bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation on Shell-Side Performance of Heat Exchanger With Axially Staggered Overlap Helical Baffles
Authors: Ji Shui , Du Wenjing , Cheng Lin
Published: March 2011

Since its appearance in the 80’s of the 20th century, the heat exchanger with helical baffles (HEHBs) has attracted lots of attention. Benefiting from its relatively simple manufacture procedure and low cost, the heat exchanger with overlapped helical baffles receives much concern. However, there are few reports on the influence of the specific overlap size of helical baffles on the shell-side heat transfer performance and fluid friction property. In this paper, numerical investigation on this open issue is carried out by means of numerical method. The emphasis is laid on the relationship between the overlap size of helical baffles and the shell-side performance. Baffles with the shape of a quarter-ellipse are simulated and the heat-transfer oil is selected as the working fluid. Results show that in the condition of same helix angles and same flow rates, 10% increase of the specific overlap size brings an increase of 23–42% on the pressure drop and an increase of 2–8% on the convection heat transfer coefficient; hence the corresponding heat transfer coefficient pre unit pressure drop is decreased by 11–22%. Compared with the continuously overlap configuration, the axially staggered overlap helical baffles can improve the comprehensive performance of HEHBs on the condition of an identical helical pitch, and hence it is favorable for the situation with strict constrain on pressure drop.
Tornado Like Flows: Experiments on Generation and Stability
Authors: Varaksin Aleksei Yu. , Romash Michael E. , Kopeitsev Viktor N. , Gorbachev Maxim A.
Published: March 2011

In the present work the investigation of possibility of controlling the behavior of free concentrated vortexes generated under laboratory conditions over an underlying surface heated from below was carried out experimentally. The method consists of the creation of obstacles on the vortex path in the form of vertical and horizontal grids with different height and size of cells. Comparison is performed of the mechanisms and of the degrees of effect made by vertical and horizontal grids on the behavior of vortex structures.
The Application of Entransy Dissipation Theory in Optimization Design of Heat Exchanger
Authors: Guo Jiangfeng , Li Mengxun , Xu Mingtian , Cheng Lin
Published: March 2011

The heat conduction and fluid friction are two main detrimental irreversibilities in heat exchanger. According to the entransy dissipation theory, the entransy dissipation can be employed to quantify these two irreversibilities. In the present work, the optimization of heat exchanger design is investigated by applying the entransy dissipation theory and genetic algorithm. Firstly, by taking the total dimensionless entransy dissipation caused by heat conduction and fluid friction as objective function, a single-objective optimization approach to heat exchanger design is developed. However, it is found that the role played by the fluid friction is not fully taken into account in this approach when the working fluid of heat exchanger is liquid. In order to circumvent this problem, the non-dimensional entransy dissipations associated with heat conduction and fluid friction are taken as two separate objective functions and a multi-objective optimization approach to heat exchanger design is established. In comparison with the single-objective optimization approach, the multi-objective optimization approach demonstrates more advantages and flexibilities for heat exchanger design.
Probabilistic Flow-Pattern-Based Heat Transfer for Condensing Intermittent Flow in Smooth Horizontal Tubes
Authors: Meyer Josua P. , Christians Marcel , Liebenberg Leon , van Rooyen Eugene
Published: March 2011

A signal analysis method used for condensing refrigerants is presented, which resulted in a modification of the prediction method for heat transfer based on probabilistic time-fractional results. An objective visual method for discrimination of flow patterns and determining probabilistic time-fractions in intermittent flow was developed. The frequency domain was identified as the main candidate for discrimination of sub-regimes present in intermittent flow. Experimental work was conducted using refrigerants R-22 and R-134a, at an average saturation temperature of 40°C, with mass fluxes ranging from 200–700 kg/m2s, and with test section inlet vapour qualities ranging from 0.65 down to 0.10. These test conditions mostly represented intermittent flow. The modified correlation predicted the experimental data with a mean absolute deviation of 10%.
Effect of Moisture Transport on Mixed Convection in Vertical Annulus of a Heated Clothed Vertical Wet Cylinder in Uniform Cross Wind
Authors: Ghaddar Nesreen , Ghali Kamel
Published: March 2011

Ventilation and heat and moisture transport from a vertical clothed wet and heated cylinder subject to uniform cross wind are studied by modeling and experimentation to investigate the effect of wet cylinder conditions and external wind humidity on renewal rate of the air annulus and its temperature. The coupled parabolic momentum, moisture, and heat balance equations including buoyancy are formulated and solved for uniform surface heating and uniformly wetted inner cylinder boundary to predict the air annulus vertical temperature distribution, moisture evaporation rate from the inner surface, total ventilation through the clothing and the top opening, and sensible and latent heat loss for any given environment conditions, clothing permeability and thermal properties, wind speed and annulus geometry. Experiments were performed in a low speed wind tunnel in which a uniformly heated vertical cylinder covered by a wet stretch fabric enclosed by a clothed outer cylinder is placed in uniform cross flow of known temperature and relative humidity. Good agreement was found between the model and the experimental measurements of sensible and latent heat losses, and air annulus temperature profile. A parametric study is performed to study the effect of moisture on sensible and latent heat loss and the induced mixed ventilation for constant heat flux surface condition of the heated clothed cylinder. The effect of adding wet model effect on the axial mass flow rate in vertical annulus does not exceed 3% in comparison with dry cylinder mixed convection at the same total heat flux at ambient conditions of 10°C and 50% relative humidity. For equal heat input to the wet cylinder in comparison to the dry cylinder at ambient conditions of 10°C and 50% relative humidity, the air layer temperature decreased by 1.51°C and 2.62 °C in air layer temperature for permeabilities of 0.05 and 0.25 m/s when compared to air layer temperature for the dry case.
Numerical Modeling of New Heat Exchanger Materials
Authors: Pelevic´ Nikola R. , van der Meer Theo H.
Published: March 2011

Heat transfer enhancement of carbon-nano fibers (CNF’s) attached on a wall surface within a micro-channel is investigated in this paper using a three-dimensional numerical method. Carbon-nanofibers, also known as graphite nanofibers, can be grown by catalytic decomposition of certain hydrocarbon at a metal surface such as iron, cobalt, nickel and some of their alloys. Typical sizes of CNF’s vary between 2 and 100 nm, with lengths ranging from 5 to 100 μm. Experimental research has shown that the presence of carbon-nano fibers grown on a surface of a fine metallic structure can enhance heat transfer by 50% [2]. These fibers influence the fluid flow, and enlarge the heat exchanging surface. The enhancement depends very much on the carbon-nano fibers density and on the structure of the carbon-nano fibers itself. This numerical study is giving directions in optimizing this new material. A random generation growth model has been developed to generate a stochastic structure of the CNF layer. Next to this a 3D Lattice Boltzmann model has been developed to simulate the heat transfer in a micro-channel flow with the surface covered with CNF’s. The 3D Lattice Boltzmann model has been verified on microchannel flow with heat transfer. Results of the conjugate heat transfer (including CNF’s at the wall) will be presented. The influence of carbon-nano fibers density and their structure on the heat transfer coefficient through the carbon-nano fibers layer is determined.
Experimental Investigations and Modelling of Condensation in Plate Heat Exchangers
Authors: Grabenstein Volker , Kabelac Stephan
Published: March 2011

The use of Plate Heat Exchanger (PHE) as condensers and evaporators increases every year. The easy and modular construction and the high thermal performance at very compact size make the PHE to an ideal heat exchanger for a wide range of applications. Due to the missing of authoritative models for the design of PHEs used as condensers or as evaporators they will typically be constructed larger than needed. For a physically based model more detailed knowledge is needed to take into account the flow and heat interactions during the condensation or evaporation process inside a PHE. In this work the two-phase flow in a PHE is analysed. For this purpose one gap of a PHE was built with two transparent polyurethane plates with the same geometrical parameter than the stainless steel plates of a commercial used PHE. Demineralized-water and air are used to define an exact 2-phase-flow. Measurements of the contact angles and surface energies of the stainless steel plates and the transparent polyurethane plates with different working fluids have been made to show the relation between the wettability and the surface energies of the plates. These measurements show a significant influence of the surface energies on the wettability of the steel and polyurethane plates and therefore it is necassary to take into account the plate material properties for the visualization experiments. A flow pattern map is developed and the typical flow patterns are presented and explained. The defined flow regimes converge with comparable data from the literature. The flow pattern of bubbly flow, irregular bubbly flow, film flow and slug flow can be identified. The film flow occurs over the whole range of the air flow rate and at superficial liquid velocities below 0.1 m/s. Above this limit at low flow rates of the gaseous phase a bubbly flow occurs and with increasing air flow rate the flow pattern changes at a superficial air velocity of 1.5 m/s to an irregular bubbly flow. With further rising of the gas phase fraction the irregular bubbly flow shifts at gas velocities nearly 4 m/s into a transition regime, which is marked by the mixed behaviour of the irregular bubbly and the slug flow regime. Slug flow can be identified when the superficial velocity of the air rises up to 7 m/s.
Boiling Heat Transfer Surface Capable of Transient Heating and Nucleation Control
Authors: Tange Manabu , Takagi Shu , Takemura Fumio , Shoji Masahiro
Published: March 2011

Using MEMS technique, we develop a novel boiling heat transfer surface with three types of circuits: a heater, a bubbling trigger, and thermocouples. This paper presents the design of the heat transfer surface and experimental results of bubbling behavior on this surface during highly subcooled boiling at high heat flux. The heater makes superheated liquid layer transiently. Then the bubbling trigger make a tiny hydrogen bubble playing a role of a nuclei of a boiling bubble. The thermocouple signal reveals a growth of superheated liquid layer, vaporization of the liquid layer beneath the bubble, and rewetting. It has been known that highly subcooled boiling at high heat flux results in atomization of vapor bubbles on heat transfer surfaces due to the violent condensation. Parametric experiments were conducted to clarify the occurrence condition of the atomization by changing heat flux and heating time before nucleation. Bubbling behavior was categorized into four patterns: Oscillating, Not-Oscillating, Single-bubble emission, and Multi-bubbles emission.
Experimental Study on Water Evaporation Enhanced by Surface Heating
Authors: Duan Fei
Published: March 2011

The average evaporation flux was significantly higher while water was heated at a flat surface by two aligned heating elements than that while the water surface was heated 5 mm below in the designed experiments under the similar conditions. The observation is contrary to the Stefan condition. A thermodynamic model is derived from the Gibbs dividing-surface approximation at a flat evaporating surface to demonstrate that an interfacial flow can enhance the evaporation by transporting energy from a high temperature position to a low temperature position. The measures showed that the interfacial liquid temperature was up to 6.9°C higher around the heating wires than that at the centerline between two heating wires as water was heated at the interface. The induced interfacial flow can transport the energy to maintain the evaporation by overtaking the negative thermal conduction to the interface globally.
Numerical Simulation of Droplet Impact on an Isothermal Micro-Grooved Solid Surface
Authors: Tao Yujia , Huai Xiulan , Li Zhigang
Published: March 2011

The process of a micro droplet of distilled water impact on an isothermal micro-grooved solid surface is numerical simulated in this paper. To accurately represent the droplet dynamics, special attention is given to the variation of the droplet pressure and velocity, the movement of the free surface between two fluids and the deforming of the droplet after impact. The Volume Of Fluid method is used to track the position and the shape of the liquid region. The PISO algorithm is selected to solve the pressure-velocity coupling. The influences of the droplet initial velocity, the contact angle for water on the surface perpendicular to the groove direction and the surface tension coefficient on the impact process are discussed in detail. The results show that the droplet spreading factor improves notably with the increase of the initial velocity, and reduces with the increase of the contact angle. When the surface tension coefficient increases, the spreading factor reduces greatly. The spreading factor is the largest and the time elapsing is the longest in the case of σ = 0.038 N/m.
Evaluation of Heat Transfer Models With Measurements in a Hydrogen-Fuelled Spark Ignition Engine
Authors: Demuynck Joachim , Verhelst Sebastian , De Paepe Michel , Huisseune Henk , Sierens Roger
Published: March 2011

Hydrogen-fuelled internal combustion engines are still investigated as an alternative for current drive trains because they have a high efficiency, near-zero noxious and zero tailpipe greenhouse gas emissions. A thermodynamic model of the engine cycle enables a cheap and fast optimization of engine settings for operation on hydrogen. The accuracy of the heat transfer sub model within the thermodynamic model is important to simulate accurately the emissions of oxides of nitrogen which are influenced by the maximum gas temperature. These emissions can occur in hydrogen internal combustion engines at high loads and they are an important constraint for power and efficiency optimization. The most common models in engine research are those from Annand and Woschni, but they are developed for fossil fuels and the heat transfer of hydrogen differs a lot from the classic fuels. We have measured the heat flux and the wall temperature in an engine that can run on hydrogen and methane and we have investigated the accuracy of simulations of the heat transfer models. This paper describes an evaluation of the models of Annand and Woschni with our heat flux measurements. Both models can be calibrated to account for the influence of the specific engine geometry on the heat transfer. But if they are calibrated for methane, they fail to calculate the heat transfer for hydrogen combustion. This demonstrates the models lack some gas or combustion properties which influence the heat transfer process in the case of hydrogen combustion.
A General Closure Model for the Time-Averaged Radiative Transfer Equation in Turbulent Flows
Authors: Coelho Pedro J.
Published: March 2011

The time-averaged form of the radiative transfer equation (RTE) includes emission and absorption correlations that need to be modeled. There is no general formulation to estimate the absorption coefficient-radiation intensity correlation, which is generally neglected (optically thin fluctuation approximation–OTFA). Here, a model to compute this correlation, as well as the other correlations in the time-averaged form of the RTE, is described. The formulation is based on the solution of two additional differential equations. The unclosed correlations in these equations are estimated assuming that the joint probability density function (pdf) of the radiation intensity and mixture fraction is a two-dimensional clipped Gaussian distribution. The model is applied to a turbulent jet diffusion flame, and a preliminary assessment of the model is reported. It is shown that fluctuations of the radiation intensity, caused by turbulence, imply the existence of a correlation between the radiation intensity and local properties. The assumption of the shape of the joint pdf of mixture fraction and radiation intensity yields satisfactory predictions if the turbulent fluctuations are moderate, but becomes inaccurate near the flame edge where turbulent fluctuations are very large. Nevertheless, the present results suggest that the proposed model may yield better predictions than the OTFA.
Scavenging of Soluble Gaseous Pollutants by Rain Droplets in the Atmosphere With Nocturnal Temperature Profile
Authors: Fominykh Andrew , Elperin Tov , Krasovitov Boris
Published: March 2011

We analyze non-isothermal absorption of trace gases by the rain droplets with internal circulation which is caused by interfacial shear stresses. It is assumed that the temperature and concentration of soluble trace gases in the atmosphere varies in a vertical direction. The rate of scavenging of soluble trace gases by falling rain droplets is determined by solving heat and mass transfer equations. In the analysis we accounted for the accumulation of the absorbate in the bulk of the falling rain droplet. The problem is solved in the approximation of a thin concentration and temperature boundary layers in the droplet and in the surrounding air. We assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and concentration of the absorbate and temperature are homogeneous and time-dependent in the bulk. By combining the generalized similarity transformation method with Duhamel’s theorem, the system of transient conjugate equations of convective diffusion and energy conservation for absorbate transport in liquid and gaseous phases with time-dependent boundary conditions is reduced to a system of linear convolution Volterra integral equations of the second kind which is solved numerically. Calculations are performed using available experimental data on nocturnal temperature profiles in the atmosphere. It is shown than if concentration of a trace gas in the atmosphere is homogeneous and temperature in the atmosphere increases with altitude, droplet absorbs gas during all the period of its fall. Neglecting temperature inhomogenity in the atmosphere described by nocturnal temperature inversion leads to essential underestimation of the trace gas concentration in a droplet on the ground.
Flow and Thermal Fields in a Pendant Droplet Moving on Lyophobic Surface
Authors: Sikarwar Basant Singh , Muralidhar K. , Khandekar Sameer
Published: March 2011

Clusters of liquid drops growing and moving on physically or chemically textured lyophobic surfaces are encountered in drop-wise mode of vapor condensation. As opposed to film-wise condensation, drops permit a large heat transfer coefficient and are hence attractive. However, the temporal sustainability of drop formation on a surface is a challenging task, primarily because the sliding drops eventually leach away the lyophobicity promoter layer. Assuming that there is no chemical reaction between the promoter and the condensing liquid, the wall shear stress (viscous resistance) is the prime parameter for controlling physical leaching. The dynamic shape of individual droplets, as they form and roll/slide on such surfaces, determines the effective shear interaction at the wall. Given a shear stress distribution of an individual droplet, the net effect of droplet ensemble can be determined using the time averaged population density during condensation. In this paper, we solve the Navier-Stokes and the energy equation in three-dimensions on an unstructured tetrahedral grid representing the computational domain corresponding to an isolated pendant droplet sliding on a lyophobic substrate. We correlate the droplet Reynolds number (Re = 10–500, based on droplet hydraulic diameter), contact angle and shape of droplet with wall shear stress and heat transfer coefficient. The simulations presented here are for Prandtl Number (Pr) = 5.8. We see that, both Poiseuille number (Po) and Nusselt number (Nu), increase with increasing the droplet Reynolds number. The maximum shear stress as well as heat transfer occurs at the droplet corners. For a given droplet volume, increasing contact angle decreases the transport coefficients.
1D Heat Transfer Model for the Chain Section of a Lime Kiln
Authors: Massad Michael , Hassan Samer , Kawaji Masahiro , Tran Honghi N.
Published: March 2011

This work was aimed at gaining a better understanding of heat transfer within lime kilns, by both experiments and detailed modeling of heat transfer phenomena in the chain section. Experiments were conducted using a laboratory mockup of a rotating kiln to obtain convective heat transfer coefficient data for cooling of a steel rod in dry or wet lime mud. For moisture contents of 0% and 30%, the mud heat transfer coefficient was determined to be 170 and 320 W/m2°C, respectively. A 1-D, unsteady heat conduction model was used to predict the temperature variations of all the chain rings in the chain system and calculate the amount of heat transferred by each chain ring to the lime mud. A thermal model was then developed to predict the steady axial temperature profiles of lime mud, gas and kiln wall throughout a rotating lime kiln equipped with a typical chain system.
Kinetics of Fast Component and Energy Balance of the Plasma in Gas Dynamic System With Solenoidal Magnetic Field
Authors: Chirkov Alexei , Kaskov Sergey
Published: March 2011

Numerical model of ion kinetics is considered for the axially symmetrical magnetic trap. Magnetic system of the trap consists of long solenoid and two end coils which constrict magnetic field lines and form so-called magnetic mirrors reflecting charged particles. The trap contains warm Maxwellian plasma and strongly non-Maxwellian high-energy (fast) ions. Steady-state fast ion population supported by the ionization of high-energy neutral atoms injected into the plasma. Physical model is based on the kinetic equation with two-dimensional Fokker–Planck collision operator in the velocity phase space. Regimes of plasma exhaust trough the mirrors are considered taking into account possibility of electrostatic barrier formation. Such regimes essentially differ from gas dynamic exhaust of the warm Maxwellian plasma. Parameters of power balance for the system under consideration are discussed.
Capacitive Void Fraction Measurements and Probabilistic Flow Pattern Mapping of Horizontal Refrigerant Flow
Authors: Canie`re Hugo , Bauwens Bruno , T’Joen Christophe , De Paepe Michel
Published: March 2011

A capacitive void fraction sensor was developed to study the dynamic behavior of refrigerant two-phase flows in horizontal macro-scale smooth tubes of evaporators used in air-conditioning and heat pump applications. A first prototype was successfully tested with air-water flow. The dielectric constant of liquid HFC refrigerants however is much smaller than the one of water, which results in much smaller capacitances to be measured. Therefore an improved version of the capacitance transducer was developed to make dynamic measurements of these HFC fluids possible. The sensor signal can be used for flow pattern detection and void fraction measurements of high pressure HFCs. A macro-scale test facility for two-phase flow and heat transfer studies of high pressure HFCs was designed and constructed. The adiabatic test section consists of a sight glass with high speed camera and the capacitive void fraction sensor. Two datasets of sensor signals were gathered using R410A and R134a respectively (G = 200–500kg/m2s and x from 0 to 1 in steps of 0.025 at Tsat = 15°C). To find more objectivity in flow pattern mapping, a signal analysis was performed. The average, the variance and a high frequency parameter were found suitable for flow regime classification into slug flow, intermittent flow and annular flow by using the fuzzy c-means clustering algorithm. A probabilistic flow pattern map is presented for the HFC data. These maps clearly quantify the width of the transition zones and can be applied for probabilistic heat transfer and/or pressure drop modeling.
Local Temperature and Heat Flux in the Aqueous Solution of Antifreeze Protein Near Ice Surfaces in a Narrow Space
Authors: Hagiwara Yoshimichi , Yamamoto Daichi , Ohnishi Yosuke
Published: March 2011

Experiments have been conducted into the unidirectional freezing of an aqueous solution of winter flounder antifreeze protein 0.02mm thick. It is confirmed that the instantaneous temperature field can be measured with a near-infrared camera. It is found that the difference between the conduction heat flux of pure water near the interface and that of ice is approximately equal to the heat flux for solidification, which is the product of ice density, interface velocity and the latent heat of fusion. The sum of the conduction heat flux of protein solution near the front edge of the serrated interface and the heat flux for solidification is approximately equal to the conduction heat flux of ice. On the other hand, the sum of the conduction heat flux of protein solution near the bottom edge of the serrated interface and the heat flux for solidification is much higher than the conduction heat flux of ice.
Vapor Condensation at Sudden Contact With Cold Liquid in Non-Equilibrium Conditions
Authors: Kryukov A. P. , Yastrebov A. K.
Published: March 2011

Condensation of saturated vapor at sudden contact with cold liquid was studied with use of numerical solution of the Boltzmann kinetic equation. Results were obtained for both constant and variable temperature of liquid. In the first case vapor condensation becomes stationary after very small time; in the second one this process can not be stationary due to used model of heat transfer in liquid. Possibility of use of stationary kinetic correlations was estimated. For both cases results of numerical solution are in good agreement with correlation proposed by D.A. Labuntsov and A.P. Kryukov. Therefore, this correlation can be used as non-equilibrium boundary condition at solution of applied problems.
Simulation of Forced Convection Ice Slurry Flow in a Heated Tube
Authors: Trabelsi Hamed , Galanis Nicolas , Orfi Jamel
Published: March 2011

This paper compares the numerically predicted steady state, laminar hydrodynamic and thermal fields of an ice slurry (water with 15% ethanol and 12.26% ice particles) and a homogeneous binary, single phase mixture (water with 17.1% ethanol) entering identical constant temperature tubes (Tw = 274.16 K) with the same temperature (T0 = 264.16 K) and Reynolds numbers (Re = 500). The isothermal length of the tube is preceded and followed by adiabatic zones. The fluids are considered to be Newtonian and the governing partial differential equations are coupled since their properties depend on the temperature and, in the case of the ice slurry, on the ice concentration which is not uniform due to heat transfer. The results show significant differences between local values of the wall shear stress, the friction factor, the bulk temperature and the Nusselt number of these two flows. Specifically, the local Nusselt number for the ice slurry is higher throughout the developing region and its bulk temperature decreases in the downstream adiabatic zone due to radial conduction and an axial increase of the bulk ice concentration.
General Solution of Drop Evaporation Modelling in Non-Viscous Steady-State Gaseous Environment
Authors: Tonini Simona , Cossali Gianpietro Elvio
Published: March 2011

The problem of steady-state mass transport from a spherical mono-component droplet immersed in gaseous environment is addressed to find a solution for the expected vaporisation rate under general ambient conditions. The continuity, momentum and energy equations are written on a radial coordinate system. The effect of thermal gradient in the vapour phase is taken into account, while the viscous and dissipation terms in the momentum and energy equations are neglected. The model yields a non-linear second order ODE that is numerically and analytically solved to calculate the steady-state vaporisation rate. The description in terms of non-dimensional variables introduces some new parameters that are expected to influence the vaporisation rate. The model is then compared to the existing simplified Maxwell equation and the well-known Stefan-Fuchs model. Quantitative comparisons are presented and discussed and an application to water droplets floating in hot gas environment, under the operating conditions typical of fire protection spray scenarios, is presented.
Flow Transitions and Bifurcations of Buoyancy-Induced Flows From an Enclosed Cylindrical Heat Source
Authors: Angeli Diego , Pagano Arturo , Corticelli Mauro A. , Barozzi Giovanni S.
Published: March 2011

This study presents a numerical analysis of the dynamics of transitional natural convection flow regimes, generated by a confined thermal source. The system considered is a 2D enclosure of square cross-section, containing a horizontal cylindrical source, placed in a central position. The heat carrier fluid is air with Prandtl number Pr = 0.7. The resulting flow is investigated with respect to the variation of the Rayleigh number, based on the minimum gap width H between the cylinder and the enclosure walls. Three values of the aspect ratio A are considered (A = 2.5, 3.3, 5). The first bifurcation of the low-Ra fixed-point solution is tracked for each A-value. The transition to unsteady, chaotic flow is detailed for A = 2.5. The supercritical behaviour of the system is investigated using nonlinear analysis tools and phase space representations of the computed time series.
Pressure-Based Prediction of Single Phase Spray Cooling Heat Transfer Coefficient for Low Prandtl Number Liquids
Authors: Abbasi Bahman , Kim Jungho
Published: March 2011

One of the main challenges of spray cooling technology is the prediction of local and average heat transfer coefficients on the heater surface. It is hypothesized that the local heat transfer coefficient can be predicted from the local normal pressure produced by the spray. In this study, hollow cone, full cone, and flat fan sprays operated at three standoff distances, five spray pressures, and two nozzle orientations were used to identify the relation between impingement pressure and heat transfer coefficient in the single phase regime. PF-5060, PAO-2, and PSF-3 were used as test fluids, resulting in Prandtl number variation between 12 to 75. A micro-heater array operated at constant temperature was used to measure the local heat flux. A separate test rig was used to make impingement pressure measurements for the same geometry and spray pressure. The heat flux data were then compared with the corresponding impingement pressure data to develop a pressure-based correlation for spray cooling heat transfer. The maximum deviation between the experimental data and prediction was within ±25%.
From Pore Scale Numerical Simulation of Conjugate Heat Transfer in Cellular Material to Effectives Transport Properties of Real Structures
Authors: Hugo Jean-Michel , Topin Fre´de´ric , Tadrist Loune`s , Brun Emmanuel
Published: March 2011

Pore scale numerical simulation of heat and mass transfer in several foams are realized. 3D geometry is reconstructed from X-Ray tomographic images and fully characterized using the iMorph software. Microscale quantities such as temperature, pressure and velocity fields are computed using commercial software (StarCCM+) based on finite volume method. Macroscale properties are then deduced from numerical data and compared to experimental ones. Impact of foam topology and material as well as fluid nature (Fluid dynamic viscosity, solid thermal conductivity …) on transfer properties are systematically studied. We discuss correlations of these results with geometrical characteristics of the samples by scaling the metal foam in order to change pore diameter.
Effect of Superhydrophobicity on Impinging Droplet Heat Transfer
Authors: Rosengarten Gary , Tschaut Rita
Published: March 2011

In this study we present a preliminary investigation into the effect of hydrophobicity on the heat transfer rate due to the impingement of cold water droplets on heated flat surfaces. Two extreme contact angles were compared; hydrophilic (∼20°) and superhydrophobic (∼160°) using different surface coatings on a thin metal substrates. Images of the droplet impingement were simultaneously recorded by a high speed camera and a high speed, high resolution infrared camera so we could correlate droplet shape and dynamics to the heat transfer rate. There is a large effect on both the droplet fluid dynamics and heat transfer between hydrophilic and superhydrophobic surfaces. The heat transfer difference between the superhydrophobic and hydrophilic cases is a complex interplay between the increased droplet contact line velocity due to induced slip and the insulating properties of the air gap. Overall we have shown significant reductions in both the instantaneous heat transfer rates and the overall cooling effect of droplets impinging on superhydrophobic surfaces relative to those for hydrophilic surfaces. In the range of droplet velocities varied (We = 50 to 190) there was little dependency of the heat transfer or fluid flow with impact velocity, due to the dominance of inertial forces.
Numerical Study on Heat Transfer Characteristics From Parallel Plates With Cavities Swept by a Visco-Elastic Fluid
Authors: Suzuki Hiroshi , Maeda Shinpei , Komoda Yoshiyuki
Published: March 2011

Two-dimensional numerical computations have been performed in order to investigate the development characteristics of flow and thermal field in a flow between parallel plates swept by a visco-elastic fluid. In the present study, the effect of the cavity number in the domain and of Reynolds number was focused on when the geometric parameters were set constant. From the results, it is found that the flow penetration into the cavities effectively causes the heat transfer augmentation in the cavities in any cavity region compared with that of water case. It is also found that the development of thermal field in cases of the present visco-elastic fluid is quicker compared with that of water cases. The present heat transfer augmentation technique using Barus effect of a visco-elastic fluid is effective in the range of low Reynolds number.
Sub-Atmospheric Pressure, Subcooled, Flow Boiling of Water on Screen Laminate Enhanced Surfaces
Authors: Penley S. J. , Wirtz R. A.
Published: March 2011

Subcooled flow boiling experiments with water at 0.2atm pressure assess the utility of fine filament screen-laminate enhanced surfaces as high performance boiling surfaces. Experiments are conducted on vertically oriented, multi-layer copper laminates in distilled water. The channel Reynolds number is varied from 2,000 to 20,000; and, subcooling ranges from 2K to 35K. Boiling performance is documented for ten different surfaces having pore hydraulic diameters ranging from 39μm to 105μm, and surface area enhancement ratios ranging from 5 to 37. Heat flux of up to 453W/cm2 is achieved at 35K subcooling at a channel Reynolds number of 6,000, which represents a 3.5-fold increase in Critical Heat Flux (CHF) over that of saturated pool boiling on the same surface. Results show that CHF is strongly correlated with subcooling; and the effect of subcooling is more pronounced as flow intensity is increased. It is found that CHF enhancement due to subcooling and flow intensity is intrinsically linked to the surface area enhancement ratio, which has an optimum that depends on the degree of subcooling. High speed video imagery (1200fps) and long range microscopy are use to document bubble dynamics. Boiling mechanisms inherent to subcooling, enhanced surface geometry, and CHF are discussed.
Experimental Study on a Compact Methanol-Fueled Reformer With Heat Regeneration Using Ceramic Honeycomb
Authors: Rai Yasuhiro , Tatsumi Kazuya , Nakabe Kazuyoshi
Published: March 2011

On the way to a new era of our society which will be based on hydrogen energy, it is needed to develop on-site hydrogen production systems to cover current insufficient infrastructures of hydrogen supply network systems. For this, a highly efficient compact reformer can be one of the most suitable solutions for on-site production of hydrogen which is supplied to distributed electric power-generation systems. But, the local and overall energy balance in the reformer should be precisely controlled since the reforming reaction processes of hydrocarbon fuels are very sensitive to reaction temperature in the reformer. For smaller reformers, in particular, the amount of heat loss through the outer surfaces is large enough to dominate the reactions. An appropriate way for thermal energy management, therefore, is necessary to accomplish highly efficient reformers. For these backgrounds, a compact tubular-typed fuel reformer was fabricated in this study, and was applied to produce hydrogen from methanol, focusing on the partial oxidation reaction (POR). The reformer was composed of a stainless steel pipe as the reactor exterior and ceramic honeycomb blocks inserted in two locations of the reactor. The honeycomb blocks are expected to assist the reforming reactions and transfer the thermal energy of the exhaust gas to the reaction region, acting as a heat regenerator. The upstream-side honeycomb block was aimed to perform an effective heat exchange from the reactor wall to the reactant gas. By inserting the block, the reforming reaction became stable at right after the block. The maximum hydrogen production was achieved in the condition of equivalence ratio, around 3.5. The other honeycomb block was inserted in the downstream of the reaction zone to convert the thermal energy of exhaust gas to radiation energy which can be transferred to the upstream reaction region. Comparing to the case without the downstream-side block, the temperature of the reaction region became higher. Gas temperatures in the downstream region, on the other hand, became lower. Methanol conversion ratio and hydrogen production ratio enhanced due to the higher temperature at the reaction region. These results indicate that the thermal energy possessed by the exhaust gas was regenerated in the reaction region by the downstream-side honeycomb block and contributes to enhance the efficiency of the fuel reformer.
Molecular Dynamics Simulation for the Impact of N-Decanol Surfactants on the Liquid-Vapor Interface of Lithium Bromide Aqueous Solution
Authors: Zhu Beibei , Gao Hongtao
Published: March 2011

In order to investigate the effect of n-decanol, a kind of alcohol surfactants, on the absorption of water vapor into lithium bromide aqueous solution, this study focused on microscopic structure of the liquid-vapor interface of the electrolyte solution, the impact of n-decanol molecules on the interfacial properties and the absorption dynamic process employing the method of molecular dynamics simulation. The liquid-vapor configuration of lithium bromide aqueous solution added with four concentration of n-decanol can be analyzed by examining the density profile, the radial distribution functions and orientational order parameter. The computed results revealed that n-decanol molecules tended to adsorb at the interface with the methyl group pointing into the vapor phase and hydrophilic hydroxyl group pointing into the liquid phase which do much help to form a hydrogen bond network with water, and the tendency of this kind of preferred orientation became distinct with the increase of the amount of n-decanol. The hydrocarbon chains of n-decanol molecules were inclined to close to stay upright near the interface while the monolayer of n-decanol came into being near the interface. Ions were repelled from the surface. The direct interactions between hydroxyl hydrogen of n-decanol and anion exist, and there are much stronger electrostatic interactions between oxygen of n-decanol and cation. The dynamic process of the absorption of water into aqueous electrolyte solution with or without n-decanol was explored by molecular dynamics simulation under non equilibrium conditions. The simulation results showed that in comparison to the lithium bromide aqueous solution without n-decanol, the electrolyte aqueous solution with n-decanol can absorb more water molecules distinctly for 100 ps.
Experiments on Flow Boiling Heat Transfer of Ammonia/Water Mixture Inside an Internally Spirally Grooved Horizontal Tube
Authors: Momoki Satoru , Arima Hirofumi , Asou Hiroyuki , Jambal Odgerel , Yamaguchi Tomohiko , Shigechi Toru
Published: March 2011

Experiments were performed on the flow boiling of the zeotropic mixture of water-ammonia inside an internally spirally grooved horizontal steel tube with a 12mm average inner diameter. The experimental conditions were the mass fraction of ammonia: 0.95, 1.0 kg/kg, the mass velocity: 40 to 80 kg/(m2s), the heat flux: 0 to 20 kW/m2 and the pressure: 0.7 MPa. The measured heat transfer coefficient reached its maximum as the quality approached about 0.6 but decreased abruptly as the quality increased. This sharp decrease as the quality increased beyond 0.6 may have been caused by mass diffusion resistance that increased the temperature locally at the vapor-liquid interface. The temperature increase at the vapor-liquid interface is discussed by analyzing the phase equilibrium characteristics and is explained in terms of the relationship between the bulk temperature and vapor quality. The heat transfer coefficients are also compared with those for pure ammonia.
Parametric Analysis of an Aluminium Plate-Fin Heat Exchanger (ALPHE) for LNG-FPSO
Authors: Lee Eungchan , Kang Hoon , Kim Yongchan , Lim Hyug , Park Jaehong , Cho Sungyoul
Published: March 2011

A nitrogen double expander cycle has been widely used for liquefaction of natural gas in LNG-FPSO (Floating Production, Storage, and Offloading). An aluminum plate-fin heat exchanger (ALPHE) is usually adopted in the liquefaction cycle. In general, the ALPHE has a very large heat transfer surface area per unit volume. This surface area consists of primary and secondary (finned) surfaces. Even taking into account fin efficiency of the secondary surface, the effective surface area per unit volume can be typically five times greater than that of a shell-and-tube heat exchanger. Various types of fin are available in ALPHE and the fin type should be selected properly to optimize the performance. For example, serrated, wavy and perforated fin are particularly suitable for gas streams. The selection and design of the layer arrangement and effective length of each stream are very important design parameters for ALPHE. In this paper, to optimize the design of ALPHE, the effects of the design parameters on the performance of ALPHE were studied using a simulation method. The properties of nitrogen and natural gas were calculated from proper equations of state. Because the performance of ALPHE is mainly influenced by the fin type, fin frequency, fin height, fin thickness, and layer arrangement, the effects of the geometric design parameters on the performance of ALPHE were studied, and the optimum design conditions were suggested in this paper.
An Analysis of Heat Transfer Processes in an Internal Indirect Reforming Type Solid Oxide Fuel Cell
Authors: Brus Grzegorz , Kolenda Zygmunt , Kimijima Shinji , Szmyd Janusz S.
Published: March 2011

This paper presents experimental and numerical studies on the fuel reforming process on an Ni/YSZ catalyst. Nickel is widely known as a catalyst material for Solid Oxide Fuel Cells. Because of its prices and catalytic properties, Ni is used in both electrodes and internal reforming reactors. However, using Ni as a catalyst carries some disadvantages. Carbon formation is a major problem during a methane/steam reforming reaction based on Ni catalysis. Carbon formation occurs between nickel and metal-support, creating fibers which damage the catalytic property of the reactor. To prevent carbon deposition, the steam-to-carbon ratio is kept between 3 and 5 throughout the entire process. To optimize the reforming reactors, detailed data about the entire reforming process is required. In the present paper kinetics of methane/steam reforming on the Ni/YSZ catalyst was experimentally investigated. Measurements including different thermal boundary conditions, the fuel flow rate and the steam-to-methane ratios were performed. The reforming rate equation derived from experimental data was used in the numerical model to predict synthetic gas composition at the outlet of the reformer.
3D Integrated Water Cooling of a Composite Multilayer Stack of Chips
Authors: Alfieri Fabio , Tiwari Manish K. , Zinovik Igor , Poulikakos Dimos , Brunschwiler Thomas , Michel Bruno
Published: March 2011

New generation supercomputers with three dimensional stacked chip architectures pose a major challenge with respect to removal of dissipated heat which can reach currently as high as 250 W/cm2 in multilayer chip stacks of less than 0.3 cm3 volume. Interlayer integrated water cooling [1] is a very promising approach for such high heat flux removal due to much larger thermal capacity and conductivity of water compared to air, the traditional cooling fluid. In the current work, a multiscale conjugate heat transfer model is developed for integrated water cooling of chip layers and validated with experimental measurements on a specially designed thermal test vehicle that simulates a four tier chip stack with a footprint of 1 cm2. The cooling heat transfer structure, which consists of microchannels with cylindrical pin fins, is conceived in such a way that it can be directly integrated with the device layout in multilayer chips. Every composite layer is cooled by water flow in microchannels (height: 100 μm), which are arranged in 2 port water inlet-outlet configuration. The total power removed in the stack is 390 W at a temperature gradient budget of 60 K from liquid inlet to maximal junction temperature, corresponding to about 1.3 kW/cm3 volumetric heat flow. The computational cost and complexity of detailed CFD modeling of heat transfer in stacked chips with integrated cooling can be prohibitive. Therefore, the heat transfer structure is modeled using a porous medium approach, where the model parameters of heat transfer and hydrodynamic resistance are derived from averaging the results of the detailed 3D-CFD simulations of a single stream-wise row of fins. The modeling results indicate that an isotropic porous medium model does not accurately predict the measured temperature fields. The variation of material properties due to temperature gradients are found to be large, therefore variable properties are used in the model. It is also shown that the modeling of the heat transfer in the cooling sublayers requires the implementation of a porous medium approach with a local thermal non-equilibrium as well as orthotropic heat conduction and hydrodynamic resistance. The improved model reproduces the temperatures measured in the stack within 10%. The model is used to predict the behavior of multilayer stacks mimicking the change of heat fluxes resulting from variations in the computational load of the chips during their operation.