Characteristics of hydrogen combustion in a direct injected constant volume combustion chamber using rainbow schlieren deflectometry

Abstract

The nation's need for alternative fuels for Internal Combustion Engines (ICEs) has been a major concern for automotive researchers. The need for a sustainable energy system has lead researchers to consider alternative fuels such as hydrogen and thus, several studies have been conducted on this fuel since the 1930s. In particular, understanding the combustion performance of hydrogen at varying equivalence ratios, ignition timings, and volumetric percentages with other fuels is necessary to optimize engine operations. This study investigates the combustion performance of hydrogen injected into a constant volume combustion chamber (CVCC). The properties studied include flame structure, combustion duration, flame front speed, chamber pressure, and net heat transfer rate. The fuel was injected directly into the chamber containing quiescent air at atmospheric pressure. An ignition system consisting of a coil and a spark plug was used to ignite fuel/air mixtures. This study implemented an optical technique, Rainbow Schlieren Deflectometry, to visualize fuel jet penetration, turbulent fuel-air mixing, flame structure, and flame propagation. Schlieren images were analyzed by a cross-correlation technique to compute flame front speed. A dynamic pressure sensor was used to acquire instantaneous chamber pressures which were used to estimate transient chamber net heat transfer rates. First, experiments were conducted by varying the fuel supply pressure to the chamber and the overall equivalence ratio. An investigation of the fuel jet penetration showed that it takes the fuel jet 2.25 ms to reach the igniter. This result was helpful in establishing ignition times for later experiments. Results showed that fuel supply pressure does not affect fuel jet penetration. The fuel jet, however, creates turbulence in the chamber that affects combustion processes. The equivalence ratios tested were ö = 1.0, 0.804, and 0.318. Results showed that equivalence ratio has a significant impact on flame front speed which decreased as the equivalence ratio decreased. Next, experiments were conducted to study the effects of ignition time on combustion processes. A programmable logic controller was added to the experimental setup to control ignition time and aid in sequencing events. The ignition times tested were t = 3, 5, and 10 ms in the early ignition group, t = 20, 30, and 40 ms in the mid-ignition group, and t = 60, 80, 240, and 540 ms in the late ignition group, where t = 0 refers to the start of fuel injection. Ignition time affects the flame structure and flame propagation. Results showed that at ignition times prior to the close of the fuel injector, the initial flame front speed is high because of fuel-jet generated turbulence. After the fuel injector closes, increasing the ignition time increases the combustion duration because of dissipating fuel-jet generated turbulence. Ignition time also has significant effects on chamber pressure variations and net heat transfer rates. Next, the effect of ignition time for varying equivalence ratios was studied. Experiments were conducted at three equivalence ratios, ö = 0.6, 0.8, and 1.0 and four ignition times, t = 3 ms, 10 ms, tend, and tend + 50 ms. An ultra-high speed camera was incorporated into the experimental setup to acquire schlieren images at a frame rate of 50,000 Hz and exposure time of 19.8µs. Results show that equivalence ratio has minor effects on chamber pressure variations and net heat transfer rate at early ignition times and on flame structure and flame propagation at any ignition times. Ignition time has a significant effect on all combustion processes. Finally, experiments were conducted to determine the effect of hydrogen percentages by volume on methane combustion at varying ignition times. A second high pressure injector was incorporated into the experimental setup to inject the methane into the combustion chamber. Experiments were conducted at the following methane/hydrogen percentages: 23% CH4 - 77% H2, 33% CH4 - 67% H2, 43% CH4 - 57% H2, 53% CH4 - 47% H2, and 63% CH4 - 37% H2. The two ignition times were t = tend and t = tend + 50 ms. Results show that combustion duration decreases as hydrogen percentage increases for identical ignition times, and as ignition time decreases at identical hydrogen percentages. Flame front speed increases as hydrogen percentage increases. Peak chamber pressure and peak net heat transfer rate decreases for the late ignition time at fixed hydrogen percentages.