Efficiently generated turbulence for an increased flame speed

The rate of combustion in premixed flames is to a large extent controlled by the level of turbulence. Fluctuations in the flow field deform the flame front that exists between the premixed reactants and the fully combusted products. The surface area of this flame front is increased as it becomes more wrinkled by the turbulent flow, which increases the turbulent flame speed. In this thesis two different methods to generate turbulence in an efficient way are studied. This turbulence is used to increase the flame speed of a low-swirl burner. In turn, this increases its power density and makes it more suitable for gas turbine application. The term efficient can be interpreted two-fold. The turbulence can either be increased at specific scales beneficial for the generation of flame surface. Or, alternatively, the turbulence can be intensified over the whole range of turbulence scales, but at a less than proportional increase of the energy input. The first approach adopts an active grid that is composed of a stationary and a rotating disk with characteristic hole patterns. Upon rotation it forms a time-dependent arrangement of pulsating jets. By changing the set of disks and the rotational frequency a wide variety of flow-forcing is possible. Hot-wire measurements performed in the flow downstream of the active grid show an energy spectrum with distinct and controllable peaks. The response, defined as the amount of energy contained in these peaks, is high (up to 25%) when the introduced perturbations have a timescale in the energy-containing range and decreases when these timescales are shorter and lie in the inertial range. However, there is no frequency identified for the current design and parameter range where the turbulent kinetic energy or the dissipation rate is maximized. The effect on the low-swirl flame is characterized by means of OH-LIF. The variation in turbulent flame speed, measured by the amount of flame surface, is of the same order as the measurement uncertainty. Therefore, it cannot be concluded that the specific fluctuations introduced by the active grid are directly causing additional wrinkling of the flame front. The amount of energy in these specific scales is too low to induce a significant change in the combustion rate. In the second approach so-called fractal grids are used to generate turbulence. These grids are obtained by truncating a self-similar fractal pattern at some level of refinement. A parametric study of fractal-grid-generated turbulence containing 24 different grids with variation in grid patterns, solidity and range of embedded scales was conducted. This identifies the parameters of the fractal grid that affect the level of turbulence and the turbulent flame speed. Here, a rod-stabilized, V-shaped flame is used as such stabilization mechanism allows for considerable more variation in upstream fractal grid geometry compared to low-swirl stabilization. The fractal grids provide much more intense turbulence compared to classical grids and therefore an increased turbulent flame speed. By increasing the range of embedded scales the turbulence is intensified. With respect to the reference case of a classical grid the turbulence intensity can be more than quadrupled while for the turbulent flame speed more than a doubling is observed. From the energy spectrum of the velocity it becomes clear that not only the largest scales are more energetic; also smaller scales are introduced as the spectrum is further extended into the high-frequency range. When the standard blockage grid in a low-swirl burner is replaced by fractal grids a similar increase in turbulence and combustion rate is observed as for a V-shaped flame. The turbulence is intensified when comparing the flow behind the multi-scale grid to the reference situation. This increase is expressed by more than doubling of the r.m.s. of the velocity fluctuations, while only marginal changes in pressure drop are observed. The OH-LIF experiments show an increase in flame surface density and widening of the flame brush as well as much finer wrinkling of the flame front for the cases involving a multi-scale blocking grid. The grid parameters that were varied are the range of embedded scales and the blockage ratio. The fact that the range of embedded scales mainly controls the turbulence intensity and the blockage ratio the low-swirl stabilization, engineering fractal grids for low-swirl combustion can be done with relative ease. In addition to the effect on the turbulent flame speed, it has also been verified that the low NOx emission levels, a key feature of low-swirl burners, are not affected when using fractal grids. This thesis presents a clear affirmative answer to the question whether it is possible to increase the turbulent flame speed of a low-swirl flame by efficiently generated turbulence. It is shown that with fractal grids it is possible to elevate the turbulence intensity significantly and, moreover, that these grids are a feasible option for implementation in real low-swirl burners. The active grid approach is considered to be of limited value for combustion applications as it does not introduce sufficient additional perturbations at the right scales.