bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of Liquid Water Content Inside the Gas Diffusion Layer
Authors: Tahseen Siddiq Hussain , Chen Kehan , Shahraeeni Mehdi , Yew Samuel C. M. , Hoorfar Mina
Published: July 2013

The amount of the liquid water present at the gas diffusion layer (GDL) has an impact on the diffusivity, capillary pressure and the permeability which in turn influences convective and diffusive transport. A prodigious amount of research has been conducted to study and measure the different properties (time of breakthrough and capillary pressure versus saturation) associated with the breakthrough condition. However, most of the reported data ignored the impact of expansion of different components in the set-up (such as tubing) and the condition after the time of breakthrough. The focus of this study is to measure the breakthrough pressure and time of breakthrough and hence determine the liquid water content inside the GDL before the time of breakthrough. The measurements are performed for different samples to study the effect of the thickness and hydrophobic contents. The results show that expansion has significant difference in the determination of water volume inside the GDL.
Characterization of Transport Properties in Porous Media of a PEM Fuel Cell
Authors: Pant Lalit M. , Mitra Sushanta K. , Secanell Marc
Published: July 2013

Porous media is an essential part of polymer electrolyte membrane fuel cells (PEMFC). In order to optimize fuel cell performance and reduce catalyst consumption, mass transport in fuel cells needs to be improved. Understanding and modelling of mass transport in porous media of fuel cell (e.g. gas diffusion layer (GDL), micro porous layer (MPL) etc.) requires a knowledge of transport properties like diffusivity, permeability and Knudsen diffusivity. Current research is focused on experimental measurement of transport properties of porous media. A counter-diffusion bridge (Wicke-Kallenbach setup) has been used to estimate permeability, Knudsen diffusivity and effective diffusivity of GDLs and MPLs. The obtained transport properties are used with the recent theoretical models of multicomponent mass transport to estimate transport in fuel cells. The experimental studies show that conventional effective approximations like Bruggeman correlations are highly overpredicting and do not fully account for all the frictional forces in porous media.
Development of a Cathode Electrode Model Using the ORR Dual-Trap Intrinsic Kinetic Model
Authors: Moore Michael , Putz Andreas , Secanell Marc
Published: July 2013

A major challenge facing PEM fuel cell researchers is the development of a cathode electrode model that accurately accounts for the oxygen reduction reaction (ORR) kinetics. Rotating disk electrode (RDE) and micro-electrode experiments consistently show a doubling of the Tafel slope at a cell potential of approx. 0.75V. However, current mathematical models repeatedly use a Tafel kinetic model with constant Tafel slope, which may lead to inaccurate results. Recently, Wang et al. proposed an intrinsic, kinetic model that assumes the ORR is comprised of four intermediate steps and two intermediate adsorbed species, with the free energies of adsorption and activation used as the kinetic parameters. This model is capable of predicting the doubling of the Tafel slope and the coverage of the assumed intermediate species. The model is extended to include changes in the oxygen concentration with changing overpotential and is then implemented in an in-house two dimensional, cathode model. Results show the doubling of the Tafel slope, resulting in significant losses in the low potential region compared to Tafel kinetics. Also shown is the site blocking effect of the intermediate species, in particular Pt oxide, which completely covers the platinum surface on the catalyst layer in the presence of high oxygen concentration, i.e. at low overpotentials. Increasing the cell potential favors the reductive transition intermediate step, which consumes the Pt oxide and subsequently increases the available platinum surface. The change in Tafel slope is caused by this transition from a completely oxide covered surface to an oxide free surface.
Analysis of Two-Phase Transport in Ultra-Thin Catalyst Layer Based Electrodes Using a Mixed Wettability MPL Model
Authors: Mateo-Villanueva Pedro , Secanell Marc
Published: July 2013

A new approach to analyze two-phase water transport in microporous layer used on polymer electrolyte fuel cells is presented. The effect of mixed wettability in MPLs is studied by means of a novel pore-size distribution (PSD) model, which is used to estimate saturation, liquid relative permeability, gas relative permeability, liquid-vapour interfacial surface area, and total pore surface area. With the proposed model, micro-structural information of the MPL often neglected by older models is taken into consideration, without adding the burden of large-scale computations usually associated with pore network models and Lattice-Boltzmann methods. The model is applied to study the effects of mixed wettability on ultra-thin catalyst layer fuel cells, where two-phase flow plays a key role.
Effect of Ionomer Loading in Inket Printed Catalyst Coated Membranes on PEFC Performance
Authors: Shukla Shantanu , Taufertshofer Kirstin , Bhattacharjee Subir , Secanell Marc , Halliop Ela , Malevich Dzmitry , Karan Kunal
Published: July 2013

The effect of ionomer loading on ultra-thin polymer electrolyte membrane fuel cell (PEMFC) electrodes fabricated using an inkjet printer is studied under a variety of temperatures and relative humidity conditions. Catalyst layer inks with varying amounts of Nafion were prepared and loaded in inkjet cartridges. The inks were then jetted on a Nafion membrane using an inkjet printer in order to produce fuel cell electrodes with low loadings. The Pt loading for the fabricated electrodes was estimated to be in the range of 0.009–0.01 mg/cm2 per printer pass. SEM imaging shows that the thickness of a catalyst layer fabricated using 10 passes is approximately 3 microns and the catalyst layer agglomerates are about 200nm in radius. In order to determine the optimal Nafion content for this fabrication method, the electrochemical active area and steady state performance for the fabricated catalyst coated membranes (CCMs) was measured and also compared to a conventional CCM fabricated using spray deposition. Results show that optimal performance is achieved with a 40wt% Nafion loading under the operating conditions tested. The improved performance can be attributed to decrease in cell resistance and increase in proton conductivity.
Influence of Air Utilization on Power Generation Properties of a Non-Combined Cycle Pressurized SOFC System
Authors: Hashimoto S. , Liu Y. , Asano K. , Mori M. , Funahashi Y. , Fujishiro Y.
Published: July 2013

In this study, we proposed the non-combined cycle pressurized IT-SOFC system for efficient use of compression hydrogen energy in a gas cylinder, and the possibility of the system was discussed by comparison of the SOFC power gain by pressurization and power consumption gain of an air compressor. Oxygen partial pressure pO2 management on cathode side is also important because pO2 directly influences to the polarization resistances. For estimation of the power densities as functions of air utilization Uair and pressurized air remaining factor χ(=(pO2,cathode−0.021Uair)/(0.21(pair−0.1))), the ac impedance data of a micro tubular single cell, which was developed in ceramic reactor project, was measured under the several pO2 conditions on cathode side at atmospheric pressure (0.1MPa) and 0.3MPa. Area specific activation polarization resistance Rpol.act and concentration polarization resistance Rpol.conc. were obtained from the the ac impedance data, and the power generation curves of SOFC bundle under the several Uair and χ conditions were simulated using the Rpol.act and Rpol.conc. These simulations suggested that if χ is high enough, i.e. over 0.5, pressurization could dramatically reduce Rpol.act and Rpol.conc. even at Uair = 0.95. Based on these simulated power generation curves of the SOFC bundle, the preconditions of SOFC system were expanded up to 10kW class for practical comparison between SOFC power and power loss by the air compressor. This comparison is suggested that the maximum actual power Wact. of pressurized SOFC at 0.3MPa can be higher than that of atmospheric pressure SOFC if χ is over c.a. 0.5. The χ and Uair must be kept high enough for the high performance operation. It is found out that effective exhaust of low pO2 gas from system is important for holding the high χ.
Seal Leakage Effects on the Electrical Performance of an SOFC Button Cell
Authors: DiGiuseppe Gianfranco
Published: July 2013

This paper reports a new study where leakages through a seal in an SOFC button cell is investigated in details. A 2D model is used to determine different flow characteristics of the leakages and its effects on the cell performance. The leakage effects on the performance are studied at different seal porosities and other parameters are also investigated. The results of this investigation indicate that a seal leakage can affect the cell performance significantly if the leak comes from outside the test rig. That is, an air leak must be present through the seal and must flow into the anode chamber where then it affects negatively the cell performance.
Lattice Boltzmann Modeling of the Effective Thermal Conductivity of an Anisotropic Gas Diffusion Layer
Authors: Yablecki J. , Bazylak A.
Published: July 2013

In the present work, the anisotropic and heterogeneous effective thermal conductivity of the gas diffusion layer (GDL) is determined using a thermal lattice Boltzmann model. A three-dimensional fifteen speed (D3Q15), two-phase (fibre and air) model with single relaxation time is presented to determine the effective thermal conductivity in the through-plane and in-plane directions. The model solves the energy transport equation through a reconstructed fibrous GDL geometry. The geometry of the GDL was reconstructed using microscale computed tomography imaging of commercially available GDL materials. Simulated thermal conductivities determined in this work can provide insight into the effect of heterogeneity and anisotropy of the GDL on the thermal management required for improved PEM fuel cell performance.
Crack Formation on Membrane Electrode Assembly (MEA) Under Static and Cyclic Loadings
Authors: Kai Yusuke , Kitayama Yuki , Omiya Masaki , Uchiyama Tomoaki , Kato Manabu
Published: July 2013

Mechanical reliabilities of membrane electrode assemblies (MEA) in polymer electrolyte fuel cells (PEFCs) are a major concern to fuel cell vehicles. Hygro-thermal cyclic conditions induce the mechanical stress in MEA and cracks are formed under operating conditions. The purpose of this paper is to understand the failure mechanism of MEA under several mechanical and environmental conditions for the critical design of durable PEFCs. We carried out both static tensile tests and low cycle fatigue tests for MEA. For tensile tests, temperature and humidity in a test chamber were controlled and the surface crack formation of MEA was observed by a video micro scope in situ. Low cycle fatigue tests were carried out in room condition and the number of cycles to crack formation was measured. The results showed that the mechanical properties of MEA were influenced by temperature and humidity. The observations of MEA during tensile tests revealed that cracks were formed on the surface of catalyst layers just after the yielding of MEA. This result indicates that the deformation mismatch between catalyst layer and proton exchange membrane (PEM) is an important parameter to suppress the crack formation in MEA. The results of low cycle fatigue tests revealed that the fatigue strength of MEA followed the Coffin-Manson law and the fatigue design of MEA based on the Coffin-Manson law is possible. This result is valuable for the critical design of durable PEFCs.
Mechanical Damage Propagation in Polymer Electrolyte Membrane Fuel Cells
Authors: Banan Roshanak , Zu Jean W. , Bazylak Aimy
Published: July 2013

While significant advances in polymer electrolyte membrane fuel cell (PEMFC) technology have taken place in the past decade, challenges still remain in the area of mechanical degradations [1]. Mechanical degradations and damage in PEMFCs including cracks and failure in the membrane electrode assembly (MEA) [2–5] can lead directly to fuel crossover, performance degradation, and reduced durability. It is therefore critical to identify and control the mechanisms that can contribute to damage initiation and propagation in the PEMFC. In this work, we investigate the damage propagation in the MEA, with a special focus on the gas diffusion layer (GDL)/catalyst layer (CL) and the membrane/CL interfaces. A numerical cohesive constitutive model is developed to explore the effect of geometry, the material properties of each component, and the location of the delamination on the propagation behaviour of through-plane delaminations.
Evaluation of the Activity of Metal-Oxides as Anode Catalysts in Direct Methanol Fuel Cell
Authors: El-Dera Sandra Erfan , Abd El Aziz Ahmed , Abd El Moneim Ahmed
Published: July 2013

In the present work, pure iridium oxide (IrO2), and ternary catalysts (IrSnSb-Oxides and RuIrTi-Oxides) are investigated to be used as anode electrocatalysts in The Direct Methanol Fuel Cells (DMFC). Investigations of Methanol Oxidation and Hydrogen Evolution over the catalysts are measured in sulphuric acid as a supportive electrolyte using cyclic voltammetry technique at room temperature (25°C). A specific comparison between the electrocatalytic activities of IrSnSb-Oxides and RuIrTi-Oxides systems is conducted. A comprehensive examination of IrSnSb-Oxides and RuIrTi-Oxides catalysts containing different fractions of the alloying elements are performed to study the effect of varying Iridium Ir content (%) in IrSnSb-Oxides and Ruthenium Ru content (%) in RuIrTi-Oxides on the catalytic activity of ternary catalysts and on the performance of DMFC. It is observed that the electrocatalytic performance of ternary oxides catalysts is strongly dependent on the Ir and Ru content. The generated IrO2 and 33.36% Ru – 1%Ir – 65.64%Ti – Oxides catalysts prove high stability for oxidation of methanol and more proficient electrochemical activity as an anodic electrocatalyst in DMFC at 25°C. The electrochemical measurements of the Hydrogen Evolution Reaction (HER) for metal oxides show that 46.65%Ir – 40.78%Sn – 12.57%Sb sample and 18.75%Ru – 9.35%Ir – 71.9%Ti sample are the superior hydrogen evolution catalysts at 25°C.
Enclosure/Open Cathode Fuel Cell Mathematical Model for Analyzing System Performance Under Arctic Conditions
Authors: Miller Samantha M. , Secanell Marc
Published: July 2013

Polymer electrolyte fuel cells (PEFC) provide the option of a remote power source with high efficiency and minimal green-house gases, NOx, SOx and particulate matter. To protect the PEFC stack from the environment in which remote power sources are required, an actively controlled enclosure to provide optimal temperature and relative humidity to the open-air cathode PEFC stack is studied. A mathematical model of a transient, non-isothermal, lumped parameter, open-cathode fuel cell stack is developed and coupled with an enclosure model. The open-cathode fuel cell stack mathematical model includes characterization of the cathode channel, the anode channel and the membrane electrode assembly (MEA). The transient mass and energy transport equations for the coupled system are solved to determine the optimal operating conditions for the PEFC stack within the enclosure.
Nonlinear Analysis of Two-Phase Flow in Polymer Electrolyte Fuel Cell Cathode Microchannels
Authors: Burkholder Michael , Siefert Nicholas , Litster Shawn
Published: July 2013

In this work, we apply a nonlinear chaos analysis to the two phase flow in polymer electrolyte fuel cell cathode air-delivery microchannels. The fuel cell voltage signal is analyzed using techniques designed to estimate invariants typical of deterministic systems with high sensitivity to initial conditions, such as chaotic two phase flow. Voltage data are taken under varying fuel cell operating conditions, and noise in the data is reduced using a nonlinear noise reduction algorithm. The chaotic strange attractor of the system is reconstructed in phase space using time-delay embedding. Correlation sums over the strange attractor are calculated to estimate the fractal correlation dimension of the system. Estimations of the Kolmogorov entropy provide an additional measure of the complexity of the strange attractor. The values of the chaotic invariants are compared across varying degrees of cathode flooding to discern how they change with two phase flow regimes and fuel cell operating conditions. Future work will involve leveraging the chaotic understanding of two phase flow with chaos control methods to increase the power stability.
Modeling and Performance Evaluation of a Large-Scale SOFC-Based Combined Heat and Power System for Biogas Utilization at a Wastewater Treatment Facility
Authors: Trendewicz A. A. , Braun R. J.
Published: July 2013

Biogas has been identified as an attractive fuel for solid oxide fuel cells (SOFCs) due to its high methane content and its renewable status. Current experimental and modeling research efforts in this field have focused mainly on single-cell and small-scale systems performance evaluation. In this paper a large scale biogas source (∼15.5 MW) from a large wastewater treatment facility is considered for integration with an SOFC-based system. Data concerning biogas fuel flow rate and composition have been acquired from a wastewater reclamation facility in Denver and are used as inputs to a steady-state model of an SOFC combined heat and power (CHP) system developed with Aspen Plus. The proposed system concept for this application comprises an advanced SOFC system with anode gas recirculation (AGR) equipped with biogas clean-up and a waste heat recovery system. The system performance is evaluated at near atmospheric pressure with a 725°C nominal operating temperature of the fuel cell stack and system fuel utilization of 80%. The average biogas fuel input has a composition of 60% CH4, 39% CO2, and 1% N2 on a dry molar basis. The SOFC-CHP system employs 80% internal reforming at a steam-to-carbon ratio of 1.2. The system offers a net electrical efficiency of 51.6% LHV and a net CHP efficiency of 87.5% LHV. The influence of the operating parameters on the system efficiency is investigated and discussed. The individual contribution of system components to the total inefficiency of the system is quantified with an exergy analysis. Exergy analysis results indicate that the system could offer a tremendous energy efficiency improvement when compared to biogas-supplied combustion turbines currently installed at the facility which operate with an average net electrical efficiency of 25%-LHV.
Effect of Agglomerate Microstructure on Oxygen Reduction in Catalyst Layers of Polymer Electrolyte Fuel Cells
Authors: Sadeghi Ehsan , Putz Andreas , Eikerling Michael
Published: July 2013

Cathode catalyst layers (CCLs) contribute to a major proportion, 30%–40%, of voltage losses in Polymer Electrolyte fuel cells (PEFC). The objectives of the present study are to investigate how the catalyst layer performance depends on electrostatic interaction between reacting protons and the charged metal phase and how these relations are affected by composition and microstructure of agglomerates. A model is developed to study oxygen reduction in catalyst layers based on a novel agglomerate microstructure with conical pores. The model consists of coupled relations for reactant transport, metal charging behaviour, and interfacial charge transfer kinetics, evaluated under steady state conditions. Results show an enhancement in the effectiveness factor of conical pores compared to cylindrical pores. Results of the model are evaluated by comparison with existing experimental data. The effectiveness factor calculated for catalyst layer is 3% which is comparable to existing experimental data.
Pore Network Modeling to Study the Effects of Common Assumptions in GDL Liquid Water Invasion Studies
Authors: Hinebaugh J. , Bazylak A.
Published: July 2013

Topologically equivalent pore network modelling of polymer electrolyte membrane (PEM) fuel cell gas diffusion layer (GDL) materials was employed to simulate ex-situ liquid water invasion experiments commonly conducted to investigate multiphase transport in PEM fuel cells. Stochastic, three dimensional pore spaces are numerically reconstructed to mimic carbon fibre paper based GDL materials. A watershed based method was used for extracting pore networks from a range of sample sizes. Invasion percolation was employed to simulate liquid water originating at one surface of the material and percolating through to the opposite surface. Inlet reservoir areas were chosen to mimic those used in ex-situ experiments in literature. It was found that sample size has a strong overall effect on saturation levels and inlet conditions primarily affected saturation near the inlet.
Experimental Investigation of Nanofluid Heat Transfer in a Plate Heat Exchanger
Authors: Taws Matthew , Nguyen Cong Tam , Galanis Nicolas , Gherasim Iulian
Published: July 2013

An experimental study was carried out to determine the forced convective heat transfer and hydraulic characteristics of a chevron-type two-channel industrial PHE when used with a nanofluid. The PHE is composed of two fluid passages formed by three corrugated plates, which have a herringbone pattern and the corrugations are of a trapezoidal shape. Heated water is used on the hot side. On the cold side a mixture of 29nm-diameter CuO nanoparticles in suspension in water is forced. Collected data for the nanofluid side covers two particle volume fractions, 2% and 4.65%, and the range of Reynolds number up to 1000. Results have shown that, for a given Reynolds number, CuO-water nanofluid clearly exhibits a higher friction factor compared to that of water. Calculated Nusselt numbers have shown no significant heat transfer enhancement when using the 2% nanofluid. A decrease of heat transfer was even observed with the 4.65% nanofluid. The laminar-turbulent transition was also observed for the nanofluids studied.
GPU-Based Molecular Dynamics Formulation of Thermal Conductivity Predictions for Monolayer Graphene
Authors: Wemhoff Aaron P. , Khadem Masoud H.
Published: July 2013

Equilibrium molecular dynamics (EMD) simulations have been applied on a graphical processing unit (GPU)-based computing platform for predicting the thermal conductivity of monolayer graphene. The GPU implementation features calculations of force interactions and heat autocorrelation functions (HACF) using the optimized Tersoff potential. The error associated with GPU-based single-point precision calculations is approximately 1% compared to serial CPU-based double-precision calculations. The calculated thermal conductivity decreases with graphene domain size, while the speedup of the GPU-based computations increases with graphene domain size. These results show that the parallel efficiency of GPU-based calculations for a many-body potential with EMD is low due to the requirement of storing individual potential energy and HACF contributions. The predictions from this study are comparable to those from other studies.
Highly Subcooled Flow Boiling: A Model for Estimating Heat Transfer Behind Sliding Bubbles in a Narrow Channel
Authors: Ozer Arif B. , Hollingsworth Donald K. , Witte Larry. C.
Published: July 2013

A quenching/diffusion analytical model has been developed for predicting the wall temperature and wall heat flux behind bubbles sliding in a confined narrow channel. The model is based on the concept of a well-mixed liquid region that enhances the heat transfer near the heated wall behind the bubble. Heat transfer in the liquid is treated as a one-dimensional transient conduction process until the flow field recovers back to its undisturbed level prior to bubble passage.

The model is compared to experimental heat transfer results obtained in a high-aspect-ratio (1.2×23mm) rectangular, horizontal channel with one wide wall forming a uniform-heat-generation boundary and the other designed for optical access to the flow field. The working fluid was Novec™ 649. A thermochromic liquid crystal coating was applied to the outside of the uniform-heat-generation boundary, so that wall temperature variations could be obtained and heat transfer coefficients and Nusselt numbers could be obtained. The experiments were focused on high inlet subcooling, typically 15–50°C.

The model is able to capture the elevated heat transfer rates measured in the channel without the need to consider nucleate boiling from the surface or microlayer evaporation from the sliding bubbles. Surface temperatures and wall heat fluxes were estimated for 17 different experimental conditions using the proposed model. Results agreed with the measured values within ±15% accuracy. The insight gathered from comparing the results of the proposed model to experimental results provides the basis for a better understanding of the physics of subcooled bubbly flow in narrow channels.
Experimental Characterization of the Thermal Diffusivity of Paraffin Phase Change Material Embedded With Herringbone Style Graphite Nanofibers
Authors: Warzoha Ronald J. , Rao Anthony , Weigand Rebecca , Fleischer Amy S.
Published: July 2013

Phase change materials (PCMs) are promising candidates for thermal energy storage due to their intrinsically high values of latent heat. However, PCMs are unable to effectively utilize all of their energy storage capacities due to their poor thermophysical properties. In this study, the effect of graphite nanofibers (diameter = 2 to 1000 nm, length = 100μm) on the bulk thermal properties of paraffin PCM (Tmelt = 56 °C) is investigated. Material properties including effective thermal conductivity, specific heat, latent heat, melt temperature and thermal diffusivity are measured using a Differential Scanning Calorimeter (DSC) and comparative reference bar apparatus. Results suggest that the addition of nanostructures not only increases thermal conductivity by up to 180%, but also reduces the specific heat capacity and density of nano-enhanced paraffin, leading to improved thermal diffusivity and thus greater utilization of its latent heat for transient thermal energy storage.
Experimental Study of the Startup Performance of Ferrofluidic Open Loop Pulsating Heat Pipes
Authors: Mohammadi Maziar , Taslimifar Mehdi , Kazemzadeh Hannani Siamak , Saidi Mohammad Hassan , Afshin Hossein , Shafii Mohammad Behshad
Published: July 2013

Pulsating Heat Pipes (PHPs) are new and promising heat transfer devices. To implement the novel idea to vary the startup performance of a PHP using ferrofluid with and without the application of magnetic field, an experimental investigation is conducted. The effects of several important parameters including working fluid, charging ratio, heat input, ferrofluid concentration, internal pressure, and application of magnetic field on the startup performance of Open Loop Pulsating Heat Pipes (Open Loop PHPs) have been considered and described in detail. Obtained results show that using ferrofluid instead of distilled water can improve the startup performance of PHPs in certain conditions. Furthermore, application of magnetic field on the water based ferrofluid helps PHPs to startup at lower temperatures. High concentration of ferrofluids (5 % w/v) degrades startup performance of PHPs. This study helps to design electronic cooling devices to be more efficient.
Genetic Algorithm Optimization of a Compact Heat Exchanger Modeled Using Volume Averaging Theory
Authors: Geb David , Zhou Feng , DeMoulin George , Catton Ivan
Published: July 2013

This paper proposes and implements a new methodology for optimizing Compact Heat Exchangers (CHXs) using a Volume Averaging Theory (VAT) model and a Genetic Algorithm (GA) optimizer. This method allows for multiple-parameter optimization of CHXs by design of their basic morphological structures, and is applied to a Finned-Tube Heat Exchanger (FTHX). A consistent model is used to describe transport phenomena in a FTHX based on VAT, which allows for the volume averaged conservation of mass, momentum, and energy equations to be solved point by point, with the morphology of the structure directly incorporated into the field equations. The equations differ from known equations and are developed using a rigorous averaging technique, hierarchical modeling methodology, and fully turbulent models with Reynolds stresses and fluxes in the space of every pore. These averaged equations have additional integral and differential terms that must be dealt with in order for the equation set to be closed, and recent work has provided this closure. The resulting governing equation set is relatively simple and is discretized and solved using the finite difference method. Such a computational algorithm is fast running, but still able to present a detailed picture of the temperature fields in both of the fluid flows as well as in the solid structure of the heat exchanger. A GA is integrated with the VAT-based solver to carry out the FTHX optimization, which is a ten parameter problem, and the FTHX’s effectiveness is selected as the fitness function to be optimized. This method of using the VAT-based solver fully integrated with a GA optimizer results in an all-in-one tool for performing multiple-parameter constrained optimization on FTHXs.
Cavitation-Enhanced Microchannel Heat Exchanger Demonstration and Heat Transfer Correlation Development Using R-134a
Authors: Sole Joshua D. , Shelofsky Bradley J. , Scaringe Robert P. , Cole Gregory S.
Published: July 2013

Electronics of all types, particularly those in the military aviation arena, are decreasing in size while at the same time increasing in power. As a result, newer high-heat-flux electronic components are exceeding the cooling capabilities of conventional single-phase military aviation coldplates and coolants. It is for this reason that we have been investigating new methods to cool the next generation of high-heat-flux military aviation electronics. In this work, a novel method of inducing two-phase conditions within a microchannel heat exchanger has been developed and demonstrated. Micro-orifices placed upstream of each microchannel in a microchannel heat exchanger not only cause an improvement in flow distribution, but can also induce cavitation in the incoming subcooled refrigerant and result in favorable two-phase flow regimes for enhanced heat transfer. In this study, R-134a is used as the coolant in the cavitation enhanced microchannel heat exchanger (CEMC-HX) which has been integrated into a vapor compression refrigeration system. Multiple micro-orifice geometries combined with a fixed microchannel geometry (nominally 250 μm × 250 μm) were investigated over a range of applied base heat fluxes (10–100 W/cm2) and mass fluxes (500–1000 kg/m2-s). Two-phase heat transfer coefficients exceeding 100,000 W/m2-K at refrigerant qualities of less than 5% have been demonstrated due to the achievement of preferential, cavitation-induced, flow regimes such as annular flow. To the author’s knowledge, this is the highest heat transfer coefficient ever reported in the literature for R-134a. Additionally, a four term two-phase heat transfer correlation was developed that achieved a mean absolute error (MAE) of 25.5%.
Condensation Induced Shock in Thermal/Fluid Systems
Authors: Jokar Amir , Christiansen Erik W.
Published: July 2013

Phase-change through condensation and evaporation processes takes place in many daily-use heating and cooling systems. However, if these processes experience uncontrolled pressure and temperature conditions, shock waves may be generated. Depending on the strength and conditions of the generated waves, they can decrease the effective life of equipment or even cause devastating destruction. Cavitation and water hammer phenomena are examples of flow issues that are well understood and reported in thermal/fluid systems, although not many studies have been published in open literature on condensation-induced shock (CIS) with the applications in refrigeration and air conditioning systems. This study reviews the limited literature on this topic, analyze the complex CIS phenomenon, derives and solves the governing equations, presents the results of typical conditions that may happen in different residential and industrial applications, and provides ways to avoid devastating incidents that may occur as a result of this phenomena.
Particle Scale Heat Transfer Analysis in Rotary Kiln
Authors: Amritkar Amit Ravindra , Tafti Danesh , Deb Surya
Published: July 2013

Rotary furnaces have multiple applications including calcination, pyrolysis, carburization, drying, etc. Heat transfer through granular media in rotary kilns is a complex phenomenon and plays an important role in the thermal efficiency of rotary furnaces. Thorough mixing of particles in a rotary kiln determines the bed temperature uniformity. Hence it is essential to understand the particle scale heat transfer modes through which the granular media temperature changes. In this study, numerical simulations are performed using coupled Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) to analyze heat transfer in a non-reacting rotary kiln. The microscopic models of particle-particle, particle-fluid, particle-surface and fluid-surface heat transfer are used in the analysis. The heat transfer simulations are validated against experimental data. The effect of particle cascading on the bed temperature is measured and contributions from various modes of particle scale heat transfer mechanisms are reported. Particles are heated near the rotary kiln walls by convection heat transfer as they pass through the thermal boundary layer of the heated fluid. These particles are transported to the center of the kiln where they transfer heat to the cooler particles in the core of the kiln and back to the cooler fluid at the center of the kiln. It is found that 90% of the heat transferred to particles from the kiln walls is a result of convection heat transfer, whereas only 10% of the total heat transfer is due to conduction from the kiln walls.
Large Eddy Simulation of Turbulent Mass Transfer in Ribbed Pipe Flow
Authors: Kang Changwoo , Yang Kyung-Soo
Published: July 2013

Understanding heat/mass transfer mechanisms in turbulent flow is essential for solving many engineering problems. From the view point of heat/mass transfer control, surface roughness is popular for enhancing heat/mass transfer in turbulent pipe flow, relevant to heat exchangers, mixers, and nuclear reactors to name a few. Such a surface roughness is often modeled with a rib. In the current investigation, the flow and concentration fields from the LES data base were analyzed in detail. The profiles of mean velocity components, mean concentration, root-mean-squares (rms) of concentration fluctuations are presented at the selected streamwise locations downstream of a rib. In comparison with the straight-pipe case at the same Re and Sc, the effects of the ribs are clearly identified, leading to overall enhancement of turbulent mass transfer. The budget of concentration variance was computed and presented in the form of contours. Our LES results shed light on a complete understanding of the mass-transfer mechanisms in turbulent ribbed-pipe flow which has numerous applications in engineering.
A Numerical Study of Natural Convective Flow Through a Vertical Symmetrically Heated Channel With an Unheated Upper Section
Authors: Oosthuizen Patrick H.
Published: July 2013

Natural convective flow through a vertical plane channel has been considered. The walls of the lower portion of the channel are heated to a uniform temperature, both of the heated walls being at the same temperature. The walls of the upper part of the channel are unheated, i.e., are adiabatic. The reason for undertaking the study arose from the fact that using an adiabatic upper wall section can, through the so-called chimney effect, increase the flow rate through the channel and therefore increase the heat transfer rate from the lower heated wall section. However if the upper adiabatic wall section is too long the increased pressure drop due to viscous effects can lead to a reduced flow rate through the channel and to a reduced heat transfer rate. Therefore a need existed to examine in more detail the effect that the height of the upper adiabatic wall section has on the heat transfer rate. The flow has been assumed to be steady and the Boussinesq approximation has been adopted. The solutions have been obtained using the commercial CFD code FLUENT©. The solution has the Rayleigh number, the Prandtl number, the ratio of the channel width to the height of the heated channel wall section, and the ratio of the height of the adiabatic channel wall section to the height of the heated wall section as parameters. Results have only been obtained for a Prandtl number of 0.74 (the value for air at temperatures near ambient temperature). Results have been obtained for a wide range of values of the remaining parameters and the effects of these parameters on the mean Nusselt number have been studied.
Lattice Boltzmann Simulation of Diesel Injection
Authors: Falcucci Giacomo , Jannelli Elio , Ubertini Stefano , Bella Gino , De Maio Alessandro , Palpacelli Silvia
Published: July 2013

The evaluation and improvement of internal combustion engine performance is a challenge of major importance and it has been the object of research efforts over the last centuries. Phenomena like fuel cavitation inside diesel injector nozzles, the formation of spray and its break-up in the combustion chamber as well as the impingement of fuel droplets on engine walls are known to have a great influence on energy release during combustion and on pollutant formation and emissions, as well.

In this work, a methodology based on the Lattice Boltzmann Method (LBM) is used to directly simulate phenomena affecting diesel injection. LBM is a numerical method based on Boltzmann’s Kinetic Equation, which has been successfully employed in recent years for the simulation of phenomena of technical interest.

The results of LB simulations are displayed and compared to experimental data from literature, proving the accuracy of the proposed investigation method.
Comparing One-Dimensional and Two-Dimensional Measurement in Flash Diffusivity Experiments
Authors: McMasters Robert L. , Dinwiddie Ralph B.
Published: July 2013

A well-established method for determining the thermal diffusivity of materials is the laser flash method. The work presented here compares two analysis methods for flash heating tests on anisotropic carbon bonded carbon fiber (CBCF). This material exhibits a higher conductivity in the direction in which the fibers are oriented than in the direction perpendicular to the fiber orientation.

The two analysis methods being compared in this experiment use different portions of the data in obtaining results. One method utilizes the temperature data from the entire surface of the sample by examining 201 temperature histories simultaneously, with each temperature history originating from an individual pixel within a line across the middle of the sample. The other analysis method utilizes only the temperature history from a single pixel in the center of the sample, similar to the data which is traditionally generated using the classical flash diffusivity method. Both analysis methods include accommodations for modeling the penetration of the laser flash into the porous surface of the CBCF material. Additionally, both models include a parameter which accounts for the non-uniform heating of the sample surface from the flash. Although the sample surface is ostensibly heated uniformly in flash diffusivity experiments, the heating has been found to be somewhat non-uniform, with more energy deposited more heavily in the center of the sample. This affects the analysis results, particularly in tests on anisotropic materials. The results in this work show very little difference between the thermal parameters arising from the two methods. The robustness of the method using the single-pixel temperature history shows that anisotropic thermal diffusivity can be measured using standard flash diffusivity instruments, avoiding the additional complexity associated with a thermal imaging camera.
Concentration Distribution Research of a Special DC Pulverized Coal Burner and Resistance Analysis
Authors: Yang Mo , Guo Chunsun , Zhang Yuwen , Kang Zhangyang
Published: July 2013

In this paper, a direct current (DC) anti-bias burner has been numerically simulated. This kind burner is combination of bias block and elbow bend and its bias block is located behind the bend. Effects of the bias block’s angle and location on coal distribution in the burner export are investigated. Euler-Lagrange approach is employed to study the gas-solid two-phase flow. The gas-phase is simulated using the RNG k-ε model and solid-phase is modeled using the discrete phase model (DPM). The results show that, installing a bias block behind the elbow can achieve a uniform distribution in outlet and the high concentrations region non-adherent. Relative to the location, the bias block angle changes is the main reason of generating resistance. When α = 25° L = 150mm, the distribution of pulverized coal of the burner outlet is the most uniform and the resistance is not large, which could satisfy the good anti-bias effect and moderate resistance loss at the same time.
Numerical Simulation of the Temperature Field and the Flow Field Inside the Domestic Cooling and Heating Unit’s Water Tank
Authors: Zhu Chao , Yang Mo , Zhang Yuwen , Wang Jinlong
Published: July 2013

A water tank of the domestic cooling and heating unit, which has a helix coil, is used to recover the waste heat of the unit. The temperature field and the flow field in the water tank have great effects on the variation of the water temperature in it. In order to obtain the temperature distribution, the flow distribution and the influencing factors, and then obtain the changing situation of the water temperature, the temperature field and the flow field of the water tank are simulated by using Fluent. The results showed that the water temperature will change with different coil decorate. The numerical model which is created by Fluent is appropriate and could be used to improve the layout of the coil in the water tank and speed up heating.
Numerical Study of Heat Transfer in Heat Pipes of Flue Gas Waste Heat Recovery
Authors: Yong Qingqing , Yang Mo , Zhang Yuwen , Lu Mei
Published: July 2013

In this paper, the advantages and precautions of using heat pipes to recycling flue gas waste heat in a power plant are discussed. A mathematical description of the model is established and its temperature and flow fields are simulated numerically. On this basis, effects of changing parameters and working conditions on the simulation results are studied.
A Simple Model for Prediction of Preheating and Pyrolysis Time of a Thermally Thin Charring Particle
Authors: Haseli Y. , van Oijen J. A. , de Goey L. P. H.
Published: July 2013

The aim of this paper is to present a simple model, based on a time and space integral method, for prediction of preheating and conversion time of a charring solid particle exposed to a non-oxidative hot environment. The main assumptions are 1) thermo-physical properties remain constant throughout the process; 2) temperature profile within the particle is assumed to obey a quadratic function with respect to the space coordinate; 3) pyrolysis initiates when the surface temperature reaches a characteristic pyrolysis temperature; 4) decomposition of virgin material occurs at an infinitesimal thin layer dividing the particle into char and virgin material regions; 5) the volume of the particle remains unaltered; 6) volatiles escape through the pores immediately after formation.

Employing assumption (2) allows one to convert the energy conservation equation of the particle, which is basically described in the form of a partial differential equation (PDE), into an ordinary differential equation (ODE) by performing space integration. Next, by applying approximate time integration the ODE is transformed into an algebraic equation. Applying this approach to the preheating and pyrolysis stages of a thermally thin charring solid particle leads to a set of algebraic equations which provides reactor designers with a convenient means for computation of the heating up time, mass loss history and total conversion of particle. The accuracy of the simple model is assessed by comparing its prediction with that of a one-dimensional detailed pyrolysis model. Overall, good agreement is achieved indicating that this new model can be used for engineering and design purposes.
Simulation of Fluid Flow and Heat Transfer Including Phase Change During the Impact of Semi-Molten Particles
Authors: Alavi S. , Passandideh-Fard M. , Mostaghimi J.
Published: July 2013

A numerical method is proposed in this paper which is capable of modeling the impact and deposition of semi-molten particles. By attributing a high viscosity to the solid region, simulating the motion of solid cores including solidification/melting is made possible inside the particle. The Navier-Stokes equations are solved in combination with the Volume-of-Fluid (VOF) technique to track the free surface of the particles. In addition, the heat transfer including phase change is modeled using the enthalpy method. The viscous terms are treated implicitly to relax the restrictions associated with small computational time-steps in explicit formulations. Several case studies with operating conditions of a typical thermal spray process are simulated. These cases include the impact of molten and semi-molten nickel particles in an atmospheric plasma spray process (APS). The effects of various parameters such as particle solid-core diameter, initial velocity and temperature are investigated. The simulations show that the size of the solid core has an important effect on the amount of particle splashing during the impact.
Numerical Study on Supercritical CH4/N2 Cooling in a Horizontal Tube
Authors: Du Zhongxuan , Lin Wensheng , Gu Anzhong
Published: July 2013

Coalbed methane (CBM) is a kind of mixed gas with the principal component of methane and nitrogen. Supercritical convective heat transfer of CH4/N2 cooled in horizontal circular tubes is one of the most important heat transfer processes during CBM liquefaction. In this paper, supercritical CH4/N2 cooling has been numerically investigated in a horizontal tube by using the low Reynolds number turbulence model proposed by Lam and Bremhorst. The study first focuses on the effect of nitrogen content on CBM heat transfer characteristics. The results indicate that supercritical convective heat transfer of CBM is mainly affected by the fact that the CBM properties change with nitrogen content. Then the study focuses on the buoyancy effect on heat transfer characteristics at different mass fluxes, heat fluxes and pressures. The results show that buoyancy effect increases with the decrease of mass flux or with the increase of heat flux, and the relationship Gr/Re2.7 predicts the buoyancy effect onset better than Gr/Re2. When the buoyancy effect is considerably strong, buoyancy effect on heat transfer in the top line of the horizontal circular tube is equivalent to buoyancy-opposed heat transfer, and buoyancy effect on heat transfer in the bottom line to buoyancy-aided heat transfer. The correlation of buoyancy-opposed heat transfer proposed by Bruch et al. predicts well for the supercritical heat transfer of methane. When the buoyancy effect is negligible, the calculated results agree well with the Gnielinski correlation.