Combustion – Process of combustion or burner operation – Flame shaping – or distributing components in combustion zone
Reexamination Certificate
1999-05-28
2001-01-30
Price, Carl D (Department: 3743)
Combustion
Process of combustion or burner operation
Flame shaping, or distributing components in combustion zone
C431S008000, C431S346000, C239S552000, C048S192000
Reexamination Certificate
active
06179608
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to a structure for mixing a fuel and an oxidant Specifically, the invention is a swirler with an integral flashback arresting capability. In one application, the invention is positioned in a gas turbine combustor downstream of the fuel/air-mixing region and upstream of the primary combustion zone to assist in mixing the fuel and air while simultaneously providing protection from a flashback event. In another aspect of the invention, the invention can also have a flame holding capability.
BRIEF DESCRIPTION OF THE RELATED ART
Flashback within a gas turbine can be a catastrophic event A flashback event occurs when the flame front within the primary combustion zone of a gas turbine combustor moves upstream from the primary combustion zone toward the source of the fuel to an undesired degree. When such an event occurs, the heat of combustion within the flame front has the potential to damage numerous structures within the fuel/air-mixing region of the combustor. Flashback events are becoming increasingly common as gas turbine combustors are operated ever leaner to achieve environment pollution objectives.
Using close coupled, non-aligned, multi-channel monoliths to quench a flame front as it flashes back is known in the art. U.S. Pat. No. 5,628,181 is a prime example of this type of structure. The patent teaches that two close-coupled monoliths with channels of different cell sizes that do not impart any swirl to the flow stream can effectively quench a flame front. In one application of this invention, the monolith is placed in a gas turbine combustor between the fuel injection point and swirler, which is located downstream. When a flashback arrestor of the design of '181 is used, a downstream swirler is essential for combustion flame holding or premixing since the straight channels of the flashback arrestor have straightened the flow. Thus during a flashback event, the flame front will pass through the swirler before being arrested, thereby causing potential damage to the swirler, especially should flames be held off the swirl vane surfaces after the flashback event has been arrested.
For many gas turbine applications, therefore, it would be beneficial if the swirler could be protected from a flashback event. If a swirl component could be integrated into the flashback arrestor, this would permit premixing, and under specific conditions enhancement of downstream flame holding, without a separate downstream device. Further since there would no longer be a swirler downstream of the flashback arrestor, a flashback event would no longer be catastrophic to the swirler.
SUMMARY OF THE INVENTION
It has now been found that a swirl velocity component can be added to the conventional flashback arrestor thereby creating an integral swirler flashback arrestor.
The basic invention is comprised of two multiple channel monoliths, one spatially upstream in a fluid flow from the other. Upstream is defined in relationship to the normal or desired flow direction. In a gas turbine combustor application which is one application for the present invention, the invention is placed between the fuel source and the primary combustion zone, thus the desired flow direction is then defined as the direction of flow of the fuel from the fuel's source to the primary combustion zone. The direction of flow in a flashback event, from the primary combustion zone to the fuel source, is considered abnormal and/or undesired.
The two monoliths are placed across the fluid flow, such that substantially all the fluid must go through the invention. In a gas turbine application, this means placing the two monoliths within a conduit, such that bypass is in essence eliminated. If excess bypass is present the device will not function properly, since unarrested flames could bypass the device resulting in damage to the engine.
The monoliths contain numerous channels with each channel being defined by walls having a length, a mean hydraulic diameter, and a spatial orientation. The channels of the two monoliths are offset. Offset means that the channels in the downstream monolith are aligned with the channels in the upstream monolith such that a flame front exiting a downstream monolith channel intercepts the wall of an upstream monolith channel. While it is not required that every downstream monolith channel be offset from an upstream monolith channel, the required number being application dependent, generally substantially all the channels must be offset, and it is preferred that all channels be offset.
The mean hydraulic diameters of the channels in both the upstream and downstream monolith are application dependent, considering the fuel, the fuel/air ratio, and the channel length. In general, the mean hydraulic diameters of the channels can always be less than the critical quenching diameter. The offsetting of the channels, however, permits the channel mean hydraulic diameters in either or both monoliths to exceed the critical quenching diameter. This feature of the invention permits the pressure drop of the invention to be reduced without effecting the invention's ability to arrest a flashback event. Testing with hydrogen indicates that under one set of conditions flashback was successfully arrested when the downstream monolith channels had a mean hydraulic diameter of approximately twice the critical quenching diameter and the upstream monolith had channels with a mean hydraulic diameter of approximately four times the critical quenching diameter. For each application, therefore, the operational range of mean hydraulic diameter ratios must be determined.
A gap between the monoliths is not preferred, but can be present. The length of any gap is application dependent. The gap must be less than the flame reformation distance. This distance is defined as the distance required for the flame front to reform into a flame front that can not be quenched by the upstream monolith. Incidental flame front reformation, therefore, may take place in the gap. A practical limit on the gap seems to be the largest channel mean hydraulic diameter found in the channels of the invention.
In the preferred embodiment of the invention, the channels the downstream monolith have smaller mean hydraulic diameters than the channels in the upstream monolith. While the invention will still prevent flashback if this condition is reversed, the invention is less effective, and may even permit a flame to be held by the downstream monolith. For example during two tests at the same conditions employing non-swirling hexagonal cell monoliths, one monolith with 0.108 inch mean hydraulic diameter channels and a second monolith with 0.054 inch mean hydraulic diameter channels, flame holding after a flashback event was observed when the 0.108 inch mean hydraulic diameter channel monolith was downstream of the 0.054 inch mean hydraulic diameter channel monolith damaging the monoliths, but no flame holding damage was observed when the monoliths were reversed.
The channels of the monoliths are given a spatial orientation so that the channels act as vanes to alter the direction of the entering flow field. An alteration in the flow field to add a swirl component is most beneficial. Any type of swirl is possible such as axial, radial, or axial/radial. The downstream flow field can be distributed or have vortex break down. The invention must generate a swirl with a swirl number greater than zero. A swirl number greater than 0.1 is desired with the preferred range of 0.2 to 0.6 for premixing and 0.5 to 1.8 for flame stabilization, the range of current swirlers. See Arthur H. Lefebvre,
Gas Turbine Combustion
126-135 (1983), incorporated herein by reference, for the definition of swirler number and the characteristics of swirling flows.
Various vane strategies are possible. A basic embodiment of the invention uses the channels of the upstream monolith to straighten the flow or partially turn the flow, and the channels of the downstream monolith as vanes to introduce a swirl. It is, however,
Berry Jonathan Dwight
Kraemer Gilbert O.
McCormick Paulding & Huber LLP
Precision Combustion Inc.
Price Carl D
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