Mechanism Of Turbulent Heat Transfer Research Articles

On the mechanism of turbulent heat transfer in composite porous-fluid systems with finite length porous blocks: Effect of porosity and Reynolds number

The majority of literature studies on composite porous-fluid systems involve fully-developed porous channel flows where the porous media covers the whole length of the channel. These studies utilized periodic boundary conditions at the inlet and outlet. In these systems, the stagnation at the frontal face of the porous block, turbulent separation bubble over the porous-fluid interface, and flow leakage from the porous to non-porous regions do not exist. The existence of these flow features in the case of a finite porous block immersed in a channel flow modifies turbulent interactions across the porous-fluid interface. In contrast to the previous studies, this paper investigates the flow and thermal characteristics of turbulent channel flow containing a porous block with a finite length. To this end, pore-scale large eddy simulations are performed in composite porous-fluid systems with two porosities (53% and 91%) at three Reynolds numbers of 3600, 7200 and 14400. Flow visualization shows that two distinct regions are formed over the interface in low-porosity cases: Region#1 near the leading edge with organised hairpin structures and high flow leakage; Region#2 away from the leading edge with unorganised hairpin structures and lower flow leakage. In region#1, maximum turbulent fluctuations occur far away from the interface while they approach the interface in region#2. The results showed that by increasing either the Reynolds number or porous length, the location of maximum turbulence statistics approaches the interface. This observation supports earlier findings for fully-developed porous channel flows which are only valid in region#2. Whereas, with a low Reynolds number or a short porous length, the turbulent statistics peak far from the interface, consistent with the observations in region#1. Besides, it was found that increasing the porosity and Reynolds number reduces the flow leakage (from the porous region to the non-porous region) up to 50% and 10%, respectively, which in turn disrupts the patterns of contour-rotating vortex pairs and hairpin structures over the interface. It is further found that for a fixed Reynolds number, the overall Nusselt number for the high-porosity case is 2.6 times higher than that of the low-porosity case. The pressure drop for the low-porosity cases is 1.8 times more than that for the high-porosity cases.

On the Mechanism of Turbulent Heat Transfer in Composite Porous-Fluid Systems with Finite Length Porous Blocks: Effect of Porosity and Reynolds Number

On the Mechanism of Turbulent Heat Transfer in Composite Porous-Fluid Systems with Finite Length Porous Blocks: Effect of Porosity and Reynolds Number

Structures and mechanism of heat transfer phenomena in turbulent boundary layer with separation and reattachment via DNS

This paper presents observations and investigations of the detailed structure and mechanism of turbulent heat transfer in the turbulent boundary layer with separation and reattachment by means of direct numerical simulation (DNS). In order to observe turbulent heat transfer in a boundary layer with reattachment and separation, a DNS of the boundary layer with heat transfer over a 2-dimensional block (2DB) is carried out, in which the effects of Reynolds number and block size are observed. The lengths of reattachment and maximum Stanton number points behind 2DB become longer with an increase in Reynolds number in the case of similar block size with one exception. On the other hand, these points become shorter with an increase in the width of the 2DB. Moreover, the counter gradient diffusion phenomenon (CDP) of the thermal field can be found on the 2DB. A quadrant analysis is carried out to investigate the turbulence motion which decides Reynolds shear stress and the wall-normal turbulent heat flux in the turbulent boundary layer with heat transfer over 2DB, in which it can be found that Q1 and Q3 events (i.e., interactions) affect the occurrence of CDP on 2DB. Also, the correlations among instantaneous fluctuating temperature, temperature gradient and vorticities to observe the turbulent structures of heat transfer around 2DB are clearly shown in the present DNS.

Theoretical models of the height of the atmospheric boundary layer and turbulent entrainment at its upper boundary

The planetary boundary layer (PBL), which directly interacts with the underlying surface, differs significantly in its nature from the low-turbulent stably stratified free atmosphere. Fluctuations of the Earth’s surface heat balance immediately affect the PBL and assimilate there owing to the effective mechanism of turbulent heat transfer. In this case the upper boundary of the PBL plays the role of a cover, preventing the direct penetration of thermal effects and contaminants into an overlying atmospheric layer. In view of this, air pollution is especially dangerous when associated with shallow PBL. In addition, local peculiarities of climate change are mainly determined by the PBL height due to the high sensitivity of thin stably stratified PBLs to the thermal effects. Deep convective PBLs are not very sensitive to weak thermal effects, but they significantly affect the formation of convective cloudiness and the climate system as a whole by means of the turbulent entrainment of the thermal energy, humidity, aerosols, and other admixtures through the upper boundary. The PBL height and turbulent entrainment must be calculated when simulating and forecasting air pollution, abnormal frosts and heat, and other hazardous phenomena. In this paper we discuss the state-of-the-art knowledge in the area of PBL height simulation and suggest a new model of turbulent entrainment for convective PBLs.

A physical picture of the mechanism of turbulent heat transfer from the wall

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.

Countergradient heat transfer in the atmospheric boundary layer over a rough surface

The nonlocality of the mechanism of turbulent heat transfer in the atmospheric boundary layer over a rough surface manifests itself in the form of bounded areas of countergradient heat transfer, which are diagnosed from analysis of balance items in the transport equation for the variance of temperature fluctuations and from calculation of the coefficients of turbulent momentum and heat transfer invoking the model of “gradient diffusion.” It is shown that countergradient heat transfer in local regions is caused by turbulent diffusion or by the term of the divergence of triple correlation in the balance equation for the temperature variance.

Direct numerical simulation of turbulent heat transfer modulation in micro-dispersed channel flow

The object of this paper is to study the influence of dispersed micrometer size particles on turbulent heat transfer mechanisms in wall-bounded flows. The strategic target of the current research is to set up a methodology to size and design new-concept heat transfer fluids with properties given by those of the base fluid modulated by the presence of dynamically-interacting, suitably-chosen, discrete micro- and nano- particles. We run Direct Numerical Simulation (DNS) for hydrodynamically fully-developed, thermally-developing turbulent channel flow at shear Reynolds number Re=150 and Prandtl number Pr=3, and we tracked two large swarms of particles, characterized by different inertia and thermal inertia. Preliminary results on velocity and temperature statistics for both phases show that, with respect to single-phase flow, heat transfer fluxes at the walls increase by roughly 2% when the flow is laden with the smaller particles, which exhibit a rather persistent stability against non-homogeneous distribution and near-wall concentration. An opposite trend (slight heat transfer flux decrease) is observed when the larger particles are dispersed into the flow. These results are consistent with previous experimental findings and are discussed in the frame of the current research activities in the field. Future developments are also outlined.

F223 二次元曲がり流路内の二重スケール乱流伝熱構造に関する研究(乱流・DNS)

F223 二次元曲がり流路内の二重スケール乱流伝熱構造に関する研究(乱流・DNS)

Simultaneous measurements of velocity and temperature fluctuations in thermal boundary layer in a drag-reducing surfactant solution flow

The mechanism of turbulent heat transfer in the thermal boundary layer developing in the channel flow of a drag-reducing surfactant solution was studied experimentally. A two-component laser Doppler velocimetry and a fine-wire thermocouple probe were used to measure the velocity and temperature fluctuations simultaneously. Two layers of thermal field were found: a high heat resistance layer with a high temperature gradient, and a layer with a small or even zero temperature gradient. The peak value of \( \overline{{u^{ + } \theta ^{ + } }} \) was larger for the flow with the drag-reducing additives than for the Newtonian flow, and the peak location was away from the wall. The profile of \( - \overline{{v^{ + } \theta ^{ + } }} \) was depressed in a similar manner to the depression of the profile of \( - \overline{{u^{ + } v^{ + } }} \) in the flow of the surfactant solution, i.e., decorrelation between v and θ compared with decorrelation between u and v. The depression of the Reynolds shear stress resulted in drag reduction; similarly, it was conjectured that the heat transfer reduction is due to the decrease in the turbulent heat flux in the wall-normal direction for a flow with drag-reducing surfactant additives.

A new turbulence model for predicting fluid flow and heat transfer in separating and reattaching flows—I. Flow field calculations

To calculate complex turbulent flows with separation and heat transfer, we have developed a new turbulence model for flow field, which is modified from the latest low-Reynolds-number k−g3 model. The main improvement is achieved by the introduction of the Kolmogorov velocity scale, uc &Z.tbnd; (vε)14, instead of the friction velocity uτ, to account for the near-wall and low-Reynolds-number effects in both attached and detached flows. The present model predicts quite successfully the separating and reattaching flows downstream of a backward-facing step, which involve most of the essential physics of complex turbulent flows, under various flow conditions. We have also discussed in detail the structure of the separating and reattaching flow based on the computational results, and presented several important features closely related to the mechanism of turbulent heat transfer.

Microscopic Structures in Turbulent Diffusion Flames.

Microscopic structures in turbulent diffusion flames are studied by time-resolved temperature distributions measured by a laser-sheet-illuminated Rayleigh scattering (LRS) method recorded by a high-speed VTR system, and one-point LRS measurement. The microscopic structures of temperature distribution are measured by analyzing the two-dimensional LRS pictures by image processing. Coaxial turbulent diffusion flames at moderate Reynolds numbers, which exhibit typical diffusion flame structures, are formed on laboratory-scale burners. It is found that the flame can be divided into four characteristic regions based on the distributions of macroscale temperature fluctuations. These four regions are visualized by the two-dimensional LRS images. The turbulent heat-transfer mechanisms in these four regions are discussed in terms of the two-dimensional LRS and the power spectral density of temperature fluctuations measured by one-point LRS. Clusters of temperature inhomogeneity are observed by the image analyses in Regions I and III. It is found that different structures of microscopic temperature inhomogeneity exist within Taylor's dissipation length scale defined by velocity fluctuations.

Microscopic Structure of Turbulent Diffusion Flames.

Microscopic structure of turbulent diffusion flames is studied by a time-resolved planar video picture system obtained by a laser Rayleigh scattering (LRS) method and a single-point LRS measurement. The microscopic temperature is measured by using a two-dimensional LRS signal and image processing. Coaxial turbulent diffusion flames at moderate Reynolds numbers, which exhibit typical diffusion flame structures, are formed on laboratory-scale burners. It is found that the flame can be divided into four typical regions characterized by the distribution of macroscale of temperature fluctuations. These four regions are visualized by the two-dimensional LRS pictures. The turbulent heat transfer mechanisms in these four regions are discussed in terms of the two-dimensional LRS and the power spectral density of temperature fluctuations, in terms of one-point LRS. A cluster of temperature inhomogeneity is observed by cluster analysis in Regions I and III. It is found that different structures of microscopic inhomogeneity in the mixing region of Taylor's dissipation length scale appear, corresponding to the characteristics of mixing and combustion mechanisms in each region.

乱流の伝熱機構

乱流の伝熱機構

Turbulent heat transfer in liquid metals-fully developed pipe flow with constant wall temperature

This paper is a continuation of the authors' earlier work on a mechanism of turbulent heat transfer in liquid metals [1]. Nusselt number and temperature profile for low Prandtl number fluids of constant properties flowing in a smooth pipe with constant wall temperature have been evaluated. Use is made of the theoretical expression for the ratio of eddy diffusivities for heat and momentum deduced in [1]. For practical calculation of film coefficient of heat transfer, an interpolation formula is proposed: N Nu = 5 + 0.05 N Pr 0.25 N Pe 0.77 which fits the calculated data with a maximum deviation of less than 11 per cent for N Pr < 0.1 and N pe < 15 000. Temperature profiles for several Prandtl and Reynolds numbers were compared with the case of constant wall flux.

A mechanism of turbulent heat transfer in liquid metals

A simplified mechanism of turbulent heat transfer, based on a modification of Prandtl's mixing-length hypothesis, has been proposed. It is assumed that there is a continuous change of momentum and energy during the flight of the eddy. Two expressions giving the ratio of eddy diffusivities for heat and momentum were obtained for fully developed pipe flow. One is for fluids of Prandtl number ranging from 0·6 to 15 and the other for liquid metals. Both correctly predict the influence of Reynolds number, Prandtl number and radial location across the pipe on the diffusivity ratio when compared to trends revealed by limited published data. Computation of Nusselt number and temperature profile in liquid metals were carried out under conditions of constant wall flux using the deduced expression for diffusivity ratio. They agree well with experimental results. For practical calculation of film coefficient of heat transfer, the following interpolation formula may be used: N Nu = 7 + 0·05 N Pr 0.25 N Pe 0.77 which fits the calculated data with a maximum deviation of less than 12 per cent for N Pr < 0·1 and N Pe < 15,000. Limiting values of Nusselt number as N Pr → 0 and N Re → ∞ were discussed.