bluuebunny/crossref_metadata_2025 huggingface dataset:
Infiltration and Solidification/Remelting of a Pure Metal in a Two-Dimensional Porous Preform
Authors: Tong X. , Khan J. A.
Published: February 2008

Infiltration and solidification/remelting of a pure metal in a two-dimensional porous preform are modeled numerically. It is assumed that under the action of constant applied pressure, the flow of liquid metal through the preform is within the range of the validity of Darcy’s Law. The distinguishing feature of this flow and heat transfer problem is the existence of two moving fronts: the infiltration front and the remelting front. The governing momentum and energy equations are nondimensionalized and cast into a Body-Fitted Coordinates (BFC) systems to deal with the transient and irregular physical domains. The dimensionless groups that govern the infiltration and remelting processes are: the dimensionless pressure difference, the dimensionless melting temperature, the preform permeability ratio, porosity, and the geometric parameters (inlet gate size, and the preform aspect ratio). A computational code has been developed to solve the problem and is verified by using the available published results. The key parameters describing the physical phenomena, i.e., the infiltration front and remelting front evolution, the total infiltration time, and the remelting region size, are presented as a function of the operating variables for two different aspect ratios. The results can be used to optimize the infiltration processing of Metal–Matrix Composites and other related manufacturing processes.
The Effect of Ambient Pressure on Flame Spread Over Thin Cellulosic Fuel in a Quiescent, Microgravity Environment
Authors: Bhattacharjee S. , Altenkirch R. A. , Sacksteder K.
Published: February 2008

Results from recently conducted experiments on flame spread over a thin cellulosic fuel in a quiescent, microgravity environment of a 50/50 volumetric mixture of oxygen and nitrogen (oxygen mass fraction 0.53) at three different pressures—101, 152, and 203 kPa (1, 1.5, and 2.0 atm)—are analyzed. The results are compared with established theoretical results and two different computational flame spread models: one that includes gas-phase radiation, and one that does not. The spread rate behavior from experiment, i.e., an increase of spread rate with pressure, is consistent with the theoretical model that includes gas-phase radiation, and side-view photographs of the flames compare favorably with two-dimensional temperature contours produced computationally from the same model. In contrast, neither the dependence of spread rate on pressure nor the flame shape can be predicted with favorable comparison to experiment if radiation is neglected.
Advanced Calculation Method for the Surface Temperature Distribution of Turbine Blades
Authors: Dorfman A. S.
Published: February 2008

A method of solution of the thermal boundary layer equation for a gas, together with the heat conduction equation for the turbine blade, using the boundary condition of the fourth kind (conjugate problem), is presented. The effect of the surface temperature distribution on the heat transfer coefficient (the effect of thermal history) is considered. This effect is important for gas turbine blades because the difference in temperatures between the blade’s surface and gas usually varies considerably along the blade’s surface; hence, the effect of thermal history can be significant. It is shown that the results, obtained accounting for thermal history, can differ substantially from results calculated with the assumption that the blade’s surface is isothermal. This might be one of the reasons why there is a marked difference between the actual temperature distribution of the turbine blade and the calculated one. It is important to consider the effect of thermal history since it is a fact that the major unknown in the design of turbine blade cooling systems is in the estimation of external heat transfer coefficient (Hannis and Smith, 1989).
Elastoplastic Contact Conductance Model for Isotropic Conforming Rough Surfaces and Comparison With Experiments
Authors: Sridhar M. R. , Yovanovich M. M.
Published: February 2008

A New thermal elastoplastic contact conductance model for isotropic conforming rough surfaces is proposed. This model is based on surface and thermal models used in the Cooper, Mikic, and Yovanovich plastic model, but it differs in the deformation aspects of the thermal contact conductance model. The model incorporates the recently developed simple elastoplastic model for sphere-flat contacts, and it covers the entire range of material behavior, i.e., elastic, elastoplastic, and fully plastic deformation. Previously data were either compared with the elastic model or the plastic model assuming a type of deformation a priori. The model is used to reduce previously obtained isotropic contact conductance data, which cover a wide range of surface characteristics and material properties. For the first time data can be compared with both the elastic and plastic models on the same plot. This model explains the observed discrepancies noted by previous workers between data and the predictions of the elastic or plastic models.
Unstable Aiding and Opposing Mixed Convection of Air in a Bottom-Heated Rectangular Duct Slightly Inclined From the Horizontal
Authors: Lin W. L. , Lin T. F.
Published: February 2008

Buoyancy-induced secondary vortex and reverse flows, flow transition, and the associated heat transfer processes in mixed convection of air through a bottom heated inclined rectangular duct were investigated experimentally. The local spanwise-averaged heat transfer coefficient and air temperature variations with time at selected locations were measured and the cross plane secondary flow was visualized for the Reynolds number ranging from 35 to 186, Grashof number up to 5 × 106, and −20° ≤ Φ ≤ 26°. The results indicated that the heat transfer enhancement is due to the presence of the buoyancy-driven secondary vortex flow and/or reverse flow. The onset of thermal instability was found to move upstream for a larger negative inclined angle (opposing convection) and/or a higher Grashof number and to be delayed for a larger positive inclined angle (aiding convection) and/or a higher Reynolds number. At increasing Grashof number, the instantaneous flow visualizations clearly showed the changes of the vortex flow and/or reverse flow structures in the downstream section of the duct. At slightly supercritical Grashof numbers the secondary flow is in the form of two pairs of longitudinal rolls with the vortex flow ascending along the side walls. For higher Grashof numbers the vortex rolls rotate in the opposite direction with the secondary flow descending near the side walls and the flow was found to be time periodic. At even higher Grashof numbers in opposing convection the buoyancy induced reverse flow exists and the flow is quasi-periodic. Further raising the Grashof number or lowering the Reynolds number causes the flow to change from a transitional quasi-periodic state to a chaotic turbulent state.
Combined Natural Convection–Conduction and Radiation Heat Transfer in a Discretely Heated Open Cavity
Authors: Dehghan A. A. , Behnia M.
Published: February 2008

Combined natural convection, conduction, and radiation heat transfer in an open-top upright cavity containing a discrete heat source has been modeled numerically. The surface emissivity has been varied and its effects on the flow and thermal fields have been determined for different values of Rayleigh number. The complex interaction of the three modes of heat transfer mechanisms is explored by solving the coupled convection, conduction, and radiation equations. It is noted that the inclusion of radiation has a significant effect on the flow, resulting in the formation of a recirculation zone within the cavity. Comparison of the local heat transfer coefficients for the conjugate analysis and no radiation case reveals that the inclusion of radiation has a negligible effect on the heat transfer performance of the heat source. However, comparison of the numerical results with experimental observations shows that accurate prediction of the flow and thermal fields is strongly dependent on the consideration of radiation heat transfer in the numerical case.
Absorption/Scattering Coefficients and Scattering Phase Functions in Reticulated Porous Ceramics
Authors: Hendricks T. J. , Howell J. R.
Published: February 2008

Spectral absorption and scattering coefficients and spectral scattering phase functions have been derived for partially stabilized zirconia (PS ZrO2) and oxide-bonded silicon carbide (OB SiC) reticulated porous ceramics (RPCs) across the wavelength range 0.4–5.0 μm. These spectral radiative properties were investigated and quantified for 10 ppi (pores/inch), 20 ppi, and 65 ppi materials. Radiative properties were recovered from spectral hemispherical reflectance and transmittance measurements using inverse analysis techniques based upon discrete ordinates radiative models. Two dual-parameter phase functions were investigated for these materials: one based on the physical structure of reticulated porous ceramics and the other a modified Henyey–Greenstein phase function. The modified Henyey–Greenstein phase function provided the most consistent spectral radiative properties. PS ZrO2 radiative properties exhibited strongly spectrally dependent behavior across the wavelength range studied. OB SiC radiative properties exhibited radiative behavior that was relatively independent of wavelength across the wavelength spectrum studied. OB SiC also demonstrated consistently higher absorption coefficients than PS ZrO2 at all wavelengths. Spectral scattering albedos of PS ZrO2 were discovered to be in the range 0.81–0.999 and increased as ppi rating increased, while those for OB Sic were lower in the range 0.55–0.888 and decreased as ppi rating increased. The average extinction efficiencies for 0.4–5.0 μm were discovered to be 1.45 for Ps ZrO2 and 1.70 for OB SiC. Extinction coefficients were discovered to correlate well with geometric optics theoretical models and electromagnetic wave/fiber interaction models based on independent scattering and absorption mechanisms.
Heat Transfer in Open Cell Foam Insulation
Authors: Doermann D. , Sacadura J. F.
Published: February 2008

Heat transfer in open cell foam insulation occurs by conduction through the solid material and through the gas in the cell interior and by thermal radiation, which propagates through the structure. The conductive process within these media is described using a simple parallel-series model. Spectral volumetric absorption and scattering coefficients as well as the spectral phase function are predicted using a combination of geometric optics laws and diffraction theory to model the interaction of radiation with the particles forming the foam. The particles considered are both struts formed at the juncture of three cells and strut junctures. The radiative properties can then be utilized to obtain a weighted extinction coefficient, which can be used in the Rosseland equation to obtain the radiative flux. The innovative part of the work lies in the radiative properties predictive model. This new model is compared with simpler ones.
Performance of Plate Finned Tube Heat Exchangers Under Dehumidifying Conditions
Authors: Wang Chi-chuan , Hsieh Yi-chung , Lin Yur-tsai
Published: February 2008

Systematic studies of continuous fin-and-tube tube heat exchangers under dehumidifying conditions are reported in the present study. The heat exchangers consist of nine fin-and-tube heat exchangers having plane fins. The effects of fin spacing, the number of tube row, and inlet conditions are investigated. Data are presented in terms of j factors and friction factors f. It is found that the inconsistencies in the open literature may be associated with the wet fin efficiency. A correlation is proposed for the present plate fin configuration; this correlation can describe 92 percent of j, and 91 percent of the f data within ±10 percent.
Radiation Heat Transfer of Arbitrary Three–Dimensional Absorbing, Emitting and Scattering Media and Specular and Diffuse Surfaces
Authors: Maruyama S. , Aihara T.
Published: February 2008

Analysis of radiation heat transfer using the Radiation Element Method by Ray Emission Model, REM2, is described. The REM2 is a generalized numerical method for calculating radiation heat transfer between absorbing, emitting and scattering media and specular surfaces with arbitrary three–dimensional configurations. The ray emission model for various radiation elements is expressed by polyhedrons and polygons. Arbitrary thermal conditions can be specified for each radiation element, and generalized radiation transfer can be achieved for both of surface and volume elements by introducing a new definition of view factors. The accuracy of the present method is verified using simple configurations. A cubic participating medium with a spherical cavity covered with specular and diffuse surfaces is analyzed as an example of an arbitrary configuration. The temperature distribution shows good accuracy with a small number (45) of rays emitted from each element compared with the Monte Carlo method.
Surface Temperature Profiles Due to Radiant Heating in a Thermocapillary Channel Flow
Authors: Epstein M. , Burelbach J. P. , Plys M. G.
Published: February 2008

Experiments were conducted to determine the temperature distribution at the surface of a thermocapillary channel flow generated by heating a shallow liquid layer locally from above with a radiant energy source. The measured temperature profiles are explained by an available nonlinear conduction model that incorporates the combined effects of thermocapillary and thermogravitational convection in shallow liquid layers.
Condensing and Evaporating Heat Transfer and Pressure Drop Characteristics of HFC-134a and HCFC-22
Authors: Liu X.
Published: February 2008

Condensing and evaporating heat transfer and pressure drop characteristics of an ozone friendly refrigerant HFC-134a and a HCFC refrigerant R-22, flowing inside a 9.5 mm (3/8 in) OD axially grooved tube were investigated experimentally to obtain the quasi-local heat transfer data and the correlations. When compared to R-22 at the same refrigerant flow rate, the condensing heat transfer coefficients for R-134a are 8 percent to 18 percent higher, and the pressure drop is 50 percent higher. The evaporating heat transfer coefficient with R-134a is about the same for mass velocity below 270 kg/m2-s and decreases above that velocity, relative to R-22. Performance characteristics are compared with data from literature reports.
Nonaxisymmetric Convection in Stationary Gas Tungsten Arc Weld Pools
Authors: Joshi Y. , Dutta P. , Schupp P. E. , Espinosa D.
Published: February 2008

Observations of surface flow patterns of steel and aluminum GTAW pools have been made using a pulsed laser visualization system. The weld pool convection is found to be three-dimensional, with the azimuthal circulation depending on the location of the clamp with respect to the torch. Oscillation of steel pools and undulating motion in aluminum weld pools are also observed even with steady process parameters. Current axisymmetric numerical models are unable to explain such phenomena. A three-dimensional computational study is carried out in this study to explain the rotational flow in aluminum weld pools.
Liquid Crystal Thermography on the Fluid Solid Interface of Rotating Systems
Authors: Camci C. , Glezer B.
Published: February 2008

Liquid crystal thermography is an effective method widely employed in transient and steady-state heat transfer experiments with excellent spatial resolution and good accuracy. Most of the past studies in liquid crystal thermography deal with stationary conditions. The present investigation deals with the influence of rotation on the color response of encapsulated liquid crystals attached to a flat rotating surface. A general methodology developed for the application of thermochromic liquid crystals in rotating systems is described for the first time. The investigation is performed for a rotational speed range from 0 to 7500 rpm using two different coatings displaying red at 30° and 45°C, under stationary conditions. Local liquid crystal color on the surface of a rotating disk is correlated with local temperature as measured by a non-intrusive infrared sensor at various rotational speeds. An immediate observation from the present study is that the color response (hue) of encapsulated liquid crystals is not altered by either the centrifugal acceleration of the rotating environment or the aerodynamic friction force at the rotating disk-air interface. Present investigation also shows that when a stroboscope light is introduced, the color response is not significantly altered due to additional periodic illumination. A complete and general experimental methodology including rotating surfaces with non-axisymmetric temperature distribution is presented. Results from the current liquid crystal technique agree well with the theoretical adiabatic temperature rise of a free rotating disk as predicted by an analytical method.
The Effects of Forced Convection on the Power Dissipation of Constant-Temperature Thermal Conductivity Sensors
Authors: Huang Y. , Bau H. H.
Published: February 2008

The effect of forced convection on the power dissipation of cylindrical and planar, constant temperature, thermal conductivity detectors (TCDs) is investigated theoretically. Such detectors can be used either for on-line continuous sensing of fluid thermal conductivity or for determining the sample concentrations in gas chromatography. A low Peclet number, asymptotic theory is constructed to correlate the TCD’s power dissipation with the Peclet number and to explain experimental observations. Subsequently, the effect of convection on the TCD’s power dissipation is calculated numerically for both time-independent and time-dependent flows. The theoretical predictions are compared with experimental observations.
Experimental Study of Forced Convection From Isothermal Circular and Square Cylinders and Toroids
Authors: Ahmed G. Refai , Yovanovich M. M.
Published: February 2008

Experimental studies of forced convection heat transfer from different body shapes were conducted to determine the effects of Reynolds number and different characteristic body lengths on the area-averaged Nusselt number. Although the bodies differed significantly in their shapes, they had approximately the same total surface area, A = 11,304 mm2 ± 5%. This ensured that for a given free stream velocity and total heat transfer rate all bodies had similar trends for the relationship of Nusselt and Reynolds numbers. The experimental program range was conducted in the Reynolds number range 104≤ReA≤105 and Prandtl number 0.71. Finally, the empirical models for forced convection heat transfer were developed. These empirical models were valid for a wide range of Reynolds numbers 0≤ReA≤105. The present experimental correlations were compared with available correlation equations and experimental data. These comparisons show very good agreement.
Solar Radiation Assisted Natural Convection in Uniform Porous Medium Supported by a Vertical Flat Plate
Authors: Chamkha A. J.
Published: February 2008

Natural convection flow of an absorbing fluid up a uniform porous medium supported by a semi-infinite, ideally transparent, vertical flat plate due to solar radiation is considered. Boundary-layer equations are derived using the usual Boussinesq approximation and accounting for applied incident radiation flux. A convection type boundary condition is used at the plate surface. These equations exhibit no similarity solution. However, the local similarity method is employed for the solution of the present problem so as to allow comparisons with previously published work. The resulting approximate nonlinear ordinary differential equations are solved numerically by a standard implicit iterative finite-difference method. Graphical results for the velocity and temperature fields as well as the boundary friction and Nusselt number are presented and discussed.
Boundary Layer Transition Under High Free-Stream Turbulence and Strong Acceleration Conditions: Part 2—Turbulent Transport Results
Authors: Volino R. J. , Simon T. W.
Published: February 2008

Measurements from heated boundary layers along a concave-curved test wall subject to high (initially 8 percent) free-stream turbulence intensity and strong (K = (ν/U∞2 dU∞/dx, as high as 9 × 10−6) acceleration are presented and discussed. Conditions for the experiments were chosen to simulate those present on the downstream half of the pressure side of a gas turbine airfoil. Turbulence statistics, including the turbulent shear stress, the turbulent heat flux, and the turbulent Prandtl number are presented. The transition zone is of extended length in spite of the high free-stream turbulence level. Turbulence quantities are strongly suppressed below values in unaccelerated turbulent boundary layers. Turbulent transport quantities rise with the intermittency, as the boundary layer proceeds through transition. Octant analysis shows a similar eddy structure in the present flow as was observed in transitional flows under low free-stream turbulence conditions. To the authors’ knowledge, this is the first detailed documentation of a high-free-stream-turbulence boundary layer flow in such a strong acceleration field.
Heat Transfer in the Forced Laminar Wall Jet
Authors: Quintana D. L. , Amitay M. , Ortega A. , Wygnanski I. J.
Published: February 2008

The mean and fluctuating characteristics of a plane, unsteady, laminar, wall jet were investigated experimentally for a constant wall-temperature boundary condition. Temperature and streamwise velocity profiles, including the downstream development of the thermal and hydrodynamic boundary layer thicknesses, were obtained through simultaneous hot and cold wire measurements in air. Even at relatively low temperature differences, heating or cooling of a floor surface sufficiently altered the mean velocity profile in the inner, near-wall region to produce significant effects on the jet stability. Selective forcing of the flow at the most amplified frequencies produced profound effects on the temperature and velocity fields and hence the time-averaged heat transfer and shear stress. Large amplitude excitation of the flow (up to 2 percent of the velocity measured at the jet exit plane) at a high frequency resulted in a reduction in the maximum skin friction by as much as 65 percent, with an increase in the maximum wall heat flux as high as 45 percent. The skin friction and wall heat flux were much less susceptible to low-frequency excitation.
Effects of Surface Properties on Radiative Transfer in a Cylindrical Tube With a Nonparticipating Medium
Authors: Sivathanu Y. R. , Gore J. P.
Published: February 2008

Radiative heat transfer inside a cylindrical tube is modeled using a statistical method called the discrete probability function (DPF) method. The DPF method involves solution of the equation of radiative heat transfer using Lagrangian simulations of representative photon trajectories on a discrete spatial grid. The DPF method is different from the Markov Chain method in terms of associating a probability with each state of the photon rather than a transition from one state to another. The advantages and disadvantages of the DPF method in comparison to the Markov Chain method are demonstrated in this paper using two practical applications of the cylindrical tube radiative heat transfer problem. The cylindrical tube has a hot source at one end and a detector at the other end. The cylindrical wall absorbs and reflects (both diffusely and specularly) the radiation incident on it. The present calculations have applications in: (1) intrusive pyrometry with collimating light guides, and (2) measurement of the spectral absorption and reflection coefficients of coatings using two, coated cylindrical tubes as specimen. The results show that: (1) the effect of light guide surface properties on errors in pyrometry must be carefully assessed, and (2) the method can be used for a convenient evaluation of radiative properties of coatings.
Radiative Transfer in Pulsed-Laser-lnduced Plasma
Authors: Xu X. , Song K. H.
Published: February 2008

When a high-power pulsed laser interacts with materials, a plasma layer containing micrometer-size particles is formed above the target surface. The laser induced plasma changes the energy coupling mechanism between the laser beam and the target. This work investigates the radiative heat transfer process in the excimer laser generated plasma layer on the Ni specimen, in the laser fluence range between 1.5 and 5 J/cm2. Novel diagnostic techniques are developed to measure transient transmission and scattering of the thin plasma layer within the duration of the laser pulse. Based on the measurement results, radiative heat transfer analysis is performed to evaluate the radiative properties of the plasma layer, including the optical depth, the absorption coefficient, the single scattering phase function, and the scattering size parameters. Knowledge of the radiative properties of the laser induced plasma helps to understand the energy transfer process during laser-materials interaction. Further, this work demonstrates the feasibility of using the transient scattering measurement for in situ monitoring of the size of the laser ejected particles.
Melting in a Two-Component Packed Bed
Authors: Pak J. , Plumb O. A.
Published: February 2008

The melting of a multi-component mixture that consists of melting and nonmelting components is examined. A numerical solution is obtained by solving the energy equation and continuity equations for both solid and liquid phases for a one-dimensional system. Both constant volume and constant porosity melting models are explored. Experiments were conducted to confirm the numerical results. During the experiments, heat was applied at the bottom of a rectangular packed bed containing salol (benzoic acid 2-hydroxyphenylester) and glass beads. The experiments were conducted with various sizes of glass beads ranging from 120 μm to 450 μm in diameter with salol having an average diameter of 90 μm. During the melting process, the volume fraction of the phase-change material (salol), temperature, and applied heat flux were measured. The volume fraction of the salol was measured utilizing gamma attenuation. Upon melting, the salol moves as two fronts, one downward due to gravity and one upward due to capillary action. The constant porosity model yields results which compare well with the experimental data.
Heat Transfer and Friction Correlations for Wavy Plate Fin-and-Tube Heat Exchangers
Authors: Kim N.-H. , Yun J.-H. , Webb R. L.
Published: February 2008

This paper deals with heat exchangers having plate fins of herringbone wave configuration. Correlations are developed to predict the air-side heat transfer coefficient and friction factor as a function of flow conditions and geometric variables of the heat exchanger. Correlations are provided for both staggered and in-line arrays of circular tubes. A multiple regression technique was used to correlate 41 wavy fin geometries by Beecher and Fagan (1987), Wang et al. (1995), and Beecher (1968). For the staggered layout, 92 percent of the heat transfer data are correlated within ±10 percent and 91 percent of the friction data are correlated within ±15 percent.
Effect of Suspended Particles on Crystallization Fouling in Plate Heat Exchangers
Authors: Bansal B. , Mu¨ller-Steinhagen H. , Chen X. D.
Published: February 2008

The presence of suspended particles in solutions significantly affects the crystallization rate. This study investigates the effects of calcium sulphate (crystallizing) particles and alumina (noncrystallizing) particles on calcium sulphate crystallization fouling in a plate heat exchanger. Calcium sulphate particles are formed during the preparation of calcium sulphate solution due to breakage of calcium sulphate crystals growing on the heat transfer surface. These suspended particles settle on the heat transfer surface and act as nuclei. The availability of extra nucleation sites increases the crystallization rate significantly. These particles can be removed with a filter, and the removal of the particles prevents this extra assistance available for crystallization. Therefore, the crystallization rate is reduced markedly. Alumina particles were purposely added during the preparation of calcium sulphate solutions. These particles attach loosely to the heat transfer surface compared with crystalline deposits which adhere strongly. Therefore, calcium sulphate crystals growing on these particles are removed easily. Also, alumina particles settling on the growth faces of calcium sulphate crystals may act as a distorting agent. This slows down the growth of the crystals.
A Maintenance Strategy for Heat Transfer Equipment Subject to Fouling: A Probabilistic Approach
Authors: Zubair S. M. , Sheikh A. K. , Budair M. O. , Badar M. A.
Published: February 2008

Fouling of heat transfer surfaces introduces a major uncertainty into the design and operation of heat transfer equipment. After a brief discussion on impact of fouling, we present and discuss a stochastic approach to the analysis of fouling models. In view of the performance indicator (U/Uc) of the heat exchangers, a maintenance strategy for planned maintenance schedules is presented. Various scenarios of reliability based maintenance strategy are introduced. The strategy is explained in terms of the scatter parameter (α) of the time-to-fouling distribution corresponding to a critical level of fouling, and the risk factor (p) representing the probability of tubes being fouled to a critical level after which a cleaning cycle is needed. In addition, the cost implications of the above mentioned strategy are explained and their impact on heat exchanger maintenance is highlighted.
Statistical Aspects of CaCO3 Fouling in AISI 316 Stainless-Steel Tubes
Authors: Zubair S. M. , Sheikh A. K. , Budair M. O. , Haq M. U. , Quddus A. , Ashiru O. A.
Published: February 2008

Calcium carbonate fouling is typically encountered in a cooling-water circulating system. An experimental program is initiated to study fouling growth law (s) as well as the basic mechanism of calcium carbonate (CaCO3) scaling. After a brief description of the experimental apparatus and procedure for calculating fouling resistance, we present the deposition data in terms of fouling resistance as a function of time taken at different sections of the tube. In addition, the randomness of fouling growth is illustrated by repeating the experiments several times under the same thermal-hydraulic conditions. The results are presented in terms of a set of sample functions and their associated probability density functions at various levels of fouling. In addition, basic mechanism of CaCO3 scale formation is also explained through Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX).
Marangoni Mechanism in Pulsed Laser Texturing of Magnetic Disk Substrates
Authors: Bennett T. D. , Krajnovich D. J. , Grigoropoulos C. P. , Baumgart P. , Tam A. C.
Published: February 2008

This paper proposes a mechanism for topographical features formed during pulsed laser texturing of Ni-P magnetic disk substrates. A salient feature of the process is the ability to raise a central peak in the irradiated spot, providing a low contact area bearing for the slider-head of a computer hard drive. Formation of topography is believed to involve gradient capillary forces acting at the surface of the molten pool (Marangoni effect). However, the central peak cannot be explained with thermo-capillary forces alone. Therefore, it is suggested that a compositional gradient due to the depletion of a surfactant at the molten surface provides the necessary condition to reverse the capillary force in the central region. This perspective is investigated using finite element modeling of the Lagrangian fluid mechanics coupled with heat and mass diffusion.
A Modified Analysis of Counter Flow Wet Cooling Towers
Authors: El-Dessouky H. T. A. , Al-Haddad A. , Al-Juwayhel F.
Published: February 2008

The paper describes a theoretical investigation for the steady-state counter flow wet cooling tower with modified definitions for both the number of transfer units and the tower thermal effectiveness. The modified number of transfer units is dependent on both air and water heat capacity. The effectiveness is defined by the tower cooling range and the approach to equilibrium. A new expression relating the tower effectiveness to the modified number of transfer units and the capacity rate ratio has been developed. The model considered the resistance to heat transfer in the water film, the nonunity of the Lewis number, and the curvature of the saturated air enthalpy curve. A procedure for implementing the model in designing or rating cooling towers has been outlined and demonstrated through illustrative examples. The model compares very satisfactorily with other methods such as Logarithmic Mean Enthalpy Difference (LMED) and conventional effectiveness—NTU. Within the ranges used, the obtained results showed that substantial errors varied from +4.289 to −2.536 percent can occur in calculating the cooled water outlet temperature, and errors from +42.847 to −16.667 percent can occur in estimating the tower thermal characteristics.
Reexamination of the Transmittance Formulae of a Lamina
Authors: Zhang Z. M.
Published: February 2008

Expressions for the optical properties of laminated layers have drawn much attention from researchers in recent years because of emerging optoelectronic applications (Chen and Tien, 1992; Cunsolo et al., 1992; Zhang and Flik, 1993, Engelbrecht, 1994; Grossman and McDonald, 1995; Anderson and Bayazitoglu, 1996). Different equations must be applied in particular situations (Chen and Tien, 1992; Zhang, 1994). After carefully examining the transmittance formulae of a lamina, this work shows that the geometric-optics formula may result in a significant error for a highly absorbing medium even in the incoherent limit (when interference effects are negligible).
The Effect of Imprecisions in Thermal Sensor Location and Boundary Conditions on Optimal Sensor Location and Experimental Accuracy
Authors: Emery A. F. , Fadale T. D.
Published: February 2008

Optimal sensor locations and the information content obtained when estimating thermal parameters using the inverse method are significantly affected by uncertainties in sensor position and in the system parameters. This paper describes the effects of these uncertainties. It is shown that the effect of sensor location uncertainties can be reduced by placing temperature sensors in locations of minimum heat flux. In transient experiments, the uncertainties in the boundary conditions have the greatest effect at points of high heat flux and cause the optimal sensor locations to move from the boundary with the highest convective heat transfer coefficient to the boundary with the lowest in an abrupt manner.
Thermal Contact Conductance of Spherical Rough Metals
Authors: Lambert M. A. , Fletcher L. S.
Published: February 2008

Junction thermal conductance is an important consideration in such applications as thermally induced stresses in supersonic and hypersonic flight vehicles, nuclear reactor cooling, electronics packaging, spacecraft thermal control, gas turbine and internal combustion engine cooling, and cryogenic liquid storage. A fundamental problem in analyzing and predicting junction thermal conductance is determining thermal contact conductance of nonflat rough metals. Workable models have been previously derived for the limiting idealized cases of flat, rough, and spherical smooth surfaces. However, until now no tractable models have been advanced for nonflat rough “engineering” surfaces which are much more commonly dealt with in practice. The present investigation details the synthesis of previously derived models for macroscopically nonuniform thermal contact conductance and contact of nonflat rough spheres into a thermomechanical model, which is presented in an analytical/graphical format. The present model agrees well with representative experimental conductance results from the literature for stainless steel 303 and 304 with widely varying nonflatness (2 to 200 μm) and roughness (0.1 to 10 μm).
Observation and Computation of Vortex and/or Reverse Flow Development in Mixed Convection of Air in a Slightly Inclined Rectangular Duct
Authors: Lin W. L , Lin T. F.
Published: February 2008

Combined flow visualization and conjugated numerical heat transfer analysis were carried out to study the axial evolution of the buoyancy induced secondary vortex and reverse flow in a mixed convective air flow through a bottom heated, slightly inclined rectangular duct. Results were obtained for the Grashof number Gr ranging from 1.6 × 103 to 2.8 × 105, inclined angle φ from −20 deg to 26 deg and the Reynolds number Re below 102 covering the steady and time dependent flows. For the buoyancy-opposing case, at a certain critical buoyancy-to-inertia ratio depending on the Re and φ both the experimental and numerical results clearly showed the generation of the longitudinal vortex rolls in the entry half of the duct and a slender reverse flow zone was induced near the exit end of the duct. At a higher buoyancy-to-inertia ratio the stronger reverse flow moves upstream and is in a time periodic snaking motion which is considered to result from the Kelvin-Helmholtz instability associated with the two counter flow streams, namely, the downstream moving longitudinal vortex rolls and the upstream moving reverse flow. Through the viscous shearing effects the strong snaking reverse flow induces a number of eddies moving along it and the longitudinal rolls are pushed towards the duct sides. This strong interaction between the vortex flow and reverse flow leads to an earlier transition to turbulence. A correlation equation was proposed for the penetration length of the reverse flow. However, for buoyancy-assisting flow no reverse flow is induced and the longitudinal vortex rolls prevail for the buoyancy-to-inertia ratio up to 2.8 × 105. Significant conjugated heat transfer effects were noted from the numerical results.
Optimal Geometric Arrangement of Staggered Vertical Plates in Natural Convection
Authors: Ledezma G. A. , Bejan A.
Published: February 2008

This paper documents the geometric optimization of an assembly of staggered vertical plates that are installed in a fixed volume. The heat transfer is by laminar natural convection. The objective is to maximize the overall thermal conductance between the assembly of plates and the surrounding fluid. The geometric parameters that vary are the horizontal spacing between adjacent columns of plates, the number of plates mounted in each vertical column, the plate dimensions, and the stagger between columns. The optimization is performed numerically and experimentally in the range Pr = 0.72 and 103 ≤ RaL ≤ 106, where RaL is the Rayleigh number based on the vertical dimension of the assembly (L). The results for the optimal horizontal spacing are correlated using formulas derived from the theory of the intersection of asymptotes (Bejan, 1984, 1995).
Velocity Field in Turbulent Subcooled Boiling Flow
Authors: Roy R. P. , Velidandla V. , Kalra S. P.
Published: February 2008

The velocity field was measured in turbulent subcooled boiling flow of Refrigerant-113 through a vertical annular channel whose inner wall was heated. A two-component laser Doppler velocimeter was used. Measurements are reported in the boiling layer adjacent to the inner wall as well as in the outer all-liquid layer for two fluid mass velocities and four wall heat fluxes. The turbulence was found to be inhomogeneous and anisotropic and the turbulent kinetic energy significantly higher than in single-phase liquid flow at the same mass velocity. A marked shift toward the inner wall was observed of the zero location of the axial Reynolds shear stress in the liquid phase, and the magnitude of the shear stress increased sharply close to the inner wall. The near-wall liquid velocity field was quite different from that in single-phase liquid flow at a similar Reynolds number. Comparison of the measurements with the predictions of a three-dimensional two-fluid model of turbulent subcooled boiling flow show reasonably good agreement for some quantities and a need for further development of certain aspects of the model.
A Predictive Model for Condensation in Small Hydraulic Diameter Tubes Having Axial Micro-Fins
Authors: Yang C.-Y. , Webb R. L.
Published: February 2008

A semiempirical model is proposed to predict the condensation coefficient inside small hydraulic diameter extruded aluminum tubes having microgrooves. The model accounts for the effects of vapor shear and surface tension forces. Surface tension force is effective in enhancing the condensation coefficient as long as the fin tips are not flooded by condensate. This enhancement increases as mass velocity is reduced. At high mass velocity the flow is vapor shear controlled and the surface tension contribution is very small. The surface tension effect is strongly affected by the fin geometry. A smaller fin tip radius provides a higher surface tension drainage force. A large cross sectional area in the interfin region will allow the surface tension enhancement to occur at lower vapor quality. Separate models are developed for the surface tension and vapor shear controlled regimes and the models are combined in the form of an asymptotic equation. The vapor shear model is based on use of an equivalent mass velocity and the heat-momentum transfer analogy. The surface tension model is analytically based. The model is validated by predicting the authors data for two tube geometries using R-12 and R-134a, and the model predicts 95 percent of the condensation data within ±16 percent.
Three-Dimensional Free Convection Boundary Layers in Porous Media Induced by a Heated Surface With Spanwise Temperature Variations
Authors: Rees D. A. S.
Published: February 2008

In this paper we consider the effect of a nonuniform surface temperature distribution on the steady laminar free convection boundary layer flow induced by a vertical plate embedded in a fluid-saturated porous medium. The surface temperature profile exhibits sinusoidal variations in the spanwise (horizontal) direction, but the minimum temperature remains above or equal to that of the ambient medium. The resulting boundary layer flow is three-dimensional, and the governing equations are solved using a combination of a spanwise spectral decomposition and the Keller-box method. Detailed results in terms of the evolution of the rates of heat transfer and the developing thermal field are presented. The numerical work is supplemented by an asymptotic analysis valid far downstream where it is found that the effect of nonuniform heating becomes confined to a thin layer of uniform thickness embedded within the main growing boundary layer.
Observation, Prediction, and Correlation of Geometric Shape Evolution Induced by Non-Isothermal Sintering of Polymer Powder
Authors: Kandis M. , Bergman T. L.
Published: February 2008

Sintering of powders occurs in a wide array of manufacturing technologies and geophysical phenomena. Despite the prevalence of powder sintering, little attention has been paid to sintering of macroscopic shapes under non-isothermal conditions. In this paper (1) features of a representative, experimentally grown solid shape produced by non-isothermal sintering are discussed, (2) prediction of the solid shape evolution is achieved using a hybrid heat transfer, sintering, and consolidation model, (3) comparison of the actual and predicted solid shapes is made, (4) parametric simulation of solid part growth in conjunction with void expansion is attained, and (5) analytical predictions of the void space evolution are developed and discussed.
Three-Dimensional Electron-Beam Deflection and Missed Joint in Welding Dissimilar Metals
Authors: Wei P. S. , Chung F. K.
Published: February 2008

Three-dimensional deflection of the electron beam resulting in a missed joint due to thermoelectric magnetism generated while welding dissimilar metals is systematically investigated. The incident energy rate distribution is assumed to be Gaussian and the deep and narrow welding cavity induced is idealized as a paraboloid of revolution. With a three-dimensional analytical solution for the temperature and by solving Maxwell’s electromagnetic equations, thermoelectric currents, magnetic flux densities, and deflections of the beam are found. The predictions agree with available experimental data. The results find that missed joints can be reduced by increasing the dimensionless accelerating voltage-to-Seebeck e.m.f. parameter, Peclet number, and effective electrical contact resistance parameter, and decreasing dimensionless beam power, magnetic permeabilities, and electrical conductivity ratio between metals 1 and 2. Tilting workpieces and shifting the electron gun from the joint line are also feasible. A three-dimensional analysis is required for a successful determination of beam deflection.
Multidimensional Transient Conduction Analysis by Generalized Transfer Functions Tables
Authors: Laurenti L. , Marcotullio F. , Ponticiello A.
Published: February 2008

This work provides a general tool for accurate multidimensional transient analysis by means of polynomial heat conduction transfer functions (CTF) which are valid for the whole class of objects having the same geometric shape. With this aim the governing conduction equations were turned into dimensionless variables, such as the well known Biot number (Bi) and Fourier modulus (Fo), and thereafter handled through a finite-element technique to create the CTF with respect to any arbitrary point of the conductive field. In addition, by selecting the nondimensional time interval (ΔFo) according to the sampling theorem, i.e., as a function of Bi, the CTF were scheduled in the form of one-entry tables. The Bi incremental values were furthermore assessed to provide, for any input function, temperature histories within equal percent deviations, say 10 percent. Since for many purposes a five percent tolerance will usually suffice, the CTF tables can be used without the necessity of interpolating. As an example, generalized CTF tables are presented for the central and one of the corner points of a cube.
Heat Transfer in a Thin-Film Flow in the Presence of Squeeze and Shear Thinning: Application to Piston Rings
Authors: Radakovic D. J. , Khonsari M. M.
Published: February 2008

The governing equations and appropriate numerical solutions are presented for the heat transfer and the flow of a shear thinning fluid confined between a curved and plane surface in a thin-film configuration. One surface undergoes a reciprocating motion and the fluid experiences transverse squeeze action, as in a typical piston ring of an internal combustion engine. The effect of viscosity variations with temperature, in conjunction with the non-Newtonian shear thinning behavior of multigrade oils, are included in the analysis. Extensive numerical simulations of the performance of the piston are presented and compared to the isothermal Newtonian solutions. Computations show that shear thinning can have a significant effect on parameters such as film thickness, viscous drag force, and power loss. The thermal effects from viscous dissipation in the clearance space along the piston ring also influence these parameters, but to a lesser degree.