M.J. Tummers

Confined annular swirling jet combustion

Five different flame states are identified in a compact combustion chamber that is fired by a 30-kW swirl-stabilized partially premixed natural gas burner working at atmospheric pressure. These flame states include a nozzle-attached tulip-shaped flame, an unattached lifted ring-shaped flame suitable for single-digit-ppm NO x emission, and a Coanda flame that clings to the bottom wall of the burner. Flame-state transition is generated by changing the swirl number and may be further modified by premixing the combustion air with 70% of the natural gas flow. Detailed experimental results of two ring-shaped flames are reported. The reactants flow out of the burner in a conical sheet and enter the spread-out flame. Evidence of precessing and oscillatory behavior of the internal recirculation zone (IRZ) as well as the flame envelope boundary is discussed and explained by the bimodal probability density functions of the velocity distributions. Low-pressure regions in the corner of the tight combustion chamber and altered turbulent flame speed due to a change in swirl account for the flame transition and stabilization process. Finally, the strength of the IRZ is compared with data of other researchers revealing that both ring-shaped flames feature a large and much stronger IRZ than reported in literature although limited swirl numbers apply.

Particle imaging velocimetry-based identification of coherent structures in normally impinging multiple jets

To gain a better insight into the effects of eddy structure and their thermal imprint on the impinging surface, we applied several methods to identify coherent structures in two arrangements of multiple jets. High-resolution particle imaging velocimetry measurements of the instantaneous fluid velocity have been performed to provide data for structure identification. Several methods based on critical point theory, i.e., vorticity magnitude, kinematic vorticity number, and second invariant of the velocity gradient tensor provided similar and generally useful information. However, in the flow considered they all appear inferior as compared with the proper orthogonal decomposition (POD). Despite lacking in conspicuous high energy modes, the flow showed to be well suited for POD, which provided clear identification of the near wall dominating vortical structure.

Wall imprint of turbulent structures and heat transfer in multiple impinging jet arrays

Thermal imprint and heat transfer on the target surface in multiple impinging jet arrays have been investigated in conjunction with flow pattern and large-scale eddy structure. Surface temperature was measured with liquid crystal thermography (LCT) in a range of jet configurations with hexagonal and in-line orifice arrangements for different combinations of distances between the orifices

Spectral analysis of biased LDA data

s/D_m\,{=}\,2\hbox{--}6

Impinging Jet Cooling of Wall Mounted Cubes

and between the orifice plate and the impingement surface

Multiflame Patterns in Swirl-Driven Partially Premixed Natural Gas Combustion

H/D_m\,{=}\,3\hbox{--}10

Experimental investigation of an adverse pressure gradient wake and comparison with calculations

(where

Dynamics of an oscillating turbulent jet in a confined cavity

D_m

Spectral analysis of boundary layers in Rayleigh-Bénard convection

is the orifice diameter). The hexagonal arrangement was considered with two different orifice shapes: sharp-edged and contoured. For selected configurations, the distribution of Nusselt number and its peculiarities were analysed in relation with the underlying eddy structure educed by proper orthogonal decomposition (POD) of the snapshots of fluid velocity measured with particle image velocimetry (PIV). Owing to the breakdown of the jets, heat transfer deteriorates with increasing orifice-to-plate distance. The jet interaction and breakdown become more severe as the jets are placed closer to each other. The large-scale eddies originating from the jet-edge shear layers grow as they are convected towards the impingement plate. Eddies of sizes between 0.2 and 0.3 orifice diameters are shown to break up the jets and cause mixing of fresh and spent fluid, lowering the beneficial temperature gradient. In some configurations, an asymmetric flow pattern is generated with embedded weak eddies on only one side of the diagonal symmetry line, which is reflected in an asymmetrical heat transfer distribution on the impingement plate. For

Effects of curvature and pressure gradient on a turbulent near wake

H/D_m\,{>}\,4