Local Dissipation Rate Research Articles

Particle image velocimetry in the impeller of a centrifugal pump: Relationship between turbulent flow and energy loss

Turbulent flows play a dominant role in the operation of centrifugal pumps, which find widespread use in industrial settings and various aspects of human life. The dissipation rate of turbulent kinetic energy emerges as a key parameter within these devices, with its local values exerting a significant influence on centrifugal pump performance. Recent advances in particle image velocimetry (PIV) techniques have expanded the ability to analyze complex turbulent flows across a broad spectrum of scales. In this context, this paper aims to deepen our understanding of the turbulent flow field and its correlation with energy loss in centrifugal pump impellers. To achieve this, experiments were conducted using PIV on a transparent pump operating under different conditions. Statistics of the turbulent flow were then obtained from phase-ensemble averages of velocities, vorticity, turbulence production, and local dissipation of turbulent kinetic energy. To overcome the limited spatial resolution constraint of PIV, the large-eddy PIV (LES-PIV) method was employed to estimate the local dissipation rate. In this method, it is assumed that the motion of larger scales is measured by the PIV technique, while the smaller scales (unresolved scales) are modeled by a sub-grid scale model, calculated from the strain rate tensors obtained from the measured fields. Energy losses in the impeller were studied using two methodologies: (i) a conventional method based on power measurements, and (ii) an alternative approach based on the budget of turbulent kinetic energy. Our results reveal that turbulent loss caused by turbulence production is the main source of energy loss in the pump impeller, and it is particularly pronounced in low-flow operating conditions characterized by large-scale structures. On the other hand, in situations where flow rates exceed the best efficiency point (BEP) condition, the predominant flow structures are marked by small-scale features, mainly attributed to local dissipation of turbulence. Our findings clarify the characteristics of energy losses in centrifugal pump impellers and their relationship with the turbulent flow field, and, in addition, providing a methodology for calculating the local turbulent dissipation rate and its limitations when derived from PIV measurements.

Statistics of kinetic and thermal energy dissipation rates in vibrational turbulent Rayleigh–Bénard convection with rough surface

In this paper, the statistical properties of the kinetic εu and thermal εθ dissipation rates in two-dimensional (2D) turbulent Rayleigh–Bénard convection (RBC) are studied by using the thermal lattice Boltzmann method (LBM). A series of direct numerical simulations (DNS) are implemented in a square cell with the unit aspect ratio for the Rayleigh number Ra = 108 and Prandtl number Pr = 4.38. The roughness height h* and vary the vibration frequency ω* from 0 to 1000 are fixed in the vibrational RBC with rough plates to explore their effect on the kinetic and thermal dissipation rates. The global features indicate that the εu and εθ are more intense in the regions with high gradients of velocity and temperature, which is in good agreement with the previous work. The scalings of Nu∼ω *0.55±0.1 and Re∼0.59±0.2 are proposed between the ensemble averaged Nusselt number Nu and Reynolds number Re. The probability density functions (PDFs) of both logarithmic kinetic dissipation rate lg εu and thermal dissipation rate lg εθ are not conformed to the log-normal distributions. Through the analysis of the time-averaged εu and εθ vertical profiles, it is further demonstrated that the local dissipation rates from the boundary layer region are still the major contribution in the vibrational RBC with rough plates.

Emergent quantum probability from full quantum dynamics and the role of energy conservation

We propose and study a toy model for the quantum measurements that yield the Born’s rule of quantum probability. In this model, the electrons interact with local photon modes and the photon modes are dissipatively coupled with local photon reservoirs. We treat the interactions of the electrons and photons with full quantum mechanical description, while the dissipative dynamics of the photon modes are treated via the Lindblad master equation. By assigning double quantum dots setup for the electrons coupling with local photons and photonic reservoirs, we show that the Born’s rule of quantum probability can emerge directly from microscopic quantum dynamics. We further discuss how the microscopic quantities such as the electron–photon coupling, detuning, and photon dissipation rate affect the quantum dynamics. Surprisingly, in the infinite long time measurement limit, the energy conservation already dictates the emergence of the Born’s rule of quantum probability. For finite-time measurement, the local photon dissipation rate determines the characteristic time-scale for the completion of the measurement, while other microscopic quantities affect the measurement dynamics. Therefore, in genuine measurements, the measured probability is determined by both the local devices and the quantum mechanical wavefunction.

Experimental study on the effect of hydrodynamic conditions on flocculation and settling properties of fine-grain sediment

The flocculation of cohesive sediment particles is a function of the collision efficiency of sediment particles, which is mainly influenced by local flow hydrodynamics. A detailed study on local hydrodynamic characteristics in a novel stirred tank was done to measure the flow field of a turbulent flow. Instantaneous flow velocity fields were obtained and processed, and the average flow field was computed. Particle image velocimetry (PIV) results measuring the hydrodynamic characteristics (expressed as turbulent kinetic energy (TKE), local dissipation rate of TKE, velocity gradient or shear rate, turbulent shear stress, etc.) were analyzed and discussed. Particle tracking and automated image processing techniques were utilized to analyze the impact of the hydrodynamic conditions on the floc and settling properties. The differences in floc properties under the two turbulent shear states (i.e., steady-state (SS) and unsteady-state (US)) hydrodynamic conditions were examined. The results show that floc size and floc size distribution (FSD) of pure kaolin clay flocs are significantly influenced by the local dissipation rate of TKE and the turbulent intensity states. The floc size, df, 84, at local dissipation rate of TKE of 11 × 10−4, 29 × 10−4, 44 × 10−4, 83 × 10−4, and 142 × 10−4 m2/s3 is 97, 108, 107, 124, and 150 μm, respectively. It was found that by increasing the local dissipation rate of TKE, the FSD was skewed left. The US shear condition generated larger flocs (and in higher proportions) relative to the SS condition. The settling velocity and flux settling velocities of flocs formed under SS conditions are higher than those formed under US conditions. The fractal dimension of SS flocs also is higher than the fractal dimension of US flocs. These results highlight a significant impact of the states of hydrodynamics on floc properties such as floc size and FSD as well as the settling and morphological properties of flocs.

Effects of turbulence on variations in early development of hydrogen and iso-octane flame kernels under engine conditions

The understanding and prediction of the early development of flame kernels are of high practical importance for the robust relight of aviation gas turbines and the control of cycle-to-cycle variations (CCV) of spark-ignition engines. CCV are known to correlate strongly with early flame kernel development and complicate the optimization of such engines in terms of safety, thermal efficiency, and engine emissions. The flame kernel initiated by a spark is initially small, in the very early combustion phase typically smaller than the size of the turbulent integral length scales. Therefore, the development of the flame kernel is dominated by local, intermittent flow fluctuations and can vary under the same nominal conditions. In this study, the effects of turbulence on the early development of premixed iso-octane and hydrogen turbulent flame kernels under realistic engine conditions are investigated through direct numerical simulations. Multiple realizations were simulated under the same nominal conditions for both fuels. Significant variations in flame kernel interactions with turbulence can be identified among different realizations. The fuel consumption rate varies by a factor of two, which is remarkable considering that only statistical differences in the local flow field are present between different realizations. Effects of different flow features of the initial flow fields on the flame kernel development were analyzed. It was found that the flow motion on the scale of the ignition radius, specifically the fluid deformation, which is characterized by the invariants of the strain rate tensor, determines the global shape of the kernel, while the variations of the kernel growth rate are mostly driven by the variations of the smallest turbulent scales. In particular, turbulence influences the flame surface area growth mainly through the tangential strain rate at the flame surface, which is shown to result from the small-scale turbulent motion. Due to differential diffusion effects, hydrogen and iso-octane exhibit significantly different flame responses to curvature, which is comprehensively studied for both fuels. The findings in this study will guide the development of combustion models that are capable to capture variations of the early flame kernels based on the local turbulence dissipation rate.

Flame structure and broadening in turbulent premixed jet flames

High-resolution and high signal-to-noise (SNR) planar Rayleigh scattering thermometry measurements are performed to investigate preheat and reaction zone broadening, turbulent transport of high-temperature gases, and the internal flame structure in a series of highly turbulent, lean (ϕ = 0.75), premixed methane-air turbulent jet flames. These flames are characterized by turbulence intensities ranging from u′/SL of 35 to 150 and turbulent Reynolds numbers approaching 104, as determined from separate particle image velocimetry (PIV) measurements. Average flame profiles obtained using a gradient-based flame reconstruction method and probability density functions (PDFs) of the preheat, reaction, and thermal layer thicknesses show substantial broadening of the preheat and thermal layers. Instantaneous preheat zones are broadened by as much as a factor of 30 compared to laminar flames, with an average broadening approaching a factor of 14 for the most turbulent case. Moderate broadening of the reaction layer is observed with up to a factor of eight times that of a laminar premixed flame for instantaneous realizations and an average broadening of four times that of a laminar flame for the most turbulent case. Conditional statistics of temperature obtained at isodistance contours within and upstream of the preheat zone show large shifts towards significantly higher temperatures compared to laminar flame values. In addition, broad temperature distributions and reduced thermal gradients at each position provide strong evidence of persistent turbulent transport of high-temperature gases ahead of the preheat zone. Finally, a detailed analysis of scalar dissipation rate statistics and dissipation layer topology was conducted. The conditional mean scalar dissipation rates for the turbulent flames show significant reductions compared to calculated laminar flame values and a shift towards higher values of the reaction progress variable (CT) for increasing turbulence levels. PDFs show a transition from near-Gaussian to highly negatively skewed with increasing turbulence levels, where the most probable values of the scalar dissipation rate shift towards higher values of CT, indicating that the primary reaction zone and flame front are subjected to increased strain. In terms of structure, no significant broadening of the small-scale dissipation layers was observed with increasing turbulence intensity. However, as turbulence intensity increases, there is a transition from a sparse number of continuous layers to a large number of shorter, ”broken”, nearly straight segments. The reduction of individual layer lengths (i.e., ”breaking”) is favored over increases in dissipation layer wrinkling, where the sharp decreases in the local dissipation rate values are highly correlated with regions of high curvature. A final notable outcome of this work is that there were negligible differences in all statistical and topological results when moving between the two highest turbulent cases, even though the turbulence intensity, turbulent Reynolds number, and the turbulence Karlovitz number increased substantially (70 - 120%). This implies that there may be an asymptotic limit in which turbulence affects flame behavior for the current configuration.

Effect of inner cylinder configuration on liquid droplet dispersion and size distributions in a Taylor-Couette reactor

Effect of varying surface structures of the inner rotating cylinder on characteristics of droplet formed in a liquid–liquid Taylor-Couette (TC) reactor is investigated. Two novel surface structures of inner cylinders, designed with axially corrugated surface (N40) and with typical three-dimensional roughness (NZ40), respectively, were adopted for the investigation. The high-speed camera visualization was used to measure the droplet size while CFD modelling was applied to predict the fluid dynamics of TC flows with such two inner cylinders and a smooth surface inner cylinder, thus revealing the effect of inner cylinder configuration on the droplet formation in the TC flows. The experimental results have clearly indicated that both the rotation speed and surface configuration variation of the inner cylinders affect the generated droplet size and distribution. As the rotation speed increases, the droplet size is obviously reduced for all types of inner cylinder configurations. Compared with the conventional smooth surface inner cylinder, the two new types of inner cylinders with special surface structures can produce much smaller droplets, especially for the case of N40 surface structure inner cylinder, with which the smallest droplet size was found at all the experimental conditions. CFD modelling results have demonstrated that the surface configurations of inner cylinder can significantly affect the turbulence generation, consequently altering the distributions of turbulent kinetic energy and local turbulence shear strain rate. As a result, the droplet size was found to be controlled by the change of the local turbulent dissipation rate that was strongly associated with the surface structures of inner cylinders. Empirical correlations that correlate the droplet size with the Weber number and Reynolds number were proposed based on the high-speed camera visualization data.

Cool flame wave propagation in high-pressure spray flames

Cool flame has an important impact on the performance and emission of advanced low-temperature combustion (LTC) internal combustion engines (ICEs) in which the liquid fuel is injected earlier at a lower ambient temperature (Tam) than that in the conventional diesel combustion engines. However, the cool flame characteristics of spray combustion under ICEs conditions are not fully understood, e.g., the effect of cool flame on the spray ignition and flame stabilization is not well studied. In this paper, the so-called cool flame wave propagation (CFWP) in the Engine Combustion Networks Spray A flames at three ambient temperatures Tam (800 K, 900 K and 1000 K) is analyzed using the data from large eddy simulation with an improved Eulerian based transported probability density function sub-grid combustion model. A good agreement between the LES results and experimental data is obtained for the spray liquid penetration length, vapor fuel penetration length, mean pressure rise profile, and flame liftoff length. It is shown that CFWP in the spray ignition process promotes the ignition of the fuel-rich and cold reactant mixture, leading to the most reactive mixture shifting toward fuel-richer locations due to the spatial stratification of temperature and concentration, and turbulent mixing. As a result, the high temperature ignition (HTI) can be shortened compared to the ignition in the homogeneous mixture. At Tam = 800 K, the HTI kernels are consistently formed following the CFWP propagating toward the spray head region. However, as Tam increases, the spatial correlation between HTI kernels and CFWP fronts is weakened. On the other hand, the turbulent mixing (quantified using local scalar dissipation rate) contributes more to the formation of HTI kernels at higher Tam. The present results indicate that CFWP is more profound at lower Tam in the spray ignition process. Finally, it is found that cool flame propagates mainly into pre-reacted fuel-rich mixture in an ignition wave propagation mode from the spray upstream region toward downstream region, whereas the ignition assisted flame mode is found in the spray upstream region where the combustion heat release is negligible.

Acceleration scaling and stochastic dynamics of a fluid particle in turbulence

Fluid-particle acceleration in turbulent flows is related to turbulent kinetic energy and its rate of dissipation into heat. Focusing on the statistics of the fluid-particle acceleration conditioned on both the local dissipation rate and the kinetic energy, it is shown that the acceleration grows exponentially with kinetic energy and presents a power law dependence on the dissipation. Based on these observations, we propose a vectorial stochastic model for the dynamics of a fluid particle giving the essential characteristics of Lagrangian turbulence.

Updated parameterization of internal tidal mixing in the deep ocean based on laboratory rotating tank experiments

The Coriolis effect plays an important role in the generation and dissipation of internal tides. In this study, the effects of the Coriolis parameter on internal tides generated by tidal current flowing over rough subsurface topography were investigated in the laboratory. The results show that the variation in the internal tidal local dissipation rate with variation in the Coriolis parameter in the deep layer follows a Gaussian distribution. The strongest dissipation occurs near the critical latitude where the local Coriolis parameter is half of the tidal frequency. A modified internal tidal mixing parameterization was developed by introducing a correction function related to the Coriolis effect on internal tidal local dissipation in the deep ocean, based on the original version developed by St. Laurent et al. (2002). The new parameterization was then implemented in a parallel ocean model. The results indicate that the deep global meridional overturning circulation (GMOC) is strengthened in the low and middle latitudes and the Southern Ocean by the change, compared with the original parameterization.

Ignition under strained conditions: Unsteady flamelet progress variable modeling for diesel engine conditions in the transient counterflow configuration

The ignition of non-premixed n-dodecane-air flames is studied in a highly transient counterflow configuration under diesel engine operating conditions. The one-dimensional transient counterflow is configured such that ignition is initially physically inhibited through high strain rates. In a short relaxation period the inflow velocities (thereby the strain rate) are lowered and ignition can occur, a behaviour similarly observed in spray flames. The non-premixed flame, which is igniting in this unsteady flow environment, is computed with direct chemistry and tabulated chemistry based on the unsteady flamelet progress variable approach (UFPV). The flamelet look-up table is constructed from igniting unsteady flamelets considering also the parameter range between stable and unstable branches of the characteristic S-shaped curve which is obtained via a continuation method. The mixture fraction, the progress variable, and the local stoichiometric scalar dissipation rate serve as control variables for the three-dimensional table. Particular attention is put on the inclusion of thermal expansion effects and the treatment of the progress variable source term to capture the onset of ignition. Besides varying strain effects, the igniting flame exhibits low temperature chemistry and multi-stage ignition behaviour which is particular challenging for the UFPV modeling. By direct comparison to the reference solution the predictive capabilities of the UFPV approach are analyzed and assessed in detail. Furthermore, the influence of table resolution on computational efficiency is discussed. The systematic analysis and validation of the UFPV approach in the transient counterflow configuration fills a specific gap in the literature and underlines the fidelity of the approach for flames subject to complex transient processes. Furthermore, the 1D configuration may serve as an environment for adjusting and optimizing UFPV tabulation strategies prior to their application to more complex 3D CFD simulations such as spray LES.

Evaluation of the unsteady flamelet progress variable approach in Large Eddy Simulations of the ECN Spray A

Within the Unsteady Flamelet Progress Variable – Large Eddy Simulation (UFPV-LES) approach the local scalar dissipation rate represents one key parameter, significantly affecting the ignition behaviour. In this study, the UFPV-LES approach is evaluated for ECN Spray A baseline conditions, relevant for diesel engines. After confirming its general applicability, using experimental data under non-reacting and reacting conditions, special attention is paid to the distribution of the local scalar dissipation rate. Based on the findings of this analysis, a reduced modeling approach, considering only igniting flamelets starting from the adiabatic mixing line between the fuel and oxidizer, is investigated. The performance of this reduced approach is assessed systematically, using the UFPV-LES results as a reference. Based on an a-priori analysis, regions affected by the model reduction are identified and evaluated. A subsequent evaluation in an a-posteriori analysis, i.e. a coupled LES, reveals similar results in terms of local flame structure as well as global ignition characteristics and confirms the applicability of the reduced model under the ECN Spray A baseline conditions.

A comparison of different methods for estimating turbulent dissipation rate in under-resolved flow fields from synthetic PIV images

The turbulent energy dissipation rate is an important parameter that determines the transfer rates of mass, heat and momentum in chemical engineering industry. Especially in a multiphase reactor, the local instantaneous turbulent dissipation rate affects the breakage and coalescence of bubbles or droplets. To directly determine the local turbulent dissipation rate, fluctuating velocity gradients have to be measured down to the Kolmogorov length scale. This paper compares the advantages and disadvantages of three different methods, namely, correlation method, optical flow method and hybrid method, in the evaluation of velocity fields, which are subsequently used to calculate the turbulent dissipation rate. An instantaneous flow field of homogeneous isotropic turbulence from DNS results is used as a benchmark. The velocity field and turbulent dissipation rate field obtained by these three methods are compared to the DNS results. It is shown that the hybrid method performs better in both velocity field evaluation and local turbulent dissipation rate estimation compared to the other two methods. This is because the hybrid method combines the advantages of the correlation method in achieving a stable averaged velocity field and the optical flow method in achieving pixel-level resolution measurement of the flow field.

Optimization method of a novel airfoil fin heat exchanger based on the entransy dissipation theory

<p indent=0mm>The structure of the novel airfoil fin heat exchanger was optimized by the synergistic enhancement method based on the entransy matching performance, and two types of nonuniform fin distribution structures were proposed. The heat transfer and flow performance of various airfoil fin distribution structures were numerically studied by using supercritical CO₂ as the working medium under constant heat flux boundary conditions. The results showed that considering the extremely uneven distribution of heat transfer parameters arising from the drastic changes in the physical properties of supercritical fluid, the distribution of novel airfoil fins was changed under the condition that the total heat transfer area remained constant. The matching performance between the entransy of fluid and that of the wall heat flux was improved, and the distribution uniformity of the local entransy dissipation rate in the channel was improved. Hence, the total entransy dissipation rate in the channel was reduced, and the heat transfer performance was improved. The numerical studies under the boundary condition of variable heat flux in the two-fluid coupled heat transfer process showed that the distribution mode of the novel airfoil fins changed according to the distribution characteristics of heat transfer parameters in the two-fluid coupled heat exchanger. Under the condition that the total heat transfer area remained constant, the distribution matching of the entransy of hot and cold fluid also improved, so that the local entransy dissipation rate was more uniform in the heat transfer area, and the entransy dissipation rate was reduced. The entransy distribution-matching principle provides a new idea and method for the structural optimization and improvement of heat exchangers, which has important theoretical and engineering significance.

Dissipative state transfer and Maxwell's demon in single quantum trajectories: Excitation transfer between two noninteracting qubits via unbalanced dissipation rates

We introduce a protocol to transfer excitations between two noninteracting qubits via purely dissipative processes (i.e., in the Lindblad master equation there is no coherent interaction between the qubits). The fundamental ingredients are the presence of collective (i.e. nonlocal) dissipation and unbalanced local dissipation rates (the qubits dissipate at different rates). The resulting quantum trajectories show that the measurement backaction changes the system wave function and induces a passage of the excitation from one qubit to the other. While similar phenomena have been witnessed for a non-Markovian environment, here the dissipative quantum state transfer is induced by an update of the observer knowledge of the wave function in the presence of a Markovian (memoryless) environment -- this is a single quantum trajectory effect. Beyond single quantum trajectories and postselection, such an effect can be observed by histogramming the quantum jumps along several realizations at different times. By investigating the effect of the temperature in the presence of unbalanced local dissipation, we demonstrate that, if appropriately switched on and off, the collective dissipator can act as a Maxwell's demon. These effects are a generalized measure equivalent to the standard projective measure description of quantum teleportation and Maxwell's demon. They can be witnessed in state-of-the-art setups given the extreme experimental control in, e.g., superconducting qubits or Rydberg atoms.

Analysis of the turbulent flow patterns generated in isotropic porous media composed of aligned or centered cylinders

The analysis of the turbulent flow inside porous media composed of infinite arrangements of cylinders with square and circular cross sections disposed in aligned or centered configuration is presented. The porosity (ϕ) was varied from 0.3 to 0.8, at a pore Reynolds number Rep=105, representing fully turbulent flow conditions. The flow cases were solved numerically by using the low-Reynolds Abe-Kondoh-Nagano version of the k-ε model. New correlations are provided for the domains composed of aligned particles in terms of ϕ for the volume averaged turbulence kinetic energy (k), its dissipation rate (ε), and macroscopic pressure gradient (MPG). These correlations are especially useful when it is not possible to model the local details of the porous media in macroscopic turbulence modeling. The highest energetic losses in terms of volume averaged k, ε, and MPG was found for the centered square arrangement. In all cases, these parameters increased abruptly as ϕ decreased, as the pore-scale local pressure gradients and turbulent kinetic energy dissipation rate levels increased. Depending on the case, they were triggered by a combination of sudden expansion-contraction of the flow channels, flow impingement, and the decrease of the effective flow area by flow recirculation. In all cases, the pore-scale local pressure gradients and turbulent kinetic energy dissipation rate levels increased abruptly as ϕ decreased, and consequently the energy losses increased. The tracer dispersion presented similar low homogeneity times for the centered cylinder arrangements. At each porous media, the variations of turbulence intensity, effective flow area, and tortuosity levels in terms of ϕ were not as significant as those observed for the local pressure gradient and dissipation rate. This effect may be considered to improve mixing and reduce pressure drops in devices such as static mixers, heat exchangers, cooling fins arrays, and other devices composed of slender tubes.

Using Particle Image Velocimetry to Determine Turbulence Model Parameters

The primary parameter of a standard model, , was calculated from stereoscopic particle image velocimetry (PIV) data for a supersonic jet exhausting into a transonic crossflow. This required the determination of turbulent kinetic energy, turbulent eddy viscosity, and turbulent energy dissipation rate. Image interrogation was optimized, with different procedures used for mean strain rates and Reynolds stresses, to produce useful turbulent eddy viscosity fields. The eddy viscosity was calculated by a least-squares fit to all components of the three-dimensional strain-rate tensor that were available from the PIV data. This eliminated artifacts and noise observed when using a single strain component. Local dissipation rates were determined via Kolmogorov’s similarity hypotheses and the second-order structure function. The eddy viscosity and dissipation rates were then combined to determine . Considerable spatial variation was observed in , with the highest values found in regions where turbulent kinetic energy was relatively low but where turbulent mixing was important, e.g., along the high-strain jet edges and in the wake region. This suggests that use of a constant in modeling may lead to poor Reynolds stress predictions at mixing interfaces. A data-driven modeling approach that can predict this spatial variation of based on known state variables may lead to improved simulation results without the need for calibration.

Effects of ambient pressure and nozzle diameter on ignition characteristics in diesel spray combustion

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.

Conditional scalar dissipation rate modeling for turbulent spray flames using artificial neural networks

Machine learning techniques have been used for the closure of the conditional scalar dissipation rate in turbulent spray flames. Statistical data are extracted from carrier-phase direct numerical simulation (CP-DNS) results for the generation of artificial neural networks that are trained to predict the conditional scalar dissipation rate such that they could serve as a sub-grid model for large eddy simulations of spray combustion. A quantitative error assessment suggests that predictions of the conditionally averaged dissipation rate are in excellent agreement with CP-DNS data. Further comparison with commonly used models for the unconditionally filtered dissipation rate promises significant improvements if ANNs are used for closure. The artificial neural networks also help to identify the important features that affect the local dissipation rates. The results suggest that few - mostly droplet related - parameters suffice as input features for accurate ANNs. This is in contradiction to standard modeling techniques that are solely based on gas phase properties and highlights the need to revisit scalar dissipation rate modeling for spray flames if analytical expressions are to be used.

Shear controllable synthesis of barium sulfate particles using lobed inner cylinder Taylor-Couette flow reactor

A novel lobed inner cylinder assembled in Taylor-Couette flow reactor (LTC) has been adopted to synthesize barium sulfate particles. The fluid dynamics that affects synthesis of particles using both the LTC and the classical Taylor-Couette flow reactor (CTC) was investigated through CFD modelling and experiments. The results have demonstrated that the Taylor vortices and turbulence induced shear rate distribution in the reactors have a significant influence on the final particle size distribution. The narrower shear rate distribution in the LTC is beneficial to the synthesis of particles with smaller size. The local turbulence intensification in the intra-Taylor vortices in the LTC effectively reduces the low shear strain regions. A strong correlation between the synthesized particle size and the local turbulent dissipation rate is existing. Shear induced by small turbulent eddies can inhibit particle growth. The LTC can be used for effectively shear controllable synthesis of particles.