Ichiro Hongo

Pulse combustion: The mechanisms of NOx production☆

Abstract A study has been performed to elucidate the fundamental mechanisms controlling the production of NO in a Helmholtz-type pulse combustor. Cycle-resolved measurements of velocity and temperature in the combustion chamber were made. These measurements were combined with the Zeldovich NO formation mechanism to explain the mechanism responsible for the low NO formation found in pulse combustors. The mechanism responsible for low NO formation found in pulsating flow as compared to nonpulsating flow was found to be a short residence time at high temperature. Three different possible mechanisms for this short residence time were investigated: (1) hot products from combustion are cooled by mixing with the cooler exhaust gases entering the combustion chamber from the tail pipe, thus quenching the NO formation reaction (“Automatic Exhaust Gas Recirculation”), (2) hot combustion products are quickly cooled by mixing with the incoming cold reactants, and (3) residual products with a lower overall temperature (due to an increased rate of heat transfer in the combustion chamber) readily mix with hot products producing a short residence time at high temperature. It is shown that the mechanism responsible for the low NO emission in pulse combustors is a result of item 3. A short residence time at high temperature is caused by rapid mixing with cooler residual gases that are lower in temperature due to increased rates of heat transfer in the combustion chamber.

Time-resolved velocities and turbulence in the oscillating flow of a pulse combustor tail pipe

Abstract The cyclic behavior of the oscillating velocity field in the tail pipe of a pulse combustor was studied using laser doppler velocimetry. In this flow, the oscillations result from an acoustic resonance and have amplitudes of up to 5 times the mean velocity. Oscillation frequencies were varied from 67 to 101 Hz. Streamwise velocity and turbulence-intensity boundary layer profiles were measured to within 130 μm of the wall, and transverse turbulence measurements were made to within 2 mm. The phase relationships of the velocity, turbulence intensity, and combustion chamber pressure oscillations are compared. Velocity oscillations near the wall are found to phase lead those in the center of the pipe, creating periodic flow reversals through the boundary layer. A comparison is made between this turbulent oscillating boundary layer and the laminar oscillating boundary layer for flow over a flat plate. The effects of axial position, pulsation frequency, pulsation amplitude, and mean flow rate on the velocity and turbulence profiles are discussed. Time-resolved wall shear stresses (directly calculated from the velocity measurements) are presented and compared with those of steady turbulent flow. Time-averaged velocity and turbulence profiles are also compared with those of conventional steady turbulent flows. The time-averaged velocity profile is found to be flatter than that of steady flow at the same mean Reynolds number, and both the streamwise and transverse turbulence intensities are found to be significantly higher than those of steady flow.

Development of small twin-valveless pulse combustors : effect of injection system

Abstract The performance of gas-fired residential sized twin pulse combustors with two aerodynamic valves for residential heating appliances is discussed. Two injection system are tested. Pre-mixed injection resulted in having better stability of anti-phase operating mode between the two combustion chambers which reduced the noise level less than 50 dB(A). The combustor shows NOx emissions of 25 ppm and a thermal efficiency of 93% over a wide turndown from 1,100 to 6,000 W. At a lower heat input region, the combustor can still operate in the anti-phase mode with fuel supply to only one combustor. It is shown that twin-valveless pulse combustors have the potential for compactness, low noise, and efficient operation over a wide turn down ratio for residential use.