In conventional gas turbine combustors liquid or gaseous fuel is sprayed directly into the combustion chamber for combustion in admixture with air. Consequently, fuel-air mixing and vaporization occur in the combustion zone resulting in significant regions of stoichiometric combustion and high NO.sub.X formation.
Accordingly, to achieve lower NO.sub.X levels there has been interest in and much development of premixed/prevaporized fuel combustion systems such as the dry low NO.sub.X natural gas combustors now used commercially. However, such combustors not only can have stability problems stemming from the need to operate near the lean limit but as with any premixed/prevaporized combustion system there is the potential for propagation of the flame upstream to the mixing/vaporization region with resultant stoichiometric burning and damage to the combustor. Although a safety shut-off can minimize damage, a shut down and inspection would be essential. With liquid fuels the problem is even greater. Moreover, the high combustor inlet temperatures not only of advanced stationary and aero gas turbine designs but even of most present day aero engines greatly increase the likelihood of such an occurrence. The problem is so severe that it has been questionable as to whether any premixed combustor will ever be feasible for an aero engine inasmuch as no conventional device has been deemed adequate to avoid engine damage. Not only must a device be able to block upstream flame propagation but it must impose a negligible pressure drop, i.e., less than about one percent. The present invention offers a practical low pressure drop solution to this important problem. Conventional flame arrestors installed on an exhaust have limited life and durability. Many of the flashback arrestor designs manufactured today support flame holding off the flashback arrestor after quenching a flashback event over some reducing agent/oxidizing agent ratio, inlet temperature and inlet velocity. Flame holding off the flashback arrestor can lead to thermal distress of the downstream structures, flashback arrestor. Over some period of time the held flame can ignite the upstream reducer/oxidizer mixture, usually resulting in failure of at least the flashback arrestor. The art teaches of supplying an external coolant to prevent damage to the flashback arrestor if a flame should hold off it.
The present invention is an improvement in flame arresting in that it will reduce the turbulence enough to prevent flame holding off the flashback arrestor at inlet velocities slightly higher than the turbulent flame speed. The turbulent flame speed can be defined as the sum of the laminar flame speed and the r.m.s. turbulent velocity. We have demonstrated a two monolith off-set flashback arrestor design which prevented flame holding off the face after flashback at inlet velocities similar to the estimated laminar flame speed.
The present invention provides a device that will reduce the quenching length usually required to quench flashback for a given cell geometry and width. Typically, a single channel monolith requires only 40 diameters to quench a flame.
The minimum channel diameter may be increased above the quenching diameter which allows more open, less expensive structures.