Session G12: Acoustics: Thermo I

Global Response of Spray Flames due to Oscillatory Spray Dynamics

A significant literature exists for the modeling of the response of premixed and non-premixed gaseous diffusion flames to various source fluctuations. These source fluctuations can stem from: (1) velocity fluctuations, (2) equivalence ratio/mixture fraction fluctuations, or (3) pressure fluctuations. Recently, the author has presented a reduced order modeling framework for the response of spray flames to velocity disturbances. In that study, the effect of acoustic and vortical component velocity fluctuations on the spray flame response for various spray parameters was explored, showing the sensitivity of the global response to droplet loading and vaporization. However, that model did not consider the effect of the oscillatory combustor environment on the sprays themselves. In the fluctuating environment where the fuel is injected, there exists fluctuations in the spray atomization, droplet loading and oscillatory evaporation. These spray fluctuations, collectively, can cause significant influences on the global flame response which has been explored in this study. The governing equations are reformulated to consider fluctuations in the spray properties and new parameters associated with these fluctuations are used to understand their effect on the global flame response.   

Mass and Heat Fluxes in a Phase Change Thermoacoustic Heat Pump

Thermoacoustic cooling is a promising alternative to vapor-compression based systems, possessing the potential for high energy density and efficiency, as well as increased reliability due to the lack of moving parts. Phase change Thermoacoustics is a relatively new addition to the field, based on introduction of periodic evaporation and condensation into the acoustic cycle, which results in a time-averaged mass flux accompanied by a latent heat flux.

Here, we present experimental characteristics of a phase change thermoacoustic heat pump, accompanied by calculations based on a theoretical a low-amplitude linear model. We show that for low temperature differences, such a device can outperform its “dry” equivalent by up to a factor of 3, in terms of cooling power and coefficient of performance (COP). At higher temperature differences, the trend is reversed, with the dry system outperforming the wet one. We show that this is due to a reversal in the direction of the acoustic-driven mass flux, and that this trend can be mitigated by increasing the local acoustic impedance. Finally, we outline the remaining challenges on the way to making this technology a reality, most of which are strongly related to fluid dynamics.

    

Linear flow perturbations in nozzle flows with heat transfer

Solutions for the unsteady flow perturbations in duct flows with area variations are relevant to combustors, automotive exhausts, after-burners and air-intake diffusers. Analytical models for these perturbations were initially restricted to very low frequency and isentropic flows. More recent models extend to any frequency by using a Magnus expansion solution. The current work presents an analytical method which extends to any frequency and to non-isentropic nozzle flows – in this case undergoing steady heat transfer. The quasi-one-dimensional linearised Euler equations are cast in terms of the dimensionless mass, stagnation temperature and entropy fluctuations, which are invariants of the system at zero frequency and with no heat transfer. The resulting first-order system of differential equations is solved using the Magnus expansion, where the perturbation parameters are the normalised frequency and the volumetric heat transfer. This represents the first time that a measure of the flow non-isentropicity (in this case the steady heat transfer) is used as a Magnus expansion parameter. The solution method was applied to converging–diverging nozzle flows with constant heat transfer, showing good agreement with numerical predictions for both subcritical and supercritical flows.

    

Real-time data assimilation for model-error inference

Nonlinear thermoacoustic oscillations can be estimated with reduced-order models, which provide qualitatively accurate estimates at low computational cost. Data assimilation algorithms are employed to improve the accuracy of numerical models. However, quantitative accurate predictions might not be attainable with low-order models as their governing equations do not capture all the physical mechanisms, i.e., low-order models may be statistically biased. We propose a bias-aware sequential ensemble data-assimilation method, which accounts for the model error in the Kalman update; with an echo-state network, which adaptively estimates the model error. In this framework, we perform real-time inference of the physical state, model parameters and model error from reference data. The method is tested on a nonlinear time-delayed low-order model by assimilating data from a higher-order model of the system. Current efforts focus on the application of this framework for on-the-fly state, parameter and model error estimation with experimental data.   

Author not Attending

Predicting the flame response of a premixed swirling flame poses a major challenge as it is highly dependent on the level of swirl and the geometry of the configuration. In swirling flames, characteristic troughs on the gain of the flame transfer function (FTF) can be observed at certain frequencies. While past experiments have focused on the FTF of swirling flames, a very limited number involve Computational Fluid Dynamics (CFD) and there is not sufficient understanding of the flame dynamics during the forcing cycles. In this work, we perform experiments and Large Eddy Simulations (LES) of lean premixed ethylene flames on a bluff body burner with a blockage ratio of 50%, using two swirlers with 45° and 60° vane angles. The agreement between experiments and LES is observed to be good. The level of swirl can significantly affect the flame behaviour and when the swirl number is increased the FTF trough is shifted to a higher frequency. The flame dynamics are further analysed by considering the effects of wrinkling.