bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat Transfer Enhancement in a Tube Fitted With Twisted Tape Swirl Generator
Authors: Eiamsa-ard Smith , Seemawute Panida , Wongcharee Khwanchit
Published: March 2011

Flow friction, heat transfer and thermal performance characteristics in a tube fitted with peripherally-cut twisted tape (PT) have been experimentally investigated. The twist ratio (y/W) of twisted tape was varied from 3 to 5 while the depth of peripheral cut ratio (d/W) and the width of peripheral cut ratio (w/W) were both kept constant at 0.22. The experiments were conducted for the Reynolds number ranging from 5100 to 19,700, using water as a working fluid. The plain tube and the tube equipped with the typical twisted tape (TT) with three different twist ratios (y/W = 3, 4 and 5) were also tested for comparison. The obtained results reveal that the use of PT enhances Nusselt number up to 211% and 138% compared to those of the plain tube and the tube with TT, respectively. It is also found that heat transfer rate increases with decreasing twist ratio. Additionally, the performance evaluation to assess the real benefits in using the PT as heat transfer enhancer has also been determined.
Thermal Performance Assessment of Turbulent Flow Through Dimpled Tubes
Authors: Nivesrangsan Pornchai , Pethkool Somsak , Nanan Kwanchai , Pimsarn Monsak , Eiamsa-ard Smith
Published: March 2011

This paper presents the heat transfer augmentation and friction factor characteristics by means of dimpled tubes. The experiments were conducted using the dimpled tubes with two different dimpled-surface patterns including aligned arrangement (A-A) and staggered arrangement (S-A), each with two pitch ratios (PR = p/Di = 0.6 and 1.0), for Reynolds number ranging from 9800 to 67,000. The experimental results achieved from the dimpled tubes are compared with those obtained from the plain tube. Evidently, the dimpled tubes with both arrangements offer higher heat transfer rates compared to the plain tube and the dimpled tube with staggered arrangement shows an advantage on the basis of heat transfer enhancement over the dimpled tube with aligned arrangement. The increase in heat transfer rate with reducing pitch ratio is due to the higher turbulent intensity imparted to the flow between the dimple surfaces. The mean heat transfer rate offered by the dimpled tube with staggered arrangement (S-A) at the lowest pitch ratio (PR = 0.6), is higher than those provided by the plain tube and the dimpled tube with aligned arrangement (A-A) at the same PR by around 127% and 8%, respectively. The empirical correlations developed in terms of pitch ratio (PR), Prandtl number (Pr) and Reynolds number, are fitted the experimental data within ±8% and ±2% for Nusselt number (Nu) and friction factor (f), respectively. In addition, the thermal performance factors under an equal pumping power constraint of the dimple tubes for both dimpled-surface arrangements are also determined.
Combined Measurement of Temperature and Velocity Distribution of Fluid Flow
Authors: Someya Satoshi , Li Yanrong , Tominaga Kaoru , Tange Manabu , Okamoto Koji
Published: March 2011

This paper proposes a combined method for two-dimensional temperature and velocity measurement using temperature sensitive particles (TSPs), a pulsed ultraviolet laser, and a high-speed camera. TSPs respond to temperature changes in a flow and can also serve as tracers for the velocity field. The luminescence from the TSPs was recorded at 15,000 frames per second as sequential images for a lifetime-based temperature analysis. These images were also used for the particle image velocimetry calculations. The temperature field was estimated using six images, based on the lifetime method. The decay curves for various temperature conditions fit well to exponential functions, and from these the decay constants at each temperature were obtained. The proposed technique was applied to measure the temperature and velocity fields in natural convection driven by a Marangoni force and buoyancy in a rectangular tank. The accuracy of the temperature measurement of the proposed technique was ±0.35∼0.40°C.
Mass Nature of Heat and Its Applications I: Motion of Thermomass Fluid and General Heat Conduction Law
Authors: Guo Zeng-Yuan , Cao Bing-Yang
Published: March 2011

The concept of thermomass is defined as the equivalent mass of thermal energy according to the Einstein’s mass-energy relation. Hence, the phonon gas in dielectrics can be regarded a weighty, compressible fluid. Heat conduction in the medium, where the rest mass lattices or molecules acts the porous framework, resembles the gas flow through the porous medium. Newton mechanics has been applied to establish the equation of state and the equation of motion for the phonon gas as in fluid mechanics, since the drift velocity of a phonon gas is normally much less than the speed of light. The momentum equation of the thermomass gas, including the driving, inertial and resistant forces, is a damped wave equation, which is in fact the general conduction law. This is because it reduces to the CV (Cattaneo-Vernotte) model or the single phase-lag model as the heat flux related inertial terms are neglected, and reduces to Fourier’s heat conduction law as all inertial terms are neglected. Therefore, the underlying physics of Fourier’s heat conduction law is the balance between the driving force and the resistant force of the heat motion, and Fourier’s law will break down when the inertial force is comparable to the resistant force, for instance, in the case of ultra-short pulse laser heating or heat conduction in carbon nanotubes at ultra-high heat flux.
Atomization and Droplet Dispersion of Low-Momentum Water Jet by High-Speed Crossflow Air Stream
Authors: Liu Cuicui , Jiang Zeyi , Liu Huafei , Zhang Xinxin , Xiang Shunhua
Published: March 2011

In this paper, a low-momentum water liquid jet emanating transversely into a high-speed air stream is investigated analytically and numerically. Viscous instability followed by Rayleigh-Taylor instability is used in the jet breakup analysis to obtain the Sauter Mean Diameter and the droplet group velocity after the breakup. With the analytical results, droplet dispersion in the air stream is simulated by the coupled Eulerian-Lagrangian approach, in which the root-normal distribution is adopted to represent the droplet diameter distribution. Water flux distribution and spray angle are obtained and validated by experimental data. The results show that the air velocity is a dominant factor on the Sauter Mean Diameter and droplet group velocity in the water jet breakup process and the spray angle is influenced by the water mass flux.
Mass Nature of Heat and Its Applications II: Non-Fourier Heat Conduction in Carbon Nanotubes
Authors: Cao Bing-Yang , Hou Quan-Wen
Published: March 2011

Carbon nanotubes (CNTs) have attracted much attention in nanotechnology fields because of their unique thermal properties. The thermal conductivity of CNTs was reported to be as high as several thousand W/mK. The heat flux in CNTs can reach 109−1012 W/m2 under normal heat conduction conditions. In this paper we demonstrate that Fourier’s heat conduction law breaks down for so high heat flux. Based on a novel concept of thermomanss, which is defined as the equivalent mass of thermal energy according to Einstein’s mass-energy relation, heat conduction in CNTs can be regarded as the flow of a phonon gas governed by its mass and momentum conservation equations like in fluid mechanics. The momentum conservation equation, including driving force, inertial force and resistance terms, reduces to Fourier’s law as the heat flux is not very high and the inertial force of phonon gas is negligible with respect to the driving force. However, Fourier’s law of heat conduction no longer holds if the heat flux is very high such that the inertial force of the phonon gas is not negligible. The heat conduction behavior deviates from Fourier’s law even for steady state conditions so that the heat conduction is characterized by a non-linear relationship between the heat flux and the temperature gradient. In this case, the thermal conductivity of the CNTs can no longer be defined as the ratio of the heat flux to the temperature gradient in experiments or numerical computations. Furthermore, the ratio of the phonon gas velocity to the thermal sound speed can be defined as the thermal Mach number. Heat flow in CNTs will be choked, just like gas flows in a converging nozzle, and a temperature jump will be observed when the thermal Mach number equals or exceeds unity. In this case, the predicted temperature profile of the CNTs based on Fourier’s law is much lower than that based on the thermomass theory considering a CNT electrically heated by high-bias current flows. The intrinsic thermal conductivity can be only calculated by the present thermomass theory, rather than Fourier’s heat conduction law. The present study shows that the thermomass based theory should be applied for high flux heat conduction in CNTs where Fourier’s heat conduction law breaks down.
Investigation of a Laminar Boundary Layer at Hydrogen Combustion
Authors: Volchkov Eduard P. , Lukashov Vladimir V. , Terekhov Vladimir V.
Published: March 2011

An experimental and numerical study of a laminar boundary layer with combustion has been carried out at hydrogen and nitrogen fuel mixture blow through a porous plate. At that main flow velocity ranged from 2 to 4 m/sec and the mass fraction of hydrogen in the fuel from 1 to 11%. The lower limit of stable combustion depending on the blow intensity and hydrogen content in the fuel mixture was obtained experimentally. Data on the temperature distribution in the boundary layer have been obtained and analyzed. The simulation results show that in this range of parameters combustion occurs in the kinetic mode.
Potential Distribution Measurement in PEFC Electrolyte Membrane for Membrane Degradation Analysis
Authors: Okano Yuuki , Ooishi Masato , Ohma Atsushi , Fushinobu Kazuyoshi , Okazaki Ken
Published: March 2011

Potential distribution in the electrolyte membrane in the MEA of PEFC has been measured in order to help understand a recently suggested membrane degradation mechanism caused by Pt band. A thin metallic probe, Pt or Au, is inserted in membrane to measure the potential distribution. The measured profile shows abrupt change at certain position, and the position is influenced by the operation and cell design parameters. A simple theoretical model to predict the potential distribution is developed and showed good agreement with experimental results.
Effect of Periodic Pulsations on Heat Transfer in Simultaneously Developing Laminar Flows: A Numerical Study
Authors: Mehta Balkrishna , Khandekar Sameer
Published: March 2011

Heat transfer in the channels and ducts are well understood in the steady laminar flows for engineering applications. In contrast, unsteady flows have potential for research as many aspects of such flows are still unclear. Periodic pulsating flow in a channel is a kind of unsteady flow which requires further investigation because (i) many upcoming applications, especially in mini-micro scale engineering domain e.g. enhanced mixing, MEMS applications, bio-fluidic devices and thermal management of electronics etc. (ii) critical review of literature reveals that there is prevailing confusion related to the species transport coefficients. Thus, need for a systematic parametric study, both numerical and experimental, cannot be overemphasized. In this paper, two different configurations of laminar pulsatile internal flow, i.e. Case (i): unidirectional flow with axial superimposed pulsations (flow in circular axisymmetric tube) and, Case (ii) unidirectional flow with superimposed transverse pulsations (parallel plates) have been numerically scrutinized. Effect of frequency (Womersley number, Wo), Prandtl number (Pr), Reynolds number (Re) and amplitude ratio, on the instantaneous and time averaged heat transfer and friction factor (Poiseuille number) is studied. It is found that the change in species transport is either marginal or highly limited and is primarily occurring in the developing length of the channel/ plate. Nusselt number under pulsating conditions in the fully developed flow regime is not very different from its steady counterpart. Enhancement of species transport due to such periodic pulsatile internal flows, over and above the non-pulsatile regular flow conditions, is questionable, and at best, rather limited. Enhancement in heat transfer is seen in Case (ii) under certain operating conditions. This latter configuration is more attractive than the former and further optimization studies are required to improve understanding.
Shape Optimization of a Rotating Rectangular Channel With Pin-Fins by Kriging Method
Authors: Moon Mi-Ae , Husain Afzal , Kim Kwang-Yong
Published: March 2011

This paper presents numerical optimization of a rotating rectangular channel design with the staggered arrays of pin-fins using Kriging meta-modeling technique. In the reference case, the channel aspect ratio (AR) is 4:1, the pin length to diameter ratio (H/D) is 2.0, and the pin spacing to diameter ratio is 2.0 in both the streamwise and spanwise directions. The rotation number is 0.15, while the Reynolds number based on hydraulic diameter is fixed at 10,000. Rotation of the channel slightly reduces the heat transfer on the leading surface and increases it on the trailing surface due to Coriolis effects. Two non-dimensional variables, the ratio of the height to diameter of the pin-fin and the ratio of the spacing between the pin-fins to diameter of the pin-fins, are chosen as design variables. The objective function defined as a linear combination of heat transfer and friction loss related terms with a weighting factor is selected for the optimization. Twenty training points generated by Latin hypercube sampling (LHS) are evaluated by three-dimensional Reynolds-averaged Navier-Stokes (RANS) analysis with the shear stress transport (SST) model for the turbulence closure. The predictions of objective function by Kriging meta-modeling at optimum point show reasonable accuracy in comparison with the values calculated by RANS analysis. The results of optimization show that the cooling performance of the optimized shape is enhanced significantly through the optimization.
Three-Dimensional Modelling of Heat Transfer in Micro-Channels With Synthetic Jet
Authors: Lee Ann , Timchenko Victoria , Yeoh Guan H. , Reizes John A.
Published: March 2011

An in-house computer code is developed and applied to investigate the effect of a synthetic jet on heat transfer rates in forced convection of water in silicon micro-channels etched in the rear side of the silicon substrate. To account for the deflection of the membrane located at the bottom of the actuator cavity, a moving mesh technique to solve the flow and heat transfer is purposefully adopted. The governing equations are transformed into the curvilinear coordinate system in which the grid velocities evaluated are then fed into the computation of the flow in the cavity domain thus allowing the conservation equations of mass, momentum and energy to be solved within the stationary computational domain. The fully three-dimensional model considers the SIMPLE method to link the pressure and velocity. A heat flux of 1 MW/m2 is applied at the surface of the top of the silicon wafer and the resulting complex, conjugate heat transfer through the silicon substrate is included. The hydrodynamics feature of the flow is validated against existing experimental results and verified against numerical results from commercial package ANSYS CFX 11.0. Good agreement has been achieved. To track the development of the flow and heat transfer when the actuator is switched on, numerical results of 20 full cycles of the actuator are simulated. When the actuator is switched on, noticeable temperature drop is observed at all points in the substrate from those which existed when there has been a steady water flow in the channel. At the end of 20th cycle of actuation, the maximum temperature in the wafer has reduced by 5.4 K in comparison with the steady flow values. In comparison with the two-dimensional study which account for 17K reduction, it indicates that synthetic jet has only smaller beneficial cooling and has been over-estimated in the previous two-dimensional study.
High-Temperature Heat Transfer Investigations Using Heterogeneous Gradient Sensors
Authors: Sapozhnikov Sergey Z. , Mityakov Vladimir Yu. , Mityakov Andrey V. , Snarskii Andrey A. , Zhenirovskyy Maxim I.
Published: March 2011

The local heat flux measurements are limited by low working temperature of the gradient heat flux sensors (GHFS) [1–3]. The novel heterogeneous sensors (HGHFS) made from metal-metal or metal-semiconductor layered composites (so-called anisotropic thermoelements) have high temperature level of 1300 K and more. Theory of the HGHFS allows to choose thickness and angle of inclination for the layers of composite, and to forecast volt-watt sensitivity. The sensitivity of metal-metal sensors is typically on the order of 0.02 to 0.5 mV/W, and it is much beyond when semiconductors are used. HGHFS are used for a first time for heat flux measurements in the furnace of the industrial boiler which is in operating of the Thermal Power Plant (fossil fuel power plant) in the city of Kirov (Russia). The local heat flux at the surface of refractory-faced water wall is measured in different regimes of operating. It is also shown that HGHFS may be used as indicator of furnace slugging. Small sizes (minimally 2×2×0.1 mm) and high working temperature of the HGHFS are useful for heat transfer investigations.
Falling Film Evaporation of Water on Horizontal Configured Tube Bundles
Authors: Li Wei , Wu Xiaoyu , Luo Zhong
Published: March 2011

This paper reports an experimental study on falling film evaporation of water on 6-row horizontal configured tube bundles in a vacuum. Three types of configured tubes, Turbo-CAB-19fpi and −26fpi, Korodense, including smooth tubes for reference, were tested in a range of film Reynolds number from about 10 to 110. Results show that as the falling film Reynolds number increases, falling film evaporation goes from tubes partial dryout regime to fully wet regime; the mean heat transfer coefficients reach peak values in the transition point. Turbo-CAB tubes have the best heat transfer enhancement of falling film evaporation in both regimes, but Korodense tubes’ overall performances are better when tubes are fully wet. The inlet temperature of heating water has hardly any effects on the heat transfer, but the evaporation pressure has controversial effects. A correlation with errors within 10% was also developed to predict the heat transfer enhancement capacity.
Effect of Water Temperature on Spray Cooling Heat Transfer on Hot Steel Plate
Authors: Lee Jungho , Yu Cheong-Hwan , Park Sang-Jin
Published: March 2011

Water spray cooling is an important technology which has been used in a variety of engineering applications for cooling of materials from high-temperature nominally up to 900°C, especially in steelmaking processes and heat treatment in hot metals. The effects of cooling water temperature on spray cooling are significant for hot steel plate cooling applications. The local heat flux measurements are introduced by a novel experimental technique in which test block assemblies with cartridge heaters and thermocouples are used to measure the heat flux distribution on the surface of hot steel plate as a function of heat flux gauge. The spray is produced from a fullcone nozzle and experiments are performed at fixed water impact density of G and fixed nozzle-to-target spacing. The results show that effects of water temperature on forced boiling heat transfer characteristics are presented for five different water temperatures between 5 to 45°C. The local heat flux curves and heat transfer coefficients are also provided to a benchmark data for the actual spray cooling of hot steel plate cooling applications.
Pyrolysis of Organic Compounds Using Incomplete Combustion on Ceramics Bed
Authors: Sakai Seigo , Abo Ryo , Araki Kuniomi , Amino Nobushige
Published: March 2011

Presently, recycling and disposal of organic wastes need a large amount of labor and energy due to such problems as the dioxin emission. It is thought that to develop a new pyrolysis mechanism for organic wastes will contribute to solving the environmental problem. Therefore, we propose a safe and inexpensive pyrolysis mechanism in which incomplete combustion occurs in small area on ceramics bed (thin burning layer), because ceramics are highly thermo-stability and have good thermal radiation characteristics at high temperature. Organic compounds pyrolyze in the thin burning layer only.
Theoretical Investigation of Radiative Transport and Heat Transfer of Nanofluids in a Direct Solar Absorption Collector
Authors: Zhu Qunzhi , Li Yongguang , Mu Lijuan , Cui Yun
Published: March 2011

Conversion of solar energy into heat is a common way to explore the clean and renewable energy source. Generally, a spectrally selective coating should be deposited to the solar-absorbing surface of the solar collector in order to reduce thermal loss emitted away from the surface in the form of thermal radiation. Nanofluids can be filled into a flat plate solar collector to absorb the solar radiation directly. The absorbance of the nanofluid in the solar spectrum may be tuned by various nanoparticles of different materials, sizes and mass fractions. In this paper, a theoretical investigation is carried out to model absorption, scattering and extinction of solar radiation within the nanofluid and to model the heat transfer within the liquid film in the direct solar absorption collector. The predicted extinction coefficient is much lower than the measured value in visible spectrum for the SiO2-water nanofluid. The calculated average temperature increase in the outlet of the collector using the measured radiative properties agrees with the measured value while that applying the predicted radiative properties is significantly lower. Nevertheless, the agreement in the predicted and the measured extinction coefficients of the TiO2-water nanofluid is reasonably good within the solar spectrum. The calculated average temperature increase is close to the measurement value. This work is helpful to understand radiative transport and heat transfer of direct solar absorption collectors using nanofluids.
Numerical Simulation of Heat and Mass Transfer in Metal Hydride Hydrogen Accumulators of Different Complex Designs
Authors: Lazarev Dmitriy , Artemov Valeriy , Yankov Georgiy , Minko Konstantin
Published: March 2011

A three-dimensional mathematical model of unsteady heat and mass transfer in porous hydrogen-absorbing media, accounting for presence of “passive” gas admixtures, is developed. New technique for evaluation of effective thermal conductivity of porous medium, which consists of microparticles, is suggested. Effect of “passive” gas admixtures on heat and mass transfer and sorption rate in metal hydride reactor is analyzed. It is shown that decrease of effective thermal conductivity and partial hydrogen pressure under decrease of hydrogen concentration effect on the hydrogen sorption rate considerably. It is disclosed that an intensive 3D natural convection takes place in a gas volume of reactor under certain conditions. Numerical analysis of heat and mass transfer in metal-hydride reactor of hydrogen accumulation systems was done. Sorption of hydrogen in cylindrical reactors with external cooling and central supply of hydrogen are analyzed including reactors with finned active volume and tube-shell reactor with external and internal cooling cartridge matrix. Unsteady three dimensional temperature and concentration fields in solid phase are presented. Integral curves representing the dynamic of sorption and desorption are calculated. Data on efficiency of considered reactors are presented and compared.
Subcooled Flow Boiling With Sintered Porous Coatings in Small Channel
Authors: Sun Yan , Zhang Li , Xu Hong , Ji Dongmei
Published: March 2011

Heat transfer characteristics from sintered microporous coating surfaces are experimentally investigated for subcooled flow boiling in the horizontal, rectangular small channel with the low aspect ratio. The channel has a hydraulic diameter of 1.98 mm and a heated length of 150 mm. Experiments are conducted at atmospheric pressure with fully-degassed deionized water as the working fluid. The liquid mass flux ranges from 93.6 to 187.1 kg/m2s, and the inlet subcooling ranges from 30 to 70 K. The heating surface of the channel is covered with the copper microporous coatings to enhance the nucleate boiling heat transfer. Four coatings sintered with differently sized particles are tested to reveal influences of microporous parameters and identify the optimum structure. A smooth (highly polished) surface is also tested as reference. Although few studies have been performed to quantify the heat transfer enhancement contributed by combining internal porous coatings and small channels, this combination has promising applications in many areas such as air conditioning, chip cooling, refrigeration systems, and many others involving compact heat exchangers.
Flow Field Study in Louvered Fin and Round Tube Heat Exchangers
Authors: Huisseune Henk , T’Joen Christophe , De Jaeger Peter , De Paepe Michel
Published: March 2011

Three-dimensional flow structures influence the heat exchanger’s performance. In this study flow visualization experiments were performed in six scaled-up models of a louvered fin heat exchanger with round tubes. The models have a staggered tube layout and differ only in their fin spacing and louver angle. A water tunnel was designed and built and the flow visualizations were carried out using dye injection. For small Reynolds numbers no horseshoe vortices are developed in front of the tubes and the recirculation regions downstream the tubes are small. As the Reynolds number is increased, the horseshoe vortices become larger and stronger. The recirculation bubbles grow until they cover the entire back of the tube. When the Reynolds number is further increased, the recirculation region becomes unsteady. At the same Reynolds number the vortex strength and the number of vortices in the second tube row is larger than in the first tube row. Reducing the fin pitch suppresses the vortex and wake development. Further it was found that the first unsteady flow patterns appear in the wake of the heat exchanger and these instabilities move upstream with increasing Reynolds number. The onset of unsteadiness is postponed to higher Reynolds numbers when the fin pitch or louver angle is reduced.
Gas-Liquid Distributions in Upward Multi-Pass Channels of Compact Evaporator
Authors: Razlan Zuradzman Mohamad , Isobe Ryota , Mizuno Yasuhiro , Goshima Hiroaki , Hirota Masafumi , Maruyama Naoki , Nishimura Akira
Published: March 2011

The gas-liquid flow distributions in multi-pass upward parallel channels that simulate the evaporator for the automobile air-conditioning system were examined experimentally. Attention was directed to the influences of the backpressure condition at the branch outlets and of the flow-inlet condition at the header entrance on the gas-liquid distributions to the branches. Experiments were conducted in an isothermal air-water flow system. The influence of the backpressure condition on the flow distributions changed depending on the flow-inlet condition. In the stratified-flow inlet, the backpressure condition was highly influential in both the air and water distributions, and the uniform water distribution that was ideal for the evaporators could not be achieved even if air was distributed uniformly to all branches. In the mist-flow inlet, the water distribution was insensitive to the backpressure conditions and its uniformity was improved in comparison with that in the stratified-flow inlet.
Experimental and Numerical Study on Flow Field and Heat Transfer Characteristics for a Periodically Unsteady Impinging Jet
Authors: Zhou Jing-Wei , Geng Li-Ping , Wang Yu-Gang , Hong Fei-Fei
Published: March 2011

An experimental investigation has been carried out to study the effect of unsteady periodically impinging jets on the flow field and heat transfer characteristics. The experiments are performed for steady jets and for typical periodical jets (i.e., sinusoidal and rectangular jets) at frequencies from 1.25 to 40Hz. The periodical jets are produced by a special mass flow rate controller. The investigation shows that the stagnation point heat transfer does not show any enhancement for the periodically impinging jets when the frequency is lower. Various signals of unsteady jets show distinguishing frequency dependences and the rectangular jet, which has a step change in signal function itself, is the most effective one for heat transfer improvement and the degree of enhancement is in the range 30–40% at frequency of 40 Hz. This increase is believed to be caused by higher oscillations and strong entrainments to the ambient fluid. The hotwire anemometry is used to measure the velocity at centerline of the nozzle and PIV is used to measure the phase-locked flow field of the periodically impinging jet. The flow field is also obtained by numerical simulation with CFD.
Single Phase and Flow Boiling Heat Transfer of Water and Ethanol in a Mini-Channel Array
Authors: Alnaser M. W. , Spindler K. , Mu¨ller-Steinhagen H.
Published: March 2011

A test rig was constructed to investigate flow boiling in an electrically heated horizontal mini-channel array. The test section is made of copper and consists of twelve parallel mini-channels. The channels are 1 mm deep, 1 mm wide and 250 mm long. The test section is heated from underneath with six cartridge heaters. The channels are covered with a glass plate to allow visual observations of the flow patterns using a high-speed video-camera. The wall temperatures are measured at five positions along the channel axis with two resistance thermometers in a specified distance in heat flow direction. Local heat transfer coefficients are obtained by calculating the local heat flux. The working fluids are deionised water and ethanol. The experiments were performed under near atmospheric pressure (0.94 bar to 1.2 bar absolute). The inlet temperature was kept constant at 20°C. The measurements were taken for three mass fluxes (120; 150; 185 kg/m2s) at heat fluxes from 7 to 375 kW/m2. Heat transfer coefficients are presented for single phase forced convection, subcooled and saturated flow boiling conditions. The heat transfer coefficient increases slightly with rising heat flux for single phase flow. A strong increase is observed in subcooled flow boiling. At high heat flux the heat transfer coefficient decreases slightly with increasing heat flux. The application of ethanol instead of water leads to an increase of the surface temperature. At the same low heat flux flow boiling heat transfer occurs with ethanol, but in the experiments with water single phase heat transfer is still dominant. It is because of the lower specific heat capacity of ethanol compared to water. There is a slight influence of the mass flux in the investigated parameter range. The pictures of a high-speed video-camera are analysed for the two-phase flow-pattern identification.
Geometry of Plate Fins for Maximizing Heat Transfer
Authors: Karvinen Timo , Karvinen Reijo
Published: March 2011

A method is presented for finding plate fin geometries for maximizing dissipated heat flux. The method is based on approximate analytical solutions of conjugated heat transfer which are utilized in optimization. As a result non-dimensional variables have been found that contain thermal and geometrical properties of the fin and the flow. These variables have a fixed value at the optimal point. The values are given for rectangular, convex parabolic, triangular, and concave parabolic fin shapes for natural and forced convection including laminar and turbulent boundary layers. An essential fact is that there is no need to evaluate convection heat transfer coefficients because they are already included in these variables. Easy-to-use design rules are presented for finding the geometry of fixed volume fins that gives the maximum heat transfer.
Heat Transfer in Annular Channel With Continuous Flow Twisting
Authors: Tarasevich S. E. , Yakovlev A. B.
Published: March 2011

In paper the experimental results on a heat transfer in annular channels with continuous twisting at length at one- and two-phase flows are observed. For a flow twisting the wire was spirally coiled on the central body of the annular channel (diameter of a wire is equal to annular gap altitude). Results of experimental data of a heat transfer of authors and various researchers at a single phase flow in annular channels with a continuous twisting are analyzed. Sampling of diagnostic variables (equivalent diameter and velocity) is spent and generalizing associations for heat transfer calculation on the concave and convex surfaces in a single-phase phase are offered. Also the technique of definition of temperature of the subcooled flow boiling beginning on surfaces of annular channels with a twisting is offered. Features of boiling, origination of heat transfer crisis and results of visualization of a two-phase flow structure in annular channels with twisting are described.
Experimental Investigation on Natural Circulation in a Thermosyphon Reboiler
Authors: Luo Xing , Gao Yanming , Kabelac Stephan
Published: March 2011

Thermosyphon reboilers are widely used in refineries, petrochemical industries and other chemical processes. The liquid product stream coming from the bottom of the vapor-liquid separator is heated in an evaporator consisting of a vertical tube or tube bundle. When the evaporation occurs, the specific volume of the two-phase fluid increases. The upward buoyancy force caused by the density difference between the evaporator and down-flow pipe drives the fluid flowing through the evaporator in to the separator and forms a natural circulation. The experiments were conducted in a pilot scale thermosyphon system in which the evaporator consists of 7 steel tubes (outside diameter 30 mm, wall thickness 2 mm, length 4 m). A mathematical model was set up to simulate the heat transfer and pressure drop, in which empirical equations from literature were used. With the help of the simulation, the flow boiling heat transfer coefficients inside the tubes can be evaluated from a few measured local wall temperatures.
Phonon Transport in Thin Films: A Lattice Dynamics/Boltzmann Transport Equation Study
Authors: Sellan Daniel P. , Turney Joseph E. , Landry Eric S. , McGaughey Alan J. H. , Amon Cristina H.
Published: March 2011

The cross-plane and in-plane phonon thermal conductivities of Stillinger-Weber (SW) silicon thin films are predicted using the Boltzmann transport equation under the relaxation time approximation. We model the thin films using bulk phonon properties obtained from harmonic and anharmonic lattice dynamics calculations. The cross-plane and in-plane thermal conductivities are reduced from the corresponding bulk value. This reduction is more severe for the cross-plane direction than for the in-plane direction. For the in-plane direction, we find that the predicted reduction in thermal conductivity gives a good lower bound to available experimental results. Including the effects of boundary scattering using the Matthiessen rule, which assumes that scattering mechanisms are independent, yields thermal conductivity predictions that are at most 12% lower than our more accurate results. Neglecting optical phonon modes, while valid for bulk systems, introduces 22.5% error when modeling thin films. Using phonon properties along the [001] direction (i.e., the isotropic approximation) yields bulk predictions that are 15% lower than that when all of the phonon modes are considered. For thin films, this deviation increases to 25%. Our results show that a single bulk phonon mean free path is an inadequate metric for predicting the thermal conductivity reduction in thin films.
Analysis of Temperature Distribution Over a Gas Turbine Shaft Exposed to a Swirl Combustor Flue
Authors: Aghakashi V. , Saidi M. H. , Ghafourian A. , Mozafari A. A.
Published: March 2011

Gas turbine shaft is generally exposed to high temperature gases and may seriously be affected and overheated due to temperature fluctuations in the combustion chamber. Considering vortex flow in the combustion chamber, it may increase the heat release rate and combustion efficiency and also control location of energy release. However, this may result in excess temperature on the combustor equipments and gas turbine shaft. Vortex flow in the vortex engine which is created by the geometry of combustion chamber and conditions of flow field is a bidirectional swirl flow that maintains the chamber wall cool. In this study a new gas turbine combustion chamber implementing a liner around the shaft and liquid fuel feeding system is designed and fabricated. Influence of parameters such as axial position in the combustor direction and equivalence ratio are studied. Experimental results are compared with the numerical simulation by the existing commercial software. Swirl number i.e. ratio of angular flux of angular momentum to angular flux of linear momentum multiplied by nozzle radius, in this study is assumed to be constant. In order to measure the temperature along the liner, K type thermocouples are used. Results show that the heat transfer to the liner at the inlet of combustion chamber is enough high and at the outlet of combustion chamber is relatively low. The effect of parameters such as equivalence ratio and the mass flow rate of oxidizer on the temperature of the liner is investigated and compared with the numerical solution. This type of combustion chambers can be used in gas turbine engines due to their low weight and short length of combustion chamber.
Precursor Film Formation of Spreading Droplet
Authors: Ueno Ichiro , Hirose Kanji , Kizaki Yusuke
Published: March 2011

We focus on a formation process of a quite thin liquid film, known as ‘precursor film,’ ahead a droplet spreading on a smooth solid substrate. The spreading droplet on the solid substrate is accompanied with a movement of a visible boundary line so-called ‘macroscopic contact line.’ Existing studies have indicated there exist two major regions of the precursor film, that is, a region dominated by the fluid dynamics, and a region dominated by the molecular diffusion. Our group has dedicated our special effort to detect the formation process of the precursor film by applying a convectional laser interferometry and a high-speed camera, and to evaluate the spreading rate of the precursor film. In the present study, the existing length of the precursor film at a very early stage of the droplet spreading is evaluated by applying a Brewster-angle microscopy as well as the interferometer. We extend our attention to the advancing process of the precursor film on inclined substrate.
Shape Oscillation of Bubble(s) in Acoustic Field
Authors: Ueno Ichiro , Matsumoto Keishi , Machida Atsumi , Hanyu Tsuyoshi
Published: March 2011

We focus on dynamics of multiple air bubbles exposed to acoustic pressure field while ascending in water. The bubbles are injected into the pool filled with water from a vertical capillary tube, and then the acoustic wave of designated frequency is applied toward the bubbles. The frequency of the acoustic wave is varied from 0.5 to 20 kHz. Volume and shape oscillations of the bubbles are captured by a high-speed camera at frame rates up to 40000 fps with a back-lighting system. Through this system, we succeed in capturing the dynamics of the axisymmetric shape oscillation with a distinct mode number; the bubble exhibits the volume oscillation first with a fundamental frequency f0, and then the gradual transition to the shape oscillation with a fundamental frequency fnm takes place. We evaluate the correlation through the careful observations between the f0 and fnm as f0 ∼ 2.1fnm, which brings almost perfectly confirmation of the prediction through the preceding theoretical works. We also indicate the criterion of the excitation of the shape oscillation by varying the frequencies of the adding pressure field.
Near Field Heat Transfer Between Metamaterials
Authors: Ben-Abdallah Philippe , Joulain Karl , Drevillon Je´re´mie
Published: March 2011

Composite materials structured at the scale of photon’s correlation lengths allow controlling the electromagnetic field they radiate in their surrounding both in far and near fields. Among these media, metamaterials are artificial magnetodielectric materials engineered to provide optical properties that we do not meet as it in nature. In some cases these materials enabled the manifestation of new phenomena such as the negative refraction, super-resolution, reversed Doppler effect and cloaking. In the present work, based on the fluctuational electrodynamics theory of Rytov, we study the non-radiative heat flux exchanged between two parallel metamaterials. We show that heat transfers between two metamaterials can be strongly affected by the presence of magnetic plasmons in the transverse electric polarization. The effect of relative position between the dielectric and magnetic resonances on heat transfer is discussed in details. Our results provide new perspectives for applications in radiative energy transfer and energy conversion technologies.
Effects of Dented Roughness on Laminar Flow and Heat Transfer in Microchannels
Authors: Miao Hui , Huang Yong , Xie Fa , Chen Haigang , Wang Fang
Published: March 2011

Liquid laminar flow and heat transfer characteristics for parallel plate micro-channels consisting of many triangle shape hollows to fit with the etching surfaces are investigated numerically in the present paper. The height of the channel is 50μm, with three different relative depths, three relative spacing, and three oblique angles of the triangle surface, respectively. The 2D N-S and energy equations are solved using a commercial CFD code FLUENT6.3. Water is used as the working fluid, and the Reynolds number ranges from 100 to 1500. The global Poiseuille number and average Nusselt number are obtained. It is shown that the dented shapes cause a modest influence in Poiseuille number, but a greater impact on the Nusselt numbers. In addition, both of Po and Nu increase modestly with Re. The local Nusselt numbers are always lower in dented area and larger in planar area of dented roughness microchannels, than that of conventional smooth value. Finally, geometry parameters have modest impact on heat transfer for dented roughness.
Heat Transfer Characteristics in a Slug Unit
Authors: Babin Valery , Barnea Dvora , Shemer Lev
Published: March 2011

Heat transfer mechanism in two-phase flows and particularly in vertical slug flow is of high interest both for basic hydrodynamic research and for industrial applications. Two-phase slug flow is highly complicated and only a limited number of heat transfer studies have been carried out. The flow field around a single Taylor bubble propagating in a vertical pipe can be subdivided into three distinct hydrodynamic regions: the gas bubble surrounded by a thin liquid film, a highly turbulent liquid wake in the vicinity of the bubble bottom, and the far wake region. Experimental and theoretical works have been presented during the last decades investigating the hydrodynamic parameters in each region. Due to the complexity and intermittent nature of slug flow the existing data on the heat transfer in slug flow is limited to a narrow range of operational conditions. To improve the understanding of the heat transfer mechanism in slug flow a new experimental setup was constructed. A part of the vertical pipe wall was replaced by a thin metal foil heated by electrical current. An IR video camera was used to determine the temporal variation of the instantaneous temperature field along the foil. The video camera was synchronized with a sensor that determined the instantaneous location of the Taylor bubble. The results of the instantaneous heat transfer measurements along the liquid film and in the wake of the Taylor bubble can be correlated with the detailed velocity measurements carried out in the same facility (Shemer et al. 2007). The effect of the local hydrodynamic parameters on the heat transfer coefficient in each region is examined.
Numerical Analysis of the Multiple-Horseshoe-Vortex Effects on the Endwall Heat Transfer in the Leading-Edge Region of a Symmetric Bluff Body
Authors: Levchenya A. M. , Smirnov E. M.
Published: March 2011

The present contribution covers results of a CFD analysis of the 3D flow and endwall heat transfer for a generic junction configuration with a wall-mounted symmetric bluff body experimentally investigated by Praisner and Smith [1, 2]. The computations based on the Reynolds-averaged Navier-Stokes equations (RANS) were performed using two codes of second order accuracy: the in-house code SINF and the commercial package ANSYS-CFX 12.0. For the turbulence closure problem, the Menter SST turbulence model with and without the streamline-curvature correction term was used. The grid sensitivity of solution was studied using a set of grids, the finest of which was of about five million cells. In accordance with the experiments, the computations with both the codes predict development of multiple horseshoe vortices and several bands of high values of the Stanton (St) number upstream of the body leading edge. The spatial relationships between the vorticity in individual planes and the associated endwall Stanton number are generally same in the measurements and in the computations. Some quantitative distinctions between the predictions and experimental data are attributed to the smoothing effect of the low-frequency unsteadiness of the horseshoe vortex system developing in the real flow. Simulation of this effect is outside of RANS-based formulations.
Characteristics of Two-Phase Flow Boiling Heat Transfer and Pressure Drop of NH3, C3H8 and CO2 in Horizontal Circular Small Tubes
Authors: Choi Kwang-Il , Rifaldi Maulana , Pamitran Agus S. , Oh Jong-Taek
Published: March 2011

An experimental investigation on the characteristics of two-phase boiling heat transfer of NH3, C3H8 and CO2 in horizontal small stainless steel tubes of 1.5 and 3.0 mm inner diameters are presented in this paper. Experimental data were obtained over a heat flux range of 5 to 70 kW/m2, mass flux range of 50 to 600 kg/m2s, saturation temperature range of 0 to 12°C, and quality up to 1.0. The test section was heated uniformly by applying an electric current to the tubes directly. Nucleate boiling heat transfer was the main contribution, particularly at the low quality region. Laminar flow was observed in the small tubes. The heat transfer coefficient of the present working refrigerants was compared with other correlations. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in small tubes was developed.
The Influence of External Electric Field on Heat Transfer at Boiling on Non-Uniform Surfaces
Authors: Eronin Alexey A. , Malyshenko Stanislav P. , Zhuravlev Anton I.
Published: March 2011

Characteristics of heat transfer and hydrodynamics of boiling of liquid nitrogen on the surfaces with different types of non-uniformities at the presence of external electric fields are experimentally investigated. It is shown that the formation of field traps is a major mechanism of heat transfer enhancement. And this effect result in noticeable change of two-phase hydrodynamics in vicinity of heated surface.