Basic explorations of limits of microwave assisted spark plug in constant volume combustion chamber

The enhancement of laminar flame development using a microwave-assisted spark plug has been investigated in methane-air mixtures. Experiments are conducted using a 1.6 liter constant volume combustion chamber with various levels of microwave energy input at a range of initial pressures, equivalence ratios, and nitrogen dilution. Microwave enhancement was evaluated on the basis of flame development time (FDT), flame rise time (FRT), total net heat release, and ignitability limit extension. The addition of microwave energy to a capacitive discharge spark decreased the FDT at all equivalence ratios at atmospheric pressure, with the greatest effect observed at slower-burning conditions. Increased microwave energy input further reduced the FDT for lean conditions. At stoichiometric conditions, a threshold was observed above which additional microwave energy did not reduce the FDT. Nitrogen dilution experiments indicate that the residence time of the flame front in the spark zone is important in dictating flame enhancement from microwave energy deposition. The enhancement of flame development by microwaves diminishes as the initial pressure of the mixture increases, with negligible flame enhancement observed above 3 bar. The addition of microwave energy had no effect on flame rise time or on total net heat release for the conditions studied.