Turbulent Transfer Coefficients Research Articles

Utility of Assimilating Surface Radiometric Temperature Observations for Evaporative Fraction and Heat Transfer Coefficient Retrieval

Recent advances in land data assimilation have yielded variational smoother techniques designed to solve the surface energy balance based on remote observations of surface radiometric temperature. These approaches have a number of potential advantages over existing diagnostic models, including the ability to make energy flux predictions between observation times and reduced requirements for ancillary parameter estimation. Here, the performance of a recently developed variational smoother approach is examined in detail over a range of vegetative and hydrological conditions in the southern U.S.A. during the middle part of the growing season. Smoother results are compared with flux tower observations and energy balance predictions obtained from the two-source energy balance model (TSM). The variational approach demonstrates promise for flux retrievals at dry and lightly vegetated sites. However, results suggest that the simultaneous retrieval of both evaporative fraction and turbulent transfer coefficients by the variational approach will be difficult for wet and/or heavily vegetated land surfaces. Additional land surface information (e.g. leaf area index (LAI) or the rough specification of evaporative fraction bounds) will be required to ensure robust predictions under such conditions. The single-source nature of the variational approach also hampers the physical interpretation of turbulent transfer coefficient retrievals. Intercomparisons between energy flux predictions from the variational approach and the purely diagnostic TSM demonstrate that the relative accuracy of each approach is contingent on surface conditions and the accuracy with which LAI values required by the TSM can be estimated.

A SIMPLE 3-DIMENSIONAL URBAN ENERGY BALANCE MODEL AND OUTDOOR SCALE MODEL EXPERIMENTS

A Simple 3-dimensional Urban canopy Model for Meso-scale simulation (SUMM) was proposed. The model predicts the energy balance of constituent faces (roof, floor and 4 vertical walls) without time-consuming iterations. In the model, complex 3-dimensional radiation processes (multi-reflection processes) are theoretically solved, resulting in a large reduction of computational time. As for the turbulent transfers, the network of resistance is adopted. We also conducted outdoor scale model experiments having the similar geometric structure with SUMM, to construct the databases (energy balances and surface temperatures) for model validation, and to examine unknown model parameters (e. g. turbulent transfer coefficients).

Turbulent Transfer Coefficients and Calculation of Air Temperature inside Tall Grass Canopies in Land–Atmosphere Schemes for Environmental Modeling

Abstract A method for estimating profiles of turbulent transfer coefficients inside a vegetation canopy and their use in calculating the air temperature inside tall grass canopies in land surface schemes for environmental modeling is presented. The proposed method, based on K theory, is assessed using data measured in a maize canopy. The air temperature inside the canopy is determined diagnostically by a method based on detailed consideration of 1) calculations of turbulent fluxes, 2) the shape of the wind and turbulent transfer coefficient profiles, and 3) calculation of the aerodynamic resistances inside tall grass canopies. An expression for calculating the turbulent transfer coefficient inside sparse tall grass canopies is also suggested, including modification of the corresponding equation for the wind profile inside the canopy. The proposed calculations of K-theory parameters are tested using the Land–Air Parameterization Scheme (LAPS). Model outputs of air temperature inside the canopy for 8–17 July 2002 are compared with micrometeorological measurements inside a sunflower field at the Rimski Sancevi experimental site (Serbia). To demonstrate how changes in the specification of canopy density affect the simulation of air temperature inside tall grass canopies and, thus, alter the growth of PBL height, numerical experiments are performed with LAPS coupled with a one-dimensional PBL model over a sunflower field. To examine how the turbulent transfer coefficient inside tall grass canopies over a large domain represents the influence of the underlying surface on the air layer above, sensitivity tests are performed using a coupled system consisting of the NCEP Nonhydrostatic Mesoscale Model and LAPS.

An Empirical Relation Describing Leaf-Area Density inside the Forest for Environmental Modeling

Abstract The spatial distribution of the leaf-area density is a key parameter in describing the forest canopy characteristics, which has a strong impact on its radiation balance and the mass and energy exchange with the atmosphere as well. The objective of this short study is to define an empirical relation describing the vertical distribution of leaf-area density that can be used for an improved estimation of the turbulent transfer coefficient inside the forest as well as above it. To check the validity of the method proposed, the calculated values are compared with the observations using datasets from eight observational sites located in four different types of forest, covering a broad range of mean values of leaf-area indices between 2 and 18.

The Difference in Turbulent Diffusion between the Active and Passive Scalars in a Thermally Stably Stratified Medium

The difference in the turbulent diffusion between the active (heat) and passive (mass) scalars in a thermally stably stratified medium is investigated. The axisymmetric problem is treated on the formation of a turbulent circulation flow above a heated disk and on the turbulent diffusion of a passive scalar (impurity) from a continuous surface source in a stably stratified medium. The results indicate that the thermal stratification causes appreciable differences in the coefficients of turbulent transfer between the active (heat) and passive (mass) scalars. This means that the assumption of the identity of the coefficient of turbulent diffusion of heat and mass, employed in conventional models of turbulence, produces significant errors in estimating the heat and mass transfer in a thermally stably stratified medium.

SIMULATION OF SWIRLING TURBULENT FLOWS AND HEAT TRANSFER IN AN ANNULAR DUCT

The article presents a numerical simulation of swirling turbulent flows and heat transfer in an annular duct. The time-averaged governing equations are solved, which are closed by a new algebraic Reynolds stress model (ASM). The simulation is performed under different flow conditions. The calculated results of gas axial and tangential velocities, turbulent kinetic energy, temperature, and local heat transfer coefficients on the inner and outer walls of the annulus are provided. They illustrate the effect of swirl number, inlet axial velocity, and ratio of inner to outer radius on the mean flow and turbulence properties, as well as on enhancing heat transfer in the annular duct.

Wind profile within the forest canopy and in the transition layer above it

The forest as an underlying surface has to be considered in atmospheric models of different scales. Experimental evidence shows that there can be a significant variation of the wind profile within the forest and in the so-called transition layer above it. Experimentally observed wind speed is often found below as indicated by the wind speed profile obtained by: (a) the logarithmic relationship in the transition layer and (b) K theory within the forest. This situation can seriously disturb the real physical picture concerning the transfer of momentum, heat and water vapour from the surface into the atmosphere. In order to minimise the foregoing problems, we have suggested an empirical expression for the wind profile in the transition layer above the forest as well as the expressions for the wind profile and turbulent momentum transfer coefficient within the forest canopy layer. Additionally, for the proposed wind profiles, the expressions for the displacement height, roughness length and parameters are determined as functions of the forest structural characteristics using continuity conditions and a simple mass conservation hypothesis. The validity of the proposed expressions was checked using the micrometeorological measurements from the experimental sites in the Thetford Scots pine forest in Norfolk, United Kingdom and in the Ponderosa pine forest at the Shasta Experimental Forest, California, USA.

Enhancement of natural convective heat transfer from tall, vertical heated plate to water

AbstractAn enhancement technique was developed for natural convection heat transfer from a tall, vertical heated plate to water. Rectangular grid fins attached to the base plate were utilized as a heat transfer promoter. These grid fins redirect the high‐temperature fluid ascending along the base plate toward the outside of the boundary layer and introduce the low‐temperature ambient fluid toward the base plate instead. The heat transfer coefficients of thus‐treated surfaces were measured and compared with a nontreated surface and a surface with conventional vertical plate‐fins. The highest performance was achieved for the experimental surfaces. In particular, the experimental surfaces with 5‐mm‐high, nonconducting grids and with 10‐mm‐high, conducting grid fins show 27% and 80% higher heat transfer coefficients compared to the turbulent heat transfer coefficients of the nontreated surface, respectively. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(2): 178–190, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10083

A113 核破砕中性子源用減速材容器内の衝突噴流挙動

A cold moderator using liquid hydrogen is one of the key components in a MW-scale spallation target system, which is cooled down by using a impinging jet flow. Flow visualization experi-ments with a simulated moderator vessel with cylindrical configuration were carried out in order to clarify flow patterns in the cold moderator vessel, which showed jet induced flows such as recirculation flows and stagnant regions. Analytical results of velocity distributions ob-tained with the standard k-s turbulent model agreed well with experimental results obtained under the bight and diameter ratio (h/D) of 2.0. However, analytical turbulent energy and heat transfer coefficients of a bottom of the vessel could not predict experimental values well.

Turbulent forced convection in microtubes

The anomalous and opposing trends in the published data dealing with turbulent flow friction factor and heat transfer coefficient in microchannels, and their apparent disagreement with macroscale correlations, are discussed. It is shown that the modification of turbulent eddy diffusivities, consistent with the way suspended particles may modify turbulence, can explain the observed higher-than-expected heat transfer coefficients in some data. It is thus suggested that suspended microscopic particles may be a major contributor to the aforementioned inconsistencies and disagreements in some of the published data.

A physical basis for a maximum of thermodynamic dissipation of the climate system

A mechanism is proposed by which the energy flow through a turbulent medium might be constrained to maximize its dissipation or (equivalently) its thermodynamic efficiency. The mechanism may provide a physical basis for the various findings over the years that the earth-atmosphere system has adopted a format which maximizes its overall rate of entropy production. The qualitative picture is of a system which, because of the asymmetry of its turbulent fluctuations about the locus of possible steady states determined by energy balance, moves to a preferred steady state and therefore to a preferred turbulent transfer coefficient.

冷減速材容器設計のための衝突噴流実験及び解析

A cold moderator using liquid hydrogen is one of the key components in a MW-scale spallation target system, which directly affects the neutronic performance both in intensity and resolution. Flow visualization experiments were carried out in order to clarified a flow field in a cold moderator vessel, the impinging jet induced flows such as recirculation flows and stagnant regions were verified in a moderator vessel. As for a cylindrical moderator vessel, analytical results, such as turbulent energy distribution and heat transfer coefficients, obtained with a non-linear turbulent model predicted experimental results obtained with a PIV system better than those with other turbulent models.

Turbulent Transport Coefficients with Allowance for Density Fluctuations

A model of turbulent transfer processes modified to take into account the effect of density fluctuations is proposed. The model is used for finding expressions for the turbulent transfer coefficients (viscosity, thermal conductivity, and diffusion of admixtures) in flows of the boundary-layer type in the presence of small density gradients. The turbulent Prandtl number distribution in the far wake behind a heated cylinder is compared with the results of measurements.

A physical basis for a maximum of thermodynamic dissipation of the climate system

AbstractA mechanism is proposed by which the energy flow through a turbulent medium might be constrained to maximize its dissipation or (equivalently) its thermodynamic efficiency. The mechanism may provide a physical basis for the various findings over the years that the earth‐atmosphere system has adopted a format which maximizes its overall rate of entropy production. The qualitative picture is of a system which, because of the asymmetry of its turbulent fluctuations about the locus of possible steady states determined by energy balance, moves to a preferred steady state and therefore to a preferred turbulent transfer coefficient.

Forced convective heat transfer in square-ribbed coolant channels with helium gas for fusion power reactors

Square-ribbed coolant channels with helium gas for fusion power reactors were designed to enhance the turbulent heat transfer in comparison with smooth coolant channels. Heat transfer coefficients and friction factors in the square-ribbed annular channels were investigated quantitatively under high pressure. The turbulent heat transfer coefficients in the square-ribbed annulus, with a ratio of the rib pitch to a height of ten, were 200–300% higher than those in the smooth annulus and then the thermal–hydraulic empirical correlations were derived. The average heat fluxes from the heated wall were predicted using the present empirical correlations and the maximum heat flux at p/ h=10 reached ≈1.5 MW/m 2 at Re=65 000. From the results of the present study, it can be expected that the proposed rib-roughened coolant channels are available to remove surface heat fluxes at the first wall and blanket in the fusion power reactors.

Enhancement of Natural Convective Heat Transfer from Tall, Vertical Heated Plates.

An enhancement technique was developed for natural convection heat transfer from tall, vertical heated plate to water. Rectangular grid fins attached to the base plate were utilized as a heat transfer promoter. These grid fins redirect the high-temperature fluid ascending along the base plate toward the outside of the boundary layer and introduce the low-temperature ambient fluid toward the base plate instead. The heat transfer coefficients of thus-treated surfaces were measured and compared with a non-treated, fiat surface and a conventional surface with vertical straight fins. The highest performance was achieved for the experimental surfaces. In particular, the experimental surfaces with 5 mm-high, non-conducting grid fins and with 10 mm-high, conducting grid fins show 27% and 80% higher heat transfer coefficients compared to the turbulent heat transfer coefficients of the non-treated, fiat surface, respectively.

Measurement of Turbulent Heat Transfer Coefficients Using Infrared Thermography near Ambient Conditions and Its Quantitative Error Estimation.

Infrared thermography is applied to the measurement of turbulent heat transfer in order to investigate its applicability under near-ambient conditions. Natural convection along a vertical smooth flat plate and forced convection along a smooth flat plate are realized in a large darkroom, where individual heat transfer coefficients are quantitatively measured using infrared thermography in the laminar, the transition and the turbulent regions. The measurement error is then estimated using the uncertainty analysis based on ANSI / ASME PTC 19.1-1985 to confirm the accuracy. It is obvious from a series of application experiments and error analyses that the present technique is useful for estimating turbulent heat transfer quantitatively and dynamically. Infrared thermography is an appropriate measurement procedure for engineering applications because it can be applied to diagnose a two-dimensional and dynamical temperature field instantaneously and nondestructively.

Assessment and Improvement of Condensation Models in RELAP5/MOD3.2

The condensation models of the standard RELAP5/MOD3.2 code are assessed and improved based on a database that is constructed from previous experimental data of various condensation conditions. The RELAP5/MOD3.2 default model of laminar film condensation does not give any reliable predictions, and the alternative model always predicts values higher than those of the experimental data. Therefore, a new correlation based on the experimental data of various operating ranges is needed. The Shah correlation, which is used to calculate the turbulent film condensation heat transfer coefficients in the standard RELAP5/MOD3.2, gives good agreement with the database except for Kuhn’s experimental data. The RELAP5/MOD3.2 horizontally stratified condensation model overpredicts both cocurrent and countercurrent experimental data. The Kim correlation predicts the database relatively well compared with that of RELAP5/MOD3.2.

HEAT TRANSFER TO A FALLING VISCOELASTIC FLUID FILM

Steady heat transfer to water and viscoelastic fluid films falling down a vertical tube, for the case of uniform wall heat flux thermal boundary condition, was measured experimentally. The viscoelastic fluids studied were aqueous solutions ofPraestol at 100, 200, and 500 ppm (by weight) concentration. The heat transfer to viscoelastic film was lower than the Newtonian prediction, at any given Prandtl number. A new correlation is proposed for dimensionless laminar and turbulent heat transfer coefficient in terms of ike Weissenberg number, over a range of 300 ≤ Reo ≤ 6,500, and 6 ≤Pro ≤ 30.

A Quasi-steady Shrinking Core Analysis of Wood Combustion

A shrinking core model of the combustion of individual chunkwood and particle wood elements is developed and validated by comparison with literature data. The model is formulated on the physical evidence that large wood specimens inserted into a hot convective environment lose weight mostly over a relatively thin outside layer, while the interior (core) remains relatively undisturbed. The modeling of the complete process requires a correlation of the turbulent heat and mass transfer coefficients which include explicitly the effects of transpiration of volatilized organic compounds and moisture, along with geometry, and equivalent radius. The fuel element burnout time is shown to be a function of fuel properties, moisture content, and size. Drier and smaller elements burn faster while moisture is shown to slow the shrinking rate due to the cooling effects of transpiration and the latent heat of evaporation.