NASA offers flameout technology

Another in a continuing series of articles for ISA members by the Mid-Atlantic Technology Applications Center (MTAC), one of NASA six regional technology transfer centers.

Pittsburgh, Pa.-NASA Langley Research Center's 8-ft. temperature tunnel is a large-scale wind tunnel capable of simulating hypersonic flight conditions through the use of air/methane combustion.

So it can function as a test bed for propulsion system research, liquid oxygen is injected directly into the tunnel's combustor to replenish that which would normally be consumed during conventional air/methane combustion. Because of the volatile nature of methane, liquid oxygen, and air, safety concerns over an unplanned combustor flameout and re-ignition prompted the development of a new flame detection scheme capable of responding quicker than the thermocouple-based system already in use by the facility.

Computer modeling of combustor flameout and re-ignition indicated that the proposed system must respond to such an event in less than 100 milliseconds. If, after 100 milliseconds, the unburned fuel in the combustor were to re-ignite, the resulting pressure pulse might be capable of exceeding the design limits of the combustor.

The extremely fast responding fiber-optic flame detection system was developed to detect an unplanned combustor flameout during tunnel operation and signal the facility control network to prevent additional fuel from entering the combustor.

The optical flameout detection system (OFDS) monitors light energy in discrete spectral bands (200-600 nanometers) with two independent photomultiplier tube (PMT) optical detectors. Light energy from the combustion process is optically coupled to the detectors through a pair of 20-ft.long, 0.0625-in.-diameter fiber-optic probes.

The output of the PMTs is used to activate circuitry that determines whether a Flame On/Off condition exists in the combustor.

In order to generate a main-Flame On indication from the detection circuitry, the detector outputs must exceed a preset minimum value corresponding to the light intensity associated with a low-intensity boost flame. Conversely, if the detector outputs drop beneath a minimum value, then a Flame Off condition is registered, which initiates a rapid shutdown of the main fuel supply.

The major components of the flame detection circuitry are a noninverting amplifier, a voltage comparator, and two relays-one normally open (NO) and one normally closed (NC). The amplifier conditions the detector output for the fixed voltage comparator. Variation of the amplifier gain changes the minimum detector output voltage required to generate a state change in the fixed voltage level comparator. A 2-1 comparator on/off voltage ratio is designed into the comparator circuitry to guard against false triggers from reflections within the combustor.

When the comparator undergoes a state change, a corresponding state change will also occur in the NO/NC relay. The relay combination is the flameout detection systems trigger, which is monitored directly by the facility's process control system.

Derivations of this electro-optical system could provide more detailed information on various aspects of the combustion process or in an application where the process control has a radiometric property correlation.

OFDS is an unpatented invention. NASA Langley wants to identify companies interested in commercializing this technology. For information, contact John Bacon, NASA/ISA liaison, by phone at 412/383-2530 or by e-mail at jbacon@mtac. pitt.edu.

Visit MTAC's home page on the World Wide Web to view the Instrumentation and Sensors section and check out other hot technologies: http://oracle. mtac.pitt edu/WWW/MTAC.html.

Copyright Instrument Society of America Jan 1997
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