Ronan Vicquelin

Assessment of external heat transfer modeling of a laboratory-scale combustor inside a pressure-housing environment

Many laboratory-scale combustors are equipped with viewing windows to allow for characterization of the reactive flow. Additionally, pressure housing is used in this configuration to study confined pressurized flames. Since the flame characteristics are influenced by heat losses, the prediction of wall temperature fields becomes increasingly necessary to account for conjugate heat transfer in simulations of reactive flows. For configurations similar to this one, the pressure housing makes the use of such computations difficult in the whole system. It is therefore more appropriate to model the external heat transfer beyond the first set of quartz windows. The present study deals with the derivation of such a model which accounts for convective heat transfer from quartz windows external face cooling system, free convection on the quartz windows 2, quartz windows radiative properties, radiative transfer inside the pressure housing and heat conduction through the quartz window. The presence of semi-transparent viewing windows demands additional care in describing its effects in combustor heat transfers. Because this presence is not an issue in industrial-scale combustors with opaque enclosures, it remains hitherto unaddressed in laboratory-scale combustors. After validating the model for the selected setup, the sensitivity of several modeling choices is computed. This enables a simpler expression of the external heat transfer model that can be easily implemented in coupled simu∗Address all correspondence to this author: pedro.rodrigues@centraliens.net lations.

Effects of a Diverging Cup on Swirl Number, Flow Pattern and Topology of Premixed Flames

Impact of the diverging cup angle of a swirling injector on the flow pattern and stabiliza- tion of technically premixed flames is investigated both theoretically and experimentally with the help of OH* chemiluminescence, OH laser-induced fluorescence and particle image velocimetry (PIV) measurements. Recirculation enhancement with a lower position of the internal recirculation zone (IRZ) and a flame leading edge protruding further upstream in the swirled flow are observed as the injector nozzle cup angle is increased. A theoretical analysis is carried out to examine whether this could be explained by changes of the swirl level as the diffuser cup angle is varied. It is shown that pressure effects need in this case to be taken into account in the swirl number definition and expressions for changes of the swirl level through a diffuser are derived. It is demonstrated that changes of the swirl level including or not the pressure contribution to the axial momentum flux are not at the origin of the changes observed of the flow and flame patterns in the experi- ments. The swirl number without the pressure term, designated as pressure-less swirl, is then determined experimentally with laser Doppler velocimetry (LDV) measurements at the injector outlet for a set of diffusers with increasing quarl angles under nonreacting conditions and the values found corroborate the predictions. It is finally shown that the decline of axial velocity and the rise of adverse axial pressure gradient, both due to the cross section area change through the diffuser cup, are the dominant effects that control the leading edge position of the IRZ of the swirled flow. This is used to develop a model for the displacement of the recirculation bubble as the quarl angle varies that shows very good agreement with experiments. [DOI: 10.1115/1.4041518]

Comparison of Monte Carlo Methods Efficiency to Solve Radiative Energy Transfer in High Fidelity Unsteady 3D Simulations

The present work assesses different Monte Carlo methods in radiative heat transfer problems, in terms of accuracy and computational cost. Achieving a high scalability on numerous CPUs with the conventional forward Monte Carlo method is not straightforward. The Emission-based Reciprocity Monte Carlo Method (ERM) allows to treat each mesh point independently from the others with a local monitoring of the statistical error, becoming a perfect candidate for high-scalability. ERM is however penalized by a slow statistical convergence in cold absorbing regions. This limitation has been overcome by an Optimized ERM (OERM) using a frequency distribution function based on the emission distribution at the maximum temperature of the system. Another approach to enhance the convergence is the use of low-discrepancy sampling. The obtained Quasi-Monte Carlo method is combined with OERM. The efficiency of the considered Monte-Carlo methods are compared. NOMENCLATURE DNS Direct Numerical Simulation FM Forward Method I Radiative intensity [W sr −1 m −2 ]