The effect of surface renewal due to largescale eddies on jet impingement heat transfer

The mechanism for the enhancement of stagnation-point heat transfer was explored analyzing the large-scale turbulent structures of an impinging round jet by a statistical technique with conditional sampling. It has been found that large-scale eddies impinging on the heat transfer surfaces produce a turbulent surface-renewal effect dominant for the enhancement of the jet impingement heat transfer. The effect of heat transfer enhancement can be described in terms of the turbulent Reynolds and Strouhal numbers based on the characteristic turbulence intensity and frequency of the large-scale eddies impinging on the stagnation-point boundary layer.     

圆湍射流的轴对称大涡模拟

对空间发展的不可压缩圆湍射流进行了大涡模拟研究。在流动轴对称假定下,对Re数等于11300的圆浩射流流动进行数值模拟。大涡模拟很好地再现了圆湍射流中拟序结构非定常演化的前期过程,成功地捕获到了射流中Kelvin～Helmholtz不稳定性的触发与初级涡环的卷起及其第一次和第二次配对合并现象。但在流动轴对称假设下,大涡模拟不能模拟出湍流拟序涡环结构的破碎过程。对圆湍射流的轴对称大涡模拟结果进行长时间统计平均,能够预报出圆湍射流的核心区特征,但与圆湍射流的理论分析解和经典的实验数据对比发现,核心区后大涡模拟预报的流向速度降低缓慢。从控制方程的数学本质和拟序结构的物理机制上对圆湍射流在轴对称假设下产生上述大涡模拟结果的原因进行了分析与探讨。     

Experimental investigation of the flow and heat transfer of an impinging jet under acoustic excitation

Classic and high speed particle image velocimetry and infrared thermography are used to investigate the behavior of a round jet impinging on a flat plate for a Reynolds number 28,000, for orifice-to-plate distances of 3 or 5 nozzle diameters and for two different nozzles, a contraction and a long tube. The contraction nozzle reveals a different heat transfer distribution on the impinging plate compared to the long tube case. The jet can be excited by a loudspeaker at Strouhal numbers 0.26, 0.51 and 0.79. This acoustic forcing changes the jet structure, modifying annular vortex rings in the shear layer of the jet and increasing the turbulent values. The heat transfer is therefore modified, resulting in an increase of the Nusselt number near the jet axis and an alleviation or a shift of the secondary peak.     

圆湍射流拟序结构的频谱特性

以圆湍射流为对象 ,用修正的周期图法对圆湍射流下游近场的速度场进行了频谱特性分析 ,空气射流的雷诺数为 Re=2 93 8。研究了外界主动声激励对圆湍射流功率谱的影响 ,并与流场显示的结果进行了对照。实验结果表明 ,在圆湍射流混合层内缘与核心区结合部是射流对外界声激励响应最明显的位置。外界声激励对圆湍射流的影响区域集中在射流近场。沿着射流方向 ,在射流出口和核心区尾部之间 ,功率谱分布有一个最大值。     

Experimental and numerical investigation of turbulent jets issuing through a realistic pipeline geometry: Asymmetry effects for air,helium, and hydrogen

Experiments and numerical simulations were conducted to investigate the dispersion of turbulent jets issuing from realistic pipe geometries. The effect of jet densities and Reynolds numbers on vertical buoyant jets were investigated, as they emerged from the side wall of a circular pipe, through a round orifice. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) techniques were employed simultaneously to provide time-averaged flow velocity and concentrations fields. Large eddy simulation (LES) was applied to provide further detail with regards to the three-dimensionality of air, helium, and hydrogen jets. These jets were always asymmetric and found to deflect about the vertical axis. The deflection was influenced by buoyancy, where heavier gases deflected more than lighter gases. Significant turbulent mixing was also observed in the near field. The jets from realistic pipe geometries experienced faster velocity decay and asymmetric jet spreading compared to round jets. These findings indicate that conventional round jet assumptions are, to some extent, inadequate to predict gas concentration, entrainment rates and, consequently, the extent of the flammability envelope of realistic gas leaks.     

Transfert de masse dans des jets turbulents de revolution en milieu non homogeneMass transfer in axisymmetrical turbulent jet of non homogeneous componentПepeнoc мaccы в ocecиммeтpичнoИ тypбyлeнтнoИ cтpye

This paper deals with the diffusion of an incompressible turbulent round jet issuing in an atmosphere at rest composed of a different gas. The experiments described here, have been carried out with a neat carbon dioxyde jet emitted in the air at the same temperature.The salient features of the study concern mass transfer (velocity, concentration profiles, rate of spread, laws of decrease along the axis) and are compared with similar properties of an air jet.To explain the discrepancy and similarity between the two flows, a theoretical approach is proposed without making use of the “tracer assumption”.     

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.     

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.     

Turbulent flow in the distribution header of a PEM fuel cell stack

A numerical investigation of the flowfield in a model distribution header manifold of a polymer electrolyte membrane fuel cell stack is conducted. The computational model simulates two segments of an experimental setup of a pair of model headers which replicate the headers of a fuel cell stack. The model headers consist of an inlet and outlet sections connected with a plate containing an array of holes that replicate the unit cells. The flow structures in the outlet header are rather complex and are the result of the superposition of a series of impinging jets in a confined space in the presence of crossflow. The flow from each hole, which represents an individual cell outlet, enters the outlet header as a jet stream and is subjected to a crossflow. Large Eddy Simulations (LES) are performed for a portion of the outlet header to investigate the complex turbulent flow and related structures under different crossflow conditions, and are complemented by Particle Image Velocimetry (PIV) measurements. The LES results show that two large vortical structures are formed in the header cross-section, with a high-speed round jet from the cell outlet holes forcing a diversion of the crossflow, dividing it into two separate branches. Investigation of the flow restructuring after a blockage of one of the jets is performed. Simulation results using a slot opening for the jet show flow instabilities. The results of this study highlight the unsteady and highly turbulent nature of the flow in the header and provide a characterization of the complex three-dimensional structure of the flow. The flowfield and flow structures may impact the overall pressure drop along the header and the effective cross-sectional area for the flow leaving the header. The observations and insights obtained from the LES simulation and PIV measurements point to the need to further investigate the impact on flow sharing in a stack of the flowfield development in the outlet header.     

NUMERICAL INVESTIGATION OF IMPINGEMENT HEAT TRANSFER USING LINEAR AND NONLINEAR TWO-EQUATION TURBULENCE MODELS

This article concerns development of a prediction method for turbulent impingement heat transfer. An investigation of a single round unconfined impinging air jet under different flow and geometrical conditions is carried out to assess the performance of linear and nonlinear two-equation turbulence models. The results show that these models may be used for impinging jet heat transfer predictions with reasonable success, but application of an ad hoc realizability constraint is necessary to avoid overprediction of the stagnation zone heat transfer. The constitutive relationship of the nonlinear models is shown to be dominated by the linear part for the present problem. Inclusion of quadratic and cubic level strain terms does not significantly affect predictions and the coupling to streamline curvature appears weak. The major effect of using an anisotropic formulation of the turbulent heat fluxes has been shown to reduce stagnation zone heat transfer rates.     

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.     

An experimental study of turbulent flow in attachment jet combustors by LDV

Flame stabilization in attachment jet combustors is based on the existence of the high temperature recirculation zone, provided by the Coanda effect of an attachment jet. The single attachment jet in a rectangular channel is a fundamental form of this type of flow. In this paper, the detailed characteristics of turbulent flow of a single attachment jet were experimentally studied by using a 2-D LDV. The flowfield consists of a forward flow and two reverse flows. The forward one is composed of a curved and a straight section. The curved section resembles a bent turbulent free jet, and the straight part is basically a section of turbulent wall jet. A turbulent counter-gradient transport region exists at the curved section. According to the results, this kind of combustor should have a large sudden enlargement ratio and not too narrow in width. Project supported by the National Natural Science Foundation of China.     

Simulation numérique des jets turbulents subsoniques à masse volumique variable par le modèle k–ε

A numerical investigation is reported for round free turbulent non-isothermal binary mixing incompressible jets discharging into a quiescent atmosphere. The standard k–ε model is used. The standard closure schemes in Favre averaged variables are first introduced. The parabolic numerical simulation method of Patankar and Spalding [Heat and Mass Transfer in Boundary Layer, Intertext Books, London, 1970] is followed. The numerical simulations show a satisfactory agreement with the experimental results of Chassaing [Mélange turbulent de gaz inertes dans un jet de tube libre, Thèse d'état, INPT, 1979], Birch et al. [J. Fluid Mech. 88 (3) (1978) 431–449] and Panchapakesan et Lumley [J. Fluid Mech. 246 (1993) 197–223, 225–247]. The developed numerical code is used to study the sensitivity of turbulent characteristics to the density ratio between the jet and the ambient air. The decay rate of the mean axial velocity, temperature and mass fraction are shown to increase with decreasing density ratio. This confirms a higher mixing efficiency (parameter which determines the quantity of mass or heat injected at the jet exit and found further from the axis) when the density ratio between the jet and the quiescent air decreases. Finally, it is shown that the density effects are affected by the buoyancy terms in the similarity region of the jet.     

Wall-Resolved LES and Zonal LES of Round Jet Impingement Heat Transfer on a Flat Plate

Numerical large-eddy simulation (NLES) is performed for a round jet impinging on a flat surface at a Reynolds number of Re = 23,000 for nozzle-to-plate spacings of H/D = 6 and 2, where H is the distance from the nozzle to the plate and D is the jet diameter. The Reynolds number has been set to match the experiments of Cooper et al. (Int J. Heat Mass Transfer, vol. 36, pp. 2675–2684, 1993). Two numerical large-eddy simulation approaches are examined. The first quasi-direct numerical simulation (DNS) approach resolves streaklike structures using fine near-wall grids; the second is the zonal approach of Tucker (Int J. Heat Fluid Flow, vol. 25, pp. 625–635, 2004), which uses the Wolfshtein k–l (Int J. Heat Mass Transfer, vol. 12, pp. 301–318, 1969) Reynolds-averaged Navier-Stokes (RANS) model near the walls and NLES elsewhere. A Hamilton-Jacobi equation is used to match the RANS region to the NLES zone. The use of a Spalart-Allmaras model leads to low levels of turbulent viscosity in the near-wall region. This is also observed when using detached-eddy (DES) when using a volume-based filter. The use of the standard DES filter based on maximum grid spacing prevents jet shear-layer transition. The k–l near-wall model maintains RANS levels of turbulent viscosity in the boundary layer. The results of both the near-wall quasi-DNS and hybrid RANS-NLES methods are generally encouraging.     

Ignitability limits for combustion of unintended hydrogen releases: Experimental and theoretical results

The ignition limits of hydrogen/air mixtures in turbulent jets are necessary to establish safety distances based on ignitable hydrogen location for safety codes and standards development. Studies in turbulent natural gas jets have shown that the mean fuel concentration is insufficient to determine the flammable boundaries of the jet. Instead, integration of probability density functions of local fuel concentration within the quiescent flammability limits, termed the flammability factor, was shown to provide a better representation of ignition probability. Recent studies in turbulent hydrogen jets showed that the envelope of ignitable gas composition (based on the mean hydrogen concentration), did not correspond to the known flammability limits for quiescent hydrogen/air mixtures. The objective of this investigation is to validate the flammability factor approach to the prediction of ignition in hydrogen leak scenarios. The ignition probability within a turbulent hydrogen jet was determined using a pulsed Nd:YAG laser as the ignition source. Laser Rayleigh scattering was used to characterize the fuel concentration throughout the jet. Measurements in methane and hydrogen jets exhibit similar trends in the ignition contour, which broadens radially until an axial location is reached after which the contour moves inward to the centerline. Measurements of the mean and fluctuating hydrogen concentration are used to characterize the local composition statistics conditional on whether the laser spark results in a local ignition event or complete light-up of a stable jet flame. The flammability factor is obtained through direct integration of local probability density functions. A model was developed to predict the flammability factor using a presumed probability density function with parameters obtained from experimental data and computer simulations. Intermittency effects that are important in the shear layer are incorporated in a composite probability density function. By comparing the computed flammability factor with the measured ignition probability we have validated the flammability factor approach for application to ignition of hydrogen jets.     

Mean Flow and Thermal Characteristics of a Turbulent Dual Jet Consisting of a Plane Wall Jet and a Parallel Offset Jet

The standard high-Reynolds number two-equation k ? ? model is used to study the flow and thermal characteristics of a dual jet consisting of a plain wall turbulent jet and a parallel turbulent offset jet (hereafter, dual jet). The flow and thermal characteristics are presented in the form of streamlines, mean velocity vector, turbulent kinetic energy, dissipation of turbulent energy, Reynolds stresses, and isothermal contour plots. The variation in local heat flux and local Nusselt number on the bottom wall is also presented. The finite-volume-method-based SIMPLE algorithm is utilized for understanding the complex nature of flow arising due to a dual jet. The convective flux is discretized using the power-law upwind scheme, while the diffusive term is discretized using the central difference scheme. To study the effect of offset ratio, which is defined as the ratio of height of the jet from the horizontal wall to the width of the jet (nozzle), it is varied between 3 and 15 at an interval of 2. It is noted that the presence of a wall jet in addition to the parallel offset jet has a significant effect on flow and thermal characteristics.     

Theoretical research of critical heat flux in subcooled impingement boiling on the stagnation zone

A new mechanism model for determination of the critical heat flux (CHF) in subcooled impingement boiling on the stagnation zone is proposed in this paper. It is based on the combination of the Helmholtz instability theory of macrolayer and the model of bubble induced turbulent heat transfer in subcooled impingement boiling. A semi-theoretical and semi-empirical correlation and its nondimensional form of the CHF for subcooled jet impingement boiling on the stagnation zone are also derived. Under the circumstances of CHF, the bubble induced turbulent heat transfer coefficient gets doubled as compared to the single-phase laminar heat transfer coefficient according to the theoretical model and the experimental data. And this kind of bubble induced turbulent heat transfer enhancing effect can be considered as a fixed ratio. The theoretical analysis result for the present case is successfully verified by the experimental result obtained on the smooth heating surface. Through the discussions, it is obtained that the CHF ratio of the subcooled jet impingement boiling against the saturated jet impingement boiling is theoretically related to the surface condition of the heater and the properties and impact velocity of the working fluid.     

A Novel Model of Turbulent Convective Heat Transfer in Round Tubes at Supercritical Pressures

In the present study, a novel model was established to investigate the enhanced heat transfer to turbulent pipe flow of supercritical pressure fluids. The governing equations for the steady turbulent compressible pipe flow were simplified into the one-dimensional nondimensionalized forms based on the boundary layer theory. A conventional mixing length turbulence model for constant-property pipe flows was modified by introducing the effect of density fluctuations into the equations of turbulent transport, and the modified turbulence model was applicable to both constant-property and variable-property pipe flows. With the suggested model, which was a combination of the nondimensional governing equations and the modified turbulence model, the numerical calculations were carried out for the turbulent convective heat transfer of water in round tubes at supercritical pressures. The results showed that the present model can provide a relatively precise prediction about the effect of pressure, mass flux, and wall heat flux on heat transfer for supercritical fluid flows and greatly reduce the calculation workload. The modified turbulence model showed a much better agreement with the experimental results than the original turbulence model.     

A FOUR-EQUATION MODEL FOR PREDICTION OF GAS-SOLID TURBULENT FLOWS

A four-equation model is proposed for prediction of dilute turbulent gas-solid flows where the ratio of the particle and the gas densities is large. The model is based on explicit algebraic relations for Reynolds stresses and turbulent fluxes of the void fraction, which were derived in an earlier work within the context of Reynolds-averaged Navier-Stokes (RANS) methodology. These relations are manipulated here to derive nonlinear eddy-viscosity-type models for thin-shear flows. Further, new models are proposed for third-order correlations which are also simplified for thin-shear flows. These models are used to propose four transport equations for the turbulence kinetic energy of the carrier phase and its rate of dissipation, the turbulence kinetic energy of the dispersed phase, and the velocity covariance of the two phases. The final four-equation model is implemented for prediction of a particle-laden turbulent jet, and encouraging agreements with available laboratory data are observed.     

The effect of turbulent jet induced by pre-chamber sparkplug on combustion characteristics of hydrogen-air pre-mixture

Effect of turbulent jet ignition induced by pre-chamber sparkplug (PCSP), a simper version of turbulent jet ignition pre-chamber system without fuel injection, on the air-hydrogen combustion characteristics was conducted based on an optical constant volume chamber under varied equivalence ratio conditions. The dynamic pressure sensor and schlieren system were used to evaluate the heat release and flame propagation characteristics. The results confirm the feasibility of PCSP type turbulent jet. The jet increase the flame propagation speed significantly compared to standard ignition, which shorten ignition delay and combustion duration, advance T50 largely, and increase the maximum combustion pressure slightly. As a result, the combustion intensity is increased largely, especially under lean regime, the combustion intensity index can be as high as 1.7 at certain equivalence ratio. In addition, the PCSP turbulent jet reduces the sensitivity of heat release to variation of equivalence ratio, which is helpful to simplify the combustion controlling strategy. Furthermore, with the enhancement of the flame propagation, the tendency of knocking combustion can be suppressed potentially.