A small-scale combustion-chamber testing facility has been designed and partly built for use in evaluating advanced combustor designs for future gas turbine engines. The specific model combustor for which the facility was designed is an approximation of a planar section of an ultra-compact combustor (UCC). In the full-scale UCC (Figure 1), vanes in an annular cavity are positioned and oriented to cause the combustion gases to flow in a spiral pattern and the resulting centripetal acceleration in the cavity is utilized to increase the speed of combustion and thereby make it possible to design the combustion chamber to be shorter than would otherwise be necessary. In the model combustor, the spiral aspect of the flow is approximated by means of a small flow of air directed perpendicular to a main flow. The design of the facility and the model combustor provides access for off-axis optical (including visual) observation and measurement of cavity-vane interactions. The facility can also be used to test many other combustor models.

Figure 1. This Is a Rear View of the UCC, a model of which is the original one around which the facility is designed. The facility can also be used to test other model combustors.

The design of this facility reflects considerations of accuracy, capability, safety, and flexibility expressed by users of other facilities. All systems and measurements are designed to comply with Aerospace Recommended Practice 1256, which is a standard published by SAE International (formerly known as the Society of Automotive Engineers). The facility includes systems that help to maintain a safe work environment: these include automatic fuel shutoff, heater shutoff, and general power-shutoff subsystems. The facility can safely accommodate testing of model combustors for which open flames are required.

Figure 2. Equivalence Ratios for liquid and gaseous fuels in combustion tests can be adjusted over wide ranges by adjusting flow rates.

The facility can supply air to the combustor model at a rate of as much as 260 standard cubic feet per minute (SCFM) [0.123 m3/s], split into two legs at 200 SCFM (0.0944 m3/s) and 60 SCFM (0.0283 m3/s), respectively. These legs can be independently heated to temperatures of as much as 500 ºF (260 ºC). The flow rates, pressures, and temperatures of air in the two legs are monitored and regulated, and are recorded along with other measurement data. Liquid fuel can be pumped to the testing volume at a maximum flow rate of 340 mL/min; gaseous fuel can be supplied at a maximum flow rate of 200 standard liters per minute. The combination of air and fuel rates can be adjusted to obtain a desired equivalence ratio (see Figure 2), which can range up to as much as 4.3 for JP-8 (a kerosine-based jet fuel).

The facility includes equipment for analyzing unburned fuel and combustion- product gases via their infrared spectra and for measuring oxygen concentrations in combustion products via the magnetic susceptibility of oxygen. Also included is an exhaust system that, if required, can pump gases out of the test volume.

This work was done by Eric R. Dittman of the Air Force Institute of Technology for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Manufacturing & Prototyping category. AFRL-0028

This Brief includes a Technical Support Package (TSP).
Small-Scale Combustion-Chamber Testing Facility

(reference AFRL-0028) is currently available for download from the TSP library.

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This article first appeared in the August, 2007 issue of Defense Tech Briefs Magazine.

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