Mechanisms of Pre-Chamber Hot Turbulent Jet Ignition of Ultra-Lean Mixtures
Stringent emission regulations require gas engine manufacturers to move towards ultra-lean operation to reduce the peak combustion temperature and thereby minimizing NOx formation. For heavy-duty gas engines, pre-chamber hot jet ignition is a common practice, which has advantages over traditional spark ignition. An experiment was developed to understand the ignition mechanisms of premixed CH4/air and H2/air mixtures by a turbulent hot jet generated by pre-chamber combustion. Simultaneous high-speed Schlieren, OH* chemiluminescence, particle image velocimetry, and infrared diagnostics were applied to visualize the jet penetration and ignition processes inside the main combustion chamber. Results illustrate the existence of two ignition mechanisms: jet ignition and flame ignition. The former produces a jet comprising of only hot combustion products from pre-chamber combustion. The latter produces a jet full of wrinkled turbulent flames and active radicals. A parametric study was performed to understand the effects of pressure, temperature, equivalence ratio along with geometric factors such as orifice diameter and pre-chamber spark position on the ignition mechanisms and probability. A global Damköhler number was defined to remove the parametric dependency. The limiting Damköhler number, separating these two ignition mechanisms, was presented on a turbulent combustion regime diagram. Ignition of ultra-lean H2/air mixtures using supersonic hot jets was also explored. Results showed that a supersonic jet could ignite leaner mixtures than a subsonic jet, e.g., the lean flammability limit of H2/air extended to &phis; = 0.22 from &phis; = 0.31. High-speed OH* and infrared imaging of supersonic jets indicated a high-temperature zone downstream of the shock structures. Detailed numerical simulations were carried out to understand the fundamental mechanism why supersonic jets could extend the lean flammability limit. To quantify the flow field of a turbulent hot jet, a novel, inexpensive, two-camera Schlieren technique was developed. A detailed statistical assessment of seedless velocity measurement using Schlieren Image Velocimetry (SIV) was explored using open source Robust Phase Correlation (RPC) algorithm. An axisymmetric high-speed (jet exit velocities 304 m/s (Mach = 0.3) and 611 m/s (Mach = 0.6)) turbulent helium jet was analyzed in the near and intermediate region (0 ≤ x/d ≤ 20) for two different Reynolds numbers, Re d=11,000 and Red=22,000 using four techniques. They were Schlieren with a horizontal knife-edge, Schlieren with a vertical knife-edge, shadowgraph technique, and the traditional Particle Image Velocimetry (PIV). The measured velocities using the four techniques were compared. The velocity field obtained using horizontal knife-edge Schlieren with 40% cutoff, and shadowgraph agreed well with the PIV results. However, vertical knife-edge Schlieren with 40% cutoff performed poorly. Primary Peak Ratio (PPR), Peak to Correlation Energy (PCE), the probability distribution of signal and noise were used to compare capability and potential of different SIV techniques. Thus SIV, a ‘seedless’ velocity measurement, can be used for a wide range of high-speed turbulent flows containing refractive index gradients. A strong thermo-acoustic combustion instability was observed near the ultra-lean limit. Flame dynamics at ultra-lean conditions got highly affected due to strong coupling between heat release and pressure perturbation. High-speed Schlieren imaging of flame propagation and velocity field measurements indicated an oscillating flame front because of intense pressure perturbation inside the main combustor. Strain rates along the flame edge started oscillating causing higher strain at antinodes and lower strain at nodes of pressure perturbation cycle. Controlling such instability, in active or passive manner, requires adequate knowledge about distinct types of instability modes, combustor acoustics, and perturbation energy associated with instability. The ignition characteristics of a hot turbulent jet impinging on a flat plate surrounded by an ultra-lean premixed H2/air was studied experimentally. Two important parameters controlling the impinging characteristics of the jet, the impinging distance, and the impinging angle were examined. Results illustrate the existence of two distinct types of ignition mechanisms. If the impinging distance is short and the hot turbulent jet hits the plate with high enough momentum, the temperature increases around the stagnation point and the ignition starts from this impinging region. However, if the impinging distance is long, the hot turbulent jet mixes with the unburned H2/air in the main chamber and ignites the mixture at the upstream from the plate. For such type of ignition, the impinging plate has no role in main chamber ignition. Employing the stagnation point ignition, a lower flammability limit of H2/air was achieved. We also explored the ignition characteristics of ultra-lean premixed H2/air by multiple hot turbulent jets in a dual combustion chamber system both experimentally and numerically. Ignition by multiple jets was motivated by the fact that most gas engine companies use multiple nozzle pre-chamber. We learned that multiple jets did not extend the lean flammability limit of H2/air. However, the ignition probability improved significantly near lean flammability limit. The effect of spark location and pre-chamber equivalence ratio was investigated in detail to assess the effectiveness of nozzle locations within the pre-chamber. It was found that depending on the spark location, at times the side jet or the middle jet initiated ignition in the main chamber. This knowledge would be helpful to optimum prechamber designs such as the location of the spark plug and the number of nozzles. Lastly, to understand the physics of a flame passing through a small orifice (1 – 10 mm) – whether it would survive or extinguish, CH4/air premixed flame passing through a small channel had been studied. Both straight and converging-diverging microchannels with the inlet to throat ratio of 2:1 and 3:1 were used as a model channel. The influences of the equivalence ratio, channel diameter, inlet-to-throat diameter ratio were studied parametrically. Dynamic behavior of flame propagation inside the channels was studied using CH* chemiluminescence, infrared imaging, as well as direct imaging. Flame dynamics such as flame shape, propagation speed, cyclic oscillatory motions, reignition, and local extinction were observed. Based on the experimentally calculated global strain rate, a stable flame propagation criterion was proposed.
Qiao, Purdue University.
Aerospace engineering|Mechanical engineering|Energy
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