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research-article  
Ahmed Kadari, N. Sad Chemloul and Said Mekroussi
J. Heat Transfer   doi: 10.1115/1.4039081
Laminar natural convection in differentially heated square cavity with right cold wavy wall and horizontal conducting fin attached to its left hot wall has been investigated numerically. The vertical walls are maintained at different isothermal temperature while the horizontal walls are insulated. The fluid filled the cavity is air with Prandtl number of 0.71. The investigation has been performed for Rayleigh number in the range of 1000-1000000, the thermal conductivity ratio was varied from 10 to100000, three fin length and position have been examined (0.25, 0.5 and 0.75) and three numbers of undulation were tested (one, two and three undulations). The wave amplitude and the fin thickness were kept constant at 0.05 and 0.04 respectively. The results obtained show that increasing the fin thermal conductivity or the Rayleigh number increase the average Nusselt number especially when the fin length increases. It was also found that the fin position enhances the heat transfer when the fin is placed opposite to the crest of the wavy wall. The trend of the local Nusselt number is wavy. The number of undulation has a negligible effect on the average Nusselt number. The average Nusselt number enhanced when a conducting fin is added to the cavity with wavy wall and without fin by 51.23% and 56.85% for one and three undulations respectively.
TOPICS: Natural convection, Cavities, Rayleigh number, Thermal conductivity, Wave amplitude, Temperature, Heat transfer, Fluids, Prandtl number
research-article  
Georgios Karamanis, Marc Hodes, Toby Kirk and Demetrios Papageorgiou
J. Heat Transfer   doi: 10.1115/1.4039085
We consider convective heat transfer for laminar flow of liquid between parallel plates. The configurations analyzed are both plates textured with symmetrically-aligned isothermal ridges oriented parallel to the flow, and one plate textured as such and the other one smooth and adiabatic. The liquid is assumed to be in the Cassie state on the textured surface(s) to which a mixed boundary condition of no-slip on the ridges and no-shear along flat menisci applies. The thermal energy equation is subjected to a mixed isothermal-ridge and adiabatic-meniscus boundary condition on the textured surface(s). We solve for the developing three-dimensional temperature profile resulting from a step change of the ridge temperature in the streamwise direction assuming a hydrodynamically-developed flow. Axial conduction is accounted for, i.e., we consider the Extended Graetz-Nusselt problem; therefore, the domain is of infinite length. The effects of viscous dissipation and (uniform) volumetric heat generation are also captured. Using the method of separation of variables, the homogeneous part of the thermal problem is reduced to a non-linear eigenvalue problem in the transverse coordinates which is solved numerically. Expressions derived for the local and the fully-eveloped Nusselt number along the ridge and that averaged over the composite interface in terms of the eigenvalues, eigenfunctions, Brinkman number and dimensionless volumetric heat generation rate. Estimates are provided for the streamwise location where viscous dissipation effects become important.
TOPICS: Flow (Dynamics), Heat, Energy dissipation, Plates (structures), Boundary-value problems, Eigenvalues, Temperature profiles, Shear (Mechanics), Eigenfunctions, Convection, Temperature, Separation (Technology), Composite materials, Thermal energy, Laminar flow, Heat conduction
research-article  
Abdullah Al-Sharafi, Bekir Sami Yilbas and Haider Ali
J. Heat Transfer   doi: 10.1115/1.4039013
Heat transfer analysis for a water droplet on micro-post arrays is carried out while mimicking the environmental conditions. Since the micro-post arrays spacing size alters the state of the hydrophilicity of the surface, the size of the micro-post arrays spacing is varied and the resulting heat transfer characteristics are analyzed. Spreading rate of water droplet on the micro-post arrays is considered and the adhesion force for the pinning of the water droplet on the micro-post arrays is presented. Temperature and flow fields are predicted and the predictions of flow velocity inside the water droplet are validated through the particle image velocimetry (PIV). The Nusselt number variation for various sizes of the micro-post arrays is obtained for two droplet volumes. It is found that reducing the solid fraction of micro-post array beyond ?s = 0.25, the Cassie & Baxter state of the surface changes to the Wenzel state; in which case, hydrophobic characteristics changes to hydrophilic characteristics for the water droplet. Heat transfer from the droplet bottom gives rise to development of the buoyancy and the Marangoni currents, which in turn generate two counter rotating circulation cells. The center of circulation cells moves further in the droplet upper part for the hydrophobic droplet case. The Nusselt number attains high values for the hydrophobic droplet at micro-post array spacing size b = 10 ?m and hydrophobic droplet at spacing size b = 50 ?m due to fin effects of the micro-post arrays.
TOPICS: Heat transfer, Drops, Water, Flow (Dynamics), Buoyancy, Temperature, Adhesion, Particulate matter, Currents, Hydrophilicity
research-article  
Manjinder Singh, Sasidhar Kondaraju and Supreet Singh Bahga
J. Heat Transfer   doi: 10.1115/1.4039014
We present a mathematical model for dropwise condensation heat transfer on a surface with wettability gradient. We adapt well-established population balance model for dropwise condensation on inclined surfaces to model dropwise condensation on a surface with wettability gradient. In particular, our model takes into account the effect of wettability gradient and energy released during drop coalescence to determine the drop departure size. We validate our model with published experimental data of dropwise condensation heat flux and drop size distribution. Based on various experimental studies on drop motion, we also propose a mechanism that explains how the energy released during drop coalescence on a surface with wettability gradient and in a condensation environment aids drop motion. The mechanism correctly explains the shift of center of mass of two coalescing drops on a surface with wettability gradient towards the drop on high wetting region. Using the model, we analyze the effect of wettability gradient on the dropwise condensation heat flux. Our model predictions show that the optimal choice of wettability gradient is governed by differential variations in population density and heat transfer through a drop with change in wettability of the surface. We also demonstrate that contact angle at which there is maximum heat transfer through a drop varies with thickness of coating layer leading to change in optimal wettability gradient.
TOPICS: Condensation, Heat transfer, Heat flux, Density, Coating processes, Coatings, Center of mass, Wetting
research-article  
Yu Rao
J. Heat Transfer   doi: 10.1115/1.4039015
A comparative experimental and numerical study has been done on multiple-jet impingement heat transfer in narrow channels with different pin fin configurations, which include full-height pin fins and miniature pin fins on the target surfaces respectively. Three different target plates including a flat plate, a plate with full-height pin fins and another plate with miniature pin fins are investigated in the jet impingement cooling systems comparatively. The experiments were done under maximum cross flow scheme for the jet Reynolds numbers from 15,000 to 30,000. Narrow jet impingement spacing is kept the same as 1.5 times jet diameter for all the target plates. In the experiments, detailed jet impingement heat transfer characteristics on the flat plate and the full-height-pin-fin plate were obtained by using the transient liquid crystal thermography technique, and additionally steady experiments were done to obtain the overall heat transfer performance of the jet impingement systems with all the three different target plates, which accounts for the heat transfer contribution from the pin fins' surface. Significant overall jet impingement heat transfer enhancement can be obtained with full-height pin fin and miniature pin fin roughened surfaces in the narrow channels. Furthermore, three-dimensional CFD analysis was done to analyze the detailed flow structure and heat transfer characteristics in the jet impingement systems with different pin fin configurations.
TOPICS: Heat transfer, Fins, Plates (structures), Flat plates, Cross-flow, Flow (Dynamics), Liquid crystals, Reynolds number, Thermography, Transients (Dynamics), Computational fluid dynamics, Impingement cooling
Technical Brief  
R. Sadeghi, A. Salar Elahi, M. Ghoranevis and M.K. Salem
J. Heat Transfer   doi: 10.1115/1.4039012
A study on hydrogen isotopes energy and mass transfers affected by longitudinal or transverse magnetic fields under conditions in the Plasma Focus (PF) reactor have been performed. A specific feature of the heat transfer under these conditions is the combined exposure of a strong magnetic field and plasma pressure forces, which manifests itself in previously unknown effects. Among these effects are the existence, in some modes, of MHD heat transfer areas of "degraded" heat transfer, the extremely uneven distribution of heat transfer coefficients along the perimeter of the tube and abnormally high temperature fluctuations near the wall. Energetic and high fluency hydrogen ions generated with a low energy (3.3 kJ) PF device were used to study the hydrogen energy transfer. The treatment was performed using various numbers of deuterium plasma focus shots and at a distance of 8 cm from the anode tip.
TOPICS: Heat, Mass transfer, Tokamaks, Plasmas (Ionized gases), Hydrogen, Heat transfer, Magnetic fields, Heat transfer coefficients, High temperature, Fluctuations (Physics), Ions, Isotopes, Anodes, Energy transformation, Pressure, Magnetohydrodynamics
research-article  
Kevin D. Cole, Dr. Barbaros Cetin and Yasar Demirel
J. Heat Transfer   doi: 10.1115/1.4038987
Estimation of thermal properties, diffusion properties, or chemical-reaction rates from transient data requires that a model is available that is physically meaningful and suitably precise. The model must also produce numerical values rapidly enough to accommodate iterative regression, inverse methods, or other estimation procedures during which the model is evaluated again and again. Applications that motivate the present work include process control of microreactors, measurement of diffusion properties in microfuel cells, and measurement of reaction kinetics in biological systems. This study introduces a solution method for non-isothermal reaction-diffusion problems that provides numerical results at high precision and low computation time, especially for calculations of a repetitive nature. Here the coupled heat and mass balance equations are solved by treating the coupling terms as source terms, so that the solution for concentration and temperature may be cast as integral equations using Green's functions. This new method requires far fewer discretization elements in space and time than fully numeric methods at comparable accuracy. The method is validated by comparison with a benchmark heat transfer solution and a commercial code. Results are presented for a first-order chemical reaction that represents synthesis of vinyl chloride.
TOPICS: Diffusion (Physics), Chemical reactions, Heat, Temperature, Heat transfer, Chemical kinetics, Process control, Spacetime, Transients (Dynamics), Thermal properties, Numerical analysis, Computation, Integral equations, Micro fuel cells
research-article  
Kun Yang, Hao Chen and Jiabing Wang
J. Heat Transfer   doi: 10.1115/1.4038909
Convective heat transfer in the channel partially filled with porous medium prevails in many engineering applications. Many existing literatures focused on the channel partially filled with single layer porous medium. However, the problem of heat transfer and entropy generation inside the channel partially filled with N-layer porous media is analyzed in this work, in which the Darcy-Brinkman model and the local thermal non-equilibrium model are used to describe the flow and heat transfer within the porous region, respectively. Besides, the stress jump boundary condition and the heat flux jump boundary condition are adopted to describe momentum and heat transfer at the porous-free fluid interface, while the stress continuity boundary condition and the heat flux continuity boundary condition are used at the interface between two different porous layers. The analytical solutions of the velocity and temperature profiles for the channel are derived, which are utilized to calculate the overall Nusselt number, the total entropy generation rate, the Bejan number and the friction factor. Furthermore, the flow and heat transfer performances of the channel partially filled with 3-layer porous media are studied.
TOPICS: Heat transfer, Entropy, Porous materials, Boundary-value problems, Stress, Heat flux, Flow (Dynamics), Friction, Fluids, Equilibrium (Physics), Convection, Engineering systems and industry applications, Momentum, Temperature profiles
research-article  
Heng Wang, Samuel D. Marshall, Rerngchai Arayanarakool, Lakshmi Balasubramaniam, Xin Jin, Poh Seng Lee and Peter C. Y. Chen
J. Heat Transfer   doi: 10.1115/1.4038910
In this paper, the heat transfer performance of two roll-to-roll microchannel heat exchangers in the shape of square cross-section with the side length ranged from 0.2 mm to 0.5 mm are investigated via numerical studies. To assess the heat transfer enhancements, equivalent straight channel heat exchangers are also researched numerically as comparisons. For roll-to-roll devices, numerical studies demonstrated that there were two reasons for heat transfer enhancement. Firstly, inside the microchannel, when the average Dean Number was greater than around 10, Dean Vortices started to form within the roll-to-roll microchannels, enhancing the convective heat transfer between channels. Secondly, the compact roll-to- roll structure of the heat exchangers increased the heat transfer areas compared with straight microchannel equivalents, and thus promoted the conductive heat transfer. Numerical simulations noted both higher Nusselt Numbers and higher Thermal Performance Factors of roll-to-roll microchannel heat exchangers than the equivalent straight microchannels, and were employed to optimize both the microchannel cross-section dimension and the wall thickness between channels. In addition, this study also calculated the swirling strength and the heat transfer area to characterize the convective heat transfer and conductive heat transfer respectively, and made a comparison between two roll-to-roll microchannel heat exchanger designs.
TOPICS: Heat exchangers, Heat transfer, Microchannels, Convection, Computer simulation, Dimensions, Vortices, Shapes, Swirling flow, Wall thickness
research-article  
Tim Gronarz, Robert Johansson and Reinhold Kneer
J. Heat Transfer   doi: 10.1115/1.4038912
In this work, the effect of applying different approximations for the scattering phase function on radiative heat transfer in pulverized coal combustion is investigated. Isotropic scattering, purely forward scattering and a d-Eddington approximation are compared with anisotropic scattering based on Mie theory calculations. To obtain suitable forward scattering factors for the d-Eddington approximation, a calculation procedure based on Mie theory is introduced to obtain the forward scattering factors as a function of temperature, particle size and size of the scattering angle. Also, an analytical expression for forward scattering factors is presented. The influence of the approximations on wall heat flux and radiative source term in a heat transfer calculation is compared for combustion chambers of varying size. Two numerical models are applied: A model based on the Discrete Transfer Method, representing the reference solution and a model based on the Finite Volume method to also investigate the validity of the obtained results with a method often applied in commercial CFD programs. The results show that modeling scattering as purely forward or isotropic is not sufficient in coal combustion simulations. The influence of anisotropic scattering on heat transfer can be well described with a d-Eddington approximation and properly calculated forward scattering factors. Results obtained with both numerical methods show good agreement and give the same tendencies for the applied scattering approximations.
TOPICS: Scattering (Physics), Combustion, Anisotropy, Radiation scattering, Electromagnetic scattering, Thermal radiation, Coal, Modeling, Approximation, Heat transfer, Radiative heat transfer, Combustion chambers, Computer simulation, Simulation, Finite volume methods, Particle size, Heat flux, Computational fluid dynamics, Engineering simulation, Numerical analysis, Temperature
research-article  
Tairan Fu, Jiangfan Liu, Minghao Duan and Sen Li
J. Heat Transfer   doi: 10.1115/1.4038874
A high-speed (2 kHz) near-infrared (1.0 µm~1.65 µm) multispectral pyrometer was used for noninvasive measurements of the sub-pixel temperature distribution near the sharp leading edge of a wing exposed to a supersonic plasma jet. The multispectral pyrometer operating in the field measurement mode was able to measure the spatial temperature distribution. Multiple spectra were used to determine the temperature distributions in the measurement region. The spatial resolution of the multispectral pyrometer was not restricted to one "pixel" but was extended to sub-pixel accuracy (the temperature distribution inside one pixel in the image space corresponding to the point region in the object space). Thus, this system gives high-speed, multi-channel, and long working time spatial temperature measurements with a small data stream from high speed multispectral pyrometers. The temperature distribution of a ceramic model with a wing leading edge was investigated with the leading edge exposed to extreme convective heating from a high-enthalpy plasma flow with a total enthalpy of 2.0 MJ/kg. Simultaneous measurements with a multispectral pyrometer and an image pyrometer verify the measurement accuracy of the sub-pixel temperature distribution. It's a great contribution for multispectral pyrometry for in situ noninvasive temperature diagnostics in supersonic plasma jet environments.
TOPICS: Temperature measurement, Plasma jets, Pyrometers, Temperature distribution, Wings, Enthalpy, Heating, Accuracy and precision, Plasmas (Ionized gases), Resolution (Optics), Flow (Dynamics), Temperature, Spectra (Spectroscopy), Ceramics
research-article  
Ashraf Muhammad and Almas Fatima
J. Heat Transfer   doi: 10.1115/1.4038841
Present study is devoted to the problem of oscillatory convective flow in the presence of viscous dissipation around different positions of a sphere. The system of differential equations governing the flow phenomena are transformed into dimensionless form by using suitable group of variables and then transformed into convenient form for integration by using primitive variable formulation. Numerical simulation based on finite difference method is carried out to analyze the mixed convection flow mechanism. Special focus is given on the transient shear stress and rate of heat transfer characteristics and their dependency on various dimensionless parameters that is mixed convection parameter λ, Prandtl number Pr, dissipation parameter N and angular frequency parameter ω. The angles X = 30°, 90° and 360° are the favorable positions around the sphere for different parameters, where transient rate of shear stress and heat transfer is noted maximum. Later, the obtained results are presented graphically by using Tech Plot-360 and compare with the previous work given in literature
TOPICS: Heat transfer, Computer simulation, Energy dissipation, Transients (Dynamics), Shear stress, Flow (Dynamics), Mixed convection, Finite difference methods, Prandtl number, Differential equations
research-article  
Dr. Ali J. Chamkha, Igor Miroshnichenko and Mikhail A. Sheremet
J. Heat Transfer   doi: 10.1115/1.4038842
The problem of unsteady conjugate natural convection and entropy generation within a semi-circular porous cavity bounded by solid wall of finite thickness and conductivity has been investigated numerically. The governing partial differential equations with the corresponding initial and boundary conditions have been solved by the finite difference method using the dimensionless stream function, vorticity and temperature formulation. Numerical results for the isolines of the stream function, temperature, and the local entropy generation due to heat transfer and fluid friction as well as the average Nusselt and Bejan numbers, and the average total entropy generation and fluid flow rate have been analyzed for different values of the Rayleigh number, Darcy number, thermal conductivity ratio and the dimensionless time.
TOPICS: Entropy, Convection, Cavities, Thermal conductivity, Temperature, Heat transfer, Viscosity, Rayleigh number, Vorticity, Natural convection, Boundary-value problems, Finite difference methods, Partial differential equations, Fluid dynamics
research-article  
Hossein Askarizadeh and Hossein Ahmadikia
J. Heat Transfer   doi: 10.1115/1.4038851
This study introduces an analysis of high-order Dual-Phase-Lag (DPL) heat transfer equation and its thermodynamic consistency. The frameworks of extended irreversible thermodynamics and traditional second law are employed to investigate the compatibility of DPL model by evaluating the entropy production rates (EPR). Applying an analytical approach showed that both the first and second-order approximations of the DPL model are compatible with the traditional second law of thermodynamics under certain circumstances. If the heat flux is the cause of temperature gradient in the medium (over diffused or flux precedence heat flow), the DPL model is compatible with the traditional second law without any constraints. Otherwise, when the temperature gradient is the cause of heat flux (gradient precedence heat flow), the conditions of stable solutions of the DPL heat transfer equation might be necessary to become compatible with the local equilibrium thermodynamics. Finally, an insight inspection has been carried out to declare precisely the influence of several terms of the high-order DPL model on the EPRs. Keywords: High-order Dual Phase Lag, Analytical solution, Entropy production rate, Thermodynamics second law, Extended irreversible thermodynamics
TOPICS: Heat transfer, Extended irreversible thermodynamics, Heat flux, Temperature gradient, Flow (Dynamics), Thermodynamics, Heat, Entropy, Equilibrium (Physics), Second law of thermodynamics, Approximation, Inspection, European pressurized reactors
research-article  
David Gomse, Thomas M. Kochenburger and Steffen Grohmann
J. Heat Transfer   doi: 10.1115/1.4038852
Heat exchangers are important components in many engineering applications. This paper proposes a numerical two-phase heat exchanger model with simultaneous heat transfer and pressure drop calculations. The presented model provides a modelling framework compatible with numerous different correlations for both single- and two-phase flow of pure fluids and fluid mixtures. Furthermore, it considers nonconstant fluid properties as well as longitudinal heat conduction and parasitic heat loads, which is particularly relevant in mixed refrigerant cycles for cooling of low-temperature applications. The governing equations are derived and the solution strategy is presented, followed by the model validation against analytical solutions in the corresponding limits. Finally, an exemplary heat exchanger is analysed using both homogeneous and separated flow models and the results are compared with experimental data from literature.
TOPICS: Fluids, Modeling, Heat exchangers, Low temperature, Two-phase flow, Cycles, Model validation, Pressure drop, Refrigerants, Heat conduction, Stress, Engineering systems and industry applications, Flow (Dynamics), Heat, Heat transfer, Cooling
research-article  
Alfred Davletbaev, Liana Kovaleva, Aleksey Zainulin and Tayfun Babadagli
J. Heat Transfer   doi: 10.1115/1.4038853
Production of heavy oil from deep/tight formation using traditional technologies ("cold" production, injection of hot steam, etc.) is ineffective or inapplicable. An alternative is electromagnetic (EM) heating after fracturing. This paper presents the results of a numerical study of heavy oil production from a well with hydraulic fracture under radio-frequency (RF) electromagnetic radiation. Two parameters ignored in our previous modeling studies, namely adiabatic effect and the thermal expansion of oil, are considered in the new formulation while high gradients of pressure/temperature and high temperature occur around the well. The mathematical model is simulated distribution of pressure and temperature in the system of "well-fracture- formation". The distribution of thermal heat source is given by the Abernetty expression. The mathematical model takes into account the adiabatic effect and thermal expansion of heavy oil. The latter makes a significant contribution to heavy oil production. Multi-stage heavy production technology with heating is assumed and several stages are recognized: Stage 1: "Cold" heavy oil production, Stage 2: RF-EM heating, Stage 3: RF is turned off and "hot" oil production continues until the flow rate reaches its initial (before heating) value. These stages are repeated starting from the second stage. Finally, RF-EM heating technology is compared to "cold" production in terms of additional oil production and economics.
TOPICS: Thermal expansion, Electromagnetic radiation, Computer simulation, Hydraulic fracturing, Heating, Fracture (Process), Pressure, Temperature, Flow (Dynamics), Heat, Manufacturing technology, Modeling, Steam, High temperature, Economics
research-article  
Dr. Ephraim Sparrow, John M. Gorman and Daniel Bryant
J. Heat Transfer   doi: 10.1115/1.4038843
Heat transfer coefficients for turbulent pipe flow are typically envisioned as axially varying from very high values at the pipe inlet to a subsequent monotonic decrease to a constant fully developed value. This distribution, although well enshrined in the literature, may not be universally true. Here, by the use of high accuracy numerical simulation, it was shown that the initially decreasing values of the coefficient may attain a local minimum before subsequently increasing to a fully developed value. This local minimum may be characterized as an undershoot. It was found that whenever a turbulent flow laminarizes when it enters a round pipe, the undershoot phenomenon occurs. The occurrence of laminarization depends on the geometry of the pipe inlet, on fluid-flow conditions in the upstream space from which fluid is drawn into the pipe inlet, on the magnitude of the turbulence intensity, and on the Reynolds number. However, the presence of the undershoot does not affect the fully developed values of the heat transfer coefficient. It was also found that the Fanning friction factor may also experience an undershoot in its axial variation. The magnitude of the heat transfer undershoot is generally greater than that of the Fanning friction factor undershoot.
TOPICS: Friction, Turbulence, Pipe flow, Entrance region, Heat transfer coefficients, Pipes, Geometry, Computer simulation, Reynolds number, Heat transfer, Fluids, Fluid dynamics
research-article  
Sean Hoenig and Richard Bonner
J. Heat Transfer   doi: 10.1115/1.4038854
Previous research in dropwise condensation on rough micro-textured superhydrophobic surfaces has demonstrated evidence of high heat transfer enhancement compared to smooth hydrophobic surfaces. In this study, we experimentally investigate the use of microporous sintered copper powder on copper substrates coated with a thiol-based self-assembled monolayer to attain enhanced dropwise condensation for steam in a custom condensation chamber. Although micro-textured superhydrophobic surfaces have shown advantageous droplet growth dynamics, precise heat transfer measurements are underdeveloped at high heat flux. Sintered copper powder diameters from 4µm to 119µm were used to investigate particle size effects on heat transfer. As powder diameter decreased, competing physical factors led to improved thermal performance. At consistent operating conditions, we experimentally demonstrated a 23% improvement in the local condensation heat transfer coefficient for a superhydrophobic 4µm diameter microporous copper powder surface compared to a smooth hydrophobic copper surface. For the smallest powders observed, this improvement is primarily attributed to the reduction in contact angle hysteresis as evidenced by the decrease in departing droplet size. Interestingly, the contact angle hysteresis of sessile water droplets measured in air is in contradiction with the departing droplet size observations made during condensation of saturated steam. It is evident that the specific design of textured superhydrophobic surfaces has profound implications for enhanced condensation in high heat flux applications.
TOPICS: Condensation, Drops, Heat transfer, Copper powders, Copper, Steam, Heat flux, Heat transfer coefficients, Water, Dynamics (Mechanics), Surface roughness, Design, Particle size, Self-assembly
research-article  
Dong Wang, Dan Ewing and Dr. Chan Ching
J. Heat Transfer   doi: 10.1115/1.4038844
Experiments were performed to investigate the local development of roughness and its effect on mass transfer in an S-shaped bend at Reynolds number of 200,000. The tests were performed over four consecutive time periods using a 203 mm diameter test section with a dissolving gypsum lining to water in a closed flow loop at a Schmidt number of 1200. The surface roughness and mass transfer over the test periods were measured using X-ray CT scans of the surface. Two regions of high mass transfer are found: along the intrados of the first and second bend. The surface roughness in these two regions, characterized by the height-to-spacing ratio, grows more rapidly than in the upstream pipe. There is an increase in the mass transfer with time, which corresponds well with the local increase in the height-to-spacing ratio of the roughness. The two regions of high mass transfer enhancement in the bend can be attributed to both a roughness effect and flow effect due to the bend geometry. The geometry effect was determined by normalizing the local mass transfer with that in a straight pipe with equivalent surface roughness. The mass transfer enhancement due to the geometry effect was found to be relatively constant for the two high mass transfer regions, with a value of approximately 1.5.
TOPICS: Mass transfer, Reynolds number, Surface roughness, Geometry, Pipes, Flow (Dynamics), Computerized tomography, Water, Linings (Textiles), Gypsum
research-article  
Luca Marocco and Francesco Garita
J. Heat Transfer   doi: 10.1115/1.4038858
In the present study turbulent forced and mixed convection heat transfer to a liquid metal flowing upward in a concentric annulus is numerically investigated by means of \mbox{Large Eddy Simulation (LES)}. The inner-to-outer radius ratio is 0.5. The Reynolds number based on bulk velocity and hydraulic diameter is 8900, while the Prandtl number is set to a value of 0.026. A uniform and equal heat flux is applied on both walls. LES has been chosen to provide sufficiently accurate results for validating Reynolds-Averaged turbulence models. Moreover, being the thermal sublayer thickness of liquid metals much larger than the viscous hydrodynamic one, liquid metals present a separation between the turbulent thermal and hydrodynamic scales. Thus, with the same grid resolution it is possible to perform a LES for the flow field and a ``thermal`` Direct Numerical Simulation (DNS) for the temperature field. Comparison of the forced convection results with available DNS simulations shows satisfying agreement. Results for mixed convection are analyzed and the differences with respect to forced convection at the same Reynolds number are thoroughly discussed. Moreover, where possible, a comparison with air is performed.
TOPICS: Turbulence, Liquid metals, Mixed convection, Annulus, Large eddy simulation, Reynolds number, Forced convection, Simulation, Resolution (Optics), Eddies (Fluid dynamics), Computer simulation, Flow (Dynamics), Temperature, Heat transfer, Separation (Technology), Prandtl number, Heat flux

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