Lean Premixed Hydrogen Flames: Turbulence, Chemistry, and Modelling

Citation

Yao, Matthew Xuhuai (2024) Lean Premixed Hydrogen Flames: Turbulence, Chemistry, and Modelling. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yjzw-vp60. https://resolver.caltech.edu/CaltechTHESIS:05292024-213954509

Abstract

Lean turbulent premixed hydrogen/air flames have substantially increased flame speeds, a behaviour which is attributed to differential diffusion effects. In this thesis, the relationships between turbulence, chemistry, and modelling are studied through direct numerical simulation (DNS) and large eddy simulation (LES).

The effect of turbulence on lean hydrogen combustion is studied through DNS using detailed chemistry and detailed transport. Simulations are conducted at six Karlovitz numbers and four integral length scales. A general expression for the burning efficiency is proposed which depends on the conditional mean chemical source term and gradient of a progress variable. At a fixed Karlovitz number, the normalized turbulent flame speed and area both increase almost linearly with the integral length scale ratio. The effect on the mean source term profile is minimal, indicating that the increase in flame speed can solely be attributed to the increase in flame area. At a fixed integral length scale, both the flame speed and area first increase with Karlovitz number before decreasing. Neglecting Soret diffusion is shown to reduce the flame speed, area, and burning efficiency. At higher Karlovitz numbers, the diffusivity is enhanced due to penetration of turbulence into the reaction zone, significantly dampening differential diffusion effects.

The structure of lean hydrogen flames, namely the species mass fraction dependence on the local temperature, differs significantly from that of unity Lewis number fuels due to thermodiffusive instabilities. When subjected to turbulence, the conditional mean species mass fraction profiles are observed to transition from the laminar mixture-averaged flamelet solution to the unity Lewis number flamelet solution. We assess the impact of Soret diffusion and integral length scales on an effective Lewis number model. The results show that the turbulent flame structure can be mapped onto laminar flamelets via the use of effective Lewis numbers, which are expressed by an a priori Karlovitz number model. Although the flame structure is altered by Soret diffusion, there is still strong agreement with previously derived Karlovitz number models for effective Lewis numbers. To map the turbulent flames onto laminar flames with effective Lewis numbers, the relative impact of Soret diffusion needs to be proportionally reduced.

To assess the LES modelling of lean hydrogen flames, we simulate a low-swirl burner, an alternative means of clean energy generation. The LES modelling of these flows remains challenging because the transition of small-scale instabilities into large-scale turbulent structures cannot be modelled by conventional strategies. Traditional one-equation tabulated chemistry formulations require only a progress variable, and cannot capture differential diffusion and curvature effects. In this work, we study the effects of tabulating different conditional mean source terms. It is shown that tabulating the appropriate conditional mean source term leads to improvements in the flow field prediction, however, key features such as the main recirculation region are not reproduced. Then, a two-equation tabulated chemistry model which accounts for differential diffusion and curvature effects is tested. This model provides the best agreement with experimental results. The work is a first effort in evaluating the performance of the two-equation model in the LES framework.

Thesis Files