bluuebunny/crossref_metadata_2025 huggingface dataset:
Gas Assisted Thin-Film Evaporation for Hotspot Thermal Management
Authors: Narayanan Shankar , Fedorov Andrei G. , Joshi Yogendra K.
Published: March 2011

Presence of hotspots in microprocessors requires localized dissipation of high heat fluxes. While typical background cooling solutions (e.g. air cooling with heat sinks/spreaders, heat pipes etc.) fail to address hotspots, aggressive fluidic cooling with liquid to vapor phase change can potentially meet this goal. In particular, we have been successful in cooling hotspots with a micro-fabricated device providing evaporative cooling. Fundamentally different from boiling, the device takes advantage of evaporation from the free surface of a very thin film of coolant enhanced by flow of dry sweeping gas. Heat transfer coefficients close to 0.1 MW/m2K have already been demonstrated exceeding the performance of other contemporary phase change cooling solutions. This development can potentially play a very important role in designing effective thermal solutions for next generation microprocessors, with application in 3D stacked chips, cooling on-chip optical devices and power electronics, among others. In this paper, we briefly describe the various components constituting the device and the experimental procedure employed for testing the device under different operation conditions. In particular, we investigate the effect of flow configuration of the sweep gas on the overall performance of the system. Jet impingement of air is characterized by varying two operating parameters: (1) the separation between the jet’s nozzle and the evaporation surface, and (2) the inclination of the jet with respect to the evaporation surface (i.e., oblique jet impingement). The results from these experiments help elucidate the importance of these two factors in determining the overall performance of the device.
Airflow and Heat Transfer in Sustainable Building Components: Double-Skin Facades
Authors: Jiru Teshome E. , Tao Yong X. , Haghighat Fariborz
Published: March 2011

Double skin facades (DSFs) are building envelopes comprised of two glass facades, a ventilated air cavity and shading devices placed within the cavity. In this paper airflow and heat transfer simulation was conducted for a DSF system equipped with a venetian blind using computational fluid dynamics (CFD) with RNG turbulence model. Simulation was done for a three-level combination of slat tilt angle and blind position. The heat transfer coefficient was directly obtained from CFD simulation. The CFD prediction was validated using experimental data collected for a mechanically ventilated DSF (1.6 m wide and 2.5 m high and 0.15 m wide cavity) equipped with venetian blinds. The present study indicates that the presence of venetian blinds influences the surface heat transfer coefficients (SHTCs), the temperature and the air distribution in the DSF system. Specifically, for the cases considered, the position of the blinds is more important than the slat angles.
Alternative and Relevant Representation to Heat Transfer Coefficient for Modeling the Heat Transfer Between a Fluid and a Non-Isothermal Wall in Transient Regime
Authors: Remy Benjamin , Degiovanni Alain
Published: March 2011

This paper deals with the relevant model that can be proposed for modeling the interfacial heat transfer between a fluid and a wall in the case of space and time varying thermal boundary conditions. Usually, for a constant and uniform heat transfer (unidirectional steady-state regime), the problem can be solved introducing a heat transfer coefficient h, uniform in space and constant in time that linearly links the surface heat flux and the temperature difference between the wall temperature Tw and an equivalent fluid temperature Tf. The problem we consider in this work concerns the heat transfer between a steady-state fluid flow and a wall submitted to a transient and non uniform thermal solicitations, as for instance a steady-state flow on a flat plate submitted to a transient and space reduced heat flux. We will show that the more interesting representation for describing the interfacial heat transfer is not to define as usually done a non-uniform and variable heat transfer coefficient h(x,t) because as it depends on the thermal boundary conditions, it is not really intrinsic. We propose an alternative approach, which consists in introducing a generalized impedance Z(ω,p) that links in space and time domain the heat flux and the temperature difference through a double convolution product instead of a scalar product. After the presentation of the general problem, the simple case of a stationary piston flow that can be solved analytically will be considered for validation both in thermal steady-state and transient regimes. To conclude and show the interest of our approach, a comparison between a global approach and a numerical simulation in a more complex and realistic case taking into account the thermal coupling with a flat plate will be presented.
Air-Side Heat Transfer and Pressure Drop Characteristics of Peripheral Fin Heat Exchangers
Authors: Pussoli Bruno F. , Barbosa Jader R. , da Silva Luciana W. , Kaviany Massoud
Published: March 2011

We present an experimental evaluation of the peripheral finned-tube heat exchanger. In this novel compact evaporator geometry, the air-side is composed by an arrangement of open-pore cells formed by radial fins whose bases are attached to the tubes and whose tips are connected to peripheral fins. Each fin arrangement is made up of six radial fins and six peripheral fins forming a hexagon-like structure. The air-side fin configuration is composed of three levels of fin arrangement, each characterized by the length of radial fin and mounted with a 30° offset from its neighboring level. Experimental data on the air-side heat transfer and pressure drop were generated in an open-loop wind tunnel calorimeter. A one-dimensional theoretical model based on the theory of porous media has also been developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny-Carman model with the particle diameter definition due to Whitaker and the friction factor correlation due to Ergun. The model overpredicts the air-side thermal conductance by less than 15% for air flow rates higher than 14 L/s. The air-side pressure drop is underpredicted by the model, but still within the limits encountered in the literature. The analysis is complemented with an entropy generation minimization analysis in order to demonstrate the procedure for obtaining an optimized configuration of the heat exchanger.
Columnar → Equiaxed Structure Transition in Solidifying Rolls
Authors: Wołczyn´ski Waldemar S. , Guzik Edward , Wajda Wojciech , Kania Bogusz
Published: March 2011

As the first step of simulation, a temperature field for solidifying cast steel and cast iron roll was created. The convection in the liquid is not comprised since in the first approximation, the convection does not influence the analysed occurrence of the C → E (columnar to equiaxed grains) transition in the roll. The obtained temperature field allows to study the dynamics of its behavior observed in the middle of the mould thickness. This midpoint of the mould thickness was treated as an operating point for the C → E transition. A full accumulation of the heat in the mould was postulated for the C → E transition. Thus, a plateau at the T(t) curve was observed at the midpoint. The range of the plateau existence tC ↔ tE corresponded to the incubation period, tCR ↔ tER that appeared before fully equiaxed grains formation. At the second step of simulation, the thermal gradients field was studied. Three ranges were distinguished: a/ for the formation of the columnar structure (the C–zone): (T˙≫0and(G|t<tCR−G|t=tCR)≫0), b/ for the C → E transition (from columnar to fully equiaxed structure): (T˙≈0and(G|t=tCR−G|t=tER)≈0), c/ for the formation of the fully equiaxed structure (the E–zone): (T˙<0and(G|t=tER−G|t>tER)≈0). The columnar structure formation was significantly slowed down during incubation period. It resulted from a competition between columnar growth and equiaxed growth expected at that period of time. The (G|t=tCR − G|t=tER) ≈ 0) relationship was postulated to correspond well with the critical thermal gradient, Gcrit.. A simulation was performed for the cast steel and cast iron rolls solidifying as if in industrial condition. Since the incubation divides the roll into two zones (columnar and equiaxed) some experiments dealing with solidification were made on semi-industrial scale. A macrosegregation equation for both mentioned zones was formulated. It was based on a recent equation for redistribution after back-diffusion. The role of the back-diffusion parameter was emphasized as a factor responsible for the redistribution in columnar structure and equiaxed structure.
Analysis of Radiation Reabsorption Effects on Flame Characteristics and NOx Emission in Laminar Flames
Authors: Wang Jingfu , Li Guoqiang
Published: March 2011

The radiation reabsorption effects on NOx formation and flame characteristics in CH4/Air laminar flames were numerically investigated by using full chemistry mechanism and detailed transport properties. The radiative gases were treated as non-gray gas and their spectral radiative properties were evaluated by means of the statistical narrow-band model. The radiative heat transfer equation was solved by the discrete ordinate method. It was found that the reabsorption of emitting radiation leads to substantially wider flame thickness and higher flame temperature than those calculated by using the optically thin model, and the radiation reabsorption effect on the “radiation extinction limit” becomes more important. The results show that the level of NOx is predicted to be highest in the adiabatic flames, that is, flames without radiation heat loss, and that the level of NOx is predicted to be lowest in the flames by the optically thin model. In the flames by the SNB model, the predicted amount of NOx lies between these two levels. The calculated results also show that the radiation reabsorption effect on NOx formation grows stronger as the stretch rate decreases, particularly when CO2, a strong absorber, is added to the unburned gas mixture. In this study, the effectiveness and validity of the optically thin radiation model for calculating NOx formation in laminar flames was also investigated in comparison with the SNB model.
Heat Transfer and Correlation Mapping for the Estimation of Thermophysical Properties in Microfluidic Devices
Authors: Pradere C. , Ravey C. , Batsale J. C.
Published: March 2011

The aim of this work is to present new techniques for the estimation of thermal diffusivity and velocity fields from the measurement of the front face temperature fields of microfluidic cells with infrared thermography. Recently, a method was performed in steady state condition. Here, a new method is presented in transient state in order to access to both Peclet and Fourier fields. This new method is based on the calculation of a temporal correlation matrix. Examples obtained for water flow in microfluidic chip are developed. It is shown that a Fourier number field can be estimated and then used to differentiate the pixels corresponding to the location of the microchannel from the others. The knowledge of this Fourier field is the first step for a consecutive estimation of a field of Peclet number. Results obtained for several flow rates demonstrated that the estimated Fourier number is different inside and outside the channel. This is in good agreement with the variation of the apparent thermal properties. Finally the estimated Peclet number is linearly related to the flow rate injected in the channel.
Numerical Prediction of Molten Metal Jet Dynamics, Fragmentation and Solidification in a Coolant Pool
Authors: Diwakar S. V. , Das Sarit K. , Sundararajan T.
Published: March 2011

The hydrodynamics of molten metal jet in a coolant pool is characterized by the presence of complex and diverse fluid structures whose formation is facilitated by various modes of instabilities acting on the fluid-fluid interface and the bulk material. The large spectrum of scales involved in these processes and the related non-linearities cloud a clear understanding of the associated physical phenomena. In order to overcome these difficulties, a numerical model has been developed in the current work, which aims to simulate the hydrodynamics, fragmentation and solidification of a molten metal jet in the coolant pool. The work uses an axisymmetric flow solver with the Volume of Fluid (VOF) interface tracking model to evaluate the macro features of the molten metal jet dynamics and to predict the evolution of interfacial instabilities. At the same time, the phenomena at the micro scale is predicted by a Lagrangian particle tracking model that is used to capture the dynamics and the heat interactions of the fragmented droplets formed from the disintegration of molten metal jet. The coupling between the two models is achieved by converting the molten fluid from VOF model into equivalent swarm of particles at the jet breakup length. The ability of the current coupled model is demonstrated using a sample test problem involving the dynamics of molten woods metal jet in a water pool.
Pressure-Driven Gas Flows in Long Rectangular Microchannels With Uniform Heat Flux Boundary Conditions
Authors: Sun Zhanyu , Jaluria Yogesh
Published: March 2011

This paper focuses on the numerical simulation of pressure-driven gas flow in long microchannels, with uniform heat flux wall boundary conditions. The flow is assumed to be two-dimensional and the momentum and energy equations are solved, considering variable properties, rarefaction effects, including velocity slip, thermal creep and temperature jump, compressibility effects and viscous dissipation. A combined serial-parallel algorithm is employed to simulate the flow in long microchannels. The numerical solution is found to be much more involved than that for the isothermal boundary conditions and the convergence of the scheme to be much slower, as expected. The numerical results are also quite different and, in some cases, quite unexpected. The thermal and hydraulic characteristics are carefully examined and analyzed. It is found that a nonlinear temperature profile arises along the microchannel due to the combined effects of pressure work and viscous dissipation. Similarly, compressibility effects lead to a nonlinear centerline pressure profile. The ratio of pressure work to viscous dissipation is investigated as a function of the Knudsen number and is found to increase with the Knudsen number. The rarefaction effects are found to increase the Nusselt number near the outlet and to decrease it near the inlet. An increase in the inlet/outlet pressure ratio is seen to significantly enhance microchannel cooling.
Experimental Study on Thermal Performance of Mixed Nitrate and Carbonate Salts
Authors: Wu Yu-ting , Ren Nan , Ma Chong-fang , Wang Tao
Published: March 2011

Although molten salts have been used in large scale in the experimental or commercial solar thermal power plants, systematic studies on thermal performence of mixed-salts are lacking. Four eutectic nitrate salts with low melting point were choosed by the analysis of the TG curve and DSC curves with nine binary mixtures of salts with different mass ratio, the specific heat-temperature curve were obtained after further analysis of the DSC curve and experimental correlations were fitted. Potassium carbonate, lithium carbonate, sodium carbonate are mixed in accordance with the different proportions, 36 kinds of mixed molten salt are obtained. The data of melting point, decomposition temperature, specific heat, latent heat and use temperature range were obtained by the analysis of the TGA and DSC curves of 36 salts.
Effects of Schmidt Number on Turbulent Mass Transfer Around a Rotating Circular Cylinder
Authors: Yoon Dong-Hyeog , Yang Kyung-Soo , Lee Kyongjun , Bremhorst Klaus
Published: March 2011

Characteristics of turbulent mass transfer around a rotating circular cylinder have been investigated by Direct Numerical Simulation. The concentration field was computed for three different cases of Schmidt number, Sc = 1, 10 and 100 at Re* = 336. Our results confirm that the thickness of the Nernst diffusion layer decreases as Sc increases. Wall-limiting behavior within the Nernst diffusion layer was examined and compared with those of channel flow. Concentration fluctuation was found to be time-scaled with (r+)2 while the time scale ratio equals the Schmidt number throughout the Nernst diffusion layer. Scalar modeling closure constants were determined, and turned out to vary considerably within the diffusion layer.
Bubble Growth in Surfactant Solutions
Authors: Hetsroni G. , Mosyak A.
Published: March 2011

The presence of surfactant additives in water was found to enhance significantly the boiling heat transfer. The objective of the present investigation was to compare the bubble growth in water to that of a surfactant solution with negligible environmental impact. The study was conducted to clarify the effect of the heat flux on the dynamics of bubble nucleation. The bubble growth under condition of pool boiling in water and surfactant solutions was studied using high speed video technique. The bubble generation was studied on a horizontal flat surface; where the natural roughness of the surface was used to produce the bubbles. At heat flux of q= 10 kW/m2 the life-time and the volume of bubble growth in surfactant solution did not differ significantly from those of water. The time behavior of the contact angle of bubble growing in surfactant solution is qualitatively similar to that of water. At a heat flux of q= 50 kW/m2, boiling in surfactant solution, when compared with that of pure water, was observed to be more vigorous. Surfactant promotes activation of nucleation sites; the bubbles appeared in a cluster mode; the life-time of each bubble in the cluster is shorter than that of a single water bubble. The detachment diameter of water bubble increases with increasing heat flux, whereas analysis of bubble growth in surfactant solution reveals the opposite effect: the detachment diameter of the bubble decreases with increasing heat flux. Natural convection boiling of water and surfactants at atmospheric pressure in narrow horizontal annular channels was studied experimentally in the range of Bond numbers Bo = 0.185–1.52. The flow pattern was visualized by high-speed video recording to identify the different regimes of boiling of water and surfactants. The channel length was 24mm and 36mm, the gap size was 0.45, 1.2, 2.2, and 3.7mm. The heat flux was in the range of 20–500 kW/m2, the concentration of surfactant solutions was varied from 10 to 600 ppm. For water boiling at Bond numbers Bo<1 the CHF in restricted space is lower than that in unconfined space. This effect increases with increasing the channel length. For water at Bond number Bo = 1.52, boiling can almost be considered as unconfined. Additive of surfactant led to enhancement of heat transfer compared to water boiling in the same gap size, however, this effect decreased with decreasing gap size. For the same gap size, CHF in surfactant solutions was significantly lower than that in water. Hysteresis was observed for boiling in degraded surfactant solutions.
Experimental Investigation of Solar Heat Pipes With Ethanol Solution as Working Fluid
Authors: Guo Hang , Du Hai Yan , Ye Fang , Ma Chong Fang
Published: March 2011

As working fluid of heat pipes, water is widely used in solar heat pipe water heater. But water causes poor performance of heat transfer under low temperature and freezing in cold winter. In this paper, ethanol-water solution is used as working substance in heat pipe. The static heat transfer and start-up performance of heat pipe are experimentally studied in this paper. The experimental results showed that when the heat flux is low, the heat transfer of heat pipe with 40% voluminal concentration ethanol solution is better than that of heat pipe with pure water. Start-up speed and temperature of ethanol solution heat pipe are higher than that of water heat pipe. In the low temperature condition, ethanol solution is more competent than water for working medium of solar heat pipe. In addition, heat transfer performance is also influenced by incline angle of heat pipes, charge quantity and heat flux. The effect of incline angle on heat transfer performance of heat pipes becomes distinct with the increasing of heat flux. The reason is that the heat flux becomes high and reflux condensate becomes much with the increasing of heating power. The optimal incline angle of heat pipe with 40 vol. % ethanol solution is 45° and the optimal charge quantities is 11.8 vol. %. The experimental results are helpful for widely use of solar heat pipe water heater in north China.
Simulation of Sub-Micron Thermal Transport in a MOSFET Using a Hybrid Fourier-BTE Model
Authors: Loy James M. , Singh Dhruv , Murthy Jayathi Y.
Published: March 2011

Self-heating has emerged as a critical bottleneck to scaling in modern transistors. In simulating heat conduction in these devices, it is important to account for the granularity of phonon transport since electron-phonon scattering occurs preferentially to select phonon groups. However, a complete accounting for phonon dispersion, polarization and scattering is very expensive if the Boltzmann transport equation (BTE) is used. Moreover, difficulties with convergence are encountered when the phonon Knudsen number becomes small. In this paper we simulate a two-dimensional bulk MOSFET hotspot problem using a partially-implicit hybrid BTE-Fourier solver which is significantly less expensive than a full BTE solution, and which shows excellent convergence characteristics. Volumetric heat generation from electron-phonon collisions is taken from a Monte Carlo simulation of electron transport and serves as a heat source term in the governing transport equations. The hybrid solver is shown to perform well in this highly non-equilibrium situation, matching the solutions obtained from a pure all-BTE solution, but at significantly lower computational cost. The paper establishes that this new model and solution methodology are viable for the simulation of thermal transport in other emerging transistor designs and in other nanotechnology applications as well.
Simulation Research on the Application of Thermoelectric Waste Heat Recovery of Internal Combustion Engine
Authors: Wang Maohai , Daun Thomas Josef , Zhang Yangjun , Zhuge Weilin
Published: March 2011

In this paper, the development of a thermoelectric generator (TEG) simulation model and its implementation into an internal combustion engine (ICE) system model are demonstrated. The TEG model is calibrated with respect to an experimental basis presented in a previously published paper. A TEG parameter study, an analysis of the overall system and the interaction between the TEG and the ICE are carried out. The simulation results indicate that the exhaust gas temperature has a much more significant influence on the TEG performance than the exhaust gas mass flow rate. Without considering the influence of additional backpressure, the application of a TEG shows potential to increase the effective engine power; thereby improving the overall efficiency by approximately 0.6 to 1.7% (depending on engine speed and load). However, when taking additional backpressure into account, this gain in effective engine power is reduced slightly, resulting in a change of the efficiency range to between 0.2 and 1.7%. This illustrates the importance of taking the backpressure into account when designing a real world TEG.
Data Center Energy Conservation by Heat Pipe Cold Energy Storage System
Authors: Wu Xiao Ping , Mochizuki Masataka , Mashiko Koichi , Nguyen Thang , Nguyen Tien , Wuttijumnong Vijit , Cabusao Gerald , Singh Randeep , Akbarzadeh Aliakbar
Published: March 2011

In this paper, design and economic analysis for applying a novel type of heat pipe into cold energy storage systems have been proposed and discussed. The heat pipe cold energy storage systems can be designed into several types that are ice storage, cold water storage and pre-cool heat exchanger. Those systems can be used for co-operating with conventional chiller system for cooling data centers. The heat load used for discussing in this paper is 8800 kW which represents a large scale data center. The methodology addressed in this paper can be also converted into the middle and small sizes of the data centers. This type of storage system will help to downsize the chiller and decrease its running time that would be able to save significant electricity cost and decrease green house gas emissions from the electricity generation. The proposed systems can be easily connected into the existing conventional systems without major design changes. The analysis in this paper is using Air Freezing Index AFI >= 400 °C-days/year for sizing the heat pipe modules. For the locations where AFI has different value the storage size will be varied accordingly. The paper also addressed a result that an optimum size of cold energy storage system that should be designed at a level to handle 60% of total yearly heat load of a data center.
Influence of Thermal Properties Accuracy on Transient Conduction Models
Authors: Vargas Pedro , Lo´pez de Ramos Aura L.
Published: March 2011

The influence of thermal properties accuracy on the heat transfer mathematical models in transient state for food was studied in this work. To pursue this objective, a model in transient state with temperature variable thermal properties was solved using the method of finite differences and the alternate implicit direction scheme to a food inside a cylindrical container under a sudden heating and cooling process, similar to a method of sterilization and/or pasteurization. The model takes into account a linear dependence of the properties with the temperature. An analysis of the behavior of the main thermal properties of the food based on temperature is presented in this article. The results showed that the prediction of the transient state model is more sensitive to perturbations in the values of volumetric heat capacity than thermal conductivity and diffusivity; however, the volumetric heat capacity varies considerably less with temperature. Models with constant properties yield minor deviations when they are evaluated at typical temperature of the cooling or heating processes. The use of the constant thermal diffusivity model with diffusivity is suggested to be evaluated at temperatures between the average and the final heating or cooling process to ensure deviations less than 5%, between the exact model and the simplified one. This solution is considerably simpler than the one obtained with the variable properties model.
Two-Phase Flow in Anode Serpentine Flow Field of a Direct Methanol Fuel Cell
Authors: Ye Fang , Guo Hang , Ma Chongfang
Published: March 2011

Fuel Cells have attracted much attention. Liquid fed direct methanol fuel cells (DMFCs), which have many important advantages over such as the easy storage and refilling of the aqueous methanol solution, are considered to be good choices for small mobile devices. Water and carbon dioxide are the sole fuel oxidized byproducts. The anode side of liquid fed DMFC is a two-phase system primarily consisting of methanol solution and carbon dioxide bubbles. In this paper, a transparent direct methanol fuel cell was designed and fabricated for investigations of two-phase flow characteristics in anode serpentine flow field. In situ observations of the carbon dioxide bubbles and two-phase flow patterns inside the flow field were made. The effects of cell current, methanol solution flow rate and methanol solution mole concentration on the flow patterns and cell performance were studied, respectively. Experimental results indicate that the electrochemical reaction and two-phase flow characteristics interacted each other. The mean size of CO2 bubbles and the gas void fraction increased with increase in current density. The typical flow patterns are bubbly flow and slug flow. At the upside down U type location of the serpentine channel, long slug bubbles easily formed and blocked the channel. The gas void fraction decreased with increase in methanol solution flow rate. The methanol solution mole concentration influences the two-phase flow slightly.
X-Ray Computed Tomography of CO2 Behavior in Water Saturated Sandstone for Geological Storage
Authors: Uemura Suguru , Kataoka Ryoto , Tsushima Shohji , Hirai Shuichiro
Published: March 2011

The CO2 Geological storage is considered as an effective technology for reducing the emissions of CO2 into the atmosphere. CO2 storage is a technically feasible and effective method for CO2 mitigation because it is based on enhanced oil recovery technology, and storage sites hold significant potential. Currently, field tests for CO2 geological storage are proceeding in many parts of the world. However, the behavior of injected CO2 is still not completely understood. The CO2 storage potential and risk of leakage from reservoirs must be accurately estimated to realize practicable CO2 storage. For this reason, laboratory-scale experimental analysis of the behavior of CO2 injected in sandstone are an important issues. In this study, CO2 distribution and its behavior in sandstone were observed by micro-focus X-ray computed tomography (CT). The X-ray CT can fluoroscope the CO2 in the porous media and reconstruct a three-dimensional CO2 distribution image. A sample was kept under high pressure conditions in a cylindrical pressure vessel and filled with CO2 saturated water. Pressure in the vessel was kept at 7.5 MPa, which is the same condition as a saline aquifer at 750 m depth. Liquid or supercritical CO2 was injected from the end face of water saturated samples. Temperature conditions were set to 20 or 40°C according to the experimental objectives of the CO2 phase. In the experimental results, CO2 distribution in the silica-packed bed and sandstone was clearly visualized with high spatial resolution compared to its diameter. The possibility of improvement in storage technology discussed.
Optimization of a Pyroelectric Energy Converter for Harvesting Waste Heat
Authors: Navid Ashcon , Vanderpool Damien , Bah Abubakarr , Pilon Laurent
Published: March 2011

Pyroelectric energy conversion offers a novel approach for directly converting waste heat into electricity. This paper reports numerical simulations of a prototypical pyroelectric energy converter. The two-dimensional mass, momentum, and energy equations were solved to predict the local and time-dependent pressure, velocity, and temperature. Then, the heat input, pump power, and electrical power generated were estimated, along with the thermodynamic energy efficiency of the device. It was established that reducing the length of the device and the viscosity of the working fluid improved the energy efficiency and power density by increasing the optimum operating frequency of the device. Results show that a maximum efficiency of 5.2% at 0.5 Hz corresponding to 55.4% of the Carnot efficiency between 145 and 185°C can be achieved when using commercial 1.5 cst silicone oil. The maximum power density was found to be 38.4 W/l of pyroelectric material.
Heat Transfer Enhancements Using Vibration During Solidification of Paraffin
Authors: Vadasz Johnathan , Meyer Josua , Govender Saneshan
Published: March 2011

In the current study the effects of vibration on the solidification process of phase change material (PCM) paraffin in a sphere shell are investigated. The amount of PCM used was kept constant during each experiment by using a digital scale to check the weight and a thermocouple to check the consistency of the temperature. A small amount of air was present in the sphere so that the sphere was not filled completely. Commercially available paraffin wax, RT35, was used in the experiments. Experimentations were done on a sphere of 40 mm diameter, wall temperature of 20°C below mean solidification temperature, and consistent initial temperature. A constant vibration frequency of 100 Hz was applied to the setup and results compared with that of no vibration. Samples were taken at different times during the solidification process and compared with respect to solid material present. It was found that the solidification time had been reduced significantly under the vibration. This led to the conclusion that there had been an improvement in heat transfer due to the vibration.
Thermo-Fluid Dynamics of an Array of Impinging Ionic Jets in a Crossflow
Authors: Grassi Walter , Testi Daniele
Published: March 2011

Laminar to weakly turbulent mixed convection in a square duct heated from the bottom side is highly strengthened by ionic jets generated by an array of high voltage points, opposite to the heated strip. Negative ion injection is activated within the dielectric liquid HFE-7100. Local temperatures on the heated wall are measured by liquid crystal thermography. Distributions of the Nusselt number are obtained at different forced flow rates, applied heat flows, and transiting electrical currents. In correspondence of the point emitters, higher Nusselt numbers in the impingement areas are measured and an analogy with the thermo-fluid dynamic behavior of an array of submerged impinging jets in a crossflow is drawn. The diameter of the ionic jets is evaluated and an electrohydrodynamic Reynolds number is employed for correlation and similarity purposes.
Two Dimensional Simulation of Flow and Heat Transfer Characteristics of Turbulent Jets
Authors: Abdel-Salam Tarek
Published: March 2011

In this study, flow and heat transfer characteristics of two-dimensional impinging jets are investigated numerically. Flow geometries under consideration are single and multiple impinging jets issued from a plane wall. Both confined and unconfined configurations are simulated. Effects of Reynolds number and the distance between the jets are investigated. Results are obtained with a finite volume CFD code. Structured grids are used in all cases of the present study. Turbulence is treated with a two equation k-ε model. Different jet velocities have been examined corresponding to Reynolds numbers of 5,000 to 20,000. Results show that the Reynolds number has significant effect on the heat transfer rate and has no effect on the location of the maximum Nusselt number.
Experimental Characterization and Numerical Simulation of Plumes Above Thermal Sources: Transport of a Passive Contaminant
Authors: Devienne Rene´ , Fontaine Jean-Raymond , Maalej Talel
Published: March 2011

Industrial ventilation problems are linked with plumes above heat sources. This paper proposes an experimental study of dynamic, thermal and concentration fields that develop from cylindrical complex or rectangular sources. The metrology allows the measurement of local velocities and temperatures but also of the concentration of a tracer gas (helium). The results are compared with the classical analytical formulae, more particularly in the proximal field and also to results issued from simulation.
Analysis of the Flow in the Combustor-Transition Piece Considering the Variation in the Fuel Composition
Authors: Alfaro Ayala J. Arturo , Gallegos Mun˜oz Armando , Riesco A´vila J. Manuel , Flores Lo´pez Marco Polo , Campos Amezcua Alfonso , Mani Gonza´lez Alejandro Germa´n
Published: March 2011

An analysis of the flow that depends of the fuel composition (natural gas) in the combustor-transition piece system, applying Computational Fluid Dynamics, is presented. The study defines the velocity and temperature profiles at the exit of the transition piece and the hot streak along the system. The variation of the composition in the fuel depends of the amount of N2 contained in the fuel, and the hot track influences on the temperature distribution at the input of the first stage of vanes and blades of the gas turbine. The study takes place in a three-dimensional model in steady state using FLUENT ® 6.3.26, applying the k-ε turbulence model and chemical equilibrium to the combustion process. The results show the influence of the transition piece geometry over the velocity and temperature profiles, principally, in the radial direction. The velocity profiles on the radial direction can be represented by six order polynomial and the temperature profile by third order polynomial. The temperature and velocity profiles keep a symmetry profile and they can be represented by six order polynomial at the circumferential direction. Knowing these profiles, it is possible to compute a more exact study of the heat transfer at vanes and blades of the first stage of the turbine to evaluate the performance and life of them. On the other hand, considering from 5% to 10% of N2 in the fuel composition, the maximum temperature is reduced in the combustion process and consequently the NOx emissions too.
Validation of a Novel Model for Shell and Tube Heat Exchangers Undergoing Crude Oil Fouling
Authors: Coletti Francesco , Macchietto Sandro
Published: March 2011

Fouling in refinery heat transfer units is a major problem that affects plant’s economics, operability, safety and environmental impact. Traditional heat exchanger design methodologies based on fixed values for the fouling resistance (e.g. TEMA fouling factors) have drawn several critiques in the past 40 years and were found responsible for exacerbating fouling rather than mitigating it. The fouling factors approach is, in fact, highly empirical and neglects fouling dynamics and its dependency on process conditions. The ability of capturing such dependency is therefore pivotal to overcome traditional design limitations. A novel dynamic, distributed model for a multi–pass shell–and–tube heat exchanger undergoing crude oil fouling was recently proposed by Coletti and Macchietto. The model takes into account the exchanger geometry and configuration, the variation of fluid temperature, velocity, physical properties and fouling rate along the length of each unit and captures the interactions between the fouling layer growth and the fluid–dynamics by solving a moving boundary problem. In this paper, the model is validated over a wide range of operating conditions (i.e. temperatures and flowrates) with data from four different industrial units (2 single and 2 double shells). Geometries and process conditions used are those of two refineries belonging to major oil companies (ExxonMobil and Shell). Some model parameters are estimated for each exchanger using measurements during the first 60 days after a mechanical cleaning. The model is then used in a fully predictive mode for subsequent times. Results indicate that for all units the outlet temperatures (in °C) are predicted over extended periods (i.e. 4–16 months) with an excellent accuracy of ±1% for the tube-side and ±2% for the shell-side. It is concluded that the model can be used with confidence on a wide range of operating conditions to calculate reliable temperatures and fouling resistances.
Phase Distribution of Slug-Annular Flow at a Horizontal Miniature T-Junction
Authors: Wang Shuangfeng , He Kui , Huang Jianzhen
Published: March 2011

In this paper, the behavior of nitrogen-water two-phase flow at a horizontal miniature T-junction with main and side branch of 500 μm diameter was studied. When the inlet flow pattern is slug-annular, the impact of upstream gas and liquid superficial velocity on phase distribution of two-phase flow in the miniature T-junction were investigated in detail. It is found that increasing liquid superficial velocity slightly increase the maldistribution at this work scope. Gas superficial velocity has complex impact on the maldistribution due to different proportion of sub-patterns, and pressure has no impact on phase distribution.
Internal Flows in Microscopic Polymer Solution Droplets Evaporating on Flat Surfaces
Authors: Yasumatsu Shohei , Nanri Narumi , Yoshitake Yu , Nakaso Koichi , Fukai Jun
Published: March 2011

To control the formation process of polymer thin films from polymer solution droplets using inkjet printings, internal flows of the droplets on substrates are studied. In our previous study [1], internal flow of polymer solution droplets receding on a lyophobic surface was experimentally visualized. It was found that the direction of the circulation flow in the droplet depended on the solvent and the initial solute concentration. In particular, the flow direction of polystyrene-anisole solution was reversed as the initial solute concentration increased. In this study, to clarify this reason, the conservation equations of momentum, energy and mass on two-dimensional cylindrical coordinate are numerically solved using a finite element method. The mathematical model considers the free convections derived by the dependencies of the density and surface tension on the solute concentration. As a result, the dependences of the calculated velocities on the initial solute concentration agree qualitatively with the experiments. The mathematical model predicts that double circulation flows appear after a single flow develops at high initial solute concentrations, while double circulations do not develop at low concentrations. It is concluded that the difference between the flow directions investigated experimentally is due to such a change of the flow structure. The distribution of the surface tension on the free surface is also discussed. When a local minimum of the surface tension appears on the free surface, the double circulations develop. According to the result for a low contact angle, the local minimum point shifts toward the axis of symmetry with a lapse of time, and finally erases the double circulations.
Thermal Conductivity of Pure Silica MEL and MFI Zeolite Thin Films
Authors: Coquil Thomas , Pilon Laurent , Lew Christopher M. , Yan Yushan
Published: March 2011

This paper reports the room temperature cross-plane thermal conductivity of pure silica zeolite (PSZ) MEL and MFI thin films. PSZ MEL thin films were prepared by spin coating a suspension of MEL nanoparticles in 1-butanol solution onto silicon substrates followed by calcination and vapor-phase silylation with trimethylchlorosilane. The mass fraction of nanoparticles within the suspension varied from 16 to 55%. This was achieved by varying the crystallization time of the suspension. The thin films consisted of crystalline MEL nanoparticles embedded in a non-uniform and highly porous silica matrix. They featured porosity, relative crystallinity and MEL nanoparticles size ranging from 40 to 59%, 23 to 47% and 55 to 80 nm, respectively. PSZ MFI thin films were made by in-situ crystallization, were b-oriented, fully crystalline and had a 33% porosity. Thermal conductivity of the PSZ thin films was measured at room temperature using the 3ω method. The cross-plane thermal conductivity of the MEL thin films remained constant around 1.02 ± 0.10 Wm−1K−1 despite increases in (i) relative crystallinity, (ii) nanoparticle size and (iii) yield as the nanoparticle crystallization time increased. Indeed, the effect of increases in these parameters on the thermal conductivity was compensated by the simultaneous increase in porosity. PSZ MFI thin films were found to have the same thermal conductivity as MEL thin films even though they had smaller porosity. Finally, the average thermal conductivity of the PSZ films was three to five times larger than that reported for amorphous sol-gel mesoporous silica thin films with similar porosity and dielectric constant.
Research and Engineering Practice in Nanofluids: Key Issues
Authors: Wang Liqiu
Published: March 2011

Nanofluids are a new class of fluids engineered by dispersing nanometer-size structures (particles, fibers, tubes, droplets) in base fluids. The very essence of nanofluids research and development is to enhance fluid macroscopic and megascale properties/performance such as thermal conductivity through manipulating microscopic physics (structures, properties and activities). Therefore, the success of nanofluid technology depends very much on how well we can address issues like effective means of microscale manipulation, interplays among physics at different scales, and optimization of microscale physics for the optimal megascale properties. In this poster we review methodologies available to effectively tackle these key but difficult problems and identify the future research needs as well. The reviewed techniques include nanofluids synthesis through liquid-phase chemical reactions in continuous-flow microfluidic microreactors, scaling-up by the volume averaging, and constructal design with the constructal theory. The identified areas of future research contain microfluidic nanofluids, thermal waves, and constructal nanofluids. While our focus is on heat-conduction nanofluids, the methodologies are equally valid for the other types of nanofluids. The review could serve as a coherent, inspiring and realistic plan for future research and development of nanofluid technology.
Computational and Experimental Study of Conjugate Heat Transfer From a Flat Plate With Shower Head Impinging Jets
Authors: Panda Rajesh Kumar , Sreekala P. , Prasad B. V. S. S. S.
Published: March 2011

Computational and experimental conjugate heat transfer of flat circular disk is investigated with a constant heat flux imposed on its bottom surface and a shower head of air jets impinging on its top surface. The shower head consists of a central air jet surrounded by four neighboring perimeter jets. Measured temperature data at twelve locations within the plate are compared with the conjugate heat transfer result obtained at the same locations computationally by Shear Stress Transport (SST) κ-ω turbulence model. Measurement and simulation results are in good agreement with each other. The spacing to orifice diameter ratio (H/d = 1 to 6), jet Reynolds number (7115 to 10674) and plate thickness (2.5 mm, 10 mm and 20 mm) are varied. The computationally obtained flow structure describes the complex interaction of the wall jets. Heat transfer rate is found to be independent of thickness of the impingement plate. Local variation of heat transfer rate with varying H/d is significant but its effect on the area weighted average heat transfer rate is very small. Shower head jets provide uniform temperature distribution with higher heat transfer rate in comparison with the single jet.
Turbulent Drag Reduction by Polymer Containing Paint: Simultaneous Measurement of Skin Friction and Release Rate
Authors: Motozawa Masaaki , Ito Toshihisa , Matsumoto Ayumu , Ando Hirotomo , Ashida Toshihiko , Senda Tetsuya , Kawaguchi Yasuo
Published: March 2011

Performance test for polymer containing trial antifouling paint was carried out experimentally. This trial paint is made by adding a polymer (PEO) to commercial antifouling paint for ship hull. Our developing paint has a function of leaching out the polymer and eventually reduces the skin frictional drag by the effect of dissolved polymer when the ship cruises. As the performance test, we examined the release rate of the polymer from the trial paint and measured the skin frictional drag at the same time. In addition, the velocity distribution near the painted wall was measured by Laser Doppler Velocimeter (LDV). As a result, in the first stage, the large drag reduction was obtained with releasing polymer. The Reynolds shear stress near the painted wall largely decreases comparing with the commercial antifouling paint in the water flow. However, after several hours, this drag reducing effect was hardly lost.