| It is critical to understand the process of near-wall heat transfer in designing gas turbine combustors. It is likewise important to accurately estimate the spatial and temporal patterns of heat flux to the wall, which are related to combustion processes in the turbulent boundary layer, because the thermal stresses induced by transient heat transfer into the wall material can affect the combustor lifetime. Sandia combustion researchers supported by SciDAC and other Office of Science programs, along with collaborators from the University of Trondheim in Norway, have performed threedimensional Direct Numerical Simulations (3D DNS) to study the evolution of an anchored, hydrogen-air, V-shaped, premixed flame, immersed in a turbulent Poiseuille flow, as it interacts with the channel walls. The simulations were performed with detailed hydrogen-air chemical kinetics, and allowed to run as long as was necessary to reach statistical stationarity. The 3D DNS results reveal that the formation of near-wall coherent turbulence structures shaped like hairpins, which push the flame towards the wall on one side and away from it on the other (figure 9), thereby inducing large spanwise gradients of heat flux to the wall. By examining temporal and spatial scalar spectra, the researchers discovered that the characteristic spatial and temporal pattern of wall "hot spots" is correlated with the passage of these hairpin-shaped vortical structures. Moreover, they discovered that, as the flame quenches near the wall, an exothermic radical recombination reaction which produces hydroperoxy also contributes significantly to the heat flux to the wall. |
Figure 9. Instantaneous snapshot from DNS of a hydrogen-air, V-shaped, premixed flame immersed in a turbulent Poiseuille flow as it interacts with the channel walls. Flow is from right to left. A temperature isocontour representing the flame is shown in red and a vorticity magnitude isosurface is shown in blue. |
