We examined the effect of hydrogen (H2) enrichment on the primary fuel methane (CH4) in a canonical non-premixed bluff-body stabilized burner operating under typical central jet-dominated flame mode. In the chosen mode of operation, globally, the flow field and flame feature three important successive spatial zones: the recirculation zone, the neck zone, and the jet-like flame zone. The flame is exposed to a higher stretch rate in the neck zone in such a configuration and eventually undergoes local extinction. Such local extinction and subsequent re-ignition/reconnection of broken flame branches have substantial implications for the hydrodynamic instability of the coaxial annular air shear layer. It is well known that H2 addition increases the flame extinction strain rate ([Formula: see text] and thus alters the local extinction phenomenon. To understand this, we performed experiments at 0%, 10%, 20%, 30%, 50%, 80%, and 100% hydrogen proportion in the H2-CH4 blend. High repetition rate (5 kHz) Particle Image Velocimetry and OH Planar Laser Induced Fluorescence (PLIF) measurements are simultaneously implemented to gain quantitative insight into the flow field and flame structure. A detailed analysis performed over the instantaneous OH–PLIF datasets reveals the absence of local extinctions in flames with H2 enrichment >30% due to an increased extinction strain rate ([Formula: see text]. Furthermore, it is found that H2 enrichment plays a significant role in the reconnection/re-ignition of the broken flame branches formed during the local extinction. For instance, a high reconnection probability is observed in flames with an H2 addition of ≥20%. Consequently, variations in the mean reaction zone height are witnessed for different H2 enrichment levels. Further analysis of the influence of variation in reaction zone height on flow field hydrodynamics is explored using Proper Orthogonal Decomposition (POD) and Continuous Wavelet Transform (CWT). The results obtained from POD and CWT indicated the suppression of vortex shedding at the annular air shear layer for H2 addition greater than 20% and irregular wrinkling of flame fronts. Thus, they quantified the beneficial effect of H2 addition in turbulent flame stabilization.

Air pollution is a crucial issue since it is one of the major causes for worldwide deaths. Car drivers and passengers are daily exposed to high level of pollutant concentrations. The particles emitted from the tailpipe can disperse in the wake of the vehicle and then infiltrate the following car c abin. It has been demonstrated that the dynamics of such particles are highly affected by the properties of the flow downstream the emitting vehicle. In the present paper, the flow characterization in the wake of a simplified car model followed by a second vehicle is performed. Reduced-scaled Ahmed bodies are used. The upstream velocity is set to U∞ = 14.3 m/s, corresponding to a Reynolds number based on the simplified model’s height ( hc = 54 mm) of Re = 49500. Two leading vehicles with two different rear slant angles (φ = 0°, 25°) are considered. Two sets of experiments are carried out, one in the wind tunnel at ESTACA and the other in the wind tunnel at CORIA’lab. In the first measurement campaign, 2D LDV is used and six inter-vehicle distances ranging from d = 0.93 hc to 5.56 hc are considered and presented. The ITTT (Interarrival Time and Transit Time) method is applied to LDV data in order to ensure reliable results. Mean and turbulent properties of the wake between the two vehicles are described. The role of the rear slant angle is highlighted. A critical distance above which the followed vehicle has an insignificant influence on the flow topology downstream of the leading vehicle is identified. For further details, and taking into consideration the first results (from LDV measurements), Stereo-PIV measurements are used to get access to the three components of the velocity. Due to time constraint, only three inter-vehicle distances are discussed in this paper (0.93 hc, 1.85 hc and 3.70 hc). The SPIV results validate those obtained from LDV measurements and provide complementary information. The sensitivities of the bi-stability of the transverse velocity to the short inter-vehicle distance (0.93 hc) and to the slant rear angle are investigated. SPIV measurements are then correlated with LDA measurements taking into account this behavior. These measurements are completed by further measurements of concentrations of the exhausted particles from the tailpipe.

No abstract