Numerical simulations of highly perturbed lean hydrogen-air flames. A detailed investigation on strained lean hydrogen combustion characteristics

Abstract

Premixed or partially premixed fuel configuration with turbulent combustion is widely used in all combustion technologies and applications available in the industry. Though there are many interesting alternatives exist to combustion technologies (e.g., battery technologies), combustion technologies still become relevant as of today because of the existing infrastructure built around the combustion applications and the ease of application. Many popular companies (e.g., Volvo Car, Siemens, etc) and research enthusiasts in Sweden and worldwide are developing interesting strategies to mitigate the global warming, which is one of the downsides of combustion technologies. Among various renewable fuels, hydrogen becomes one of the important carbon free fuels of interest. Also compared to the other fuels, hydrogen fuel has very good combustion characteristics (high laminar flame speed, wide flammability limits, low ignition energy, etc.) and can be produced using various technologies and renewable methods. It is important for researchers to develop efficient combustion models to make the application of hydrogen fuel more practical and adaptable in various applications. This can be done only by using the data obtained by experimental results and Computational Fluid Dynamics (CFD) tools effectively. However, there is not enough research available which can accurately predict the burning rate and combustion characteristics of turbulent hydrogen – air mixture. Hydrogen has a significant high burning rate which is usually controlled by local mixture composition changes due to higher molecular diffusivities of hydrogen air mixture. The most popular approach is based on the hypothesis that the entire turbulent flame regime is controlled by local flamelets which are highly perturbed. This thesis work mainly covers the study of such highly perturbed local flamelets. Eventually, the data can be further used to design or develop hydrogen-air turbulent combustion applications.

CHEMKIN-PRO is the main tool which is used in this project to simulate combustion characteristics of hydrogen – air mixture. CHEMKIN PRO gives flexibility to the user to setup specific boundary conditions based on user needs to obtain a desired output. This project can be divided into two parts, the first part – simulation of laminar hydrogen air flames and the second part – simulation of strained laminar flames. In laminar flame simulations, the main goal is to study the effect of different input configurations like inlet temperature, pressure, diffusion and transport model effects, gird resolution and curvature and different combustion mechanisms on laminar flame speed 𝑆𝑙 for different equivalence ratios (𝜑) ranging from 0.5 to 2.9. It is observed that the flame speed increases rapidly up to 𝜑=1.5 and then reduces sharply. In the second part, strained laminar flame simulations, the goal is to study the strain rates at which the maximum consumption velocity 𝑆𝐶𝑚𝑎𝑥 and the extinction has attained for lean hydrogen air flames ranging from 𝜑=0.36 𝑡𝑜 0.80, using Oppdif module in CHEMKIN PRO. For strained flames, consumption velocity 𝑆𝑐 becomes more relevant as it represents the global burning rate of the mixture. Later, the effects of different mechanisms, inlet temperature, different equivalence ratios, pressure and flame thickness are studied and presented at the end of this thesis. A significant amount of time is also spent on extracting and visualizing the relevant data in MATLAB from the output obtained by CHEMKIN PRO.