Experimental Investigation of Flame Wall Interaction

Motivation

A major part of pollutants in fossil fuel combustion is produced in the near wall region. Several emission limits can temporally only be achieved by additional exhaust converters. By increasing the efficiency of internal combustion engines, there is a trend of downsizing, which increases the surface-to-volume ratio. In gas turbines lean flames have been observed to be much closer to the wall, which has an impact on the durability and lifetime of engine parts and components. Hence near wall phenomena has received increased importance in future developments.

In the near wall region flow- and thermodynamic properties show a different behaviour than in the undisturbed flow field. Wall surface temperatures are always colder than those in flames, which can lead to quenching of chemical reactions and the formation of unburned hydrocarbons. At present the interaction of fluid motion, transport phenomena and chemical reaction has not been investigated and understood completely. Laser based diagnostics allow a nonintrusive detection of important tensorial quantities and scalars with a high resolution in both time and space. The approach of this work is, to apply these techniques in the near wall region. Besides a more detailed understanding of combustion phenomena close to surfaces, detailed experiments are needed providing validation data for numerical calculations.

Experimental Setup

Video 1: Chemoluminescence and phosphorescence of a thermographic phosphor after UV illumination on a wall stabilized jet burner.

Experiments are carried out on a generic burner device, where a premixed jet impinges on a perpendicular orientated wall. The flame stabilizes depending on flow and wall conditions between nozzle and wall surface. The behaviour of structured and/or catalytic coated wall materials can be investigated for several surface temperatures through an interchangeable wall with an internal cooling plenum. Flow properties are attainable between 2500 < Re < 10 000 and the turbulence intensity can be pushed by the additional use of a turbulence generation grid inside the nozzle.

Diagnostics

Video 2: Mie scattering of inert TiO2 particles in a ethane flame stabilized close to a wall.

The key to a better understanding of reactive flows is the usage of simultaneous measurement techniques of important quantities. Coherent Anti-Stokes Raman Spectroscopy (CARS) thermometry is applied as well as Laser Induced Fluorescence (LIF) for the detection of CO. For measurements of wall surface temperature Thermographic Phosphors are deployed. Besides playing a key role for chemical reaction rates the measured gas phase temperature is used for density and quenching correction of the CO LIF signal. Future experiments will simultaneously employ particle image velocimetry (PIV) and OH-PLIF to characterize the flow field and flame structure near the wall.