Velocity and size characteristics of liquid-fuelled flames stabilized by a swirl burner

Abstract

Velocity and droplet size characteristics of an unconfined quarl burner, of 16 mm quarl inlet diameter, have been measured with a phase-Doppler anemometer at a swirl number of about 0.29: the Reynolds number of the flow was 30000, based on the cold bulk velocity of 30.4 m s -1 and the hydraulic diameter. The atomization was achieved by shear between the swirling air and six radial kerosene jets and the resulting Sauter and arithmetic mean diameters were about 70 and 50 μm respectively after injection: velocity characteristics are presented for three 5 μm-wide size classes, 10, 30 and 60 μm. The flows correspond to no combustion and combustion of natural gas with a heat release of 8 kW supplemented by liquid kerosene flow rates sufficient to generate 21.6 and 37.2 kW : the gas equivalence ratio was 0.45 and atomized kerosene at two flow rates increased the overall ratios to 1.64 and 2.53. In non­-reacting flow, droplets 30 μm and smaller are sufficiently small to be entrained by the mean air velocity towards the central part of the flow and into the swirl-induced recirculating air bubble. The 60 μm droplets are able to travel through the bubble uninfluenced by turbulent fluctuations in the air and are ‘centrifuged’ away from the centreline, through acquisition of a mean swirl velocity component, so that a large proportion of the kerosene volume flow rate lies at the edge of the swirling jet. Because larger droplets are centrifuged to the outer part of the flow, whereas the smaller are entrained towards the centreline, the Sauter and arithmetic mean diameters are, by 1.22 quarl exit diameters downstream of the quarl, approximately 65 and 36 μm at the outer part of the flow and 35 and 12 μm near the centreline in the inert flow. In reacting flow, droplets evaporate rapidly in regions of elevated temperatures and hence no droplets are found within the flame brush and recirculation region. The aerodynamic response of each size class to the air velocity is similar to inert flow so that the majority of the kerosene flow is centrifuged away from the flame. On exit from the quarl, the evaporation and burning rates cause the Sauter and arithmetic mean diameters to be about 70 and 50 μm and 60 and 30 μm at the inner and outer edges of the spray respectively. By 1.22 quarl exit-diameters from the exit of the quarl, the air motion entrains droplets smaller than about 30 μm towards the flame, at the inner edge of the spray, so that the Sauter and arithmetic mean diameters are 60 and 40 μm at the outer edge of the jet. There is comparatively little effect of changing the flow rate of kerosene because the combustion is controlled by the low available number of smaller droplets, although the Group combustion number corresponds to ‘cloud’ burning. The relative response of droplets to the mean and turbulent components of air motion, including the ‘centrifuging’ effect, can be scaled to other flows through dimensionless numbers defined in the text.

Dynamics of Spray – Swirl – Acoustics Interactions

“…The phase averaged droplet diameter of the spray in swirl flow varies in an acoustic cycle. A variation of 50% in D 10 phase averaged diameter within an acoustic cycle over the mean 20 Dynamics of spray -swirl -acoustics interactions P h a se a n g le (d e g ) P h a se a n g le (d e g )…”

“…The data is phase locked with the acoustic signal generated by the electromechanical acoustic drivers. The PDPA system was set-up to optimal measurement conditions [19,20]. The transmitting optics focal length is 261 mm, resulting in probe volume of 61 µm.…”

Interaction of droplet dispersion and evaporation in a polydispersed spray

“…Since the air velocity around the droplets was not measured in our experiments, the gas velocity is assumed to be the same as the velocity of small droplets of size 15-30 µm. These droplets are expected to faithfully follow the gas flow since the measurement location is situated far downstream of the injector exit, and so the 'ballistic' motion of atomized droplets that strongly determines droplet motion near the atomizer (Hardalupas et al 1990) is attenuated. This is supported by the Stokes number analysis presented below.…”

“…Even this apparently simplified configuration poses tough challenges for simultaneous measurements of droplet and vapour phases. In the past, the phase Doppler anemometer (PDA) has been used extensively to measure the dispersed phase (for instance, Hardalupas, Taylor & Whitelaw 1990;Hardalupas, Taylor & Whitelaw 1994;Sornek, Dobashi & Hirano 2000;Nijdam et al 2004;Chen et al 2006), while planar laser induced fluorescence (PLIF) is usually used for droplet vapour concentration measurements (Bazile & Stepowski 1995;Ritchie & Seitzman 2001;Cochet et al 2009). However, for a single particle counter type instrument like the PDA, measurement of instantaneous inter-droplet distance and droplet number density is not straightforward, and also, correlating the single point information of droplet properties with planar measurement of vapour concentration by PLIF is complicated (e.g.…”

Evaluation of the topological characteristics of the turbulent flow in a ‘box of turbulence’ through 2D time-resolved particle image velocimetry

“…There are two proposed mechanisms to explain the formation of clusters of the dispersed phase. One is the centrifuging of particles by the turbulent eddies due to inertia (e.g., Maxey 1987;Wang and Maxey 1993;Eaton and Fessler 1994;Sundaram and Collins 1997;Hardalupas et al 1990Hardalupas et al , 1992) and the second is a sweepstick mechanism, which suggests that the fluid acceleration field is swept by the local fluid velocity and particles tend to stick to and move with the zero acceleration points (e.g., Chen et al 2006;Vassilicos 2006, 2008;Bragg et al 2015). To be able to assess experimentally these two mechanisms for dispersed phase clustering, the topology of the flow structure in a turbulent flow must be detected and quantified across the different scales.…”