Turbulent separated inert and reactive flows over a triangular bluff body

Application Challenge AC2-12 © copyright ERCOFTAC 2019

Test Data

Overview of Tests

The description of the validation test rig strictly follows the original papers [1,2,3]. The test data contain LDA, CARS and gas analysis flow measurements for several operational conditions. For the sake of completeness, there are several alternative / additional experimental data (provided below), which replicate the inert and reactive bluff-body flows and can be used independently or in addition to the Volvo test – rig data.

A flexible modular combustor with optical access has been developed to generate experimental data for model validation. It was designed to enable the use of non-intrusive optical measurement techniques and to allow various combustion systems to be studied in an idealized fashion. The test set-up consists of a straight channel, with a rectangular cross-section, divided into an inlet section and a combustor section as shown in Fig. 3. The inlet section is used for flow straightening, turbulence control as well as fuel and seeding injection. A triangular – shaped bluff body was used for flame stabilization as shown in Fig. 3.

The air entering the inlet section is distributed over the cross-section by a critical orifice plate that, at the same time, isolates the combustor acoustically from the air supply system. Gaseous propane is injected and premixed with air 0.07 m downstream of the critical orifice plate by a multi-orifice, critical flow, fuel injector. The turbulence level in the combustor inlet is controlled by installing grids, honeycomb and/or screens at several axial locations in the inlet section. The examined experiments were run with premixed propane-air mixtures. A premixed combustion system was chosen to simplify the experiment, avoiding gradients in the fuel-air mixture and the influence of droplet evaporation or high speed jets from gaseous fuel injection.

The validation rig was designed to make several hours of test runs possible in order to allow for extensive traversing with a variety of sampling measurement equipment. The combustor section is of a modular design with the walls split into a number of interchangeable sections. The side-wall elements are either air cooled quartz windows for optical access or they may consist of pure water cooled sections. The upper and lower walls are water cooled, but may be replaced by other designs to make other laser diagnostic techniques, such as laser induced fluorescence, LIF, or particle image velocimetry, PIV, possible. The combustor section ends in a circular duct with a larger diameter and the acoustic outlet condition can be considered as a “sudden expansion”. The flame holder used in the present study had a triangular shape (Fig. 3) and was chosen because of the simple geometry, the resemblance to real afterburner geometries and because strong instationary flow and combustion phenomena were bound to occur.

Gas analysis and instrumentation

Local concentrations of combustion products were sampled with a traversing probe. The probe was mounted in the center of the combustor and could be used at three different axial positions where vertical profiles of combustion species could be measured. The gas analysis equipment consisted of chemiluminescence analyzer for Nox, two nondispersive infrared instruments for CO and CO₂, a paramagnetic analyzer for O₂ and a flame ionization detector for total unburned hydrocarbons (UHC). All instruments, except for the UHC detector, were able to cover the whole range of concentrations in the flame. The maximum level of UHC that could be measured with reasonable accuracy was 4000 ppm.

The water content, equivalence ratio, wet species concentrations and flame temperature were calculated from the measured gas composition, assuming equilibrium chemistry and conservation of species. The accuracy of a single species concentration measurement was estimated to be 5% and the accuracy of the equivalence ratio and calculated flame temperature to be about 10%. Fluctuations of the static pressure were measured with a piezo-electric pressure transducer located at the bottom wall, 0.03 m downstream of the reference position. Frequency spectra were recorded and analyzed with a HP 355660A analyzer. The airflow and fuel flow were measured with critical and sub-critical orifices yielding an accuracy of 3% respectively.

Contributed by: D.A. Lysenko and M. Donskov — 3DMSimtek AS, Sandnes, Norway