Flow Investigation and Acoustic Measurements of an Unconfined Turbulent Premixed Jet Flame

Abstract

A turbulent jet emanating from an unconfined, premixed burner is investigated by using large eddy simulation (LES) and direct numerical simulation (DNS) as well as experimentally by means of optical (OH* chemiluminescence), acoustic (microphone), and laser-optical measurement techniques (Particle Image Velocimetry). Comparison of the results obtained through experiments, LES, and DNS indicate a reasonable agreement. In order to analyze the impact of mesh refinement on the resolved flame properties and acoustic radiations, computational grids with varying resolutions are used for the LES. As large coherent flow motion exists in the considered flow case, due to an over-predicted diffusion the flame length calculated with LES is underestimated. On the other hand, DNS exhibits a similar intensity distribution for OH* as the experiment and, hence, the flame length is predicted accurately by DNS. The emitted noise spectrum has a tonal shape with peaks at the burner's resonance frequency for the non-reacting flow which changes to broadband noise and, in general, is raised in amplitude for reacting flows. In addition, it is shown that an increase in Reynolds number, preheat temperature, or a decrease in equivalence ratio close to stoichiometric ratios yields more noise being emanated from the burner. The latter indicates the fact that direct combustion noise is linked to interactions of turbulent fluctuations with the flame front. When using an equivalence ratio closer to stoichiometric ratio, a thinner reaction zone is expected and the intrinsic interaction between the flame and turbulent flow is more pronounced. Nomenclaturea 1 = thermal diffusivity d = nozzle diameter D 1 = diffusion coefficient f = frequency f res = resonance frequency of the burner H = shape factor r/d = relative radial location with respect to nozzle diameter Re = Reynolds number t DNS = duration of Direct Numerical Simulation T = temperature 1 Ph.D. student, Chair of Fluid Dynamics, Hermann-Föttinger-Institut, 2 Tu = degree of turbulence/ turbulence level u = axial velocity u 0 = bulk velocity of the jet u/u 0 = normalized axial velocity u RMS = fluctuation of the axial velocity component v RMS = fluctuation of radial velocity component w = radial velocity w RMS = fluctuation of radial velocity component w/u 0 = normalized radial velocity x/d = relative axial location with respect to nozzle diameter y 10/50/95 = location of 10%/ 50%/ 95% of bulk velocity in radial direction δ 1 = displacement thickness Δ = cut-off scale ε = error due to numerical diffusion  = momentum thickness  DNS = progress variable ν 1 = laminar viscosity ν eff = effective viscosity ν sgs = sub grid scale (turbulent) viscosity  = equivalence ratio of methane-air mixture

Experimental Investigation of an Unconfined Swirl-Stabilized Turbulent Premixed Flame

“…In addition and especially towards the end of the recirculation zone, both radial and tangential velocity fluctuations increase for the lean flame compared to the rich flame which is probably triggered by the shorter flame at such lean conditions yielding higher fluctuations. Similar effects are also observed for an unconfined jet flame emanating from the same burner without swirl-stabilization 9 . Another effect associated with the increase of fuel content in the gas-air mixture is the change in flame shape.…”

Combustion‐Generated Noise: An Environment‐Related Issue for Future Combustion Systems

“…[43] Thet est rigs main characteristics are the emanated premixed, turbulent jet flame and its simple design. As shown in Figure 8, the burner is deliberately kept simple by consistingo fo nly an annulari njector far upstream for injectiono fp remixed fuel/airm ixture (A), ap lenum of constantd iameter (B), and ac onverging nozzle (C);i ti sc oncluded with ap ilot burner extension (D), which stabilizes the turbulent jet flame at high Reynolds numbers, lean equivalencer atios,o rb oth.…”