Melting in a vertical tube rotating about a vertical,colinear axis

Experiments were performed in which melting of a phase-change medium occurred in a closed vertical tube which was rotated about a vertical axis colinear with that of the tube. The melting was initiated and maintained by a step-change increase in the wall temperature of the containment tube. During the course of the experiments, parametric variations were made in the rotational speed, in the temperature difference which drives the melting, and in the duration of the melting period. The phase-change medium was 99% pure n-eicosane paraffin with a melting temperature of 36.3°C. It was found that rotation gave rise to considerably more rapid melting than that for no rotation, with the time required to achieve a given amount of melting being halved due to rotation. The rate at which energy could be stored was also significantly increased by rotation. Furthermore, at any duration of the melting period, the shape of the unmelted solid differed markedly in the presence or absence of rotation, being either straight-sided or sloped-sided. The melted mass results for all of the investigated conditions were very tightly correlated in terms of the Froude, Stefan, Grashof, and Fourier numbers.     

Numerical investigation of the heat and mass transfer in a vertical tube evaporator with the three-zone analysis

A numerical study for the flow, heat and mass transfer characteristics near the inflow region of the vertical evaporating tube with the films flowing down on both the inside and outside tube walls has been carried out. Condensation occurs along the outside wall and evaporation at the free surface of the inside film. The transport equations for momentum and energy are parabolized by the boundary-layer approximation and solved by using the marching technique. In this kind of numerical approach, the accurately predicting the early stage is really important because a small error at the previous step can produce the amplified big error at the next step. To accurately predict the flow at the inflow region of the vertical evaporating tube, the calculation domain of two film flow regions and tube wall is solved simultaneously. The interesting heat transfer characteristics revealed through this three-zone simulation, such as the evaporation delay and the temperature inflection at the very near inflow region are found and discussed along the discrepancy between the inner film inlet temperature and the saturation temperature. The case that the inner film comes in with the saturation temperature shows a good performance. The velocity and temperature fields as well as the amounts of the condensed and evaporated mass in both inner and outer films are predicted for the various conditions.     

Numerical investigation of an enhanced PTC absorber tube using cylindrical inserts

The main purpose of this study is to investigate numerically the thermal performance of a parabolic trough solar collector's absorber tube that contains a novel kind of inserts with the objective to improve the heat transfer between the heat transfer fluid and the absorber tube. In the first part of this paper, the diameter and the length of the cylindrical inserts are investigated based on finite volume method and Monte Carlo ray tracing method for Reynolds number ranges from 2.36 × 10⁴ to 7.09 × 10⁴. In the second part, the eccentricity of the cylindrical inserts is investigated under the same operating conditions. The Therminol®VP1 is the HTF that is used in this investigation's intermediate fluid. The numerical simulation indicates that the perturbators enhance the thermal behavior of the receiver and reduces the absorber tube's temperature difference.     

Numerical investigation of boiling flow of nitrogen in a vertical tube using the two-fluid model

The two-fluid model has been extensively used in modeling boiling flow of water, however, there are few equivalent studies of boiling flow of cryogenic liquids. In the present study, the two-fluid model which is basically included in the CFX code was modified by incorporating new closure correlations, then boiling flow of liquid nitrogen in a vertical tube was numerically simulated using the basic model and the modified model, respectively. Comparison with experimental data shows that the modified model is satisfactorily improved in accuracy. This study demonstrated that the following parameters and models are important for accurate prediction: the lift force, the bubble diameter distribution and the active site density, among which, the active site density has the most significant effect.     

Melting of a vertical ice cylinder inside a rotating cylindrical cavity filled with binary aqueous solution

The melting of a vertical ice cylinder into a homogeneous calcium chloride aqueous solution inside a rotating cylindrical cavity with several rotating speeds is considered experimentally. The melting mass and temperature are measured on four initial conditions of the solution and four rotating speeds of the cavity. The temperature of the liquid layer becomes uniform by the mixing effect resulting from cavity rotation and it enhances the melting rate of the ice cylinder. As the cavity‐rotating speed increases, the melting rate increases. The dimensionless melting mass is related to the Fourier number and the rotating Reynolds number in each initial condition, therefore an experimental equation that is able to quantitatively calculate the dimensionless melting mass is presented. It is seen that the melting Nusselt numbers increase again in the middle of the melting process. The ice cylinder continues to melt in spite of the small temperature difference between the ice cylinder and the solution. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(6): 359–373, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20211     

Melting of frozen, porous media contained in a horizontal or a vertical, cylindrical capsule

Melting of ice in porous media has been investigated experimentally and analytically for a horizontal and vertical cylindrical capsule. Quantitative results of the temperature distribution and solid-liquid interface motion and shape were obtained for inward melting with different size and types of spherical beads used as the porous media. Predictions from an analysis which considers conduction as the only mode of heat transfer in both the solid and liquid were compared to experimental data to show where natural convection becomes significant. It was found that the melting rate was augmented by natural convection in the liquid. For large differences in the thermal conductivity of the phase-change material and porous medium (e.g. water and aluminum), the effective thermal conductivity of the system was not predicted accurately by the model used, resulting in a further discrepancy between data and predictions. Moreover, the assumption of local temperature equilibrium between the void constituent and the porous medium becomes invalid for a water-aluminum bead system.     

Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement

The present paper describes the analysis of the melting process in a single vertical shell‐and‐tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid‐dynamic (CFD) model that takes into account of the phase‐change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase‐change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase‐change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano‐enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano‐enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Numerical investigation on mixed convection in a non-Newtonian fluid inside a vertical duct

This paper reports a numerical study of the thermal and fluid-dynamic behaviour of laminar mixed convection in a non-Newtonian fluid inside a vertical duct enclosed within two vertical plates that are plane and parallel, having linearly varying wall temperatures. The other inlet conditions consist of a parabolic distribution of the velocity field and a constant fluid temperature. The problem is assumed to be steady and two-dimensional. The formulation of a mathematical model in dimensionless co-ordinates and the discretisation of the governing equations by means of the finite difference method, have made it possible to create a numerical code developed in Matlab environment. The study was focused on the simultaneous presence and on the mutual interaction of natural and forced convection, starting from the effects of the re-circulation on the heat transfer. The quantitative results of the analysis, which are strongly affected by the variation of the Grashof number and of the exponent of the power law, are given in terms of graphic visualisations of the fluid velocity profiles and, when the governing parameters vary, of the various geometries characterising the heat transfer.     

Free convection in a vertical cylindrical enclosure

The problem of transient natural convection which occurs in a vertical cylinder opened at both ends, filled with a fluid saturated porous medium and heated with a periodical lateral heat flux density is outlined. The present study is carried out by the use of the Darcy flow model, and it assumes local thermal equilibrium between the solid and fluid phases. The wall heat conduction is taken into account. Numerical simulations provide us with the evolution of flow and temperature fields within the cylinder. The analysis of flow and thermal field response to any changes in the period of heat pulsation values, the ratio of the wall thermal diffusivity to the porous medium thermal diffusivity and the thickness of the wall are reported in the course of this study.     

Numerical simulation of the upward propagation of a flame in a vertical tube filled with a very lean mixture

Upward propagation of a premixed flame in a vertical tube filled with a very lean mixture is simulated numerically using a single irreversible Arrhenius reaction model with infinitely high activation energy. In the absence of heat losses and preferential diffusion effects, a curved flame with stationary shape and velocity close to those of an open bubble ascending in the same tube is found for values of the fuel mass fraction above a certain minimum that increases with the radius of the tube, while the numerical computations cease to converge to a stationary solution below this minimum mass fraction. The vortical flow of the gas behind the flame and in its transport region is described for tubes of different radii. It is argued that this flow may become unstable when the fuel mass fraction is decreased, and that this instability, together with the flame stretch due to the strong curvature of the flame tip in narrow tubes, may be responsible for the minimum fuel mass fraction. Radiation losses and a Lewis number of the fuel slightly above unity decrease the final combustion temperature at the flame tip and increase the minimum fuel mass fraction, while a Lewis number slightly below unity has the opposite effect.     

Numerical investigation on heat transfer of supercritical water in a roughened tube

ABSTRACT

The turbulent mixed convection heat transfer of supercritical water flowing in a vertical tube roughened by V-shaped grooves has been numerically investigated in this paper. The turbulent supercritical water flow characteristics within different grooves are obtained using a validated low-Reynolds number κ-ε turbulence model. The effects of groove angle, groove depth, groove pitch-to-depth ratio, and thermophysical properties on turbulent flow and heat transfer of supercritical water are discussed. The results show that a groove angle γ = 120° presents the best heat transfer performance among the three groove angles. The lower groove depth and higher groove pitch-to-depth ratio suppress the enhancement of heat transfer. Heat transfer performance is significantly decreased due to the strong buoyancy force at T_b = 650.6 K, and heat transfer deterioration occurs in the roughened tube with γ = 120°, e = 0.5 mm, and p/e = 8 in the present simulation. The results also show that the rapid variation in the supercritical water property in the region near the pseudo-critical temperature results in a significant enhancement of heat transfer performance.     

导叶式升力型垂直轴风力机的数值研究

分析传统H型Darrieus风力机整体气动特性低的原因,从改善风力机极小攻角处极差气动特性出发,提出一种在极小攻角处添加导叶的导叶式升力型垂直轴风力机。采用非定常RNG湍流模型和滑移网格技术,对导叶式升力型垂直轴风力机的空气动力场进行二维数值模拟。比较分析0°方位角导叶对叶轮流场、叶片表面压差系数及叶片力矩的影响。研究结果表明,导叶改善了叶片表面负压差区,使叶片表面压差系数提高了56.2%,叶轮转矩增加了25.4%,有效地提高了风力机整体气动性能。     

An investigation of the brazier effect of a cylindrical tube under pure elastic-plastic bending

The Brazier effect of originally straight cylindrical tubes under pure elastic-plastic bending is investigated by the deformation theory based on some deformation assumptions agreeing with numerous experiments. The expressions for bending moment and flattening ratio in terms of curvature are finally obtained.     

Experimental and numerical investigation of two-phase pressure drop in vertical cross-flow over a horizontal tube bundle

Experimental pressure drop data for vertical two-phase air–water flow across horizontal tubes is presented for gas mass fractions in the range 0.0005–0.6 and mass fluxes in the range 25–700 kg/m² s. The square in-line tube bundle had one column containing ten tubes and two columns of half tubes attached to the walls. The tubes had a diameter of 38 mm and a pitch to diameter ratio of 1.32. This data and air–water and R113 vapour–liquid data available in the literature are compared with the predictions from two kettle reboiler models, the one-dimensional model and a one-dimensional formulation of the two-fluid model. The one-dimensional model was implemented with three separate void fraction correlations and one two-phase friction multiplier correlation. The results show that the two-fluid model predicts air–water void fraction data well but R113 data poorly with pressure drop predictions for both being unsatisfactory. The one-dimensional model is shown to predict pressure drop and void fraction data reasonably well, provided a careful choice is made for the void fraction correlation.     

Numerical and experimental investigation of two phase flow during boiling in a coiled tube

A numerical simulation, using the VOF multiphase flow model, and the corresponding experiments were conducted to investigate the boiling flow of R141B in a horizontal coiled tube. The numerical predictions of phase evolution were in a good agreement with the experimental observations, and the two phase flow in the tube bends was much more complicated due to the influence of liquid–vapor interaction with the interface evolution. The associated heat transfer was also considered. It was found that the temperature profile in the two phase flow was significantly affected by the phase distribution and higher temperature always appears in the vapor region.     

Numerical heat transfer analysis of laminar film condensation on a vertical fluted tube

The purpose of this study is to find a convenient and practical procedure for calculating heat transfer of laminar film condensation on a vertical fluted tube. The condensate film on the tube surface along the axial direction was divided into two portions: the initial portion and the developing portion. The developing portion was analyzed in details. The film thickness equation of condensate film over the crest and the momentum equation of condensate film in the trough were established respectively after some simplifications and coupled with two-dimensional thermal conduction equation. The relationship between the heat transfer rate and the length of the flute was obtained through solving the equations numerically. The present procedure was tested on a sinusoidal fluted tube. The amount of heat transfer rates Q_t of the tube were calculated at different temperature differences by using this procedure. The calculation was compared with the experimental data quoted. The results were in good agreement with a maximum deviation of 18%. So the present procedure is reliable and can be used in the parameter design of sinusoidal fluted tubes.     

Experimental investigation of heat transfer of supercritical carbon dioxide flowing in a cooled vertical tube

This work presents an experimental investigation on the heat transfer characteristics of a cooled vertical turbulent flow of supercritical carbon dioxide. The test section consists of two sectors (vertical tube-in-tube) connected in series by means of a U-bend. An integral method is used to treat the experimental data. Experimental results illustrate the influence of buoyancy forces on the heat transfer process. Results are presented in dimensionless form commonly used in the studies of mixed convection in heating. Correlations have been developed for upward and downward flows. The present results complete the literature on turbulent vertical mixed convection under cooling conditions.     

Numerical investigation of the thermal separation in a Ranque–Hilsch vortex tube

The application of a mathematical model for the simulation of thermal separation in a Ranque–Hilsch vortex tube is presented in this paper. The modelling of turbulence for compressible, swirling flows used in the simulation is discussed. The work has been carried out in order to provide an understanding of the physical behaviors of the flow, pressure, temperature in a vortex tube. A staggered finite volume approach with the standard k–ε turbulence model and an algebraic stress model (ASM) is used to carry out all the computations. To investigate the effects of numerical diffusion on the predicted results, the second-order upwind (SOU) and the QUICK numerical schemes are used and compared with the first-order upwind and the hybrid schemes. The computations show that the differences of results obtained from using the various schemes are marginal. In addition, results predicted by both turbulence models generally are in good agreement with measurements but the ASM performs better agreement between the numerical results and experimental data. The computations with selective source terms of the energy equation suppressed show that the diffusive transport of mean kinetic energy has a substantial influence on the maximum temperature separation occurring near the inlet region. In the downstream region far from the inlet, expansion effects and the stress generation with its gradient transport are also significant.