The structure of turbulent nonpremixed flames, formed by fuel jets issuing into quiescent or laminar air flow, is appeared to be different in the three regions-the region near jet exit, the transient region, and the downstream region. The behavior of the flame zone as well as the mixing chracteristics between fuel and air in the near region dominates the flame structure of downstream. Therefore knowledge about the flame structure of the near region is quite important to understand overall reactive flow exactly. When the exit velocity of fuel jet is high enough, small scale turbulent eddies are generated before coming out from nozzle and exist within the jet flow. In addition, large scale vortices are produced at the jet boundary. Such flow feature of turbulent jets affects the mixing of fuel and oxidant controlling chemical reactions and is affected reversely by heat release and concentration variation due to reactions.
This study has been carried out in order to better understand the structure of turbulent nonpremixed flames. Attention is mainly focused on the location and laminar feature of flame zone of hydrocarbon-air flames in the near region. Physical meaning of the radial distribution of temperature, concentration, and velocity and similarity with counterflow diffusion laminar flames are also dicussed. A schematic illustration of the flames is proposed to concentrate the results of this research consequently and to help more accurate theoretical models developed.
Experiments were performed for hydrocarbon jets issuing upwards from a circular tube in a coflowing low-velocity air stream. The principal experimental parameters are the Reynolds number and the type of tuel. Three values of the Reynolds number, $Re_j$ = 2,000, 6,000, or 9,500, were selected for the experiments, based on the condition of jet exit. Three types of fuel were used-commercial propane, mixture gas of propane and 30% nitrogen by volume, and 99.99% methane.
Two types of synchronized photography originated in this study were attempted to visualize the structure of the jet flames. Schlieren and Mie scattering images of $TiO_2$ particles were simultaneously superimposed on the same frame of a film exposed to the flame luminescence itself. Entrainment of surrounding air was also visualized by using the smoke line method. Mean temperature and concentration were measured with a Pt-Pt/13%Rh thermocouple and a gas chromatograph respectively. Instantaneous radial and axial velocities were also measured with a phase doppler anemometer.
It was found that the synchronized photography would be easy to apply and useful for the visual study on the structure of turbulent nonpremixed flames. From the synchronized photographs of nonpremixed hydrocarbon flames, the flame zone in the near region was proved to be located radially outwards away from the jet boundary, not within the mixing layer. Combining the entire data of flame zone location, it was concluded that the diffusion velocity of fuel to air, as well as the stoichiometric volume ratio and the entrainment of air, is an important factor to shape up the overall structure of flame zone.
In the synchronized photographs, it was easy to know that the flame zone is very stable. Measured rms values of turbulence near the flame zone were also as small as ones in laminar flames. Some researchers insisted that it is because turbulence is locally laminarized due to flame. However, the flame zone is located airside apart from the mixing layer. The barrier zone between flame zone and fuel jet boundary, in which highly viscous mixture gas exists, prevents turbulent eddies in fuel jet from being trasnferred to the flame zone. These facts make it obvious that the flow in the neighbourhood of flame zone is intrinsically laminar rather than a locally laminarized turbulent state.
The results of concentration measurement exhibited that leakage of oxygen through the flame zone occurred. Due to the fact, great attention must be taken in analyzing nonpremixed flames by using the fast chemistry. However, it was appeared that relationship between mixture fraction and mass fraction could be decided uniquely for various cross-sections. Therefore, the prediciton of the composition of any cross-section is possible under assumptions of equal diffusivities of species and the fast chemistry without so much errors. Radial temperature and concentration distribution of the flames had hood similarity with those of counterflow laminar nonpremixed flames when distance from the flame zone to the jet axis for jet flames or to the burner surface for counterflow flames was taken as the reference coordinates.
A schematic illustration was proposed which showed a structure of the turbulent nonpremixed flame in the near region. This may be a useful model for the design of nonpremixed burners using gaseous hydrocarbon fuels.