bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Marangoni Convection on Solidification of Phase Change Materials Infiltrated in Porous Media in Presence of Voids
Authors: Moeini Sedeh Mahmoud , Khodadadi J. M.
Published: December 2013

Void formation is encountered in the form of air pockets during preparation of composite thermal energy storage systems, consisting of phase change materials (PCM) infiltrated into a high-conductivity porous structure. The presence of voids within the pores of a porous structure degrades the thermal and phase change behavior of such composites. Recent work devoted to multiphase modeling of the infiltration of PCM in liquid state into porous media and formation of voids showed that among the various contributing driving forces (i.e. gravity, pressure gradient and interfacial forces), the interfacial forces (resulting from surface tension and contact angle) play a significant role at the pore level. Additionally, modeling the solidification and melting of PCM within the pores in presence of a void revealed that there is a temperature gradient along the interface between the PCM and void. Considering the surface tension as the major driving force at the pore level, this temperature gradient is large enough to give rise to a gradient in surface tension that then triggers the Marangoni convection at the interface. Thus, as a convection mechanism, it affects the phase change process as well as the interface shape. Therefore, in this paper, the effects of the Marangoni convection on PCM solidification time and shape of the interface was investigated at the pore level. A numerical approach was employed for solidification of a PCM based on the combination of the Volume-of-fluid (VOF) and enthalpy-porosity methods, including the variation of the surface tension with temperature, i.e. Marangoni effects. A two-dimensional model of a pore was developed based on the average geometric features of the pores in a porous structure with interconnecting pores. Following the grid independence study, the transient simulation of solidification was performed, whereas the PCM within the pore and the air within the void were treated as incompressible liquid and compressible gas, respectively. The liquid density change during the solidification was included to explicate the formation of shrinkage void and its distribution within the pores. The PCM solidification time and shape of the final interface between the PCM and air pocket (representing the amount and distribution of the shrinkage void evolving during the solidification) were extracted and compared between the cases with and without Marangoni convection. For verification purposes, the volume of the predicted infiltration void is in agreement with experimental measurements and the volume of the shrinkage void shows a good agreement with theoretical volume change. The final shape of the interface was justified and turned out to be in agreement with the prevailing Marangoni convection pattern.
Microstructure-Dependent Heat Transfer Modeling of Carbon Nanotube Arrays for Thermal Interface Applications
Authors: Sadasivam Sridhar , Hodson Stephen L. , Fisher Timothy S.
Published: December 2013

A growing interest has developed in the use of carbon nanotube (CNT) arrays as thermal interface materials (TIMs). However, theoretical modeling of CNT TIMs has largely been limited to semi-empirical methods without detailed consideration of array microstructure, primarily due to the inherent randomness of the microstructure and the computational complexity involved in full atomistic modeling of CNTs. In this work, we report combined thermo-mechanical simulation of CNT arrays with a coarse-grain approach for the mechanics modeling and a thermal network approach for the heat transfer modeling. Parametric studies on the effects of CNT height on the Young’s modulus and buckling load of CNT arrays are reported. The thermal network model is used to estimate the pressure dependence of diffusive and tip contact resistances of CNT arrays; the predictions are compared with thermal resistance measurements using the photoacoustic method. The resulting simulation framework enables a particularly rich and broad thermo-mechanical data set. Selected parametric variations are computed to assess the stress-strain behavior, effective conductivity within the CNT array, and aspects of the contact topologies of the CNT-substrate interface.
Experimental Characterization of an Aluminum-Based Minichannel Solar Water Heater
Authors: Diaz Gerardo
Published: December 2013

Water heating accounts for a significant fraction of the energy consumption in the residential and commercial sectors. A wide variety of water heater configurations are available some of which have higher operating costs than others. The use of solar water heaters is becoming more widespread in many countries usually due to the low operating costs associated with the technology. This paper describes the first implementation of an aluminum-base minichannel solar water heater currently being tested at the University of California Merced.
A Molecular Dynamics Simulation of Au Nanoparticles Aggregation in Ionic Solution
Authors: Lv Xiaoxing , Yue Kai , Lei Qingchun , Zhang Xinxin
Published: December 2013

Due to unique and tunable optical properties, gold nanoparticles (GNPs) are becoming more widely used in biological and biomedical applications. However, nanoparticles in fluid tend to lose the specific function because of aggregation in the transport process of use. Therefore, it is necessary to investigate the aggregation behavior for having a good understanding of aggregation mechanism and inhibiting GNPs aggregation. A MD simulation in this study was performed to investigate the physical aggregation behavior of GNPs in biological media. By analyzing the aggregation proportion of GNPs in different conditions and the changes in center-to-center distance between GNPs with the time, the effects of the hydrophilic/hydrophobic characteristics of GNPs, velocity of ionic solution, size of GNPs, initial distance between two GNPs, and surface charge were discussed. The simulation results indicate that the aggregation proportion of GNPs with hydrophilic modification is 62.5%, which is less than 87.5% in the model without surface modification, while the final aggregation proportion of GNPs with hydrophobic modification increased to 100%. When the velocity of the NaCl aqueous solution is 0.1 m/s, the final aggregation proportion of GNPs is 87.5%, which is similar with the model without flow velocity. But the final aggregation proportion increased to 100% when the velocity is 1m/s. Under the same conditions, the GNPs of 1 nm diameter aggregated at 0.16 ns, but the GNPs of 1.5 nm and 2 nm diameters aggregated at 0.6 ns and 0.8 ns, respectively. For the GNPs of 1 nm diameter, the GNPs can only get close to each other very slowly when the distance between the surfaces of GNPs is within the range of 0.8–1.2 nm, whereas the GNPs will aggregate quickly when the distance is close enough. GNPs can retain stable by modified with appropriate negative charge. But ions in the solution will weaken this effect.
Optimizing a Functionally Graded Metal-Matrix Heat Sink Through Growth of a Constructal Tree of Convective Fins
Authors: Kephart Jacob , Jones G. F.
Published: December 2013

This work focuses on an analytical approach to understand optimal material utilization in metal matrix heat exchangers. An objective of this work is to develop a bridge between a fully defined and discrete structure to that of a functionally graded porous media. A porous media heat exchanger is a structure which uses porous material, such as a metal foam, to achieve large convective surface areas in a small volume while also using the media as a conductive path from the heat source or sink. Therefore, a functionally graded porous media heat exchanger has a porosity that is specified as a function of position. Constructal theory is used here to develop increasingly complex convective fin structures, optimized at each level of complexity, which have a resulting characteristic of 2-D functional grading. The approach described here is developed from first principles by using Fourier’s law to develop analytical solutions and seeks to yield an optimized heat exchanger configuration that maximizes total heat transfer subject to a fixed amount conductive material, total volume, and flow condition.
Design of a Fluid-Based Micro-Scale Electrocaloric Refrigeration System
Authors: Guo Dongzhi , Gao Jinsheng , Yu Ying-Ju , Santhanam Suresh , Fedder Gary K. , McGaughey Alan J. H. , Yao Shi-Chune , Slippey Andrew
Published: December 2013

The electrocaloric effect (ECE) is a phenomenon in which reversible temperature and entropy changes of a material due to polarization appear under the application and removal of an electric field. Materials with a giant ECE have recently been reported, suggesting practical application in cooling devices. In this paper, a refrigeration system composed of silicon MEMS cooling elements is designed based on the ECE in a terpolymer. Finite element simulations are performed to explore the system performance. The effect of the form of the applied electric field is studied. The time lag between the electric field and the diaphragm motion is found to affect the cooling power significantly. A parametric study of the operating frequency is also conducted. The results indicate that when the system is operated at a temperature difference of 5 K, a cooling power density of 2 W/cm2 is achieved for one element.
Natural Convection in an Asymmetrically-Heated Open-Ended Channel: A Three-Dimensional Computational Study
Authors: Lau Ghar Ek , Timchenko Victoria , Reizes John , Fossa Marco , Yeoh Guan Heng
Published: December 2013

Buoyancy-driven flows in an asymmetrically heated open-ended channel which occur in façade and roof building-integrated photovoltaic systems were investigated using large-eddy simulation. The channel inclination angle was varied from 30° to 90° to the horizontal, whereas the channel height-to-width aspect ratio remained at 20. In each case, a uniform heat flux was applied along the top wall whereas the bottom wall was assumed to be adiabatic. It is shown that typical dynamics of large-scale structures in the flow and thermal fields of natural convection in the channels are successfully modeled numerically by the use of LES. The effects of varying the inclination angle on the heat transfer in the channel are explored by examining the mean flow fields and in addition, the effects of radiation have been considered. Both experimental and numerical results show that open-ended channels with low inclination angles are characterized by a low chimney effect which leads to a decreased flow rate and a delay in transition to turbulence, thereby decreasing the heat transfer coefficient and leading to higher temperatures on the heated wall. A correlation describing the local Nusselt number in the channel is also developed in order to characterize the global heat transfer behavior.
Thermal Contact Conductance of Radiation-Aged Thermal Interface Materials for Space Applications
Authors: Sayer Robert A. , Koehler Timothy P. , Dalton Scott M. , Grasser Thomas W. , Akau Ronald L.
Published: December 2013

Thermal interface materials (TIMs) serve a critical role in thermal management by enhancing heat transfer across contact interfaces. Specifically, they are most commonly used in electronics to enhance the flow of heat from source to sink by decreasing the overall thermal resistance of the system. In space, these materials are exposed to high doses of Gamma radiation due to the lack of an atmosphere to serve as an absorbing medium. With typical design lifetimes of 5 to 10 years, total radiation exposure can be significant and can adversely affect the thermal contact resistance (TCR) of the TIM. In this manuscript, we report the effect of radiation-aging on the TCC of several commercially available electrically insulating, thermally conductive interface materials that are commonly used in satellite systems. Although radiation dose levels can vary significantly during the course of a space mission, a dosing of 10 Mrad per year for TIMs is a reasonable estimate. The TIMs were aged in a Gamma cell at a rate of 250 rad/s to total doses of 50 and 100 Mrad to simulate mission lengths of 5 and 10 years, respectively. The TCR of each radiation-aged sample, as well as un-aged samples, were measured under vacuum (less than 3 × 10−4 Pa). Radiation-aging of the TIMs led to a significant increase in the TCR of the tested samples. For example, the pressure-dependent TCR was shown to increase 20–150% for Cho-Therm 1671 and 50–250% for ThermaCool R10404 samples subjected to 50 Mrad of gamma-ray irradiation. These results show that radiation-aging of TIMs cannot be ignored in the design and simulation of space systems.
Off Set Strip Fins Thermo Hydraulic Performance Analysis Through CFD Simulation
Authors: Polo Guisselle , Bula Antonio
Published: December 2013

A computational simulation of the thermo hydraulic performance of a trapezoidal Off Set Strip Fin (OSF) is performed. The simulations are carried for Reynolds number ranging from 300 to 9000 considering three-dimensional conditions (3D). Calculations were developed assuming a constant flow. Colburn factor j, and the friction factor f, were calculated. Uniform temperature condition is assumed at the solid surface and at the inlet. The results have been compared with studies made by Kays and London, (1984), Manglik and Bergles (1990), and Muzychka and Yovanovich, (2001). According to the obtained results, it can be concluded that Computational Fluid Dynamics (CFD) is able to explore the behavior and performance of this kind of fin, showing that the hydraulic performance is well represented by correlations widely used in literature, while thermal performance seems to be under predicted.
Flow Boiling in a Short Narrow Gap Channel
Authors: Janssen D. D. , Dixon J. M. , Young S. J. , Kulacki F. A.
Published: December 2013

Heat transfer coefficients in sub-cooled flow boiling in symmetrically heated narrow gap channels are reported at power densities of 1 kW/cm3 and greater. A pair of parallel ceramic resistance heaters in a nearly adiabatic housing forms the flow passage with length-to-gap ratios of 16:1 and 34:1. Water, Novec™ 7200 and 7300 are used as the heat transfer fluids at a mass flux of 100 to 1000 kg/m2s. Reynolds numbers range from ∼200 to ∼5600, Weber numbers range from ∼0.75 to ∼173, and boiling numbers from O(10−4) to O(10−2). Flow regimes span single-phase convection to nucleate flow boiling depending on mass flux and inlet sub-cooling, and exit quality can reach 40% in some cases. Results include overall two-phase heat transfer coefficients, wall temperature, exit quality and coefficient of performance. The initiation of flow boiling demonstrates that mean heater temperatures can be maintained below 95 °C over a wide range of power density and up to and exceeding 1 kW/cm3. A super position principle is suggested as an analytical framework to estimate exit quality and heat transfer coefficients. Highly favorable coefficients of performance across the data set indicate that the pumping power penalty within the heated zone is very small. Thus convective boiling in which the mechanism is nucleate boiling appears to hold the greatest potential to increase heat transfer coefficients, especially in small scale, inter-chip cooling strategies.
Experimental Investigation of Flow Boiling Performance of Open Microchannels With Uniform and Tapered Manifolds (OMM)
Authors: Kalani Ankit , Kandlikar Satish G.
Published: December 2013

Boiling can provide orders of magnitude higher cooling performance than a traditional air cooled system especially related to electronics cooling application. It can dissipate large quantities of heat while maintaining a low surface temperature difference. Flow boiling with microchannels has shown a lot of potential due to its high surface area to volume ratio and latent heat removal. Flow instabilities and early critical heat flux have however prevented its successful implementation. A novel flow boiling design is experimentally investigated to overcome the above mentioned disadvantages while presenting a very low pressure drop. The design uses open microchannels with a tapered manifold (OMM) to provide stable and efficient operation. Distilled, degassed water at atmospheric pressure is used as the fluid medium. Effect of tapered block with varied dimension is investigated. Heat transfer coefficient and pressure drop data for uniform and tapered manifolds for plain and microchannel chips are presented. A maximum heat flux of 281.2 W/cm2 at 10.1 °C wall superheat is obtained with microchannel chips using a tapered manifold. The CHF was not reached as the performance exceeded the heater capacity. The maximum pressure drop obtained for the above mentioned configuration was only 3.3 kPa.
Development of Heat Transfer in a Two-Dimensional Wavy Falling Film of Water and its Influence on Wave Stability
Authors: Haustein Herman D. , Rohlfs Wilko , Al-Sibai Faruk , Kneer Reinhold
Published: December 2013

Heat exchangers employing falling films are relevant to a multitude of industrial applications using water-based liquids. In the present study, periodic, two-dimensional waves are imposed by excitation on a vertically falling film of water, which is then heated by a uniform heat flux, within the laminar and transitional flow range (39<Re<200). Liquid-film thickness is measured by confocal chromatic imaging and surface temperature is measured by high-speed IR thermography. As the 2D waves travel downstream they destabilize in the spanwise direction and evolve 3D structures (bumps). Further wave destabilization, under relatively low heating, was observed to coincide with the appearance of local thermal flows (“hot streaks”), though no deformation of the liquid surface could be measured. These flows are understood to be induced by thermo-capillary forces, which in extreme cases are known to lead to the formation of rivulets, film rupture and heater burnout. Understanding these initial stages of thermo-capillary flow is crucial to its suppression.

Analysis of the thermal images reveals several significant streamwise length scales: a thermal inlet length based on the emergence of the thermal boundary layer (Lt), a thermal inlet length based on reaching thermally developed conditions (Lh), and the length at which “hot-streaks” first appear (Ls). In addition the dominant (most unstable) spanwise wavelength of the hot streaks, Lz, was identified through FFT analysis of the thermal profile beyond Ls. First the independence of the thermal inlet lengths from the heat-flux was established. Next, the influence of the nominal flow conditions (Reynolds number and excitation frequency) on Lt, Lh and Lz was examined — thereby extending the range of previous studies to higher Reynolds numbers. The thermal inlet lengths Lt and Lh were found to increase with flow rate, whereas they had opposing trends with regard to frequency. Lz consistently decreased with an increase of the flow rate, as smaller (turbulent) scales became more dominant, and it was found to be indifferent to excitation frequency over a wide range. Some future directions and methods of hot streak suppression are discussed, as well.
Experimental and Computational Evaluation of Flow Characteristics for Advanced Film Cooling Hole Geometries
Authors: Gomez-Ramirez David , Srinivasan Shreyas , Ramesh Sridharan , Miranda Marco , Ekkad Srinath V. , Alvin Mary Anne
Published: December 2013

Film cooling is crucial in the field of gas turbines to protect the blade surfaces from the hot combustion gases. Several hole geometries have been studied in the past in an effort to optimize the cooling effectiveness of the holes while maintaining the structural integrity of the blade and low manufacturing costs. To understand the cooling effectiveness of the various hole geometries, the flow structures that develop as the coolant jet interacts with the hot mainstream must be understood. The present paper compares the results obtained from 2D Particle Image Velocimetry (PIV) measurements with CFD predictions using standard Reynolds-Averaged Navier Stokes (RANS) models with a commercially available code. The study is conducted for flat plate film cooling via conventional cylindrical holes, shaped holes (10° flare/laidback), and a tripod anti-vortex hole (AV) design. A constant blowing ratio (BR) of 0.5 was used for all the experiments, except for an additional measurement for the AV design at a BR of 1.0. Computational fluid dynamic (CFD) calculations were made with a standard k-epsilon model and compared to PIV results. The results show the counter-rotating vortices developing for cylindrical and shaped holes up to 5D and 3D respectively from the hole exit. AV holes showed no vortex formation, further supporting its higher cooling performance. Moreover, the present results indicate no separation of the coolant jet for AV or shaped holes as expected, while cylindrical holes displayed a small separation with a vertical extent of ∼0.1D. The CFD model was able to capture the main structures of the flow, but further efforts will concentrate in improving the representation of the flow normal to the flat plate surface.
A New Device and Technique for Thermal Conductivity Measurements of Glass-Forming Materials With Application to Cryopreservation
Authors: Ehrlich Lili E. , Feig Justin S. G. , Schiffres Scott N. , Malen Jonathan A. , Rabin Yoed
Published: December 2013

The current study is aimed at developing a device and technique to measure the thermal conductivity of materials relevant to cryopreservation — the preservation of biomaterials at very low temperatures. It is well established that ice formation is the cornerstone of low-temperature injury [1]. In an effort to improve the outcome of cryopreservation, ice crystallization can be controlled by the addition of cryoprotective agents (CPAs), such as dimethyl sulfoxide (DMSO). CPA solutions are characterized by exponentially increasing viscosity with the decreasing temperature. If cooled rapidly enough, the crystalline phase can be completely suppressed and the material is trapped in a solid-like state known as vitrification (vitreous in Latin means glassy). While correlating the quality of the cryopreserved product with the thermal history may be straightforward to obtain in small specimens, characterized by close-to-uniform temperature distribution, analysis of larger specimens requires integration of mathematical tools to estimate the spatial temperature distribution at any instant along the cryogenic protocol. The data developed in the current study is aimed at enabling the corresponding thermal analysis, while exploring the variation in thermal conductivity between the crystalline and glassy states.
Design of an Interrupted-Plate Heat Exchanger Used in a Liquid-Piston Compression Chamber for Compressed Air Energy Storage
Authors: Zhang Chao , Shirazi Farzad A. , Yan Bo , Simon Terrence W. , Li Perry Y. , Van de Ven James
Published: December 2013

In the Compressed Air Energy Storage (CAES) approach, air is compressed to high pressure, stored, and expanded to output work when needed. The temperature of air tends to rise during compression, and the rise in the air internal energy is wasted during the later storage period as the compressed air cools back to ambient temperature.

The present study focuses on designing an interrupted-plate heat exchanger used in a liquid-piston compression chamber for CAES. The exchanger features layers of thin plates stacked in an interrupted pattern. Twenty-seven exchangers featuring different combinations of shape parameters are analyzed. The exchangers are modeled as porous media. As such, for each exchanger shape, a Representative Elementary Volume (REV), which represents a unit cell of the exchanger, is developed. The flow through the REV is simulated with periodic velocity and thermal boundary conditions, using the commercial CFD software ANSYS FLUENT. Simulations of the REVs for the various exchangers characterize the various shape parameter effects on values of pressure drop and heat transfer coefficient between solid surfaces and fluid. For an experimental validation of the numerical solution, two different exchanger models made by rapid prototyping, are tested for pressure drop and heat transfer. Good agreement is found between numerical and experimental results. Nusselt number vs. Reynolds number relations are developed on the basis of pore size and on hydraulic diameter.

To analyze performance of exchangers with different shapes, a simplified zero-dimensional thermodynamic model for the compression chamber with the inserted heat exchange elements is developed. This model, valuable for system optimization and control simulations, is a set of ordinary differential equations. They are solved numerically for each exchanger insert shape to determine the geometries of best compression efficiency.
Flow Boiling Dynamics of Water and Nanofluids in a Single Microchannel at Different Heat Fluxes
Authors: Edel Zachary , Mukherjee Abhijit
Published: December 2013

The preferable cooling solution to the problem of thermal management of modern electronics for increasing power dissipation could be micro heat exchangers based on forced flow boiling. Nanoparticle deposition can affect nucleate boiling heat transfer coefficient via alteration of surface thermal conductivity, roughness, capillary wicking, wettability, and nucleation site density. It can also affect heat transfer by changing bubble departure diameter, bubble departure frequency, and the evaporation of the micro and macrolayer beneath the growing bubbles. In this study, flow boiling was investigated using degassed, deionized water, and 0.001 vol% aluminum oxide nanofluids in a single rectangular brass microchannel for one inlet fluid temperature of 63°C, one flow rate of Re = 100, and two heat fluxes of 130 kW/m2 and 300 kW/m2. High speed images were taken periodically for water and after durations of 25, 75, and 125 minutes of nanofluid flow boiling. The change in regime timing revealed the effect of nanoparticle suspension and nanoparticle deposition on the Onset of Nucelate Boiling (ONB) and the Onset of Bubble Elongation (OBE). Single phase flows at the channel outlet were recorded and compared for different durations of nanofluid flow boiling. The addition of nanoparticles was found to stabilize bubble nucleation and growth and increase heat transfer in the thin film regions of the evaporating menisci.
Investigation of Optimal Wavenumber Range and Resolution for Determination of Combustion Gas Temperature and Concentration
Authors: Ren Tao , Reeder Todd A. , Modest Michael F.
Published: December 2013

An inverse radiation algorithm was developed to reconstruct temperature and species concentrations from a homogeneous gas column to determine the optimal wavenumber range and resolution for such reconstruction. By minimizing the error between calculated and measured transmissivity spectra, temperature and species concentrations are deduced as results of the inversion process. The error is minimized while considering multiple factors, including spectral region, spectral resolution, temperature and concentration range, and susceptibility to measurement drift (systematic error) and noise (random error). Results are obtained for homogeneous mixtures containing CO2, H2O or CO with N2. Measured spectra were synthesized through calculations from HITEMP2010, the high-temperature spectral database. These optimal parameters can be further used for guiding experiment and inversion of gas temperature and concentration in nonhomogeneous media.
Experimental Characterization of Viscosity and Thermal Conductivity of Aluminum Oxide Nanofluid
Authors: Tang Clement C. , Tiwari Sanjib , Cox Matthew W.
Published: December 2013

Experiments have been conducted to measure the dynamic viscosity and thermal conductivity of aluminum oxide (Al2O3) nanoparticle dispersions at 6% vol. in liquid water. Rheological characterization of the Al2O3–water nanofluid has shown that it exhibits a Newtonian fluid behavior for the shear rate range of 6 to 122 s−1 between 6 and 75°C. Measurements of the nanofluid thermal conductivity have been performed for temperatures between 7 and 55°C, and have been found to be 8 to 17% higher than that of water. Thermal conductivity of the nanofluid responded to temperature increase with appreciation in value. When compared with correlations available in the literature, the measured thermal conductivities are in better agreement with correlation that includes the effect of temperature.
Application of a Script Language “Cemmath” to Thermal Engineering
Authors: Kim Charn-Jung , Shin Dongwoo
Published: December 2013

A script language “msharpmath” or “cemmath” that can be downloaded from www.msharpmath.com has been successful to handle mathematical equations in engineering and mathematics. It is here extended to solve thermal problems. Although the example problems are limited to very simple geometries, the possibility of growing as a commerical code can be acknowledged.
Energy Performance of Sustainable Roofing Systems
Authors: Sailor David J. , Vuppuluri Prem
Published: December 2013

This study presents efforts to analyze how sustainable roofing technologies can contribute to the energy budget of buildings, and the resulting implications for heating and cooling energy use. The data analyzed in this study were obtained from a field experiment performed on a four story warehouse/office building in Portland, Oregon USA. The building’s roof includes a 216 panel, 45.6 kW solar photovoltaic array in combination with 576 m2 of vegetated green roofing. While most of the surface consists of green roof shaded by photovoltaic panels, the roof also has test patches of dark membrane, white membrane and un-shaded green-roofing. Interior and exterior surface temperatures were monitored over a period of two years and heat flux into the building is estimated using a finite difference conduction model.

On average, the black roof membrane was the only roof that caused a net heat gain into the building in the summer. In the winter, all four roofing technologies resulted in net heat losses out of the building. Both the PV-shaded and un-shaded green-roofs indicated a net heat loss out of the interior of the building during both the summer and winter. This latter effect is largely a result of green-roof evaporative cooling — which can benefit air conditioning demand in summer but may be undesirable during heating-dominated seasons.
Lattice Boltzmann Simulation of High-Diffusivity Problems With Application to Energy Transport in a High-Temperature Solar Thermochemical Reactor
Authors: Li Like , Singh Abhishek , AuYeung Nicholas , Mei Renwei , Petrasch Jörg , Klausner James F.
Published: December 2013

We describe a general technique for rescaling the high-diffusivity convection diffusion equation (CDE) when it is simulated with the lattice Boltzmann equation (LBE) method. The macroscopic CDE is recovered from the kinetic-based LBE when the mean free path of the particles is much smaller than the lattice grid size. As the relaxation time and the mean free path are proportional to the diffusion coefficient of the CDE in LBE models, direct use of a large diffusion coefficient would lead to large numerical errors in LBE simulations. To improve accuracy, we rescale the CDE by choosing a large time scale and a moderate relaxation coefficient so that the characteristic Fourier number for the diffusion process remains the same. The rescaled LBE model is first validated with two numerical tests for which analytical solutions are available: the transient heat conduction in a semi-infinite solid and that inside a circle. The comparison between the LBE results and analytical solutions shows that the numerical errors are greatly reduced when the high diffusion coefficient is rescaled down. It is then applied in a diffusion-radiation coupled model to simulate the energy transport in a high-temperature solar thermochemical reactor for hydrogen production. Rescaling of the solar flux boundary conditions and the chemical reaction source terms due to the rescaling of the diffusion coefficient is also discussed and the simulation results will be used to optimize the cavity-reactor design.
Implementation of the PN-Approximation for Radiative Heat Transfer on OpenFOAM
Authors: Marquez Ricardo , Modest Michael
Published: December 2013

This work presents an OpenFOAM implementation of the PN approximation for radiative heat transfer, including higher orders P3, P5, and P7. Also described is a procedure which enables the sequential numerical computations of the coupled partial differential equations (PDEs) by re-expressing the boundary conditions in matrix form so that individual boundary conditions can be associated with each PDE. The implementation of the software programs are verified with derived analytical solutions for 1-D slabs with constant and variable properties, and are also tested with various orientations in order to demonstrate the geometric invariance properties of the 3-dimensional PN formulation. A few examples taken from the literature are also considered in this work and could be taken as benchmark solutions for the PN approximations.
Discharge of Carbon Dioxide Using a Non-Thermal Plasma Reactor
Authors: Taylan Onur , Berberoglu Halil
Published: December 2013

This paper reports a numerical study on the discharge of carbon dioxide using a non-thermal dielectric barrier discharge (DBD) plasma reactor at ambient conditions. DBD plasma reactors have been used for various applications due to their ease of production, process control, operation at different conditions. The applications of DBD plasma reactors include discharge of gases. Carbon dioxide is a greenhouse gas formed as a byproduct of fossil fuel combustion. Use of DBD non-thermal plasma reactor can be a promising technology for carbon dioxide mitigation due to its operation at low temperatures, lack of need for catalysts, and flexibility in controlling the products generated. In this study, a tubular DBD non-thermal plasma reactor was modeled with different electrode materials separated by different dielectric materials. The aim was to provide guidelines for the design and material selection for optimizing DBD plasma reactors for CO2 discharge. A parametric set of simulations was performed using a finite element solver to investigate how electrode and dielectric materials affect the discharge volume of CO2 and power requirement of the non-thermal plasma discharge of CO2. The results showed that electrode material did not affect the discharge or the power requirement. However, dielectric material with higher permittivity or lower conductivity increased the gas discharge and power requirement. Among the analyzed materials, aluminum electrode and mica tube were suggested based on the simulation results for the maximum gas discharge and low power requirement.
Fundamental Basis and Application of Cold-Room Project Design: A Case Study of Frigoship
Authors: Öztürk Merve , Göktepe Emine , Celen Ali , Çebi Alican , Dalkılıç Ahmet Selim , Wongwises Somchai
Published: December 2013

In this paper, a cold-room system in a frigoship was analyzed as a case study to demonstrate the application of the proposed model. The model was also evaluated in terms of the alteration of alternative refrigerants and insulation thickness of the cold rooms’ refrigeration system. The coefficients of performance (COP), refrigerant charge rates, and capacities of each component of the refrigeration system for the refrigerants CFC-12, HCFC-22 and their alternatives, such as HFC-134a, HFC-410a, HFC-404a, HFC-407c, and HFC-507 were determined by considering the effects of the main parameters of the performance analysis, such as refrigerant type, Based on the results of the energy analyses, all of the alternative refrigerants have a slightly lower COP and require higher compressor work than CFC-12 and HCFC-22 for condensation temperature of 40°C. The hunted fishes have an evaporation temperature of −25°C regarding with the preserved product in the case studies. Effects of isolation thickness of wall on evaporator capacity and sea water temperature on condenser are also investigated. Also variation capacity of system components such as evaporator, condenser and compressor with relative humidity of neighbor volume is studied.
Heat Transfer Characteristics Over the Interface of Alkanethiolate SAM and Alkane Liquid
Authors: Kikugawa Gota , Ohara Taku , Kawaguchi Tohru , Kinefuchi Ikuya , Matsumoto Yoichiro
Published: December 2013

In the present study, we performed molecular dynamics (MD) simulations of the self-assembled monolayer (SAM) interface system in order to investigate heat transfer characteristics over the SAM/alkane solvent interface. 1-dodecanethiol (C12H25S-) SAM chemisorbed on a gold substrate contacting with n-dodecane (C12H26) solvent was examined to compare the microscopic heat transfer mechanisms inside both the SAM and solvent phases. The direct nonequilibrium MD (NEMD) simulation, in which a constant heat flux across the SAM interface was imposed, was performed. The heat flux through the system was decomposed into the microscopic “building blocks”, i.e., the contribution of energy transfer associated with molecular motion and that of energy exchange by intermolecular (nonbonded) and intramolecular (covalent bond) interactions. Interestingly, inside the SAM layer, almost all of the energy is transferred by the intramolecular interaction along the alkyl chain. On the other hand, in the alkane liquid phase, the intramolecular and intermolecular interactions have comparable contributions to the total heat flux in spite of the same molecular structure and alkyl chain length as the SAM molecules. This difference in the heat transfer mechanism implies the relation between the ordering structure of alkyl chains and thermal conductivity in organic materials.
Numerical Simulation of an Industrial Fluid Catalytic Cracking Riser
Authors: Silaen Armin , Tang Guangwu , Wu Bin , Zhou Chenn Q. , Meng Qingjun , Agnello-Dean Dwight , Wilson Joseph , Khanna Samir
Published: December 2013

A three-dimensional multi-phase, multi-species, turbulent reacting flow computational fluid dynamics (CFD) model was established to simulate fluid catalytic cracking (FCC) process inside an industrial FCC riser. FCC catalyst, oil, and air were used as the solid, liquid, and gas phases, respectively. A hybrid technique for coupling chemical kinetics and hydrodynamics computations was employed, where the simulation was divided into (a) reacting flow hydrodynamic simulation with a small but sufficient number of lumped reactions to compute flow filed properties and (b) and reacting flow hydrodynamics with many subspecies where complex chemical reactions occur. A four-lump kinetic model was used for the major species and a fourteen-lump kinetic model was used for the subspecies model. The results were validated against measurements from the actual riser.
Experimental Flow Field Investigations of Nozzle Film Cooling Scheme on a Flat Plate Using Stereo PIV
Authors: Zheng Yingjie , Hassan Ibrahim
Published: December 2013

This paper presents experimental flow field investigations of a film cooling scheme, referred to as nozzle scheme, on a flat plate using stereo PIV. The nozzle scheme has a cylindrical hole and internal obstacles to change the velocity distribution near the hole exit and hence the jet-mainstream interaction. Counter-rotating vortex pair (CRVP) is known to be one of the detrimental effects that affect the film cooling effectiveness. Previous CFD simulations demonstrated nozzle hole’s capability of reducing CRVP strength and enhancing film cooling effectiveness in comparison with a normal cylindrical hole. The present study examines the nozzle hole flow filed experimentally at blowing ratio ranged from 0.5 to 2.0 and compares with cylindrical hole. The experiments were conducted in a low-speed wind tunnel with a mainstream Reynolds number of 115,000 and the density ratio was 1.0 during all the investigations. The experimental results show that nozzle hole reduces streamwise vorticity of CRVP by an average of 55% at low blowing ratio, and 34%–40% at high blowing ratios. The velocity field and vorticity field of nozzle jet are compared with cylindrical jet. The result reveals that the nozzle jet forms a round bulk in contrast to the kidney shape jet core in cylindrical hole case. In addition, it is found that CRVP strength may not be a primary contributor to the jet lift-off.
Numerical Study of Water Droplet Evaporation on a Superhydrophobic Surface
Authors: Pan Zhenhai , Dash Susmita , Weibel Justin A. , Garimella Suresh V.
Published: December 2013

A comprehensive numerical model is developed to predict evaporation of a water droplet from an unheated superhydrophobic substrate. Analytical models that only consider vapor diffusion in the gas domain, and assume the system to be isothermal, over-predict the evaporation rates by ∼25% compared to experiments conducted on such surfaces. The current model solves for conjugate heat and mass transfer in the solid substrate, liquid droplet, and surrounding gas. Evaporative cooling of the interface is accounted for, and vapor concentration is coupled to local temperature at the interface. Buoyancy-driven convective flows in the droplet and vapor domains are also simulated. A droplet evaporating in a constant-contact-angle mode with an initial volume of 3 μl and contact angle of 160 deg is considered at an ambient temperature of 21°C and 29% relative humidity, to match conditions of related experiments. The interface cooling effect suppresses the evaporation rate significantly; however, natural convection in the gas and liquid domains has a negligible impact on the evaporation rate. The local evaporation flux along the droplet interface predicted by the model is compared to that predicted by an analytical diffusion-based model. The numerically calculated total evaporation rate agrees with experimental results to within 2%. The large deviations between past analytical models and the experimental data on superhydrophobic surfaces are reconciled.
A Novel Technique for Reducing Cooling Load of an Air Conditioning System Operating in Hot and Humid Climates
Authors: Al-Farayedhi Abdulghani A. , Ibrahim Nasiru I. , Gandhidasan P.
Published: December 2013

Cooling load is the energy needed to be removed from a space by a cooling system to provide the desired level of comfort. Large space load requires high energy from the cooling system. A new technique of reducing the cooling load using condensate to pre-cool air stream entering the evaporator of a vapor compression air-conditioning system is presented in this paper. In a cooling process, water vapor condensation normally occurs when the evaporator coil surface temperature becomes lower than the dew point temperature of the humid air entering the evaporator. The cooling process results in appreciable amount of condensate in climatic conditions with high relative humidity and temperature such as those in Dhahran, Saudi Arabia. The rate of condensate yield is calculated using actual climate data of three typical summer days of Dhahran area for the months of June, July and August. These months are the most humid and hottest during the year. Each month is represented by a typical day determined by the average of the three hottest and humid days during the same months of the past three years. It is found that the condensate obtained during night time is more than the day time because of the high relative humidity at night. The results indicate that the cooling load can be reduced up to 10 % when the air entering the evaporator is pre-cooled by 4 °C using the condensate. In addition, the daily condensate yields from the evaporator coil in June, July and August are 1.27, 0.92 and 1.31 kg/kW-CDD, respectively.
Analysis of Flow and Heat Transfer Characteristics Around Oval-Shaped Cylinder
Authors: Zhang Guan-min , Tian Mao-cheng , Zhou Nai-xiang , Li Wei , Kukulka David
Published: December 2013

Numerical simulation and experimental study were carried out to investigate the flow and heat transfer characteristics of air flowing across different types of oval-shaped cylinders. These cylinders have axis ratios, ε, of 1, 1.5, 2, 3, 4, and 5 with the major axis parallel to the free-stream for Reynolds numbers, based on the hydraulic diameter, varying from 4000 to50000. When ε = 1 the tube is a circular cylinder and when 1/ε = 0 a flat plate is represented. Numerical results show that the wake size decreases as ε increases from 1 to 5. The minimum value of Cp takes place at an angular position decrease as ε decreases and the maximum value of Cf gradually increases with the increasing ε. Simulated results agree very well with those available in the existing literature. Oval-shaped cylinders have a higher favorable pressure gradient at the front of the cylinder and a lower adverse pressure gradient at the back of the cylinder for flows in inhibiting separation. Empirical correlations for each tube have been obtained by numerical simulation relating the dimensionless heat transfer coefficient with the Reynolds Number and Prandtl Number. Field synergy theory and performance evaluation criteria (PEC) were used to analyze the mechanisms of heat transfer enhancement for oval-shaped cylinders. It was found that an oval-shaped tube with ε = 2 has the best comprehensive heat transfer performance at Re >11952. In order to verify the effectiveness and correctness of our numerical model, an experiment was carried out for cylinders for values of ε equal to 1, 2, 3 and 4.
Gas Dynamics of Sublimed Carbon Nano-Clusters in High Fluence Time-Resolved Laser-Induced Incandescence
Authors: Memarian F. , Daun K. J.
Published: December 2013

High fluence time-resolved laser-induced incandescence (TiRe-LII) measurements show a discrepancy between experimental observations and model predictions. Several hypotheses have been proposed to explain this discrepancy, including the possibility of back flow of sublimed species and the possibility of formation of shock waves. This is the first study that uses transient Direct Simulation Monte Carlo (DSMC) to investigate the abovementioned effects in high fluence TiRe-LII. This study verifies that back flow of sublimed species occurs, and must be included in analytical models. On the other hand, shock waves were not observed for the fluences and predetermined temperature curves used in this study.
Application of Artificial Neural Networks to Predict Heat Transfer From Buried Pipe for Ground Source Heat Pump Applications
Authors: Demir Hakan , Atayılmaz Ş. Özgür , Agra Özden , Dalkılıç Ahmet Selim
Published: December 2013

The earth is an energy resource which has more suitable and stable temperatures than air. Ground Source Heat Pumps (GSHPs) were developed to use ground energy for residential heating. The most important part of a GSHP is the Ground Heat Exchanger (GHE) that consists of pipes buried in the soil and is used for transferring heat between the soil and the heat exchanger of the GSHP. Soil composition, density, moisture and burial depth of pipes affect the size of a GHE. Design of GSHP systems in different regions of US and Europe is performed using data from an experimental model. However, there are many more techniques including some complex calculations for sizing GHEs. An experimental study was carried out to investigate heat transfer in soil. A three-layer network is used for predicting heat transfer from a buried pipe. Measured fluid inlet temperatures were used in the artificial neural network model and the fluid outlet temperatures were obtained. The number of the neurons in the hidden layer was determined by a trial and error process together with cross-validation of the experimental data taken from literature evaluating the performance of the network and standard sensitivity analysis. Also, the results of the trained network were compared with the numerical study.