Collaborators 2021

The formation and evaporation of nanodroplets in steam ejectors is neglected in many numerical simulations. We analyse the influence of a primary nozzle on steam ejector performances considering phase change processes. The numerical model is validated in detail against experimental data of supersonic nozzles and steam ejectors available in the literature. The results show that the first nonequilibrium condensation is observed within the primary nozzle, while under-expanded supersonic flow causes a second nucleation-condensation process to achieve a large liquid fraction of 0.26 in the steam ejector. The compression process of the supersonic flow results in a steep decrease of the degree of subcooling leading to droplet evaporations. The condensation and evaporation processes repeat alternatively depending on the flow behaviour in the mixing section. The increasing area ratio leads to the transition of the flow structure from under-expanded flows to over-expanded flows in the mixing section. The droplet diameter is about 7 nm in the constant section and the entrainment ratio can reach approximately 0.75 for an area ratio of 8, which achieves a good performance of the steam ejector.

In the present study, large-eddy simulations (LES) are used to identify the underlying mechanism that governs the ignition phenomena of spray flames from different nozzle diameters when the ambient temperature (Tam) varies. Two nozzle sizes of 90µm and 186µm are chosen. They correspond to the nozzle sizes used by Spray A and Spray D, respectively, in the Engine Combustion Network. LES studies of both nozzles are performed at three Tam of 800K, 900K, and 1000K. The numerical models are validated using the experimental liquid and vapor penetration, mixture fraction (Z) distribution, as well as ignition delay time (IDT). The ignition characteristics of both Spray A and Spray D are well predicted, with a maximum relative difference of 14% as compared to the experiments. The simulations also predict the annular ignition sites for Spray D at Tam ⩾ 900K, which is consistent with the experimental observation. It is found that the mixture with Z ⩽ 0.2 at the spray periphery is more favorable for ignition to occur than the overly fuel-rich mixture of Z > 0.2 formed in the core of spray. This leads to the annular ignition sites at higher Tam. Significantly longer IDT for Spray D is obtained at Tam of 800K due to higher scalar dissipation rates (χ) during high temperature (HT) ignition. The maximum χ during HT ignition for Spray D is larger than that in Spray A by approximately a factor of 5. In contrast, at K, the χ values are similar between Spray A and Spray D. This elucidates the increase in the difference of IDT between Spray D and Spray A as Tam decreases. This may explain the contradicting findings on the effects of nozzle diameters on IDT from literature.

In the present study, conjugate heat transfer (CHT) calculations are applied in a computational fluid dynamics (CFD) simulation to simultaneously solve the in-cylinder gas phase dynamics and the temperature field within the liner of the engine. The effects of different initial temperatures with linear profiles across the liner are investigated on the wall heat transfer as well as on the sulfuric acid formation and condensation. The temporal and spatial behavior of sulfuric acid condensation on the liner suggests the importance of CHT calculations under large two-stroke marine engine relevant conditions. Comparing the mean value of the heat transfer through the inner and outer sides of the liner, an initial temperature difference of 15 K with a linear profile is an appropriate initial condition to initiate the temperature within the liner. Moreover, the effect of the amount of water vapor in the air on the sulfuric acid formation and condensation is studied. The current results show that the sulfuric acid vapor formation is more sensitive to the variation of the water vapor amount than the sulfuric acid condensation.

The present work performs unsteady Reynolds-averaged Navier–Stokes simulations to study the effect of turbulence–chemistry interaction (TCI) on diesel spray flames. Three nozzle diameters (d0) of 100, 180, and 363 μm are considered in the present study. The Eulerian stochastic fields (ESF) method (with the TCI effect) and well-stirred reactor (WSR) model (without the TCI effect) are considered in the present work. The model evaluation is carried out for ambient gas densities (ρam) of 30.0 and 58.5 kg/m3. The ESF method is demonstrated to be able to reproduce the ignition delay time (IDT) and lift-off length (LOL) with an improved accuracy than that from the WSR method. Furthermore, TCI has relatively more influence on LOL than on IDT. A normalized LOL (LOL*) is introduced, which considers the effect of d0, and its subsequent effect on the fuel-richness in the rich premixed core region is analyzed. The RO2 distribution is less influenced by the TCI effect as ambient density increases. The ESF model generally predicts a longer and wider CH2O distribution. The difference in the spatial distribution of CH2O between the ESF and WSR model diminishes as d0 increases. At ρam = 30.0 kg/m3, the ESF method results in a broader region of OH with lower peak OH values than in the WSR case. However, at ρam = 58.5 kg/m3, the variation of the peak OH value is less susceptible to the increase in d0 and the presence of the TCI model. Furthermore, the influence of TCI on the total OH mass decreases as d0 increases. The total NOx mass qualitatively follows the same trend as the total OH mass. This present work clearly shows that the influence of TCI on the global spray and combustion characteristics becomes less prominent when d0 increases.

For the past three decades a significant amount of research has been conducted on bridge flutter. Wind tunnel tests for a 2000 m class twin-box suspension bridge have revealed that a twin-box deck carrying 4 m tall 50% open area ratio wind screens at the deck edges achieved higher critical wind speeds for onset of flutter than a similar deck without wind screens. A result at odds with the well-known behavior for the mono-box deck. The wind tunnel tests also revealed that the critical flutter wind speed increased if the bridge deck assumed a nose-up twist relative to horizontal when exposed to high wind speeds – a phenomenon termed the “nose-up” effect. Static wind tunnel tests of this twin-box cross section revealed a positive moment coefficient at 0⁰ angle of attack as well as a positive moment slope, ensuring that the elastically supported deck would always meet the mean wind flow at ever increasing mean angles of attack for increasing wind speeds. The aerodynamic action of the wind screens on the twin-box bridge girder is believed to create the observed nose-up aerodynamic moment at 0⁰ angle of attack. The present paper reviews the findings of the wind tunnel tests with a view to gain physical insight into the “nose-up” effect and to establish a theoretical model based on numerical simulations allowing flutter predictions for the twin-box bridge girder.

Turbulent reactive flows are ubiquitous in industrial processes. Decoupling transport effects from intrinsic chemical reactions requires an in-depth understanding of fluid flow physics; computational fluid dynamics (CFD) methods have been widely used for this purpose. Most CFD simulations of reactive liquid-phase flows, where the Schmidt numbers are large, rely on isotropic eddy viscosity models. However, the assumption of turbulent isotropy in most stirred reactors and wall-bounded flows is fundamentally incorrect and leads to erroneous re- sults. Here, we apply a systematic CFD approach to simulate liquid-phase diffusive and convective transport phenomena that occur in a Taylor-Couette (TC) reactor. We resolve the turbulent flow by extracting statistics from large eddy simulation which is used to tune the anisotropic Reynolds stress model. In addition, we con- ducted a series of turbulent precipitation and mixing studies in a TC reactor that was designed and fabricated in- house. The numerical model is successfully validated against a published torque correlation and it is found to accurately describe the advection and diffusion of chemical species. The validated model is then used to demonstrate key flow properties in the reactor. We define new local turbulent Pecl ́et numbers to characterize the relative increase in diffusivity from turbulent advection and observe a 29% increase in the turbulent contribution as Reynolds number is doubled. Both reactive simulations and experiments show an increase in overall reaction rates with increased turbulence. The results from reactive simulations provide a deeper understanding of flow- kinetics interactions at turbulent conditions.

This article investigates the effect of ambient oxygen (O2) levels and ambient density on the primary soot size under diesel engine-like conditions via the Lagrangian soot tracking (LST) method. The numerical studies and soot analysis are carried out for an n-heptane spray flame in the Sandia constant volume combustion chamber. Numerical studies are carried out at two O2 levels of 15% and 12%, as well as two ambient densities of 14.8 kg/m3 and 30 kg/m3. The LST model involves treating the soot particles formed in the spray flame as Lagrangian particles, and their individual soot information is stored. Based on the primary soot size distribution for soot particles in the core of the spray jet, an increase in ambient density from 14.8 kg/m3 to 30 kg/m3 is shown to increase the peak and mean soot size by a factor of 1.5. Furthermore, the peak and mean primary soot size decreases with decreasing O2 levels from 15% to 12%. The larger primary soot size at higher O2 levels and ambient densities can be attributed to the higher net growth rate experienced by the soot particles. At low density, the span of the soot cloud is shorter O2 level is low. In contrast, the highdensity cases show a comparable soot cloud span at both O2 levels before steady-state is reached. Soot age is introduced to predict the soot residence time in the spray flame. The results show that the soot residence time is dependent on both the span of the soot cloud and the initial onset location of the soot formed.

A prediction method, known as the Coupled Time Scale (CTS) method, is proposed in the current work to estimate the ignition delay time (IDT) of liquid spray combustion by only performing an inert spray simulation and a zero-dimensional (0-D) homogeneous reactor (HR) simulation. The method is built upon the assumption that if the majority of the vapor regions in a spray has a composition close to the most reactive mixture fraction, which can be obtained by performing 0-D HR calculations, these regions will then have a high probability to undergo high-temperature ignition in the spray. The proposed method is applied to estimate the high-temperature IDT of n-dodecane sprays. Two nozzle diameters (Dnoz) of 90 um and 186 um which correspond to Spray A and Spray D in the Engine Combustion Network [1] respectively, are considered. Both are tested at three ambient temperatures (Tam) of 800 K, 900 K, and 1000 K. The fidelity of the proposed CTS method is verified by comparing the predicted IDT against CFD simulated IDT and measured IDT. Comparison of the estimated IDT from the CTS method to the measured IDT yields a maximum relative difference of 24%. Meanwhile, a maximum relative difference of 33% is found between the IDTs computed from the CTS method and the large eddy simulations of the associated reacting sprays across the different Tam, Dnoz and chemical mechanisms considered in this study.

Choanoflagellates are unicellular microscopic organisms that are believed to be the closest living relatives of animals. They prey on bacteria through the act of the continuous beating of their flagellum, which generates a current through a crown-like filter. Subsequently, the filter retains bacterial particles from the suspension. The mechanism by which the prey is retained and transported along the filter remains unknown. We report here on the hydrodynamic effects on the transportability of bacterial prey of finite size using computational fluid dynamics. Here, the loricate choanoflagellate Diaphaoneca grandis serves as the model organism. The lorica is a basket-like structure found in only some of the species of choanoflagellates. We find that although transportation does not entirely rely on hydrodynamic forces, such forces positively contribute to the transportation of prey along the collar filter. The aiding effects are most possible in non-loricate choanoflagellate species, as compared to loricate species. As hydrodynamic effects are strongly linked to the beat and shape of the flagellum, our results indicate an alternative mechanism for prey transportation, especially in biological systems where having an active transport mechanism is costly or not feasible. This suggests an additional potential role for flagella in addition to providing propulsion and generating feeding currents.

Numerical simulations using large eddy simulation (LES) and Unsteady Reynolds Averaged Navier–Stokes (URANS) are carried out to identify the underlying mechanisms that govern the early soot evolution process in an n-dodecane spray flame at 21% O2 by molar concentration. A two-equation phenomenological soot model is used here to simulate soot formation and oxidation. Both ignition delay time (IDT) and lift-off length (LOL) are found to agree with experimental measurements. The transient evolution of soot mass, in particularly the soot spike phenomenon, is captured in the present LES cases, but not in the URANS cases. Hence, a comparison of numerical results from LES and URANS simulations is conducted to provide a better insight of this phenomenon. LES is able to predict the rapid increasing soot mass during the early stage of soot formation due to having a large favorable region of equivalence ratio (ϕ > 1.5) and temperature (T > 1800 K) for soot formation. This favorable region increases and then decreases to reach a quasi-steady state in the LES case, while it continues to increase in the URANS simulation during the early time. In addition, the soot spike is a consequence of the competition between soot formation and oxidation rates. The time instance when the total soot mass reaches peak value coincides with the time instance when the total mass of soot precursor reaches a plateau. The soot spike is formed due to the continuous increase of oxidizing species in the LES case which leads to a more dominant oxidation process than the formation process.

Railway ballast affected by heavy cyclic loading degrades and spreads resulting in an uncomfortable transportation caused by undesirable vibrations. Restoring a well sorted track ballast can be expensive. This paper analyzes track ballast deformation using the Discrete Element Method (DEM). The simulations are performed using the STAR-CCM+ software in a three-dimensional domain. Four track ballast models are studied. The first two models describe the ballast as spheres with and without rolling resistance, respectively. The third model uses a clump model that allows breaking of the ballast, whereas the fourth model describes the ballast as composite particles generated from 3D-scanned ballast stones. The sleepers and rails are modelled as DEM particles. As a supplement to the study of different ballast models, the influence of variation in the loading profile is investigated. The largest obtained deformation is observed in the ballast modelled as spheres and the smallest deformation in the ballast modelled from the 3D scanned ballast stones. The results highlight the importance of describing the ballast as non-spherical geometries.

Computer-Aided Engineering (CAE) has supported the industry in its transition from trial-and-error towards physics-based modelling, but our ways of treating and exploiting the simulation results have changed little during this period. Indeed, the business model of CAE centers almost exclusively around delivering base-case simulation results with a few additional operational conditions. In this contribution, we introduce a new paradigm for the exploitation of computational physics data, consisting in using machine learning to enlarge the simulation databases in order to cover a wider spectrum of operational conditions and provide quick response directly on field. The resulting product from this hybrid physics-informed and data-driven modelling is referred to as Simulation Digital Twin (SDT). While the paradigm can be equally used in different CAE applications, in this paper we address its implementation in the context of Computational Fluid Dynamics (CFD). We show that the generation of Simulation Digital Twins can be efficiently accomplished with the combination of the CFD tool TransAT and the data analytics platform eDAP.

Numerical simulations are performed to investigate the effects of ambient density (ρam) and nozzle diameter (Dnoz) on the ignition characteristic of diesel spray combustion under engine-like conditions. A total of nine cases which consist of different ρam of 14.8, 30.0, and 58.5 kg/m3 and different Dnoz of 100, 180, and 363μm are considered. The results show that the predicted ignition delay times are in good agreement with measurements. The current results show that the mixture at the spray central region becomes more fuel-rich as Dnoz increases. This leads to a shift in the high-temperature ignition location from the spray tip towards the spray periphery as Dnoz increases at ρam of 14.8 kg/m3 . At higher ρam of 30.0 and 58.5 kg/m3 , the ignition locations for all Dnoz cases occur at the spray periphery due to shorter ignition timing and the overly fuel-rich spray central region. The numerical results show that the first ignition location during the high-temperature ignition occurs at the fuel-rich region at ρam⩽30.0 kg/m3 across different Dnoz. At ρam = 58.5 kg/m3 , the ignition occurs at the fuel-lean region for the 100 and 180μm cases, but at the fuel-rich region for the 363μm nozzle case. This distinctive difference in the result at 58.5 kg/m3 is likely due to the relatively longer ignition delay time in the 363μm nozzle case. Furthermore, the longer ignition delay time as Dnoz increases can be related to the higher local scalar dissipation rate in the large nozzle case.