Antonie Alex Verbeek

Flow Separation and Turbulence in Jet Pumps for Thermoacoustic Applications

The effect of flow separation and turbulence on the performance of a jet pump in oscillatory flows is investigated. A jet pump is a static device whose shape induces asymmetric hydrodynamic end effects when placed in an oscillatory flow. This will result in a time-averaged pressure drop which can be used to suppress acoustic streaming in closed-loop thermoacoustic devices. An experimental setup is used to measure the time-averaged pressure drop as well as the acoustic power dissipation across two different jet pump geometries in a pure oscillatory flow. The results are compared against published numerical results where flow separation was found to have a negative effect on the jet pump performance in a laminar flow. Using hot-wire anemometry the onset of flow separation is determined experimentally and the applicability of a critical Reynolds number for oscillatory pipe flows is confirmed for jet pump applications. It is found that turbulence can lead to a reduction of flow separation and hence, to an improvement in jet pump performance compared to laminar oscillatory flows.

Efficiently generated turbulence for an increased flame speed

In this study 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, which makes this low NOx burner more applicable for gas turbine application. The first approach adopts an active grid that forms a time-dependent arrangement of pulsating jets. By changing the disks and the frequency a wide variety of flow-forcing is possible. Hot-wire measurements performed downstream of the active grid show an energy spectrum with distinct peaks. However, there is no frequency identified where the turbulent kinetic energy or the dissipation rate is maximized. The variation in turbulent flame speed 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. 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. First, a rod-stabilized, V-shaped flame is used as such stabilization mechanism allows for considerable more variation in upstream fractal grid geometry. It is shown that fractal grids provide much more intense turbulence compared to classical grids. By increasing the range of embedded scales the turbulence is intensified. With respect to the reference case the turbulence intensity can be more than quadrupled while for the turbulent flame speed more than doubling is observed. When the standard grid in a low-swirl burner is replaced by fractal grids a similar increase in turbulence and combustion is observed as for a V-shaped flame. There is an increase in flame surface density and a widening of the flame brush as well as much finer flame wrinkling for the cases involving a multi-scale grid. Since 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. It has also been verified that the low NOx emissions, a key feature of low-swirl burners, are not affected when using fractal grids.