Search

This paper is aiming at numerically demonstrating the interrelationship and consistency between field synergy principle (FSP) via the field synergy number (Fc) and the entransy dissipation extremum principle (EDEP). Numerical simulation is conducted by using the FLUENT software and the user defined function programs (UDF) for fin-and-tube surfaces (plain plate and slotted fins) and composite porous materials. The thermal boundary conditions include given heat flux and given surface temperature. The flow includes laminar and turbulent. The air properties may be constant or vary with temperature. Based on the numerical data the analyzed results from the FSP via Fc are totally consistent with the results analyzed by the EDEP for all the cases studied. Such consistency between the FSP and the entransy theory can be regarded as a kind of demonstration of the reliability and correctness of both the FSP and the entransy theory.

The fully-developed mixed turbulent convective heat transfer characteristics in dimpled tubes of parabolic trough receiver are numerically studied at a certain Reynolds number of 2 × 104 and different Grashof numbers ranged from 0 to 3.2 × 1010 to produce substantial surface heat transfer augmentations with relatively small pressure drop penalties. The Boussinesq approximation is applied, in which variations in fluid properties other than density are ignored. The Realizable k-e two-equation turbulence model with enhancement wall treatment is adopted. The influences of outer wall heat flux distributions and dimple depth on flow resistance and heat transfer rate are illustrated and analyzed. The results indicate that the average friction factor and Nusselt number in dimpled receiver tubes under non-uniform heat flux (NUHF) are larger than those under uniform heat flux (UHF). In most cases, the comprehensive performance of dimpled receiver tube under NUHF is also better than that under UHF. The deep dimples (d/Di = 0.875) are far superior to the shallow dimples (d/Di = 0.125) at a same Grashof number.

This paper numerically investigated the coupled flow and heat transfer of a parabolic trough collector (PTC), with the non-uniform heat flux boundary condition on the absorber wall and the rarefied gas effects in the annular vacuum gap being taken into consideration. A fully coupled cross-sectional heat transfer model is established with Direct Simulation Monte Carlo (DSMC) method for the rarefied gas flow and heat transfer in the vacuum annual gap. The PTC tube efficiency can be obtained from the above simulation for a given HTF temperature. Such simulation is conducted for several specified HTF temperature and different efficiency data are obtained. These data are fitted by an equation. This equation is then used to advance the HTF temperature in the axial direction. In such a way a simplified 3D model for the design of a PTC receiver is obtained. Cross-sectional simulation results show that when the gas pressure is less than 0.1 Pa further decrease in pressure makes no further contribution to reduce the heat loss. The effects of periphery non-uniform distribution of heat flux, coating material emissivity, envelope diameter and HTF inlet velocity on the PTC efficiency are discussed. An operation variant is proposed by using the 3D model by which the total PTC tube length can be reduced for a given thermal load.

The present work focuses on the fully developed mixed turbulent flow and heat transfer in receiver tube heated by non-uniform heat flux, especially the effect of local buoyancy force induced by the non-uniform heat flux at Reynolds number of 2 × 104–105, Prandtl number of 1.5 and Grashof number of 0–1012. The friction factor and Nusselt number between forced convection and mixed convection under uniform heat flux and non-uniform heat flux are analyzed quantitatively. The effect of solar elevation angle on the fluid flow and heat transfer is also investigated. It is concluded that the mixed fluid flow and heat transfer under non-uniform heat flux is different from that under uniform heat flux. The solar elevation angle has strong influence on the mixed fluid flow and heat transfer characteristics. A criterion for the buoyancy free is proposed. It is not feasible to perform the heat transfer design and prediction for parabolic trough solar collector based on the experimental correlations for forced convection or conventional mixed convention.

This study combines both concepts of solar ventilation technology and solar air collector. This is a quite innovative and potential facility to effectively use thermal energy and reduce the accumulation of heat in the indoor space simultaneously. The purpose of this study is to create a prototype and implement the experiments. Computational fluid dynamics (CFD) approach is employed to validate the characteristics of the flow and heat transfer. For the accuracy of numerical predictions, the method of Solar Ray Tracing was used for thermal radiation flux as boundary condition on the wall. The local heat transfer correlation was investigated to predict surrounding wind speed upon device cover. Three sorts of glasses and several aspect ratios of flow channels have been compared to conclude the optimal configuration. In addition, four important factors, such as the stagnant layer thickness, emissivity on the illustrated surface, mass flow rate and the height of the device, are also considered and discussed in detail. The result showed that the optimal design is dominated by the combination of an aspect ratio of 50 mm:10 mm, and appropriate mass flow rate to the height of the device. The present work on thermal energy collection can assist us in designing a powerful solar air collector in some potential applications.

The intermediate heat exchanger (IHX), which consists of thousands of tubes assembled along concentric circles, has a complex geometry that makes it extremely difficult to analyze through the 3-D CFD approach, especially when there exists coaxial expansion bends. The motivation of this work is to study its thermal hydraulic characteristics and figure out the influence of the coaxial bends on its performance. In addition, both of the 3-D detail simulation method and the distributed parameter method are employed in this research. And the respective results by these two methods show that the shell-side main flow direction is generally consistent with the tube direction and the rear straight tube section is the main working section with the heat transfer rate of nearly 45% of the total heat transfer rate, and that the bending section plays an important role in controlling the non-uniformity of temperature field.

In this paper, a numerical investigation is presented, aiming at the effect of the buoyancy force induced by the non-uniform heat flux on the laminar flow and heat transfer characteristics in the solar receiver tube of parabolic trough collector. The flow and heat transfer performances are analyzed for forced and mixed laminar convection in receiver tube heated by uniform and non-uniform heat fluxes with different Grashof numbers, Reynolds numbers and solar elevation angles. The results show that the natural convection can increase heat transfer rate of laminar forced convection by more than 10% when the Grashof number is greater than a threshold value. The mixed fluid flow and heat transfer characteristics vary with solar elevation angle. Heat transfer deterioration occurs when the Richardson number is greater than 12.8.

The intermediate heat exchanger (IHX) is a type of shell-and-tube heat exchanger. It is assembled with thousands of straight or expansion bend tubes in concentric circles and extremely huge and complex in geometry which leads to the non-uniform flow in the shell-side. The motivation of this work is to design a 6 m long, 290 MW IHX and clarify the influence of the tube specification on its performance. Traditional thermal design method is employed here for the design process, while 2-D porous medium model with anisotropic properties is adopted for simulation to support the design process adjusting the tube specification. The results show that the tube specification mainly affects the flow uniformity in the inlet region but has little effects on outlet region flow distribution. Comparing the IHX thermal-hydraulic performance under different tube specifications, it is found that the specification with 19 mm outer diameter, 25 mm radial pitch and 0.8 mm wall thickness is the best choice.

In this work, the condensation of refrigerants on a single, high-density, low-fin tube and full-sized shell and tube condensers were investigated experimentally. The low-fin tube had an external fin density of 56 fins per inch (fpi) and fin height 1.023 mm. Another three-dimensional (3D) finned tube was also tested for comparison. The condensing heat transfer coefficient of the refrigerant R134a was first investigated outside a single horizontal tube at saturation temperature of 40 °C. The overall heat transfer coefficients of the two tubes were similar in magnitude. The condensing heat transfer coefficient of the low-fin tube was 16.3–25.2% higher than that of 3D enhanced tube. The experiments of the two condensers mounted with low-fin and 3D enhanced tubes were then conducted in centrifugal and screw chiller test rigs. It was found that chillers with the two different condensers generally had the same refrigeration capacity under the same experiment conditions. The refrigeration capacity of the screw chiller was smaller. It had fewer tube rows and elicited fewer inundation effects owing to the falling condensate. The heat transfer coefficients of the condensers with R134a in centrifugal chillers equipped with high-density low-finned tubes were higher than those in the screw chillers. The total number of tubes for low-fin tube condensers, in the two chillers, was reduced by approximately 15% compared with the use of domestic advanced condensers equipped with the 3D enhanced tubes.

The present work focuses on the fully developed mixed turbulent flow and heat transfer in receiver tube heated by non-uniform heat flux, especially the effect of local buoyancy force induced by the non-uniform heat flux at Reynolds number of 2×104-105, Prandtl number of 1.5 and Grashof number of 0-1012. The friction factor and Nusselt number between forced convection and mixed convection under uniform heat flux and non-uniform heat flux are analyzed quantitatively. The effect of solar elevation angel on the fluid flow and heat transfer is also investigated. We found that it is not feasible to perform the heat transfer design and prediction for parabolic trough solar collector based on the experimental correlations for forced convection or traditional mixed convention.

Guyer-Krumhansl (G-K) equation is a promising macroscopic model to explore heat transport in nanoscale. In the present work, a new nonlocal characteristic length is proposed by considering the effects of heat carriers-boundaries interactions to modify the nonlocal term in G-K equation, and a slip heat flux boundary condition is developed based on the local mean free path of heat carriers. Then an analytical solution for heat flux across 2-D nanolayers and an in-plane thermal conductivity model are obtained based on the modified G-K equation and the slip heat flux boundary. The predictions of the present work are in good agreement with our numerical results of direct simulation Monte Carlo (DSMC) for argon gas nanolayer and the available experimental data for silicon thin layers. The results of this work may provide theoretical support for actual applications of G-K equation in predicting the thermal transport properties of nanolayers.

This article presents a 2-D numerical simulation on nucleate boiling using the VOSET method. Heat transfer through a liquid microlayer around a three-phase contact point is incorporated by a multiscale system method. A temperature interpolation method is presented for solving temperature field in cells containing liquid-vapor interface. Apart from the single-bubble model, computations are carried out for two-bubble and bubble merger models. Heat fluxes predicted by simulation are compared with experimental correlations and good agreement is obtained. In addition, simulation results of bubble's behavior also verify some boiling mechanisms.

In this paper, the heat transfer characteristics of porous material adopted in the receiver of a concentrated solar power (CSP) with different structure parameters are numerically investigated. The commercial software FLUENT and the user defined function program (UDF) are adopted to implement the simulation. The porous material geometry is represented by periodic structures formed with packed tetrakaidecahedron. The air flow and heat transfer characteristics under the boundary conditions of constant heat flux and constant wall temperature are studied. The field synergy principle (FSP) and the entransy dissipation extremum principle (EDEP) are used to analyze the flow and heat transfer performance of the composite porous material. From the numerical results the best composite of the porous material is obtained. The effects of different boundary conditions are revealed. It is also demonstrated that the FSP and the EDEP are inherently consistent.

In this paper, large eddy simulations with the dynamic Smagorinsky eddy viscosity model are performed for a fully developed turbulent flow and heat transfer in a square duct with unstable natural convection on the cross section at a turbulent Reynolds number of 400, a Prandtl number of 0.7 and different Grashof numbers from 105 to 5 × 107. The MPI parallel programming language is implemented to speed up the calculations. The influences of the buoyancy force on the mean flow and heat transfer, turbulent intensity and Reynolds stresses are analyzed. The results show that the mean velocity decreases while the subgrid viscosity, turbulent intensity and heat transfer increase obviously with the increase of Grashof number. The turbulent intensity in the streamwise direction decreases considerably in the near-wall region while the turbulent intensity in the spanwise direction increases clearly in almost whole regions with an increase in Grashof number. The spatial distribution of Reynolds stresses are mainly influenced by the thermal buoyancy force. The turbulence production rate and the buoyancy force production term of Reynolds stress component 〈v′v′〉 increase with the increase of Grashof number.

This article presents a theoretical and numerical study on the heat conduction of gas confined in a nanoscale cube. An effective thermal conductivity model of confined gas using a modified mean free path is proposed for the heat conduction in transition regime inside a closure. Excellent agreement of the present model with the results from our simulations by the method of direct simulation Monte Carlo (DSMC) has been achieved for different boundary conditions of side walls. The temperature jumps and the reduction of local heat flux near the side walls are observed from the DSMC results.Copyright © 2016 by ASME

Direct numerical simulations (DNS) of turbulent flow and convective heat transfer in a square duct with axial rotation were carried out. The pressure-driven flow is assumed to be hydrodynamically and thermally fully developed, for which the Reynolds number based on the friction velocity and hydraulic diameter is kept at constant (Reτ = 400). In the finite length duct, two opposite walls are perfectly insulated and another two opposite walls are kept at constant but different temperatures. Four thermal boundary conditions were chosen in combination with axial rotation to study the effects of rotation and Grashof number on mean flow, turbulent quantities and momentum budget. The results show that thermal boundary conditions have significant effects on the topology of secondary flows, profiles of streamwise velocity, distribution of temperature and other turbulent statistic quantities but have marginal effects on the bulk-averaged quantities; Coriolis force affects the statistical results very slightly because it exerts on the plane normal to main flow direction and the rotation rate is low; Buoyancy effects on the turbulent flow and heat transfer increase with the increase of Grashof number (Gr), and become the major mechanism of the development of secondary flow, turbulence increase, and momentum and energy transport at high Grashof number.

The flow field and temperature distributions of free molecular micro-electro-thermal resist jet (FMMR) were studied resorting to DSMC–FVM coupled method. Direct simulation Monte Carlo (DSMC) method is the most useful tool to simulate the flow field of FMMR and unstructured grid is suitable for the flow simulation in a complicated region with tilted wall surface. DSMC code based on unstructured grid system was developed and the result was compared with that of structured grid and analytical solution to validate the reliability of the developed code. The DSMC method was then used to simulate the fluid flow in the micro-nozzle (Kn > 0.01) and the temperature distribution in the nozzle wall was obtained by the finite volume method (FVM). The Dirichlet–Neumann method was used to couple the wall heat flux and temperature between flow field and solid area. The effect of different income pressure was studied in detail and the results showed that the temperature of solid area changed drastically at different income pressure, so the commonly-adopted method of pre-setting boundary temperature before simulation was unreasonable.

In this paper, the basic idea of the field synergy principle (FSP) is briefly reviewed and is validated experimentally by incompressible flow through a square duct with an imposed temperature difference between vertical walls and perfectly insulated on the horizontal walls. This creates a situation where the steamwise flow velocity is normal to the cross section temperature gradient. The experimental results show the independency of crosswise heat transfer rate on the steamwise flow velocity. Detailed discussion is provided to account for some minor deviation from the expected results of FSP.

In this paper, a direct numerical simulation of a fully developed turbulent flow and heat transfer are studied in a square duct with an imposed temperature difference between the vertical walls and the perfectly insulated horizontal walls. The natural convection is considered on the cross section in the duct. The numerical scheme employs a time-splitting method to integrate the three dimensional incompressible Navier-Stokes equation. The unsteady flow field was simulated at a Reynolds number of 400 based on the Mean friction velocity and the hydraulic diameter (Re m = 6200), while the Prandtl number (Pr) is assumed 0.71. Four different Grashof numbers (Gr = 104, 105, 106 and 107) are considered. The results show that the secondary flow and turbulent characteristics are not affected obviously at lower Grashof number (Gr ≤ 105) cases, while for the higher Grashof number cases, natural convection has an important effect, but the mean flow and mean temperature at the cross section are also affected strongly by Reynolds stresses. Compared with the laminar heat transfer at the same Grashof number, the intensity of the combined heat transfer is somewhat decreased.

Direct numerical simulation (DNS) has been carried out to investigate the effect of weak rarefaction on turbulent gas flow and heat transfer characteristics in microchannel. The Reynolds number based on the friction velocity and the channel half width is 150. Grid number is 64 × 128 × 64. Fractional time-step method is employed for the unsteady Navier–Stokes equations, and the governing equations are discretized with finite difference method. Statistical quantities such as turbulent intensity, Reynolds shear stress, turbulent heat flux and temperature variance are obtained under various Knudsen number from 0 to 0.05. The results show that rarefaction can influence the turbulent flow and heat transfer statistics. The streamwise mean velocity and temperature increase with increase of Kn number. In the near-wall-region rarefaction can increase the turbulent intensities and temperature variance. The effects of rarefaction on Reynolds shear stress and wall-normal heat flux are presented. The instantaneous velocity fluctuations in the vicinity of the wall are visualized and the influence of Kn number on the flow structure is discussed.

In the finite volume approach for incompressible flow and heat transfer, the discretization of the convection term and the treatment of the coupling between velocity and pressure are the two major issues affecting solution stability, accuracy and convergence rate. In this review paper, some recent advances made in the CFD/NHT & Enhanced Heat Transfer Center of Xi'an Jiaotong University, China, are presented and their applications in the study of heat transfer enhancement are also briefly summarized.

Experimental and numerical studies were conducted to investigate the forced convection and flow friction of a turbulent airflow in a horizontal air-cooled rectangular duct, with square-sectioned cross-ribs mounted on its bottom surface. Cross-sectional dimensions of the cross-ribs were 6.37 mm × 6.37 mm. Reynolds number of the fully turbulent flow was maintained constant at 12,380. Heat was supplied uniformly to the airflow via bottom surface of the duct only. Effects of varying the angle formed by the cross-ribs between 30° and 120° on the forced convection and flow friction were studied. It was found that an optimum angle corresponding to the highest heat transfer coefficient occurred between 60° and 70°. Computational predictions of forced convection and flow friction of the same rectangular duct mounted with cross-ribs were performed with the zonal k– e model, the stability-guaranteed second-order difference scheme and the block implicit method. A comparison with the experimental results indicated that a reasonably good agreement had been achieved. Existence of an optimum angle formed by the cross-ribs between 60° and 70° was also indicated, except the numerical predictions were lower than the experimental findings by approximately 2–10%. However, the flow friction had been slightly over-estimated by 2–11% with the numerical model. It was shown that forced convection could be enhanced sufficiently by mounting cross-ribs on the internal surfaces of a rectangular duct, especially when the optimum angle was used.

The flow field and temperature distributions of free molecular micro-electro-thermal resist jet (FMMR) were studied resorting to DSMC-FVM coupled method. Direct Simulation Monte Carlo (DSMC) method is the most useful tool to simulate the flow field of FMMR and unstructured grid is suitable for the flow simulation in a complicated region with tilted wall surface. DSMC code based on unstructured grid system was developed and then the result was compared with that of structured grid and analytical solution to validate the reliability of the developed code. The DSMC method was then used to simulate the fluid flow in the micro-nozzle (Kn>0.01) and the temperature distribution in the nozzle wall was obtained by the Finite Volume Method (FVM). The Dirichlet-Neumann method was used to couple the wall heat flux and temperature between flow field and solid area. The effect of different income pressure was studied in detail and the results showed that the temperature of solid area changed drastically at different income pressure, so the commonly-adopted method of pre-setting boundary temperature before simulation was unreasonable. The results showed that the influence of boundary layer decreased as the pressure increased.Copyright © 2008 by ASME

Direct numerical simulation has been carried out to investigate the effect of weak rarefaction on turbulent gas flow and heat transfer characteristics in mirochannel. The Reynolds number based on the friction velocity and the channel half width is 150. Grid number is 64×128×64. Fractional time step method is employed for the unsteady Navier-Stokes equations, and the governing equations are discretized with Finite Difference Method. Statistical quantities such as turbulent intensity, Reynolds shear stress, turbulent heat flux and temperature variance are obtained under various Knudsen number from 0 to 0.05. The results show that rarefaction can influence the turbulent flow and heat transfer statistics. The streamwise mean velocity and temperature increase with increase of Kn number. In the near wall region rarefaction can increase the turbulent intensities and temperature variance. The effect of rarefaction on Reynolds shear stress and wall-normal heat flux are presented. The instantaneous velocity fluctuations in the vicinity of the wall are visualized and the influence of Kn number on the flow structure is discussed.Copyright © 2004 by ASME

Vacuum tubular receiver is the core component of parabolic trough solar collectors (PTCs). To enhance the heat transfer rate from solar radiation to heat transfer fluid (HTF), one of the effective techniques is to improve the convective heat transfer inside the inner tube. In this study, we focus the heat transfer of the HTF side. A numerical simulation on the fully developed turbulent flow and heat transfer in the inner tube with and without helical fins, protrusions and dimples has been investigated. The results show that the receiver tubes with dimples have superior performance of heat transfer augmentation compared with that with protrusions or helical fins. Then, the effects of the geometry sizes and arrangements of dimples on the convective heat transfer performance are further studied. The performance evaluation plot of heat transfer enhancement techniques show that dimples with deeper depth, narrower pitch and more numbers in the circumference direction is benefit for improving the performance of heat transfer enhancement while the varied arrangements have no obvious influence.