bluuebunny/crossref_metadata_2025 huggingface dataset:
Air-Side Performance Characterization of Air-to-Refrigerant Heat Exchangers Using Parallel Parameterized CFD
Authors: Saleh Khaled , Han Weizhe , Aute Vikrant , Radermacher Reinhard
Published: December 2013

The goal of the study presented in this paper is to use Computational Fluid Dynamics (CFD) to characterize the heat transfer and friction performance of fins used in air-to-refrigerant heat exchangers. Five different types of fins used in air-cooled heat exchangers (HXs) are studied using Parallel Parameterized CFD (PPCFD) approach described in this paper. The fin types considered in this paper are; Plain, Wavy, Slit, Super Slit, and Louver. 3-D CFD models are built and tested for these fin types. Based on the CFD results, air side heat transfer coefficient (HTC), Colburn j factor, Fanning f factor, and pressure drop are calculated. The results from CFD simulations are compared against experimental data from the literature for the different fin types and a good agreement is found between the two. In addition, the results from CFD simulations are used to evaluate the thermal and hydraulic performance for a wide range of heat exchanger parameters such as tube diameters, fin pitch, number of rows, and frontal air velocity. The results show the advantages of using PPCFD to efficiently develop correlations for different types of fins used in air-cooled HX, with significant reduction in engineering time. The PPCFD approach can be extended to efficiently optimize novel heat transfer surfaces.
Comparing the Enhancement of Heat Transfer Caused by Sliding Gas Bubbles and by Sliding Vapor Bubbles in Subcooled Flow in a Minichannel
Authors: Albahloul Khaled E. , Hollingsworth D. Keith , Witte Larry C. , Ozer Arif B.
Published: December 2013

Studies published over the past decade have established the importance of sliding bubbles in enhancing the heat transfer in subcooled two-phase flow in channels small enough to confine the bubbles. Recent work in this laboratory (Ozer et al., 2011, 2012) proposed that the primary enhancement mechanism is a single-phase convective mechanism: the transport of cold fluid nearer the wall due to the mixing behind the bubble. This is in contrast to two phase-change mechanisms: distributed bubble nucleation and the evaporation of the liquid microlayer between a sliding bubble and the surface.

The work reported here explores this hypothesis by comparing the heat transfer enhancement produced by injected air bubbles to Ozer’s measurements obtained with naturally nucleated vapor bubbles. Data were collected under similar conditions in a highly subcooled laminar flow of Novec 649 in a horizontal rectangular minichannel of 1.21 to 1.484 mm channel spacing. The channel was formed by an electrically heated metallic upper wall and an unheated transparent lower wall. For the air/liquid flow, bubbles were injected at either a single point on the lower wall or through a sintered metal plug. The latter system produced a more channel-filling distribution of bubbles. A high-speed imaging system recorded the bubble motion and liquid crystal thermography recorded time-averaged surface temperature data. The comparison is presented in the form of the streamwise evolution of surface temperatures and the enhancement in time-averaged Nusselt number. Also, results for the passage of a single air bubble are presented.

The air/liquid flow produced a Nu enhancement of between 120–350% compared to a single-phase flow at the same conditions. The passage of the single gas bubble produced a decrease in the wall temperature directly behind the bubble of 2–5 °C.

The Nu enhancement produced by the air/liquid data and the nucleated vapor data is well correlated to appropriate dimensionless groups involving bubble diameter and frequency. The results from both data sets support the contention that a transient transport/mixing model developed previously for the vapor/liquid case captures the dominant single-phase convective mechanism in sliding bubble flows in highly confined channels.
Modeling of Fine Annealing of Large Size Optical Glass
Authors: Ma Yefeng , Wu Nan , Li Lihua , Zhang Song , Zheng Lili , Zhang Hui
Published: December 2013

Large size optical glass has attracted much attention due to its applications in large optical devices. Due to stringent requirement in applications, extra thermal treatment should be investigated to increase the quality of product. During fine annealing, glass temperature is crucial for final quality of optical glass as a result of its influence on thermal stress and optical homogeneity. To ensure a high performance, temperature distribution in the glass should be homogeneous and symmetric, and maximum temperature difference is expected to be small. This paper proposed two approaches to improve the glass temperature uniformity during fine annealing. Firstly, the glass blocks are packaged with heat exchange blocks on the top and bottom surfaces and insulation layer on the side. Thickness of layers and materials usage are investigated. Simulation results show that the homogeneity and symmetry of glass temperature distribution can be improved. Temperature difference in the horizontal direction can be further reduced in the case of 10mm copper heat exchange block together with 50mm insulation layer. Secondly, a muffle apparatus is utilized and symmetry of temperature distribution can be improved. Furthermore, above two approaches can be combined. Knowledge learned in this work can be used to guide industrial fine annealing process to reduce the stress level and improve the symmetric of residual stress.
Utilizing the Integral Technique to Determine the Similarity Variable in Classical Heat Transfer Problems: One Dimensional Heat Conduction in a Finite or Semi-Infinite Solid
Authors: Kobus Chris J.
Published: December 2013

In advanced heat transfer courses, a technique exists for reducing a partial differential equation where the dependent variable is a function of two independent variables, to an ordinary differential equation where that same dependent variable becomes a function of only one independent variable. The key to this technique is finding out what the similarity variable to make this transformation is. The difficulty is that the form of the similarity variable is not intuitive, and many heat transfer textbooks do not reveal how this variable is found in classical problems such as viscous and thermal boundary layer theory. It turns out that one way to find this variable is by utilizing the integral technique. By employing the integral technique to boundary layer theory, it will be shown that when the approximate functional relationship for the dependent variable (temperature, velocity, etc) can be represented by an nth order polynomial, the similarity variable can be found very simply. This is seen to be a good tool especially in heat transfer education, but has applications in research as well.
Numerical Simulation and Structure Optimization of Converter Gas Evaporative Cooler
Authors: Jiang Zeyi , Chen Pengyuan , Liu Pan , Tian Guanghe
Published: December 2013

In Lurgi-Thyssen dedusting system of steelmaking converter, the evaporative cooler represents a crucial operating unit, in which the hot dust-laden flue gas has to be cooled by saturation with water. The cooling process of the gas consists of gas-liquid two phase flow and interphase heat and mass transfer. In this paper, k-epsilon standard equations and Lagrange discrete phase model are employed to describe the gas turbulent flow and the heat/mass transfer with droplet evaporation individually. The computational fluid dynamics (CFD) simulation for practical engineering project shows that the large-flux cooling gas is commonly constructed in a non-uniform flow caused by the sharp turnings at the inlet and outlet channels. The unevenness of velocity distribution and the effective cooling height are defined in this paper to evaluate the cooling flow process. A series of newly designed structures with guide plate are investigated by CFD method to eliminate the problems with the non-uniformity. The results of numerical simulation show that optimal designed guide ring plate could improve the flow uniformity and the heat transfer. The investigations have been used to guide the engineering application.
Modeling of Thermo-Magnetic Phenomena in Active Magnetic Regenerators
Authors: Trevizoli Paulo V. , Barbosa Jader R. , Tura Armando , Arnold Daniel , Rowe Andrew
Published: December 2013

The active magnetic regenerator (AMR) is at the heart of the thermo-magnetic Brayton cooling cycle. It consists of a porous matrix heat exchanger whose solid phase is a magnetocaloric material (solid refrigerant) that undergoes a reversible magnetic phase transition when subjected to a changing magnetic field. The cooling capacity of the cycle is proportional to the mass of solid refrigerant, operating frequency, volumetric displacement of the working fluid (generally an aqueous solution) and regenerator effectiveness. AMRs can be modeled via a porous media approach and a model has been developed to simulate the time-dependent fluid flow and heat transfer processes. Gadolinium (Gd) is usually adopted as a reference material for magnetic cooling at near room temperature and, in this study, its magnetic temperature change and physical properties were accounted for using a combination of experimental data and the Weiss-Debye-Sommerfeld theory. In this paper, the influence of the applied magnetic field waveform and of demagnetizing effects on the AMR performance is investigated numerically. An evaluation of the model is also carried out in the light of a comparison against experimental data for a regenerator containing spherical Gd particles.
The Impact of Fin Deformation on Condensation Heat Transfer Coefficients in Internally Grooved Tubes
Authors: Mehendale Sunil
Published: December 2013

In HVACR equipment, internally enhanced round tube (microfin) designs such as axial, cross-grooved, helical, and herringbone are commonly used to enhance the boiling and condensing performance of evaporators, condensers, and heat pumps. Typically, such tubes are mechanically expanded by a mandrel into a fin pack to create an interference fit between the tube outside surface and the fin collar to minimize the thermal contact resistance between tube and fin. However, during this expansion process, the internal enhancements undergo varying amounts of deformation, which degrades the in-tube thermal performance.

Extensive data on condensing heat transfer coefficients in microfin tubes have been reported in the open literature. However, researchers have seldom used expanded tubes to acquire and report such data. Hence, it is always questionable to use such pristine tube data for designing heat exchangers and HVACR systems. Furthermore, the HVACR industry has been experiencing steeply rising copper costs, and this trend is expected to continue in coming years. So, many equipment manufacturers and suppliers are actively converting tubes from copper to aluminum. However, because of appreciable differences between the material properties of aluminum and copper, as well as other manufacturing variables, such as mandrel dimensions, lubricant used, etc., tube expansion typically deforms aluminum fins more than copper fins.

Based on an analysis of the surface area changes arising from tube expansion, and an assessment of the best extant in-tube condensation heat transfer correlations, this work proposes a method of estimating the impact of tube expansion on in-tube condensation heat transfer. The analysis leads to certain interesting and useful findings correlating fin geometry and in-tube condensation thermal resistance. This method can then be applied to more realistically design HVACR heat exchangers and systems.
Dependence of Thermal Conductivities of the AlN Film in the LED Architecture on Surface Roughness and Lattice Mismatch
Authors: Su Zonghui , Malen Jonathan A. , Freedman Justin P. , Davis Robert F. , Leach Jacob H. , Preble Edward A.
Published: December 2013

Thickness dependent thermal conductivity measurements were made on aluminum nitride (AlN) thin films grown by two methods on the (0001) surfaces of silicon carbide (SiC) and sapphire substrates with differing surface roughness. We find that the AlN layer itself makes a small contribution to the overall thermal resistance. Instead, the thermal boundary resistance (TBR) of 5.1±2.8 m2-K/GW between the AlN and substrate is equivalent to 240 nm of highly dislocated AlN, or 1450 nm of single crystal AlN. An order-of-magnitude larger TBR was measured between AlN films and SiC substrates with increased surface roughness (1.2 nm vs. 0.2 nm RMS). High resolution TEM images reveal near-interface planar defects in the AlN films grown on the rough SiC that we hypothesize are the source of increased TBR.
Effects of Pressure and a Microporous Coating on HFC-245fa Pool Boiling Heat Transfer
Authors: Moreno Gilberto , Jeffers Jana R. , Narumanchi Sreekant
Published: December 2013

A study was conducted to experimentally characterize the pool boiling performance of hydrofluorocarbon (HFC)-245fa. The motivation for this research is to characterize the performance of candidate refrigerants for potential use in automotive power electronics two-phase cooling systems. The HFC-245fa pool boiling experiments were conducted using horizontally oriented 1-cm2 heated surfaces to quantify the effects of pressure and a microporous-enhanced coating on heat transfer coefficients and critical heat flux (CHF) values. Experiments were carried out at pressures ranging from 0.15 MPa to 1.1 MPa (reduced pressure range: 0.04–0.31). To enhance boiling heat transfer, a copper microporous coating was applied to the test surfaces. The coating was found to enhance heat transfer coefficients by as much as 430% and CHF by approximately 50%. Increasing pressure decreased the magnitude of the heat transfer coefficient enhancements but had minimal effect on CHF enhancements. The experimental data were then used to generate correlations for the boiling heat transfer coefficients and CHF values. Finally, the performance of HFC-245fa was compared to the performance of hydrofluoroolefin (HFO)-1234yf and HFC-134a at conditions of equivalent saturation temperatures and reduced pressures.
Mixed Pressure and AC Electroosmotically Driven Flow With Asymmetric Wall Zeta Potential and Hydrophobic Surfaces
Authors: Lesani Mostafa
Published: December 2013

The present study examines Alternating Current (AC) electroosmotic flows in a parallel plate microchannel subject to constant wall temperature. Numerical method consists of a central finite difference scheme for spatial terms and a forward difference scheme for the temporal term. Asymmetric boundary conditions are assumed for Poison-Boltzmann equation for determining the electric double layer (EDL) potential distribution. The potential distribution is then used to evaluate the velocity distribution. The velocity distribution is obtained by applying slip boundary conditions on the walls which accounts for probable hydrophobicity of surfaces. After determining the velocity distribution numerically, the energy equation is solved by taking into account the effects of viscous dissipation and non-uniform Joule heating. The results reveal that the effect of increasing Knudsen number is a slight increase in the dimensionless temperature profile. Furthermore, the effect of increasing dimensionless time is an increase in dimensionless oscillatory temperature which leads to a steady oscillatory condition. Also, the effect of increasing dimensionless Debye-Huckel parameter is a decrease in mean oscillation value and an increase in the required time for reaching steady oscillatory condition. In addition, increasing forward or backward pressure leads to increased viscous heating near the walls. Furthermore, the effect of increasing zeta potential is an increase in the dimensionless mean velocity oscillation amplitude.
Transient Internal Forced Convection Under Step Wall Heat Flux Condition
Authors: Fakoor-Pakdaman M. , Bahrami Majid
Published: December 2013

A new closed-form analytical model is developed to predict transient laminar forced convection inside a circular tube following a time-wise step change in the wall heat flux. The proposed all-time model is based on a blending of two asymptotes; i) short-time asymptote: transient pure conduction in an infinite cylinder and ii) long-time asymptote: steady-state convective heat transfer inside a circular duct. Different fluid velocity profiles are taken into consideration and the model covers: i) Slug Flow (SF); ii) Hydrodynamically Fully Developed Flow (HFDF); and iii) Simultaneously Developing Flow (SDF) conditions. The present model is developed for the entire range of the Fourier and Prandtl numbers. As such, short- and long-time asymptotes for the fluid bulk temperature are obtained. The Nusselt number is defined based on the local temperature difference between the tube wall temperature and the fluid bulk temperature. It is shown that irrespective of the velocity profile, at the initial times the Nusselt number is only a function of time. However, at the steady state condition it depends solely upon the axial location. In addition, during the transient period, the Nusselt number is much higher than that of the long-time response. We also performed an independent numerical simulation using COMSOL Multiphysics to validate the present analytical model. The comparison between the numerical and the present analytical model shows good agreement; a maximum relative difference less than 9.1%.
A Comparative Cost and Performance Analysis of Structured and Packed-Bed Thermocline Thermal Energy Storage Systems
Authors: Strasser Matthew N. , Selvam R. Panneer
Published: December 2013

Integrated thermal energy storage (TES) is necessary for concentrating solar power (CSP) plants to be implemented as a cost-competitive and reliable power production option. This work presents and discusses existing utility scale CSP plants with integrated TES. The current standard TES system configuration, indirect two-tank, is presented, and the single-tank thermocline is proposed as a cost-reducing alternative. The traditional thermocline concept involves storing energy in a “packed-bed” of solid aggregate; however, concerns exist that settlement of aggregate over repeated thermal cycles may lead to thermal ratcheting and rupture of the tank’s walls. To address this issue, a structured thermocline is proposed, in which the aggregate bed is replaced with plates of concrete. Finite-difference-based numeric models are introduced and used to study the performance of packed-bed and structured thermoclines. A complete cost analysis of a 2,165 MWht TES system, considering both thermocline configurations, is provided. System Advisory Model (SAM) is used to perform life-cycle cost and performance analysis of a central receiver CSP plant incorporating three unique TES configurations: none, packed-bed thermocline, and structured thermocline. Conclusions are drawn as to which TES configuration is the most viable.
Evaluation of the Integration of Oscillating Heat Pipes in High Power DC-DC Converters for Pulsed Power Applications
Authors: Huhman B. M. , Hathaway A. , Ma H. B.
Published: December 2013

The Pulsed Power Physics Branch at the U.S. Naval Research Laboratory (NRL) is developing a battery-powered, rep-rate charger for a 60-kJ capacitor bank. The goal is to charge a 4800μF capacitor to 5kV in five seconds for a fifty shot burst. A bank of LiFePO4 batteries is used with a full H-bridge converter and transformer to elevate the 500V battery voltage to a 5kV secondary voltage. The operation of the Integrated Gate Bipolar Transistor (IGBT) generates heat as a byproduct of the energy transfer from the batteries to the capacitor, which must be effectively removed. The traditional method of cooling the IGBTs involves a passive heat sink and forced air cooling, which can be quite large if the dissipated power load is high enough. This work investigates the replacement of the forced air cooling method with an oscillating heat pipe (OHP). The OHP investigated herein was made of aluminum with dimensions of 130.1 mm × 101.9 mm × 2.5 mm. The OHP channel dimension imbedded in the aluminum block is 1.0 mm by 1.0 mm. Utilizing high effective thermal conductivity, the integrated OHP has the potential to reduce the overall system volume and enable the design of a sealed converter package. Numerical analysis and experimental results demonstrate that the OHP can significantly increase the effective thermal conductivity and enable a fast time response of the pulsed power DC-DC converter. Comparison with the numerical analysis show that the heat transfer resistance occurring in the cooling block is the primary resistance for the investigated IGBT OHP cooling.
Enhancement of Interfacial Thermal Transport by Carbon Nanotube-Graphene Junction
Authors: Bao Hua , Luo Shirui , Hu Ming
Published: December 2013

Thermal transport across material interfaces is crucial for many engineering applications. For example, in microelectronics, small interfacial thermal resistance is desired to achieve efficient heat dissipation. Carbon nanotube (CNT) has extremely high thermal conductivity and can potentially serve as an efficient thermal interface material. However, heat dissipation through CNTs is limited by the large thermal resistance at the CNT-material interface. Here we have proposed a CNT-graphene junction structure to enhance the interfacial thermal transport. Non-equilibrium molecular dynamics simulations have been carried out to show that the thermal conductance can be significantly enhanced by adding a single graphene layer in between CNT and silicon. The mechanism of enhanced thermal transport is attributed to the efficient thermal transport between CNT and graphene and the good contact between graphene and silicon surface.
A Two-Dimensional Cylindrical Transient Conduction Solution Using Green’s Functions
Authors: McMasters Robert L. , Beck James V.
Published: December 2013

There are many applications for problems involving thermal conduction in two-dimensional cylindrical objects. Experiments involving thermal parameter estimation are a prime example, including cylindrical objects suddenly placed in hot or cold environments. In a parameter estimation application, the direct solution must be run iteratively in order to obtain convergence with the measured temperature history by changing the thermal parameters. For this reason, commercial conduction codes are often inconvenient to use. It is often practical to generate numerical solutions for such a test, but verification of custom-made numerical solutions is important in order to assure accuracy. The present work involves the generation of an exact solution using Green’s functions where the principle of superposition is employed in combining a one-dimensional cylindrical case with a one-dimensional Cartesian case to provide a temperature solution for a two-dimensional cylindrical. Green’s functions are employed in this solution in order to simplify the process, taking advantage of the modular nature of these superimposed components. The exact solutions involve infinite series of Bessel functions and trigonometric functions but these series sometimes converge using only a few terms. Eigenvalues must be determined using Bessel functions and trigonometric functions. The accuracy of the solutions generated using these series is extremely high, being verifiable to eight or ten significant digits. Two examples of the solutions are shown as part of this work for a family of thermal parameters. The first case involves a uniform initial condition and homogeneous convective boundary conditions on all of the surfaces of the cylinder. The second case involves a nonhomogeneous convective boundary condition on a part of one of the planar faces of the cylinder and homogeneous convective boundary conditions elsewhere with zero initial conditions.
Conjugate Heat Transfer CFD Predictions of the Influence of the Impingement Gap on the Effect of Crossflow
Authors: El-Jummah Abubakar M. , Andrews Gordon E. , Staggs John E. J.
Published: December 2013

This conjugate heat transfer (CHT) CFD predictions were carried out for a square array impingement holes with X/D of 5 and at a single high mass flow cooling rate G of 1.84kg/sm2bar. CHT was modelled with a 100kW/m2 fixed heat flux imposed at the hot face of the target wall, which gives a temperature at the cold face that is typical of normal operating conditions of a gas turbine. The wall temperatures as a function of distance along the gap and through the wall thickness were computed, together with the gap aerodynamics. The main parameter investigated was the gap Z/D (0.83–7.0). The results showed that the surface averaged Nu/Pr0.33 was predicted to be reduced from 73 to 48 over the range of Z/D, which is in good agreement with literature values. The cause was shown to be reduced surface turbulence levels as the crossflow convected turbulence away from the surface. There was also heating of the impingement jets at large Z/D so that the original cool jet air did not reach the surface.
Application of Genetic Algorithm-Polynomial Neural Network for Modelling Heat Transfer and Fluid Flow Characteristics of a Double-Pipe Heat Exchanger
Authors: Mehrabi Mehdi , Pashaee Tuhid , Sharifpur Mohsen , Meyer Josua P.
Published: December 2013

In this paper a genetic algorithm-polynomial neural network approach is used in order to model the effect of important parameters on heat transfer as well as fluid flow characteristics for a double-pipe helical heat exchanger by using numerical-certified results. In this way, overall heat transfer coefficient (Uo), inner and annular pressure drop (ΔPin, ΔPan) are modeled with respect to the variation of inner and annular dean number, inner and annular Prandtl number, and pitch of coil which are defined as input (design) variables. The numerical-certified data was randomly divided into test and train sections which the former is used for benchmark. The GA-PNN structure was instructed by 75 percent of the numerical-validated data. 25 percent of the primary data which had been considered for testing procedure were entered into GA-PNN proposed models and results were compared by statistical criteria.
Micro-Scale Two-Phase Flow Heat Transport Device Driven by Electrohydrodynamic Conduction Pumping
Authors: Patel Viral K. , Seyed-Yagoobi Jamal
Published: December 2013

Micro-scale two-phase flow heat transport involves specialized devices that are used to remove large amounts of heat from small surface areas. They operate by circulating a working fluid through a heated space which causes phase change from liquid to vapor. During this process, a significant amount of heat is transported away from the heat source. Micro-scale heat transport devices are compact in size and the heat transfer coefficient can be orders of magnitude higher than in macro-scale for similar operating conditions. Thus, it is of interest to develop such devices for cooling of next-generation electronics and other applications with extremely large heat fluxes. The heat transport device presented in this paper is driven by electrohydrodynamic (EHD) conduction pumping. In EHD conduction pumping, when an electric field is applied to a dielectric liquid, flow is induced. The pump is installed in a two-phase flow loop and has a circular 1 mm diameter cross section with electrode spacing on the order of 120 μm. It acts to circulate the fluid in the loop and has a simple yet robust, non-mechanical design. Results from two-phase flow experiments show that it is easily controlled and such electrically driven pumps can effectively be used in heat transport systems.
Experimental Observations of Nitrogen Diluted Ethylene and Methane Jet Flames
Authors: Hutchins Andrew R. , Kribs James D. , Muncey Richard D. , Reach William A. , Lyons Kevin M.
Published: December 2013

While the liftoff mechanisms of nitrogen-diluted methane jet flames have been well documented, higher order fuels, such as ethylene, have not been studied as extensively with regards to flame stabilization and behavior. Higher order fuels generally burn more intensely, and thus produce much different stabilization patterns than those of simple hydrocarbon fuels, such as methane. The purpose of this study was to observe the effects of nitrogen dilution on ethylene combustion and compare to that witnessed in typical methane jet flames; specifically, the influence on the liftoff height, blowout, and flame chemiluminescence. Liftoff and blowout velocities were compared for various mixtures of ethylene without nitrogen. It was observed that the reason behind the varying stabilization patterns is due to the higher thermal diffusivity of ethylene as well the higher flame speeds that are characterized in the combustion of ethylene. Using a sequence of images from each mixture, the flame liftoff heights were recorded. Due to the strong chemiluminescence of ethylene flames, little fluctuation between liftoff parameters was observed, with respect the velocity; however, there was a significant effect on the liftoff height, with respect to dilution. Blowout for fuel mixtures was much more difficult to achieve due to the higher thermal diffusivity of ethylene, meaning the flame would stabilize at positions much farther downstream than those of simple hydrocarbon fuels.
Numerical Analysis of a Single Minichannel Within a High-Temperature Hydrogen Heat Exchanger for Beamed Energy Propulsion Applications
Authors: Gould Daniel W. , Hoff Brad W. , Young Marcus P. , Webb Rebecca N.
Published: December 2013

The requirement that the propellants used in launch vehicle systems must also provide the thermal energy to be converted to kinetic energy in the rocket nozzle has plagued rocket designers since the dawn of the space age. Beamed propulsion systems, however, avoid this constraint by placing the energy source on the ground and transmitting the energy to the spacecraft via microwaves. This computational work evaluates three different heat exchanger channel designs for use in a beam propulsion spacecraft. The working fluid was hydrogen and the input energy was 1.3 kW. The increase in axial temperature along the 0.1 m long channel was as high as 2000 K. In addition, it was found that despite the very small diameter of the minichannels, 1 mm, each design produced extreme temperature gradients across the channel cross section. These temperature gradients affected the velocity profile; the maximum velocity was not located at the channel center.
Analytical Modeling of Mist Condensation by Natural Convection Over Inclined Flat Surfaces
Authors: Fayazbakhsh Mohammad Ali , Bahrami Majid
Published: December 2013

Dew formation on a transparent surface creates a pattern that can cause blurred view over it. This fogging phenomenon should be avoided in many applications. Mist condensation of water on a cold surface exposed to humid air is studied in this work. In order to analyze the misting process, the fluid flow and heat transfer of humid air as well as the heat transfer across the solid surface are considered. A dew formation model is used to predict the shape and size of the droplets. Analytical models have been proposed to solve the heat and mass transfer for the simple arrangement of a vertical flat surface. The analytical model is then combined with the dew formation model to introduce an analytical model for mist condensation over vertical and inclined surfaces. Due to the proposed method, complex numerical calculations can be avoided for solving the heat and mass transfer equations.
Enhancement of Forced Convection Heat Transfer Using Twisted Heater With Exponential Heat Inputs
Authors: Shibahara Makoto , Liu Qiusheng , Fukuda Katsuya
Published: December 2013

Forced convection transient heat transfer coefficients have been measured for nitrogen gas flowing over a twisted heater due to exponentially increasing heat inputs (Q0exp(t/τ)). And then, the effect of heater configuration on transient heat transfer by a twisted heater has been investigated comparing to that of the plate heater. In the experiment, the platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as a test heater. For heat transfer enhancements in single-phase flow, it was twisted at the central part of the heater with an angle of 90 degrees with respect to the upper part of the heater. The heat generation rate was exponentially increased with a function of Q0exp(t/τ). The gas flow velocity ranged from 1 to 4 m/s for the gas temperatures of 313K. The periods of heat generation rate ranged from 46 ms to 17 s. The surface temperature difference and heat flux increased exponentially as the heat generation rate increased with the exponential function. The heat transfer coefficients for twisted heater have been compared to those of the plate heater. They were 24 % higher than those of the plate one. The geometric effect (twisted effect) of heater in this study showed an enhancement on the heat transfer coefficient. It was considered that the heat transfer coefficients are affected by the change in the flow due to swirling flow on the twisted heater. Finally, the empirical correlations for quasi-steady-state heat transfer and transient one have been obtained based on the experimental data.
Feature-Section-Criterion for Predicting Lean Blowout and Comparison Research
Authors: Zhang Zhibo , Zheng Hongtao , Yang Honglei , Yang Ren , Liu Qian , Liu Xiao
Published: December 2013

Lean blowout (LBO) plays an important role in combustor performance. A new method named Feature-Section-criterion (FSC) for predicting the LBO of annular combustor has been put forward and expounded in this paper. A CFD software FLUENT has been used to simulate the combustion flow field of an annular combustor. The process of blowout and effects of flow split among swirlers and primary holes have been researched by using of FSC. The result shows that the predictions of FSC are in agreement with corresponding experimental data. So this method for predicting lean blowout is reliable and can be used for engineering applications.
Experimental Investigation on Flow Patterns and Frictional Pressure Drop of Downward Air-Water Two-Phase Flow in Vertical Pipes
Authors: Xue Yuqing , Li Huixiong , Sheng Tianyou , Liao Changjiang
Published: December 2013

A large amount of air need be transported into the reservoir in the deep stratum to supply oxygen to some microbes in Microbial Enhanced Oil Recovery (MEOR). Air-water two-phase flows downward along vertical pipeline during the air transportation. Base on the experiment data described in this paper, the characteristics of air-water two phase flow patterns were investigated. The flow pattern map of air-water two phase flows in the pipe with inner diameter of 65 mm was drawn, criterions of flow pattern transition were discussed, and the dynamic signals of the pressure and the differential pressure of the two phase flow were recorded to characterize the three basic flow regimes indirectly. The frictional pressure drop of downward flow in vertical pipe must not be disregarded contrast with upward two phase flow in the vertical pipe because the buoyancy must be overcame when the gas flows downward along pipe, and there would be a maximum value of frictional when the flow pattern translated from slug flow to churn flow.
Analysis of Heat Transfer and Pressure Drop Through Idealized Open Cell Ceramic Foams: Comparison Between Kelvin and Weaire-Phelan Cell Structures
Authors: Bianco N. , Cunsolo S. , Chiu W. K. S. , Naso V. , Migliozzi A. , Oliviero M.
Published: December 2013

In the applications of metal foams, the knowledge of the thermal transport properties is of primary importance. Thermal properties of a foam heavily depend on its microstructure. However, the influence of some geometric characteristics of the foam cells on their properties is far from being understood. Foam models are promising tools to study the above said effects. The effect of the cell architecture on heat transfer and pressure drop in open cell foams is investigated numerically using two foam models. The Kelvin and the Weaire-Phelan foam models are developed in an open source software “Surface Evolver”. Heat transfer and pressure drop in samples with different porosities and cell dimensions are studied using COMSOL® Multiphysics. Finally, a comparison between the numerical results obtained from two foam models is carried out in order to evaluate the feasibility to substitute the Weaire-Phelan foam structure, which is more complex and computationally heavier, with the simpler Kelvin foam representation.
In Situ Particle Size Measurements of Gas-Borne Silicon Nanoparticles by Time-Resolved Laser-Induced Incandescence
Authors: Sipkens T. A. , Petermann N. , Daun K. J. , Titantah J. , Karttunen M. , Wiggers H. , Dreier T. , Schulz C.
Published: December 2013

The functionality of silicon nanoparticles is strongly size-dependent, so there is a pressing need for laser diagnostics that can characterize aerosolized silicon nanoparticles. The present work is the first attempt to extend time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used for sizing soot, to size silicon nanoparticles. TiRe-LII measurements are made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon. Molecular dynamics (MD) is used to predict the accommodation coefficient between silicon nanoparticles and argon and helium, which is needed to interpret the TiRe-LII data. The MD-derived thermal accommodation coefficients will be validated by comparing them to experimentally-derived values found using transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis.
Effect of Fuel and Oxygen Carriers on the Hydrodynamics of Fuel Reactor in a Chemical Looping Combustion System
Authors: Harichandan Atal B. , Shamim Tariq
Published: December 2013

The hydrodynamics of fuel reactor in a chemical looping combustion (CLC) system has been analyzed by using a multiphase CFD-based model with solid-gas interactions and chemical reactions. In this paper, the fuel reactors of two CLC systems are numerically simulated independently by using hydrogen with calcium sulfide as oxygen carrier, and methane with nickel as oxygen carrier in similar conditions. Kinetic theory of granular flow has been adopted. Conservation of mass, momentum and species equations, and reaction kinetics of oxygen carriers are used for the numerical calculation. The present results obtained are in good agreement with the experimental and numerical results available in open literature. The bubble hydrodynamics in both the fuel reactors are analyzed. The salient features of bubble formation, rise and burst are prominent in hydrogen-fueled reactor as compared to methane-fueled reactor. The fuel conversion rate is found to be larger in the case of hydrogen-fueled reactor.
On Computational Heat Transfer in Industrial Applications
Authors: Sundén Bengt
Published: December 2013

Industrial energy systems involve many components where fluid flow, heat and mass transfer are the important transport processes. For design and development as well as investigation of innovative ideas, computational methods are of vital importance. Since the early pioneering works (by, e.g., the Spalding group) in the 1960s and 1970s, the development has been tremendous and nowadays CFD (computational fluid dynamics) is an established methodology. During the same time period computer capacities in addition have been brought extensively forward. Although still many topics need further development, applications to industrial problems are constantly increasing. In this paper examples of computational heat transfer, fluid flow and related transport phenmena applied to plate heat exchangers (PHEs), gas turbine heat transfer, and fuel cells are highlighted.
Self-Organized Micro-Honeycomb Network Structure of Single-Walled Carbon Nanotubes for Photovoltaic Devices
Authors: Maruyama Shigeo , Cui Kehang , Chiba Takaaki , Einarsson Erik , Chiashi Shohei
Published: December 2013

Single-walled carbon nanotubes (SWNTs) are expected to be a promising nanomaterial because of their outstanding electronic, mechanical, and thermal properties. For macroscopic device applications, an assembly of SWNTs is a critical issue. We propose a self-organized micro-honeycomb network structure of SWNTs obtained by water vapor treatment of as-synthesized vertically-aligned SWNTs (VA-SWNT) for solar cell devices with higher performance. The micro-honeycomb structure was realized by simply exposing VA-SWNT to water vapor and drying in ambient condition. Honeycomb cell walls consist of capillary-aggregated vertically aligned SWNTs with heavily bundled top part. Within each cell, collapsed spaghetti-like SWNTs make contact to the substrate. The SWNT/n-Si heterojunction solar cell was built by placing the micro-honeycomb SWNTs network film on top of the substrate which has a 3 mm × 3 mm bare n-type silicon contact window in the center. The contact window is surrounded by SiO2 as insulating layer and Pt as electrode. Our preliminary tests showed that optimal photovoltaic conversion efficiency (PCE) under AM1.5 was 5.91%, with the fill factor of 72%. The open-circuit voltage and short-circuit current are 0.53V and 15.5 mA/cm2, respectively. This showed a substantial improvement compared with heterojunction solar cells using spaghetti-like SWNTs. Furthermore, the superior performance of dye-sensitized solar cells with the micro-honeycomb SWNTs was demonstrated.
Experimental Investigation on Mixed Convection in Horizontal Channels Heated Below and Partially Filled With Aluminum Foam
Authors: Buonomo Bernardo , Ferraro Giuseppe , Manca Oronzio , Marinelli Lorenzo , Nardini Sergio
Published: December 2013

In this paper an experimental investigation on mixed convection in air in a heated channel partially filled with an aluminum foam is carried out. The aluminum foam layer is sets on the lower heated wall of the channel. The channel upper plate has a heat transfer toward the external ambient. The investigation allows to evaluate the effect of the aluminum foam on the mixed convection in the heated channel by wall temperature measurements and flow visualization. Results are given for heated channel without and with foam in terms of wall temperature profiles for different Reynolds number value, form 10 to 300, wall heat flux and for aluminum foam with 10 and 20 pore per inch.

Lower wall temperature values are detected for the channel with foam with respect to the channel clean without foam. Foams with higher pore per inch shows lower average wall temperature values whereas the local wall temperatures showed different behaviors for the different pore per inch values. The presence of foam in heated channel determined weaker secondary motions with respect to the clean cases. The effect of the foams seems more significant for the high Reynolds number values and average Nusselt number increases with the foam presence in the heated channel.
Numerical Solution of a Ground Source Heat Pump System Using Foundation Piles
Authors: Oguma Masahito , Matsumoto Takeshi , Funabiki Ayako , Miyaoka Futoshi , Ito Kosuke , Kakizaki Takao
Published: December 2013

A ground source heat pump (GSHP) system has higher cooling and heating performances than an air source heat pump system, so the GSHP system has attracted attention in the cold regions. Particularly after the 2011 earthquake off the Pacific coast of Tohoku, which damaged nuclear power plants, the GSHP utilizing renewable energy, has become attractive in heating system in the Tohoku region. However, it is necessary to install wells to collect ground source heat, and so, together with the cost of the heat pump itself, the installation cost is a barrier to widespread adoption of this technology in Japan.

On the other hand, due to poor subsoil, foundation piles are often required in the construction of buildings in Japan. By using foundation piles as heat exchangers, which are commonly used in residential construction, the cost of using GSHP systems in houses may be reduced. However, since the placement of the piles depends on the floor layout of the residence, that is arbitrary sequence. Moreover, an arbitrary floor layout requires a complicated multi-dimensional numerical analysis to design the GSHP, and the analysis is burdensome for general designers. Therefore, the use of the model unit of the two-dimensional cylindrical heat exchange well is proposed. The use of this model, which includes an unused volume of soil, reduces the analysis burden for general design tools.

On experience, the arrangement rate is 4 m2 per pile, and the well separation is narrow. And the foundation piles will form a group. So thermal interference between heat exchanger wells might be working hard. In addition, the foundation piles are very short, compared to the traditional borehole depth of 50 to 100 m. Therefore, the thermal performance of the well may degrade compared to that of the traditional GSHP system, although the initial costs are less. Therefore, we examined a GSHP system with heat exchanger piles by using the heat exchange well unit model for residences in the Tohoku region.

As a result, the two-dimensional analysis and more is needed to predict the thermal performances of the heat exchanger piles so that the effect of the pitch of the heat exchange wells on the heat transfer in the axial direction will be large. In cold climates, the heating demand is large and, in the summer, the amount of waste heat to the ground is small, so a large amount of heat penetration into the ground from the atmospheric air is important for continuous GSHP operation in the Tohoku region, and in Fukushima Prefecture in particular.