bluuebunny/crossref_metadata_2025 huggingface dataset:
An Experimental Study of Thermal Performance of a Two-Phase Loop Thermosyphon (TPLT)-Based Steam Generator: Effects of Thermal Boundary Conditions
Authors: Hua Meng , Zhang Liang , Zhu Zi-Qin , Fan Li-Wu , Yu Zi-Tao , Hu Ya-Cai
Published: December 2013

For the Parabolic trough collector (PTC) system, thermal boundary condition of the receiver (or heating section) is important for the thermal optimization. In this work, effects of thermal boundary on thermal performance of the two-phase loop thermosyphon (TPLT) natural circulation PTC system was investigated experimentally. Three kinds of thermal boundary heating conditions (upper and lower half, and whole circular heated) and two filling ratios (FR = 0.6, 1.2) were adopted in this paper. The results show that half heating condition can improve heat transfer performance in receiver and system thermal resistance. But the preferred half heating boundary was varied as the filling ratio was changed. However, a lower thermal efficiency was observed for the partly heating boundary conditions. For a low heat flux condition in this work, the effects of thermal boundary on flow instability were not obvious, especially for the bigger filling ratio condition.
A New Nusselt Number for Complicated Configurations
Authors: Shih Tom I-Ping , Sathyanarayanan Srisudarshan Krishna
Published: December 2013

Convective heat transfer over surfaces is generally presented in the form of the heat-transfer coefficient (h) or its nondimensional form, the Nusselt number (Nu). Both require the specification of the free-stream temperature (Too) or the bulk (Tb) temperature, which are clearly defined only for simple configurations. For complicated configurations with flow separation and multiple temperature streams, the physical significance of Too and Tb becomes unclear. In addition, their use could cause the local h to approach positive or negative infinity if Too or Tb is nearly the same as the local wall temperature (Twall). In this paper, a new Nusselt number, referred to as the SCS number, is proposed, that provides information on the local heat flux but does not use h and hence by-passes the need to define Too or Tb. CFD analysis based on steady RANS with the shear-stress transport model is used to compare and contrast the SCS number with Nu for two test problems: (1) compressible flow and heat transfer in a straight duct with a circular cross section and (2) compressible flow and heat transfer in a high-aspect ratio rectangular duct with a staggered array of pin fins. Parameters examined include: Reynolds number at the duct inlet (3,000 to 15,000 for the circular duct and 15,000 and 150,000 for the rectangular duct), wall temperature (Twall = 373 K to 1473 K for the circular duct and 313 K and 1,173 K for the rectangular duct), and distance from of the inlet of the duct (up to 100D for the circular duct and up to 156D for the rectangular duct). For the circular duct, Nu was found to decrease rapidly from the duct inlet until reaching a minimum and then to rise until reaching a nearly constant value in the “fully” developed region if the wall is heating the gas. If the wall is cooling the gas, then Nu has a constant positive slope in the “fully” developed region. The location of the minimum in Nu and where Nu becomes nearly constant in value or in slope are strong functions of Twall. For the SCS number, the decrease from the duct inlet is monotonic with a negative slope, whether the wall is heating or cooling the gas. Also, different SCS curves for different Twall approach each other as the distance from the inlet increases. For the rectangular duct, Nu tends to oscillate about a constant value in the pin-fin region, whereas SCS tends to oscillate about a line with a negative slope. For both test problems, the variation of SCS is not more complicated than Nu, but SCS yields the local heat flux without need for Tb, a parameter that is hard to define and measure for complicated problems.
A Level-Set Method for Multi-Species Particle Deposition
Authors: Yap Y. F. , Vargas F. M. , Chai J. C.
Published: December 2013

Often in a general deposition process, there exist multiple species particle depositing simultaneously. These species particles is different from each other both chemically and physically. Prediction of these processes requires separate treatment for each species. This article presents a level-set formulation for multi-species particle deposition where each species particles depositing with its respective deposition reaction order. The approach is verified via a one-dimensional multi-species deposition and the limiting case of one-species deposition. Applications of the framework to multi-species particle deposition in a square enclosure, a channel with flowing fluid and a lid-driven cavity are then demonstrated.
Investigation of Applicability of Using Water Mist for Cooling High-Pressure Turbine Components via Rotor Cavity Feed Channels
Authors: Ragab Reda , Wang Ting
Published: December 2013

Mist cooling has been considered as a potential means to significantly enhance cooling the hot components of a gas turbine. However, it is a great challenge to transport mist to locations where cooling is needed due to the high temperature environment and rotating components in the gas turbines. One plausible way is to transport high-pressure water to the rotor cover-plate cavity via a feed channel passing through the vane. The high-pressure water is then atomized to fine droplets (mist) that can be used to cool the rotor cover-plate cavity and possibly the blades as well. This study is focused on investigating whether pressurized liquid water can resist boiling during its transportation through the hot vane and gas turbine interior walls. Both 1-D empirical correlations and a 3-D flow computational scheme are used. Results show what could happen during the water transport in the feed channel with various inlet and boundary conditions. Conditions that can avoid boiling have been identified.
Numerical Study of the Effect of Cold Air Vent Flow on the Convective Heat Transfer Rate From a Hot Window Covered by a Top-Down, Bottom-Up Plane Blind System
Authors: Oosthuizen Patrick H.
Published: December 2013

In summer when the air-conditioning system is in use cool air from a floor-mounted vent located beneath a window often flows over the warm window. The presence of a blind system over the window will, in general, influence the effect of the vent flow on the convective heat transfer rate from the window. The effect of a Top-Down, Bottom-Up plane blind system and a cool air vent flow on the heat transfer rate from a recessed window has therefore been numerically studied here. The actual situation considered in this study is an approximate model of real situations. The window is represented by a plane isothermal section recessed into the wall, this window section being hotter than the room air far from the window. The floor-mounted vent is assumed to be located against the wall and to have a uniform discharge velocity which is normal to the vent surface. The flow has been assumed to be two-dimensional, i.e., the effect of the window and vent width has not been considered. The flow has been assumed to be steady and situations involving both laminar and turbulent flow have been considered. The fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being dealt with using the Boussinesq approach. The governing equations have been solved using the commercial CFD code ANSYS FLUENT©, the k-epsilon turbulence model having been used. The solution has the following parameters: the Rayleigh number, the Reynolds number based on the vent discharge velocity, the dimensionless depth that the window is recessed, the Prandtl number, the dimensionless top and bottom blind opening, the dimensionless size of the air vent, and the dimensionless vent discharge temperature to undisturbed air temperature difference. Results have only been obtained for a Prandtl number of 0.74 and for fixed values of the dimensionless depth that the window is recessed, the dimensionless size of the air vent, and the dimensionless vent discharge temperature difference. The effects of the other dimensionless variables on the window Nusselt number have been numerically studied.
Improved Thermal Conductivity of 13X/CaCl2 Composite Adsorbent by CNT Embedment
Authors: Chan K. C. , Chao Christopher Y. H.
Published: December 2013

Adsorption cooling systems utilize the principle of adsorption to generate cooling effect. Composite adsorbents synthesized from zeolite 13X and CaCl2 have previously been shown to have a high adsorption capacity and high adsorption rate with lower desorption temperature where the adsorption capacity and adsorption rate are 420% and 122% of zeolite 13X under the same condition respectively. This results in more compact design and a lower temperature waste-heat source can be used. The system performance is, however, limited by the low thermal conductivity of the 13X/CaCl2 composite adsorbent which is common for many adsorbents. Due to the low thermal conductivity of the adsorbent, poor heat transfer and slow temperature change in the adsorbent bed lead to longer time for the adsorbent to achieve the adsorption/desorption temperature. This directly reduces the adsorption/desorption rate of the adsorbate on the adsorbent, such as water on zeolite, and results in lower system coefficient of performance (COP) and specific cooling power (SCP). It was proposed that embedding carbon nanotube (CNT) into the 13X/CaCl2 composite absorbents can increase the thermal conductivity of the adsorbent bed to improve the system performance. Thus, the properties of the multi-wall CNT (MWCNT) embedded zeolite 13X/CaCl2 composite adsorbents were investigated to find out the optimized composition for the cooling system. The material properties of the MWCNT embedded zeolite 13X/CaCl2 composite adsorbent were measured. The thermal conductivities of the MWCNT embedded 13X/CaCl2 composite adsorbents were predicted by developing a new theoretical model modified based on area contact model. The performance of the adsorption cooling system using zeolite 13X and MWCNT embedded composite adsorbent were studied numerically. It is found that the COP and SCP are improved by 3.6 and 26 times respectively. This results in a much more compact and energy efficient cooling system.
In-Plane Thermal Conductivity of Ultra-Thin Al2O3 Films Measured by Micro-Raman
Authors: Luo Zhe , Liu Han , Feng Yanhui , Ye Peide , Chen Yong P. , Xu Xianfan
Published: December 2013

In the past decades, heat transport in thin film structures has been extensively studied due to its importance in performance optimization of micro/nano-scale devices. A large amount of heat is generated during the operation of electronic devices and must be dissipated efficiently via dielectric electrical insulating films, thus it is crucial to investigate and engineer the thermal properties of thin film structures. Numerous methods have been developed to measure thin film thermal conductivity, many of which took advantage of electrical heating and temperature sensing such as the 3ω method [1], scanning thermal microscopy (SThM) [2], etc. These methods require the heaters/sensors to directly contact the samples surface, inducing the risk to bias the temperature field; also, the contacting components add a significant complexity to the originally simple 2-D film structure, which increases the difficulty of modeling and data analysis.
Dynamics of the Liquid Microlayer Underneath a Vapor Bubble Growing at a Heated Wall
Authors: Guion A. , Langewisch D. , Buongiorno J.
Published: December 2013

Evaporation of the liquid microlayer developing underneath a bubble in the initial (inertia controlled) phase of its growth can be a significant vapor source in the later (heat-diffusion controlled) phase of bubble growth. In the literature, representation of this microlayer is typically limited to a very short (order of microns) region near the apparent Triple Phase Line (TPL) between the bubble and the wall. However, experimental observations show that the microlayer may actually extend hundreds of microns beyond the TPL region. Guided by this observation, we develop a simple model to predict the time evolution of the extended microlayer, and the associated corresponding evaporation rate and heat flux underneath a bubble. The model is derived as a special case of the complete governing equations, which account for the complicated effects of disjoining pressure, capillarity, vapor recoil and interfacial resistance. The predictions of the model are in reasonable quantitative agreement with the experimentally observed behavior of the microlayer.
The Outside Transport Phenomena of the Passive Containment Cooling System
Authors: Li Cheng , Fan Huanran , Zhao Ruichang
Published: December 2013

Based on the outside cooling of the Passive Containment Cooling System (PCCS), a simplified containment mathematical model was built and was numerically solved. Air circulation driven by natural forces for a steady condition along with the coupled thermal-hydraulic phenomena was then obtained and analyzed. Variations of humid air temperature and velocity, water film sub-cooling and heat fluxes along the flow direction were discussed and driving forces were compared to obtain the ranked important phenomena of the PCCS outside cooling. The conclusions are significant to reasonably understand the PCCS outside transport phenomena.
A Parametric Numerical Study of Radiative Transfer in Thermotropic Materials
Authors: Gladen Adam C. , Mantell Susan C. , Davidson Jane H.
Published: December 2013

A thermotropic material is modeled as an absorbing, thin slab containing anisotropic scattering, monodisperse, spherical particles. Monte Carlo ray tracing is used to solve the governing equation of radiative transfer. Predicted results are validated by comparison to the measured normal-hemispherical reflectance and transmittance of samples with various volume fraction and relative index of refraction. A parametric study elucidates the effects of particle size parameter, scattering albedo, and optical thickness on the normal-hemispherical transmittance, reflectance, and absorptance. The results are interpreted for a thermotropic material used for overheat protection of a polymer solar absorber. For the preferred particle size parameter of 2, the optical thickness should be less than 0.3 to ensure high transmittance in the clear state. To significantly reduce the transmittance and increase the reflectance in the translucent state, the optical thickness should be greater than 2.5 and the scattering albedo should be greater than 0.995. For optical thickness greater than 5, the reflectance is asymptotic and any further reduction in transmittance is through increased absorptance. A case study is used to illustrate how the parametric study can be used to guide the design of thermotropic materials. Low molecular weighted polyethylene in poly(methyl methacrylate) is identified as a potential thermotropic material. For this material and a particle radius of 200 nm, it is determined that the volume fraction and thickness should equal 10% and 1 mm, respectively.
High Temperature Heat Transfer During Hot Forming Die Quenching of Boron Steel
Authors: Caron Etienne , Daun Kyle J. , Wells Mary A.
Published: December 2013

Distributed mechanical properties can be obtained in ultra high strength steel parts formed via hot forming die quenching (HFDQ) by controlling the cooling rate and microstructure evolution during the quenching step. HFDQ experiments with variable cooling rates were conducted by quenching Usibor® 1500P boron steel blanks between dies pre-heated up to 600°C. The heat transfer coefficient (HTC) at the blank / die interface, which is used to determine the blank cooling rate, was evaluated via inverse heat conduction analysis. The HTC was found to increase with die temperature and stamping pressure. This heat transfer coefficient increase was attributed to macroscopic flattening of the boron steel blank as well as microscopic deformation of surface roughness peaks. At the end of the hot stamping process, the HTC reached a pressure-dependent steady-state value between 4320 and 7860 W/m2·K when the blank and die temperatures equalize.
Double MRT-Lattice Boltzmann Simulation of Natural Convection Melting in a Rectangular Cavity
Authors: Feng Yongchang , Li Huixiong , Cheng Can
Published: December 2013

In this letter, a double multiple relaxation time (MRT) lattice Boltzmann method (LBM) based on double distribution lattice Boltzmann model is developed for numerical simulations of solid-liquid phase change. In contrast to previous lattice Boltzmann models, we solve the flow and temperature fields using the MRT-D2Q9 model and the MRT-D2Q5 model, respectively. The one-dimensional conduction melting and two-dimensional convection melting in a rectangular cavity for pure substance are investigated using the proposed model. Simulation results agree well with analytical solutions and scaling laws. In addition, temperature and liquid fraction fields suggest that natural convection has important influence on total melting process. We conclude by showing that the developed model is a promising tool to investigate solid-liquid phase change in complex media and geometries.
Assisted Turbulence Convective Heat Transfer for Cooling the Photovoltaic Cells
Authors: Arianmehr I. , Ting D. S.-K. , Ray S.
Published: December 2013

Solar PV (photovoltaic) is a rapidly advancing renewable energy technology which converts sunlight directly into electricity. One of the outstanding challenges of the current PV technology is the reduction in its conversion efficiency with increasing PV panel temperature, which is closely associated with the increase in solar intensity and the ambient temperature surrounding the PV panels. To more effectively capture the available energy when the sun is most intense, significant efforts have been invested in active and passive cooling research over the last few years. While integrated cooling systems can lead to the highest total efficiencies, they are usually neither the most feasible nor the most cost effective solution. This work examines some simple passive means of manipulating the prevailing wind turbulence to enhance convective heat transfer over a heated plate in a wind tunnel. Specific turbulence generators are devised based on knowledge gained from orificed perforated plate turbulence study.
Numerical Modelling of Condensation of Multiple Bubbles in Subcooled Flow Boiling With VOF Method
Authors: Liu Zhenyu , Sunden Bengt , Yuan Jinliang
Published: December 2013

The understanding of condensation of multiple bubbles is of importance to improve the continuum models for the large-scale subcooled flow boiling. The CFD modeling for condensation of multiple bubbles has been developed with the Volume of Fluid (VOF) method in this work. An explicit transient simulation is performed to solve the governing equations along with the VOF equation and source terms for condensation. The Geometric Reconstruction Scheme, which is a Piecewise Linear Interface Calculation (PLIC) method, is employed to keep the interface sharp. The surface tension is modeled by the Continuum Surface Force (CSF) approach and is taken into account in the computations. Numerical simulation predicts the behavior of the actual condensing bubbles. The condensation rate of a single bubble can be influenced by the velocity of the fluid flow and the temperature difference between the bubble and fluid. The effect of interaction of the bubbles on the condensation is analyzed based on the numerical predictions. The condensation rate of lower bubbles increases due to the random perturbation induced by other bubbles. The influence of other bubbles on the condensation rate can be neglected if the distances between the bubbles are large enough.
Optical Analysis of a New CPC-Based Solar Collector Designed for Hydrogen Production
Authors: Gu Xiaoguang , Taylor Robert A. , Rosengarten Gary
Published: December 2013

Methanol reforming to produce hydrogen is an excellent way to provide fuel for hydrogen-based fuel cells. Since methanol reforming is an endothermic process, requiring an energy input, it is possible to use this reaction as a way to store primary energy. In this paper, we propose that this reaction can be driven with a new type of solar collector which has high overall efficiency. The advantage of the proposed design is that it can achieve high temperatures (up to 250°C) without tracking thus reducing capital and running costs. A CPC (compound parabolic concentrator) collector was designed with a half angle of 27.4 degrees and a concentration ratio between 1.5–1.75 over the entire cone angle. Furthermore, due to the small size of the designed type of collector, it would be easy to manually orient it so that the axis is aligned east-west, which would allow it to concentrate all day. The fabricated collector shown later in this paper has the advantage of being portable with a thickness of just 70mm. In this design, we use a vacuum layer between the receiver and the frame to minimize the convective heat loss and to allow for thermal concentration. Selective surfaces, such as TiNOx, are employed in the receiver to absorb solar (short wavelength) radiation while minimizing emission of thermal (long wavelength) radiation. An optical analysis via ray tracing shows an optical efficiency of 80% to 85% in the range of half incident angle. Also, a prototype of the designed CPC collector is manufactured and shown in this paper.
Inlet and Wall Effects on Fluid Flow in Doubly-Periodic Arrays of Spacer-Filled Passages
Authors: Mojab S. M. , Beale S. B. , Pollard A. , Hanff E. S. , Pharoah J. G.
Published: December 2013

A numerical investigation has been carried out to study the effect of the sidewalls and the number of cells in arrays of spacer-filled channels on the local flow distribution, for Reynolds number, Re = 100, for a spacer-configuration typically employed in process industries. It was found that the channel sidewalls have a significant effect on the velocity profile near the walls. Numerically calculated values of velocity are compared with those measured experimentally, with good agreement being obtained; a maximum deviation of 4.5% was observed. Particle traces emitted from a cell at the channel entrance revealed that, unexpectedly, the flow moves parallel to the spacer filaments within each channel layer and changes 90° direction mostly at the cell adjacent to the channel side walls. The effects of the number of cells and the type of boundary condition imposed on the channel transverse sidewalls on the pressure drop and friction factor are considered.
Modelling and Optimization of a Batch Furnace for Hot Stamping
Authors: Twynstra M. G. , Daun K. J. , Caron E. , Adam N. , Womack D.
Published: December 2013

Non-uniform heating during the austenization stage of hot-stamping can lead to inhomogeneous mechanical properties, coating thickness variations, and residual stresses in the stamped components. This paper presents a heat transfer model of an electrically-powered batch austenization furnace, with the objective of diagnosing and correcting factors that cause non-uniform blank heating. Radiation view factors are calculated with the Monte Carlo method. Convection with the furnace air and conduction through the blank and furnace door is also incorporated into a transient global domain energy balance. The model incorporates temperature- and phase-dependent radiative and thermophysical properties of the steel blanks and ceramic insulation of the furnace walls. The heat transfer model is validated with data obtained from instrumented blanks in an actual production cycle. The model is then used to optimize heater settings for the existing furnace using simulated annealing.
Angular False Scattering in Radiative Heat Transfer Analysis Using the Discrete-Ordinates Method With Higher-Order Quadrature Sets
Authors: Hunter Brian , Guo Zhixiong
Published: December 2013

The SN quadrature set for the discrete-ordinates method is limited in overall discrete direction number in order to avoid physically unrealistic negative directional weight factors. Such a limitation can adversely impact radiative transfer predictions. Directional discretization results in errors due to ray effect, as well as angular false scattering error due to distortion of the scattering phase function. The use higher-order quadrature schemes in the discrete-ordinates method allows for improvement in discretization errors without an overall directional limitation. In this analysis, four higher-order quadrature sets (Legendre-Equal Weight, Legendre-Chebyshev, Triangle Tessellation, and Spherical Ring Approximation) are implemented for determination of radiative transfer in a 3-D cubic enclosure containing participating media. Radiative heat fluxes, calculated at low direction number, are compared to the SN quadrature and Monte Carlo predictions to gauge quadrature accuracy. Additionally, investigation into the reduction of angular false scattering with sufficient increase in direction number using higher-order quadrature, including heat flux accuracy with respect to Monte Carlo and computational efficiency, is presented. While higher-order quadrature sets are found to effectively minimize angular false scattering error, it is found to be much more computationally efficient to implement proper phase function normalization for accurate radiative transfer predictions.
Measurement of Heat Transfer Enhancement and Pressure Drop Across Eddy Promoter Air Screens in a Liquid-to-Air-Membrane Energy Exchanger (LAMEE)
Authors: Oghabi Ashkan , Ghadiri Moghaddam Davood , Simonson Carey , Besant Robert W.
Published: December 2013

In liquid-to-air membrane energy exchangers (LAMEEs), the heat and mass transfer resistances in the air channel are dominant. An eddy promoter air screen can effectively enhance the heat and mass transfers in the air channel. In this study, the heat transfer enhancement and pressure drop across three different eddy promoter air screens in an air channel are experimentally investigated. Eddy promoter air screens are comprised of plastic ribs in the stream-wise direction and aluminum cross-bars normal to the air flow direction. A low speed wind tunnel test facility, which simulates the air channel of a LAMEE is designed to measure the friction factor and enhanced convective heat transfer coefficient in the air channel with an eddy promoter air screen. Tests were conducted at Reynolds numbers of 920, 1550, and 2160. In this paper, the effects of the spacing of the cylindrical bars and plastic ribs on the heat transfer performance are studied experimentally. Also, the performance of eddy promoter air screens as a function of enhanced heat transfer coefficient and increased pressure drop is investigated. Results show that the eddy promoter air screens have the highest efficiencies at Reynolds of 1550 and double the convective heat transfer coefficient of the air with respect to a smooth channel.
A Parametric Numerical Study of the Effects of Freestream Turbulence Intensity and Length Scale on Anti-Vortex Film Cooling Design at High Blowing Ratio
Authors: Repko Timothy W. , Nix Andrew C. , Heidmann James D.
Published: December 2013

An advanced, high-effectiveness film-cooling design, the anti-vortex hole (AVH) has been investigated by several research groups and shown to mitigate or counter the vorticity generated by conventional holes and increase film effectiveness at high blowing ratios and low freestream turbulence levels. [1, 2] The effects of increased turbulence on the AVH geometry were previously investigated and presented by researchers at West Virginia University (WVU), in collaboration with NASA, in a preliminary CFD study [3] on the film effectiveness and net heat flux reduction (NHFR) at high blowing ratio and elevated freestream turbulence levels for the adjacent AVH. The current paper presents the results of an extended numerical parametric study, which attempts to separate the effects of turbulence intensity and length-scale on film cooling effectiveness of the AVH. In the extended study, higher freestream turbulence intensity and larger scale cases were investigated with turbulence intensities of 5, 10 and 20% and length scales based on cooling hole diameter of Λx/dm = 1, 3 and 6. Increasing turbulence intensity was shown to increase the centerline, span-averaged and area-averaged adiabatic film cooling effectiveness. Larger turbulent length scales were shown to have little to no effect on the centerline, span-averaged and area-averaged adiabatic film-cooling effectiveness at lower turbulence levels, but slightly increased effect at the highest turbulence levels investigated.
Experimental Study of Boiling in Porous Media
Authors: Sapin Paul , Duru Paul , Fichot Florian , Prat Marc , Quintard Michel
Published: December 2013

Following a long-lasting failure in the cooling system of a pressurized water reactor (PWR), the superheated core can be efficiently cooled down by reflooding. The macroscopic model used at the French Institute of Radioprotection and Nuclear Safety (IRSN) to simulate this process is based on strong assumptions on the microscopic flow patterns. This paper describes the experimental setup designed for the study of boiling in porous media with the emphasis on various pore-scale boiling regimes. The final experimental setup is a two-dimensional porous medium made of 392 cylinders randomly placed between two ceramic plates. Each heating cylinder is a RTD probe (Resistance Temperature Detector), that can give thermal measurements in every point of the test section as well as heat generation. This paper presents preliminary results: pool boiling is characterized for a single cylinder mounted in the test section and reflooding of a line of 9 cylinders is observed.
New Predictive Methodology for the Onset of Flow Instability in Single Horizontal Microtube With an Inlet Orifice
Authors: Fan Yanfeng , Hassan Ibrahim
Published: December 2013

A new methodology to predict the onset of flow instability (OFI) in single horizontal microtube with inlet orifice is proposed. The predictive methodology states that OFI occurs as the pumping power under no heating condition is equal to the pumping power under heating condition in the microtube at the same volume rate. Since the pumping power can be simply described as the product of volume rate and pressure drop cross the microtube, the heat flux at OFI is determined as the two-phase pressure drop under heating condition is equal to the single-phase pressure drop under no heating condition at same flow rate. The addition of inlet orifice increases the pumping power under no heating condition. The increased pumping power by orifice delays the onset of flow instability. The predictive methodology is validated by comparing the predicted heat flux at OFI with our previous experimental data in the microtubes with three different inlet restriction ratios. The result shows that the proposed method is capable of prediction of heat flux at OFI with a deviation of 30% and mean absolute error of 13% at mass flux less than 2000 kg/m2·s.
The Effect of Inlet Orifice on Critical Heat Flux and Flow Boiling Heat Transfer in Single Horizontal Microtube
Authors: Fan Yanfeng , Hassan Ibrahim
Published: December 2013

Flow oscillation is a crucial issue for the development of flow boiling heat transfer in the applications. Inlet orifice has been proven be an option to eliminate the oscillation. However, the effects of inlet orifice on critical heat flux and flow boiling heat transfer coefficient are lack of study. In this work, the effects of inlet restriction on critical heat flux and heat transfer coefficient in single horizontal microtube under uniform heating condition is experimentally investigated using FC-72 as working fluid. A stainless steel microtube with an inner diameter of 889 μm is selected as main microtube. Two smaller microtubes are assembled at the inlet of main microtube to achieve the restriction configurations of 50% and 20% area ratios. The experimental measurement is carried out at mass fluxes ranging from 160–870 kg/m2·s and heat fluxes varying from 6–170 kW/m2. Two saturation pressures, 10 and 45 kPa, are tested. The experimental results of critical heat flux and two phase heat transfer coefficient obtained in the microtube without orifice are compared with the existing correlations. The addition of an orifice does not enhance the normal critical heat flux but increases the premature critical heat flux. In aspect of heat transfer, the orifice shows improvement on heat transfer coefficient at low mass flux and high saturation pressure.
One-Dimensional Heat and Mass Transfer and Discrete Granule Model of a Tubular Packed-Bed Reactor for Thermochemical Storage of Solar Energy
Authors: Ströhle Stefan , Jovanovic Zoran , Haselbacher Andreas , Steinfeld Aldo
Published: December 2013

Thermochemical storage of high-temperature (450°C – 1000°C) thermal energy can be applied to concentrated solar power systems for round-the-clock electricity dispatchability. Reversible, non-catalytic gas-solid reactions are used to convert thermal into chemical energy during endothermic charging, and vice versa during exothermic discharging. To assist the experimental investigation of such chemical reactors, a numerical model of the heat and mass transfer in a tubular packed bed reactor has been developed. The fluid serves as both heat transfer medium and gas reactant supplier and is modeled as a homogeneous phase using unsteady one-dimensional mass and energy balances. Solid reactants are modeled as spherical porous granules. The unsteady radial energy and mass balances are solved in the granules. This allows for the accurate treatment of larger granules with a diameter in the range of mm to cm in which radial gradients may occur. The temperature profile within a granule is calculated from an energy balance, whereas mass balances track the local concentration of the gases within the pores, affected by diffusion and an eventual mass source/sink due to the gas-solid reaction. The fluid and granule phases are coupled through local mass and energy exchange terms. The numerical implementation of the model is tested through a thorough code-verification study and a general assessment on mass and energy conservation within the coupled fluid and granule phases.
A Fast Coupled Solver for Phonon Transport in Composites
Authors: Loy James M. , Vadakkepatt Ajay , Mathur Sanjay R. , Murthy Jayathi Y.
Published: December 2013

In recent years, computational techniques for solving phonon transport have been developed under the framework of the semiclassical Boltzmann Transport Equation (BTE). Early work addressed gray transport, but more recent work has begun to resolve wave vector and polarization dependence, including that in relaxation times. Because the relaxation time in typical materials of interest spans several orders of magnitude, typical solution techniques must address an enormous range of Knudsen numbers in the same problem. Calculation procedures which solve the BTE in phase space sequentially work well in the ballistic limit, but are slow to converge in the thick limit. Unfortunately, both extremes may be encountered simultaneously in typical wave-number (K) -resolved phonon transport problems. In previous work, we developed the coupled ordinate method (COMET) to address this problem. COMET employs a point-coupled solution to resolve coupling in K-space, and embeds this point solver as a relaxation sweep in a geometric multigrid method to maintain spatial coupling. We have demonstrated speedups of up to 200 over conventional sequential solution procedures using this method. COMET also exhibits excellent scaling on multiprocessor platforms, far beyond those obtained by sequential solvers.

In this paper, we extend COMET to address interface transport in composites. Just as scattering couples phonons of different wave vectors in the bulk, reflection and transmission couple different wave vectors together at interfaces. Again, sequential solution procedures perform poorly because of the poor algorithmic coupling in K space. A computational procedure based on COMET is developed for composites, addressing multigrid agglomeration strategies to promote stronger K-space coupling at interfaces. The technique is applied to canonical superlattice geometries and superior performance over typical sequential solvers is demonstrated. Furthermore, the method is applied to realistic particle composites employing computational meshes developed from x-ray computed tomography (CT) scans of particulate beds. It is demonstrated to yield solutions where sequential solution techniques fail to converge at all.
Experimental Investigation of Bubble Departure Diameter and Bubble Departure Frequency in Subcooled Flow Boiling in a Vertical Annulus
Authors: Brooks Caleb S. , Silin Nicolás , Hibiki Takashi , Ishii Mamoru
Published: December 2013

Wall nucleation experiments have been performed in a vertical annulus test section for investigation of the bubble departure diameter and bubble departure frequency. The experimental data in forced convective subcooled boiling flow is presented as a parametric study of the effect of wall heat flux, local bulk liquid sub-cooling, liquid flow rate, and system pressure. The data is shown to extend the database currently available in literature to a wider range of system conditions. Along with the current database in forced convective flow, the available models for bubble departure size and frequency are reviewed and compared with the new data. The prediction of the bubble departure frequency is shown to require accurate modeling of the bubble departure diameter which has poor agreement with the experimental database.
Numerical Simulation of Thermal Transit-Time Flow Meter for High Temperature, Corrosive and Irradiation Environment
Authors: Alidoosti Elaheh , Ma Jian , Jiang Yingtao , Moazzeni Taleb
Published: December 2013

In the environments of high temperature (>300 °C – 1000 °C), corrosive and even irradiation application, the challenges of providing reliable and accurate flow rate measurement is significant. In comparing with many other existing technologies for normal operation environments, correlated thermal transit-time flow meter show its advantages of resolving the challenges encountered in those harsh conditions. The correlated thermal signals can be detected by two separated thermal sensors (for example, thermocouples) in series alignment along the pipe, and derive the flow rate. It was evaluated to have accurate measurement for small pipe at slow fluid speed. In the higher flow rate and big pipe size application, this technology shows its weakness due to the limitations associated with slow response time of thermal sensor, dimension, and low strength of thermal signal. In this paper, we present a sophisticated layout of thermal transit-time flow meter with numerical simulation and experiments. By numerical results, we observed that the obtained flow in the bypass route is linearly proportional to the main flow over higher range of flows showing that the measured flow is successfully extended to high range and with stable and accurate measurement results.
Strip-and-Zone Micro-Channel Liquid Cooling of Integrated Circuits Chips With Non-Uniform Power Distributions
Authors: Zhang Yu , Li Yubai , Li Xin , Yao Shi-Chune
Published: December 2013

For a high-power integrated circuit (IC), it is desirable to cool with the liquid micro-channels. However, the non-uniform power distribution of the IC is a great challenge. In this paper, the strip-and-zone strategy is presented. First, the optimal channel-width assuming a uniform power distribution of the total chip power is considered as a nominal situation. Then, according to the distribution of the power density of the power blocks on chip, the micro-channels are divided into several parallel strips with various zones in these strips. A further optimization of the channel-width of each zone in the strips shall be made that a higher heat transfer coefficient will occur in the zones of higher heat flux, while the strips of higher total power will have same or less flow resistances. As a result, under the same pressure drop among all the strips, same or more flow will occur at the strips of higher total power and the maximum temperature on the chip is reduced. Illustration of this strip-and-zone micro-channel liquid cooling design is provided through a design case of an IC chip with realistic power distribution. Comparing with the same chip at the air cooling and at the micro-channel cooling with the nominal channel-width, the chip at strip-and-zone micro-channel liquid cooling yields the lowest surface temperature as expected.
Effects of an Unsteady Wake on Heat Transfer of Endwall Surface in the Linear Cascade
Authors: Park Jun Su , Jung Eui Yeop , Lee Dong Hyun , Kim Kyung Min , Kim Beom Soo , Chang Byoung Moon , Cho Hyung Hee
Published: December 2013

The present study aimed to investigate the effect of an unsteady wake on the heat transfer for the endwall surface of a linear turbine blade cascade. A naphthalene sublimation method was implemented to obtain the detailed heat/mass transfer distributions on the endwall surface. Tests were conducted on a five-passage linear cascade in a low-speed wind tunnel. The effects of unsteady wakes were simulated in the facility by a wake generator consisting of circular rods that were traversed across the inlet flow. The test conditions were fixed at a Reynolds number of 70,000 based on the inlet velocity and chord length. The flow coefficients were varied from 1.3 to 4.2. and range of Strouhal number was 0.1 to 0.3. The results showed that the heat transfer distributions differed between steady and unsteady test cases. The overall heat transfer for the unsteady test cases was higher, and the heat transfer was enhanced with increasing the Strouhal number due to the resulting thin boundary layer and high turbulence intensity. Therefore, a cooling system for the endwall of a rotor should focus on decreasing the high temperatures of the endwall surface induced by the unsteady wakes.
An Enthalpy Formulation for Thermocline With Encapsulated PCM Thermal Storage and Benchmark Solution Using the Method of Characteristics
Authors: Tumilowicz Eric , Chan Cho Lik , Xu Ben , Li Peiwen
Published: December 2013

An enthalpy-based model of thermocline operation general to both single phase and encapsulated phase change filler materials was created in MATLAB. The method of characteristics is applied in space and time, mapping fluid temperature and filler enthalpy to a numerical grid, and in the case of a melting filler, allowing accurate tracking of phase state interfaces to fractional positions of the grid. Careful consideration of various possible heat transfer conditions along with placement of phase state interfaces in the numerical grid allows for extreme versatility and accuracy in model application. Input of specific fluid and filler properties, tank size, time of operation, and initial and boundary conditions returns a full representation to any desired amount of charge/discharge processes or cycles. The paper covers mathematical formulation, certain intricacies of numerical implementation, model verification, and the beginnings of application to prove proper operation and generality.
Heat Transfer in Multiple Parallel High Aspect Ratio Ducts With Triangular Trench Enhancement Features
Authors: Lamont Justin , Chatterjee Krishnashis , Ekkad Srinath V. , Ledezma Gustavo , Kaminski Christopher , Tolpadi Anil
Published: December 2013

Detailed heat transfer coefficient distributions and pressure drop have been obtained for high aspect ratio (AR = Width/Height = 12.5) ducts with triangular trench enhancement features oriented normal to the coolant flow direction. Numerical and experimental approaches analyze the performance of triangular trenches for six geometrically identical ducts branching from a common plenum. The numerical approach is based on a Reynolds Averaged Navier Stokes (RANS) turbulence model with an unstructured mesh. A transient liquid crystal (TLC) technique is used to experimentally calculate Nu on the ducts surfaces. Reynolds number (Re = 7080, 14800, and 22400, with respect to the duct hydraulic diameter are explored. As Computational Fluid Dynamics (CFD) and TLC results are both detailed, qualitative and quantitative comparisons are made. Experimental results show the closest and furthest ducts from the entrance of the plenum are considerably affected, as recirculation zones develop which partially choke the inlet the respective ducts. Results from the experiments are compared to CFD predictions from Duct 4. In addition, the experimental data are recalculated with the maximum bias in TLC temperature to indicate an improved matching between CFD and experimental methods to demonstrate that CFD captures the wall heat transfer coefficient trends similar to experimental results. The triangular trenches enhance heat transfer in the ducts two-fold compared to smooth wall Dittus-Boelter Nusselt number correlation for flow in tubes.
Effective Thermal Conductivity of Ordered Mesoporous Carbon CMK-3
Authors: Feng Daili , Feng Yanhui , Zhang Xinxin , Wang Ge
Published: December 2013

CMK-3 is a typical of carbon rods which are arranged in relatively regular two-dimensional hexagonal array. In our study, the effective thermal conductivity of CMK-3 composite is investigated. For the thermal conductivity of carbon rods, the equilibrium molecular dynamics (EMD) is performed with Tersoff potential. The influences of porosity and temperature are also considered. For the thermal conductivity of air confined in mesoporous can be estimated by the frequently used Kaganer model. Then, the effective thermal conductivity models developed for coupled heat transfer of air and solid are obtained by the unit cell method. ETCs along the X and Y directions are extremely poor, due to the overwhelming effect of air thermal resistance. However, in the Z direction, the ETC improves almost linearly as the porosity decreases, and the value is much higher than those of X and Y directions. This study is in attempts to explore the possibility of CMK-3 being a proper substrate for thermal usage.
Heat Transfer in Physical Vapor Deposition of Polycrystalline Multilayers and Residual Stress
Authors: Zhang Song , Zhang Hui , Zheng Lili , Wu Nan , Ma Yefeng
Published: December 2013

This paper studied the stress generation in the deposition process of multilayer polycrystalline films by physical vapor deposition. A stress prediction model was proposed which is capable of describing the generation of both the intrinsic stress and thermal stress by analyzing heat/mass transfer and growth process during the deposition process. An alternative deposition of Cu and Ag layers on Si (100) was simulated. The thermal stress in the multilayer system was discussed in detail by analyzing the influence of different parameters on thermal stress, such as deposition temperature and film thickness. It is found that the thermal residual stress strongly depends on the deposition temperature and film thickness ratio and thus the thermal stress can be modified by adjusting the related parameters.