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1.
The Reynolds analogy concept has been used in almost all turbulent reacting flow RANS (Reynolds-averaged Navier–Stokes) simulations, where the turbulence scalar transfers in flow fields are calculated based on the modeled turbulence momentum transfer. This concept, applied to a diffusion flame model combustor, is assessed in this paper. Some of the numerical results, obtained from a flamelet combustion model with the turbulent Prandtl/Schmidt number varying from 0.25 to 0.85, are presented and compared with a benchmark experimental database. It is found that the turbulent Prandtl/Schmidt number has significant effects on the predicted temperature and species fields in the combustor. This is also true for the temperature profile along the combustor wall. In contrast, its effect on the velocity field is insignificant in the range considered. With an optimized turbulent Prandtl/Schmidt number, both velocity and scalar fields can be reasonably and quantitatively predicted. For the present configuration and operating conditions, the optimal Prandtl/Schmidt number is 0.5, lower than the traditionally used value of ~0.85. This study suggests that for accurate prediction of turbulence scalar transfers in practical reacting flows, the Reynolds analogy concept should be improved and new approaches should be developed. 相似文献
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The Reynolds analogy concept has been used in almost all turbulent reacting flow RANS(Reynoldsaveraged Navier–Stokes) simulations, where the turbulence scalar transfers in flow fields are calculated based on the modeled turbulence momentum transfer. This concept, applied to a lean premixed combustion system, was assessed in this paper in terms of exit temperature distribution. Because of the isotropic assumption involved in this analogy, the prediction in some flow condition, such as jet cross f... 相似文献
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The general method of Izakson and Millikan for the derivation of the well-known Prandtl-Nikuradse skin friction law is applied to the analysis of turbulent heat and mass transfer in pipes, channels, and boundary layers. The formula for the heat (or mass) transfer coefficient (or the Nusselt number) is obtained which contains the dimensionless coefficients of the universal logarithmic equations for the velocity and temperature profiles as parameters of the formula. One of these parameters is a universal function of Prandtl (or Schmidt) number and all the others are constants. The existing velocity and temperature profile measurements in various turbulent wall flows permit the determination of all the necessary coefficients with fair accuracy. The resulting calculations are in satisfactory agreement with numerous experiments on heat and mass transfer in pipes and boundary layers on a flat plate over the Prandtl (or Schmidt) number range from 6 × 10−3 to 106 and over two orders of magnitude of Reynolds (or Péclét) number variations. 相似文献
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《International Journal of Heat and Mass Transfer》2005,48(21-22):4569-4578
A direct numerical simulation (DNS) of turbulent heat transfer in a channel flow with a Giesekus model was carried out to investigate turbulent heat transfer mechanism of a viscoelastic drag-reducing flow by additives. The configuration was a fully-developed turbulent channel flow with uniform heat flux imposed on both the walls. The temperature was considered as a passive scalar with the effect of buoyancy force neglected. The Reynolds number based on the friction velocity and half the channel height was 150. Statistical quantities such as root-mean-square temperature fluctuations, turbulent heat fluxes and turbulent Prandtl number were obtained and compared with those of a Newtonian fluid flow. Budget terms of the temperature variance and turbulent heat fluxes were also presented. 相似文献
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AbstractThe Reynolds-averaged computation of turbulent flow with heat transfer most commonly models the turbulent heat flux as directly related to the turbulent flux of momentum through the turbulent Prandtl number. Its significant deviation from a uniform bulk flow value for high molecular Prandtl numbers needs to be adequately described to predict accurately the heat transfer. The present study derives a model for the near-wall variation of this important parameter, used as input into an analytical solution of heated turbulent pipe flow. The basic functional form of the profile of the turbulent Prandtl number is determined from direct numerical simulations (DNS), and experimental data are used for model calibration. The analytically predicted Nusselt numbers agree very well with experimental measurements, proving the reliability of the proposed model for the turbulent Prandtl number also for Reynolds numbers well beyond the scope of DNS. The validation against experiments further highlights the significant effect of the temperature-dependent material properties of the considered high Prandtl number liquids. Numerical simulations often discard this aspect to reduce the computational effort. The present combination of DNS, analytical solution, and experiments appears as a convenient approach for modeling turbulent key quantities such as the turbulent Prandtl number, which is well applicable to other convective flow conditions and Prandtl number regimes, as well. 相似文献
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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. 相似文献
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以甲烷/空气的湍流射流非预混燃烧为对象,建立二维稳态湍流非预混火焰的小火焰模型.利用湍流流动模型和小火焰模型耦合求解,计算出速度、混合分数、温度以及反应标量的摩尔分数在燃烧室内的分布,模拟结果表明小火焰模型能够用来描述燃烧室内燃烧机理. 相似文献
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Jong Gyu Kim Kang Y. Huh Il Tae Kim 《Numerical Heat Transfer, Part A: Applications》2013,63(6):589-609
Three-dimensional analysis is performed for the turbulent reactive flow and radiative heat transfer in the walking-beam-type slab reheating furnace by FLUENT. A simplified burner is validated against the results of the actual burner with the detailed grid resolution to avoid an excessive number of grids. The predicted temperature distribution in the furnace and global energy flow fractions are in reasonable agreement with available data. Distribution of the heat flux to the slabs, velocity vectors, and all major scalar variables in the furnace also are predicted. This study shows that three-dimensional analysis may be a useful tool to understand quantitatively the complicated combustion and heat transfer characteristics in the furnace. 相似文献
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Iwao Hosokawa 《International Journal of Heat and Mass Transfer》2004,47(5):959-968
It has recently been known that local high-gradient regions of an advected scalar, such as temperature or mass contaminant, in a turbulent state of fluid form thin sheets, randomly oriented and moving around with turbulent motion. Here is presented a joint multifractal model for velocity and scalar dissipations in isotropic turbulence which can predict the statistical distributions of the worms, vorticity-concentrated regions, as well as the above-described scalar-gradient sheets. This model allows us to derive turbulent diffusion coefficient in isotropic turbulence scaled by Reynolds number and Prandtl number, which predominates far over molecular diffusion coefficient. 相似文献
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为研究湿度对燃烧特性的影响,采用湍流雷诺应力模型和层流小火焰模型,对湿空气透平(HAT)循环燃气轮机带有旋流器的燃烧室内甲烷扩散燃烧过程进行了数值模拟对比了在4种不同空气含湿量(0、100、200、300g/kg(DA))情况下的燃烧室内部温度场、速度场以及NO组分分布的情况,分析了湿度对HAT循环燃烧室扩散燃烧特性的影响结果表明,加湿降低了整个燃烧室的温度,并使其内部温度分布更加均匀;加湿使燃烧室的NO浓度大大降低;加湿减小了回流区长度。 相似文献
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Phuong M. Le Dimitrios V. Papavassiliou 《International Journal of Heat and Mass Transfer》2009,52(21-22):4873-4882
The present work investigates the correlation between the velocity and the temperature field in wall turbulence using direct numerical simulation of turbulent channel flow and plane Couette flow in conjunction with a Lagrangian method. Characteristic length scales for heat transfer are calculated for fluids with Prandtl numbers between 0.1 and 100. Structures of larger scales are found to contribute to the transport of heat as the distance from the wall increases. Turbulent Prandtl numbers are then calculated, showing that the turbulent Prandtl number is a function of the distance from the wall, but it does not depend on the fluid Prandtl number for high Prandtl numbers. 相似文献
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This communication deals with the determination of mass, momentum and heat transport in turbulent mixing layers and with the determination of momentum transport in concentric round jets. Mixing length, k/? and k/ω models are employed in the calculations. The constants appearing in these models are evaluated to yield the correct spreading rate, mean axial velocity, concentration and temperature profiles. A single-point probability density function (pdf) has been employed to calculate heat and mass transport in mixing layers. It is shown that the concentric jet mean velocity profiles are accurately predicted if the constants multiplying the production terms in the ?? and ω? equation have values of 1.52 and 1.50. These constants have to be taken equal to 1.44 and 3.80 for mixing layers. The calculated turbulent Schmidt/Prandtl number is 0.70 for mixing layers. 相似文献
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C.D. Dritselis N.S. Vlachos 《International Journal of Heat and Mass Transfer》2011,54(15-16):3594-3604
A numerical study is carried out of the magnetic field effects on the coherent structures and the associated heat transfer in a turbulent channel flow with constant temperature at the bottom (cold) and top (hot) walls. Results from direct numerical simulations are conditionally sampled in order to extract the dominant coherent structures in the near-wall region for flows with and without a uniform external magnetic field in the wall-normal direction. The Reynolds number based on the bulk velocity and the wall distance is 5600, while only a representative small Stuart number of 0.01 is explored. Two fluids with Prandtl numbers of 0.01 and 0.71 are studied. It is shown that the conditionally averaged quasi-streamwise vortices are modified by the magnetic field with their size being increased and their strength decreased. The underlying organized fluid motions are damped by the Lorentz force and the turbulent heat transfer related to the action of quasi-streamwise vortices is decreased by the magnetic field. For the higher Prandtl number fluid, a similarity between the coherent temperature and the coherent streamwise velocity fluctuations is observed for both types of flow. This is diminished for the lower Prandtl number fluid, especially in the magnetohydrodynamic flow, inhibiting the intrusion of cold (hot) fluid from the cold (hot) wall towards the central region. 相似文献
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《Progress in Energy and Combustion Science》1999,25(4):353-385
Lean premixed combustion (LPC) of natural gas is of considerable interest in land-based gas turbines for power generation. However, modeling such combustors and adequately addressing the concerns of LPC, which include emissions of nitrogen oxides, carbon monoxide and unburned hydrocarbons, remains a significant challenge. In this paper, characteristics of published simulations of gas turbine combustion are summarized and methods of modeling turbulent combustion are reviewed. The velocity–composition PDF method is selected for implementation in a new comprehensive model that uses an unstructured-grid flow solver. Reduced mechanisms for methane combustion are evaluated in a partially stirred reactor model. Comprehensive model predictions of swirl-stabilized LPC of natural gas are compared with detailed measurements obtained in a laboratory-scale combustor. The model is also applied to industrial combustor geometries. 相似文献
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Hang Seok Choi 《Numerical Heat Transfer, Part A: Applications》2013,63(3):230-245
The characteristics of heat transfer in confined multiple jet flows of a micro can combustor is investigated by means of large eddy simulation (LES). The micro combustor can be employed for a hybrid system, which consists of a micro gas turbine and a solid oxide fuel cell. In the present study, the focus is brought into heat transfer, which has a great effect on combustion stability as heat loss to the outside of combustor. The study is made for the three cases of different baffle plate configurations with changing the velocity ratio between fuel and oxidant jets. Downstream of the baffle plate, the flow recirculation regions appear and they can affect the enhancement of the turbulent heat transfer to the wall. In particular, the near-wall flow recirculation region formed between the oxidant jet and the combustor wall plays an important role for wall heat transfer. We study the turbulent thermal fields and conjugate heat transfer which show peculiar characteristics corresponding to the three different baffle plate shapes and different velocity ratios. 相似文献
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对马蹄形火焰玻璃窑炉燃烧空间内的流动、燃烧及辐射传热等过程进行了数值模拟研究,得到了炉内燃烧空间的速度场、温度场、组分浓度分布及燃烧空间向玻璃液面传递的热流分布。探讨了燃烧空间入口的进气角度对炉内温度场和向玻璃面传递的热流的影响,模拟结果表明,当入口的进气角度在5°~10°之间时,传热效果较好。 相似文献