Evaporation and clustering of ammonia droplets in a hot environment

Recent developments in the transition to zero-carbon fuels show that ammonia is a valid candidate for combustion. However, liquid ammonia combustion is difficult to stabilize due to a large latent heat of evaporation, which generates a strong cooling effect that adversely affects the flame stabilization and combustion efficiency. In addition, the slow burning rate of ammonia enhances the undesired production of NOx and N2O. To increase the flame speed, ammonia must be blended with a gaseous fuel having a high burning rate. In this context, a deeper understanding of the droplet dynamics is required to optimize the combustor design. To provide reliable physical insights into diluted ammonia sprays blended with gaseous methane, direct numerical simulations are employed. Three numerical experiments were performed with cold, standard, and hot ambient in nonreactive conditions. The droplet radius and velocity distribution, as well as the mass and heat coupling source terms are compared to study the effects on the evaporation. Since the cooling effect is stronger than the heat convection between the droplet and the environment in each case, ammonia droplets do not experience boiling. On the other hand, the entrainment of dry air into the ammonia-methane mixture moves the saturation level beyond 100% and droplets condense. The aforementioned phenomena are found to strongly affect the droplet evolution. Finally, a three-dimensional Voronoi analysis is performed to characterize the dispersive or clustering behavior of droplets by means of the definition of a clustering index

Space-time adaptive reduction of unsteady flamalets

The Wavelet Adaptive Multiresolution Representation (WAMR) code and the G-Scheme framework are used for the numerical time integration of the flamelet model. The steep gradients are efficiently captured by the WAMR algorithm with an a-priori defined accuracy and an associated large reduction of the number of degrees of freedom (DOFs). A further opportunity to reduce the complexity of the problem is represented by the G-Scheme, to achieve multi-scale adaptive model reduction along-with the time integration of the differential equations

Numerical generation of multidimensional flamelet databases using an adaptive wavelet method

The Wavelet Adaptive Multiresolution Representation (WAMR) code is used for the numerical time integration of the one-dimensional laminar diffusion flames equations in trans-critical and supercritical conditions, where the thermodynamic and transport properties exhibit large changes. These steep gradients are efficiently captured by the WAMR algorithm with an a-priori defined accuracy and an associated large reduction of the number of degrees of freedom, allowing a highly efficient flamelet database generation critical conditions

Phase II study of lonidamine in metastatic breast cancer.

Thirty patients with previously treated metastatic breast cancer were entered in a phase II study with oral lonidamine. Twenty-eight patients are evaluable for toxicity and 25 for response. A partial remission was obtained in four patients (16%) and disease stability in 11 (44%): 10 patients progressed (40%). Toxicity was acceptable, consisting mainly of myalgias (39% of patients) and asthenia (21.4%). No myelotoxicity was observed. The drug is active in previously treated metastatic breast cancer and, because of its peculiar pattern of action and toxicity, deserves to be evaluated in combination with cytotoxic chemotherapy

Low-mach number simulations of transcritical flows

A numerical framework for the direct simulation, in the low-Mach number limit, of reacting and non-reacting transcritical flows is presented. The key feature are an efficient and detailed representation of the real fluid properties and an high-order spatial discretization. The latter is of fundamental importance to correctly resolve the largely non-linear behavior of the fluid in the proximity of the pseudo-boiling. The validity of the low-Mach number assumptions is assessed for a previously developed non-reacting DNS database of transcritical and supercritical mixing. Fully resolved DNS data employing high-fidelity thermodynamical models are also used to investigate the spectral characteristic as well as the differences between transcritical and supercritical jets

Local combustion regime identification using machine learning

A new combustion regime identification methodology using the neural networks as supervised classifiers is proposed and validated. As a first proof of concept, a binary classifier is trained with labelled thermochemical states obtained as solutions of prototypical one-dimensional models representing premixed and nonpremixed regimes. The trained classifier is then used to associate the regime to any given thermochemical state originating from a multi-dimensional reacting flow simulation that shares similar operating conditions with the training problems. The classification requires local information only, i.e. no gradients are required, and operates on reduced-dimension thermochemical states, in order to cope with experimental data as well. The validity of the approach is assessed by employing a two-dimensional laminar edge flame data as a canonical configuration exhibiting multi-regime combustion behaviour. The method is readily extendable to additional classes to identify criticality phenomena, such as local extinction and re-ignition. It is anticipated that the proposed classifier tool will be useful in the development of turbulent multi-regime combustion closure models in large scale simulations

Detached-eddy simulation of shock unsteadiness in an overexpanded planar nozzle

This work investigates the self-excited shock-wave oscillations in a three-dimensional planar overexpanded nozzle turbulent flow by means of detached-eddy simulations. Time-resolved wall pressure measurements are used as primary diagnostics. The statistical analysis reveals that the shock unsteadiness has common features in terms of the standard deviation of the pressure fluctuations with other classical shock-wave/boundary-layer interactions, like compression ramps and incident shocks on a flat plate. The Fourier transform and the continuous wavelet transform are used to conduct the spectral analysis. The results of the former indicate that the pressure in the shock region is characterized by a broad low-frequency content, without any resonant tone. The wavelet analysis, which is well suited to study non stationary process, reveals that the pressure signal is characterized by an amplitude and a frequency modulation in time

Assessment of detached eddy simulation of a separated flow in a planar nozzle

The sea-level start-up of rocket engines is characterized by the nozzle experiencing a high degree of overexpansion and a consequent internal flow separation with a strong unsteady shock wave boundary layer interaction (SWBLI). This may produce side-loads dangerous for the engine structure. Despite the important amount of experimental, theoretical and numerical work that can be found in literature, the physical mechanism behind this unsteadiness is still not clear and research is going on. In this work, a 3D planar overexpanded nozzle has been simulated by means of the detached eddy simulation (DES) method. In high Reynolds number flows, simulating only the separated flow with the LES method and the attached boundary layer with the RANS method allows the computational cost to be reduced. The unsteady pressure signals have been analyzed by the wavelet decomposition in order to characterize their spectral content. The results indicate that the DES is able to capture the shock oscillations and that the computed characteristic frequency is close to the ones available from literature for the same test case. The value of the fundamental frequency computed in this work is lower than the one predicted by the model of the longitudinal acoustic frequency. The self sustained oscillation is driven by a pressure imbalance between the pressure level downstream the recompression shock and the ambient. The turbulent separated zone seems to have an important part, even if the mechanism is still not clear

Direct Numerical Simulations of the Evaporation of Dilute Sprays in Turbulent Swirling Jets

The effects of swirled inflows on the evaporation of dilute acetone droplets dispersed in turbulent jets are investigated by means of direct numerical simulation. The numerical framework is based on a hybrid Eulerian\u2013Lagrangian approach and the point-droplet approximation. Phenomenological and statistical analyses of both phases are presented. An enhancement of the droplet vaporization rate with increasing swirl velocities is observed and discussed. The key physical drivers of this augmented evaporation, namely dry air entrainment and swirl-induced centrifugal forces acting on the droplets, are isolated with the aid of additional simulations in which the inertial properties of the droplets are neglected. The correlation between swirl and dry air entrainment rate is found to be responsible for the increase of the global evaporation rate and the spray penetration length reduction, while swirl-induced centrifugal forces are found to be effective only in the jet shear layer, close to the injection orifice, for the analyzed cases