The reactivity of hydrogen enriched turbulent flames
PROCESS SAFETY AND ENVIRONMENTAL PROTECTION
Authors: Hampp, F.; Goh, K. H. H.; Lindstedt, R. P.
The use of hydrogen enriched fuel blends, e.g. syngas, offers great potential in the decarbonisation of gas turbine technologies by substitution and expansion of the lean operating limit. Studies assessing explosion risks or laminar flame properties of such fuels are common. However, there is a lack of exper-imental data that quantifies the impact of hydrogen addition on turbulent flame parameters including burning velocities and scalar fluxes. Such properties are here determined for aerodynamically stabilised flames in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re-t = 314 +/- 19) using binary H-2/CH4 and H-2/CO fuel blends. The binary H-2/CH4 fuel blend is varied from alpha = X-H2/(X-H2 + X-F) = 0.0, 0.2 and 0.4-1.0, in steps on 0.1, and the binary H-2/CO fuel blend from alpha = 0.3 - 1.0 also in steps of 0.1. The equivalence ratio is adjusted between the mixture specific lower limit of local flame extinction and the upper limit of flashback. The flames are characterised using PIV measurements combined with a flame front detection algorithm. The study quantifies the impact of hydrogen enrichment on (i) turbulent burning velocity (S-T), (ii) turbulent transport and (iii) the rate of strain acting on flame fronts. Scaling relations (iv) that correlate S-T with laminar flame properties are evaluated and (v) flow field data that permits validation of computational models is provided. It is shown that CH4 results in a stronger inhibiting effect on the reaction chemistry of H-2 compared to CO, that turbulent transport and burning velocities are strongly correlated with the rate of compressive strain and that scaling relationships can provide reasonable agreement with experiments. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Stereoscopic PIV measurements of flow over a riblet surface at high Reynolds number
EXPERIMENTAL THERMAL AND FLUID SCIENCE
Authors: Ozkan, G. M.; Elsinga, G. E.; Breugem, W-P; Stuebing, D.; Reynolds, K. J.; Westerweel, J.
The effect of drag reducing riblets on the flow structure was examined experimentally for a turbulent boundary layer at Re-theta = 9890 and riblet spacing s (+) = 13.4. Trapezoidal riblets were used, which were attached to the water tunnel wall as a coating. Force measurements were performed to quantify the amount of drag reduction. Then, the mechanism underlying this reduction was investigated by stereo-PIV measurements in the cross-stream plane. To determine the effect of the drag reducing riblets, the results were compared with the smooth flat plate. Time-averaged turbulent statistics such as turbulent kinetic energy and Reynolds shear stress were found to be lower over the riblets compared to the flat surface. Two-point correlations of the fluctuating velocity components were calculated to elucidate the average flow structure size and strength, where riblets significantly suppressed the turbulent structures. Quadrant analysis of the Reynolds shear stress was performed to assess the change in ejection and sweep events and the results were found to be in correspondence with previous works.