Rheology,friction, and heat transfer study of a discontinuously shear-thickening antimisting polymer solution

Measurements of the viscosity of an antimisting polymer solution in capillaries indicated that this fluid exhibits a complex non-Newtonian nature involving both shear-thinning at low shear rate and abrupt time-dependent shear-thickening past a critical shear rate. Various parameters such as concentration, aging, photodegradation, intentional mechanical degradation, and temperature were shown to have unexpectedly similar effects on the viscous characteristics of the fluid.Extensive friction and heat transfer measurements were also carried out, and it was found that meaningful correlations for these results can be developed if an adequate apparent viscosity model is used for the computations. In particular, all fluid properties must be evaluated at the inner wall temperature, and both the gelation and degradation induced in the test tube itself must be taken into account.It was then possible to show that this peculiar fluid is in fact an asymptotic drag and heat transfer reducer, even though it exhibits dramatic discontinuous shear-thickening characteristics. Various turbulent flow results are presented that were obtained under fully-developed conditions as well as in the very long entrance region observed. It was additionally verified that after severe degradation this fluid can revert to a Newtonian nature as evidenced by its viscosity, friction, and heat transfer properties.     

Prediction of flow and heat transfer in narrow rotating rectangular channels using Reynolds stress model

A computational study is performed on three-dimensional turbulent flow and heat transfer in a rotating rectangular channel with aspect ratio (AR) of 10:1, oriented 120° from the direction of rotation. The Focus is on high rotation and high-density ratios effects on the heat transfer characteristics of the 120° orientation. The Reynolds stress model (RSM), which accounts for rotational effects are used to compute the turbulent flow and heat transfer in the rotating channel. The effects of rotation and coolant-to-wall density ratio on the fluid flow and heat transfer characteristics is reported on a range of rotation numbers and density ratios (0 < Ro < 0.25 and 0.07 < Δρ/ρ < 0.4). The computational results are in good agreement with experimental data within ±15%. The results show that the density ratio, rotation number and channel orientation significantly affect the flow field and heat transfer characteristics in the rotating rectangular channel. Flow reversal occurs at high rotation number and density ratio.     

Numerical simulation of heat transfer in a pipe with non-homogeneous thermal boundary conditions

Direct numerical simulations of heat transfer in a fully-developed turbulent pipe flow with circumferentially-varying thermal boundary conditions are reported. Three cases have been considered for friction Reynolds number in the range 180–360 and Prandtl number in the range 0.7–4. The temperature statistics under these heating conditions are characterized. Eddy diffusivities and turbulent Prandtl numbers for radial and circumferential directions are evaluated and compared to the values predicted by simple models. It is found that the usual assumptions made in these models provide reasonable predictions far from the wall and that corrections to the models are needed near the wall.     

Transient forced convection heat transfer in a channel

A numerical analysis is made of incompressible transient turbulent flow heat transfer between two parallel plates when there is a step jump in space along the channel in wall heat flux or wall temperature. The variation of the fluid velocity and effective diffusivity over the channel cross section are accounted for. The fluid is assumed to have a fully-developed turbulent velocity profile throughout the length of the channel. The thermal responses of the system are obtained by solving energy equation for air by a digital computer. The results are presented in graphical forms. The stability of the finite difference solution is studied and condition for the stability of the difference solution is derived. A method is given to obtain velocity distributions from the distribution of turbulent eddy diffusivity of momentum. Variations of Nusselt numbers are obtained as a function of time and space. Steady-state values are also given and compared with the published results.     

Investigation of flow and heat transfer in corrugated-undulated plate heat exchangers

 An experimental and numerical investigation of heat transfer and fluid flow was conducted for corrugated-undulated plate heat exchanger configurations under transitional and weakly turbulent conditions. For a given geometry of the corrugated plates the geometrical characteristics of the undulated plates, the angle formed by the latter with the main flow direction, and the Reynolds number were made to vary. Distributions of the local heat transfer coefficient were obtained by using liquid-crystal thermography, and surface-averaged values were computed; friction coefficients were measured by wall pressure tappings. Overall heat transfer and pressure drop correlations were derived. Three-dimensional numerical simulations were conducted by a finite-volume method using a low-Reynolds number k–ɛ model under the assumption of fully developed flow. Computed flow fields provided otherwise inaccessible information on the flow patterns and the mechanisms of heat transfer enhancement. Received on 5 February 1999     

Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube

An experimental investigation on the convective heat transfer and friction factor characteristics in the plain and helically dimpled tube under turbulent flow with constant heat flux is presented in this work using CuO/water nanofluid as working fluid. The effects of the dimples and nanofluid on the Nusselt number and the friction factor are determined in a circular tube with a fully developed turbulent flow for the Reynolds number in the range between 2500 and 6000. The height of the dimple/protrusion was 0.6 mm. The effect of the inclusion of nanoparticles on heat transfer enhancement, thermal conductivity, viscosity, and pressure loss in the turbulent flow region were investigated. The experiments were performed using helically dimpled tube with CuO/water nanofluid having 0.1%, 0.2% and 0.3% volume concentrations of nanoparticles as working fluid. The experimental results reveal that the use of nanofluids in a helically dimpled tube increases the heat transfer rate with negligible increase in friction factor compared to plain tube. The experimental results showed that the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% (for 0.1%, 0.2% and 0.3% volume concentrations respectively) higher than the Nusselt number obtained with plain tube and water. The experimental results of isothermal pressure drop for turbulent flow showed that the dimpled tube friction factors were about 2-10% higher than the plain tube. The empirical correlations developed for Nusselt number and friction factor in terms of Reynolds number, pitch ratio and volume concentration fits with the experimental data within ±15%.     

3D-PTV measurements in a plane Couette flow

Genuine plane Couette flow is hard to realize experimentally, and no applications of modern spatially resolving measurement techniques have been reported for this flow so far. In order to resolve this shortcoming, we designed and built a new experimental facility and present our first results here. Our setup enables us to access the flow via 3D particle tracking velocimetry and therefore to obtain truly three-dimensional flow fields for the first time experimentally in plane Couette flow. Results are analyzed in terms of basic flow properties, and a clear distinction of flow regimes (laminar for Re < 320, transitional for 320 < Re < 400, and turbulent when Re > 400) could be made. Comparison with DNS data shows good agreement in the turbulent regime and builds trust in our data. Furthermore, vortical coherent structures are studied in detail with the additional help of kalliroscope imaging, and the typical vortex spacing is determined to be roughly one gap width. As a noteworthy result, we find that the onset of the turbulent regime coincides with the range of Reynolds numbers at which a distance of 100 wall units is comparable to the gap width.     

Fully-developed conjugate heat transfer in porous media with uniform heating

We propose a computational method for approximating the heat transfer coefficient of fully-developed flow in porous media. For a representative elementary volume of the porous medium we develop a transport model subject to periodic boundary conditions that describes incompressible fluid flow through a uniformly heated porous solid. The transport model uses a pair of pore-scale energy equations to describe conjugate heat transfer. With this approach, the effect of solid and fluid material properties, such as volumetric heat capacity and thermal conductivity, on the overall heat transfer coefficient can be investigated. To cope with geometrically complex domains we develop a numerical method for solving the transport equations on a Cartesian grid. The computational method provides a means for approximating the heat transfer coefficient of porous media where the heat generated in the solid varies “slowly” with respect to the space and time scales of the developing fluid. We validate the proposed method by computing the Nusselt number for fully developed laminar flow in tubes of rectangular cross section with uniform wall heat flux. Detailed results on the variation of the Nusselt number with system parameters are presented for two structured models of porous media: an inline and a staggered arrangement of square rods. For these configurations a comparison is made with literature on fully-developed flows with isothermal walls.     

Velocity-temperature correlation in strongly heated channel flow

Velocity-temperature correlations in a strongly heated channel flow were investigated experimentally by a LDV and a resistance thermometer. The wall heat flux is varied up to 50,000 W/m² with reference mean-velocity of 15 m/s, and then, the wall temperature reaches up to 1,000 K. The results show that the ejection fluid motion is intensified by the strong heating near the wall increasing the turbulent heat flux from the wall. The intensification of ejection motion balances the destruction of turbulent heat flux. Then, the overall turbulent heat transfer does not change clearly. Part of this work was supported by Tanikawa Fund, Promotion of Thermal Technology     

Optimization of convective fin systems: a holistic approach

 A numerical analysis of natural convection heat transfer and entropy generation from an array of vertical fins, standing on a horizontal duct, with turbulent fluid flow inside, has been carried out. The analysis takes into account the variation of base temperature along the duct, traditionally ignored by most studies on such problems. One-dimensional fin equation is solved using a second order finite difference scheme for each of the fins in the system and this, in conjunction with the use of turbulent flow correlations for duct, is used to obtain the temperature distribution along the duct. The influence of the geometric and thermal parameters, which are normally employed in the design of a thermal system, has been studied. Correlations are developed for (i) the total heat transfer rate per unit mass of the fin system (ii) total entropy generation rate and (iii) fin height, as a function of the geometric parameters of the fin system. Optimal dimensions of the fin system for (i) maximum heat transfer rate per unit mass and (ii) minimum total entropy generation rate are obtained using Genetic Algorithm. As expected, these optima do not match. An approach to a `holistic' design that takes into account both these criteria has also been presented. Received on 22 February 2001 / Published online: 29 November 2001     

Numerical investigations on the heat transfer in turbulent oscillating pipe flow

 The examinations on the heat transfer in developing laminar oscillating pipe flow presented before [1] have been extended to include turbulence as well. A suitable low-Reynolds-number k-ɛ-turbulence model was incorporated in an existing 2D-simulation code for oscillating flow conditions and subsequent examinations focused on the heat transfer associated with turbulent oscillating flow. The calculations cover a wide range of the characteristic parameters and the results are summarised in form of new heat transfer correlations to suit with the operating conditions of regenerative thermal machines. Received on 5 July 1999     

Experimental investigation of heat transfer in vertical upward and downward supercritical CO2 flow in a circular tube

An experimental investigation of turbulent heat transfer in vertical upward and downward supercritical CO₂ flow was conducted in a circular tube with an inner diameter of 4.5 mm. The experiments were performed for bulk fluid temperatures from 29 to 115 °C, pressures from 74.6 to 102.6 bar, local wall heat fluxes from 38 to 234 kW/m², and mass fluxes from 208 to 874 kg/m² s. At a moderate wall heat flux and low mass flux, the wall temperature had a noticeable peak value for vertical upward flow, but increased monotonically along the flow direction without a peak value for downward flow. The ratios of the experimental Nusselt number to the value obtained from a reference correlation were compared with Bo^* and q⁺ distributions to observe the buoyancy and flow-acceleration effects on heat transfer. In the experimental range of this study, the flow acceleration predominantly affected the heat-transfer phenomena. Based on an analysis of the shear-stress distribution in the turbulent boundary layer and the significant variation of the specific heat across the turbulent boundary layer, a new heat-transfer correlation for vertical upward and downward flow of supercritical pressurized fluid was developed; this correlation agreed with various experimental datasets within ±30%.     

Direct numerical simulation of the turbulent energy balance and the shear stresses in power-law fluid flows in pipes

The results of direct numerical simulation of turbulent flows of non-Newtonian pseudoplastic fluids in a straight pipe are presented. The data on the distributions of the turbulent stress tensor components and the shear stress and turbulent kinetic energy balances are obtained for steady turbulent flows at the Reynolds numbers of 10⁴ and 2×10⁴. As distinct from Newtonian fluid flows, the viscous shear stresses turn out to be significant even far from the wall. In power-law fluid flows the mechanism of the energy transport from axial to transverse component fluctuations is suppressed. It is shown that with decrease in the fluid index the turbulent transfer of the momentum and the velocity fluctuations between the wall layer and the flow core reduces, while the turbulent energy flux toward the wall increases. The earlier-proposed models for the average viscosity and the non-Newtonian one-point correlations are in good agreement with the data of direct numerical simulation.     

湍流冲击射流流动与传热的数值研究进展

湍流冲击射流由于其冲击表面时具有很高的局部传热率和冲击力,被广泛应用于如表面的加热、电子元件的冷却、纸张的干燥和材料的切割等工程应用和工业过程中.由于其流动的复杂性,也常被作为一种理想的测试实例来评价湍流模型的性能.此外,湍升力射流与地面之间的空气动力作用对V/STOL (垂直或短距离起落)飞机的性能具有很大的影响.长期以来,人们从理论分析、实验测量和数值模拟方面对冲击射流进行了广泛而系统的研究,积累了丰富的资料.本文在分析了湍流冲击射流的数值研究现状的基础上,对近年来有关湍流冲击射流流动与传热的数值研究方面的文献有选择地进行了综述,重点评述了不同湍流模型对冲击射流流动与传热的预测能力,讨论了存在的问题并对该领域今后的研究方向进行了展望.      

Simulation and measurement of turbulent heat transfer in a channel with a surface-mounted rectangular heated block

Present study numerically and experimentally investigates the turbulent forced convective flow over a heated block mounted on one principal wall of an adiabatic channel. In the computation, thek-?, low-Reynolds-number, two-equation model was adopted for the turbulence closure. In the experiment, the flow measurement was performed by the laser Doppler velocimetry and the mass transfer measurement was carried out via the naphthalene sublimation technique. By virtue of the analogy between heat and mass transfer, the results could then be converted to predict the heat transfer coefficient. The effects of the Reynolds number and the aspect ratio of the block on heat transfer and fluid flow are thoroughly investigated. Distributions of the velocity and the turbulent kinetic energy are presented to gain an insight into the influence of the fluid flow on the heat transfer from the block. The Nusselt number hump is found on every face of the block, which is attributed to the separating bubble there. It is worth noting that the Nusselt number hump is located near the reattachment point of the separating bubble. In the absence of the separating bubble, the Nusselt number decreases or increases monotonously. Comparisons between numerical and experimental results of the local velocity and the heat transfer coefficient show reasonable agreement.     

添加剂湍流减阻流动与换热研究综述

添加剂湍流减阻是指在液体的管道湍流中添加少量的高分子聚合物或某种表面活性剂从而使湍流阻力大大降低的现象.从其被发现至今,经过近半个世纪的研究(实验研究、理论分析、数值模拟和实际系统的应用研究),尽管对这一现象及其实际应用价值已有了较为深入的认识,但仍有许多方面尚有欠缺,例如对湍流减阻的机理仍然在探索中.本文归纳评述了高分子聚合物或表面活性剂添加剂湍流减阻流动与换热现象的研究现状,从湍流减阻剂的特性、减阻剂的湍流减阻机理、湍流减阻发生时的换热机理、减阻流动速度场分布和换热控制等几个方面综述了添加剂湍流减阻流动与换热特性,并综述了湍流减阻剂在实际工业系统中的应用情况,在对添加剂湍流减阻机理、有湍流减阻发生时的对流换热机理等的理解方面进行了新的总结.      

Measurements of the gas-particle convective heat transfer coefficient in a packed bed for high-temperature energy storage

Experimental measurements of the forced convection gas-particle heat transfer coefficient in a packed bed, high-temperature, thermal energy storage system were performed using a custom-made experimental facility. Special attention was paid to the application of uncertainty analysis (a very important concept in experimentation). General and detailed uncertainty analyses were carried out, which identified the choices that were made in the experimental planning and procedure to ensure reliable final results. The experimental data reduction program used the governing equations and the results of the uncertainty analysis while making allowance for media property variations with temperature. Results were correlated in terms of Nusselt number, Prandtl number and Reynolds number and comparisons were made with existing correlations developed with similar storage media. The maximum temperature for the bed was about 1000°C (1830°F) with flue gas as the operating fluid in the storage mode and atmospheric air in the recovery mode. Because most related previous studies were not necessarily focused on high-temperature applications, the published gas-particle heat transfer correlations were obtained at relatively low temperature ranges, generally at room temperature or at temperatures slightly above room temperature. Moreover, only a few of the previously reported correlations associated the results with the corresponding uncertainty margins. The results from this study give a convective gas-particle heat transfer correlation for high-temperature thermal energy storage applications. Also, due to substantial uncertainties normally associated with the measurements of this heat transfer coefficient, it is significant to note that no firm conclusions can be reached on the validity or non-validity of previously reported related correlations for which the uncertainty margins were not reported.     

Studies on heat transfer in flow of silver nanofluid through a straight tube with twisted tape inserts

Convective heat transfer in the flow of silver nanofluid through a straight tube with twisted tape inserts was investigated experimentally. This straight tube was used as absorber/receiver tube in parabolic trough collector. The experiments were conducted for Reynolds number range 500 < Re < 6000 with twisted tape inserts of different twist ratio range 0.577 < H/D < 1.732. This experimental study shows that twisted tape inserts enhances heat transfer rate in the tube. The heat transfer coefficient and friction factor in the flow of silver nanofliud with 5 % volume fraction (concentration) are higher compared to the flow of water. From this study, Nusselt number, friction factor and enhancement factor are found as 2.0–3.0 times, 10–48.5 and 135–175 %, respectively with silver nanofliud. Finally new possible correlations for predicting heat transfer and friction factor in the flow of silver nanofliud through the straight tube with twisted tape inserts are proposed.     

Experimental study of the condensation heat transfer characteristics of CO2 in a horizontal microfin tube with a diameter of 4.95?mm

The condensation heat transfer characteristics for CO₂ flowing in a horizontal microfin tube were investigated by experiment with respect to condensation temperature and mass flux. The test section consists of a 2,400?mm long horizontal copper tube of 4.6?mm inner diameter. The experiments were conducted at refrigerant mass flux of 400–800?kg/m²s, and saturation temperature of 20–30?°C. The main experimental results showed that annular flow was highly dominated the majority of condensation flow in the horizontal microfin tube. The condensation heat transfer coefficient increases with decreasing saturation temperature and increasing mass flux. The experimental data were compared against previous heat transfer correlations. Most correlations failed to predict the experimental data. However, the correlation by Cavallini et al. showed relatively good agreement with experimental data in the microfin tube. Therefore, a new condensation heat transfer correlation is proposed with mean and average deviations of 3.14 and ?7.6?%, respectively.     

Flow and heat transfer characteristics of drag reducing surfactant solution in a helically coiled pipe

The reduction characteristic of turbulent drag and heat transfer of drag reduction surfactant solution flowing in a helically coiled pipe were experimentally investigated. The drag reduction surfactant used in the present study was the amine oxide type nonionic surfactant of oleyldihydroxyethylamineoxide (ODEAO, C₂₂H₄₅NO₃=371). The zwitterion surfactant of cetyldimethylaminoaciticacidbetaine (CDMB, C₂₀H₄₁NO₂=327) was added by 10% to the ODEAO solution in order to avoid the chemical degradation of ODEAO by ionic impurities in a test tape water. The experiments of flow drag and heat transfer reduction were carried out in the helically coiled pipe of coil to pipe diameter ratio of 37.5 and the helically coiled pipe length to pipe diameter of 1180.5 (pipe diameter of 14.4 mm) at various concentrations, temperatures and flow velocities of the ODEAO surfactant solution. The ODEAO solution showed a non-Newtonian behavior at high concentration of the ODEAO. From the experimental results, it was observed that the friction factor of the ODEAO surfactant solution flowing through the coiled pipe was decreased to a great extent in comparison with water as a Newtonian fluid in the turbulent flow region. Heat transfer measurements for water and the ODEAO solution were performed in both laminar and turbulent flow regions under the uniform heat flux boundary condition. The heat transfer coefficients for the ODEAO solution flow were the same as water flow in the laminar region. On the other hand, heat transfer reduction of the ODEAO solution flow was remarkedly reduced as compared with that of the water flow in the turbulent flow region.