59170 - Characterization of a Designed Test Bench for Near-Wall Reactions of Ch4 and H2 

The thermal design of combustion chambers and turbine blades in jet engines is critically depends on a detailed knowledge of the combustion and of the heat loads to the walls is necessary. In general, high operating temperatures and reduced combustor size are strived for in order to increase the efficiency and reduce weight. Consequently, the components are exposed to heat loads above the melting point of the materials and there is a growing risk of incomplete combustion within the combustion chambers.

To protect the heat stressed components, effective cooling methods like film cooling are essential. Air is tapped from the compressor section and injected through effusion holes drilled in the components. However, incomplete combustion products, which contain unburnt fuel, may form a reactive mixture with the oxygen of the cooling air. With temperatures being above the ignition point of the fuel, the mixture ignites and the opposite of cooling takes place – near-wall reactions and heat loads to the surface.

Hence, extensive efforts were made to set up a new test bench for fundamental investigation of chemical near-wall reactions at atmospheric pressure. First, a hot gas generator provides hot oxygen-rich exhaust gas around 1400 K and 20 m/s, which enters a water-cooled test section constructed from Hastelloy C22. Then, within the test section, cold fuel is injected through 5 angled nozzles near the surface and ignites after mixing with the hot air. The test section allows for optical access to the secondary reaction zone by fused silica and its wall is equipped with a series of thermocouples (TC). The TC data are used to calculate the heat loads to the wall numerically, while planar laser-induced fluorescence (PLIF) of hydroxyl radical (OH) is applied to characterize the reaction zone. Due to growing interest in renewable energies and for handling reasons, gaseous methane (CH4) and hydrogen (H2) are injected.

The experimental results serve as validation data for in-house numerical codes. Hence, a solid basis of data is necessary, resulting from different sources. Based on TC data and supplied fuel and oxidizer, the species distribution of the exhaust gas is calculated in the chemical kinetic tool Cantera. The flow condition at the outlet of the burner and inlet of the test section is determined by cold measurements of hot wire anemometry at the same Reynolds number. The mass flow and the temperature of the injected cold CH4 and H2 are also controlled and measured by mass flow controllers and TCs.

In this work, a detailed description of the test bench is given and first PLIF results are presented. Significant differences in ignition delay and reaction zone characteristics of CH4 and H2 fuel are observed. Additionally, the PLIF images are in good agreement to the TC data. The TCs show a good sensitivity to the near-wall reactions.

In future studies, the temperature of the reaction zone is determined by two-line PLIF and high-speed OH*-chemiluminescence is applied for a time resolved investigation. Further, the heat fluxes to the wall are calculated numerically by means of inverse heat conduction. The approach allows for a 2D distribution of temperature and heat flux.