The prediction of turbulent swirling jet flow

A space marching integration procedure is used to solve the Reynolds equations governing the axisymmetric incompressible turbulent swirling jet flow. Turbulence is modelled by the k−ε model with an isotropic turbulent viscosity. Besides mean velocity field, turbulent properties—such as Reynolds stresses, turbulent kinetic energy and dissipation rate—are obtained and the results are compared with experimental data. Agreement is quite encouraging and shows that the assumption of isotropic turbulent viscosity seems plausible.     

The thermal efficiency of near-wall gas-droplets screens. Part I. Numerical modeling

Present work is directed to further perfection of a modeling of the structure and heat transfer processes in gas-droplets near-wall screens in a tube. The problem statement in the present work is based on the Eulerian-Eulerian two-fluid model for transport processes in the gas and dispersed phases in the turbulent gas-droplets jet. We used the model [High Temp. 40(1) 2002 78] for computation of averaged motion, heat transfer and squared velocity fluctuations of dispersed phase. The gas phase turbulence was modeled with the LRN k- [AIAA J. 36(1) 1998 38] model with taking into account the effect of evaporating droplets on the kinetic turbulence energy and the rate of its dissipation. The present paper takes into account the influence of droplets deposition and vaporization of deposited particles on the tube wall and particles evaporation in the near-wall jet. Developed numerical model was tested by comparison with experimental and numerical data for gas-droplets non-isothermal flow in a tube and in a near-wall screen.     

The interaction of high-speed turbulence with flames: Global properties and internal flame structure

We study the dynamics and properties of a turbulent flame, formed in the presence of subsonic, high-speed, homogeneous, isotropic Kolmogorov-type turbulence in an unconfined system. Direct numerical simulations are performed with Athena-RFX, a massively parallel, fully compressible, high-order, dimensionally unsplit, reactive flow code. A simplified reaction-diffusion model represents a stoichiometric H₂-air mixture. The system being modeled represents turbulent combustion with the Damköhler number Da=0.05 and with the turbulent velocity at the energy injection scale 30 times larger than the laminar flame speed. The simulations show that flame interaction with high-speed turbulence forms a steadily propagating turbulent flame with a flame brush width approximately twice the energy injection scale and a speed four times the laminar flame speed. A method for reconstructing the internal flame structure is described and used to show that the turbulent flame consists of tightly folded flamelets. The reaction zone structure of these is virtually identical to that of the planar laminar flame, while the preheat zone is broadened by approximately a factor of two. Consequently, the system evolution represents turbulent combustion in the thin reaction zone regime. The turbulent cascade fails to penetrate the internal flame structure, and thus the action of small-scale turbulence is suppressed throughout most of the flame. Finally, our results suggest that for stoichiometric H₂-air mixtures, any substantial flame broadening by the action of turbulence cannot be expected in all subsonic regimes.     

Experimental study on the turbulent premixed combustion characteristics of 70%H2/30%CO/air mixtures

To investigate the effect of equivalence ratio and turbulence intensity on the combustion characteristics of syngas/air mixtures, experiments involving premixed combustion of 70% H₂/30% CO/air mixtures at various equivalence ratios and turbulence intensities were conducted in a turbulent combustion bomb at atmospheric temperature and pressure. The turbulent burning velocity and flame curvature were used to study turbulent combustion characteristics. The results show that the turbulent burning velocity grew nonlinearly as the equivalence ratio increased, while the normalized turbulent burning velocity tended to decrease. When the equivalence ratio was relatively low, the turbulence intensity was a greater determinant of the burning velocity. The normalized turbulent burning velocity increased as the turbulence intensity increased. Re and Da were found to be directly and inversely proportional to u’/u_L, respectively. A linear relationship was observed between u_T/u_L and ln Re. As the turbulence intensity increased or equivalence ratio decreased, the wrinkle degree of the flame front increased, and the maximum and minimum values of flame front curvature increased and decreased, respectively. Meanwhile, the range of the flame front curvature increased gradually. The proportion of components with smaller absolute value of flame front curvature gradually decreases.     

Experimental study of Markstein number effects on laminar flamelet velocity in turbulent premixed flames

Effects of turbulent flame stretch on mean local laminar burning velocity of flamelets, , were investigated experimentally in an explosion vessel at normal temperature and pressure. In this context, the wrinkling, At/Al, and the burning velocity, ut, of turbulent flames were measured simultaneously. With the flamelet assumption the mean local laminar burning velocity of flamelets, , was calculated for different turbulence intensities. The results were compared to the influence of stretch on spherically expanding laminar flames. For spherically expanding laminar flames the stretched laminar burning velocity, un, varied linearly with the Karlovitz stretch factor, yielding Markstein numbers that depend on the mixture composition. Six different mixtures with positive and negative Markstein numbers were investigated. The measurements of the mean local laminar burning velocity of turbulent flamelets were used to derive an efficiency parameter, I, which reflects the impact of the Markstein number and turbulent flame stretch—expressed by the turbulent Karlovitz stretch factor—on the local laminar burning velocity of flamelets. The results showed that the efficiency is reduced with increasing turbulence intensity and the reduction can be correlated to unsteady effects.     

Direct numerical simulation of ignition in turbulent n-heptane liquid-fuel spray jets

Direct numerical simulation was used for fundamental studies of the ignition of turbulent n-heptane liquid-fuel spray jets. A chemistry mechanism with 33 species and 64 reactions was adopted to describe the chemical reactions. The Eulerian method is employed to solve the carrier-gas flow field and the Lagrangian method is used to track the liquid-fuel droplets. Two-way coupling interaction is considered through the exchange of mass, momentum, and energy between the carrier-gas fluid and the liquid-fuel spray. The initial carrier-gas temperature was 1500 K. Six cases were simulated with different droplet radii (from 10 to 30 μm) and two initial velocities (100 and 150 m/s). From the simulations, it was found that evaporative cooling and turbulence mixing play important roles in the ignition of liquid-fuel spray jets. Ignition first occurs at the edges of the jets where the fuel mixture is lean, and the scalar dissipation rate and the vorticity magnitude are very low. For smaller droplets, ignition occurs later than for larger droplets due to increased evaporative cooling. Higher initial droplet velocity enhances turbulence mixing and evaporative cooling. For smaller droplets, higher initial droplet velocity causes the ignition to occur earlier, whereas for larger droplets, higher initial droplet velocity delays the ignition time.     

Current status of droplet evaporation in turbulent flows

This article reviews the available literature results concerning the effects of turbulence on the transport (heat and mass transfer) rates from a droplet. The survey emphasizes recent findings related specifically to physical models and correlations for predicting turbulence effects on the vaporization rate of a droplet. In addition, several research challenges on the vaporization of fuel droplets in turbulent flow environments are outlined.     

The effect of ignition delay time on the explosion behavior in non-uniform hydrogen-air mixtures

The purpose of this study is to examine the explosion characteristics of non-uniform hydrogen-air mixtures with turbulent mixing. In the experiment, hydrogen is first filled into a 20 L spherical chamber to a desired initial pressure, then air is introduced into the same chamber through a fast response solenoid valve, by adjusting the ignition delay time (t_d), i.e., the time period between the end of air injection and the action of ignition, the turbulent mixing strengthen (or called uniformity of hydrogen-air mixture) is then changed. The experimental results show that the explosions are overall enhanced as t_d decreases, which indicates that turbulence plays a leading role in enhancing the explosion behaviors. In addition, it is found that the effect of turbulence on p_max is more prominent in end-wall ignition than that in center ignition. This is because the heat loss per unit time is higher in end-wall ignition due to the flame front continuously contacts with inner wall of the chamber throughout the explosion process, although the explosion duration time t_e for both ignition cases is reduced when turbulence is introduced, heat loss reduction for end-wall ignition is generally larger than that in center ignition. Lately, a systematical analysis of the turbulent effect associated with various equivalence ratios on the explosion characteristics is conducted in end-wall ignition. Those experimental results illustrate that the turbulence-enhancing influence is more noticeable when hydrogen-air mixtures move toward the lower explosion limit. However, no significant influence of turbulence on explosion process can be found as combustible mixtures tend to the fuel-rich side. This is mainly because that when hydrogen-air mixtures tend to fuel-rich side, τ_e reduction caused by the presence of turbulence is relatively weak as compared with that under quiescent condition, resulting in heat loss during explosion process changes slightly, hence there is no significant impact on explosion parameters.     

A Burke-Schumann analysis of dual-flame structure supported by a burning droplet

Droplet combustion experiments carried out onboard the International Space Station, using pure fuels and fuel mixtures, have shown that quasi-steady burning can be sustained by a non-traditional flame configuration, namely a “cool flame” burning in the “partial-burning” regime where both fuel and oxygen leak through the low-temperature-controlled flame-sheet. Recent experiments involving large, bi-component fuel (n-decane and hexanol, 50/50 by volume) droplets at elevated pressures show that the visible, hot flame becomes extremely weak while the burning rate remains relatively high, suggesting the possible simultaneous presence of “cool” and “hot” flames of roughly equal importance. The radiant output from these bi-component droplets is relatively high and cannot be accounted for only by the presence of a visible hot flame. In this analysis we explore the theoretical possibility of a dual-flame structure, where one flame lies close to the droplet surface, called the “cool flame”, and the other farther away from the droplet surface, termed the “hot flame”. A Burke-Schumann analysis of this dual structure seems to indicate that such flame structures are possible over a limited range of initial conditions. These theoretical results can be compared against available experimental data for pure and bi-component fuel droplet combustion to test how realistic the model may be.     

The influence of turbulence on the structure and propagation of enclosed flames

This investigation was undertaken to examine the influence of turbulence on burning velocity and on the physical structure of the flame surface under flow conditions similar to those experienced in turbojet afterburner systems. Uniform propane-air mixtures were supplied to a combustion chamber 12 in. long and of 4 in. × 4 in. cross section. Control over the turbulence level was achieved by means of grids located at entry to the chamber. Schlieren photographs were taken through transparent side walls at turbulence levels ranging from 2 to 14 per cent and at velocities up to 250 ft/sec. These photographs provided the basic data for the investigation. Turbulent flume velocities were derived as the product of the inlet velocity and the sine of the angle between the flow direction and the mean surface of the flame.

The results fully supported the wrinkled laminar flame concept of turbulent flame propagation. Turbulent flame speed was found to increase with increases in laminar flame speed, turbulent velocity and flow velocity. Under turbulent flow conditions the flame surface was characterized by a cellular structure, the average cell size diminishing with increase in approach stream velocity and turbulence. However, the main effect of turbulence was in lacerating and disrupting the flame and thereby increasing its surface area.

The results of previous investigations were confirmed in regard to the relatively slight dependence of flame spreading rate on inlet velocity, especially at high velocities. However, flame spreading rate was found to vary appreciably with turbulence and also with fuel-air ratio, a result which was consistent with the wrinkled laminar flame model, but which contradicted previous findings on enclosed flames.     

DNS of spark ignition and edge flame propagation in turbulent droplet-laden mixing layers

A parametric study of forced ignition at the mixing layer between air and air carrying fine monosized fuel droplets is done through one-step chemistry direct numerical simulations to determine the influence of the size and volatility of the droplets, the spark location, the droplet-air mixing layer initial thickness and the turbulence intensity on the ignition success and the subsequent flame propagation. The propagation is analyzed in terms of edge flame displacement speed, which has not been studied before for turbulent edge spray flames. Spark ignition successfully resulted in a tribrachial flame if enough fuel vapour was available at the spark location, which occurred when the local droplet number density was high. Ignition was achieved even when the spark was offset from the spray, on the air side, due to the diffusion of heat from the spark, provided droplets evaporated rapidly. Large kernels were obtained by sparking close to the spray, since fuel was more readily available. At long times after the spark, for all flames studied, the probability density function of the displacement speed was wide, with a mean value in the range 0.55-0.75S_L, with S_L the laminar burning velocity of a stoichiometric gaseous premixed flame. This value is close to the mean displacement speed in turbulent edge flames with gaseous fuel. The displacement speed was negatively correlated with curvature. The detrimental effect of curvature was attenuated with a large initial kernel and by increasing the thickness of the mixing layer. The mixing layer was thicker when evaporation was slow and the turbulence intensity higher. However, high turbulence intensity also distorted the kernel which could lead to high values of curvature. The edge flame reaction component increased when the maximum temperature coincided with the stoichiometric contour. The results are consistent with the limited available experimental evidence and provide insights into the processes associated with ignition of practical spray flames.     

Large-eddy simulation of turbulent heat convection in a spanwise rotating channel flow

In this paper, we investigate the effects of the Coriolis force in a heated plane channel flow subjected to spanwise rotation using the method of large-eddy simulation. We present both the general and simplified transport equations for the resolved turbulent stresses, which are essential for understanding the unique pattern of turbulent kinetic energy production in a rotating system. Numerical simulations are performed using primarily two dynamic subgrid-scale stress models and one dynamic subgrid-scale heat flux model; namely, the conventional dynamic model (DM) and a novel dynamic nonlinear model (DNM) for closure of the filtered momentum equation, and an advanced dynamic full linear tensor thermal diffusivity model (DFLTDM) for closure of the filtered thermal energy equation. The turbulent flow field studied herein is characterized by a Reynolds number Re_τ = 150 and various rotation numbers Ro_τ ranging from 0 to 7.5. In order to validate the LES approach, turbulent statistics obtained from the simulations are thoroughly compared with the available experimental results and direct numerical simulation (DNS) data. A detailed comparative study has been conducted in order to evaluate the performance of the DM and DNM in terms of their prediction of characteristic features of the velocity and temperature fields and their capability of reflecting both forward and backward scatter of kinetic energy between the filtered and subgrid scales.     

Ignition transition in turbulent premixed combustion

Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion in a centrally-ignited, fan-stirred cruciform burner capable of generating intense isotropic turbulence. Using the same methodology, this paper presents new complete MIE data sets for stoichiometric and rich cases at three different equivalence ratios ? = 1.0, 1.2 and 1.3, each covering a wide range of a turbulent Karlovitz number (Ka) indicating a time ratio between chemical reaction and turbulence. Thus, ignition transition in premixed turbulent combustion depending on both Ka and ? can be identified for the first time. It is found that there are two distinct modes on ignition in randomly stirred methane–air mixtures (ignition transition) separated by a critical Ka where values of Ka_c ≈ 8–26 depending on ? with the minimum Ka_c occurring near ? = 1. For Ka < Ka_c, MIE increases gradually with Ka, flame kernel formation is similar to laminar ignition remaining a torus, and 2D laser tomography images of subsequent outwardly-propagating turbulent flames show sharp fronts. For Ka > Ka_c, MIE increases abruptly with Ka, flame kernel is disrupted, and subsequent randomly-propagating turbulent flames reveal distributed-like fronts. Moreover, we introduce a reaction zone Péclet number (P_RZ) indicating the diffusivity ratio between turbulence and chemical reaction, such that the aforementioned very scattering MIE data depending on Ka and ? can be collapsed into a single curve having two drastically different increasing slopes with P_RZ which are separated by a critical P_RZ ≈ 4.5 showing ignition transition. Finally, a physical model is proposed to explain these results.     

Turbulent open channel flow with heat transfer subjected to the control of a spanwise travelling wave

Turbulent open channel flows with heat transfer subjected to the control of a spanwise travelling wave have been investigated by means of direct numerical simulation (DNS). The three-dimensional Navier–Stokes and energy equations are numerically solved using a fractional-step method. The spanwise travelling wave is induced by a body force that is confined within the viscous layer with its maximum at the bottom wall and decaying exponentially away from it. The objective of this study is to reveal the near-wall turbulence behaviours, the turbulent heat transfer, and thermal structures under the control of the spanwise travelling wave. Three typical frequencies of the spanwise travelling wave, i.e., high-, middle- and low-frequency, corresponding to the exciting periods at T⁺ = 25, 50 and 100, are investigated to reveal the dynamics of turbulent motions and heat transfer. The Prandtl number (Pr) varies from 1 up to 100. To elucidate the behaviours of turbulence statistics and heat transfer, some typical quantities, including the mean velocity, velocity and vorticity fluctuations, temperature and its fluctuation, turbulent heat fluxes, and the structures of the temperature fluctuation, are exhibited and analyzed.     

Effects of H2 or CO2 addition, equivalence ratio, and turbulent straining on turbulent burning velocities for lean premixed methane combustion

Using hydrogen or carbon dioxide as an additive, we investigate the bending effect of turbulent burning velocities (ST/SL) over a wide range of turbulent intensities (u^′/SL) up to 40 for lean premixed methane combustion at various equivalence ratios (?), where SL is the laminar burning velocity. Experiments are carried out in a cruciform burner, in which a sizable downward-propagating premixed CH₄/diluent/air flame interacts with intense isotropic turbulence in the central region without influences of ignition and unwanted turbulence from walls. Simultaneous measurements using the pressure transducer and pairs of ion-probe sensors at various positions of the burner show that effects of gas velocities and pressure rise due to turbulent combustion on ST of lean CH₄/H₂/air flames can be neglected, confirming the accuracy of the ST data. Results with increasing hydrogen additions (δ=10, 20, and 30% in volume) show that the bending of ST/SL vs u^′/SL plots is diminished when compared to data with δ=0, revealing that high reactivity and diffusivity of hydrogen additives help the reaction zone remaining thin even at high u^′/SL. In contrast, the bending effect is strongly promoted when CO₂ is added due to radiation heat losses. This leads to lower values of ST/SL at fixed u^′/SL and ?, where the slope n can change signs from positive to negative at sufficiently large u^′/SL, suggesting that the reaction zone is no longer thin. All ST data with various δ can be well approximated by a general correlation (ST−SL)/u^′=0.17Da^0.43, covering both corrugated flamelet and distributed regimes with very small data scatter, where Da is the turbulent Damköhler number. These results are useful in better understanding how turbulence and diluents can influence the canonical structures of turbulent premixed flames and thus turbulent burning rates.     

Numerical solution of film condensation from turbulent flow of vapor–gas mixtures in vertical tubes

A complete two-phase model is presented for film condensation from turbulent downward flow of vapor–gas mixtures in a vertical tube. The model solves the complete parabolic governing equations in both phases including a model for turbulence in each phase, with no need for additional correlation equations for interfacial heat and mass transfer. A finite volume method is used to form the discretized mean flow equations for conservation of mass, momentum, and energy. A fully coupled solution approach is used with a mesh that automatically adapts to the changing film thickness. The results of using three turbulence models involving combinations of mixing length and k–ε models in the film and mixture regions are compared. This new model is extensively compared with previous numerical and experimental studies. In the experimental comparisons, it was found that a model consisting of a k–ε turbulence model for both the film and the mixture flows produced the best agreement. Results are also presented for a parametric study of condensation from steam-air mixtures. The effects of changes to the inlet Reynolds number, the inlet gas mass fraction, and the inlet-to-wall temperature difference on the film thickness and heat transfer are presented and discussed. Local profiles of axial velocity, temperature, and gas mass fraction are also presented.     

A rule of minimal energy dissipation in modeling of channel flow turbulence

ABSTRACT

A model of turbulence is proposed to solve Reynolds equations for fully developed flow in a wall-bounded straight channel. We show that for the channel flow the Reynolds number can be defined as a ratio of flow kinetic energy to the work of friction/dissipation forces. Then, we introduce a turbulent Reynolds number as a balance between energy losses due to the momentum exchange by turbulent vortices traveling from lowto high-velocity areas and wall friction. The main idea of the model is expressed in the following phenomenological law: The minimal energy dissipation rule requires that a local deformation of the axial velocity profile can and, in the presence of finite-size instabilities, should generate turbulence with such intensity that it keeps the local turbulent Reynolds number below the critical value. Thus, the only empirical parameter in the model is the critical Reynolds number.

The model is applied to several basic channel flows such as the fully developed flow in a circular tube, in an infinite plane channel, and in an annulus. The application of the minimal energy dissipation rule requires an additional integral equation, and this can be considered as an integral-equation algebraic model of turbulence.     

Near-wake characteristics of a model horizontal axis tidal stream turbine

The results of a detailed experimental investigation of the near-wake (up to seven turbine diameters downstream) of a model horizontal axis tidal turbine (HATT) device in a large-scale recirculating water channel facility are reported. An Acoustic Doppler Velocimeter is used to provide detailed three-dimensional mean and turbulent flow field information at five different depths across the full width of the channel downstream of the turbine, giving the most complete three-dimensional velocities and Reynolds normal and shear stress data set yet available. In addition the Reynolds-stress anisotropy tensor is used to illustrate the degree of anisotropy of the Reynolds stress within the turbine's wake. These results reveal the strongly anisotropic nature of the near-wake turbulence suggesting isotropic turbulence models should not be used to model near-wake dynamics. Finally the power-law decay rates of the maximum normalised turbulent kinetic energy differ significantly from those found downstream of grids, meshes or perforated disks, suggesting that previous modelling approaches, which neglected swirl effects and modelled the turbine by absorption discs, may significantly over predict the turbulent kinetic energy decay rate of HATT wakes.     

Evaporation model of a single hydrocarbon fuel droplet due to ambient turbulence at intermediate Reynolds numbers

A general evaporation model of a single liquid fuel droplet under various ambient turbulence conditions at intermediate droplet Reynolds numbers is proposed combining the current and previous experimental results. Extensive evaporation experiments of single fuel droplets are conducted at room temperature in quasi-laminar and turbulent environments at Reynolds numbers in the range of 72-333. Tested hydrocarbon fuels include by pentane, hexane, heptane, octane and decane. Flow fields, behind a perforated plate or a circular disk in a vertical low-speed wind tunnel, provide the required ambient turbulent environments for droplet evaporation experiments. The ambient turbulence intensities vary in the range of 1-60% and the integral length scales in the range of 0.5-20 times of the initial droplet diameter. Results show that normalized evaporation rate (K/K_L) decreases with increasing effective Damköhler number (0.0001<Da_v<0.1) and is approximately equal to unity with further increasing Damköhler number (0.1<Da_v<1).