DNS Study Research Articles

Carrier-phase DNS study of particle size distribution effects on iron particle ignition in a turbulent mixing layer

The ignition and combustion of iron particles in a turbulent mixing layer is studied by means of three-dimensional carrier-phase direct numerical simulations (CP-DNS). A particular focus is set on particle size distribution (PSD) effects on the ignition behaviour by comparing CP-DNS results from using a realistic experimental PSD to DNS data based on a monodisperse (MD) particle cloud with the same equivalence ratio. The CP-DNS solves the Eulerian transport equations of the reacting gas phase and resolves all turbulent scales, while the particle boundary layers are modelled in the Lagrangian point-particle framework. A previously validated sub-model for the oxidation of iron to Wüstite (FeO) that accounts for both diffusion- and kinetically-limited combustion is employed. The mixing layer is initialised with an upper stream of air carrying cold iron particles and an opposed lower stream of hot air. Simulation results show distinct differences in the ignition behaviour between the MD and PSD cases. The ignition of the PSD case is delayed compared to the MD case and does not show any significant particle clustering prior to ignition. Further investigations indicate that the particle size has a crucial effect on the mixing process and ignition time. Small particles start their oxidation process early and already consume some of the available oxygen, while not crucially affecting the gas temperature due to their limited iron mass contribution. Conversely, a slower entrainment into the lower stream combined with higher thermal inertia and the prior oxygen depletion by the small particles leads to a delayed oxidation of the larger particles. As a net result, the PSD case shows a wide spread of individual particle ignition delay times and overall delayed bulk ignition compared to the MD case, where the majority of the particles ignites over a shorter period of time.

Wall friction and heat transfer in turbulent pulsating flow in a square duct

For the first time, a DNS study of pulsating turbulent flows and heat transfer in a square duct is performed. Calculations were carried out at Re=2200 in the current-dominance mode for eight values of the oscillation period and three values of the Prandtl number. It was found that the time-mean friction coefficient in the studied pulsating flows has a lower value than in the stationary flow. The difference reaches −14.7%. The values of the Nusselt numbers are also decreasing, albeit by a smaller amount. The features of secondary flows of Prandtl’s 2nd kind at different phases of the flow oscillations are studied. The profiles of the wall friction stress and heat transfer coefficients and their evolution over time are determined. The time-oscillations of the wall friction are entirely determined by the oscillations of the main flow, being ahead of it in phase by π/4, which coincides with the behavior of laminar pulsating flows at high frequencies. The oscillations in heat transfer on the wall are determined by oscillations in the intensity of turbulent heat exchange, and the change in the latter occurs almost in phase with a change in the volume-average intensity of the transverse velocity fluctuations.

DNS Study on Turbulent Transition Induced by an Interaction between Freestream Turbulence and Cylindrical Roughness in Swept Flat-Plate Boundary Layer

Laminar-to-turbulent transition in a swept flat-plate boundary layer is caused by the breakdown of the crossflow vortex via high-frequency secondary instability and is promoted by the wall-surface roughness and the freestream turbulence (FST). Although the FST is characterized by its intensity and wavelength, it is not clear how the wavelength affects turbulent transitions and interacts with the roughness-induced transition. The wavelength of the FST depends on the wind tunnel or in-flight conditions, and its arbitrary control is practically difficult in experiments. By means of direct numerical simulation, we performed a parametric study on the interaction between the roughness-induced disturbance and FST in the Falkner–Skan–Cooke boundary layer. One of our aims is to determine the critical roughness height and its dependence on the turbulent intensity and peak wavelength of FST. We found a suppression and promotion in the transition process as a result of the interaction. In particular, the immediate transition behind the roughness was delayed by the long-wavelength FST, where the presence of FST suppressed the high-frequency disturbance emanating from the roughness edge. Even below the criticality, the short-wavelength FST promoted a secondary instability in the form of the hairpin vortex and triggered an early transition before the crossflow-vortex breakdown with the finger vortex. Thresholds for the FST wavelengths that promote or suppress the early transition were also discussed to provide a practically important indicator in the prediction and control of turbulent transitions due to FST and/or roughness on the swept wing.

DNS Study of Spherically Expanding Premixed Turbulent Ammonia-Hydrogen Flame Kernels, Effect of Equivalence Ratio and Hydrogen Content

In this study, 3D premixed turbulent ammonia-hydrogen flames in air were studied using DNS. Mixtures with 75%, 50% and 25% ammonia (by mole fraction in the fuel mixture) and equivalence ratios of 0.8, 1.0 and 1.2 were studied. The studies were conducted in a decaying turbulence field with an initial Karlovitz number of 10. The flame structure and the influence of ammonia and the equivalence ratio were first studied. It was observed that the increase in equivalence ratio smoothened out the small scale wrinkles while leading to strongly curved leading edges. Increasing the amount of hydrogen in the fuel mixtures also led to increasingly distorted flames. These effects are attributed to local increases in the equivalence ratio due to the preferential diffusion effects of hydrogen. The effects of curvature on the flame chemistry were studied by looking at fuel consumption rates and key reactions. It was observed that the highly mobile H2 and H species were responsible for differential rates of fuel consumption in the positively curved and negatively curved regions of the flame. The indication of a critical amount of hydrogen in the fuel mixture was observed, after which the trends of reactions involving H radical reactions were flipped with respect to the sign of the curvature. This also has implications on NO formation. Finally, the spatial profiles of heat release and temperature for 50% hydrogen were studied, which showed that the flame brush of the lean case increases in width and that the flame propagation is slow for stoichiometric and rich cases attributed to suppression of flame chemistry due to preferential diffusion effects.

Use of 2-D and 3-D unsteady RANS in the computation of wall bounded buoyant flows

The present study numerically investigates the performance of a range of RANS turbulence closures with different near-wall treatments in the prediction of natural convection flows. Comparisons are performed of both 2-D and 3-D steady and time-dependent computations in differentially heated cavities, which involve very simple geometries, but give rise to complex flow physics of relevance to cooling applications, including safety features of nuclear reactors. The turbulence closures assessed include eddy-viscosity and second-moment closures and involve low-Reynolds-number versions (LRN) which fully resolve the viscous sublayer, as well as high-Reynolds-number versions (HRN) which adopt wall function treatments to prescribe boundary conditions for the wall shear stress and heat flux.In the 2-D computations of the tall rectangular cavity, there are only small deviations in the predictions of different models, and these are mainly confined to the local Nusselt number comparisons. Among the LRN models, the second-moment EBRSM is in good agreement with the data, while the LRN eddy-viscosity models under-estimate the Nusselt number levels. The HRN models, when used with the standard log-law-based wall function, over-estimate the Nusselt number. During the course of the present investigation, a new, more general, version of the Analytical Wall Function (AWF) has been developed and introduced to the open-source CFD code utilised. Here, this more general numerical wall treatment brings the HRN k-ε predictions of the Nusselt number to very close agreement with the experimental data.In the 2-D square cavity computations, the simulations focus on a high Rayleigh number (Ra) of 1011, for which DNS data has recently become available. At such high Ra value all the models are challenged due to the unsteadiness of the flow and the presence of a largely stationary and laminar core, and thin turbulent boundary layers. Assessment of different 3-D configurations of the same square cavity demonstrates that the effect of the turbulent mixing in the third (z) direction is significant, but can be taken into account by introducing a cavity depth of D=0.15H, matching that used in the DNS study. The deviations in the predictions of the different models are stronger than those observed in the corresponding rectangular cavity comparisons, especially for the local Nusselt number distribution. As is also the case in the rectangular cavity, the eddy-viscosity models under-estimate the Nusselt number, while the EBRSM with the more elaborate turbulent heat flux model once more is closest to the DNS data. All HRN models with the log-law wall function severely under-estimate the Nusselt number levels. Introduction of the buoyancy-adapted AWF leads to substantial predictive improvements. The HRN AWF approach is thus shown to be a reliable and cost-effective modelling strategy for natural convection flows.

A DNS study of extreme and leading points in lean hydrogen-air turbulent flames - part II: Local velocity field and flame topology

Data obtained in recent direct numerical simulations (Lee et al.) of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by three different Karlovitz numbers Ka ranging from 3 to 33 are further analyzed in order to explore local characteristics and structure of (i) extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) and (ii) leading points that are also characterized by a high FCR or HRR, but advance furthest into unburned reactants. Results show that, on the one hand, common characteristics of flame perturbations (curvature, strain and stretch rates, displacement speed) fluctuate significantly in the extreme or leading, FCR or HRR points and are different in different flames. Moreover, other two-point local quantities such as the local gradients of combustion progress variables or species (e.g., the radical H) mass fractions are different in different flames. Therefore, a common simple configuration of a perturbed laminar flame cannot be used as a catchall model of the entire local structure of zones surrounding the discussed points at various Ka. On the other hand, single-point local characteristics (temperature, species mass fractions, rates of their production) of the FCR extreme points are comparable in all three turbulent flames and in the critically strained planar laminar flame. In particular, the FCRs in the extreme points fluctuate weakly and are approximately equal to each other and to the peak FCR in the critically strained laminar flame. The latter finding implies that (i) the maximum FCR evaluated in the critically strained laminar flame could be used to characterize, in a first approximation, the local FCR in the extreme or leading points in turbulent flames, thus, supporting the leading point concept, and (ii) almost the same extreme FCR can be reached in substantially different local burning structures.

On the flame stabilization of turbulent lifted hydrogen jet flames in heated coflows near the autoignition limit: A comparative DNS study

Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflows are performed with a detailed H2/air chemical mechanism to understand their ignition dynamics and stabilization mechanisms. Turbulent lifted jet flames with four different coflow temperatures, Tc, between 750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditional means of heat release rate and species critical to ignition, and by performing a displacement speed analysis and a local combustion mode analysis with an indicator, α. Although Tc at 950 K is higher than the autoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, while at 1100 K, flame stabilization is found to be highly affected by autoignition. The local combustion mode analysis further reveals that at 950 K, even if a local ignition mode with |α|<1 first appears in the near field of the jet, it develops into a local extinction mode with α<−1 as local temperature decreases due to the excessive mixing of heated coflow and cold H2 within vortical structures, which inhibits the ignition kernel development upstream of the flamebase. At 1100 K, however, a local ignition mode prevails upstream of the flamebase. To further identify the effect of a vortex on the early development of an ignition kernel in a mixing layer between the heated coflow and cold H2, a series of two-dimensional DNSs are performed, varying several vortex parameters and air temperature, as a reference for the more complicated corresponding 3-D turbulent DNS cases. The results substantiate that the development of a vortex in the mixing layer tends to retard the autoignition within the vortex, especially when its temperature is slightly above the autoignition limit.

DNS Study on Structures of Turbulent Heat Transfer in Round Impinging Jet

DNS Study on Structures of Turbulent Heat Transfer in Round Impinging Jet

A 2-D DNS study of the effects of nozzle geometry, ignition kernel placement and initial turbulence on prechamber ignition

A parametric direct numerical simulation study was conducted to investigate the effects of the initial flow field (quiescent or turbulent), nozzle inlet sharpness and width, main chamber composition (lean and stoichiometric), and ignition kernel placement in a two-dimensional prechamber (PC) ignition system. The strongly coupled operating and geometric parameters determine the time at which the flame exits the prechamber, the transient structure and penetration of the initially cold and subsequently hot reactive jet and their impingement on the lower main chamber (MC) wall, affecting the combustion mode and the fuel consumption rate. The temperature of the flame reaching and crossing the nozzle is affected by the flame exit time and is significantly lower than the adiabatic flame temperature of the planar flame, although no quenching is observed. Interaction with the flow field (strong small scale vortices for narrow and sharp entry nozzles, large vortices for wide nozzles) generated close to the exit increases the surface area of the flame and its interaction with the MC mixture. Jet penetration and impingement on the lower MC wall is determined by combustion in the PC and the flow field it generates in the main chamber. Impingement results in large scale vortical structures, which further contribute to the flame area increase and accelerate the consumption of the MC charge at later times. For the conditions studied, budget analysis shows that the main combustion mode is premixed deflagration with locally enhanced or reduced reactivity. Local flame–flame interactions which are more pronounced close to the nozzle exit and the lower MC wall can increase the propagation speed up to six times compared to the planar flame. The evolution of the probability density functions of different quantities is used to characterize the strongly transient process.

DNS study on turbulent heat transfer induced by virtual turbulence at inlet in boundary layer on a flat plate

DNS study on turbulent heat transfer induced by virtual turbulence at inlet in boundary layer on a flat plate

DNS study on turbulent heat transfer induced by turbulence at the inlet in entrance region of pipe

DNS study on turbulent heat transfer induced by turbulence at the inlet in entrance region of pipe

A DNS study on temporally evolving jet flames of pulverized coal/biomass co-firing with different blending ratios

Biomass co-firing within the existing pulverized coal boiler is thought as a practical near-term way of biomass utilization, while its detailed combustion characteristics and pollutant formation have not yet been fully understood. In the present study, we report a Carrier-phase Direct Numerical Simulation study coupled with detailed mechanism to provide a deep insight into the coal/biomass co-firing (CBCF) jet flames under different blending ratios. It is found that compared with the pure coal flame, the CBCF could (i) prompt the volatiles ignition, produce higher H2O and similar CO2 mass fractions at blending ratios of 20% and 40%, and obviously reduce the gas temperature and CO2 mass fraction at the blending ratio of 50%; (ii) prompt the coal devolatilization and char burnout at blending ratios of 20% and 40%, while the char burnout is reduced when blending ratio is 50% due to the local enrichment of large particles and lack of oxygen; (iii) reduce the thermal, prompt, NNH and N2O-intermediate routes of NO formation, but show limited effect on the NO-reburning route of NO destruction, therefore, resulting in an obvious NO reduction.

Direct numerical simulation of drag reduction by spanwise oscillating dielectric barrier discharge plasma force

DBD (dielectric barrier discharge) plasma actuators have in recent years become increasingly attractive in studies of flow control due to their light structures and easy implementation, but the design of a series of actuators enabling drag reduction depends on many parameters (e.g., the length of the actuator, the space between actuators, and voltage applied) and remains a significant issue to address. In this study, velocities created by the DBD plasma actuators in stagnant flow obtained by the numerical model are compared with experimental results. Then, a DNS study is carried on, and spanwise oscillated DBD plasma actuators are examined to obtain a drag reduction in a fully developed turbulent channel flow. This study connects the conventional spanwise oscillated force in drag reduction studies with DBD plasma actuators. While the former is one of the most successful applications for the drag reduction, the latter is a most promising tool with its light and feasible structure.

DNS of an Oblique Jet in a Particle-Laden Crossflow: Study of Solid Phase Preferential Concentration and Particle-Wall Interaction

A DNS study of the interaction between an oblique laminar jet and a particle-laden crossflow is presented. The delivery tube is included in the simulation and jet in crossflow blowing ratio is set equal to 0.5, typical for gas turbine film cooling applications. The solid phase is polydisperse and it is simulated by adopting a Lagrangian two-way coupling point-particle approach. Wall-particle interaction is also taken into account. The fluid flow in the jet outflow region was found to be dominated by a strong vorticity field and by hairpin shaped vortices that are shed periodically in the crossflow. Hairpin legs are associated to a counter-rotating vortex pair that persist in the far-field of the jet. The spatial distribution of the dispersed phase is strongly influenced by these large-scale coherent structures. A three-dimensional Voronoi analysis demonstrated a particle preferential concentration induced by hairpin vortices downstream from the jet exit. Volume fractions curves are presented as a function of the spanwise direction for different transversal sections of the crossflow region. A void particle region is induced by vortices in the central near-wall zone downstream of the jet exit. Particles tend to accumulate along two symmetric regions placed on the lateral side of the structures generated by the jet injection. By an analysis of particle impacts on the wall it was observed that particles characterized by lower values of the $${\textit{St}}$$ number, whose response time is comparable with the characteristic time of the hairpin vortices, tend to impact on the wall on two symmetric side of the jet exit in the proximity of hairpin legs. This demonstrated that the generated large-scale coherent structures play a major role in the not homogeneous dispersion of the solid phase.

Prechamber ignition: An exploratory 2-D DNS study of the effects of initial temperature and main chamber composition

A numerical investigation was conducted to study ignition and the initial phases of methane combustion in a two-dimensional setup consisting of a pre- (PC) and main (MC) chamber. The effect of the wall thermal boundary condition (isothermal Tw=500 K or adiabatic), initial mixture temperature (Tu=300 and 800 K) and equivalence ratio in the main chamber (ϕMC=0.5 and 1.0) was studied. A stoichiometric mixture was used in the PC and the mixtures were initially quiescent in all cases. Flame propagation in the prechamber is affected mainly by the initial temperature, while the thermal state of the wall plays a minor role. The transient jet that is generated at the exit of the orifice connecting the two chambers creates an intense flow field with vortical structures that, depending on the initial temperature, persist for a long time or dissipate quickly affecting combustion in the main chamber. Depending on the local flow and mixing conditions close to the orifice exit, the hot jet can be broken into small kernels at low Tu or forms quickly a flame torch at hot conditions, strongly affecting ignition of the MC mixture and flame propagation in the main chamber. In the lean MC cases, the intense mixing with the stoichiometric PC mixture creates local compositions that are more favorable for ignition by the hot turbulent reactive jet that subsequently exits from the PC at a temperature that is significantly lower than the adiabatic flame temperature of the corresponding mixture. Despite the short residence time, the reactive state of the mixture is affected as it flows through the nozzle. The flame structures in the MC are described in terms of the progress variable and mixture fraction and compared to flamelet-type calculations. The local flame structure differs strongly from that of the 1-D unstrained premixed flame, particularly for the low Tu cases. The flamelet-type calculations show that ignition of the most reactive mixture is enhanced by the radicals in the hot reactive jet, while scalar dissipation rate accelerates the ignition of the whole mixture. The 2-D simulations show that ignition is significantly longer than what is predicted by the flamelet calculations.

DNS study of the global heat release rate during early flame kernel development under engine conditions

Despite the high technical relevance of early flame kernel development for the reduction of cycle-to-cycle variations in spark ignition engines, there is still a need for a better fundamental understanding of the governing in-cylinder phenomena in order to enable resilient early flame growth. To isolate the effects of small- and large-scale turbulent flow motion on the young flame kernel, a three-dimensional DNS database has been designed to be representative for engine part load conditions. The analysis is focussed on flame displacement speed and flame area in order to investigate effects of flame structure and flame geometry on the global burning rate evolution. It is shown that despite a Karlovitz number of up to 13, which is at the upper range of conventional engine operation, thickening of the averaged flame structure by small-scale turbulent mixing is not observed. After ignition effects have decayed, the flame normal displacement speed recovers the behavior of a laminar unstretched premixed flame under the considered unity-Lewis-number conditions. Run-to-run variations in the global heat release rate are shown to be primarily caused by flame kernel area dynamics. The analysis of the flame area balance equation shows that turbulence causes stochastic flame kernel area growth by affecting the curvature evolution, rather than by inducing variations in total flame area production by strain. Further, it is shown that in local segments of a fully-developed planar flame with similar surface area as the investigated flame kernels, temporal variations in flame area rate-of-change occur. Contrasting to early flame kernels, these effects can be exclusively attributed to curvature variations in negatively curved flame regions.

DNS Study of the Bending Effect Due to Smoothing Mechanism

Propagation of either an infinitely thin interface or a reaction wave of a nonzero thickness in forced, constant-density, statistically stationary, homogeneous, isotropic turbulence is simulated by solving unsteady 3D Navier–Stokes equations and either a level set (G) or a reaction-diffusion equation, respectively, with all other things being equal. In the case of the interface, the fully developed bulk consumption velocity normalized using the laminar-wave speed SL depends linearly on the normalized rms velocity u′/SL. In the case of the reaction wave of a nonzero thickness, dependencies of the normalized bulk consumption velocity on u′/SL show bending, with the effect being increased by a ratio of the laminar-wave thickness to the turbulence length scale. The obtained bending effect is controlled by a decrease in the rate of an increase δAF in the reaction-zone-surface area with increasing u′/SL. In its turn, the bending of the δAF(u′/SL)-curves stems from inefficiency of small-scale turbulent eddies in wrinkling the reaction-zone surface, because such small-scale wrinkles characterized by a high local curvature are smoothed out by molecular transport within the reaction wave.

DNS Study on Effect of Prandtl Number for Turbulent Boundary Layer with various External Forces

DNS Study on Effect of Prandtl Number for Turbulent Boundary Layer with various External Forces

Characteristics of turbulent premixed oxy-fuel combustion - A DNS study

A 3D DNS numerical study with detail chemistry mechanism has been carried out to investigate turbulent premixed combustion with oxyfuel mixtures under similar operating conditions as happened in spark ignition Internal Combustion Engine (ICE). H2O and CO2 are adopted as the dilution in oxy-fuel combustion. The temperature profiles of oxy-H2O and oxy-CO2 combustion are consistent with those of air-fired conditions in laminar premixed flame when the molar fraction of H2O and CO2 are 73% and 66% in oxidizer, respectively. 79%, 67% molar fraction of H2O and 79%, 56% molar fraction of CO2 are also conducted to learn the effects of the dilution molar fraction on the process of flame propagation. With the molar fraction of dilution increases, the mass of C2H2 increases the flame propagation speed and the mass of CO does an opposite influence. With the investigation for effects of turbulent intensity under conditions of 73% H2O and 66% CO2 with the initial u′ of 0.8, 1.6 and 2.4 m/s, respectively, results show that the turbulent intensity has little effect on the formation of CO. It is also demonstrated that for oxy-fuel combustion, due to the disparity in laminar flame speed, an appropriate u′ is necessary to keep consistent with the flame propagation speed meanwhile to maintain suitable temperature profiles.

A DNS Study of Sensitivity of Scaling Exponents for Premixed Turbulent Consumption Velocity to Transient Effects

3D Direct Numerical Simulations of propagation of a single-reaction wave in forced, statistically stationary, homogeneous, isotropic, and constant-density turbulence, which is not affected by the wave, are performed in order to investigate the influence of the wave development on scaling (power) exponents for the turbulent consumption velocity UT as a function of the rms turbulent velocity u′, laminar wave speed SL, and a ratio L11/δF of the longitudinal turbulence length scale L11 to the laminar wave thickness δF. Fifteen cases characterized by u′/SL = 0.5,1.0,2.0,5.0, or 10.0 and L11/δF = 2.1, 3.7, or 6.7 are studied. Obtained results show that, while UT is well and unambiguously defined in the considered simplest case of a statistically 1D planar turbulent reaction wave, the wave development can significantly change the scaling exponents. Moreover, the scaling exponents depend on a method used to compare values of UT, i.e., the scaling exponents found by processing the DNS data obtained at the same normalized wave-development time may be substantially different from the scaling exponents found by processing the DNS data obtained at the same normalized wave size. These results imply that the scaling exponents obtained from premixed turbulent flames of different configurations may be different not only due to the well-known effects of the mean-flame-brush curvature and the mean flow non-uniformities, but also due to the flame development, even if the different flames are at the same stage of their development. The emphasized transient effects can, at least in part, explain significant scatter of the scaling exponents obtained by various research groups in different experiments, thus, implying that the scatter in itself is not sufficient to reject the notion of turbulent burning velocity.