A spatially advancing turbulent flow and heat transfer in a curved channel

Direct numerical simulation was performed for a spatially advancing turbulent flow and heat transfer in a two‐dimensional curved channel, where one wall was heated to a constant temperature and the other wall was cooled to a different constant temperature. In the simulation, fully developed flow and temperature from the straight‐channel driver was passed through the inlet of the curved‐channel domain. The frictional Reynolds number was assigned 150, and the Prandtl number was given 0.71. Since the flow field was examined in the previous paper, the thermal features are mainly targeted in this paper. The turbulent heat flux showed trends consistent with a growing process of large‐scale vortices. In the curved part, the wall‐normal component of the turbulent heat flux was twice as large as the counterpart in the straight part, suggesting active heat transport of large‐scale vortices. In the inner side of the same section, temperature fluctuation was abnormally large compared with the modest fluctuation of the wall‐normal velocity. This was caused by the combined effect of the large‐scale motion of the vortices and the wide variation of the mean temperature; in such a temperature distribution, large‐scale ejection of the hot fluid near the outer wall, which is transported into the near inner‐wall region, should have a large impact on the thermal boundary layer near the inner wall. Wave number decomposition was conducted for various statistics, which showed that the contribution of the large‐scale vortex to the total turbulent heat flux normal to the wall reached roughly 80% inside the channel 135^° downstream from the curved‐channel inlet. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20275     

Enhancement of heat transfer in a turbulent channel flow with insertion of a flat body

Experiments have been performed for turbulent channel flow obstructed with a flat body. The local heat transfer coefficient and the wall static pressure were measured on two kinds of flat bodies for which the trailing edge shape differed. The length of the body, the thickness of the body, and the distance between the wall and the body were changed in several steps. The total performance between heat transfer and pressure drop was estimated under conditions of equal pumping power. The total performance hardly changed, even if the trailing edge shape and length of the bodies were different. The averaged heat transfer coefficient increased with increasing thickness of the bodies. However, as the friction factor increased, the performance became worse. When a comparatively thin body was installed near the heating surface, good performance was obtained. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 354–366, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10100     

Unsteady turbulent heat transfer of mixed convection in a reciprocating circular ribbed channel

The SIMPLE-C scheme is used to solve the mass, momentum, energy conservation equations and turbulent k-ε equations with a two-layer model near wall for a fluid past a reciprocating circular ribbed channel when changing Reynolds number (4250-10,000), Grashof number (0-400,000,000), pulsating number (0-9.3) and cooling mediums. The average time-mean Nusselt number for the reciprocating circular ribbed channel can be 45-182% larger than that for the equivalent stationary smooth channel. The heat transfer enhancement produced by buoyancy for the reciprocating circular ribbed channel decreases as the pulsating number increases. The oscillating amplitude of Nusselt number with crank angle in the oil-cooling is less than in the water-cooling.     

Thermal boundary condition effects on forced convection heat transfer

We propose a numerical solution of an adjoint problem of forced convection heat transfer to evaluate the mean heat transfer characteristics under arbitrary thermal boundary conditions. Using the numerical solution of the adjoint problem under the Dirichlet condition, which can be computed by slightly modifying a conventional heat transfer code, we obtain an influence function of local surface temperature on total heat transfer. As a result, the total heat transfer for arbitrary surface temperature distributions can be calculated by the influence function. Similarly, using the numerical solution of the adjoint problem under the Neumann condition, we can also obtain an influence function of the local heat flux on the mean surface temperature. The influence functions for a circular cylinder and for an in-line square rod array are presented to illustrate the capability of this method. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 227–238, 1999     

Effects of an unsteady thermal boundary condition on forced convection heat transfer

A numerical analysis based on adjoint formulation of unsteady forced convection heat transfer is proposed to generally evaluate effects of the thermal boundary condition on the heat transfer characteristics. A numerical solution of the adjoint problem enables us to predict the heat transfer characteristics, such as the total heat transfer rate or the temperature at a specific location, when the thermal boundary conditions change arbitrarily with time. Moreover, using the numerical solution of the adjoint problem, we can obtain the optimal thermal boundary conditions in both time and space to maximize the heat transfer at any arbitrary time. Numerical solutions of the adjoint problem in a lid‐driven cavity are presented to illustrate the capability of the present method. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 237–247, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10032     

流体流过带折流板通道时的流动及传热数值研究

对常物性流体在通道内的周期性充分发展层流流动和换热特性进行了二维数值计算分析。所研究的通道是由两平行平板布置于中心线位置的一系列折流板构成。平行平板保持温度恒定，折流板则分成完全导热和绝热两种情况，对不同几何参数，Re数和Pr数下的流动和换热性能进行了数值研究。文章还给出了系统流函数图和局部换热系数分布情况。     

Turbulent heat transfer in a channel flow with arbitrary directional system rotation

Arbitrary directional system rotation of a channel flow can be decomposed into simultaneous componential rotations in the three orthogonal directions. In order to study its effect on turbulent heat transfer, three typical cases, i.e., combined spanwise and streamwise (Case I), streamwise and wall-normal (Case II), and wall-normal and spanwise rotations (Case III), are simulated with two of the three coordinate-axial rotations imposed on the system. In Case I, the effect of spanwise rotation dominates the heat transfer mechanism when the two componential rotation rates are comparable. However, if the streamwise rotation is much stronger than the spanwise rotation, the turbulent heat transfer can be enhanced on the two walls, but more strikingly on the suction side. In Case II, even though no explicit spanwise rotation is imposed on the system, the combined rotations also bring the enhancement/reduction of turbulent heat transfer on the pressure/suction side, respectively, which is similar to that in a spanwise rotating channel flow. In Case III, the spanwise rotation effect is still obvious, however, its effect is reduced somewhat due to the redirection of the mean flow by the wall-normal rotation.     

Characteristics of heat transfer and fluid flow in a channel with single-row plates array oblique to flow direction for photovoltaic/thermal system

This study has been carried out to investigate the characteristics of convective heat transfer and fluid flow for a single row of oblique plates array to the flow direction inside a channel. The flow inside the channel is laminar and the plates array have spanwise distance between the plates and heated by radiation. This configuration has been designed to be used for Photovoltaic/Thermal system (PV/T) applications. The theoretical results are validated with measured values, and a good agreement prevailed. The results show that an increase in the plate oblique angle (γ) in the range from 0 to 15 degrees, leads to an increase in the Nusselt number (Nu) up to a maximum value and then decreases. The oblique angle at the maximum value of Nu depends on the flow Reynolds Number (Re), and (?_w/?_pl), where (?_w/?_pl) is defined as the ratio of the plates’ spacing at zero oblique angle to the plate length. Furthermore, increasing (?_w/?_pl) results in a significant increase in the heat transfer coefficient depending on the values of Re, and plate oblique angle (γ). In addition, increasing (γ) from 0 to 15 degrees results in a decrease in the friction factor up to a certain value, after which the friction value approaches a constant value depending on Re value and (?_w/?_pl). It was found that for any value of the plate oblique angle (γ), the friction factor decreases with the increase of the values of (?_w/?_pl) and Re, respectively.     

Heat transfer in channel flow around a rectangular cylinder

Heat transfer and flow visualization experiments have been made in a channel with a rectangular cylindrical section having various width-to-height ratios. Vortices were observed to shed periodically from the cylinder and then reattach to the channel wall. This reattachment of the vortices induces a periodic fluctuation in heat flux at the wall and enhances the heat transfer in the downstream region of the cylinder. The streamwise position of the maximum Nusselt number moves downstream with decreasing width-to-height ratio, b/h, of the cylinder. When b/h = 2.0, however, the heat flux periodicity disappears because the wake narrows intermittently owing to reattachment of the separated flows to the upper and lower surfaces of the cylinder. © 1998 Scripta Technica. Heat Trans Jpn Res, 27(1): 84–97, 1998     

Numerical prediction of turbulent flow and heat transfer with phase change in crystallizer of inverse casting

horoductionInverse casting is a technique for Producing nearnet-shape cast strips. T'he main idea of this technique isdeveloped from a successful inveshgation['], in which ithas been Proved that the inverse cashng teclmiquewould be applicable to produce the steel strip with aabackness of 0.5-3 nun, with a good material prOPertyand with a lower energy consumption in contrast withconventional conhnuous cashng process. The POssibilitytO p~ce composite strips is also one of the mostattrachve pro…     

Investigation of fluid flow and heat transfer in a vertical channel heated from one side by PV elements, part I - Numerical Study

The impetus of this paper is to analyse numerically the fluid flow and heat transfer characteristics of buoyancy-driven convection between two vertical parallel walls, heated from one side. Both convection and radiation heat exchanges are considered as the heat transfer mechanisms by which the thermal energy is transferred into the air. A steady-state two-dimensional model is used for the analysis. Numerical results are derived for a channel of 6.5 m in height and different widths of the channel. Various heat fluxes are considered in order to show the effect of the input heat on the heat transfer across the air layer. Detailed studies of the flow and thermal fields in the air are presented in order to explore the thermal behavior of air in the channel. Velocity and temperature profiles of the outlet air and the surface temperature of the heated and insulated wall are presented. In Part II of this paper the findings from an experimental study are reported.     

Enhancement of heat transfer from a flat surface in a channel flow by attachment of rectangular blocks

Enhancement of the heat transfer from a flat surface in a channel flow by attachment of rectangular cross‐sectional blocks has been investigated as a function of Reynolds number (Re), arrangement of the blocks with respect to the main flow direction as well as each other, and the numbers (spacing) of the blocks. The channel had a cross‐sectional area of 80×160 mm² (i.e. an aspect (width‐to‐height) ratio of 2). Re, based on the hydraulic diameter of the channel (D_e) and bulk mean velocity (u), changed in the range of 6670–40000. The blocks were positioned both transverse and parallel with respect to the main flow direction. The parallel blocks were arranged in both in‐line and staggered orientation with respect to each other. The effect of the blocks on the flow pressure drop was also measured. The results indicated that the heat transfer could be enhanced or reduced depending on the spacing between the blocks and their positioning and arrangement. For a given pressure drop, the best heat transfer enhancement by the blocks over that from a smooth surface (without blocks) was obtained when the blocks were positioned parallel to the flow and arranged in a staggered manner with respect to each other. Copyright © 2001 John Wiley & Sons, Ltd.     

Direct numerical simulation of convective heat transfer in a zero-pressure- gradient boundary layer with supercritical water

Experimental research has long shown that forced-convective heat transfer in wall-bounded turbulent flows of fluids in the supercritical thermodynamic state is not accurately predicted by correlations that have been developed for single-phase fluids in the subcritical thermodynamic state. In the present computational study, the statistical properties of turbulent flow as well as the development of coherent flow structures in a zero-pressuregradient flat-plate boundary layer are investigated in the absence of body forces, where the working fluid is in the supercritical thermodynamic state. The simulated boundary layers are developed to a friction Reynolds number of 250 for two heat-flux to mass-flux ratios corresponding to cases where normal heat transfer and improved heat transfer are observed. In the case where improved heat transfer is observed, spanwise spacing of the near-wall coherent flow structures is reduced due to a relatively less stable flow environment resulting from the lower magnitudes of the wall-normal viscosity-gradient profile.     

Effects of wave number and average radius on flow and heat transfer in a curve-wave channel

The demand for high cooling capacity heatsinks has been increasingly promoted due to the fast elevated heat generation of modern electronic devices. High-performance microscale heat exchangers are often accompanied by considerable pressure drop penalty that limits their applications. In order to avoid large pressure drop while maintaining high heat transfer rate, a macroscale curved channel was proposed with applying the periodical wave structure on the channel sidewalls. A three-dimensional conjugated numerical model was established, and the effects of wave number and average radius on the hydrothermal performance were investigated. It was found that the heat transfer performance in the smooth-curve channel is significantly improved due to the wavy sidewalls with a moderate pressure drop penalty. The heat transfer enhancement is more noticeable in the channels with more wave units and smaller average channel radius. Furthermore, it was noted that the periodical thermal entrance effect exists on the inner and the outer convective surfaces of curve-wave channels, which can be attributed to the variations of the secondary flow with the wave direction along the channels.     

Transitional processes of flow and heat transfer in a circular pipe with short static mixer

The transitional processes of flow and heat transfer in a circular pipe fitted with a short static mixer were studied with Newtonian and pseudoplastic fluids. An experimental formula, which was derived from the same concept as a well-known transition model of boundary layer flow on a flat plate, coincided well with the experimental results of friction and heat transfer. The transitional Reynolds number for heat transfer was larger than that for flow for both fluids. In heat transfer experiments, the transitional Reynolds number for a pseudoplastic fluid was smaller than that for a Newtonian fluid, and heat transfer augmentation in the transitional region was larger in a pseudoplastic fluid than in a Newtonian fluid. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 254–266, 1996     

3D numerical modeling of non-isotropic turbulent buoyant helicoidal flow and heat transfer in a curved open channel

A 3D non-isotropic algebraic stress/flux turbulence model is employed to simulate turbulent buoyant helicoidal flow and heat transfer in a rectangular curved open channel. The prediction shows that, unlike the isothermal flow, there are two major and one minor secondary flow eddies in a cross section of thermally stratified turbulent buoyant helicoidal flow in a curved open channel. The results compare favorably with available experimental data. The thermocline in a curved channel is thicker than that in a straight channel. All of these is the result of complex interaction between the buoyant force, the centrifugal force and the Reynolds stresses. The turbulent flow in a curved channel is obviously non-isotropic: the turbulence fluctuations in vertical and radial directions are lower in magnitude than that in the axial direction, which illustrates the suppression of turbulence due to buoyant and centrifugal forces. The results are of significant practical value to engineering works such as the choice of sites for intake and pollutant-discharge structures in a curved river.     

Analytical study of heat and mass transfer effects on unsteady Casson fluid flow over an oscillating plate with thermal and solutal boundary conditions

In this study, the impacts of heat and mass transfer characteristics on an isotropic incompressible Casson fluid flow over an oscillatory plate with the incidences of solutal and thermal boundary conditions have been investigated. Exact solutions of the fundamental equations governing the fluid flow are determined by using the Laplace transform technique. Numerical results based on analytical solutions are presented in graphical and tabular illustrations to clarify the behaviors of the fluid. Most interestingly, both fluid velocity and species concentration increase with an increment of mass transfer coefficient, whereas the fluid velocity diminishes as oscillating frequency increases near the surface of the plate. This happens due to the presence of high fluctuation of the plate in the flow system. Finally, this investigation is helpful to the scientific community, and the obtained results can be used as benchmark solutions for solving nonlinear flow governing problems fully via various numerical methods.     

Natural convection heat transfer along a vertical flat plate with a projection in the turbulent region

In the present study, the heat transfer coefficients occurring with a projection in the turbulent region of a vertical flat plate were measured experimentally for various projection heights in the range of 0 to 20 mm. The wall temperature and fluid flow fields were also visualized using a liquid crystal sheet and nylon 12 powder, respectively. The average and local Nusselt numbers reach 1.07 to 1.22 and 1.2 to 1.7 times those for pure turbulent natural convection, respectively. The maximum enhancement rates of heat transfer are attained at a position of 2.3 to 3.3 times the projection height from the upper projection surface toward the downstream, and these positions are in good agreement with those of the reattachment of the fluid flow and with centers of dark red regions in the liquid crystal. On the other hand, the heat transfer coefficients in the just upstream and downstream regions of the projection are small compared with those for no projection. By introducing the nondimensional parameter R, the present experimental results are rearranged quantitatively and effectively. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 222–233, 2001     

Influence of thermal and flow boundary perturbations on convection heat transfer characteristics: Numerical analysis based on adjoint formulation

A numerical approach based on adjoint formulation of convection heat transfer is proposed to predict the change of heat transfer characteristics for arbitrary thermal and flow boundary perturbations. In order to obtain the adjoint system of the convection heat transfer problem, we formally linearize the governing equations by the perturbation method and then derive the adjoint system for the perturbation system. As a result, it is shown that the numerical solutions of the base and the adjoint problems enable us to predict the changes of heat transfer characteristics, such as the change of total heat transfer rate or the change of temperature at a specific location, when the thermal and flow boundary conditions are perturbed. An application example is presented to demonstrate the proposed method. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 1–12, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10065