within the chamber. The pressure is tracked through use of a high-frequency pressure transducer, the diaphragm rupture moment is captured with a high-speed video camera, and the flame within the pre-chamber is detected by a custom-built ionization probe. The experimental apparatus was used in three configurations to study any potential buoyancy effects and utilized three different gaseous fuels, including a 50%-50% methane-hydrogen blend, pure methane, and pure hydrogen. Additionally, the equivalence ratio within the pre-chamber was varied from values of 0.9 to 1.2, and the initial pressure was set to either 1.0, 1.5, or 1.75 atm. In all cases, combustion was initiated from a spark plug, causing a flame to develop until the diaphragm breaks, releasing a hot jet of fuel and air from the nozzle inserted into the pre-chamber. In the pressure transducer tests, it was found that hydrogen produced the highest pressures and fastest rupture times, and methane produced the lowest pressures and slowest rupture times. The methane-hydrogen blend provided a middle ground between the two pure fuels. An equivalence ratio of 1.1 consistently provided the highest pressure values and fastest rupture out of all tested values. It was also found that the orientation has a noticeable impact on both the pressure development and rupture moment as higher maximum pressures were achieved when the chamber was laid flat in the “vertical jet” orientation as compared to when it was stood upright in the “horizontal jet” orientation. Additionally, increasing the initial pressure strongly increased the maximum developed pressure but had minimal impact on the rupture moment. The tests done with the ion probe demonstrated that an equivalence ratio of 1.1 produces a flame that reaches the ion probe faster than an equivalence ratio of 1.0 for the methane-hydrogen blend. In its current form, the ion probe setup has significant limitations and should continue to be developed for future studies. The properties analyzed in this study deepen the understanding of the processes that occur within the pre-chamber and aid in understanding the conditions that may exist in the hot jet produced by it as the nozzle ruptures. The knowledge gained in the study can also be applied to develop models that can predict other parameters that are difficult to physically measure.