Numerical study of the influence of wall roughness on laminar boundary layer flashback

Modification of flow characteristics by wall roughness has been intensively analyzed in the field of fluid mechanics. However, few studies have considered wall roughness in boundary layer flame flashback. In the present study, laminar boundary layer flashback in channels with roughness at the walls is studied using two-dimensional simulations. The laminar premixed flame interacts with a linear shear flow along a wall with triangle-shaped fine roughness. The simulation results show that wall roughness in this configuration can attenuate flashback tendency due to enhanced heat loss to the boundary. By comparing results for isothermal and adiabatic boundary conditions at the walls, it is shown that the heat loss enhancement due to wall roughness may be a dominant effect in determining flame propagation characteristics for the walls with low thermal resistance. The critical velocity gradient is shown to decrease with wall roughness level and increase with thermal gas expansion ratio.

Figure 1

A sketch of the numerical configuration.

Figure 2

A sketch of the roughness element configuration.

Figure 3

Temperature field and backflow region for adiabatic walls. $\mathrm{\Theta }=8$, $g=\frac{dU}{dy}=0.1S_L/{\delta }_L$, $k=4$.

Figure 4

Stationary flame tip velocity $U_{\mathrm{tip}}$ and stagnation zone thickness $H_{\mathrm{stag}}$ for different roughness slopes $k$. Adiabatic walls, $\mathrm{\Theta }=8$, $g=\frac{dU}{dy}=0.1S_L/{\delta }_L$.

Figure 5

(Top) Vertical cross sections of horizontal component $U$ of flow velocity ahead of the flame for different $k$. $\mathrm{\Theta }=8$, $g=\frac{dU}{dy}=0.1S_L/{\delta }_L$, $k=1$, 2, 3, 4. (Bottom) Velocity magnitude for $k=1$. Upper threshold of $1S_L$ is applied to visualize the near-wall region. Stagnation zone thickness $H_{\mathrm{stag}}$ corresponds to $0.1S_L$.

Figure 6

Stationary flame tip velocity $U_{\mathrm{tip}}$ and stagnation zone thickness $H_{\mathrm{stag}}$ for different roughness slopes $k$. Isothermal walls, $\mathrm{\Theta }=8$, $g=\frac{dU}{dy}=0.1S_L/{\delta }_L$.

Figure 7

Critical inlet velocity gradient (for isothermal walls) versus thermal expansion Θ for different $k$.

Figure 8

(Left) Temperature field and streamlines, and (right) scaled reaction rate field, under critical condition. Isothermal wall, $\mathrm{\Theta }=8$, $g=\frac{dU}{dy}=0.175S_L/{\delta }_L$, $k=1$.

Figure 9

(Left) Temperature field and streamlines, and (right) scaled reaction rate field, under critical condition. Isothermal wall, $\mathrm{\Theta }=8$, $g=\frac{dU}{dy}=0.1564S_L/{\delta }_L$, $k=4$.

Figure 10

Radial profile of consumption speed at different roughness slopes $k$, for $\mathrm{\Theta }=8$, $g=\frac{dU}{dy}=0.1S_L/{\delta }_L$.

Figure 11

Comparison of critical gradient model [2] and simulation results for $\mathrm{\Theta }=8$, $g=\frac{dU}{dy}=0.1S_L/{\delta }_L$, $k=1$, and $k=4$. $S_c$ is consumption speed profile in the radial direction, $U$ is radial velocity profile ahead of the flame.

Figure 12

Radial velocity profile in front of the flame tip for critical conditions, $\mathrm{\Theta }=8$, $k=1$, and $k=4$. Arrows show the vertical position of flame tip.

Figure 13

Comparison of the consumption speed profile $S_c\left(y\right)$ and velocity along the curved flame front at critical condition for $\mathrm{\Theta }=8$, $k=1$, and $k=4$.