bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow Structure and Heat Transfer in Stagnation Flow CVD Reactor
Authors: Memon Nasir , Jaluria Yogesh
Published: March 2011

An experimental study is undertaken to investigate the flow structure and heat transfer in a stagnation flow Chemical Vapor Deposition (CVD) reactor at atmospheric pressure. It is critical to develop models that predict flow patterns in such a reactor to achieve uniform deposition across the substrate. Free convection can negatively affect the gas flow as cold inlet gas impinges on the heated substrate, leading to vortices and disturbances in the normal flow path. This experimental research will be used to understand the buoyancy-induced and momentum-driven flow structure encountered in an impinging jet CVD reactor. Investigations are conducted for various operating and design parameters. A modified stagnation flow reactor is built where the height between the inlet and substrate is reduced when compared to a prototypical stagnation flow reactor. By operating such a reactor at certain Reynolds and Grashof numbers it is feasible to sustain smooth and vortex free flow at atmospheric pressure. The modified stagnation flow reactor is compared to other stagnation flow geometries with either a varied inlet length or varied heights between the inlet and substrate. Comparisons are made to understand the impact of such geometric changes on the flow structure and the thermal boundary layer. In addition, heat transfer correlations are obtained for the substrate temperature. Overall, the results obtained provide guidelines for curbing the effects of buoyancy and for improving the flow field to obtain greater film uniformity when operating a stagnation flow CVD reactor at atmospheric pressure.
Study on Collector Efficiency of Flat-Plate-Type Evacuated Solar Collector to Get Hot Water Near 100°C
Authors: Hirasawa Shigeki , Kawanami Tsuyoshi
Published: March 2011

We studied effects of parameters on collector efficiency of evacuated solar collector system to get hot water near 100°C. Change of temperature in the solar collector system is calculated for a daily change of solar radiation with 5 minutes cloud. Six operation-control methods are examined. Calculation results show that the effect of the control methods on average collector efficiency for one day is small as 1%. Best control method to minimize effect of the cloud on exit temperature fluctuation of the water is that the flow rate of the water is controlled proportional to the solar radiation. Two types of heat storage system are examined: a non-circulating type (supply new water and accumulate heated water in the heat storage tank) and a circulating type (circulating water from the heat storage tank). The non-circulating type is effective to use the solar energy in the daytime, and the circulating type is effective to use solar energy in the evening. Also, we measured temperature of a collector plate under actual solar radiation in a fine day.
Heat Transfer to Suspensions of Microencapsulated Phase Change Material (MEPCM) Flowing Through Minichannels
Authors: Dammel Frank , Stephan Peter
Published: March 2011

The heat transfer to water-based suspensions of microencapsulated phase change material (MEPCM) flowing laminarly through rectangular copper minichannels was investigated both experimentally and numerically. The MEPCM-particles had an averags size of 5μm and contained as phase change material neicosane, which has a theoretical melting temperature of 36.4°C. Water and suspensions with particle mass fractions of 10% and 20% were considered. While the experiments result in rather global values such as wall temperatures at certain points, suspension in- and outlet temperatures and the pressure drop, the numerical simulations allow additionally a more detailed insight for example into the temperature distribution in the flowing suspension. The results show that MEPCM suspensions are only advantageous in comparison to water in a certain range of parameter combinations, where the latent heat is exploited to a high degree. The available latent heat storage potential, which depends on the particle fraction in the suspension and on the mass flow rate, has to be in the same order of magnitude as the supplied heat. Moreover, the mean residence time of the particles in the cooling channels must not be considerably shorter than the characteristic time for heat conduction perpendicular to the flow direction. Otherwise the particles in the center region of the flow leave the cooling channels with still solid cores and their latent heat is not exploited. Furthermore, the benefit of the added MEPCM particles depends on the inlet temperature, which has to be slightly below the theoretical melting temperature, and on the subcooling temperature after the heat supply, which has to be sufficiently low to guarantee that the entire phase change material solidifies again before it reenters the cooling channels. The suspensions showed Newtonian behavior in the viscosity measurement. The actual pressure drop determined in the experiments is smaller than the pressure drop estimation based on the measured viscosities. The difference between the two values increases with increasing particle mass fraction. This shows that the particles are not evenly distributed in the flowing suspension, but that there is a particle depleted layer close to the channel walls. This reduces the required pumping power, but makes it even more important to provide conditions, in which a sufficiently large amount of the supplied heat is conducted to the center region of the channels.
Effect of Vapour Velocity on Condensation of Ethylene Glycol on Horizontal Integral Fin Tubes: Heat Transfer and Retention Angle Measurements
Authors: Fitzgerald Claire L. , Briggs Adrian , Wang Huasheng , Rose John W.
Published: March 2011

Heat-transfer data are reported for forced-convection filmwise condensation of ethylene glycol flowing vertically downward over two single, horizontal instrumented integral-fin tubes and one plain tube. Vapor-side, heat-transfer coefficients were obtained by direct measurement of the tube wall temperature using two specially manufactured, instrumented tubes with thermocouples embedded in the tube walls. Both tubes have fin height of 1.6 mm and fin root diameter and 12.7 mm, with fin thickness and spacings of 0.3 mm and 0.6 mm, respectively for one of the tubes and 0.5 mm and 1 mm, respectively for the other. Tests were performed at low pressures; 5.59kPa, 8.15kPa and 11.23kPa, at nominal vapour velocities from 13m/s to 82 m/s. All the data show that both of the finned tubes provided an increase in heat flux at the same vapour-side temperature difference with increasing vapour velocity. Visual observations were made and photographs obtained of the fluid retention angle φf at each combination of vapor velocity and pressure tested. It was observed that the curvature of the meniscus was distorted by the increase in vapor velocity and in many cases, the extent of condensate flooding decreased compared to its value in the quiescent vapor case.
Characterisation of a Wavy Convective Instability in Poiseuille-Rayleigh-Be´nard Flows: Linear Stability Analysis and Non Linear Numerical Simulations Under Random Excitations
Authors: Nicolas Xavier , Xin Shihe , Zoueidi Noussaiba
Published: March 2011

The aim of the present paper is to characterize a secondary convective instability of Poiseuille-Rayleigh-Be´nard (PRB) mixed convection flows in air that takes the shape of wavy thermoconvective rolls, for 70≤Re≤300 and 3000<Ra<15000. At first, the linear stability analysis by Clever and Busse [JFM, 1991] in the case of PRB flows between two infinite plates is extended to the case of confined channels with a 10 transversal aspect ratio. In the second part, using 3D non linear direct numerical simulations, the space and time development of the chaotic wavy rolls obtained by maintaining a permanent random excitation at channel inlet is analyzed. As the perturbation is designed to cover all the modes, it is possible to detect the modes that are naturally amplified by the flow and those that are damped. It is shown that the wavy roll characteristics obtained in this way vary a lot with Ra increasing and stabilize for Ra>3Ra*. Comparisons with the experiments by Pabiou et al. [JFM, 2005] are proposed.
Experimental and 3D Numerical Investigation of Buoyancy-Driven Instabilities Around a Heated Horizontal Square Cylinder Within an Enclosure
Authors: Leplat Gillian , Reulet Philippe , Millan Pierre , Fraigneau Yann , Le Que´re´ Patrick
Published: March 2011

This study addresses the natural convection flow driven by a heated square cylinder within an air-filled enclosure for parameter values corresponding to the onset of unsteadiness and weakly transitional flow regime. 2D time-resolved PIV measurements are jointly used with 2D and 3D numerical simulations, either with full or linearized equations, to study the intrinsic 3D flow instabilities. Two types of vortical structures develop within the enclosure in the parallel and transverse directions to the cylinder axis. The interaction between these convective cells initiates an unsteady flow regime characterized by large scale eddies swaying around the cylinder and wave oscillations which lead to the generation of peculiar hook-shaped structures. Statistical and spectral analyses of the flow help pointing up some noticeable features of this unsteady flow such as the spectrum of the longitudinal spatial modes that reveals a turbulence-like pattern with unconventional slopes in the inertial and dissipative zones.
Numerical Study of Partial Oxidation of a Single Coal Particle in a Stream of Air
Authors: Kestel Matthias , Nikrityuk Petr , Meyer Bernd
Published: March 2011

This work is devoted to the numerical study of the impact of the Reynolds number on the partial oxidation of a single moving coal particle. The model includes four gaseous chemical species (O2, CO2, CO, N2). Two heterogeneous surface reactions and one homogeneous gas reaction are employed. The Navier-Stokes equations coupled with the energy and species conservation equations were used to solve the problem. Based on the results of the numerical simulations we found the existence of three basic regimes characterizing the distribution of CO around the particle. The first regime, which we call gasification regime, exists by low surface temperatures of the particle and high Reynolds numbers and is characterized by absence of CO2 gases around the particle. The second regime is transitional. It is characterized by a horseshoe-shaped CO distribution around the particle. And the third regime is defined by a flame sheet (thin CO2 layer) surrounding the particle and elongated in the flow direction. This regime corresponds to the combustion of the particle and exists at higher temperatures of the particle surface.
The Effect of Outlet Inclination on Two-Phase Flow Splitting in an Equal-Sided Impacting Tee Junction
Authors: Mohamed M. A. , Soliman H. M. , Sims G. E.
Published: March 2011

Phase-distribution data have been generated for two-phase (air-water) flow splitting at an impacting tee junction with a horizontal inlet and inclined outlets. This investigation also considers the full-separation capabilities of the junction and the effect of the outlet angle of inclination on partial separation at various inlet conditions. A flow loop with the ability to incline the outlets from −90° downward to +90° upward was constructed. The operating conditions were as follows: test-section inside diameter of 13.5 mm, nominal junction pressure (Ps) of 200 kPa (abs), near ambient temperature (Ts), inlet superficial gas velocities (JG1) ranging from 2.0 to 40 m/s, inlet superficial liquid velocities (JL1) ranging from 0.01 to 0.18 m/s, mass split ratios (W3/W1) from 0 to1.0 and inlet flow regimes of stratified, wavy, and annular. The data reveal that the degree of maldistribution of the phases depended on the inlet conditions, the mass split ratio at the junction, and the inclination angle of the outlets.
Critical Heat Flux During Subcooled Flow Boiling of Alumina Nanofluids in a Narrow Channel
Authors: Vafaei Saeid , Wen Dongsheng
Published: March 2011

This work reported an experimental study of flow boiling of pure water and aqueous alumina nanofluids in a 0.5 millimetre narrow channel. An open-ended stainless steel microchannel system is constructed where liquids are supplied by a syringe pump and heated by a variable DC power source, with synchronized measurement of the surface temperature distribution along the channel and pressure fluctuations associated with bubbles. The effect of nanoparticle concentration, inlet subcooling and mass flow rate and on critical heat flux in a microchannel is investigated. A modest increase in CHF is observed for nanofluids, being higher at higher particle concentrations and higher inlet subcoolings. Regular fluctuations in temperature and pressure signals suggest a cyclic boiling events occurring in the narrow channel; the large pressure fluctuation, coupling with the surface temperature and inlet temperature fluctuations, would affect flow boiling mechanisms in narrow channels.
The Application of Entransy Dissipation Theory in the Volume-to-Point Heat Conduction Problem
Authors: Ge Liang , Xu Mingtian , Cheng Lin
Published: March 2011

Recent advances in semiconductor technologies are accompanied by an accelerated increase in power density level from high performance chips such as microprocessors. The thermal management of electronic devices, especially meeting the limitations on maximum operating temperature and ensuring temperature uniformity across the chip, becomes one of the most critical issues in the electronic industry. Much effort is devoted to devising the efficient electronic cooling technologies. One cooling strategy proposed by Bejan is that the heat generated in a finite volume may be removed to a point on the boundary through some embedded conducting paths with high conductivity in the substrate. The problem is to optimize the allocation of the conducting paths so that the generated heat can be most effectively dissipated and the highest temperature in the domain is minimized, which is called the volume-to-point conduction problem. Bejan developed the constructal method to optimize the high conductivity material allocation. However the uniformly distributed heat source is always assumed in Bejan’s discuss, which is not consistent with the reality in the chips or other integrated circuits. In the present work, the volume-to-point conduction problem with non-uniform heat sources is formulated in the framework of the entransy dissipation theory and is solved by the variational method and finite element method. The tree-shape distribution of the conducting paths is obtained, which agrees with the constructal theory.
Inverse Conduction Technique in Hankel Domain Using Infrared Thermography: Application to Droplet Stream Quenching a Metal Disk
Authors: Lelong Franck , Gradeck Michel , Re´my Benjamin , Ouattara Aboubacar , Maillet Denis
Published: March 2011

Cooling of a hot metal by a spray occurs in various situations. Such is the case for a loss of coolant accident in a nuclear reactor, where a generated spray impacts the fuel rod assemblies. Design of an experimental characterization setup for cooling a hot (600°C) disk shape Nickel sample by a stream of monodisperse droplets is presented here. Non-invasive excitation/measurement techniques have been used in order to implement an inverse technique for quantitative estimation of both wall heat flux and temperature: heating is made by induction and infrared thermography is used for rear face temperature measurement. Control and calibration of the losses are key points here: their level is of the same order of magnitude as the flux removed by the droplets. Examples of inversion are presented.
Semi-Analytical Calculation of the Inlet Zone Performance of Ideal Shell and Tube Heat Exchangers
Authors: Mohammadi K. , Heidemann W. , Mu¨ller-Steinhagen H.
Published: March 2011

A semi-analytical model is presented for the evaluation of the performance factor of the inlet zone of an E type shell and tube heat exchanger without leakage flows. The performance factor is defined as the ratio of dimensionless heat transfer coefficients and pressure drops of both vertical and horizontal baffle orientation and therefore facilitates the decision between horizontal and vertical baffle orientation of shell and tube heat exchangers. The model allows the calculation of the performance factor of the inlet zone as a function of the baffle cut, the shell-side Reynolds number at the inlet nozzle and the Prandtl number of the shell-side fluid. The application of the model requires the knowledge of the performance factor of water at baffle cut equal to 24% of the shell inside diameter. For the development of the model a numerical data basis is used due to the lack of experimental data for shell and tube heat exchangers with different baffle orientations. The numerical data are obtained from CFD calculations for steady state conditions within a segmentally baffled shell and tube heat exchanger following the TEMA standards. Air, water and engine oil with Prandtl numbers in the range of 0.7 to 206 are used as shell-side fluids. The semi-analytical model introduced for the performance factor predicts the CFD results with a relative absolute error less than 5%. The presented model has to be validated with further experimental data and/or numerical results which explain the effect of baffle orientation on the shell-side heat transfer coefficient and pressure drop in order to check the general applicability.
The MHD Flows for a Heated Generalized Oldroyd-B Fluid With Fractional Derivative
Authors: Liu Yaqing , Zheng Liancun , Zhang Xinxin , Zong Fenglei
Published: March 2011

In this paper, we present a circular motion of magnetohydrodynamic (MHD) flow for a heated generalized Oldroyd-B fluid. The fractional calculus approach is introduced to establish the constitutive relationship of a viscoelastic fluid. The velocity and temperature fields of the flow are described by fractional partial differential equations. Exact analytical solutions of velocity and temperature fields are obtained by using Hankel transform and Laplace transform for fractional calculus. Results for ordinary viscous flow are deduced by making the fractional order of differential tend to one and zero. It is shown that the fractional constitutive relation model is more useful than the conventional model for describing the properties of viscoelastic fluid.
Lattice Boltzmann Flow Simulation in Micro-Nano Transitional Porous Media
Authors: Suga K. , Takenaka S. , Ito T. , Kaneda M. , Kinjo T. , Hyodo S.
Published: March 2011

In order to simulate heat and mass transfer in porous media whose scales range micron to nanometers, this work intends to provide a scheme for flow field simulation for such porous media. Since Navier-Stokes equations are no longer applicable to high Knudsen number (Kn) flow regimes, the conventional lattice Boltzmann method (LBM) cannot be applicable to flows in nanoscale porous media. Hence, a modified lattice Boltzmann method is applied for computing flows in micro-nano porous media in the transitional flow regimes at moderately high Knudsen numbers. The lattice Boltzmann equation applied is an extended version using an effective relaxation time associated with Kn and a regularization procedure coupled with Maxwell’s diffuse-scattering boundary condition for walls. For the flow field where the representative molecular mean free path varies (effectively the Kn varies locally), the locally defined Kn is introduced. In order to verify the LBM scheme, the results are compared with those of the molecular dynamics (MD) simulations by the Leonard-Jones potential. The flow fields considered are in modeled nano-porous media whose porosity is around 0.9. The results of micro-nanoscale porous media flows at Knudsen numbers: Kn = 0.04–0.24 show reasonable agreement in both the simulation methods and confirm the reliability of the presently applied LBM. Interestingly, in complex flow geometry, the advantage of higher order discrete velocity models of the LBM is not notable. Therefore, it is concluded that conventional discrete velocity models, say the D2Q9 and D3Q19 models are reasonably enough for flows in micro-nanoscale porous media.
Comparisons of Pins/Dimples/Protrusions Cooling Concepts for a Turbine Blade Tip-Wall at High Reynolds Numbers
Authors: Xie Gongnan , Sunde´n Bengt , Zhang Weihong
Published: March 2011

The blade tip region encounters high thermal loads because of the hot gas leakage flows, and it must therefore be cooled to ensure a long durability and safe operation. A common way to cool a blade tip is to design serpentine passages with 180° turn under the blade tip-cap inside the turbine blade. Improved internal convective cooling is therefore required to increase blade tip lifetime. Pins, dimples and protrusions are well recognized as effective devices to augment heat transfer in various applications. In this paper, enhanced heat transfer of an internal blade tip-wall has been predicted numerically. The computational models consist of a two-pass channel with 180° turn and arrays of circular pins or hemispherical dimples or protrusions internally mounted on the tip-wall. Inlet Reynolds numbers are ranging from 100,000 to 600,000. The overall performance of the two-pass channels is evaluated. Numerical results show that the heat transfer enhancement of the pinned tip is up to a factor of 3.0 higher than that of a smooth tip while the dimpled-tip and protruded-tip provide about 2.0 times higher heat transfer. These augmentations are achieved at the cost of an increase of pressure drop by less than 10%. By comparing the present cooling concepts with pins, dimples and protrusions, it is shown that the pinned-tip exhibit best performance to improve the blade tip cooling. However, when disregarding the added active area and considering the added mechanical stress, it is suggested that the usage of dimples is more suitable to enhance blade tip cooling, especially at low Reynolds numbers.
Influence of Surface Roughness on Contact Heat Transfer
Authors: Ustinov V. A. , Kneer R. , Al-Sibai F. , Schulz S. G. , El-Magd E.
Published: March 2011

Almost all technical devices in use today are assemblies of individual pieces. For all force-based assembly methods, such as bolting or press-fitting, the thermal behavior is influenced by the contact resistance at the joint surface. For metal pieces in contact with each other, the authors have developed a measurement method and analysis tools enabling the determination of the contact heat transfer coefficient. Previously published results [1, 2, 3] have shown the dependence of the contact heat transfer coefficient on surface structure, contact pressure and material properties. The present work provides experimental and analytical data for the contact heat transfer coefficient and also proposes a model for calculating the real contact area of two surfaces which are placed under different contact pressures. Experiments were conducted for two material combinations with three different surface structures, while varying the contact pressures from 7 MPa to 230 MPa. When selecting average surface roughness (Rz) as a characterizing parameter for surface structure, the results did not show a consistent trend. Thus, in this paper Rz was replaced by the real contact area between the two surfaces of interest. This area was determined by applying a refined method based on surface roughness measurements. The experimental data show a better consistency, when plotting the contact heat transfer coefficient relative to real contact area (Fk) rather than the previously used Rz–values.
Non-Equilibrium Boundary Effects Influence on the Interface Surface Formation
Authors: Kryukov Alexei , Puzina Yulia
Published: March 2011

A film boiling model on a hemispherical heater in subcooled water is presented based on convection heat transport in liquid, thermal conduction in the vapor film, radiation heat transport. The numerical solution of the model has been obtained. Different cases are then analyzed with this model. The calculation results have predicted good-enough agreement with experimental data for lower part of liquid–vapor interface but significantly under-predicted the data for the part near free surface of liquid.
On the Analysis of Turbulent Transient Flows in Channels
Authors: Kraev Vyacheslav M. , Yanyshev Dmitry S.
Published: March 2011

The problem of designing different devices under unsteady conditions is still remaining one of the critical ones, and cannot be fully resolved yet. So analysis of unsteady thermal and hydrodynamic processes is one of the determinants in aviation, astronautics, shipbuilding, cryogenics, chemical industry, etc. By now, considerable test data had been obtained for unsteady single-phase flows in round tubes, flat ducts, and in shaped channels. It allows carrying out one-dimensional analysis in a variety of heat-stressed structures and heat exchangers. Experimental researches and theoretical estimations show that values of heat transfer and friction coefficients may be three or even four times larger than the results of quasi-steady analysis. Turbulent flow structure researches conducted in Moscow Aviation Institute (MAI) showed that unsteadiness of mass flow rate at the inlet of the channel influences greatly on the flow structure. According to this, hydrodynamic unsteadiness should considerably affect on heat transfer. This influence may be explained by flow thermal inertia, changing of the flow turbulent structure and gas radial flow-over provided by velocity profile restructuring. Basing on the experiment, one can get optimized flow acceleration law, so that pressure losses due to friction will be minimal. The experimental data was generalized into some empirical formulae for Nusselt number and friction coefficient, so that one-dimensional engineering calculations could be provided. Nevertheless, the problem of numerical analysis of flow in tubes with transient mass flow at the inlet using RANS equations is yet unresolved. Analysis performed in MAI showed that most popular and widely used k-epsilon turbulent model cannot be applied in this particular case. The results of calculations performed with this model do not match with experimental data. For example, the results of calculation performed at Moscow Power Engineering Institute are directly opposite to the experimental data, viz the flow acceleration leads to Nusselt number and friction coefficient decreasing and vice versa. Results of calculations with k-epsilon performed in MAI are also unsatisfactory and do not go with the experimental data. The equations of k-omega based models contain dynamic viscosity terms and do not need wall functions. Results of MAI calculations with Menter’s Shear Stress Transport model go with the experimental data qualitatively. The results of these calculations are stated.
Heat and Mass Transfer of Drying Particles in a Fluidized Bed
Authors: Bruchmu¨ller J. , van Wachem B. G. M. , Gu S. , Luo K. H.
Published: March 2011

In this study, the heat transfer and drying process of arabica coffee beans in a batch fluidized bed roaster has been studied. Herein, the discrete element method (DEM) has been used and modified to account for resolved 1D temperature and moisture content profiles within each single coffee bean. This approach has the strength to provide much more information on the global (fluidization, mixing) and local (particle data) level compared to existing coffee roaster models. Therefore, the product quality can be evaluated on-line by many more specific criteria beyond the averaged global particle temperature and moisture content. Instead, information of every single particle is available which includes heat and mass transfer coefficients, its local position inside the bed, collision forces, etc. Furthermore, the overall roaster performance is based on e.g. fluidization stability, mixing efficiency or uniformity of quality properties among all particles. More data are presented to account for a broader coffee bean roasting evaluation. Modeling results are in good agreement with experimental data.
Study on the Effective Thermal Conductivity of Macro, Mirco and Nano Porous Materials in the Light of the Knudsen Conduction/Radiation Coupling Effect
Authors: Gross Ulrich , Raed Khaled
Published: March 2011

Thermal transport phenomena in porous media are characterized by conduction through solid matrix and filling gas, and also by radiation. The gas is dispersed in the porous system depending on the pore size distribution. In each pore, the gas contributes to the heat transfer between the pore surfaces. This effect is strongly influenced by pore size, gas atmosphere, accommodation coefficient and other factors. A recent publication of the present authors focused on modeling the change of the effective thermal conductivity when the gas atmosphere is changed. In the current contribution, the effect of pore size distribution on heat transfer in macro, micro, and nano insulation materials is presented. Samples were chosen from five different highly porous materials with different pore size distribution within the macro, micro, and nano classes. Porosity and pore size distribution of the samples were chosen to get a clear characterization of the materials. The effective thermal conductivity was measured by applying the radial heat flow method at temperatures up to 1000 °C. Evaluating Knudsen effect from the pore size distribution alone does not give plausible explanation for the measured thermal conductivity. However, it is important to consider the kind of connections between the pores. In case of nano materials, the radiation effect proves to be strongly dependent on the Knudsen number.
The Surface Tension of Nanobubbles and the Effect of the Potential Cutoff Radius
Authors: Cosden Ian A. , Lukes Jennifer R.
Published: March 2011

Molecular dynamics (MD) simulations are carried out to calculate the surface tension of bubbles formed in a metastable Lennard-Jones (LJ) fluid. The calculated normal and transverse pressure components are used to compute a surface tension which is compared to the surface tension computed from the Young-Laplace equation. Curvature effects on surface tension are investigated by performing various sized simulations ranging from 6,912 to 256,000 LJ particles. Density profiles, pressures, and calculated surface tension are shown to have a strong dependence on the choice of the interaction cutoff radius. A cutoff radius of 8σ, significantly larger than that commonly used in the literature, is recommended for accurate calculations in liquid-vapor systems.
Heat Transfer Characteristics of CuO-Base Oil Nanofluid Laminar Flow Inside Flattened Tubes Under Constant Heat Flux
Authors: Razi P. , Akhavan-Behabadi M. A.
Published: March 2011

An experimental investigation has been carried out to study the heat transfer characteristics of CuO-Base oil nanofluid flow inside horizontal flattened tubes under constant heat flux. The nanofluid flowing inside the tube is heated by an electrical heating coil wrapped around it. The convective heat transfer coefficients of nanofluids are obtained for laminar fully developed flow inside round and flattened tubes. The effect of different parameters such as Reynolds number, flattened tube internal height, nanoparticles concentration and heat flux on heat transfer coefficient is studied. Observations show that the heat transfer performance is improved as the tube profile is flattened. The heat transfer coefficient is increased by using nanofluid instead of base fluid. Also, it can be concluded that decreasing the internal height of the flattened tubes and increasing the concentration of nanoparticles both contribute to the enhancement of heat transfer coefficient.
Heat Transfer Experiments in a Confined Jet Impingement Configuration Using Transient Techniques
Authors: Hoefler Florian , Dietrich Nils , von Wolfersdorf Jens
Published: March 2011

A confined jet impingement configuration has been investigated in which the matter of interest is the convective heat transfer from the airflow to the passage walls. The geometry is similar to gas turbine applications. The setup is distinct from usual cooling passages by the fact that no crossflow and no bulk flow direction are present. The flow exhausts through two staggered rows of holes opposing the impingement wall. Hence, a complex 3-D vortex system arises, which entails a complex heat transfer situation. The transient Thermochromic Liquid Crystal (TLC) method was used to measure the heat transfer on the passage walls. Due to the nature of the experiment, the fluid as well as the wall temperature vary with location and time. As a prerequisite of the transient TLC technique, the heat transfer coefficient is assumed to be constant over the transient experiment. Therefore, additional measures were taken to qualify this assumption. The linear relation between heat flux and temperature difference could be verified for all measurement sites. This validates the assumption of a constant heat transfer coefficient which was made for the transient TLC experiments. Nusselt number evaluations from all techniques show a good agreement, considering the respective uncertainty ranges. For all sites the Nusselt numbers range within ±9% of the values gained from the TLC measurement.
Electrical Supercooling Mitigation in Erythritol
Authors: Jankowski Nicholas R. , McCluskey F. Patrick
Published: March 2011

This report describes an experimental investigation into the effect of electric current in reducing the supercooling of erythritol. Previous studies have identified erythritol as a prime material candidate for moderate temperature thermal energy storage (TES) systems due to its high latent heat of fusion and melting temperature (118°C), but it has also shown excessive supercooling, sometimes exceeding 65°C [1]. Various methods for controlling or reducing supercooling are reviewed, including work by Shichiri and Hozumi showing that a small electric current passed through supercooled water is highly effective in initiating nucleation [2,3]. In the present study, the authors demonstrate a similar effect with erythritol by subjecting a sample to repeated thermal cycles with and without the application of a direct electric current. The control cases without electric current showed a highly variable recrystallization temperature ranging from 67°C to 109°C (or supercooling magnitudes from 9 to 51°C). Passing a direct current through the sample using silver wire electrodes significantly shifted the material’s nucleation behavior. The local nucleation temperature only varied from 108°C to 112°C (or 6–10°C of supercooling), and nucleation always occurred on the positive electrode surface. Control cases both before and after the electrical trials indicated no noticeable change in sample crystallization behavior.
A New Criterion for Assessing Heat Exchanger Performance
Authors: Guo Jiangfeng , Xu Mingtian , Cheng Lin
Published: March 2011

The principle of minimum entropy production has played an important role in the development of non-equilibrium thermodynamics. Inspired by this principle, Bejan derived the expression of the local entropy production rate for heat convection and established the entropy production minimization approach for the heat exchanger optimization design. Although one can obtain the entropy production distribution in the heat exchanger numerically, it can not directly been employed to examine the heat exchanger performance. Tondeur and Kvaalen found that the entropy production uniformity is closely related to the heat exchanger performance. In the present work, based on Tondear and Kvaalen’s work, an entropy production uniformity factor is defined, which quantifies the uniformity of the local entropy generation distribution in heat exchanger. Numerical results of the heat transfer in a rectangular channel show that the larger entropy production uniformity factor implies less irreversible loses. Therefore, this factor can serve as a thermodynamic figure of merit for assessing the heat exchanger performance.
The Development of Performance Prediction Methods for an Automotive CO2 Air Conditioning Cycle
Authors: Katsuta Masafumi , Oshiro Takahiro , Kaneko Akira , Bae Sangchul , Komatsu Shunji , Ohno Yohei
Published: March 2011

In previous researches, we have been focusing on the performance of the each element heat transfer and hydraulic performance of refrigeration cycle. Experimental investigations have been repeated several times and, finally, we have substantial data base including the effect of lubricant oil. Moreover, the mal-distribution of two-phase in an evaporator can be also predicted from the experimental data base. Under these circumstances, this study is intended to effectively put the construction of an automotive CO2 air conditioning system into practical design use through the simulation using the above-mentioned data base. This paper describes the refrigeration cycle performance prediction of each element (e.g. an evaporator, a gas-cooler, and so on) by a simulation using substantial data base and various available correlations proposed by us and several other researchers. In the performance prediction model of heat exchangers, local heat transfer and flow characteristics are considered and in addition, the effects of lubricant oil on heat transfer and pressure drop are duly considered. The comparison is also made between simulation results and bench test results using a real automotive air conditioning system. Finally, the developed simulation method can predict the cooling ability successfully within ±5%. By incorporating the lubricant oil effect, the simulation results are improved to ±5% and ±15% for the cooling ability and pressure drop respectively.
Thermal and Electrical Transport Characteristics of Polycrystalline Gold Nanofilms
Authors: Ma Weigang , Miao Tingting , Zhang Xing
Published: March 2011

The in-plane thermal and electrical conductivities of several suspended polycrystalline gold nanofilms with thickness of 20.0–54.0 nm have been measured simultaneously at 100–310 K. Both the thermal and electrical conductivities drop greatly compared to the corresponding bulk value, and the electrical conductivity reduction is larger. Fits to the temperature-dependent electrical conductivity confirm that the scattering of electrons by softened phonons is significant and cannot be reconciled with the classical size-effect model considering only surface and grain boundary. Taking into account the enhanced electron-phonon scattering, the electrical conductivity is well predicted over the whole temperature range and the obtained Debye temperature agrees well with the calculated value from the elastic continuum model. Furthermore, a new model on the thermal transport of metallic nanofilm is proposed based on the Energy Conservation Law, in which the electron-phonon scattering induced electron energy decrease is supposed to be counteracted by the phonon energy increase. The present model greatly improves the prediction of thermal conductivity in thin films compared to the corresponding result directly from electrical thermal analogy applied to bulk metals.
Operational Mechanism of Square Capillary Looped Pulsating Heat Pipe
Authors: Qu Wei , Yang Bin
Published: March 2011

Pulsating Heat Pipe (PHP) has the turn structure of capillary and the mechanism is paid much more attention. The circulation of Looped Pulsating Heat Pipe (LPHP) is modelled to couple the heat transfers, volume flow rates, operational driving forces and flow resistances. The analysis and calculation show that, the square capillary LPHP can be optimized by adjusting the coupled dominating parameters to improve the flow and performance. The working fluid, the transferred power and the dynamic diameter will exert essential influences on the LPHP performance, the thermal resistance of the LPHP can be as small as 0.05°C/W.
Effects of Air Induct Structure to Flow Uniformity and Resistance for Primary Surface Recuperator of a Micro Gas Turbine
Authors: Qu Wei , Gao Shan
Published: March 2011

Primary surface recuperator is important for micro gas turbines, the flow distribution and pressure loss are sensitive to the induct structure design significantly. The air induct structure for one recuperator is modelled and simulated. Several flow fields and pressure losses are obtained for different designs of air induct structure. The air induct structure can affect the flow uniformity, further influence the pressure loss a lot. For several changes of air induct structure, the non-distribution of air flow can be decreased from 67% to 13%, and the pressure loss can be decreased to 50% of the original. Considering the recuperator design and the gas turbine, one optimized structure is recommended, which has less local pressure loss and better flow distribution.
Numerical Modeling and Physical Simulation of Vortex Heat Transfer Enhancement Mechanisms Over Dimpled Reliefs
Authors: Leontiev Alexander I. , Isaev Sergey A. , Kornev Nikolai V. , Chudnovsky Yaroslav , Hassel Egon
Published: March 2011

The paper presents a comprehensive analysis of conditions for numerical simulation and physical modeling of convective heat transfer in the vicinity of dimpled surface relief. Contradictory results, unreasonable assumptions, and non-justified conclusions are marked. Based on the analysis of physical experiments the correlation between the predictions and measured data is discussed. Detailed numerical study of turbulent air flow and heat transfer in the narrow channel with three types of dimples (spherical, conic and oval) was carried out. Various mathematical and discrete models, including, those based on solving Reynolds-averaged Navier-Stokes equations (RANS/URANS-SST), and also adaptive scale models (SAS-SST) are compared. The influence of flow parameters (Reynolds number) and geometric sizes (dimple diameter, depth, radius of rounding off of an edge, channel width and height) on local and integral characteristics of flow and heat transfer (total heat output and hydraulic losses) is determined. Special attention is given to reorganizing vortex structures and flow regime (with periodic fluctuations) with increasing relative dimple depth and Reynolds number. For the first time the influence of the scale factor of a constant cross-section channel is detailed. Thermal-hydraulic characteristics of various dimpled reliefs are compared, and the advantage of an oval dimple over a spherical one is shown.
Double-Diffusive Natural Convection in an Enclosure Having Finite Thickness Walls
Authors: Kuznetsov Geniy V. , Sheremet Mikhail A.
Published: March 2011

This study is a computational work devoted to mathematical simulation of three-dimensional conjugate heat and mass transfer in an enclosure having finite thickness walls with local heat and mass sources. The mathematical model written in dimensionless variables “vorticity vector – vector potential functions – temperature – concentration” has been solved using an iterative implicit finite-difference method. The heat and mass transfer characteristics of the three-dimensional enclosure have been analyzed using the streamlines, temperature and concentration fields. Main attention was paid to the effects of the Rayleigh number, buoyancy ratio and transient factor on the heat and mass transfer modes. The correlations describing the dependences of average Nusselt and Sherwood numbers on the thermal Rayleigh number have been obtained.
Heat Transfer Modelling of a Novel Concentrating Photovoltaic System
Authors: Mallick Tapas Kumar , Natarajan Sendhil Kumar
Published: March 2011

In this work, an experimentally validated two dimensional finite element model was refined to predict the solar cell temperature of the novel Concentrating Photovoltaic’s (CPV) system. The corresponding 2-D numerical simulation of solar cell temperature for novel concentrating PV system of 10x was presented in this work. Based on the 2-D thermal model, solar cell temperature has been predicted for various boundary conditions. In addition to that, the effects of the ambient temperature and the solar radiation on the solar cell temperature have also been investigated for the proposed CPV system. Based on the influencing parameters of ambient temperature and solar radiation, a separate solar cell temperature correlation has been proposed to predict the cell temperature for the given range of parameters.
Thermal Conductivity of Biomorphic Porous SiC Based Ceramics
Authors: Popovska Nadejda , Alkhateeb Emad , Kugler Tanja , Fro¨ba Andreas P. , Leipertz Alfred
Published: March 2011

Biomorphic porous SiC composite ceramics were produced by chemical vapor infiltration and reaction (CVI-R) technique using paper preforms as template. The thermal conductivity of four samples with different composition and microstructure was investigated: a) C-template b) C-SiC, c) C-SiC-Si3N4 and d) SiC coated with a thin layer of TiO2. The SiC-Si3N4 composite ceramic showed enhanced oxidation resistance compared to single phase SiC. However; a key property for the application of these materials at high temperatures is their thermal conductivity. The later was determined experimentally at defined temperatures in the range 298–373K with a laser flash apparatus. It was found that the thermal conductivity of the porous ceramic composites increases in the following order: C-template < C-SiC < C-SiC-Si3N4 < SiC-TiO2. The results were interpreted in regard to the porosity and the microstructure of the ceramics.
Compact Heat Sink With Combined Structures of Dimples-Protrusions With Pin-Fins or Micro-Channels: Review, Simulation and Experimental Data
Authors: Spokoyny Mikhail , Trofimov Vladimir , Qiu Xinliang , Kerner James M.
Published: March 2011

This work presents survey and selected results in the design of compact heat sinks for semiconductor industry that use concavities, dimples or protrusions, combined structures of dimples and pin-fins with or without micro-channels, and methods of manufacturing based on “wave” technology. Analysis and observations in this work are devoted to the study of flow structure and jet-vortex nature of heat transfer enhancement mechanisms in a single dimple, in an array of dimples and in dimpled channels at a different Reynolds number, dimple depth and dimple shape. Impulse and heat transfer analogy in channels with dimples and protrusions are also considered. The direct numerical simulation and experimental analysis are performed for channels with combined structure of dimples and pin-fins. The thermal resistance and dependences of Nusselt number on Reynolds number are reported for studied arrangements at which dimples do not act as surface vortex generators.