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We investigate the effect of increasing levels of hydrogen enrichment on the nonlinear response and saturation of premixed bluff-body stabilized methane/hydrogen flames submitted to acoustic forcing. The thermal power is kept approximately constant to preserve the nozzle velocity while increasing the flame speed through hydrogen enrichment. The flame describing function (FDF) is measured for a fixed frequency and three hydrogen-methane blends ranging from 10% to 50% by power, corresponding to 25% to 75% by volume. We show that when the flame is forced at the same frequency at similar power and bulk velocities, increasing levels of hydrogen enrichment increase the saturation amplitude of the flame. To provide insight into the flame dynamics responsible for the change in the global nonlinear response and saturation amplitude, the flames were investigated using high-speed imaging in combination with OH planar laser-induced fluorescence (OH-PLIF) at a range of forcing amplitudes. At lower hydrogen concentrations, the flame is stabilized along the inner shear layer and saturation in the heat release rate (HRR) occurs at lower forcing amplitudes due to large-scale flame-vortex interactions causing flame annihilation as observed in several previous studies. At increased levels of hydrogen enrichment, distinctly different flame dynamics are observed. In these cases, the flame accelerates and propagates across to the outer shear layer, which acts to suppress large-scale flame annihilation during roll-up of both the inner and outer shear layers. This results in a coherent increase in flame surface area with forcing amplitudes significantly increasing the saturation amplitude of the flame. These results show that high levels of hydrogen increase the amplitude response to acoustic forcing leading to higher saturation amplitudes. This suggests that substituting natural gas with hydrogen in gas turbines increases the risk of much higher limit-cycle amplitudes if self-excited instabilities occur. [ABSTRACT FROM AUTHOR]

The hydrodynamic response of an axisymmetric jet placed at various positions in a standing wave oriented normally to the jet is investigated. At the velocity and pressure nodes the axisymmetric (m=0) and first azimuthal (m=±1) modes are excited, respectively, through manipulation of the jet exit boundary conditions. At positions between the nodes, both the m=0 and m=±1 modes are simultaneously excited resulting in asymmetric forcing due to the phase difference between the transverse and longitudinal acoustic fluctuations. This leads to the asymmetric formation of vortices in the near field and bifurcation into two or more momentum streams further downstream. The dominant momentum stream is deflected in the direction of the velocity node. It is shown that the asymmetric response can be well approximated by a superposition of the boundary conditions at the pressure and velocity nodes where the contributions from each mode are proportional to the acoustic pressure and velocity. A method is proposed to characterize the bifurcation behaviour statistically via moments of the probability density functions constructed from profiles of streamwise momentum. The jet symmetry and momentum spreading are shown to be proportional to the magnitude of the transverse acoustic velocity. Finally, the streamwise velocity is reconstructed as a superposition of Gaussian profiles providing a robust method to characterize the number of individual momentum streams which also shows that each of the streams behave self-similarly. [ABSTRACT FROM AUTHOR]

We present a newly developed, improved method for deducing the depth profile of a sub-surface current from measurement of the current-modified dispersion relation of surface waves, and test and validate the method with laboratory experiments.Understanding and characterizing near-surface currents is important for a wide variety of applications including climate modeling, coastal engineering, and operational safety near coastlines, among others. Many conventional in situ methods struggle to measure the currents in a top few meters of the ocean, and measure the depth profile at only one point in the horizontal plane. An attractive alternative is to determine near-surface currents from measurements of the wave spectra, enabling remote sensing methods such as X-band radar. The new method is iterative in nature, starting with the existing state-of-the art method as an initial guess, and estimating a correction factor that improves the accuracy of the reconstructed shear profile. The method is tested using experimental data of waves traveling atop a controlled shear flow. Various shear depth-profiles are produced in the laboratory and measured using particle image velocimetry which serve as "truth" measurements. Waves are produced with a piston-type wavemaker and measured using a synthetic schlieren method. Using the new inversion method, we demonstrate a factor two improvement in accuracy in the reconstructed current profile relative to state-of-the-art methods. [ABSTRACT FROM AUTHOR]

This paper presents a Reynolds averaged Navier Stokes turbulent combustion model for CH4/H2/air mixtures which includes the effect of heat losses and flame stretch. This approach extends a previous model concept designed for methane/air mixtures and improves the prediction of flame stabilization when hydrogen is added to the fuel. Heat loss and stretch effects are modeled by tabulating the consumption speed of laminar counterflow flames in a fresh-to-burnt configuration with detailed chemistry at various heat loss and flame stretch values. These computed values are then introduced in the turbulent combustion model by means of a turbulent flame speed expression which is derived as a function of flame stretch, heat loss, and H2 addition. The model proposed in this paper is compared to existing models on experimental data of spherical expanding turbulent flame speeds. The performance of the model is further validated by comparing computational fluid dynamics predictions to experimental data of an atmospheric turbulent premixed bluff-body-stabilized flame fed with CH4/H2/air mixtures ranging from pure methane to pure hydrogen. [ABSTRACT FROM AUTHOR]