Laser doppler velocimetry investigation of the turbulence structure of axisymmetric diffusion flames

Measurements of mean velocity components, turbulent intensities, velocity probability density functions, power spectra and autocorrelation functions of axial velocity fluctuation, and spatial turbulence macroscale, are reported in a turbulent round jet flow, issuing vertically into stagnant air, in non-combusting and combusting situations. The fuel density (a mixture of methane and argon) is chosen to be equal to the cold flow gas density (a mixture of air and helium) in order to minimize cold fuel/cold gas mixture density difference effects on measured turbulence properties. The objectives are to study the influence of the combustion process on the turbulence structure of the combustible jet flows considered, and to provide data against which results of numerical prediction methods for such flows embodying various turbulence and combustion models can be compared, with a view to improving our understanding of relevant transport processes and on guiding modelling and prediction efforts of such flows. A one-dimensional laser velocimeter operating in forward scatter differential Doppler mode was used to obtain the measurements. Gas temperatures were measured by thermocouples. A visual study by schlieren photography has also been conducted. It is found that the existence of the flame suppresses turbulence in the upstream region of the jet flow and enhances it in the downstream region, where turbulence intensities are substantially higher than in the corresponding cold jet flow. However, the relative intensities, i.e. the ratio of the local turbulent intensity to the local mean velocity, are smaller in the jet diffusion flame and become comparable to relative turbulent intensities found in the cold jet flow in the downstream region of the flow. Turbulence in the jet diffusion flame is appreciably more anisotropic than in the corresponding cold jet in all regions of the flow, suggesting the eventual desirability of multi-stress models of turbulence for the prediction of such flames. The combustion process has been found to have also a marked influence on the turbulence macroscale. It is significantly smaller than in the cold jet flow in the upstream region and increases appreciably at downstream distances, the rate of this increase closely following the rate of temperature increase. The experimental results obtained will guide the development of an improved prediction method for such combusting systems.     

One-dimensional turbulence model simulations of autoignition of hydrogen/carbon monoxide fuel mixtures in a turbulent jet

The autoignition of hydrogen/carbon monoxide in a turbulent jet with preheated co-flow air is studied using the one-dimensional turbulence (ODT) model. The simulations are performed at atmospheric pressure based on varying the jet Reynolds number and the oxidizer preheat temperature for two compositions corresponding to varying the ratios of H₂ and CO in the fuel stream. Moreover, simulations for homogeneous autoignition are implemented for similar mixture conditions for comparison with the turbulent jet results. The results identify the key effects of differential diffusion and turbulence on the onset and eventual progress of autoignition in the turbulent jets. The differential diffusion of hydrogen fuels results in a reduction of the ignition delay relative to similar conditions of homogeneous autoignition. Turbulence may play an important role in delaying ignition at high-turbulence conditions, a process countered by the differential diffusion of hydrogen relative to carbon monoxide; however, when ignition is established, turbulence enhances the overall rates of combustion of the non-premixed flame downstream of the ignition point.     

Effects of turbulence on nonpremixed ignition of hydrogen in heated counterflow

Experiments were conducted to determine the effects of turbulence on the temperature of a heated air jet required to ignite a counterflowing cold hydrogen/nitrogen jet. In contrast to pseudo-turbulent flows, where turbulence was generated by only a perforated plate on the fuel side, resulting in little effect on ignition in a hydrogen system, fully turbulent flows with perforated plates on both sides of the flow were found to produce noticeable effects. The difference was attributed to the fact that in fully turbulent flows, a significantly larger range of turbulent eddies extend to smaller scales than in pseudo-turbulent flows. At atmospheric pressure, the lowest turbulence intensity studied had ignition temperatures notably lower than laminar ones, while further increases in turbulence intensity resulted in rising ignition temperatures. As a result, optimal conditions for nonpremixed hydrogen ignition exist in weakly turbulent flows where the ignition temperature is lower than can be obtained in other laminar or turbulent flows at the same pressure. Similar trends were seen for all fuel concentrations and at all pressures in the second ignition limit (below 3-4 atm). At higher pressures, turbulent flows caused the ignition temperatures to continue to follow the second limit resulting in ignition temperatures higher than the laminar values. The extension of the second limit ends at the highest pressures (7 to 8 atm) where evidence of third limit behavior appears. Three mechanisms were noted to explain the experimental results. First, turbulent eddies similar in size to the ignition kernel can promote discrete mixing of otherwise isolated pockets of gas. Second, this mixing can promote HO₂ chain branching pathways, which can account for the enhanced ignition noted in the second limit where reaction is governed by crossover temperature chemistry. Third, turbulence limits the excursion times available for reaction, inordinately affecting the slower HO₂ reactions. This is responsible for the increasing ignition temperature with turbulence intensity and pressure.     

应用激光多普勒测速仪研究湍流有旋自由射流

用激光多普勒测速仪对旋流器产生的强湍流有自由射流的速度场和湍流场进行了实验研究。实验结果表明,这种旋流器十分有利于中心回流区的形成,射流的平均速度分布沿轴向快速地衰减,有旋自由射流呈现湍流各向异性,特别是在回流区域中,本文对流场中湍流动量传递的方向进行了讨论。     

LES of a Turbulent Slot Impinging Jet to Predict Fluid Flow and Heat Transfer

In this article, large eddy simulation (LES) is performed for a turbulent slot jet impingement heat transfer at a Reynolds number of 13,500 and a nozzle to plate spacing of 10. Various aspects of predicting a turbulent jet impinging flow in an optimum domain size and grid resolution for LES have been assessed. Two inflow conditions, one without any fluctuations and the other with fluctuations generated by the spectral synthesizer, were tested and comparisons of various mean flow, turbulence, and heat transfer data showed that LES without any inflow fluctuations provides good agreement with the corresponding experimental and numerical results reported in the literature. Further, various important dynamical flow structures have been visualized from the instantaneous computed data. Finally, mean flow and turbulence statistics have been presented in the wall jet region close to the stagnation point, which could be useful as data for validation of RANS-based turbulence models.     

Scalar transport in a turbulent jet

A model equation for the scalar dissipation rate, based on the Two Scale Direct Interaction Approximation (TSDIA) of Yoshizawa [1] was solved and applied to a turbulent round jet in conjunction with turbulence modelling based on the eddy viscosity and diffusivity. The model coefficients were adjusted by using a similarity analysis for the round jet. This led to an improvement in the prediction of concentration fluctuations on the axis of a jet with respect to results obtained with the equal length scales model. The turbulent Schmidt number, no longer assigned an ad-hoc constant value, displays experimentally observed behaviour in the jet.     

Prediction of the entrainment of ambient air into a turbulent argon plasma jet using a turbulence-enhanced combined-diffusion-coefficient method

The entrainment of the ambient air into a turbulent argon plasma jet is studied numerically using a turbulence-enhanced combined-diffusion-coefficient method. Namely, the Navier-Stokes equations and two-equation turbulence model coupled with the turbulence-enhanced combined-diffusion-coefficient approach are employed to predict the turbulent plasma jet characteristics including the evolution of air mole fraction along the plasma jet in air surroundings. Although complicated gas species always exist in the plasma jet due to rather high gas temperatures being involved, it is shown that the entrainment of ambient air into the turbulent argon plasma jet can still be treated simply by the combined turbulent and molecular diffusion between only two different gases (argon and air). Good agreement between the predicted results with corresponding experimental data reported by Fincke et al. [Int. J. Heat Mass Transfer 46 (22) (2003) 4201] demonstrates the applicability of the present modeling approach.     

Experimental study of turbulent jet induced by steam jet condensation through a hole in a water tank

A turbulent jet induced by steam jet condensation in a water pool was investigated experimentally. An experimental apparatus equipped with a steam boiler, a single-hole steam sparger, and a water pool, etc. was used. For the measurements, a pitot tube and thermocouples were used for turbulent flow velocity and temperatures, respectively. Overall flow shapes of the turbulent jet by the steam jet condensation are similar to those of axially symmetric turbulent jet flows. The angular coefficients of turbulent rays are quantitatively comparable between the traditional turbulent jet flows and the turbulent jet flows induced by the steam jet condensation in this work. Although the turbulent flows were induced by the steam jet condensation, general theory of turbulent jets was found to be applicable to the turbulent flows of this work.     

Numerical study of transient conjugate heat transfer of a turbulent impinging jet

This study presents the numerical study of transient conjugate heat transfer in a high turbulence air jet impinging over a flat circular disk. The numerical simulation of transient, two-dimensional cylindrical coordinate, turbulent flow and heat transfer is adopted to test the accuracy of the theoretical model. The turbulent governing equations are resolved by the control-volume based finite-difference method with a power-low scheme, and the well-known low-Re κ–ω turbulence model to describe the turbulent structure. The SIMPLE algorithm is adopted to solve the pressure–velocity coupling. The parameters studied include turbulent flow Reynolds number (Re = 16,100–29,600), heated temperature of a circular disk (T_h = 373 K) or heat flux (q″ = 63–189 kW/m²), and orifice to heat-source spacing (H/D = 4–10). The numerical results of the transient impinging process indicate that the jet Reynolds number has a significant effect on the hydrodynamics and heat transfer, particularly in the stagnation region of an impinging jet. High turbulence values lead to greater heat transfer coefficients in the stagnation region and cause a bypass of the laminar-to-turbulent transition region in the wall jet region. Induced turbulence from the environment around the jet also influences the variation of the stagnation heat transfer. The modeling approach used here effectively captures both the stagnation region behavior and the transition to turbulence, thus forming the basis of a reliable turbulence model.     

Large eddy simulation of coherent structure of impinging jet

The flow field of a rectangular exit, semi-confined and submerged turbulent jet impinging orthogonally on a flat plate with Reynolds number 8500 was studied by large eddy simulation (LES). A dynamic sub-grid stress model has been used for the small scales of turbulence. The evolvements such as the forming, developing, moving, pairing and merging of the coherent structures of vortex in the whole regions were obtained. The results revealed that the primary vortex structures were generated periodically, which was the key factor to make the secondary vortices generate in the wall jet region. In addition, the eddy intensity of the primary vortices and the secondary vortices induced by the primary vortices along with the time were also analyzed.     

LES analysis of flow and turbulent mixing in an unsteady jet

The flow and mixing process of unsteady jets are fundamentally analyzed by large eddy simulations. The effects of nozzle velocity and turbulence intensity on the turbulent eddy structure and mixing process between the nozzle fluid and ambient fluid were investigated. The results show that a toroidal‐shaped vortex, which emerges around the jet tip, primarily accelerates the entraining flow. Also, increasing the turbulence intensity in the nozzle encourages mixing in the jet without changing the jet‐contour. Furthermore, when the rise‐up time of the initial nozzle velocity is elongated, turbulent mixing is suppressed. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 303–313, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20158     

A MULTIPLE-TIME-SCALE TURBULENCE MODEL BASED ON VARIABLE PARTITIONING OF THE TURBULENT KINETIC ENERGY SPECTRUM

A multiple-time-scale turbulence model based on a single point closure and a simplified split-spectrum method is presented. In the model the effect of the ratio of the production rate to the dissipation rate on the eddy viscosity is modeled using multiple time scales and a variable partitioning of the turbulent kinetic energy spectrum. Example problems included are a wall-jet flow, a wake-boundary-layer interaction flow, a backward-facing step flow, and a confined coaxial swirling jet. The multiple-time-scale turbulence model yields significantly improved computational results compared with those obtained using the k-c turbulence model. It is also shown that the present turbulence model can resolve the viscous superlayer of turbulent flows.     

STREAMLINE ADAPTIVE GRID METHOD FOR COMPLEX FLOW COMPUTATION

A concept of generating and adapting grids according to flow streamlines is employed in the prediction of complex fluid flows. In general, this method retains the merits of body-fitted coordinates in the near-wall regions. Moreover, this approach generates new coordinate lines according to the streamline patterns of the preceding solution based on either Cartesian or curvilinear grid systems and adapts sufficient grids in regions where high resolutions are needed so that the difficulty of numerical diffusion is significantly reduced, To give systematic coordinate transformations, conservation equations applicable in the general nonorthogonal curvilinear coordinate system are derived by the method of generalized tensors. A treatment of near-wall turbulence suitable in the general curvilinear coordinate system is also presented. The superiorities of the present approach are demonstrated by studying the laminar/turbulent flow over a backward facing step and the flow induced by a two-dimensional turbulent jet injected into cross flow. The results show remarkable successes for the case of laminar flow over a backward facing step in the whole domain and the case of a turbulent jet into cross flow in the prediction of detailed flow structures.     

倾斜射流对移动平板表面紊动和传热特性的影响

采用雷诺应力湍流模型和Simplic算法对半封闭槽道内倾斜射流冲击移动平板的流动和传热特性进行了数值模拟,研究了不同射流角度和不同平板移动速度下平板近壁湍动能和板面努塞尔数的变化.结果表明：射流角度和平板运动速度对平板近壁湍动能和表面努塞尔数值分布影响显著;当入射角与平板运动方向相同时,板速的升高提高了近壁面的湍动能,但是降低了冲击区域的局部努塞尔数值;平板表面的平均努塞尔数值随板速的提高先降低后大幅升高,高速下角度对平板表面的平均传热效果影响较小;当入射角为80°,平板运动方向与入射方向相反且板速和射流速度相同时,在移动平板表面能够获得较佳的紊动和传热效果.     

Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure

Partially premixed combustion is involved in many practical applications, due to partial premixing of combustible and oxidant gases before ignition, or due to local extinctions, which lead to mixing of reactants and burned gases. To investigate some features of flames in stratified flows, the stabilization processes of lifted turbulent jet flames are studied. This work offers a large database of liftoff locations of flames stabilized on turbulence-free jets for different fuels and nozzle diameters studied over their flame stability domains. Methane, propane, and ethylene flames are investigated for nozzle diameters of 2, 3, 4, and 5 mm. Blowout velocities are measured and compared with an approach based on large-scale structures of the jet. The axial and radial locations of the flame base are measured by planar laser-induced fluorescence (PLIF) of the OH radical through high sampling (at least 5000 points). From this large database the average locations of the flame base are analyzed for the fuels investigated. The pdfs exhibit an evolution of their shapes according to the region of the turbulent jet where the flame stabilizes (potential core, transition to turbulence, or fully developed turbulence regions). This dependence is probably due to the interaction of the flame with the jet structures. This is confirmed by the comparison between the amplitude of the height fluctuations and the local size of the large-scale structures deduced from particle image velocimetry measurements and self-similarity laws for velocity. The results show the flame can be carried over a distance equal to the local diameter of the jet within the region of fully developed turbulence for propane and ethylene, and over a slightly larger distance for methane.     

Concerning the calculation of plane turbulent jets on the basis of the k-ε model of turbulence

Within the framework of the standard k-ε-T² model of turbulence, self-similar equations for the velocity and temperature fields of a plane jet have been integrated numerically. Tables of solutions for velocity, temperature, kinetic energy of turbulence, rate of its dissipation, and for the mean square of temperature fluctuation at different Prandtl numbers are presented. The quantitative parameters for the evolution of mean and turbulent characteristics of free jet flows have been determined. A new numerical solution scheme for a non-similar partial differential equation is described. Comparison between the results obtained and experimental/numerical data of other authors is made.     

Visualization of a turbulent jet using wavelets

ho-cttonCoherent strUctUres are known to ealst and areresPOnsible for most of the momentUIn transfer inndulent jets. Many identification techniques, such asimage processing, sPeCtI'a analysis, spatial correlationfimctions, education schemes, PrOper OrthOgonaldecomposihon, stOChastic eshInation, pattem recoghhon,and wave1et tusform, are wen established to detennincoheret stI'Ucwts. Howevee the local scales with resPeCtto spacehme change continously for the turblence andthe coheren stheA…     

Counter-gradient transport in the combustion of a premixed CH4/air annular jet by combined PIV/OH-LIF

A combination of PIV/OH laser induced fluorescence technique is used to measure the conditional - burned and unburned - gas velocity in a turbulent premixed CH₄/air annular bluff-body stabilized burner. By changing the equivalence ratio from lean to almost stoichiometric, the energy budget of the recirculating region anchoring the flame is altered in such a way to increasingly lift the flame away from the jet exit. The overall turbulence intensity interacting with each flame is thus systematically varied in a significant range, allowing for a parametric study of its effect on turbulent scalar transport under well controlled conditions, always well within the flamelet regime. The component of the flux normal to the average front is found to reverse its direction, confirming the Bray number as a good indicator of gradient/counter-gradient behavior, once the actual incoming turbulence level felt locally by the flame is assumed as the proper control parameter.     

Hydrogen autoignition in a turbulent jet with preheated co-flow air

The autoignition of hydrogen in a turbulent jet with preheated air is studied computationally using the stand-alone one-dimensional turbulence (ODT) model. The simulations are based on varying the jet Reynolds number and the mixture pressure. Also, computations are carried out for homogeneous autoignition at different mixture fractions and the same two pressure conditions considered for the jet simulations. The simulations show that autoignition is delayed in the jet configuration relative to the earliest autoignition events in homogeneous mixtures. This delay is primarily due to the presence of scalar dissipation associated with the scalar mixing layer in the jet configuration as well as with the presence of turbulent stirring. Turbulence plays additional roles in the subsequent stages of the autoignition process. Pressure effects also are present during the autoignition process and the subsequent high-temperature combustion stages. These effects may be attributed primarily to the sensitivity of the autoignition delay time to the mixture conditions and the role of pressure and air preheating on molecular transport properties. The overall trends are such that turbulence increases autoignition delay times and accordingly the ignition length and pressure further contribute to this delay.     

On the calculation of heat and momentum transport in a round jet

This communication deals with the determination of the turbulent Prandtl number and some constants which appear in the mixing length, k/ε and k/w turbulence models. The determination is carried out in a heated round jet by means of a finite-difference algorithm whose results are compared with the available experimental data. It is shown that in round jets the constants that multiply the production terms in the ε- and w- equations have values of 1.52 and 1.50, and the turbulent Prandtl number is 0.80.