Direct numerical simulation of stably and unstably stratified turbulent open channel flows

Summary Direct numerical simulation of stably and unstably stratified turbulent open channel flow is performed. The three-dimensional Navier-Stokes and energy equations under the Boussinesq approximation are numerically solved using a fractional-step method based on high-order accurate spatial schemes. The objective of this study is to reveal the effects of thermally stable and unstable stratification on the characteristics of turbulent flow and heat transfer and on turbulence structures near the free surface of open channel flow. Here, fully developed weakly stratified turbulent open channel flows are calculated for the Richardson number ranging from 20 (stably stratified flow) to 0 (unstratified flow) and to −10 (unstably stratified flow), the Reynolds number 180 based on the wall friction velocity and the channel depth, and the Prandtl number 1. To elucidate the turbulent flow and heat transfer behaviors, typical quantities including the mean velocity, temperature and their fluctuations, turbulent heat fluxes, and the structures of velocity and temperature fluctuations are analyzed.     

Computational Analysis of the Oscillatory Mixed Convection Flow along a Horizontal Circular Cylinder in Thermally Stratified Medium

The present work emphasizes the significance of oscillatory mixed convection stratified fluid and heat transfer characteristics at different stations of non-conducting horizontally circular cylinder in the presence of thermally stratified medium. To remove the difficulties in illustrating the coupled PDE’s, the finite-difference scheme with efficient primitive-variable formulation is proposed to transform dimensionless equations. The numerical simulations of coupled non-dimensional equations are computed in terms velocity of fluid, temperature and magnetic field which are computed to examine the fluctuating components of skin friction, heat transfer and current density for various emerging parameters. The governing parameters namely, thermally stratification parameter     

Sc数对湍流被动标量扩散的影响

对无剪切湍流混合层中被动标量的扩散进行了直接数值模拟。结合烟粒子扩散风洞实验研究了不同Sc数对被动标量扩散的影响。DNS中被动标量的Sc数取为0.2和3。平均标量场基本上与Sc数无关,脉动标量方差随着Sc数的增加而增加,当Sc>1时,无量纲的质量通量与Sc数基本无关,而Sc=0.2时的质量通量与Sc>1的质量通量有较大的差别。     

An investigation of the Prandtl number effect on turbulent heat transfer in channel flows by large eddy simulation

Summary Fully developed turbulent channel flow with passive heat transfer has been calculated to investigate the turbulent heat transfer by use of the large eddy simulation (LES) approach coupled with dynamic subgrid-scale (SGS) models. The objectives of this study are to examine the effectiveness of the LES technique for predicting the turbulent heat transfer at high Prandtl numbers and the effects of the Prandtl number on the turbulent heat transfer in a fully developed turbulent channel flow. In the present study, the Prandtl number is chosen as 0.1 to 200, and the Reynolds number, based on the central mean velocity and the half-width of the channel, is 10⁴. Some typical cases are computed and compared with available data obtained by direct numerical simulation (DNS), theoretical analysis and experimental measurement, respectively, which confirm that the present approach can be used to predict the heat transfer satisfactorily, even at high Prandtl numbers. To depict the effect of the Prandtl number on turbulent heat transfer, the distributions of mean value and fluctuation of resolved flow temperatures, the heat transfer coefficient, turbulent heat fluxes, and some instantaneous iso-thermal sketches are analyzed.     

稳定线性热分层环境下火灾烟气羽流=积分解及实验分析

对稳定线性热分层环境下湍浮力羽流无量纲积分方程首次采用数值解法进行求解，得出浮力通量、动量通量等参量在轴线上的变化规律以及羽流中性浮力点的高度和羽流最大上升高度。2种不同尺度空间及不同烟气羽流出口条件的实验表明，实际火灾烟气羽流最大上升高度与理论计算结果及文献经验公式相符，从而表明积分模型数值解的有效性，并为高大空间火灾烟气输运规律研究及探测技术发展提供了理论及实验依据。     

Numerical investigation of supercritical LNG convective heat transfer in a horizontal serpentine tube

The submerged combustion vaporizer (SCV) is indispensable general equipment for liquefied natural gas (LNG) receiving terminals. In this paper, numerical simulation was conducted to get insight into the flow and heat transfer characteristics of supercritical LNG on the tube-side of SCV. The SST model with enhanced wall treatment method was utilized to handle the coupled wall-to-LNG heat transfer. The thermal–physical properties of LNG under supercritical pressure were used for this study. After the validation of model and method, the effects of mass flux, outer wall temperature and inlet pressure on the heat transfer behaviors were discussed in detail. Then the non-uniformity heat transfer mechanism of supercritical LNG and effect of natural convection due to buoyancy change in the tube was discussed based on the numerical results. Moreover, different flow and heat transfer characteristics inside the bend tube sections were also analyzed. The obtained numerical results showed that the local surface heat transfer coefficient attained its peak value when the bulk LNG temperature approached the so-called pseudo-critical temperature. Higher mass flux could eliminate the heat transfer deteriorations due to the increase of turbulent diffusion. An increase of outer wall temperature had a significant influence on diminishing heat transfer ability of LNG. The maximum surface heat transfer coefficient strongly depended on inlet pressure. Bend tube sections could enhance the heat transfer due to secondary flow phenomenon. Furthermore, based on the current simulation results, a new dimensionless, semi-theoretical empirical correlation was developed for supercritical LNG convective heat transfer in a horizontal serpentine tube. The paper provided the mechanism of heat transfer for the design of high-efficiency SCV.     

A numerical analysis of passive scalar transport in turbulent shear flows

Abstract

The development of the formation and vortex pairing process in a two‐dimensional shear flow and the associated passive scalar (mass concentration or energy) transport process was numerically simulated by using the Vortex‐in‐Cell (VIC) Method combined with the Upwind Finite Difference Method. The visualized temporal distributions of passive scalars resemble the vortex structures and the turbulent passive scalar fluxes showed a definite connection with the occurrence of entrainment during the formation and pairing interaction of large‐scale vortex structures. The profiles of spatial‐averaged passive scalar ø, turbulent passive scalar fluxes, u'ø’ and v'ø’, turbulent diffusivity of mean‐squared scalar fluctuation, v'ø‘ ², mean‐squared turbulent passive scalar fluctuation, √ø‘ ², skewness, and flatness factor of the probability density function of scalar fluctuation ø’ at three different times are calculated. With the lateral dimension scaled by the momentum thickness and the velocity scaled by the velocity difference across the shear layer, these profiles were shown to be self‐preserved. The probability density function of turbulent scalar fluctuation was found to be asymmetric and double‐peaked.     

On the theory of turbulent heat and mass transfer with allowance for intermittence effects

A new viewpoint of the mechanism of turbulent heat and mass transfer has been proposed; a mathematical apparatus of statistical theory and a method of construction of mathematical models under the conditions of intermittent dynamic and scalar fields of nonuniform turbulent flow have been developed. A model of a turbulent axisymmetric jet for evaluation testing of the method has been constructed and the conditional and total means of the basic statistical characteristics of the jet have been calculated. It has been shown that the reason for the “different mechanism” of turbulent transfer of momentum, heat, and substance is the intermittence of not coincident dynamic and scalar fields of turbulent flow. An equation for the conditional probability density function of the concentration of a passive impurity has been derived and its solution for the axisymmetric jet in a strongly intermittent region has been given. Good agreement between the performed calculations and experimental data confirms the adopted concept of the transfer mechanism.     

Multicomponent heat and mass transfer during the turbulent flow of liquid films

A method is proposed for calculating the parameters of simultaneous heat and mass transfer in turbulent multicomponent liquid films which is based on solving the system of differential equations for convective heat conduction and multicomponent convective diffusion.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 1, pp. 16–22, July, 1989.     

Numerical modeling of the aerodynamics, heat exchange, and combustion of a polydisperse ensemble of coke-ash particles in ascending axisymmetric two-phase flow

A two-dimensional stationary model of motion, heat and mass exchange, and chemical reaction of polydisperse coke and ash particles in ascending gas-suspension flow has been constructed with allowance for the turbulent and pseudoturbulent mechanisms of transfer in the dispersed phase. The system of equations that describes motion and heat transfer in the solid phase has been closed at the level of the equations for the second moments of velocity and temperature pulsations, whereas the momentum equations of the carrying medium have been closed using the equation for turbulent gas energy, which allows for the influence of the particles and heterogeneous reactions.     

Experimental investigation of ice slurry heat transfer in horizontal tube

Heat transfer of ice slurry flow based on ethanol–water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 °C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus–Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. It was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus–Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re_cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results.     

Formation and Transport of Atomic Hydrogen in Hot-Filament Chemical Vapor Deposition Reactors

In this paper we focus on diamond film hot-filament chemical vapor deposition reactors where the only reactant is hydrogen so as to study the formation and transport of hydrogen atoms. Analysis of dimensionless numbers for heat and mass transfer reveals that thermal conduction and diffusion are the dominant mechanisms for gas-phase heat and mass transfer, respectively. A simplified model has been established to simulate gas-phase temperature and H concentration distributions between the filament and the substrate. Examination of the relative importance of homogeneous and heterogeneous production of H atoms indicates that filament-surface decomposition of molecular hydrogen is the dominant source of H and gas-phase reaction plays a negligible role. The filament-surface dissociation rates of H2 for various filament temperatures were calculated to match H-atom concentrations observed in the literature or derived from power consumption by filaments. Arrhenius plots of the filament-surface hydrogen dissociation rates suggest that dissociation of H2 at refractory filament surface is a catalytic process, which has a rather lower effective activation energy than homogeneous thermal dissociation. Atomic hydrogen, acting as an important heat transfer medium to heat the substrate, can freely diffuse from the filament to the substrate without recombination.     

Materials selection for thermal comfort in passive solar buildings

This paper presents the results of a combined analytical, computational, and experimental study of the key parameters for selecting affordable materials and designing for thermal comfort in passive solar buildings. The heat transfer across the walls of buildings is modeled using a simple heat diffusion model. In this way, the passive heat storage from the sun (passive solar) and the heat load from internal heat sources are stored in the walls of buildings that provide internal cooling during the day and internal heating at night. The simple analytical model of heat diffusion is used to identify the merit indices for the optimization of affordable passive solar performance. The time dependence of wall/internal temperature is then simulated using a simple finite difference model. The results from the analytical model and finite difference model are validated by conducting temperature measurements in two affordable housing complexes in Egypt. The implications of the results are then discussed for the design of thermal comfort in affordable housing.     

Nonsteady evaporation and growth of drops in gas(EOUS) medium

The process of heat and mass transfer during the evaporation or condensational growth of single drop in gas is investigated in quasistatic approach. The convection and diffusion in gas and heat conductivity in gas and drop are taken into account. For single drop in infinite gas medium the new analytical solution, which describes the asymptotic quasi-steady regime of phase transfer is obtained. There is the discussion of the numerically obtained distributions of the temperature and concentration and the laws of phase transfer and pressure change for particle in finite and infinite medium.     

Heat and mass transfer processes in two phase He II/vapor

Modeling heat and mass transfer characteristics of two phase He II is discussed. The case considered assumes that the channel flow is one-dimensional and stratified, with mass exchange between the two phases. Two specific examples are considered in some detail. The first is the heat and mass transfer characteristics for small liquid flow rate. Use of several simplifying assumptions allows the problem to be reduced to solution of a one-dimensional ordinary differential equation. The result is a non-dimensional expression for the liquid level or void fraction along the channel. A set of dimensionless parameters are defined that establish the relative contributions of vapor mass transport and counterflow in the He II. The model also predicts the temperature profile and vapor mass flow rate. The second case concerns the flow of liquid under nearly isothermal conditions with relatively small vapor mass flow rate. Under these conditions, the flow may be modeled using classical hydrodynamics taking into consideration the unique characteristics of the He II. Results of these models are compared to experimental data for heat and mass transfer in a two phase He II/vapor flow.     

CFD model of ITER CICC. Part VI: Heat and mass transfer between cable region and central channel

Dual-channel cable-in-conduit conductors (CICC) are used in the superconducting magnets for the International Thermonuclear Experimental Reactor (ITER). As the CICC axial/transverse size ratio is typically ∼1000, 1D axial models are customarily used for the CICC, but they require constitutive relations for the transverse fluxes. A novel approach, based on Computational Fluid Dynamics (CFD), was recently proposed by these authors to understand the complex transverse thermal-hydraulic processes in an ITER CICC from first principles. Multidimensional (2D, 3D) Reynolds-Averaged Navier-Stokes models implemented in the commercial CFD code FLUENT were validated against compact heat exchanger and ITER-relevant experimental data, and applied to compute the friction factor and the heat transfer coefficient in fully turbulent spiral rib-roughened pipes, mimicking the central channel of an ITER CICC. That analysis is extended here to the problem of heat and mass transfer through the perforated spiral separating the central channel from the cable bundle region, by combining the previously developed central channel model with a porous medium model for the cable region. The resulting 2D model is used to analyze several key features of the transport processes occurring between the two regions including the relation between transverse mass transfer and transverse pressure drop, the influence of transverse mass transfer on axial pressure drop, and the heat transfer coefficient between central channel and annular cable bundle region.     

Experimental investigation on TBAB clathrate hydrate slurry flows in a horizontal tube: Forced convective heat transfer behaviors

Tetra-n-butyl-ammonium bromide (TBAB) clathrate hydrate slurry (CHS) is one kind of secondary refrigerants, which is promising to be applied into air-conditioning or latent-heat transportation systems as a thermal storage or cold carrying medium for energy saving. It is a solid–liquid two phase mixture which is easy to produce and has high latent heat and good fluidity. In this paper, the heat transfer characteristics of TBAB slurry were investigated in a horizontal stainless steel tube under different solid mass fractions and flow velocities with constant heat flux. One velocity region of weakened heat transfer was found. Moreover, TBAB CHS was treated as a kind of Bingham fluids, and the influences of the solid particles, flow velocity and types of flow on the forced convective heat transfer coefficients of TBAB CHS were investigated. At last, criterial correlations of Nusselt number for laminar and turbulent flows in the form of power function were summarized, and the error with experimental results was within ±20%.     

Symmetry-Preserving Discretization of Heat Transfer in a Complex Turbulent Flow

Convective and diffusive operators are discretized such that their symmetries are preserved. The resulting discretization inherits all symmetry-related properties of the continuous formulation. It is shown that a symmetry-preserving discretization is unconditionally stable and conservative. A fourth-order, symmetry-preserving discretization method is developed and tested for the numerical simulation of turbulent (flow and) heat transfer in a channel with surface-mounted cubes, where the temperature is treated as a passive scalar. The Reynolds number (based on the channel width and the mean bulk velocity) is Re=13,000. The results of the numerical simulation agree well with available experimental data.     

Calculation of radiative-convective heat transfer in the combustor of diesel engine

An integral method is suggested for the calculation of local heat transfer in the cylinder of diesel engine in view of radiation-convection interaction. The turbulent boundary layer is investigated on the combustor surface of arbitrary curvature subjected to the flow of a working medium which radiates and absorbs thermal energy. The thermal radiation transfer is described within the Schwarzschild model. Integral relations of boundary layer are used for solving equations of complex heat transfer. The integral characteristics of radiative heat transfer in the combustor of a diesel engine are determined (emissivity factor of the working medium, Bouguer and Boltzmann numbers, attenuation factor, and coefficient allowing for the diffuseness of radiation). The calculated values of local radiative-convective heat flux on the combustor surface are in good agreement with the experimental data obtained in a high-speed diesel engine under conditions of bench tests.     

Heat Exchange between an Infiltrated Granular Bed and a Surface

Comparative analysis of the existing relations for calculation of convective heat exchange between an infiltrated granular bed and surfaces of different geometry has been carried out. It is shown that different characteristic scales of the process of heat and mass transfer correspond to laminar and turbulent modes of filtrational flow. The occurrence of an analogy with heat transfer of a singlephase medium in a laminar mode has been found. A correlation for calculation of heat exchange between a vertical cylinder and an infiltrated granular bed has been obtained.