Turbulent hydrogen premixed flames at high pressure and high temperature

TL;DR

The paper addresses turbulent lean premixed hydrogen flames under simultaneous high pressure and temperature to emulate gas-turbine compression, using DNS with three cases chosen to keep $Re_{ m jet}$ and nominal $Ka$ constant in the unburnt mixture. A 9-species chemistry model including the Soret thermodiffusion effect is employed on a fine Cartesian grid with Strang splitting and CVODE, ensuring $rac{ ext{Δ}}{ ext{η}}\,\le 2$ and $rac{ ext{δ}_F}{ ext{Δ}}\approx 10$. The main finding is that the coupled increase in $p$ and $T$ yields only moderate overall changes due to compensating effects, but reduces turbulence dissipation inside the flame, strengthens turbulence within the flame, and enhances thermodiffusive coupling, with the tangential strain rate normalized by the Kolmogorov time $ au_ ext{η}$ remaining near the universal value of ~0.23. This supports extrapolating ambient-condition results to gas-turbine conditions and provides a framework to study higher in-flame turbulence levels at no extra computational cost, aided by preserved laminar thermodiffusive behavior and a robust tangential-strain scaling.

Abstract

The combined influence of elevated pressure and temperature, representative of gas-turbine operating conditions, on lean premixed hydrogen flames is investigated using Direct Numerical Simulations (DNS) of a turbulent jet. Three cases are considered: 1 atm/298 K, 5 atm/472 K, and 20 atm/700 K, scaled to maintain the same jet Reynolds number and nominal Karlovitz number in the unburnt mixture, enabling a direct comparison of flame-turbulence interactions. Although the combined effects are moderate overall due to compensating influences, measurable differences arise in flame structure and turbulence-flame coupling. They are driven by reduced turbulence dissipation within the flame at high pressure and temperature, which enhances the interaction between turbulence and thermodiffusive effects. Finally, the tangential strain rate exhibits the same universal Kolmogorov scaling observed in homogeneous-isotropic turbulence and in methane flames, confirming its robustness for modelling turbulence

Figures (5)

• Figure 1: Laminar references: (a) Normalised one-dimensional profiles (hydrogen mass fraction $\rm{H_2}$, temperature $T$, hydrogen reaction rate $\dot{\omega}_{\rm H_2}$ and Bilger mixture fraction $Z_{\rm Bilger}$), (b) dispersion relations, (c) non-linear regime values, (d.1-3) snapshots of progress variable for the different cases. The lines with no symbols in (b) correspond the Darrieus-Landau instability.
• Figure 2: Two-dimensional slices of the turbulent flames for the three cases: Progress variable based on temperature $\theta$ (left), turbulent kinetic energy $\tilde{k}$ normalised with the bulk velocity $U_{\rm bulk}$ (right). Isocontours of the progress variable based on hydrogen $C_{\rm H_2}=0.93$, corresponding to the peak reaction rate, are represented on top of $\tilde{k}$ to help locate the flame.
• Figure 3: Evolution of the turbulent flame speed $s_{\rm T}$, flame surface area $A_{\rm T}$ and stretched factor $I_0$ along the flame. $s_{\rm T}$ and $A_{\rm T}$ are respectively normalised with the laminar flame speed $s_{\rm L}$, and the reference flame surface $A_{C}$ obtained from an isocontour of mean progress variable.
• Figure 4: Evolution of turbulent quantities with pressure and temperature at different streamwise locations in the unburnt jet and within the flame. From left to right: kinematic viscosity $\nu$, Kolmogorov scale $\eta$, Taylor microscale Reynolds number $Re_{\lambda}$ and Karlovitz number $Ka$. Blue circles and red squares represent values in the unburnt jet and within the flame, respectively. The colour shades become progressively darker downstream. Viscosity and Kolmogorov scale are normalised with the jet properties.
• Figure 5: Evolution of tangential strain rate $\left\langle K_{\rm S}\right\rangle _{\rm S}$ inside the flame at different streamwise locations. $\left\langle K_{\rm S}\right\rangle _{\rm S}$ is the surface-weighted average of tangential strain conditioned on $C_{\rm H_2}=0.8$ and is normalised with the Kolmogorov time $\tau_{\eta}$. Universal strains obtained for homogeneous isotropic turbulence ($\rm{HIT}$) Girimaji1990427 and methane (${\rm {CH_4}}$) LUCA20192451 are also represented.