bluuebunny/crossref_metadata_2025 huggingface dataset:
Control of Heat Transfer in Separated Flows With the Help of Miniturbulators
Authors: Bogatko T. V. , D’yachenko A. Yu. , Terekhov V. I. , Yarygina N. I.
Published: March 2011

In the present paper, the influence of vorticity layer on the turbulent separated flow and heat transfer in a cross-flow cavity was experimentally examined. The vorticity layer was generated by a miniturbulator installed in the upstream region of the flow separation point. As the miniturbulator, a small cross-flow rib was used whose height was one order of magnitude smaller than the cavity depth. The variable parameters were the angle of wall inclination in the cavity, the rib height, and the rib-to-cavity separation. The additional vortical disturbances introduced into the recirculation zone were found to exert an appreciable influence on the vortex formation pattern and on the distribution of pressure and heat-transfer coefficients. The experimental data were compared to computation data obtained with the Fluent 6 software. Numerical data on the dynamic and thermal characteristics of flows past a system comprising a sudden pipe expansion and a low-height diaphragm installed in the upstream region of the flow separation point are also presented. It is found that such a diaphragm, used to modify the characteristics of the separated flow, results in a change of the length and intensity of the eddying flow in the separation zone. The vortex sheet produced by the diaphragm interacts with the primary eddy, makes the separation zone more extended, and shifts, even to a greater extent, the point at which the heat-transfer coefficient attains its maximum in the downstream direction. The maximum heat-transfer coefficient turns out to be increased in comparison with undisturbed flow. Both the location of the diaphragm and the diaphragm height strongly affect the heat-transfer characteristics.
Turbulent Heat Transfer Over a Moving Plate Due to Impinging Slot Jets
Authors: Chattopadhyay Himadri , Benim Ali Cemal
Published: March 2011

In the present paper, turbulent heat transfer characteristics of slot jets impinging on a moving surface up to a Reynolds number of 50,000 have been studied. The turbulent flow field was resolved using the realizable k-ε model due to Shi et al. (1995) after rigorously establishing the adequacy of the model by comparison with large-eddy simulation data. A periodic element from a jet-bank configuration was chosen in the direction of the surface movement. The distribution of heat transfer on impinging surface is found to be significantly affected by the plate motion. However, the mean velocity distribution along vertical direction in the stagnation region is not affected by the plate motion. With increasing surface motion, the initial symmetric distribution changes to an inclined-S type pattern in the direction of the surface movement up to a certain level of surface velocity and the average heat transfer reduces. When the surface motion crosses this level, the net heat transfer starts increasing. The amount of heat transfer was found to be linked with the level of turbulent kinetic energy near the impingement region. The surface velocity at which the heat transfer reaches the value corresponding to the fixed surface value increases with increasing Reynolds number.
Nanofluid-Based Absorbers for High Temperature Direct Solar Collectors
Authors: Lenert Andrej , Perez Zuniga Yoshio S. , Wang Evelyn N.
Published: March 2011

Solar collectors with surface receivers have low overall energy conversion efficiencies due to large emissive losses at high temperatures. Alternatively, volumetric receivers promise increased performance because solar radiation can be transferred into a fluid medium, which subsequently reduces the concentrated heat at the surface. Nanofluid-based direct solar receivers, where nanoparticles in a liquid medium can scatter and absorb solar radiation, have recently received interest to efficiently distribute and store the thermal energy. We present a combined modeling and experimental study to investigate the efficiency of fluid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A simple one-dimensional transient heat transfer model was developed to compare idealized surface receivers to idealized volumetric receivers. In the limit of idealized behavior, volumetric receivers were shown to be more efficient than selective-surfaces for high concentration levels (C > 100) and/or tall receiver designs (H > 10 cm). Furthermore, volumetric receivers allowed the working range of the fluid to be extended to higher temperatures while maintaining relatively high receiver efficiency. We also designed and custom built a cylindrical receiver to experimentally demonstrate the concept of nanofluid-based receivers and validate the modeling efforts. We explored the effect of particle characteristics such as size, distribution, and material properties, as well as collector parameters such as absorbing depth and level of solar concentration. The work offers design guidelines for the development of efficient volumetric receivers for future solar thermal energy conversion systems.
Experimental Investigation of Boiling Heat Transfer and Pressure Drop in Swirl Flow Under Conditions of One-Sided Heating
Authors: Komov Alexander T. , Dedov Alexey V. , Varava Alexander N. , Yagov Victor V.
Published: March 2011

The paper gives the basic results of experimental investigation of hydrodynamics and nucleate boiling heat transfer in heat-absorbing devices of the ITER thermonuclear reactor, which are subjected to one-sided heating. Experimental data are obtained on pressure drop, heat transfer and critical heat fluxes (CHF) in the following range of parameters of water flow: pressure p = 0.7–2.0 MPa, mass flux G = 340–25 000 kg/(m2s), and water temperature at the inlet Tin = 20–60 °C. The investigations were performed for flow swirled by tapes with different values of flow swirl coefficient, as well for test sections without tape. The test-sections with a cooled channel of 2.0, 4.0, and 8.0 mm inside diameter were used. One-sided heating of the test sections were provided by the scanning electron beam injected by the electron gun. More than 500 experimental points on nucleate boiling heat transfer were obtained. The predicting equation for nucleate boiling heat transfer at one-sided heating has been proposed. A simple approach was used for the calculation of CHF. The predicting equation for CHF reliably generalizes the existing experimental data.
Simultaneous Heat and Mass Transfer in Inclined Channel With Asymmetrical Conditions
Authors: Oulaid Othmane , Benhamou Brahim , Galanis Nicolas
Published: March 2011

This work deals with a numerical study of simultaneous heat and mass transfer with phase change in an inclined channel formed by two parallel plates. The lower one is covered by a thin liquid water film and the upper one is considered impermeable. The plates are maintained at a constant temperature TW. Ambient air with uniform dry bulb temperature Tin and relative humidity φin enters the channel with a uniform upward velocity Uin. The liquid film is assumed to be extremely thin and its temperature is equal to the wall temperature. Steady state conditions are considered and the flow is assumed to be laminar. Viscous dissipation, radiation heat transfer and other secondary effects (pressure work, energy transport by the inter-diffusion of species, Dufour and Soret effects) are neglected. The physical properties are taken constant except for the density in the body forces, which is considered to be a linear function of temperature and mass fraction. Results show that buoyancy forces have an important effect on the hydrodynamic, thermal and mass fraction fields and this effect depends on the channel inclination. A flow reversal chart and analytical correlations for the corresponding critical values of the thermal and solutal Grashof numbers are presented for different channel inclinations.
Heat Transfer Enhancement by Artificial Roughness at Reynolds Numbers Related With Laminar and Transitional Regimes for High-Viscous Liquids
Authors: Gotovsky Mikhail A. , Isaev Sergey A.
Published: March 2011

Artificial roughness (AR) formed by annular rolling or dimpling is one of the most well-known examples of Reynolds analogy (RA) breaking in a favor of heat transfer. Surfaces which can be called ARPD - Artificial Roughness, are manufactured by wall Pressure Deformation. ARPD surfaces have some similar thermal hydraulic properties which permit to unite them in the common group. General characteristics of ARPD surfaces are considered here. But the main attention is paid to such surface performance for coolants with high Prandtl numbers. It is important that Reynolds numbers must be close sufficiently to its critical value for smooth tube. Some experimental data show that extremely high heat transfer enhancement ratio can be obtained under such conditions for substantially less pressure loss ratio increase. Similar qualitative results obtained for several types of ARPD — dimpled, annular rolled and spirally corrugated tubes — are demonstrated. These results are related partially with critical Reynolds number decrease and partially with specific character of heat transfer evolution in laminar–turbulent transition range for high-viscous liquids. Such enhancement method can be used effectively for heat exchangers with high-viscous liquids (oils, for example).
Study on the Route to Chaos of Natural Convection in Cylindrical Envelope With an Internal Concentric Cylinder With Slots
Authors: Zhang Kun , Yang Mo , Zhang Yu Wen , Lu Mei
Published: March 2011

Natural convection heat transfer was investigated numerically in a cylindrical envelope with an internal concentric cylinder with slots. Governing equations are discretized using finite volume method and solved using SIMPLE algorithm with QUICK scheme. Calculations were performed on certain parameters with a Rayleigh number varying from 700 to 20000. The effect of the Rayleigh number on the route to the chaos of the system was analyzed by the phase space of velocity at the sample point. The results show that the system can reach to steady state and symmetric when the Rayleigh number is below 700, and to steady state and asymmetric when the Rayleigh number is equal to 1000. For a Rayleigh number ranged between 1500 and 3000, an asymmetric periodical solution is obtained although the initial field and boundary conditions were symmetric. As the Rayleigh numbers increase further, a quasi-periodic solution of the system is achieved at Ra = 2000. There is one more bifurcation and period doubling at successive critical values of Rayleigh numbers from to. It is ascertained that periodicity is lost at Ra = 20000. The results show that the oscillatory flow undergoes several bifurcations and ultimately evolves to a chaotic flow.
Flow Characteristics and Structures of Three-Dimensional Unsteady Thermal Convection in a Container
Authors: Fujita Satoshi , Tanigawa Hirochika , Funaki Jiro , Hirata Katsuya
Published: March 2011

In this study, we numerically investigates the flow and thermal characteristics of the three-dimensional thermal convection in a cubic cavity heated below in the gravitational field, concerning about spatially-averaged kinetic energy K, Nusselt number Nu and flow structure. We assume Prandtl number Pr = 7.1 (water) and Rayleigh number Ra = 1.0×104 – 3.5×105. As a result, we have specified two of three important values of the Rayleigh number which demarcate different flow bifurcations and are referred to as the second and third critical Rayleigh numbers Rac2 and Rac3. We have found that Rac2 and Rac3 are roughly 2.6×105 and 3.1×105, respectively. We have observed a histerisis effect upon the value of Rac2 with chaotic behaviour at Ra ≈ Rac2, and revealed flow structures. In addition, we investigate the relationship between Ra and the oscillatory-convection frequency. The increasing rate of the Kmean with increasing Ra shows a different manner from that of Nuinflow-ave, mean. That is, the former is progressive and the latter is asymptotic, as Ra increases. Both the values of Kmean and Nuinflow-ave, mean in oscillatory flow tend to be smaller than those in steady flow, respectively. Then, there exist small jumps/drops of Kmean and Nuinflow-ave, mean at Ra = Rac2.
Infrared Based Wall Shear Stress Measurement Techniques
Authors: Rudolph Ilka , Reyer Matthias , Nitsche Wolfgang
Published: March 2011

The paper presented here introduces two novel, infrared based wall shear stress measurement techniques. The first provides wall shear stress visualizations with a high spatial and temporal resolution as well as spatial quantitative information. The other technique enables sensor based measurements of the wall shear stress magnitude and direction. Both techniques are based on the close link between momentum and heat transport in the boundary layer and correlate the surface temperature distribution of a heated surface (first technique) or a heated spot (second technique), which is measured using infrared thermography, with the wall shear stress. For the spatial qualification and quantification, the temporal surface temperature evolution of a heated structure subjected to a flow is linked to the wall shear stress distribution. The second, sensor based technique heats a small spot on an otherwise unheated surface. The temperature distribution, or thermal tuft, around this heated spot is closely related to the local wall shear stress magnitude and direction. Results are presented for both techniques and compared to reference measurements and visualizations respectively. Reference measurements of the wall shear stress were obtained using a skin friction balance, oilflow visualizations were used as a reference visualization technique.
Correlations for Saturated Critical Heat Flux in Microchannels
Authors: Wu Zan , Li Wei
Published: March 2011

Experimental results of the saturated-flow boiling critical heat flux (CHF) in microchannels for both multi- and single-channel configurations were obtained from the literature. The collected database contains 629 data points, covering 5 refrigerants, nitrogen, and water, for a wide range of operational conditions, and different micro-channel dimensions. The whole database was analyzed by using four empirical correlations to verify their respective accuracies. However, none of the existing correlations could predict the entire database precisely. A saturated CHF correlation was proposed by using boiling number, length-to-diameter ratio, and exit quality. The new correlation can predict the overall micro-channel database accurately on the whole. It predicts almost 97.0% of the non-aqueous data (except R12 data points located in the macro-scale region) and 94.0% of the water data within the ± 30% error band.
Experimental Investigation of a Contact Line Dynamic Induced by Liquid Evaporation Heat and Mass Transfer
Authors: Tachon Loi¨c , Guignard Stephan , Tadrist Loune`s
Published: March 2011

The aim of the present study is to propose an original method to compute the shape of the interface in the vicinity of a contact line after a film break. The principle of the experiment consists in a controlled volume of evaporating liquid (HFE7100) deposited in a millimetric cylindrical vertical well. During the evaporation process, the liquid-gas interface takes a toroidal shape delimiting an axisymmetric film with the well bottom and side walls. At the beginning, the evaporation process occurs without triple line on the bottom. Then a circular triple line appears on the bottom. The meniscus interface shape and the triple line location are instantaneously measured by laser sheet sounding from underneath and numerical inversion: This technique analyses the variation of the light intensity along a laser sheet due to its refraction trough consecutive interfaces (solid-liquid-vapor).The laser sheet impacts intensity on a perpendicular screen is inverted. The inversion result is the shape of the interface and the position of the triple line during the evaporation process.
Homogeneous Nucleation Boiling During Jet Impingement Quench of Hot Surfaces
Authors: Hasan Mohammad Nasim , Monde Masanori , Mitsutake Yuichi
Published: March 2011

In the present study, we focused on the rapid liquid heating process and the subsequent boiling explosion that occurs when a liquid jet comes in contact with a very hot surface during jet impingement quenching. Assuming the liquid jet as 1-D semi-infinite solid during its brief contact with the surface, a model has been proposed based on the ideas of 1-D heat conduction and homogeneous nucleation. In this model, a liquid control volume having the size of a critical cluster at the boundary is considered and the corresponding energy balance is obtained by accounting for the two parallel competing processes that takes place inside the liquid control volume, namely, transient external heat deposition and internal heat consumption due to liquid superheat, bubble nucleation and subsequent growth. Results obtained are presented in terms of the liquid temperature escalation within the control volume, the limit of maximum attainable liquid temperature and the time necessary to reach the temperature limit at the boiling explosion. The boiling explosion condition as defined in the present model is also compared with the theoretical boiling explosion condition denoted by the upper bound of evaporative heat flux across the liquid-vapor interface, qmax,max. The time duration of the solid-liquid contact prior to the boiling explosion at different surface temperatures as obtained by the proposed model may be helpful for better understanding the possible surface temperature oscillations due to repetitive solid-liquid contact in the first few microseconds of jet impingement quenching.
Numerical Simulation of Droplet Impact and Solidification Including Thermal Shrinkage in a Thermal Spray Process
Authors: Alavi Sina , Passandideh-Fard Mohammad
Published: March 2011

In this paper, we performed a numerical study on the effects of thermal shrinkage on deposition of molten tin and nickel droplets on a steel substrate in thermal spray processes using Volume-of-Fluid (VOF) method. Thermal shrinkage is a phenomenon caused by variation of density during solidification and cooling of molten metals. In our model, the Navier-Stokes equations along with energy equation including phase change are solved using a 2-D axisymmetric mesh. We used the VOF method to track the free surface of droplet. For solidification, we used an enthalpy-porosity formulation. The simulations performed in this study are accomplished using a commercial code (Fluent). Results of these scenarios are presented: the normal impacts of 2.7mm tin droplets at 1m/s and 2m/s, initially at 240°C, onto a 27°C steel substrate. When the droplet impacts the substrate with a velocity of 1m/s, the final splat has a single cavity inside due to shrinkage. In other cases with the scales of a typical thermal spray process, the results of normal impact of nickel droplets with a velocity of 73m/s, initial temperature 1600°C and diameter 60μm to steel substrate with different temperatures are presented. In these cases shrinkage decreases the droplet splashing on the substrate.
Sensitivity of Diffusive-Convective Transition to the Initial Conditions in a Transient Be´nard-Marangoni Problem
Authors: Che´nier Eric , Desceliers Christophe , Delcarte Claudine , Trouette Benoiˆt , Doumenc Fre´de´ric , Guerrier Be´atrice
Published: March 2011

Sensitivity of a transient Be´nard-Marangoni problem is studied using stochastic models to simulate the uncertainties of thermal initial conditions. Using different assumptions, three probabilistic models are developed and compared. Statistics are performed on flow velocities and temperatures. Transitions are examined with respect to the stochastic models.
Heat Transfer During Drop Impact Onto Wetted Heated Smooth and Structured Substrates: Experimental and Theoretical Study
Authors: Gambaryan-Roisman Tatiana , Budakli Mete , Roisman Ilia V. , Stephan Peter
Published: March 2011

Spray cooling is a very effective means of heat removal from hot surfaces. Its efficiency can be further improved using structured wall surfaces. One of the fundamental processes governing spray cooling is an impact of a single cold droplet onto a heated wetted wall. The hydrodynamics of drop impact governs the transient heat transport in the film and in the wall. We study hydrodynamics and heat transfer during impact of a single drop onto heated smooth and structured heated plates. The temperature distribution in the heated plates has been measured with seven thermocouples. The splash dynamics and the evolution of interface temperature distribution have been visualized using high-speed infrared thermography. The film thickness evolution in the inner region has been measured using chromatic confocal imaging technique. Initial film thickness and drop impact parameters have been varied in the experiments. The evolution of the temperature distribution at the liquid-gas interface and the instationary temperature distribution in the heated plate depend on the initial film thickness, impact parameters and the plate topography. A self-similar analytical solution of the full Navier-Stokes equations and of the energy equation is obtained for the velocity and temperature fields in the spreading film. The theory allows prediction of the contact temperature and the residual film thickness.
Heat Transfer Enhancement From a Circular Cylinder to Distilled Water by Ultrasonic Waves at Different Subcooling Degrees
Authors: Baffigi Federica , Bartoli Carlo
Published: March 2011

The main aim of this work is to investigate experimentally the influence of ultrasonic waves, on the heat transfer enhancement, from a stainless steel circular cylinder to distilled water, in subcooled boiling conditions. This study has carried on for a few years at the Department of Energetics “L.Poggi”. The effect was observed since ’60s: different authors had investigated the cooling effect due to the ultrasonic waves at different heat transfer regimes, especially from a thin platinum wire to water. They had found out that the highest heat transfer coefficient enhancement was been in subcooled boiling conditions. So this paper has the purpose to clarify the physical phenomenon and optimize a large range of variables involved in the mechanism. It reports the experimental results obtained with ultrasound at the frequency of 38 kHz, at two different subcooling degrees, ΔTSUB = 25 and 35°C. The heat fluxes applied on the cylinder, the ultrasonic generator power, Pgen and also the placement of the heater inside the ultrasonic generator tank, are been varied. The ultrasonic waves seem to be very useful for a practical application in the last generation electronic components cooling: they need dissipating huge heat fluxes and avoiding high temperatures (≈150°C), after that they could damage themselves.
Development of Combined Particle Image Velocimetry and Particle Image Thermography for Air Flows
Authors: Schmeling Daniel , Czapp Marek , Bosbach Johannes , Wagner Claus
Published: March 2011

Simultaneous measurements of instantaneous velocity and temperature fields of air flows by means of Particle Image Velocimetry (PIV) and Particle Image Thermography (PIT) enables highly demanded studies on thermal plumes, their dynamics and the resulting heat transfer for Pr ≈ 0.7. Thereby, small particles of thermochromic liquid crystals (TLCs), which reveal temperature depending reflection properties are used as tracer particles for combined PIT and PIV. The feasibility of the method is demonstrated in a Rayleigh-Be´nard convection experiment in a cubical enclosure. Furthermore, a new particle generator being able to produce continuously very small monodisperse droplets of TLCs has been designed. The improvement of the developmental process for mixed and Rayleig-Be´nard convection studies is discussed. Thereby, special focus is laid on the production process of small TLCs, the generation of monodisperse acetone-TLC droplets and the temperature depending colour play of the produced particles.
Bio-Heat Transfer Model of Human Eye Subjected to Retinal Laser Irradiation
Authors: Narasimhan Arunn , Jha Kaushal Kumar
Published: March 2011

Retinopathy is a surgical process in which maladies of the human eye are treated by laser irradiation. A two-dimensional numerical model of the human eye geometry has been developed to investigate steady and transient thermal effects due to laser radiation. In particular, the influence of choroidal pigmentations and choroidal blood convection — parameterized as a function of choroidal blood perfusion are investigated in detail. The Pennes bio-heat transfer equation is invoked as the governing equation and a finite volume formulation is employed in the numerical method. The numerical model is validated with available experimental and two-dimensional numerical results. For a 500 μm diameter spot size, laser power of 0.2 W, with 100% absorption of laser radiation in the Retinal Pigmented Epithelium (RPE) region, the peak RPE temperature is observed to be 175 °C at steady state, with no blood perfusion in choroid. It reduces to 168.5 °C when the choroidal blood perfusion rate is increased to 23.3 kgm−3s−1. However, under transient simulations, the peak RPE temperature is observed to remain constant at 104 °C after 100 ms of the laser surgery period. A truncated three-dimensional model incorporating multiple laser irradiation spots is also developed to observe the spatial effect of choroidal blood perfusion. For a circular array of seven uniformly distributed spots of identical diameter and laser power of 0.2 W, steady and transient temperature evolution are presented with analysis.
Investigation of Interfacial Thermal Resistance on Nano-Structures Using Molecular Dynamics Simulations
Authors: Singh Navdeep , Banerjee Debjyoti
Published: March 2011

Due to their very high thermal conductivity carbon nanotubes have been found to be an excellent material for thermal management. Experiments have shown that the heaters coated with carbon nanotubes increase the heat transfer by as much as 60%. Also when nanotubes are used as filler materials in composites, they tend to increase the thermal conductivity of the composites. But the increase in the heat transfer and the thermal conductivity has been found to be much less than the calculated values. This decrease has been attributed to the interfacial thermal resistance between the carbon nanotubes and the surrounding material. MD simulations were performed to study the interfacial thermal resistance between the carbon nanotubes and the liquid molecules. In the simulations, the nanotube is placed at the center of the simulation box and a temperature of 300K is imposed on the system. Then the temperature of the nanotube is raised instantaneously and the system is allowed to relax. From the temperature decay, the interfacial thermal resistance between the carbon nanotube and the liquid molecules is calculated. In this study the liquid molecules under investigation are n-heptane, n-tridecane and n-nonadecane.
Spectral Gas Absorption Coefficient Model Effects on Radiative Source Term in a 2D Axisymmetric Diffusion Flame
Authors: Mossi Anderson C. , Barve Vinayak V. , Galarc¸a Marcelo M. , Vielmo Hora´cio A. , Franc¸a Francis H. R. , Ezekoye Ofodike A.
Published: March 2011

A calculation of the radiative source term in combustion processes is an important part of the simulation process, because high temperatures are involved and the coupling of radiation to chemistry affects the overall flame characteristics. While relatively simple gas absorption coefficient models have been used in the recent past, it is becoming clearer that more accurate gas models alter the distribution of radiative sources in the flame. To accurately evaluate the radiative losses, it is necessary to use gas models in which the gas absorption coefficient is wavelength dependent. Such analyses can be computationally expensive depending on the particular treatment of the spectral dependence. It is important to understand the relative costs and benefits of different treatment of these effects. In this work, the divergence of the radiative heat flux is calculated for a two-dimensional cylindrical axisymmetric chamber using four different models: a simple gray gas model, the weighted-sum-of-gray-gases (WSGG) model, the spectral line-based weighted-sum-of-gray-gases (SLW) model, and the cumulative wavenumber (CW) model. The gray gas model and the WSGG model are widely used in recent studies and in most commercial software, because they are simple to implement and provide fast results. In general, however, they are not able to accurately predict the radiative losses. On the other hand, the SLW and CW models detail the variations of the absorption coefficient with the wavelength, and can give more accurate answers for the radiative source term, but require bigger computational effort. The divergence of the radiative flux predictions are compared with these four models, using temperature and concentration fields obtained from previous numerical simulations. The overall differences in radiation properties and in the overall cost of computations are detailed.
Turbulence Characteristics in a Natural Convection Boundary Layer Above a Heated Round Plate
Authors: Hattori Yasuo , Suto Hitoshi
Published: March 2011

A turbulent natural-convection boundary layer in air near the upper surface of a heated round plate is experimentally investigated. The heated round plate, the diameter of which is 300 mm, is horizontally placed in a test section of chamber. The upper-surface temperature is 473 K. Instantaneous temperature and two-dimensional velocity vectors near the upper surface are measured by using a cold wire and a particle image velocimetry, respectively. The measurements reveal a multi-layer structure near the upper surface. In the lower layer, velocity fluctuation of horizontal component is active compared with that of vertical component. This intermittently causes the incursion of low temperature fluids. On the other hand, in the upper layer, the intensity of vertical velocity fluctuation with high temperature fluid is much larger than that of horizontal velocity fluctuation. This multi-layer structure is mainly generated by the large-scale fluid motions, such as winding of high temperature fluids and entrainment of low temperature fluids.
High Temperature Facility Under Vacuum for the Thermal Characterization of Anisotropic Materials
Authors: Souhar Youssef , Remy Benjamin , Degiovanni Alain
Published: March 2011

New applications in Aerospace or in Energy industries require the development of new insulating materials at high temperature exhibiting anisotropic properties. Their thermal characterization requires the development of new experimental facilities. The In-Plane measurements can be very difficult due to the thermal coupling that can appear between the sample and air in its vicinity, especially for low conductivity materials. We will show that this problem can be solved either through theoretical models or by working under vacuum. A new facility developed in LEMTA for the thermal characterization at high temperature and under vacuum of anisotropic materials is presented. This type of measurements allows us to get rid of convection effects and non-homogenous oxidation of the material. This method is fast and accurate thank to the inverse method based on an analytical model and allow to estimate through only one experiment the three diffusivities in each direction.
Vortex Dynamics of Synthetic Jets: A Computational and Experimental Investigation
Authors: Arik Mehmet , Utturkar Yogen
Published: March 2011

Seamless advancements in electronics industry resulted in high performance computing. These innovations lead to smaller electronics systems with higher heat fluxes than ever. However, shrinking nature of real estate for thermal management has created a need for more effective and compact cooling solutions. Novel cooling techniques have been of interest to solve the demand. One such technology that functions with the principle of creating vortex rings is called synthetic jets. The jets are meso-scale devices operating as zero-net-mass-flux principle by ingesting and ejection of high velocity working fluid from a single opening. These devices produce periodic jet streams, which may have peak velocities over 20 times greater than conventional, comparable size fan velocities. Those jets enhance heat transfer in both natural and forced convection significantly over bare and extended surfaces. Recognizing the heat transfer physics over surfaces require a fundamental understanding of the flow physics caused by micro fluid motion. A comprehensive computational and experimental study has been performed to understand the flow physics of a synthetic jet. Computational study has been performed via Fluent commercial software, while the experimental study has been performed by using Laser Doppler Anemometry. Since synthetic jets are typical sine-wave excited between 20 and 60 V range, they have an orifice peak velocity of over 60 m/s, resulting in a Reynolds number of 2000. CFD predictions on the vortex dipole location fall within 10% of the experimental measurement uncertainty band.
Multi-Factor Coupling Analysis of Altitude Performance of Lubrication System in Helicopter
Authors: Li Ming , Gao Hongxia , Yu Jianzu , Zhou Yi , Niu Xuemin
Published: March 2011

Good altitude performance of lubrication system ensures flying performance of the helicopter. Both engine and reducer need lubricating oil to cool themselves, to keep stable and decrease malfunction in flying tasks. In this article, a multi-factor coupling calculation method is put forward for altitude performance analysis of lubrication system in “Zhi X” main reducer system, taking into consideration of heat transfer and flow resistance characteristics of every component, off-design performance of the fan and dynamic characteristic of heat exchanger. Performance parameters of each component are calculated, such as temperature, pressure and flux. The results are in good agreement with numerical results of FM2. Then an analysis software for altitude performance of lubrication system is developed for “Zhi X” based on the multi-factor coupling calculation method, which is regarded as a scientific and practical tool for altitude performance prediction and integrated design of lubrication system in helicopter.
Boiling Heat Transfer in Microtubes at High Flow Rates
Authors: O¨zdemir Mehmed Rafet , Kos¸ar Ali
Published: March 2011

Boiling heat transfer is an important heat removal mechanism for cooling applications in micro scale and finds many applications. Many studies were conducted to shed light on boiling heat transfer in microchannels. They were concentrated on saturation boiling at low mass fluxes (G<1000 kg/m2s). In the current study, the emphasis is on high mass fluxes unlike in the literature. Thus, the current study addresses to the lack of information about boiling heat transfer at high flow rates and aims at presenting necessary experimental data. For this, an experimental study was conducted at high flow rates in microtubes. The microtube sizes were used as 250 micrometers. Deionized water was used as working fluid, and the test section was heated by Joule heating. Mass flux was changed from 1000 kg/m2s to 7500 kg/m2s, and heat transfer coefficients and qualities were deduced from local temperature measurements. The effects of mass velocity and quality on boiling heat transfer coefficient were investigated.
Experimental Investigation of Low Weber Number Post-Impact Drop Spreading on Solid Substrates
Authors: Jog Milind A. , Manglik Raj M.
Published: March 2011

The post-impact spreading and recoil behaviors of droplets of pure liquids (water and ethanol) and aqueous solution of Triton X-100 (a surfactant) on a dry horizontal hydrophilic (glass) substrate are investigated for low Weber numbers. The evolution of drop shape during spreading and recoil are captured using a high-speed (4,000 frames per second) digital video camera. Digital image-processing was used to determine the spread and height of the liquid film on the surface from each frame. Unlike pure liquids, the liquid-gas interfacial tension for surfactant solution is a function of surface age, where surface tension is that of the solvent at zero time and then reaches an equilibrium value with increasing surface age. Furthermore, the equilibrium surface tension is a function of the surfactant concentration, which decreases from that of the solvent at zero concentration to that at the critical micelle concentration (CMC), and remains essentially constant thereafter. The surface tension of aqueous Triton X-100 solution varies from that of pure water to nearly that of ethanol. As such the comparison of transient droplet-impact-spreading-recoil behavior of the three liquids, or their temporal variations of the spread and the flattening factor, provides a basis for understanding the role of dynamic surface tension and surface wettability.
Optical Fiber Ring Sensor for Measuring Temperature
Authors: Rahman Anisur
Published: March 2011

A new Optical-Fiber Ring Resonator Temperature (OFRRT) sensor has been presented for measuring temperature. The sensor is designed based on Morphology Dependent Resonances (MDR) which follows Maxwell’s electromagnetic equations. The OFRRT sensor is designed based on optical ring deformation that leads to change in output signal. A finite element model of the prototype sensor shows encouraging result to detect the temperature. The sensor design is very simple and robust. The developed expression for size parameter for transverse electric wave is very simple and can be used to characterize the resonances in dielectric micro-ring resonator. Parametric studies of the sensor have been performed by using finite element analysis. The proposed OFRRT sensor can be used to measure the temperature of micro-structures from remote places. More importantly, the OFRRT sensor can also be used for biomedical applications.
Steady State and Pulsed Thermal Simulation of Gateless AlGaN/GaN Heterostructures
Authors: James William T. , Venkatachalam Anusha , Graham Samuel
Published: March 2011

Due to the complex nature of the failure in reliability of GaN electronics, a multiphysics model was developed to describe the evolution of electrical, thermal, and mechanical stress fields in devices under operational conditions. Electrical, thermal and mechanical characteristics of an AlGaN/GaN transmission line measurement (TLM) structure were modeled under steady state and pulse operational conditions for varying input powers. The electric field and power dissipation in the device was determined by solving Maxwell’s equations while the peak temperature was obtained by solving the thermal energy equation. By coupling the solutions, the impact on device current-voltage characteristics was obtained. Under pulse conditions, devices showed higher currents and lower temperature rises owing to reduced Joule heating compared to steady state. Finally, a one way coupling to a thermo-mechanical model of the device allowed for the analysis of the thermal-induced stress fields which have an impact on the overall device reliability. It was found that the peak temperature and stress in the simulated device at 40 V in steady state was 765 K and −0.60 GPa, while in pulsed mode the peak temperatures and stress were significantly lower at 562 K and −0.38 GPa for a 5 μs pulse and 652 K and −0.48 GPa for a 25 μs pulse.
Numerical Simulation of Thermofluid Characteristics of Two-Phase Slug Flow in Microchannels
Authors: Mehdizadeh A. , Sherif S. A. , Lear W. E.
Published: March 2011

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number (Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nu number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.
Investigation of High Temperature Nanofluids for Solar Thermal Power Conversion and Storage Applications
Authors: Shin Donghyun , Jo Byeongnam , Kwak Hyun-eun , Banerjee Debjyoti
Published: March 2011

The aim of this study is to investigate the enhancement of thermal properties of various high temperature nanofluids for solar thermal energy storage application. In concentrating solar power (CSP) systems, the thermo-physical properties of the heat transfer fluids (HTF) and the thermal energy storage (TES) materials are key to enhancing the overall system efficiency. Molten salts, such as alkali nitrates, alkali carbonates, or eutectics are considered as alternatives to conventional HTF to extend the capabilities of CSP. However, there is limited usage of molten salt eutectics as the HTF material, since the heat capacity of the molten salts are lower than that of conventional HTF. Nanofluid is a mixture of a solvent and nanoparticles. Well dispersed nanoparticles can be used to enhance thermo-physical properties of HTF. In this study, silica (SiO2) and alumina (Al2O3) nanoparticles as well as carbon nanotubes (CNT) were dispersed into a molten salt and a commercially available HTF. The specific heat capacity of the nanofluids were measured and applicability of such nanofluid materials for solar thermal storage applications were explored. Measurements performed using the carbonate eutectics and commercial HTF that are doped with inorganic and organic nano-particles show specific heat capacity enhancements exceeding 5–20% at concentrations of 0.05% to 2.0% by weight. Dimensional analyses and computer simulations were performed to predict the enhancement of thermal properties of the nanofluids. The computational studies were performed using Molecular Dynamics (MD) simulations.
Thermal Conductivity Characterization and Modeling of P-Type Metal/Semiconductor Nanocomposites
Authors: Xu Dongyan , Feser Joseph P. , Zhao Yang , Lu Hong , Burke Peter , Gossard Arthur C. , Majumdar Arun
Published: March 2011

Semiconductor alloys with epitaxially embedded nanoparticles have been shown to be very promising materials for thermoelectric energy conversion applications. In this work, we report on thermal conductivity characterization of two classes of p-type nanoparticle-in-alloy composite materials: compensated InGaAs semiconductor matrix with randomly distributed ErAs nanoparticles, and GaSb and its alloys with embedded ErSb nanoparticles. The three omega method is used to measure thermal conductivity of all materials. It is shown that thermal conductivity of compensated p-type ErAs:InGaAs is comparable to the n-type ErAs:InGaAs and it reduces with the increase in the erbium concentration. ErSb:GaSb nanocomposites are intrinsically p-type and show a thermal conductivity substantially lower than the pure GaSb compound. By comparing nanostructured samples from alloyed (InGaSb) and unalloyed (GaSb) matrix materials, we show that alloying is complimentary to the role of the nanostructure in reducing thermal conductivity. We also discuss Boltzmann transport modeling that indicates an optimum nanocrystal size, and the prospects for further reductions in the lattice thermal conductivity.
Effects of Protrusions on Conjugate Heat Transfer in a Microtube or Microchannel
Authors: Rahman Muhammad M. , Injeti Phaninder
Published: March 2011

Effects of protrusions on heat transfer in a microtube and in a two-dimensional microchannel of finite wall thickness were investigated for various shapes and sizes of the protrusion. Calculations were done for incompressible flow of a Newtonian fluid with developing momentum and thermal boundary layers under uniform and discrete heating conditions. It was found that the local Nusselt number near the protrusions changes significantly with the variations of Reynolds number, height, width, and distance between protrusions, and the distribution of discrete heat sources. The results presented in the paper demonstrate that protrusions can be used advantageously for the enhancement of local heat transfer whereas the global performance may be enhanced or diminished based on the tube geometry.
Heat-Transfer at Supercritical Pressures
Authors: Pioro Igor
Published: March 2011

The first works devoted to the problem of heat transfer at supercritical pressures started as early as the 1930s. E. Schmidt and his associates investigated free-convection heat transfer to fluids at the near-critical point with the objective of developing a new effective cooling system for turbine blades in jet engines. In the 1950s, the idea of using supercritical “steam”-water appeared to be rather attractive for steam generators / turbines to increase thermal efficiency of fossil-fired power plants. Intensive work on this subject was mainly performed in the former USSR and in the USA in the 1950s–1980s. Therefore, the most investigated flow geometry at supercritical pressures is circular tubes with water as the coolant. Currently, using supercritical “steam” in fossil-fired power plants is the largest industrial application of fluids at supercritical pressures. At the end of the 1950s and the beginning of the 1960s, some studies were conducted to investigate the possibility of using supercritical water as a coolant in nuclear reactors. Several concepts of nuclear reactors were developed. However, this idea was abandoned for almost 30 years, and then regained momentum in the 1990s as a means to improve the performance of water-cooled nuclear reactors. Main objectives of using supercritical water in nuclear reactors are increasing the efficiency of modern nuclear power plants, which is currently 30–35%, to circa 43–50%, and decreasing operational and capital costs by eliminating steam generators, steam separators, steam dryers, etc. Therefore, objectives of the current paper are to assess the work that was performed and to understand specifics of heat transfer at supercritical pressures.