Turbulent Mixing Characteristics Research Articles

Experimental research on the flow field and flame geometry of free buoyant diffusion flames with low dimensionless heat release rates

The flame geometry of turbulent diffusion flames is dominated by the turbulent mixing between the gaseous fuel and the entrained air. The significant variations of the power law for the mean flame height regarding the dimensionless heat release rate (Q˙*) in different Q˙* ranges implied that the turbulent mixing characteristics would change greatly among these Q˙* regimes. This paper presents a combined experimental and analytical study to reveal the turbulent transport properties and flame shape of free turbulent buoyant diffusion flames of relatively low Q˙* prevalent in large-scale fires. The instantaneous velocity fields of free buoyant methane diffusion flames (burner diameter: d = 0.30 m, Q˙*= 0.22–0.40) were measured by the stereo particle image velocimetry (SPIV) technique. The results indicate that the radial profiles of the time-averaged axial velocity, axial normal stress, radial normal stress, shear stress, and turbulent kinetic energy exhibited self-similarity at different heights under all the heat release rates. The turbulent flow in the buoyant flame was found to be anisotropic. Based on the gradient transport approximation, the mean turbulent viscosity within the mean flame height (νt=) was scaled by g1/2d3/2, where g is the gravitational acceleration. The semi-physical correlations for the mean flame height and width were derived based on the concepts of turbulent mixing and equal axial convection and radial diffusion times, respectively. The two correlations agreed reasonably with the experimental data of this work.

Exploration of Multishafts Stirred Reactors: An Investigation on Experiments and Large Eddy Simulations for Turbulent Chaos and Mixing Characteristics

Exploration of Multishafts Stirred Reactors: An Investigation on Experiments and Large Eddy Simulations for Turbulent Chaos and Mixing Characteristics

Numerical investigation on flow and heat transfer characteristics of helium–xenon gas mixture in rod bundles

The helium-xenon(He–Xe) gas mixture with low Prandtl number and good heat transfer characteristics suits for the advanced high heat flux and compact gas-cooled reactors. A numerical simulation was conducted on the flow, heat transfer, and turbulent mixing characteristics of He–Xe gas mixture in a tightly arranged triangular rod bundle subchannel. The transition characteristics of He–Xe gas mixture in rod bundle structure were obtained. The friction resistance coefficient, Nusselt number(Nu), and turbulent mixing factor β was corrected within the Reynolds number(Re) range of 3 × 102 - 1.8 × 105. The research results indicate that the transition occurs at Re equal to 2000 and forms a transition zone within the Re range of 2000–5000; The Blasius correlation can describe the wholly turbulent flow precisely, but it is suitable to adopt a new formula in the transition zone; The local Nu prediction performance of the modified Peckett correlation is in good agreement with the numerical simulation results, and the Petukhov correlation with shape correction and temperature correction factors has good prediction results on global Nu; The turbulent mixing factor using Re and shape parameters can better describe the mixing characteristics. This study has reference value for the design of rod bundle reactors using He–Xe gas mixtures as working fluids.

Turbulent mixing in a choked shallow lagoon and the impacts of remediation engineering

Turbulent mixing is an important mechanism that controls the transport of materials and energy in coastal transitional water bodies such as lagoons, thereby affecting their internal environments. In this study, a three-dimensional numerical model was employed to simulate the hydrodynamics and salinity distribution in a large, shallow, choked lagoon, yielding variables related to momentum and salinity mixing processes, such as shear stress, salinity mixing flux, and buoyancy flux. The results revealed the turbulent mixing pattern of the lagoon during tidal cycles. The characteristics of turbulent mixing are different between the downstream region with a complex topography and the flat upstream region. The frictional shear in the downstream channel and deep groove fully mixed the water column, resulting in a relatively unstratified state. In the upstream region, buoyancy effects dominate stratification in the middle-upper part of the water column, which supported turbulent mixing near the bottom therein and throughout the water column in the downstream region. The turbulent mixing in the lagoon was regulated by the spring-neap tidal cycle and was intensified during spring tides due to the increased tidal flow. Given the hydrodynamic and environmental issues in Xiaohai Lagoon (XHL), the government proposed a series of remediation schemes in connection with the inlet and channel of the lagoon. In this study, based on the characteristics of actual engineering, several typical scenarios were set up to analyze the impact of remediation projects on lagoon mixing processes. The openness parameter was selected to evaluate the extent of the lagoon regulations. As the openness parameter increased, the overall mixing of the lagoon was enhanced, whereas a larger increase in shear dissipation caused a drop in overall mixing efficiency. Excessive connectivity between lagoons and the ocean can lead to inefficiencies in turbulent mixing and intense marine effects, which are issues in lagoon management.

Numerical Study of Fluid Flow and Mixing in the Argon Oxygen Decarburization (AOD) Process

A three-dimensional (3D) model has been developed based on the Eulerian multiphase flow approach to investigate the fluid flow behavior and mixing efficiency in the multi-tuyere AOD process. The interphase forces, including drag force, lift force, virtual force, turbulent dispersion force, and wall lubrication force, were incorporated into this model. The model was used to simulate six-tuyere and seven-tuyere AOD processes. The phenomena of multi-jet penetration, bubble plume merging, 3D turbulent flow and mixing characteristics were considered. The results indicate that the bubble plume merging occurs in the upper part of the liquid bath, forming a typical plume cluster. The predicted penetration length for a single tuyere jet agrees well with the previous work. For the multi-jet system, the side jets penetrate deeper than the inside ones. The six-tuyere AOD has a good flow condition in the center of the liquid bath, while the seven-tuyere AOD has a better flow pattern in the sidewall region and the lower bath. Overall, the seven-tuyere AOD performs better in mixing efficiency than the six-tuyere AOD under the same gas flow rate. These findings increase the understanding of the AOD process, allowing further optimization of process parameters. This model can be further extended to incorporate the thermochemical reactions into the modeling of the AOD reactor.

Observed characteristics of flow, water mass, and turbulent mixing in the Preparis Channel

Observed characteristics of flow, water mass, and turbulent mixing in the Preparis Channel

Diffusion of point source pollution at the cross joint

Abstract The cross joint is one of the standard connection types for urban water supply pipelines. A pipeline containing cross joints was taken as the research object using NaCl as the tracer. The turbulent mixing characteristics of water and pollutants at the cross joints and pollutant migration and diffusion were studied and analysed by numerical analysis and experimental measurement. The primary purpose is to check the comprehensive influence of the six factors of pipe diameter, inlet flow ratio, outlet flow ratio, location of damage point, flow of brine, and density of brine on the turbulent mixing of water and brine in the pipeline. The coefficient of variation is used as an evaluation index to evaluate the mixing in the pipeline, and the effective mixing length LEML is used to quantify the uniform mixing position of brine in the pipeline. The results show that the inlet flow ratio, outlet flow ratio, and pipe diameter significantly affect LEML in the east outlet of the cross joint. Outlet flow ratio and pipe diameter significantly affect LEML in the cross joint's south outlet direction. In addition, the dimensionless relationship equation representing LEML is fitted through dimensional analysis.

Effect of chemical reaction on mixing transition and turbulent statistics of cylindrical Richtmyer–Meshkov instability

Direct numerical simulations of a three-dimensional cylindrical Richtmyer–Meshkov instability with and without chemical reactions are carried out to explore the chemical reaction effects on the statistical characteristics of transition and turbulent mixing. We adopt 9-species and 19-reaction models of non-premixed hydrogen and oxygen separated by a multimode perturbed cylindrical interface. A new definition of mixing width suitable for a chemical reaction is introduced, and we investigate the spatio-temporal evolution of typical flow parameters within the mixing regions. After reshock with a fuller mixing of fuels and oxygen, the chemical reaction becomes sufficiently apparent at affecting the evolution of the flow fields. Because of the generation of a combustion wave within the combustion regions and propagation, the growth of the mixing width with a chemical reaction is accelerated, especially around the outer radius with large temperature gradient profiles. However, the viscous dissipation rate in the early stage of the chemical reaction is greater because of heat release, which results in weakened turbulent mixing within the mixing regions. We confirm that small-scale structures begin to develop after reshock and then decay over time. During the developing process, helicity also begins to develop, in addition to kinetic energy, viscous dissipation rate, enstrophy, etc. In the present numerical simulations with cylindrical geometry, the fluctuating flow fields evolve from quasi-two-dimensional perturbations, and the generations of helicity can capture this transition process. The weakened fluctuations during shock compression can be explained as the inverse energy cascade, and the chemical reaction can promote this inverse energy cascade process.

Single-phase flow in helical cruciform fuel assembly with conjugate heat transfer

The helical cruciform fuel (HCF) is promising to improve the power density and safety margin of reactors core by its advantages in the heat transfer area-to-volume ratio and continual inter-channel mixing. In this work, the turbulent mixing and heat transfer characteristics of HCF assembly for pressurized water reactor was studied. A novel mesh generation strategy was used to discretize the computational domain of helical fuel assembly with hexahedron meshes. The velocity field, inter-channel cross flow intensity, heat transfer coefficient and the circumferential distribution of wall temperature and heat flux were obtained based on the CFD simulation. The cross flow intensity in the side gaps was greater than that in the internal gaps, which were all significant larger than that of traditional circular rod bundle. The heat transfer coefficient of helical fuel assembly was slightly larger than traditional circular rod bundle under same mass flow rate condition. The helical fuel assembly can offer 38.4% more heat transfer area. The peak-to-average heat flux in the circumferential direction was larger than 2.0, which may result in early boiling crisis.

Data‐Driven Identification of Turbulent Oceanic Mixing From Observational Microstructure Data

AbstractCharacterizing how ocean turbulence transports heat is critically important for accurately parameterizing global circulation models. We present a novel data‐driven approach for identifying distinct regions of turbulent mixing within a microstructure data set that uses unsupervised machine learning to cluster fluid patches according to their background buoyancy frequency N, and turbulent dissipation rates of kinetic energy ϵ and thermal variance χ. Applied to data collected near the Velasco Reef in Palau, our clustering algorithm discovers spatial and temporal correlations between the mixing characteristics of a fluid patch and its depth, proximity to the reef and the background current. While much of the data set is characterized by the canonical mixing coefficient Γ = 0.2, elevated local mixing efficiencies are identified in regions containing large density fluxes derived from χ. Once applied to further datasets, unsupervised machine learning has the potential to advance community understanding of global patterns and local characteristics of turbulent mixing.

Statistical characteristics of turbulent mixing in spherical and cylindrical converging Richtmyer–Meshkov instabilities

In this paper, the Richtmyer–Meshkov instabilities in spherical and cylindrical converging geometries with a Mach number of approximately 1.5 are investigated by using the high resolution implicit large eddy simulation method, and the influence of the geometric effect on the turbulent mixing is investigated. The heavy fluid is sulphur hexafluoride (SF6), and the light fluid is nitrogen (N2). The shock wave converges from the heavy fluid into the light fluid. The Atwood number is 0.678. The total structured and uniform Cartesian grid node number in the main computational domain is 20483. In addition, to avoid the influence of boundary reflection, a sufficiently long sponge layer with 50 non-uniform coarse grids is added for each non-periodic boundary. Present numerical simulations have high and nonlinear initial perturbation levels, which rapidly lead to turbulent mixing in the mixing layers. Firstly, some physical-variable mean profiles, including mass fraction, Taylor Reynolds number, turbulent kinetic energy, enstrophy and helicity, are provided. Second, the mixing characteristics in the spherical and cylindrical turbulent mixing layers are investigated, such as molecular mixing fraction, efficiency Atwood number, turbulent mass-flux velocity and density self-correlation. Then, Reynolds stress and anisotropy are also investigated. Finally, the radial velocity, velocity divergence and enstrophy in the spherical and cylindrical turbulent mixing layers are studied using the method of conditional statistical analysis. Present numerical results show that the geometric effect has a great influence on the converging Richtmyer–Meshkov instability mixing layers.

Simulation study of turbulent mixing characteristics of tight lattice with triangular arrangement under blockage condition

The study of turbulent mixing in the gaps of fluid sub-channels in the tight lattice under blockage conditions is of great significance to the prediction of the thermal-hydraulic behavior of fuel assemblies under accident conditions. The CFD method was used to simulate the fluid flow phenomenon in the blockage condition in the tight lattice, and the velocity field and the turbulent mixing coefficient distribution in the section and downstream of the blockage under different blockage conditions were further compared and analyzed. The analysis results show that the blockage mainly affects the flow field of the sub-channel where it is located and the adjacent sub-channels. The turbulent mixing coefficient in these channels will increase significantly; when the sub-channel blockage rate is small, the turbulent mixing coefficient downstream of the blockage will be reduced; the short length will make the flow field near the top of blockage segment more complicated. The obtained changes of transverse and axial turbulent mixing coefficients under different blockage conditions can provide a reference for the parameter setting of the sub-channel analysis program.

Experimental investigation of turbulent characteristics in pore-scale regions of porous media

The study of flow dynamics in porous media or randomly packed beds is essential because these systems are commonly applied in a wide array of engineering applications, such as thermal energy storage, catalytic reactors for processing and distillation, oil recovery, fuel cells, and pebble bed nuclear reactor cores. The flow mixing and turbulence characteristics in pore-scale regions of an experimental facility composed of randomly packed spheres with an aspect ratio of 4.4 were experimentally investigated in this study. Velocity measurements at several pores in the test facility were conducted by combining the matched-index-of-refraction and time-resolved particle image velocimetry techniques, and these measurements were performed at Reynolds numbers of 490, 1023, and 1555. The results obtained from the velocity measurements in the pore regions revealed different and complex flow patterns depending on the pore geometries. In pore region 1, it was found that a strong axial flow entered the pore and impinged into the sphere’s surface when it encountered another flow entering laterally. These dynamic interactions created a high level of turbulent mixing, a recirculation flow region, and a shear layer, as observed in the computed statistical results for $$u^{\prime }_\mathrm{{rms}}$$ , $$v^{\prime }_\mathrm{{rms}}$$ , and $$u^{\prime }v^{\prime }$$ . In other pore regions, the flow field results exhibited complex interactions among jet flows discharging into the pores from various flow gaps created by neighboring spheres. These mutual interactions between the entering jet flows were found to create highly turbulent mixing regions at the pore’ centers and low-velocity or stagnation-flow regions in the vicinity of the sphere surfaces. Flapping shear layers were also observed when two jet flows with unequal strengths entered the pores. The characteristics of turbulent flow mixing in different pore regions were investigated via the spatiotemporal two-point correlation of fluctuating velocities along the velocity streamlines. This approach mitigated the constraints of the random geometries of pore regions on the extent of spatial separation lengths in the two-point velocity correlations. Using this approach, spatial distributions of integral length scales and convection velocities were estimated along the streamlines within the random geometries of pore regions. It was found that the estimated values of the integral length scales and convection velocities were spatially dependent on the localized flow characteristics within the pores. The integral length scales were found to be less than $$0.4D_\mathrm{{sp}}$$ , experimentally confirming the pore-scale prevalence hypothesis that turbulent structures in porous media are restricted by the pore sizes. In addition, the ratio of the convection velocity to the interstitial velocity (or local velocity magnitude) decreased as the Reynolds number increased.

Quantification of turbulent structures in and around the boundary region of a turbulent round jet released into counter-flow

This experimental study reports the turbulent flow structure of the interaction of turbulent round jet issued into a counter turbulent flow for improved understanding of the turbulent mixing characteristics including pollutant dispersal at the estuarine region for tidal river flows. A high-frequency data acquisition system (Acoustic Doppler velocimeter) was employed to quantify the turbulent flow structures within the mixing region of the turbulent jet. It was found that the cross-sectional mean axial jet velocity decayed faster in counter turbulence compared to a quiescent background. A reduced occurrence of engulfment and nibbling processes were evident which signified that the entrainment rate of the moving fluid reduced owing to the jet released into the counter flow. Further, the intermittency and diffusion depth of eddies decreased for jets issued against the current. The kinetic energy budget revealed that the pressure diffusion term of the turbulent kinetic energy budget equation supplied energy to the jet from the mean background flow at the self-similar region of the jet.

Numerical and Experimental Study of Turbulent Mixing Characteristics in a T-Junction System

The mixing, migration, and degradation of pollutants in sewers are the main causes for pipeline corrosion and the increased pollution scope. The clarification of the turbulent mixing characteristics in pipelines is critical for finding the source of pollution in a timely fashion and inspecting pipelines’ damaged locations. In this paper, numerical simulations and experiments were conducted to investigate the turbulent mixing characteristics in pipelines by studying a T-junction system, of which four variables (main pipe diameter φ, cross-flow flux Q, mixing ratio δ, the incident angle of T-junctions θ) were considered. The coefficient of variation (COV) of the salt solution was selected as the evaluation index and effective mixing length (LEML) was defined for quantitative analysis. The numerical results were found to be in good agreement with the experimental results. The results reveal that the values of LEML rise as Q or φ increase and decrease with the increase of δ, where the influence of φ is much greater than Q and δ, and there is no obvious regularity between LEML and θ. By dimensional analysis and multivariate nonlinear regression analysis, a dimensionless relationship equation in harmony with the dimensional analysis was fitted, and a simplified equation with the average error of 4.01% was obtained on the basis of correlation analysis.

Observation of Turbulent Mixing Characteristics in the Typical Daytime Cloud-Topped Boundary Layer over Hong Kong in 2019

Turbulent mixing is critical in affecting urban climate and air pollution. Nevertheless, our understanding of it, especially in a cloud-topped boundary layer (CTBL), remains limited. High-temporal resolution observations provide sufficient information of vertical velocity profiles, which is essential for turbulence studies in the atmospheric boundary layer (ABL). We conducted Doppler Light Detection and Ranging (LiDAR) measurements in 2019 using the 3-Dimensional Real-time Atmospheric Monitoring System (3DREAMS) to reveal the characteristics of typical daytime turbulent mixing processes in CTBL over Hong Kong. We assessed the contribution of cloud radiative cooling on turbulent mixing and determined the altitudinal dependence of the contribution of surface heating and vertical wind shear to turbulent mixing. Our results show that more downdrafts and updrafts in spring and autumn were observed and positively associated with seasonal cloud fraction. These results reveal that cloud radiative cooling was the main source of downdraft, which was also confirmed by our detailed case study of vertical velocity. Compared to winter and autumn, cloud base heights were lower in spring and summer. Cloud radiative cooling contributed ~32% to turbulent mixing even near the surface, although the contribution was relatively weaker compared to surface heating and vertical wind shear. Surface heating and vertical wind shear together contributed to ~45% of turbulent mixing near the surface, but wind shear can affect up to ~1100 m while surface heating can only reach ~450 m. Despite the fact that more research is still needed to further understand the processes, our findings provide useful references for local weather forecast and air quality studies.

Study of Variable Turbulent Prandtl Number Model for Heat Transfer to Supercritical Fluids in Vertical Tubes

In order to improve the prediction performance of the numerical simulations for heat transfer of supercritical pressure fluids, a variable turbulent Prandtl number (Prt) model for vertical upward flow at supercritical pressures was developed in this study. The effects of Prt on the numerical simulation were analyzed, especially for the heat transfer deterioration conditions. Based on the analyses, the turbulent Prandtl number was modeled as a function of the turbulent viscosity ratio and molecular Prandtl number. The model was evaluated using experimental heat transfer data of CO2, water and Freon. The wall temperatures, including the heat transfer deterioration cases, were more accurately predicted by this model than by traditional numerical calculations with a constant Prt. By analyzing the predicted results with and without the variable Prt model, it was found that the predicted velocity distribution and turbulent mixing characteristics with the variable Prt model are quite different from that predicted by a constant Prt. When heat transfer deterioration occurs, the radial velocity profile deviates from the log-law profile and the restrained turbulent mixing then leads to the deteriorated heat transfer.

The gas-liquid two-phase flow in reciprocating enclosure with piston cooling gallery application

With the specific power of diesel engines steadily advancing, heat removal from the piston has become a determining factor to ensure engine reliability and durability of engines. However, piston head complex structure makes it very difficult to accurately capture the flow and heat transfer processes of cooling engine oil within the piston gallery. The current study used high-speed photography to obtain periodic interface motions of gas and liquid phases under dynamic conditions, investigated the influences of engine speed and filling ratio on the interface motion, and derived the mechanism for heat transfer enhancement. Numerical simulations further explored turbulent mixing characteristics of gas-liquid mixture and reciprocating impinging effect on walls under various dynamic conditions. Coupled with a geometric reconstruction scheme, gas and liquid flow patterns were tracked by the Eulerian model, and then compared with results from VOF and CLSVOF models. A rough criterion was proposed to qualitatively estimate heat transfer enhancement of gas-liquid two-phase flow during periodic piston motion.

Role of overturns in optimal mixing in stratified mixing layers

Turbulent mixing plays a major role in enabling the large-scale ocean circulation. The accuracy of mixing rates estimated from observations depends on our understanding of basic fluid mechanical processes underlying the nature of turbulence in a stratified fluid. Several of the key assumptions made in conventional mixing parameterizations have been increasingly scrutinized in recent years, primarily on the basis of adequately high resolution numerical simulations. We add to this evidence by compiling results from a suite of numerical simulations of the turbulence generated through stratified shear instability processes. We study the inherently intermittent and time-dependent nature of wave-induced turbulent life cycles and more specifically the tight coupling between inherently anisotropic scales upon which small-scale isotropic turbulence grows. The anisotropic scales stir and stretch fluid filaments enhancing irreversible diffusive mixing at smaller scales. We show that the characteristics of turbulent mixing depend on the relative time evolution of the Ozmidov length scale$L_{O}$compared to the so-called Thorpe overturning scale$L_{T}$which represents the scale containing available potential energy upon which turbulence feeds and grows. We find that when$L_{T}\sim L_{O}$, the mixing is most active and efficient since stirring by the largest overturns becomes ‘optimal’ in the sense that it is not suppressed by ambient stratification. We argue that the high mixing efficiency associated with this phase, along with observations of$L_{O}/L_{T}\sim 1$in oceanic turbulent patches, together point to the potential for systematically underestimating mixing in the ocean if the role of overturns is neglected. This neglect, arising through the assumption of a clear separation of scales between the background mean flow and small-scale quasi-isotropic turbulence, leads to the exclusion of an highly efficient mixing phase from conventional parameterizations of the vertical transport of density. Such an exclusion may well be significant if the mechanism of shear-induced turbulence is assumed to be representative of at least some turbulent events in the ocean. While our results are based upon simulations of shear instability, we show that they are potentially more generic by making direct comparisons with$L_{T}-L_{O}$data from ocean and lake observations which represent a much wider range of turbulence-inducing physical processes.

Characteristics of turbulent mixing at late stage of the Richtmyer-Meshkov instability

We present a model for the mixing width, h, at self-similar stage of the Richtmyer-Meshkov instability (RMI). The derivation of the model is based on the formula of the growth rate of mixing width [Gao et al., Phys. Fluids 28, 114101, (2016)], which is essentially equals to the first principle (Navior-Stokes equation). The model predicts that, in the self-similar mixing stage of the RMI h2 is linearly proportional to time. This is substantially different from the classical h∼τ𝜃 description. The linearity is validated by various experimental and numerical data. The exponent, 𝜃, in the classical relation is also discussed according to the present model.