Forced-convection freezing heat transfer characteristics in a return bend with a rectangular cross section

Experiments have been performed to investigate the freezing heat transfer characteristics in a return bend with a rectangular cross section. The experiments were carried out for two kinds of duct heights of 30 and 50 mm under the fixed size of 300 mm in duct width and 159 mm in curvature radius of convex wall. Both the convex and concave walls of a return bend were kept less than the freezing temperature of water. It was found that the freezing characteristics on the convex wall are markedly different from those on the concave wall of a return bend, and that the cooling temperature ratio is one of the most important parameters on the forced-convection freezing heat transfer in a return bend.     

Turbulent heat-transfer characteristics along the heated convex wall of a rectangular cross-sectional return bend

The turbulent heat-transfer characteristics along the heated convex wall of a return bend which has rectangular cross section with large ratio have been examined for various clearances of the duct in detail. The experiments are performed under condition that the convex wall is heated at uniform heat flux while the concave wall is insulated. Water as a working fluid is utilized. Using four kinds of clearances of 15, 40, 60 and 80 mm, the Reynolds number in the turbulent range is varied from 8×10³ to 8×10⁴ with Prandtl number ranging from 6.5 to 8.5. In consequence, it is found that both the local and the mean heat-transfer rates are always smaller than those for straight parallel plates or for the straight duct. It is also found that the local heat-transfer characteristics in the outlet region of the return bend are more sensitively influenced by the variation of duct clearance than those in the inlet region.     

Local Nusselt number and temperature field in turbulent flow through a heated square-sectioned U-bend

Local heat transfer coefficients and temperature distributions within the fluid for air flow around a 180° square-sectioned bend have been measured. The ratio of bend radius to hydraulic diameter of the duct is 3.35:1 and the flow entering the bend is sensibly fully developed. Measurements of air and wall temperatures span a range of Reynolds numbers from 9.9 × 10³ to 9.2 × 10⁴ with the principal emphasis given to the case of Re ? 5.6 × 10⁴. This Reynolds number and geometric configuration coincide with that of a companion LDA study carried out by Chang et al¹ which provides detailed maps of the mean and turbulent velocity fields. The data show that by 45° into the bend the heat transfer coefficients on the inner convex wall of the bend drop markedly while those on the other walls increase. By 90° the ratio of the heat transfer coefficients at the mid positions of the concave and convex walls is more than 2:1. Nevertheless this ratio is less than would be anticipated from considering two-dimensional flow on weakly curved surfaces. There is a general consistency between the velocity and the temperatyre field data in the heated fluid     

An experimental study of ice-layer transition phenomena in a rectangular duct containing water flow

Measurements of both the velocity and turbulence-intensity distributions above an ice-layer surface along flow direction have been performed to clarify the ice-layer transition phenomena observed in a rectangular duct. The test duct which has a lower cooled wall kept less than the freezing temperature of water with cross-sectional dimension of 50 mm by 19 mm was used in the present measurements. The velocity and turbulence-intensity distributions in the test duct were measured using Laser Doppler Velocimeter set up on the two-dimensional traversing table. The freezing experiments were carried out under the condition of uniform water-flow rate even after the ice layer has developed in the test duct. It was found that inlet water flow tended to be laminarized under an influence of developing ice layer, and that onset of the ice-layer transition phenomena might be closely related to an increase in turbulence intensity in the water flow above the developing ice-layer surface.     

On swirl development in a square cross-sectioned, S-shaped duct

The flow in a uniform square cross-sectioned, S-shaped duct was investigated experimentally, at Reynolds number (Re) = 4.73 × 10⁴ and 1.47 × 10⁵, using three S-ducts of different curvature and turning angle. The hydraulic diameter (D) for each S-duct is 150 mm. Besides studying the square cross-sectioned S-duct flow at moderately higher Re than current literature, the S-ducts’ geometry used in this study also have larger curvatures and higher turning angles than those reported in the literature. With surface pressure measurement and smoke wire flow visualization, flow separation at the inside wall of the first bend was detected. Using surface oil flow visualization on the bottom wall of the S-duct and cross-wires measurement at the duct exit, it is shown here that the swirl developed in the first bend was partly attenuated in the second bend due to the formation of swirl of opposite direction. The swirl of an opposite sign results in the formation of a clear dividing or separation line on the bottom wall (and top wall) of the duct. Additional flow features include the formation of streamwise vortices on the outer-wall of the second bend. These streamwise vortices can either be a pair of counter-rotating vortices or a single vortex. The formation mechanism of these streamwise vortices is explained using the Squire and Winter [J Aeronaut Sci 18(4):271–277, 1951] formula and it is shown that the said mechanism is applicable to both Re in the present study.     

弯管内爆轰波传播的流场显示和数值模拟

采用激光纹影系统拍摄了爆轰波在不同位置的流场照片. 用二阶附加半隐的龙格- 库塔法和五阶WENO格式 分别离散欧拉方程时间和空间导数项，用基元反应来描述爆轰化学反应过程，获得了压力、 温度、典型组元质量分数分布及数值胞格结构和爆轰波平均速度. 结果表明：受壁面稀疏波 和压缩波影响，爆轰波阵面发生畸变. 但由于弯管曲率半径较大，未出现爆轰波熄灭. 靠近 凹壁面的激波强度大于凸壁面侧，且凹壁面侧的反应区宽度较凸壁面侧要窄. 弯管出口处的 三波点数目较入口处减少，爆轰波衰减. 在出口直段，受扰动的爆轰波可恢复为自持爆轰波. 爆轰波流场、胞格结构、平均爆轰波速度的计算和实验结果定性一致.     

Investigation of temperature statistic characteristics in turbulent water flow with unsteady heat transfer

The results of an experimental study of a temperature field and its statistical characteristics in turbulent water flow upon a sudden change of heat generation in the channel wall are reported. Measurements were performed in 5 mm × 40 mm, 10 mm × 40 mm, and 20 mm × 40 mm channels in the regions of thermal stabilization and stabilized heat transfer at Reynolds numbers of (0.8–6.8) × 10⁴. The measurement results are generalized using a dimensionless time scale. The results of the calculation of heat transfer coefficients at unsteady heat transfer are presented.     

Application of a reflection-free DSM to turbulent flow and heat transfer in a square-sectioned U-bend

The paper reports the outcome of applying two differential second-moment (DSM) closures to resolve the complex three-dimensional motion that arises in turbulent flow in a square-sectioned duct passing around a 180°C bend. The initial results showed (in accord with a number of recent studies) that, with the same underlying closure hypotheses, a DSM scheme produces better agreement with experiment than does the corresponding algebraic second-moment (ASM) treatment, although the differences were acceptably small. Thereafter, applications are reported for a new type of DSM that employs no wall-reflection terms. This leads to markedly better predictions of the turbulence field and thus of the wall heat transfer than the conventionally adopted version.     

Turbulent forced convective heat transfer in two-phase evaporating droplet flow through a vertical pipe

A physical model was developed to study heat transfer in turbulent dispersed flow at very high vapor quality in a vertical pipe by numerically solving the coupling governing differential equations for both phases. Major heat transfer mechanisms included in the model were the thermal nonequilibrium effects, droplet vaporization, droplet deposition on the duct wall and thermal radiative transfer. The predicted results indicated that vapor superheating is dominant for the cases with high wall superheat, otherwise droplet vaporization dominates the energy transport processes. Heat transfer during the droplet-wall interaction only exists at low wall superheat but in small amounts.     

Effects of temperature-dependence of viscosity and viscous dissipation on laminar flow heat transfer in circular tubes

Heat transfer effects of variable viscosity and viscous dissipation for heated developing laminar flows in circular tubes have been investigated. Three studies are reported covering a comprehensive range of input data for the case of constant wall heat flux. Initially the program was used to predict the effect on heat transfer of temperature-dependent viscosity via a general temperature power relation. In addition, predictions were made for nine particular fluids covering a range of Prandtl numbers from 0.025 to 12 500, and a range of Brinkman numbers from 1.8 × 10^?10 to 6.8 × 10³. A more detailed study was made for two particular oils covering a range of practical interest. For the liquids considered their viscosity temperature-dependence resulted in enhancement of heat transfer, whereas for fluids with a Prandtl number <200 the effect of viscous dissipation was negligible, and for fluids of a Brinkman number > × 10^?2 the outcome was a reduction of heat transfer. A numerical instability problem occurred for situations of very high viscous dissipation which limited the length of duct that could be examined.     

水-甲醇混合工质两相闭式热虹吸管传热特性的研究

对水－甲醇混合工质两相闭式热虹吸管的传热特性进行了实验研究,实验表明,水－甲醇混合工质热管与纯工质（水）热管之间,以及在不同倾角条件下,热管轴向壁温分布均有较大差别,这是由于甲醇（易挥发组分）轴向浓度分布不同造成的。     

Numerical investigation of fully developed turbulent fluid flow and heat transfer in a square duct

Three-dimensional fully developed turbulent fluid flow and heat transfer in a square duct are numerically investigated with the author's anisotropic low-Reynolds-number k-ε turbulence model. Special attenton has been given to the regions close to the wall and the corner, which are known to influence the characteristics of secondary flow a great deal. Hence, instead of the common wall function approach, the no-slip boundary condition at the wall is directly used. Velocity and temperature profiles are predicted for fully developed turbulent flows with constant wall temperature. The predicted variations of both local wall shear stress and local wall heat flux are shown to be in close agreement with available experimental data. The present paper also presents the budget of turbulent kinetic energy equation and the systematic evaluation for existing wall function forms. The commonly adopted wall function forms that are valid for two-dimensional flows are found to be inadequate for three-dimensional turbulent flows in a square duct.     

Particulate Dispersion in a Turbulent Serpentine Channel

Particulate dispersion in an S-shaped duct, with periodicity between inlet and exit, is studied by direct numerical simulation. Stokes numbers range from 0.125 to 6.0. In a straight, turbulent channel flow, eddies are responsible for particulate impact. Turbophoresis causes a mean drift toward the wall. In a curved channel, particle inertia can be the dominant cause of impact. Above the lowest Stokes number, particles form into a plume that leaves the inner bend and flows toward the outer wall. Turbulence then disperses the plume. Heavier particles cross the bend and reflect from the outer wall, forming a high concentration layer near the surface. The heaviest particles reflect again from the wall and are dispersed across the duct by turbulence. An empirical formula is used to analyze the propensity for particle impacts to erode the wall. The region of maximum erosion is not the region of maximum number of impacts, nor is it where the impact velocity is highest: the impact angle determines where erosion is largest.     

Visualization of a confined accelerated bubble

High-speed photography was used to study the collapse of a confined two-dimensional, air cavity in water, subjected to a propagating pressure disturbance. The 5–6 mm diameter cavity was confined in a rectangular duct. A sustained pressure disturbance was created by an accelerating piston in contact with the water 240 mm away from the bubble. The pressure increased from 0.1 MPa to about 0.12 MPa with a rise time of the order of 2 ms. The pressure pulse was not reflected until its arrival at the end of the duct, 320 mm from the piston. A microjet was produced at the proximal wall which penetrated the distal cavity wall, thereby producing a pair of bubbles which was thought to be regions of intense vorticity. The features of such confined bubble collapse were not found in previous investigations of unconfined bubble accelerations by weak pressure disturbances. Confinement apparently intensified the effect of the disturbance significantly. Received 18 August 1998 / Accepted 12 May 1999     

An icing physics study by using lifetime-based molecular tagging thermometry technique

An experimental investigation was conducted to quantify the unsteady heat transfer and phase changing process within small icing water droplets in order to elucidate underlying physics to improve our understanding of the important micro-physical process of icing phenomena. A novel, lifetime-based molecular tagging thermometry (MTT) technique was developed and implemented to achieve temporally-and-spatially resolved temperature distribution measurements to reveal the time evolution of the unsteady heat transfer and dynamic phase changing process within micro-sized water droplets in the course of icing process. It was found that, after a water droplet impinged onto a frozen cold surface, the liquid water at the bottom of the droplet would be frozen and turned to solid ice rapidly, while the upper portion of the droplet was still in liquid state. As the time goes by, the interface between the liquid phase water and solid phase ice was found to move upward continuously with more and more liquid water within the droplet turned to solid ice. Interestingly, the averaged temperature of the remaining liquid water within the small icing droplet was found to increase, rather than decrease, continuously in the course of icing process. The temperature increase of the remaining liquid water is believed to be due to the heat release of the latent heat during solidification process. The volume expansion of the water droplet during the icing process was found to be mainly upward to cause droplet height growth rather than radial to enlarge the contact area of the droplet on the test plate. As a result, the spherical-cap-shaped water droplet was found to turn to a prolate-spheroid-shaped ice crystal with cusp-like top at the end of the icing process. The required freezing time for the water droplets to turn to ice crystals completely was found to depend on the surface temperature of the test plate strongly, which would decrease exponentially as the surface temperature of the frozen cold test plate decreases.     

Three-dimensional laminar flow and heat transfer in the entrance region of trapezoidal ducts

The laminar convective flow and heat transfer in a duct with a trapezoidal cross-sectional area are studied numerically. The governing equations are solved numerically by a finite volume formulation in complex three-dimensional geometries using co-located variables and Cartesian velocity components. Details of the numerical method are presented. The accuracy of the method was also established by comparing the calculated results with the analytical and numerical results available in the open literature. The Nusselt numbers are obtained for the boundary condition of a uniform wall temperature whereas the friction factors are calculated for no-slip conditions at the walls. The asymptotic values of the Nusselt numbers, friction factors. incremental pressure drops, axial velocity and momentum rate and kinetic energy correction factors approach the available fully developed values. Various geometrical dimensions of the cross-section are considered.     

Turbulence in cooling channels of rocket engines: Large Eddy Simulations

The aim of this Note is to predict by means of large eddy simulations the three-dimensional structures and secondary mass and heat fluxes which develop within a heated curved duct, for applications to rocket engines cooling channels. We show the existence of unsteady Görtler-type vortices above the concave wall, as well as intense secondary vortices taking the shape of two quasi-steady counter-rotating cells of Ekman type close to the convex wall. These cells control heat exchanges. They induce ejections and sweeps close to the convex wall when it is heated. In this case the Nusselt number undergoes strong transverse fluctuations which might induce material alterations. To cite this article: C. Münch, O. Métais, C. R. Mecanique 333 (2005).     

Interaction of Rarefaction Waves and Vacuum in a Convex Duct

The motion of the supersonic compressible flow governed by the Euler system in a two-dimensional convex duct is studied. The rarefaction waves in the compressible flow propagate and reflect on the walls of the convex duct, so that interaction occurs and a vacuum may appear. The existence of the global piecewise smooth solution to the steady Euler system in the interaction region is established. Meanwhile, the appearance of a vacuum is carefully considered. It is found that a vacuum is always adjacent to one of the walls and the appearance of a vacuum depends on the limit of the slope of the wall at the infinity.     

Comparison of the heat transfer characteristics of supercritical pressure water to that of subcritical pressure water in vertically-upward tubes

A systematic comparison was made between the forced convection heat transfer characteristics of the supercritical pressure water and that of the subcritical pressure water in vertically-upward tubes. It was found that, severe heat transfer deterioration did not occur in the vertically-upward internally-ribbed tube at supercritical pressures, and the variations in the inside wall temperature with the bulk fluid enthalpy experienced three stages, namely, the continuously increasing stage, the smoothly changing stage and another continuously increasing stage at the supercritical pressures; however, at subcritical pressures, there existed at least four stages for the variation of the inside tube wall temperature, i.e., the continuously increasing stage, the basically unchanging stage, the sharply rising stage and another continuously increasing stage. The heat transfer coefficients in the subcritical two-phase region, in which the heat transfer deterioration did not occur, were much greater than those in the heat transfer enhancement region of supercritical pressure water. In the large specific heat region of supercritical pressure water, the enhanced heat transfer was impaired by increasing the heat flux; however, in the subcritical two-phase region, the higher the heat flux, the greater the heat transfer coefficient would be. It was also found that the heat transfer deterioration of supercritical pressure water was similar in mechanism to the DNB (departure from nucleate boiling) at subcritical pressures.     

Hybrid turbulence simulation of spray impingement cooling: The effect of vortex motion on turbulent heat flux

Spray impingement has been a major interest of researchers in the areas of spray cooling, internal combustion, fire suppression and spray cooling, etc. for a long time. Numerous studies have been done in the area of spray cooling. Spray cooling with phase change takes advantage of relatively large amounts of latent heat and is capable of removing high heat fluxes from the surface, which has generated the interest of many researchers. In this paper, the turbulent characteristics of vapor formed during the spray impingement are studied. Water and gasoline are used in the numerical analysis of the two‐phase spray impingement on a heated wall. Hybrid turbulence modeling was used for the analysis where the subgrid scale model was employed away from the wall and k–ε model was used near the wall. Gasoline, at 298 K, was sprayed on the heated wall, kept constant at 650 K. The surrounding temperature was maintained at 400 K at the start of the simulation. In case of water and gasoline at Reynolds number 2750, the heated wall was kept constant at 400 K and the surrounding temperature was maintained at 298 K at the start of the simulations. The nozzle diameter of 100µm was used for this study, with the nozzle plate spacing ratio at 10. The spray was impinged on the flat plate at angles of 0, 15, and 30^°. Root mean‐squared velocities and turbulent heat flux were plotted in the water spray impingement for the different angles of impingement. The effect of turbulence on the heat transfer was observed. The effect of vortex motion on the turbulent heat flux values was analyzed using different Reynolds numbers of impingement and at different angles in case of gasoline. The turbulent heat flux attained the maximum values with high vortex formation. Upwash of fluid transported heat away from the wall, producing higher heat flux values in the region. Copyright © 2008 John Wiley & Sons, Ltd.