Laminar burning velocity of hydrogen and flame structure of related fuels for detailed kinetic model validation

Popular Abstract in English
The word “combustion” describes a number of physical and chemical processes, whose common characteristic is an interaction between fuel and oxygen and their subsequent transformation into products, such as CO2 and water. Even though the process is often described as a single chemical reaction between fuel and oxygen, in reality, their chemical transformation requires many intermediate stages and involves many reactions. The simplest combustion system is hydrogen + oxygen (H2 + O2), which can be described with 8 species and about 20 elementary reactions. The smallest hydrocarbon fuel, methane, requires at least 35 species and 170 reactions. If all the species and reactions are defined, the combustion process can be formulated in a mathematical model. Such simulations have become widely used since they can provide a deeper understanding of the underlying processes, which might not be accessible in experiments. However, even the simplest system of hydrogen + oxygen is still not completely characterized under all conditions. Further development of our understanding becomes even more important since at the moment hydrogen combustion is receiving increased attention in industry due to reduced pollutant formation if hydrogen is used as a fuel.

One of the most important parameters of a combustible mixture is the laminar burning velocity, which describes how fast the flame can propagate in space. It is important from both practical and fundamental points of view. Knowledge of the laminar burning velocity is required in the design and development of combustion devices, such as internal combustion engines or gas turbines. In addition, the laminar burning velocity is a parameter that is used to develop combustion models and/or judge their performance. Flame structure, i.e. the distribution of species inside the flame, can also serve this objective.

Due to a constant improvement in combustion models, there is an increasing need to provide accurate experimental values of the laminar burning velocities. It is defined theoretically as the speed of an infinitely large freely propagating planar flame. Such conditions can not be reproduced in the laboratory, therefore, the accuracy of the measurements is determined not only by the quality of experimental equipment, it also depends on whether the laboratory system is close enough to these ideal theoretical conditions. A part of the work reported in this thesis concerns the accuracy of the heat flux method, which is one of the three widely used methods for burning velocity measurement. As a result of the present work, some of the practical issues that can lead to inaccurate values of the burning velocity were identified and recommendations were made with the aim of improving the accuracy of the method.

A major part of this thesis concerns the laminar burning velocity of hydrogen flames and how it changes with increasing temperature of the initial combustible mixture. This was analyzed both experimentally and using combustion models. In some cases, hydrogen flames can lose stability, i.e. they start to form irregular structures, or cells. When this occurs, the experimental procedure for determination of the burning velocity has to be modified. The approach applied in this thesis made it possible to perform measurements in such unstable flames without losing the accuracy. As for the temperature dependence of the burning velocity, it has a complex behavior, which is often disregarded in engineering applications. In the present work, this behavior was discussed and analyzed.

The last part of the thesis is related to the flame structure of fuels relevant to hydrogen energy, ammonia (NH3) and methane (CH4). Such fuels are often referred to as hydrogen carriers, i.e. they can be stored, transported and later converted to H2. This procedure can be advantageous due to the explosive nature of hydrogen. In this thesis, CH4 systems were studied under conditions relevant to hydrogen production, for which the combustion models are still underdeveloped. On the other hand, ammonia is a simple fuel which does not contain carbon, so the aim of the ammonia project was therefore to study fundamental nitrogen chemistry. Several existing combustion models were applied to simulate the structure of ammonia and methane flames, with the aim to find out how these models can be developed in the future.