bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat Transfer Characteristics of Swirling Impinging Jets
Authors: Brown Karl J. , Persoons Tim , Murray Darina B.
Published: March 2011

Impinging jets are well renowned for their abilities to achieve significantly high heat transfer coefficients. It has been found in previous studies that the incorporation of a swirl in the flow can enhance heat transfer but it can also decrease it. The objective of this research is to discover the defining characteristics of swirling impinging jets and to find an optimal swirl geometry for enhancing the heat transfer of the system. Using thermal imaging and hot film sensor techniques the heat transfer distribution of the swirling jet is examined in the context of the flow field; heat flux fluctuations at the impinging surface are investigated also.
Effect of Wall Heat Flux on Entropy Generation in Turbulent Mixed Convection Heat Transfer of a Pipe Flow With Highly Variable Properties
Authors: Bazargan Majid , Mohseni Mahdi
Published: March 2011

There are many engineering systems with working fluids which have properties varying significantly with temperature. This causes the effect of the wall heat flux on the velocity and temperature fields to become larger with respect to the constant property flows. In this study the effect of the wall heat flux on the entropy generation in the mixed turbulent convection heat transfer of a fluid flow with high property variations has been investigated. The local and total entropy generation is calculated. In addition, the region in which the entropy production is larger has been determined. Furthermore, the contribution of each of the mechanisms of entropy production which is depended on the wall heat flux is determined. It should be noted that the implementation of different heat fluxes at the tube wall affects all mechanisms of entropy generation. The results show that the bulk entropy generation reaches a minimum value when the heat transfer coefficient has a maximum value. The wall heat flux also has an opposite effect on the heat transfer coefficient and entropy generation which is a favorable result.
Study of Post Dryout Heat Transfer in Annulus With Flow Obstacles
Authors: Anghel I. G. , Anglart H. , Hedberg S.
Published: March 2011

The purpose of this paper is to present the experimental setup, experimental method and results of the recent post-dryout heat transfer investigations in an annulus with pin spacers. The experiments were performed in the thermal-hydraulic laboratory at the Royal Institute of Technology (KTH), Stockholm, Sweden. The experimental facility has an annular test section which consists of two double-heated concentric tubes manufactured of Inconel 600. Five levels of pin spacers were installed along the test section to keep the rod and the tube equidistant during experiments. The experimental matrix includes measurements of wall temperature distributions for single phase and two-phase flows, for both convective boiling and post-dryout heat transfer regimes. The influence of variations in mass flux (500–1500) kg/(m2s) and inlet subcooling 10 and 40 K at system pressure of 7 MPa were investigated. The experimental results indicate that post dryout heat transfer is influenced by the pin spacers. In particular it has been observed that the dry patch appearing in the test section can be quenched downstream of the pins-spacer. The current results provide additional experimental database which can be used for validation of post-dryout heat transfer models including the flow obstacle effects.
Thermal Design of a Protruding Heater in Laminar Channel Flow
Authors: Alves Thiago Antonini , Altemani Carlos A. C.
Published: March 2011

The conjugate heat transfer from a single block heater mounted on a conductive wall of a parallel plates channel was investigated under conditions of laminar airflow. The heater cooling was attained by direct forced convection to the airflow and by conduction through its contact with the channel wall. The investigation was performed for a two-dimensional configuration with fixed channel geometry and variable heater height, where the heater upstream edge was centered on the channel wall. At the channel entrance the flow velocity and temperature were uniform. The channel wall thickness was constant and its thermal conductivity was varied in the range from 0 to 80 that of the air, while the heater block thermal conductivity was equal to 500 that of the air. The conservation equations were solved numerically by the control volumes method with the SIMPLE algorithm. The results were expressed in dimensionless form, considering three distinct constraints: fixed flow rate, fixed channel flow pressure drop and fixed pumping power. The last two constraints present an optimum heater height, but care was taken to avoid that the flow recirculation downstream the heater extended beyond the channel length. The heat transfer enhancement promoted by wall conduction was clearly indicated.
Dynamics of Pool Boiling on Plain and Nanotube Coated Silicon Surfaces
Authors: Sathyamurthi Vijaykumar , Banerjee Debjyoti
Published: March 2011

Saturated pool boiling experiments are conducted over silicon substrates with and without Multi-walled Carbon Nanotubes (MWCNT) with PF-5060 as the test fluid. Micro-fabricated thin film thermocouples located on the substrate acquire surface temperature fluctuation data at 1 kHz frequency. The high frequency surface temperature data is analyzed for the presence of chaotic dynamics. The shareware code, TISEAN© is used in analysis of the temperature time-series. Results show the presence of low-dimensional deterministic chaos, near Critical Heat Flux (CHF) and in some parts of the Fully Developed Nucleate Boiling (FDNB) regime. Some evidence of chaotic dynamics is also obtained for the film boiling regimes. Singular value decomposition is employed to generate pseudo-phase plots of the attractor. In contrast to previous studies involving multiple nucleation sites, the pseudo-phase plots show the presence of multi-fractal structure at high heat fluxes and in the film boiling regime. An estimate of invariant quantities such as correlation dimensions and Lyapunov exponents reveals the change in attractor geometry with heat flux levels. No significant impact of surface texturing is visible in terms of the invariant quantities.
Investigation of Flow Boiling on Nanostructured Surfaces
Authors: Jeon Saeil , Roy Pratanu , Anand N. K. , Banerjee Debjyoti
Published: March 2011

Flow boiling experiments were performed on copper, bare silicon and carbon nanotube (CNT) coated silicon wafer using water as the test fluid. Wall heat flux was measured by varying the wall superheat. The experiments were performed under pool boiling conditions (zero flow rate) as well as by varying the flow rates of water. The liquid sub-cooling was varied between 40 ∼ 60 °C. An infra–red camera was used to calibrate the surface temperature of the silicon wafers and the copper surface. Heat flux measurements were performed by using a calorimeter apparatus. High speed visualization experiments were performed to measure the bubble departure diameter, bubble departure frequency and bubble growth rate as a function of time. Heat flux values for all three surfaces were calculated from the temperature differences obtained by sheathed thermocouples inside the copper block in the calorimeter apparatus. Flow boiling curves were plotted to enumerate the enhancements in heat transfer. It was observed that MWCNT coated silicon surface enables higher heat fluxes compared to bare silicon surface. This enhancement can be ascribed to be due to the high thermal conductivity of the carbon nanotubes, micro-layer effect, enhancement of transient heat transfer due to periodic solid-liquid contact and increase in active nucleation sites on nanostructured surfaces.
Experimental Investigation of Flow Boiling Pressure Drop of R134a in a Micro-Scale Horizontal Smooth Tube
Authors: Tibiric¸a´ Cristiano Bigonha , Ribatski Gherhardt
Published: March 2011

This paper presents new experimental flow boiling pressure drop results in a microscale tube. The experimental data were obtained under diabatic conditions in a horizontal smooth tube with internal diameter of 2.3 mm. Experiments were performed with R134a as working fluid, mass velocities ranging from 100 to 600 kg/m2s, heat flux ranging from 10 to 55 kW/m2, saturation temperatures of 31 °C, and exit vapor qualities from 0.20 to 0.99. Flow pattern characterization was also performed from images obtained by high-speed filming. Pressure drops up to 48 kPa/m were measured. These data were carefully analyzed and compared against 13 two-phase frictional pressure drop prediction methods, including both macro- and micro-scale methods. Comparisons against these methods based on the data segregated according to flow patterns were also performed. Overall, the method by Cioncolini et al. [1] provided quite accurate predictions of the present database.
Numerical Modeling of Munitions Liquid Agent With Solid Heel in the Metal Parts Furnace
Authors: Zheng Yunhan , Webster Alfred , Vanoni Mike
Published: March 2011

The Metal Parts Furnace (MPF) is a custom-designed incinerator and has been successfully demonstrated for treatment of munitions containing chemical agents since the first full-scale prototype baseline incineration facility on Johnston Island in 1990. The development of a numerical model to predict peak vaporization rate and total vaporization time during processing can provide the ability to control operating parameters to minimize the potential for any upset conditions such as agent overload, and to ensure that appropriate processing conditions are chosen to achieve desired safety factors. The numerical model will also provide the capability to maximize operating efficiency and to help adjust conditions during processing of different agents and munitions configurations. The Continental Research and Engineering (CR&E) Peak Vaporization Rate (PVR) model was initially developed in 1988 for the MPF to provide munitions tray vaporization rates as a function of the MPF temperature and munitions fill level. The model was updated and calibrated to test data in 1994. The calibrated PVR model is suitable for munitions with varying quantities of liquid chemical agent; however, this model can only be used to predict the vaporization rate of the liquid chemical agent. In order to simulate the effect of the solidified agent found in the munitions, CR&E has developed a Computational Fluid Dynamics (CFD) model based the vaporization rate model to simulate heating, melting, and vaporization of liquid chemical agent mixed with a varying solid heel mass fraction.
Correlation Between Fractal Dimension of Matrix and Extraction Behavior of Plant Materials
Authors: Yang Junhong , Di Qianqian , Zhao Jun , Wang Liqiu
Published: March 2011

This paper studies the correlation between fractal dimension of matrix microstructure of plant materials and extraction behavior using Astragalus root slices as examples. This work compared the yield of extracts on the conventional solvent soaking extraction of samples irradiated 3min by microwave 600W and 900W, respectively. Regarding to microwave treated samples, the area fractal dimensions (DL) of aperture in shape were estimated by using the slit island method on the basis of SEM images analysis, the volume fractal dimensions (DV) of pore size distribution inside matrix were also determined by the mercury injection method on the basis of measured results (6nm–40×105nm) by automatic mercury injection apparatus. Our findings are that, 900W treated sample behaves higher yield of extracts than 600W. The values of DL and DV both increase with increasing of microwave power. The higher values of DL correspond to the much irregular and deformed shape of aperture, which seems un-benefit for the extraction of component inside matrix. However, the higher values of DV correspond to the less concentration of pore size distribution, implying better connectivity of pore or channel at multi-scale (including trachea 20μm–50μm, aperture 0.1μm–1μm and plasmodesma 1nm–10nm in size) and permeability inside matrix during extraction, higher yield of extracts. It demonstrates that combining the two fractal dimensions can present much more information for better understanding of mass transfer behavior and the knowledge of material properties.
Bubble Lift-Off Diameter in Forced Convective Boiling Flow
Authors: Chu In-Cheol , Song Chul-Hwa
Published: March 2011

A series of experiments were carried out to investigate the bubble nucleation to lift-off phenomena for a subcooled boiling flow in a vertical annulus channel. A high speed digital video camera was used to capture the dynamics of the bubble nucleation to lift-off process. A total of 148 recordings were made, and the bubble lift-off diameter and the bubble nucleation frequency were evaluated for 118 recordings up to now. The basic features of the lift-off diameter and nucleation frequency were addressed based on the present observation. A database for the bubble lift-off diameter was built by gathering and summarizing the data of Prodanovic et al., Situ et al., and the present work. The prediction capability of Unal’s model, Situ et al.’s model, and Prodanovic et al.’s correlation was evaluated against the database. The best prediction results were obtained by modifying the wall superheat correlation in Unal’s model.
Effect of Energy Deposition Modes on Ultrafast Solid-Liquid-Vapor Phase Change of a Thin Gold Film Irradiated by a Femtosecond Laser
Authors: Huang Jing , Zhang Yuwen , Chen J. K. , Yang Mo
Published: March 2011

Effects of different parameters on the melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are systematically studied. The classical two-temperature model was adopted to depict the non-equilibrium heat transfer in electrons and lattice. The melting and resolidification processes, which was characterized by the solid-liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, was obtained by considering the interfacial energy balance and nucleation dynamics. Vaporization process which leads to ablation was described by tracking the location of liquid-vapor interface with an iterative procedure based on energy balance and gas kinetics law. The parameters in discussion include film thickness, laser fluence, pulse duration, pulse number, repetition rate, pulse train number, etc. Their effects on the maximum lattice temperature, melting depth and ablation depth are discussed based on the simulation results.
Pool Boiling Heat Transfer of Dowtherm-A and Dowtherm-J on Smooth and Roughened Vertical Heater Surfaces
Authors: Nahra Ziad , Næss Erling , Lo̸vland Martin
Published: March 2011

Heat transfer coefficients in nucleate pool boiling of the organic heat transfer fluids Dowtherm A and Dowtherm J were obtained experimentally using a vertical electrically heated cylindrical carbon steel surface. The experiments were carried out at system pressures between 0.2–4.9 bar and heat fluxes in the range 20 to 230 kW/m2 and using different surface roughnesses. The heater surface was polished with emery paper 1200 to an average surface roughness (Ra) of 0.02 and 0.13 μm for Dowtherm A and J respectively, and roughened to an average surface roughness (Ra) of 2.42 μm by emery paper 40, and by sandblasting to an average (Ra) of 1.3 and 3.6 μm. The experimental results on the roughened surfaces showed that the heat transfer coefficient increased with increasing pressure, increasing surface roughness and increasing heat flux. A new correlation for predicting the pool boiling heat transfer coefficient for Dowtherm A and Dowtherm J is proposed. The correlation is an adapted Gorenflo [1], [2] correlation, and is valid for pressures between 0.2 and 5 bar, heat fluxes up to 230 kW/m2 and surface roughnesses between 0.02 and 3.6 μm. The accuracy of the correlation is within ±15% compared to experimental data.
Thermal Conductance and Constriction Resistance of Single-Layer Graphene Nano Ribbons
Authors: Huang Zhen , Murthy Jayathi , Fisher Timothy S.
Published: March 2011

This paper considers phonon transport behavior in graphene nanoribbons (GNRs) that bridge semi-infinite graphene contacts. The work employs an atomistic Green’s function method to investigate phonon wave effects in zigzag and armchair edge ribbons. Phonon transmission functions and thermal conductances are found to be sensitive to the edge shape of structures. The thermal conductances of GNRs with different widths are normalized by the quantum of thermal conductance to reveal the relation between number of phonon modes and conductance as a function of temperature. In addition, the phonon transmission functions of nano ribbons with defects are evaluated by artificially creating mismatches at interfaces. By comparing the transmission function of different defect patterns and the corresponding thermal conductances, the reduction of phonon transport is quantified. The length of defects is found to be important to phonon transport. Constriction effects are also studied at abrupt mismatched interfaces, and the reduction of thermal conductance is found to be moderately high.
Numerical Investigation of Heat Transfer Effects in Microchannels Under H2 Boundary Condition
Authors: Dharaiya V. V. , Srivastava R. R. , Kandlikar S. G.
Published: March 2011

Study of fluid flow characteristics at microscale level is gaining importance with shrinking device sizes. Better understanding of fluid flow and heat transfer in microchannels will have important implications in biomedical industry, MEMS, electronic chip cooling, heat exchangers, and microfluidic devices. Also, due to short lengths employed in microchannels, entrance header effects can be significant and needs to be investigated. In this work, three dimensional model of microchannels, with aspect ratios (α = a/b) ranging from 0.1 to 10, are numerically simulated using CFD software, FLUENT. Heat transfer effects in the entrance region of microchannel are presented by plotting average Nusselt number as a function of non-dimensional thermal entrance length x*. The numerical simulations with both circumferential and axial uniform heat flux (H2) boundary conditions were performed for four wall, three wall and two wall cases. Large numerical data sets, generated in this work for rectangular cross sectional microchannels for three walls and two walls H2 boundary condition, can provide better understanding and insight into the transport processes in the microchannel.
CFD Simulation of a Hydrogen Reformer Furnace
Authors: Zheng Dezhi , Wu Bin , Fleitz Jeff , Trajkovski Robert , Zhou Chenn Q.
Published: March 2011

A hydrogen reformer furnace is a combustion chamber which is used to supply heat for the catalytic process that converts natural gas into hydrogen. The reforming reaction that happens inside the catalyst tubes is endothermic, requiring high levels of heat input. The combustion process in the hydrogen reformer furnace provides the heat to maintain the chemical reaction inside the catalyst tubes. Temperature control of the catalyst tubes is a fundamental design requirement of the hydrogen reformer furnace, as the temperature should be maintained in the range which could maximize catalyst reactivity while minimizing any damage to the catalyst pipes. As the furnace has two complicated chemical systems, the heat effect inside the tubes has been simplified by estimating the heat flux based on industry operation. Utilizing the multiphase and non-premixed combustion model using CFD (Computational Fluid Dynamic), the temperature and velocity distribution in the hydrogen reformer furnace have been investigated. Results show that parts of the catalyst tubes are overheated causing hot spots which could lead to premature aging of the pipes. Both the location of burners and maldistribution of the hot flue gas have a great impact on this issue.
Study of Thermal Constriction Phenomena for Drops Spreading Encountered During Plasma Spraying Processes
Authors: Laraqi N. , El Ganaoui M. , Timchenko V.
Published: March 2011

During a thermal spraying process, the heat flux reaches a few hundreds MW/m2 while the spreading duration is extremely short, being about a microsecond. The interfacial phenomena of thermal deposit/substrate constriction play an extremely important role since they control the coating cooling rate, and as a consequence, the thermo-mechanical properties of the deposited layer. We present in this work an analytical solution intended for the computation of the thermal constriction resistance during the spreading. This solution is derived by using the Hankel finite transform and the Duhamel theorem leading to a simple expression as a series expansion with fast convergence. The effect of the spreading velocity on the thermal constriction resistance has been studied. Results show the existence of a critical threshold of spreading velocity for which the transient problem has to be considered. On the other hand (below the threshold) the steady state regime should be sufficient for the constriction estimate.
Thermal Conductivity of Highly Ordered Amorphous and Crystalline Mesoporous Titania Thin Films
Authors: Coquil Thomas , Pilon Laurent , Reitz Christian , Brezesinski Torsten , Nemanick Joseph E. , Tolbert Sarah H.
Published: March 2011

This paper reports the cross-plane thermal conductivity of amorphous and crystalline templated mesoporous titania thin films synthesized by evaporation-induced self-assembly. Both sol-gel and nanocrystal-based films were considered, with respective average porosities of 30% and 35%. The pore diameter ranged from 7 to 25 nm and film thickness from 60 to 370 nm while the average wall thickness varied from 3 to 25 nm. Nanocrystals in crystalline mesoporous films featured diameters between 9 and 13 nm. The thermal conductivity was measured at room temperature using the 3ω method. The experimental setup and the associated analysis were validated by comparing the thermal conductivity measurements with data reported in the literature for dense titania films with thickness ranging from 95 to 1000 nm. The cross-plane thermal conductivity of the amorphous mesoporous titania thin films did not show strong dependence on pore size, wall thickness, or film thickness. This can be attributed to the high atomic scale disorder of amorphous materials. Heat is thus mainly carried by localized non-propagating vibrational modes. The average thermal conductivity of the amorphous mesoporous titania films was identical to that of the nanocrystal-based films and equal to 0.37 W/m.K. Thermal conductivity of sol-gel crystalline mesoporous titania thin films was significantly larger than that of their amorphous counterparts. It also depended on the organic template used to make the films. The results indicated that the pore size was not an important factor. Instead thermal conductivity depended only on porosity, crystallinity, nanocrystal size and connectivity.
Investigation of Carbon Dioxide Explosive Boiling in Single Evaporator of a Two-Phase Mechanically-Pumped Thermal Control Loop
Authors: Sun Xi-Hui , Huang Zhen-Cheng , Xiao Wen-Jia , Chen Yue , He Zhen-Hui , Pauw A. , van Es J.
Published: March 2011

We investigated experimentally the start-up characteristics of a mechanically pumped two-phase loop (MPTCL), with CO2 as working fluid, and a single evaporator that consists of a bent inner ring and an outer ring constructed by stainless tubes with hydraulic diameter of 2.6 mm and length of 9 m, along which totally 54 pieces of heating element are distributed. Experiments were performed in the following conditions: mass flow rates of 1.1, 2.1, and 3.3g/s; heat loads ranged from 50 to 300W, with the heat-load ratios of the inner ring to the outer ring 2.2:1, 1:1, and 1:2.2 at the operational temperature of −15°C, respectively. During the start-up cases, we detected a reverse flow accompanying with pressure spike, which can be understood as explosive boiling, and a subsequent temporal dry-out phenomenon at the outlet of the evaporator, as a result of explosive boiling. The back flow together with the pressure spike is helpful to set up a two-phase flow all along the evaporator, though it may have negative effect on the loop, especially, when coincident explosive boiling happens. However, such a pressure spike that depends on initial superheating should be controlled to avoid possible harm to the loop.
Experimental Study and Numerical Analysis of Water Single-Phase Pressure Drop Across a Micro-Pin-Fin Array
Authors: Konishi Christopher A. , Hwu Ruey , Qu Weilin , Pfefferkorn Frank E.
Published: March 2011

This study investigates the hydraulic performance of a copper micro-pin-fin array subjected to water liquid single-phase flow conditions. The test section contains an array of 1950 staggered square micro-pin-fins with 200 micron × 200 micron cross-section by 670 micron height. The ratios of longitudinal pitch and transverse pitch to pin-fin equivalent diameter are equal to 2. Seven water inlet temperatures from 22°C to 80°C, and seventeen maximum mass velocities for each inlet temperature, ranging from 181 to 1649 kg/m2s, were tested. The test module was well insulated to maintain adiabatic conditions. Comparison of predictions of eleven existing friction factor correlations with the experimental data show relatively large discrepancies. The experimental study was complemented with a numerical analysis of single-phase flow in the micro-pin-fin array. Numerical results show excellent agreement with experimental data for Reynolds numbers below 700.
An LES Insight Into Convective Mechanism of Heat Transfer in a Wall-Bounded Pin Matrix
Authors: Delibra Giovanni , Borello Domenico , Hanjalic Kemal , Rispoli Franco
Published: March 2011

We report on an LES (large-eddy-simulations) study of flow and heat transfer in a longitudinal periodic segment of a matrix of cylindrical rods in a staggered arrangement bounded by two parallel heated walls. The configuration replicates the set-up investigated experimentally by Ames et al. (ASME Turbo Expo, GT2007-27432) and mimics the situation encountered in internal cooling of gas-turbine blades. LES have been performed using the in-house finite-volume computational code T-FlowS. Considered are two Reynolds numbers, 10000 and 30000, based on the rod diameter and maximum velocity in the matrix. The unstructured grid contained around 5 and 15 million cells for the two Re numbers respectively. After validating the simulations with respect to the available experimental data, the paper discusses the characteristic vortex and plume structures, streamline and heatline patterns and their evolution along the pin matrix, around individual pins and at the pin-endwall junctions. It is concluded that the convection by organized vertical structures originated from vortex shedding govern the thermal field and play the key role in endwall heat transfer, exceeding by far the stochastic turbulent transport.
Heat Transfer and Friction Augmentation in High Aspect Ratio, Ribbed Channels With Dissimilar Inlet Conditions
Authors: Slabaugh Carson D. , Tran Lucky V. , Kapat J. S. , Warren Bobby A.
Published: March 2011

This work is an investigation of the heat transfer and pressure-loss characteristics in a rectangular channel with ribs oriented perpendicular to the flow. The novelty of this study lies in the immoderate parameters of the channel geometry and transport enhancing features. Specifically, the aspect ratio (AR) of the rectangular channel is considerably high, varying from fifteen to thirty for the cases reported. Also varied is the rib-pitch to rib-height (p/e), studied at two values; 18.8 and 37.3. Rib-pitch to rib-width (p/w) is held to a value of two for all configurations. Channel Reynolds number is varied between approximately 3,000 and 27,000 for four different tests of each channel configuration. Each channel configuration is studied with two different inlet conditions. The baseline condition consists of a long entrance section leading to the entrance of the channel to provide a hydrodynamically-developed flow at the inlet. The second inlet condition studied consists of a cross-flow supply in a direction perpendicular to the channel axis, oriented in the direction of the channel width (the longer channel dimension). In the second case, the flow rate of the cross-flow supply is varied to understand the effects of a varying momentum flux ratio on the heat transfer and pressure-loss characteristics of the channel. Numerical simulations revealed a strong dependence of the local flow physics on the momentum flux ratio. The turning effect of the flow entering the channel from the cross-flow channel is strongly affected by the pressure gradient across the channel. Strong pressure fields have the ability to propagate farther into the cross-flow channel to ‘pull’ the flow, partially redirecting it before entering the channel and reducing the impingement effect of the flow on the back wall of the channel. Experimental result shows a maximum value of Nusselt number augmentation to be found in the 30:1 AR channel with the aggressive augmenter (p/e = 37.3) and a high momentum flux ratio: Nu/Nuo = 3.15. This design also yielded the friction with f/f0 = 2.6.
Viscosity Effect on Thermocapillary Rupture of Falling Liquid Films
Authors: Zaitsev Dmitry , Semenov Andrey , Kabov Oleg
Published: March 2011

Rupture of a subcooled liquid film flowing over an inclined plate with a 150×150 mm heater is studied for a wide range of liquid viscosity (dynamic viscosity μ = (0.91–17.2)x10−3 Pa·s) and plate inclination angle with respect to the horizon (Θ = 3–90 deg). The main governing parameters of the experiment and their respective values are: Reynolds number Re = 0.15–54, heat flux q = 0–224 W/cm2. The effect of the heat flux on the film flow leads to the formation of periodically flowing rivulets and thin film between them. As the heat flux grows the film thickness between rivulets gradually decreases, and, upon reaching a certain threshold heat flux, qidp, the film ruptures in the area between the rivulets. The threshold heat flux increases with the flow rate of liquid and with the liquid viscosity, while the plate inclination angle has little effect on qidp. Criterion Kp, which is traditionally used in the literature to predict thermocapillary film rupture, was found to poorly generalize data for high viscous liquids (ethylene glycol, and aqueous solutions of glycerol) and also data for Θ≤45 deg. The criterion Kp was modified by taking into account characteristic critical film thickness for film rupture under isothermal conditions (no heating), deduced from existing theoretical models. The modified criterion has allowed to successfully generalize data for whole ranges of μ, Re, Θ and q, studied.
Heat Transfer Asymptote in Laminar Tube Flows of Non-Linear Viscoelastic Fluids
Authors: Siginer Dennis A.
Published: March 2011

The fully developed thermal field in constant pressure gradient driven laminar flow of a class of nonlinear viscoelastic fluids with instantaneous elasticity in straight pipes of arbitrary contour ∂D with constant wall flux is investigated. The nonlinear fluids considered are constitutively represented by a class of single mode, non-affine constitutive equations. The driving forces can be large. Asymptotic series in terms of the Weissenberg number Wi are employed to expand the field variables. A continuous one-to-one mapping is used to obtain arbitrary tube contours from a base tube contour ∂D0. The analytical method presented is capable of predicting the velocity and temperature fields in tubes with arbitrary cross-section. Heat transfer enhancement due to shear-thinning is identified together with the enhancement due to the inherent elasticity of the fluid. The latter is to a very large extent the result of secondary flows in the cross-section but there is a component due to first normal stress differences as well. Increasingly large enhancements are computed with increasing elasticity of the fluid as compared to its Newtonian counterpart. Order of magnitude larger enhancements are possible even with slightly viscoelastic fluids. The coupling between inertial and viscoelastic nonlinearities is crucial to enhancement. Isotherms for the temperature field are discussed for non-circular contours such as the ellipse and the equilateral triangle together with the behavior of the average Nusselt number Nu, a function of the Reynolds Re, the Prandtl Pr and the Weissenberg Wi numbers. Analytical evidence for the existence of a heat transfer asymptote in laminar flow of viscoelastic fluids in non-circular contours is given for the first time. Nu becomes asymptotically independent from elasticity with increasing Wi, Nu = f (Pe,Wi) → Nu = f(Pe). This asymptote is the counterpart in laminar flows in non-circular tubes of the heat transfer asymptote in turbulent flows of viscoelastic fluids in round pipes. A different asymptote corresponds to different cross-sectional shapes in straight tubes. The change of type of the vorticity equation governs the trends in the behavior of Nu with increasing Wi and Pe. The implications on the heat transfer enhancement is discussed in particular for slight deviations from Newtonian behavior where a rapid rise in enhancement seems to occur as opposed to the behavior for larger values of the Weissenberg number where the rate of increase is much slower. The asymptotic independence of Nu from elasticity with increasing Wi is related to the extent of the supercritical region controlled by the interaction of the viscoelastic Mach number M and the Elasticity number E, which mitigates and ultimately cancels the effect of the increasingly strong secondary flows with increasing Wi to level off the enhancement. The physics of the interaction of the effects of the Elasticity E, Viscoelastic Mach M, Reynolds Re and Weissenberg Wi numbers on generating the heat transfer enhancement is discussed.
Preliminary Results for Estimating the Backside Heat Losses of a Composite Panel
Authors: Emery A. F.
Published: March 2011

The procedure for the estimation of heat losses from a composite panel that must be known in order to optimize repairs is described. Many papers for estimating parameters describe either simulated experiments or experiments conducted under very controlled situations and the results based upon models of the experiment are usually favorable [1,2]. In contrast when experiments are conducted in the field or under adverse conditions, even when the system is a simple one and amenable to modeling, the estimation can be unexpectedly difficult. The paper describes experiments to estimate the heat losses from the back side of a composite panel undergoing repairs and the modeling of the system. Although the temperatures based on the estimated parameters successfully match the measured steady state temperatures, the model does not accurately predict the transient behavior, raising questions about its ability to determine the magnitude of the observed heat losses and, more importantly, what must be done to improve the model.
Wind Effects on Air-Cooled Condensers for Power Plant Cooling
Authors: Maulbetsch John S. , DiFilippo Michael N. , Owen Michael , Kroger Detlev G.
Published: March 2011

The use of large, air-cooled condensers (ACC’s) for the cooling of turbine exhaust steam at steam/electric power plants is chosen more frequently as concerns over water conservation and water-related environmental issues become more prevalent. While dry cooling achieves significant reductions in plant water consumption, it does so at increased cost and reduced plant efficiency and output when compared to the more commonly used closed-cycle wet cooling systems. Maintaining full cooling capability under all operating conditions is crucial to the efficiency and economic viability of the plant. The effect of wind on ACC performance is the most significant challenge associated with ACC specification, design and performance. Extensive field measurements have been made on five utility-scale ACC’s to determine their operation and performance under varying wind conditions. The primary wind-related effects are shown to be hot air recirculation and degraded fan performance. The total effect on performance plus the relative importance of the two mechanisms are related to wind conditions and ACC configurations. Brief comparisons of field data to the results of CFD modeling are discussed.
An Experimental Investigation of a New Hybrid Film Cooling Scheme
Authors: Ghorab M. , Hassan I. , Lucas T.
Published: March 2011

This paper presents an experimental investigation of the film cooling performance of a New Hybrid Film Cooling Scheme using Thermochromic Liquid Crystal technique. The new scheme has been designed to improve the film cooling performance of gas turbine airfoils. The scheme includes two consecutive film hole configurations with interior bending. The cooling performance of the new scheme was analyzed across blowing ratios of 0.5, 1.0 and 1.5, at a density ratio of 0.94. The results showed that the new scheme enhanced the local and the laterally averaged film cooling performance in terms of effectiveness, and net heat flux reduction in compared to other film hole configurations. The bending effect of the new scheme throttled the secondary flow causing it to spread widely over the downstream surfaces, hence enhancing the film cooling performance at low and high blowing ratios. The hybrid scheme provided an average heat transfer ratio near unity over the downstream surfaces at low and high blowing ratios.
Numercial Simulation of Sinter Cooling Process
Authors: Tian Fengguo , Huang D. Frank , Zhou Chenn Q.
Published: March 2011

A 2-D sinter cooling model is built to simulate the hot iron ore sinter cooling process in a sinter cooler. In this model the convection heat transfer is applied for the heat transfer between the sinter particle skin and the cooling air flow. Thermal conduction is used for the heat conduction within the sinter particles, and fluid dynamics is applied tothe cooling gas distributions. This model will be able to analyze the effects of sinter particle size, size distribution, hot sinter initial temperature, initial temperature distribution, sinter cooler size, cooler configuration and cooling air flow rate as well as cooling air temperature on the sinter cooling process. In this paper the 2-D sinter cooling model is presented along with certain parametric study examples.
Thermal Performance of an Air-Cooled Micro-Grooved Heat Sink
Authors: Kalinowski Paul , Cetegen Edvin , Dessiatoun Serguei , Lawler John
Published: March 2011

The development and testing of a new low-profile, air-cooled heat sink for use in cooling electronic modules are presented. This heat sink consists of a micro-groove surface with a special manifold that generated high heat transfer coefficients but low pressure drops. The optimization of the various geometries that make up a micro-grooved heat sink is discussed, as well as the various CFD simulations that were employed to estimate and optimize the thermal performance of a micro-grooved heat sink. The thermal performance of a prototype micro-grooved heat sink is compared with that of a finned heat sink. Other characteristics of the micro-grooved heat sinks will also be contrasted with the finned heat sink.
Probing Nanoscale Heat and Force Interactions Using Atomic Force Microscopes (AFM)
Authors: Shen Sheng , Mavrokefalos Anastassios , Sambegoro Poetro , Chen Gang
Published: March 2011

Many devices and instruments such as magnetic hard disk drives and atomic force microscopes (AFM) rely on the stable operation of their small probing heads at nanoscale gaps. Due to the small scale of the probing heads, the force interactions (Casimir force and electrostatic force) between the small probes and the surrounding become more significant. The local heating caused by read/write electric currents in hard disk drives or probing laser beams in AFM on the probes inevitably leads to the heat transfer between them and the surrounding. The nanoscale heat and force interactions play a critical role in the performances of those instruments. In this paper, we use a bimaterial AFM cantilever to measure the nanoscale air heat conduction, radiation and force between a microsphere and a substrate. The resulting “heat transfer-distance” and “force-distance” curves clearly show the strong dependence of nanoscale interactions with gap distances.
Simulation of Electron Beam Heating of a Cu Thin Film on a Si Substrate
Authors: Robinson Vance S. , Zhang Xi , Hopkins Forrest
Published: March 2011

Electron beam interaction with thin films is a critical phenomenon in applications such as nanostructure fabrication, surface treatment and curing, surface sterilization, scanning electron microscopy, and electron beam lithography. Unlike bulk solids, thin films whose thickness is on the same order of magnitude as the penetration depth require consideration of interface effects: namely resistance to heat transfer and more electron scattering. In such cases, the energy deposition profile, the location of interfaces and the associated change in material properties must be accounted for. In this paper we describe the thermal simulation of a thin copper film (0.5 μm, 1 μm, 2μm) on a Si substrate irradiated by an electron beam (10 keV, 20 keV, 40 keV). We explore the effect of the interface position relative to the electron range and the local heating effects associated with continuous or long pulse beams.
Acoustic Streaming Induced Thermal Convection in an Enclosure Subject to Uniform Heat Flux
Authors: Aktas Murat K.
Published: March 2011

The effects of acoustic streaming motion on transient convective heat transfer in an air-filled shallow enclosure with a vibrating side wall are investigated. The acoustic streaming phenomenon has been extensively studied by using theoretical and experimental methods. However, the investigations on the effects of longitudinal or transverse temperature gradients on acoustic streaming formation, associated transport phenomena and influence of various parameters on streaming structures are relatively scarce. To our knowledge, the influence of a transverse temperature gradient created by a uniform wall heat flux on streaming patterns has not been studied. In the present study, the fluid motion is driven by the periodic vibration of the enclosure side wall. The vertical walls of the enclosure are adiabatic while the bottom wall is subject to uniform heat flux and the top wall is isothermal or both bottom and top walls are subject to uniform, symmetric heat flux. The fully compressible form of the Navier–Stokes equations are considered to compute the primary oscillatory and secondary mean flow fields. A control-volume method based, explicit time-marching Flux-Corrected Transport (FCT) Algorithm is used to simulate the transport phenomena in the enclosure. The results of an isothermal test case simulation are compared with the existing literature for code validation. Transverse temperature gradient induced by uniform wall heating (symmetrically or non-symmetrically) strongly affects the acoustic streaming structures and velocities. The streaming motion significantly changes the transient behavior of heat transfer in the enclosure compared to pure conduction.
Thermal Management Issues in Fuel Cell Technology
Authors: Brandon N. P. , Matian M. , Marquis A. J. , Adjiman C. S.
Published: March 2011

Fuel cells are electrochemical energy conversion devices that convert chemical energy in fuels directly into electrical energy, without the process of combustion. As a result, they are not constrained by the thermodynamic limitations of heat engines and therefore have the potential to achieve higher efficiencies. Various fuel cell types exist, operating from room temperature to over 1000 °C. This paper focuses on two of the leading fuel cell types, namely the lower temperature (80–120 °C) polymer electrolyte membrane fuel cell (PEMFC) and the higher temperature (500–1000 °C) solid oxide fuel cell (SOFC), with particular attention paid to the importance of thermal management and heat transfer in these systems, as it is thermal transients, and the appropriate design of the thermal management sub-system, that frequently limit fuel cell system performance and durability. Two examples of research from the authors’ laboratories are given; the first relates to the measurement and modelling of heat transfer in PEMFCs; the second to the thermal management of SOFCs.
The Prediction of CHF at Near Critical Pressures by High Order Neural Network (HONN)
Authors: Rostamifard Dariush , Fallahnezhad Mehdi , Zaferanlouei Salman , Setayeshi Saeid , Moradi Mohammad Hassan
Published: March 2011

The critical heat flux (CHF) condition is characterized by a sharp reduction of the local heat transfer coefficient that results from the replacement of liquid by vapor adjacent to the heat transfer surface. We used High Order Neural Network (HONN) as a stronger open box intelligent unit than traditional black box neural networks to predict of CHF at near critical pressures. The process of training and testing this model is done using a set of available published filed data. The CHF values predicted by the HONN model are satisfying compared with the measured data. The predicted values were also compared with those predicted using Feed Forward Neural Network (FNN) and available empirical equations that have been suggested in the literature. It was found that the HONN model with Root Mean Square (RMS) errors of 3.23% in critical pressures conditions has superior performance in predicting the CHF than the best accurate prediction of the methods. Results show by having an HONN model of our nonlinear input-output mapping, there are many advantageous than ANN model including faster running for new data, lesser RMS error and better fitting properties.