A wide variety of processes in energy and process engineering require mathematical investigations using simulations that fully resolve the physical domain in time and space. Often, these processes involve phase transitions from at least one phase to the other (i.e., solid to gas phase and vice versa). Our work presents advanced models and resulting massively parallel simulations developed and performed by the authors at the Chair of Fluid Dynamics at the University of Duisburg-Essen. The presentation focuses on novel approaches for modeling nanoparticle synthesis in a top-to-bottom process (gas to particle), as well as coal combustion for low-emission flames and the validation of the presented models.

The first case presents the modeling of heterogeneous nanoparticles (platinum on alumina support) in a turbulent spray flame reactor, which is done by introducing a multi-population balance model that handles the size spectra of dispersed (gas phase) and deposited (on alumina carriers) nanoparticle synthesis.

The second case presents FPV-LES of the co-firing of coal and ammonia. A four-mixture fraction FPV approach is developed, considering all necessary mixture fractions to accurately describe coal and ammonia combustion. The influence of char combustion is found to be negligible in the investigated domain.

The presented simulations, using the newly developed models, show the advantage of new model developments in this field, together with state-of-the-art high-performance computing, as the results shown provide new insights that contribute to the general understanding of these delicate processes.

Though decades of studies of coherent pulsar radiation, the physical mechanism of the emission at kinetic micro-scales is still under investigation. One of the proposed mechanisms is the linear acceleration emission that  is coherent antenna-type of radiation. The mechanism is based on plasma particles oscillating along magnetic field lines and producing linear acceleration emission.

We studied how plasma bunches/clouds of electron-positron pairs created during spark events in the polar gap region evolve as a function of the plasma temperature and drift velocity between electrons and positrons and how the bunches radiate by the linear acceleration emission. We utilized particle-in-cell simulations of relativistically hot bunches to investigate the non-linear evolution of the bunches. Also, we have implemented a novel treatment by utilizing the plasma current in the simulations to obtain properties of the coherent radio emission mechanism. Our treatment does not require tracking individual plasma macro-particles but can directly utilize aggregated information from the currents about the collective particle motion.

We found that the initial drift velocity between electrons and positrons is the main parameter influencing the bunch evolution. For zero drift, the bunches can expand and overlap in the phase space and form relativistic streaming instability. Otherwise, the bunches are constrained from expansion by ambipolar diffusion effects, oscillating electrostatic fields are formed, and the plasma is strongly heated. Furthermore, we calculated the radio emission properties of both types of the bunch evolutions. We found that bunches constrained from expansion have similar observational characteristics as observed for pulsars. The radiation power oscillates at micro-second time scales, and the spectrum contains a flat part for low frequencies and power-law profiles for higher frequencies. Also, the emitted radiation is relativistically beamed along the pulsar dipole axis.