bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimisation of Annular Cylindrical and Spherical Fins in an Internal Combustion Engine Under Realistic Conditions
Authors: Illa´n Fernando , Alarco´n Mariano
Published: March 2011

The heat transfer from cylinder to air of a 2-stroke internal combustion finned engine has been simulated. For this purpose, a 2-D model of the block and head has been done. Starting from the dimensions of a real engine, annular cylindrical and spherical symmetric walls and fins have been used to obtain an equivalent simplified geometry, where the heat transfer rate is the same as that in the real engine. The cylinder body, cylinder head (both provided with fins) and piston have been numerically analyzed and optimized in order to minimize engine dimensions. The maximum temperature admissible at the hottest point of the engine has been adopted as the limiting condition. An engine temperature map has been presented as additional information. Cyclically variable inner cylinder conditions (thermodynamic, thermal and chemical) proceed from a previous work; convective and radiative inner boundary conditions and convective external conditions have also been taken into account in this work. A network simulation model (NSM) has been used as the numerical tool in the analysis presented.
Study of CHF Condition for Flow Boiling of R134a in Circular Microchannels
Authors: Basu Saptarshi , Ndao Sidy , Michna Gregory J. , Peles Yoav , Jensen Michael K.
Published: March 2011

An experimental study was carried out to investigate the CHF condition for flow boiling of R134a in single circular micro-tubes with inner diameters of 0.50 mm and 0.96 mm. The effects of mass flux, saturation pressure, inlet subcooling, tube diameter, and vapor quality on CHF were studied. The flow parameters investigated were as follows: exit pressures of 670, 890, and 1160 kPa; mass fluxes of 300–1500 kg/m2s; and inlet subcooling of 5, 20, and 40 °C. CHF occurred under saturated conditions. CHF was found to increase with an increase in mass flux, tube diameter, and inlet subcooling. CHF decreased with increasing saturation pressures and increasing vapor qualities. The experimental data were compared to three CHF correlations — the Bowring correlation, the Katto-Ohno correlation, and the Thome correlation. The correlations predicted the correct experimental trend.
Universal Curves for Optimizing Longitudinal Rectangular Fins Radiating to Free Space
Authors: Luna-Abad J. P. , Alhama Francisco , Campo A.
Published: March 2011

The relative inverse admittance is used to obtain the geometry of the optimum longitudinal rectangular fin of a set of fins of the same volume that radiate to free space. With this information a set of new universal curves that permits direct optimization of 1-D and 2-D practical fins. To ensure a dissipation effect, lines of constant effectiveness are introduced in the optimization graph. Examples are shown.
Internal Temperature Distributions of Vaporizing Droplets: Effect of Their Spacing
Authors: Castanet G. , Labergue A. , Lemoine F.
Published: March 2011

A line of mono-sized and periodically spaced droplets is moving in the diffusion flame sustained by the droplet fuel evaporation. The temperature field within the droplets is measured using the two-color laser induced fluorescence technique. Experiments are undertaken on droplets made of different fuels including acetone, ethanol, 3-pentanone, n-heptane, n-decane and n-dodecane which have very different volatility and viscosity. A simplified model of the heat transfer within the droplet is developed, taking into account both heat conduction and heat advection by the droplet internal fluid circulation. Streamlines are assumed to follow those of a spherical Hill vortex, the intensity of which can be related to the friction coefficient. Comparisons between the measurements and the simulations reveal that the heat convection within interacting droplets is strongly reduced compared to the model of the isolated droplet.
Influence of Fiber Orientation and Aspect Ratio on the Effective Conductivity of Parallelepipedonal-Cell Short-Fiber Composites
Authors: Bezerra Juliana S. , Cruz Manuel E. , Matt Carlos F.
Published: March 2011

Modern applications of short-fiber composite materials demand accurate characterization of their macroscopic thermal properties. Many physical and microstructural factors influence the effective thermal conductivity of a composite material body. The objective of the current work is to investigate the influence of the microstructure configuration on the effective conductivity of a parallelepipedonal-cell composite containing equal-sized short fibers; the orientation and aspect ratio of the fibers are varied. The possible presence of voids in the matrix or of an interfacial thermal resistance between the constituent phases are not considered. A previous continuous formulation and computational implementation of heat conduction in a statistically homogeneous and periodic composite material are employed. The approach is based on the application of homogenization theory to the variational form of the original heat conduction boundary value problem for the multiscale composite medium. The variational form is well suited for subsequent numerical solution by the finite element method. The expression for the composite effective conductivity is here computed for several parallelepipedonal-cell microstructures. The numerical results are critically compared with available experimental data for short-fiber composites, and indications for important future research efforts are drawn.
Heat and Mass Transfer in a Permeable Fabric System Under Hot Air Jet Impingement
Authors: Lee Sangsoo , Park Chanwoo , Kulkarni Devdatta , Tamanna Sanjida , Knox Ted
Published: March 2011

Heat and mass transfer in a permeable fabric system used for Personal Protective Equipment (PPE) was investigated using hot air jet impingement conditions to mimic the jet exhaust of Short Take-Off and Vertical Landing (STOVL) aircraft. The STOVL aircraft uses a thrust-vectoring nozzle of the jet engine and a lift fan in order to vertically land and take off a short runaway. The jet engine exhaust is a new kind of thermal hazard for military personnel operating within an affected zone of the jet exhaust. An experimental approach was used to measure the thermal response of a fabric system consisting of permeable fabric samples and air pocket using a high-speed jet impingement. The jet impingement conditions consisted of two different temperatures: one of 100°C and another of 200°C at a jet impingement velocity of 32 m/s. Air was used as the working fluid. In this study, two permeable fabrics, (NOMEX IIIA and Cotton) commonly used for the Personal Protective Equipment (PPE) were investigated. The physical properties (porosity, permeability, Ergun coefficient, and density) and the thermo-physical properties (thermal diffusivity, thermal conductivity, and specific heat) of the fabrics were measured. A one-dimensional, two-medium formulation assuming thermal non-equilibrium between solid (fabric) and gas (air) phases in the fabric layer was used for the numerical analysis. The measurement results from the fabric experiment were used to define boundary conditions and adjust various heat transfer correlations and input data used in the numerical model. The experimental and numerical results of the temperatures of the fabric system were compared. The effects of the air temperature of the jet impingement on the thermal response of the fabric system were discussed.
Heat and Water Transport in a Polymer Electrolyte Fuel Cell Electrode
Authors: Mukherjee Partha P. , Ranjan Devesh , Mukundan Rangachary , Borup Rodney L.
Published: March 2011

In the present scenario of a global initiative toward a sustainable energy future, the polymer electrolyte fuel cell (PEFC) has emerged as one of the most promising alternative energy conversion devices for various applications. Despite tremendous progress in recent years, a pivotal performance limitation in the PEFC comes from liquid water transport and the resulting flooding phenomena. Liquid water blocks the open pore space in the electrode and the fibrous diffusion layer leading to hindered oxygen transport. The electrode is also the only component in the entire PEFC sandwich which produces waste heat from the electrochemical reaction. The cathode electrode, being the host to several competing transport mechanisms, plays a crucial role in the overall PEFC performance limitation. In this work, an electrode model is presented in order to elucidate the coupled heat and water transport mechanisms. Two scenarios are specifically considered: (1) conventional, Nafion® impregnated, three-phase electrode with the hydrated polymeric membrane phase as the conveyer of protons where local electro-neutrality prevails; and (2) ultra-thin, two-phase, nano-structured electrode without the presence of ionomeric phase where charge accumulation due to electro-statics in the vicinity of the membrane-CL interface becomes important. The electrode model includes a physical description of heat and water balance along with electrochemical performance analysis in order to study the influence of electro-statics/electro-migration and phase change on the PEFC electrode performance.
Numerical Simulation of Heat Transfer Mechanisms in Spray Cooling
Authors: Gehring Eelco , Trujillo Mario F.
Published: March 2011

A primary mechanism of heat transfer in spray cooling is the impingement of numerous droplets onto a heated surface. This mechanism is isolated in the present and ongoing work by numerically simulating the impact of a single train of FC-72 droplets employing an implicit free surface capturing methodology. The droplet frequency and velocity ranges from 2000–4000 Hz, and 0.5–2 m/s, respectively, with a fixed drop size of 239 μm. This gives a corresponding Weber and Reynolds range of 10–170 and 330–1300, respectively. Results show that the impingement zone is largely free of phase change effects due to the efficient suppression of the local temperature field well below the saturated value. Due in part to the relatively high value of the Prandtl number and the compression of the boundary layer from the impingement flow, a cell size on the order of 1 μm is necessary to adequately capture the heat transfer dynamics. It is shown that the cooling behavior increases in relation to increasing frequency and impact velocity, but is most sensitive to velocity. In fact, for sufficiently low velocities the calculations show that the momentum imparted on the film is insufficient to maintain a near stationary liquid crown. The consequence is a noticeable penalty on the cooling behavior.
Three Dimensional Numerical Simulation of Gas Heat Transfer in Rectangular Microchannels
Authors: Yan Xiaohong , Wang Qiuwang
Published: March 2011

Rectangular microchannel is the typical component of the micro heat exchangers and micro heat sinks. Three-dimensional compressible Navier-Stokes equations are solved for gas flow and heat transfer in microchannels under uniform heat flux boundary condition. The numerical methodology is based on the control volume SIMPLE scheme. It is found that the heat removal characteristic for compressible flow is better than the incompressible flow and it is not suitable to use conventionally defined Nu to measure the heat transfer characteristic for compressible heat transfer. The effect of the aspect ratio (width to height) on the cross-sectional averaged wall temperature and the Nu is negligible under the uniform heat flux boundary condition. However, the local uniformity of the wall temperature is significantly influenced by the aspect ratio. The square cross-section exhibits the best local uniformity of the wall temperature.
Controlling Condensation of Water Using Hybrid Hydrophobic-Hydrophilic Surfaces
Authors: Varanasi Kripa K. , Deng Tao
Published: March 2011

Heterogeneous nucleation of water plays an important role in wide range of natural and industrial processes. Though heterogeneous nucleation of water is ubiquitous and everyday experience, spatial control of this important phenomenon is extremely difficult. Here we show, for the first time, that spatial control in the heterogeneous nucleation of water can be achieved by manipulating the local nucleation energy barrier and nucleation rate via the modification of the local intrinsic wettability of a surface by patterning hybrid hydrophobic-hydrophilic regions on a surface. Such ability to control water nucleation could address the condensation-related limitations of superhydrophobic surfaces, and has implications for efficiency enhancements in energy and desalination systems.
Simplified Method for Prediction of Cooling Load
Authors: Yoo Seong-Yeon , Han Kyu-Hyun , Kim Jin-Hyuck
Published: March 2011

A simplified method is developed to predict the hourly cooling load of the next day. Only four building specifications and the estimated temperature and humidity of the next day are used to predict the cooling load. The sensible heat loads are calculated using the sensible heat coefficient, outdoor air coefficient, and sensible heat constant. Similarly, the latent heat loads are calculated using the outdoor air coefficient, latent heat constant. Four building specifications, that is, sensible heat coefficient, outdoor air coefficient, sensible heat constant and latent heat constant are determined from the building design data for cooling load. In order to validate the prediction model, two benchmarking buildings are selected, and cooling loads are measured in the actual operation conditions. The predicted results show fairly good agreement with the measured data for both buildings.
Infrared Electric Emitters for Drying Paper
Authors: Abdelmessih Amanie N. , Beakley Michael A. , Campbell Steven B. , McKnight Eric W. , Roberts Michael P. , Woodward Erin R.
Published: March 2011

The paper industry is the second largest consumer of energy in the Northwest. The majority of energy consumption in any paper mill occurs in the paper dryer section. This article presents a unique, complete dryer section design that is more economical than conventional paper dryers. The proposed dryers use infrared radiation as the primary form of heat transfer. The infrared dryer section design is compared to the conventional steam dryer section for a high speed newsprint paper machine. Since conventional steam dryer sections in paper machines are a major capital investment and last for decades, a simple, economical retrofit design of a conventional dryer section is included.
A Meshless Method for Modeling Convective Heat Transfer
Authors: Pepper D. W. , Wang X. , Carrington D. B.
Published: March 2011

A meshless method is used in a projection-based approach to solve the primitive equations for fluid flow with heat transfer. The method is easy to implement in a MATLAB format. Radial basis functions are used to solve two benchmark test cases: natural convection in a square enclosure and flow with forced convection over a backward facing step. The results are compared with two popular and widely used commercial codes: COMSOL, a finite element model, and FLUENT, a finite volume-based model.
Thermal Conductance of Biporous Evaporator Wicks in Thin Film Evaporation and Boiling Regimes
Authors: Srinivasan Vinod , Lu Ming-Chang , Coso Dusan , Majumdar Arun
Published: March 2011

We present detailed data on the performance of microstructured geometries for use in the evaporator section of a vapor chamber heat pipe. The central innovation of the geometries is their hierarchical structure, involving the use of large microchannels in order to reduce liquid flow drag while fabricating microscale pin fin arrays whose small pores increase capillary suction. The overall conductance in such a geometry is dependent on the extent of thin liquid film (thickness ∼few microns), which is manipulated by increasing the surface area-to-volume ratio through the use of microstructuring. Experiments were conducted for a heater area of 1cm2, with the wick in a vertical orientation. Results are presented for fixed microchannel widths of 30–60 microns, with pin fin diameters ranging from 4 to 32 microns, and pin fin array widths of 150 to 300 microns. The competing effects of increase in surface area due to microstructuring, and the suppression of evaporation due to reduction in pore scale are explored. In the evaporative regime, conductances of the order of 6 W/cm2-K are attained at heat fluxes of up to 140 W/cm2, until the capillary limit is reached and the wick dries out.
Heat Transfer Enhancement Observed in Nucleate Boiling of Alcohol Aqueous Solutions at Very Low Concentration
Authors: Sakai Takashi , Yoshii Shinsaku , Kajimoto Kazutoshi , Kobayashi Hiroyuki , Shinmoto Yasuhisa , Ohta Haruhiko
Published: March 2011

Experiments on saturated pool boiling were conducted by using alcohol aqueous solutions of 1-Propanol/Water, 2-Propanol/Water and Water/Ethylene glycol mixtures at atmospheric pressure. The heating surface was a horizontal upward-facing circular flat plate of 40mm in diameter. Despite that only the deterioration of heat transfer coefficient due to the existence of diffusion resistance was reported so far in nucleate boiling of mixtures, enhancement of heat transfer coefficients was observed for 1-Propanol/Water and 2-Propaonl/Water mixtures at extremely low alcohol concentrations. In these concentration ranges, Marangoni force acting on the vapor-liquid interface towards the three-phase interlines was expected to enhance the heat transfer, and this heat transfer enhancement can overcome the heat transfer deterioration due to the mass transfer resistance inherent in mixture boiling. In the intermediate concentration range, however, no heat transfer enhancement but only deterioration was observed, where Marangoni effect had no important role. On the other hand, the measured CHF values were gradually decreased with the increase of alcohol concentration in the moderate concentration range. In the low alcohol concentration range, however, CHF values were clearly decreased for 1-Propanol/Water and 2-Propanol/Water. To confirm the observed heat transfer trends of three different alcohol aqueous solutions, Marangoni effect on the nucleate boiling heat transfer mechanisms was discussed.
Modeling Size Effects of a Portable Two-Phase Electronics Cooling Loop With Different Refrigerants
Authors: Saenen Tom , Baelmans Martine
Published: March 2011

A one dimensional dynamic system model is developed to accurately simulate a two-phase microchannel electronics cooling loop. This model is based on the single component mixture equations for mass, momentum and energy. These equations are solved numerically using a finite volume method in conjunction with the SIMPLE algorithm. To calculate the pressure losses and heat transfer state of the art empirical correlations are used. Furthermore size effects of a typical microchannel cooling system are investigated with the new model. Special attention is given to the accumulator size and its limitations for portable applications. A simple model to investigate the accumulator size effect on the loop is developed and compared to numerical results obtained from the system model. The influence of various loop parameters and possible improvements are also investigated. Finally the effect of using different coolants is studied.
Experimental and Numerical Investigation of Biomass Pyrolysis Process Focusing on Intra-Particle Heat Transfer
Authors: Watanabe Hirotatsu , Morinaga Yosuke , Okada Takuya , Okazaki Ken
Published: March 2011

The purpose of this research was to investigate biomass pyrolysis process focusing on intra-particle heat transfer. Thermal decomposition characteristics of wood cylinder with a diameter of 8mm were studied experimentally and numerically. In an experiment, a thermobalance reactor was used to investigate weight loss of wood cylinder during the pyrolysis. Three K-type thermocouples with a diameter of 0.5 mm were placed in the sample to measure the intra-particle temperature. Wood cylinders were heated by infrared furnace under inert gas at 1 Ks−1 and 30 Ks−1. In a calculation, unsteady two-dimensional heat and mass transfer equations were discretized by using Finite Volume Method with first order implicit scheme. The reaction kinetics of biomass pyrolysis were modeled by using a multi-step kinetic scheme. To investigate the effect of intra-particle heat transfer, calculations with considering temperature gradient and with uniform temperature were carried out. As a result, the calculation results with considering intra-particle heat transfer were in good agreement with experimental ones, while calculation results without considering temperature gradient were quite different from experimental ones at high heating rate. Intra-particle heat transfer mechanism at low heating rate was quite different from that at high heating rate. Both numerical and experimental results showed that there was a distinct peak of intra-particle temperature due to strong exothermic reaction at low heating rate. Meanwhile, endothermic reactions were dominant at high heating rate, and there was no temperature peak. Moreover, an increase in the slope of temperature history observed at high heating rate. It was difficult to explain the slope increase by only weak exothermic reactions. This was because that heat capacity was decreased significantly during pyrolysis. When the heating rate was high, the yield of volatile matter whose heat capacity was quite less than that of char or wood was increased. It was shown that volatile and char formation characteristics were strongly related with intra-particle heat transfer characteristics.
Computational Study of Flow Reversal for Improved Thermal Management in a PEMFC Stack With Forced Air Convection Cooling
Authors: Sasmito Agus Pulung , Birgersson Erik , Mujumdar Arun Sadashiv
Published: March 2011

In a proton exchange membrane fuel cell (PEMFC) stack equipped with a forced-convection open-cathode manifold, significant temperature gradients can develop from the inlet to the outlet due to the incoming cool air heating up as it passes through the cathode flow fields. In order to reduce these gradients, we consider flow-reversal of the air by reversing the flow direction of the fans that are providing the forced convection: This allows cool air to alternately enter the stack from the front and the rear instead of only from the front. The impact of the flow-reversal is studied with a model considering two-phase flow and conservation of mass, species, momentum, charge, and energy in the PEMFC stack and ambient; the fans are treated as interface conditions. The results indicate that the temperature, water and current density distributions become more uniform with an overall reduction of the maximum temperature in the stack.
Characteristics of Wetting Temperature and Maximum Heat Flux During Spray Cooling of Hot Surface
Authors: Mitsutake Yuichi , Monde Masanori
Published: March 2011

An experimental investigation has been done to elucidate the effects of mass flux G, degree of subcooling ΔTsub and initial solid temperature Tb0 on transient spray cooling of a downward facing φ89 mm hot block surface. The spray impact diameter was adjusted to φ110mm and φ36mm which simulate uniform and non-uniform spray cooling of the surface. The block made of copper, brass and carbon steel at an initial temperature of 200–500 °C was cooled with subcooled water and ethanol spray. The subcooling was from 10 to 80 K and the mass flux was from 1 to 72 kg/m2s. Surface temperature and surface heat flux were evaluated with an axisymmetric 2D inverse heat conduction analysis. A transient transition regime was characterized with a wetting temperature and a quenching temperature. The wetting and quenching temperatures were correlated fairly with GΔTsub. Effects of G, ΔTsub, Tb0 and a thermal inertia of the solid (ρcλ)s on a maximum heat flux are evaluated.
A Study on Inverted Temperature Profile Based on Nonequilibrium Molecular Dynamics Simulation
Authors: Tokunaga Atsushi , Nagayama Gyoko , Tsuruta Takaharu
Published: March 2011

Problem of the inverted temperature profile in condensation has been discussed at length in kinetic theory. The possible occurrence of the inverted temperature profile raised serious doubts about the validity of the previous theory, because the result seemed physically unreasonable. In this study, nonequilibrium molecular dynamics (NEMD) simulation with two facing surfaces of evaporation and condensation is carried out in order to obtain further evidence for the occurrence of the inverted temperature profile. Heat and mass transfer rates across the liquid-vapor interface during the condensation of argon are calculated for different nonequilibrium conditions given by changing the temperature of evaporating surface. From a view point of irreversible thermodynamics, the criteria for the inverted temperature profile are examined with the NEMD data and we have a conclusion that the inverted temperature profile may occur without contradiction to the second law of thermodynamics. In addition, we found that the molecular reflection at the condensing surface has an important role in the inverted temperature phenomenon. The reflected molecules do not accommodate with the condensing surface so that those molecules raise the temperature in the vicinity of the condensing surface under the nonequilibrium conditions.
Impinging Oil Jet Behaviour for Planar Wall Heat Transfer
Authors: El-Khawankey Sarah , Al-Sibai Faruk , Kneer Reinhold
Published: March 2011

Impinging jets for convective cooling are used in several technical applications. Piston cooling with impinging oil jets is one key application. To improve the heat transfer between the surface of the piston and the oil film it is necessary to understand the underlying mechanisms of heat transfer at the boundary face. For this reason it is important to analyze the oil flow and to identify and evaluate the influence of the parameters governing film formation. Also the oil jet is investigated, because the film formation can be influenced by the jet. In the experiments the oil temperature is set to 30 °C or 60 °C and the pressure at the nozzle inlet is varied between 1.6 bar and 4.2 bar. The minimal Reynolds number is 125 and the maximum is 1924. The liquid Weber number varies between 2.2 × 10−2 and 52.9 × 10−2. The results of the visualization measurements reveal the influence of the exit velocity, oil temperature and the related material properties on the film formation process. On the one hand the results show the macroscopic relation between Reynolds number and the level of instability. On the other hand the relation between Weber number and the break-up at the surface of the jet and accordingly of the film surface can be demonstrated.
Maximum Heat Transfer From Multi-Scale Fins Arranged in a Row With Non-Uniform Geometry
Authors: Bello-Ochende T. , Meyer J. P. , Bejan A.
Published: March 2011

This paper described numerical the procedure used to determine the optimum configuration of two rows of pin fins so that the total heat transfer rate is maximized. The heat transfer across the fins is by laminar forced convection bathed by a free-stream that is uniform and isothermal. The optimization is subjected to fixed volume of fin materials. The dimensions of the optimized configuration are the result of balancing conduction along the fins with convection transversal to the fins. Numerical results on the effect of dimensionless pressure drop and the thermal conductivity ratio on the optimal configuration are reported. Results obtained from numerical analyses are comparable to those in the open literature. The results also show that the flow structure performs best when the fin diameters and heights are non-uniform.
Carbon Nanotube Interfaces for Magneto Thermoelectric Actuation
Authors: McCarthy Patrick T. , Hodson Stephen L. , Sands Timothy D. , Fisher Timothy S.
Published: March 2011

Magneto thermoelectric generator cell technology uses the ferromagnetic phase transition of gadolinium to drive the movement of a diaphragm ‘shuttle’ whose mechanical energy can be converted to electrical form and which enhances heat transfer through both conduction and convection. This paper describes the thermal behavior of gadolinium foils used in magneto thermoelectric generator cells that, in conjunction with a planar array of similar devices, would form a thermal backplane to a solar photovoltaic panel. In this scenario, the backplane operates as a self-powered cooling device that can simultaneously convert thermal energy to electrical energy as well as improve photovoltaic efficiency through active cooling. This form of energy harvesting and enhancement shows the potential of increasing the energy density of silicon photovoltaic panels. The synthesis and characterization of thermal interfaces applied to the gadolinium shuttles and hot/cold substrates are described. Carbon nanotube arrays are implemented as the thermal interfaces, and their performance under static conditions is assessed. Optimization of the carbon nanotube interfaces on the gadolinium shuttles is achieved using photoacoustic experiments for measuring the thermal interface resistances above and below the gadolinium foil. Carbon nanotube growth studies on gadolinium demonstrated a reduction in thermal interface resistances from 28.8 ± 2.1 mm2K/W to as low as 17.9 ± 0.8 mm2K/W. Initial design, fabrication, and experimental techniques and results are presented in this paper.
Performance of Glass Heat Pipes for Solar Thermoelectric Energy Systems
Authors: Miljkovic Nenad , Wang Evelyn N.
Published: March 2011

The development of high performance thermal management strategies offers opportunities to increase energy conversion efficiencies for solar thermoelectric systems. In particular, heat pipes and two-phase thermosyphons have received increased interest due to their ability to passively and efficiently transport heat using liquid phase-change. However, the high manufacturing and materials costs limit the broad utilization of such cooling solutions. In this work, we investigate heat pipe designs using glass where the manufacturing methods can be inexpensive for solar thermoelectric systems. We developed an analytical model based on dimensionless thermal resistances that determines how material properties, geometry, scaling and solar concentration affect heat transfer performance. We simultaneously investigated the feasibility of three realistic heat pipe solar thermoelectric systems. The results from the work offer design guidelines for the development of high performance, low cost heat pipes for solar thermoelectric systems.
Heat Transfer and Pressure Drop Characteristics for a Turbine Casing Impingement Cooling System
Authors: Ben Ahmed F. , Weigand B. , Meier K.
Published: March 2011

Flow mechanisms, heat transfer and discharge coefficient characteristics for a representative part of a turbine casing cooling system, consisting of an array of 20 impinging jets, were numerically investigated. The study focused on the influence of the jet Mach number while maintaining the Reynolds number constant at Re = 7,500. Therefore, the orifice bore diameter or the fluid density had to be varied. The objectives of the current CFD simulations have not been adressed before in literature, not only because heat transfer characteristics and pressure drop are given for impingement jet Mach numbers up to 0.72 at a constant relatively low Reynolds number, but also because fundamental understanding of physical phenomena of the flow in the cylindrical plenum and in the small sharp-edged orifices at the bottom side of the tube is provided. Increasing the Mach number by simultaneously reducing the orifice diameters led to slightly decreasing Nusselt numbers, with average deviations of the order of 14%. However, the heat transfer coefficient increased considerably with increasing Mach number. On the contrary, the variation of the Mach number by varying the density showed only a slight influence on the heat transfer coefficient. The predicted discharge coefficients increased significantly by augmenting the Mach number.
Significant Heat Transfer Enhancement in Microchannels With a Segmented Flow of Two Immiscible Liquids
Authors: Asthana Ashish , Zinovik Igor , Weinmueller Christian , Poulikakos Dimos
Published: March 2011

With the advancement in microfabrication technologies the size of electronic devises keeps decreasing. At the same time high density of circuits and the directly related heat generation requires simultaneous increase in the performance of cooling systems to keep the electronics operating at optimum temperatures. Liquid cooling incorporating microchannel networks as the heat sinks is known to be a promising approach to this heat transfer problem because of its plausible manufacturability at the chip level. However, even the upper range of heat fluxes achieved in such microchannel heat sinks will not meet the increasing demands of the electronics industry. We present herein a novel experimental study of convective heat transfer in serpentine microchannels with segmented liquid-liquid emulsions. It is demonstrated that this concept yields significant heat transfer enhancement in micro channel heat sinks compared to traditional single phase liquid cooling. The flow of water and mineral oil droplets is examined experimentally in microchannels with cross section 100 by 100 μm and flow rates up to 130 μl/min. The study is focused on the investigation of the local temperature of the liquid and on the influence of the oil droplets on the temperature distribution and heat transfer. Laser Induced Fluorescence (LIF) is employed to measure the temperature of the flow fields with and without droplets, and PIV measurements are conducted to determine the velocity field. The range of flow conditions which led to a stable segmented flow with enhancement of heat transport was identified. The increase of the oil fraction was accompanied by an increase of the recirculation zone in the transverse direction to the flow and by higher velocity gradients inside the zones thus resulting in enhanced mixing of water across the channel. The LIF measurements indicate that for the same total flow rate, the local temperature of the water region separating the droplets will be higher than in the case of single phase liquid flow. For segmented flow, up to a four-fold increase of the Nusselt number compared to pure water flow was observed.
Heat Transfer Characteristics of Fuel Based Emulsions With Alcohol Additives
Authors: Yamasaki Hiroshi , Suzuki Kei , Furukawa Shigeki , Lai Ming-Chia
Published: March 2011

Experimental study has been conducted to study the heat transfer characteristics of the hydrocarbons as the base fuel and water emulsions. The focus is mainly paid on the effect of the ambient emulsion temperature and alcohol additives on the boiling and its inception of fine water droplet dispersed in the emulsions. Heat transfer was measured using an electrically heated horizontal thin Pt wire. It is concluded that there is the transition point in the heat transfer coefficient, which is just above the boiling point of emulsified component of water. The heat transfer of water in fuel emulsions and effect of alcohol additives on it were also revealed.
Evaporation of Pure Liquids at High Prandtl Numbers in a Scale-Up Capable Falling Film Evaporator
Authors: Arndt Stefanie , Scholl Stephan
Published: March 2011

In industrial application heat transfer to temperature sensitive products in falling film evaporation is often linked to the evaporation at elevated viscosities. In the present study a scale-up capable falling film evaporator has been used to investigate the heat transfer to liquids with Prandtl numbers up to 150. The focus was on heated falling films during surface evaporation. Film-Reynolds numbers were varied from 48 to 10,000. As pure liquids water and cyclohexanol were used. In the results a distinct transition zone between laminar and turbulent flow can be observed for elevated Prandtl numbers. The comparison to literature models shows that more parameters have to be taken into account to properly predict the heat transfer in falling film evaporators for different equipment and fluids. The optical monitoring of the film on the inside of the evaporation tube through an endoscope showed that the fully turbulent regime could not been reached for high viscosities.
Cluster Mechanism of New Phase Formation on a Surface of Solid-State Body
Authors: Egorov Boris V. , Geliev Alexander V. , Do Xuan D. , Markachev Yuri E. , Shogin Dmitry M.
Published: March 2011

One of the aspects of aircraft icing problem study is research of the mechanism of new phase formation on a surface of solid-state body. Similar problems arise at studying of heat and mass transfer in devices of film type in chemical industry. In the present work discussion of new approach to the given problem, dealing with cluster mechanism of new phase formation on a surface of a solid-state body is offered. The research carried out shows the need to pay attention to a role of molecular clusters in developing of gas flow turbulence that essential impact on processes of inter-phase heat and mass transfer.
Heat Transfer Behaviour and Flow Field Characterisation of Impinging Synthetic Jets for a Wide Range of Parameters
Authors: Farrelly Rayhaan , McGuinn Alan , Persoons Tim , Murray Darina
Published: March 2011

Impinging synthetic jets are considered as a potential solution for convective cooling, in applications that match their main characteristics (high local heat transfer rates, zero net mass flux, scalability, active control). Nevertheless the understanding of heat transfer to synthetic jets falls short of that available for steady jets. To address this, this paper uses detailed flow field measurements to help identify the main heat transfer mechanisms in impinging synthetic jets. Local heat transfer measurements have been performed for an impinging round synthetic jet at a range of Reynolds numbers between 1000 and 3000, nozzle to plate spacings between 4D and 16D and stroke lengths (L0) between 2D and 32D. The heat transfer results show evidence of distinct regimes in terms of L0/D and L0/H ratios. Based on appropriate scaling, four heat transfer regimes are identified which justifies a detailed study of the flow field characteristics. High speed particle image velocimetry (PIV) has been employed to measure the time-resolved velocity flow fields of the synthetic jet to identify the flow structures at selected L0/H values corresponding to the identified heat transfer regimes. The flow measurements support the same regimes as identified from the heat transfer measurements and provide physical insight for the heat transfer behaviour.
Effect of Heat Flux on Droplet Entrainment Using Annular Flow Dryout Model
Authors: Ahmad Masroor , Burlutskiy Evgeniy , Walker Simon P. , Hewitt Geoffrey F.
Published: March 2011

Annular film dryout depends upon the competition of entrainment, deposition and evaporation processes between the droplet-laden core and wall liquid film. In this paper, effect of heat flux on droplet entrainment is analyzed by modeling different low and high pressure diabatic annular flow experiments numerically using an annular flow dryout model (AFM). Overall, the AFM predicted the experimental data reasonably accurately. It is concluded that at high pressures increasing heat flux may enhance net entrainment considerably but this effect diminishes at low pressures.
Flow Pattern Transition and Related Heat Transfer in an Annular Gap Around a Truncated Conical Rotor in a Cylinder
Authors: Matsumoto Ryosuke , Ozawa Mamoru
Published: March 2011

Viscous flows in a concentric annulus with rotating inner cylinder was characterized by Taylor-Couette flow. When the inner cylinder is replaced with a truncated conical rotor, the effect of imbalanced centrifugal force induces the meridional circulation flow with a single toroidal vortex. Under non-isothermal condition at cooling the rotor and heating the vessel wall, the flow pattern is drastically changed with the thermal conditions and the rotating speed. This paper describes the vortex flow-pattern transition and related heat transfer of the mixed convection in such a system. The temperature and flow field were visualized by means of thermo-sensitive liquid-crystal tracers. At the same time, the local heat transfer distributions on the cylindrical vessel wall were measured under a uniform heat flux by the electrically-conductive transparent film. Based on the experimental findings is described the interaction between the heat transfer and the thermal flow field during the unsteady mixed convection in the annular gap.
Fluorescence Thermometry for Measuring Wall Surface and Bulk Fluid Temperatures
Authors: Kim Myeongsub , Yoda Minami
Published: March 2011

Cooling the next generation of microelectronics with heat fluxes of more than 1 kW/cm2 over hot spots less than 103 μm2 in area will require new single- and two-phase thermal management technologies with micron-scale addressability. Thermal transport models using heat transfer correlations may be the most efficient approach for the initial design and optimization of such micron-scale heat exchangers which will likely involve arrays of microchannels. It is unclear, however, whether classic macroscale convective heat transfer correlations are applicable to these devices given their complex geometries and the possibility of significant thermal coupling between channels. There is therefore a need for new techniques that can measure both bulk fluid and wall surface temperatures at micron-scale spatial resolution without disturbing the flow of coolant. We report here the use of a nonintrusive technique, fluorescence thermometry (FT), to determine bulk fluid temperatures and, for the first time, wall surface temperatures, with a spatial resolution of O(10 μm) for water flowing through a heated channel. Fluorescence thermometry is typically used to estimate temperature distributions in water flows based on variations in the emission intensity of a fluorophore dissolved in the water. The accuracy of FT can be improved by taking the ratio of the emission signals from two different fluorophores (dual-tracer FT, or DFT) to eliminate variations in the signal due to (spatial and temporal) variations in the excitation intensity. In this work, two temperature-sensitive fluorophores, fluorescein and sulforhodamine B, with emission intensities that increase and decrease, respectively, with increasing temperature, are used to further improve the accuracy of the temperature measurements. Temperature profiles were measured in the steady Poiseuille flow of water at Reynolds numbers of 3.3 and 8.3 through a 1 mm square channel heated with a thin-fim heater. Temperatures in the bulk flow were measured using DFT with an average uncertainty of 0.2 °C at a spatial resolution of 30 μm. Fluid temperatures within the first 0.3 μm next to the wall were measured using evanescent-wave illumination of a single temperature-sensitive fluorophore with an average uncertainty of less than 0.2 °C at a spatial resolution of 10 μm. The results are compared with numerical predictions, which suggest that the fluid temperatures within 0.3 μm are effectively the wall surface temperature.
Performance of a Heat Sink With Interrupted and Staggered Elliptic Fins
Authors: Gururatana Suabsakul , Li Xianchang
Published: March 2011

The power density of electronic devices has been increasing along with the rapid technology development. Cooling of electronic systems is therefore essential in controlling the component temperature and avoiding any hot spot. Heat sinks are commonly adopted in electronics cooling together with different technologies to enhance heat transfer process. Fin-based heat sinks are commonly designed so that coolants (gas or liquid) are forced to pass through the narrow straight channel. A driving fan is then needed to overcome the viscous pressure loss and maintain the coolant flow. As part of effort to improve the heat sink performance, this study simulated the details of the flow and temperature fields of heat sinks with interrupted and staggered elliptic fins cooled by forced convection. The focus of this study lies on three scenarios: Heat transfer before the flow reaches the periodic condition in the flow direction; effect of the heat sink base surface on flow and heat transfer; and conjugate heat transfer between convection and heat conduction inside the fins. In addition, studies were also conducted on the effect of the Reynolds number. The results of this paper can help design heat sinks for electronics cooling by employing the new concept of interrupted and staggered fins.
A Study of the Thermal-Hydraulic Performance and System-Level Effects of Aluminum Oxide-Propanol Nanofluid
Authors: Sommers Andrew D. , Yerkes Kirk L. , Runyon Augusta R.
Published: March 2011

The application of nanofluids in real systems could lead to smaller, more compact heat exchangers and reductions in material cost. However, few studies have been conducted which have carefully measured the thermo-physical properties and thermal performance of these fluids as well as examine the system-level effects of using these fluids in traditional cooling systems. In this work, dilute suspensions of 10 nm aluminum oxide nanoparticles in propanol (0.5 wt%, 1 wt%, and 3 wt%) were investigated. Changes in density, specific heat, and thermal conductivity with particle concentration were measured and found to be linear whereas changes in viscosity were nonlinear and increased sharply with particle loading. Nanofluid heat transfer performance data were generally commensurate with that measured for the baseline. For the 1 wt% concentration, a small but significant enhancement in the heat transfer coefficient was recorded for 1800 < Re > 2800 which is attributed to an earlier transition to turbulent flow. In the case of high particle loading (i.e. 3 wt%), the thermal performance was observed to deteriorate with respect to the baseline case. Discoloration of the fluid was also observed after being cycled at high flow rates and increased temperature.
Numerical and Experimental Investigation of the Behaviour of Non-Contacting Droplets During the Reflood Phase After a LOCA
Authors: Chatzikyriakou D. , Walker S. P. , Hale C. P. , Lakehal D. , Hewitt G. F.
Published: March 2011

During the reflood phase, following a Large Loss of Coolant Accident (LOCA) in a Pressurised Water Reactor (PWR), a flow of vapour containing small saturated droplets (of order 1mm diameter) is responsible for the precursory cooling before the quenching of the rods by the liquid water. The main mechanism for this cooling process is convective heat transfer to the vapour, with the vapour being cooled by the evaporation of the entrained saturated droplets. If the fuel rod temperature exceeds the Leidenfrost [1] value, the droplets do not wet it, but rather bounce off from it due to the formation of a vapour film between the droplet and the metal. Secondary cooling of the rods is provided by this process. Both the hydrodynamics of these impacts and the droplet-vapour-wall heat transfer mechanisms affect the degree of this secondary cooling. We investigate here the heat transfer attributable to such droplets in typical reflood conditions by a combination of new experimental observations, numerical simulations and correlations based on earlier studies [2], [3], [4]. Using an infrared technique we obtain spatial temperature measurements of the area below a non-contacting droplet [5]. At the same time we observe the hydrodynamic behaviour of the droplet by means of a high speed optical camera. Combining our experimental results with an analytically-computed droplet-wall interaction rate we estimate the cooling by those droplets in typical reflood conditions. These measurements are used for the validation of numerical simulations which are conducted using the CFD code TransAT©, to support its application to cases beyond the present reach of the experimental technique.