E.C. Fernandes

Swirl flow structure and flame characteristics in a model lean premixed combustor

The swirling flow in a lean premixed prevaporized (LPP) combustor model (LPP chamber) is studied, making use of high-speed photography, Laser Doppler Velocimetry (LDV), sound probes, fine-wire thermocouples, and suction probes for chemical gas analysis. The experimental study, which involved parametric changes of Re , S , and { , extends from the combustor primary zone to the premixing chamber and was conducted with and without reaction, burning either gaseous propane or liquid fuel assisted with preheated air at 300 °C. Measurements show that the swirling flow mixture that enters the primary zone of the combustor chamber shows evidence of a PVC structure, for S > 0.5, that embraces the central recirculation zone (CRZ). Furthermore, reaction test results show that increasing the swirl number decreases CO and NO x concentrations at the combustor exit and reduces flame stability limits to levels close to the lean limit because of flashback. In addition, a detailed study of the reacting condition ( S =1.05, { =0.5) was performed; the results indicate that the PVC is still present, embracing the base of the CRZ and staying in the inner shear layer, where the flame should also be stabilized, and contributing particularly to an abnormal radial distribution of W rms . Large gradients of mean temperature andUnburnedHydrocarbons(UHC) concentration were found to be limited to the base of the CRZ.

Coherent helical structures in swirl flows

A nonstationary flow with rotating helical filaments formed in the region of swirl flow breakdown has been studied using the method of phase averaging of the output signal of a laser Doppler velocimeter. Vortices in the measured flow field were separated using the λ2 technique based on an analysis of the velocity gradient tensor. It is shown for the first time that precession of the central monopolar vortex is accompanied by the formation of a pair of secondary vortices with the opposite directions of circulation, which form a helical dipole structure.

Velocity — temperature correlations in recirculating flames with and without swirl

Abstract Simultaneous measurements of temperature and velocity have been performed with digitally compensated thermocouples and a laser velocimeter in swirling and nonswirling recirculating flames stabilized downstream of a baffle. The velocity measurements are used to obtain magnitude estimates of the mean pressure gradients from the conservation equations of momentum in order to analyze the interaction between force fields induced by these gradients and the large density fluctuations typical of practical flames. The results are analysed in terms of the effects of swirl on the rate of turbulent heat transfer in the Favre-averaged form. It is shown that for the two types of flames analysed the radial heat flux follow conventional scalar velocity concepts, but the axial heat flux show large values in regions of negligible ∂ T /∂x . The evidence is that the interaction between gradients of mean pressure and density fluctuations are important in the conservation of the fluxes, with comparatively large contributions in the swirling flame. Swirl is also shown to induce large gradients of the radial mean velocity component, which may influence the conservation of the turbulent shear stress .

Rich–Lean Flame Interaction in a Lamella-Type Burner

ABSTRACT An expanded version of a practical rich–lean burner consisting of lamella geometry with lean flames in the central part and surrounded by two slits of rich flames (to prevent peripheral lean flames from lifting) is evaluated in the present study. The central part of this burner was studied in the search for an optimal geometry that maximized the stability of a methane-air lean flame, especially with regard to peripheral rich–lean flame interaction. For the burner central part, a slit thickness of 2 mm was found to be the optimum value for maximizing the overall burner stability of simple lean flames if the slit interspace was higher than 2 mm. This lean stability limit was then increased by about 9 ± 2% when the burner was operated with peripheral rich–lean flames. Detailed experimental characterization of the rich–lean flame interaction zone was performed for a lean flame of ϕL = 0.65, revealing the structure of a triple flame. In this rich–lean interaction, the lean flame was contaminated by diffusion of heat and radicals forcing the laminar flame speed to increase (improving overall burner stability) and, from chemiluminiscence signals, a change in the lean equivalence ratio could be quantified. An analytical expression was derived that showed how the apparent equivalence ratio of contaminated lean flames is linearly dependent on the equivalence ratio and velocity gradients as the driving forces of the mechanism of triple flame interaction. Experimental results validated the analytic expression, giving support to a holistic view of rich–lean flame interaction.

Velocity-temperature correlations in recirculating flames with and without swirl

Simultaneous measurements of temperature and velocity have been performed with digitally-compensated thermocouples and a laser velocimeter in swirling and non-swirling recirculating flames stabilized downstream of a baffle. The velocity measurements are used to obtain magnitude estimates of the mean pressure gradients from the conservation equations of momentum in order to analyse the interaction between force fields induced by these gradients and the large density fluctuations typical of practical flames. The results are analysed in terms of the effects of swirl on the rate of turbulent heat transfer in the Favre-averaged form. It is shown that for the two types of flames analysed the radial heat flux follow conventional scalar-velocity concepts, but the axial heat flux show large values in regions of negligible The evidence is that the interaction between gradients of mean pressure and density fluctuations are important in the conservation of the fluxes, with comparatively large contributions in the swirling flame. Swirl is also shown to induce large gradients of the radial mean velocity component, which may influence the conservation of the turbulent shear stress