bluuebunny/crossref_metadata_2025 huggingface dataset:
Free-Stream Turbulence and Concave Curvature Effects on Heated, Transitional Boundary Layers
Authors: Kim J. , Simon T. W. , Russ S. G.
Published: May 2008

An experimental investigation of transition in concave-curved boundary layers at two free-stream turbulence levels (0.6 and 8.6 percent) was performed. For the lower free-stream turbulence intensity case, Go¨rtler vortices were observed in both laminar and turbulent flows using liquid crystal visualization and spanwise velocity and temperature traverses. Transition is thought to occur via a vortex breakdown mode. The vortex locations were invariant with time but were nonuniform across the span in both the laminar and turbulent flows. The upwash regions between two vortices were more unstable than were the downwash regions, containing higher levels of u’ and u’ v’, and lower skin friction coefficients and shape factors. Turbulent Prandtl numbers, measured using a triple-wire probe, were near unity for all post-transitional profiles, indicating no gross violation of Reynolds analogy. No streamwise vortices were observed in the higher turbulence intensity case. This may be due to the high eddy viscosity, which reduces the turbulent Go¨rtler number to subcritical values, thus eliminating the vortices, or due to an unsteadiness of the vortex structure that could not be observed by the techniques used. Based upon these results, predictions that assume two-dimensional modeling of the flow over a concave wall with high free-stream turbulence levels, as on the pressure surface of a turbine blade, seem to be adequate—there is no time-average, three-dimensional structure to be resolved. High levels of free-stream turbulence superimposed on a free-stream velocity gradient (which occurs within curved channels) cause a cross-stream transport of momentum within the flow outside the boundary layer. The total pressure within this region can rise above the value measured at the inlet to the test section.
Heat Transfer in Thin, Compact Heat Exchangers With Circular, Rectangular, or Pin-Fin Flow Passages
Authors: Olson D. A.
Published: May 2008

We have measured heat transfer and pressure drop of three thin, compact heat exchangers in helium gas at 3.5 MPa and higher, with Reynolds numbers of 450 to 36,000. The flow geometries for the three heat exchanger specimens were: circular tube, rectangular channel, and staggered pin fin with tapered pins. The specimens were heated radiatively at heat fluxes up to 77 W/cm2. Correlations were developed for the isothermal friction factor as a function of Reynolds number, and for the Nusselt number as a function of Reynolds number and the ratio of wall temperature to fluid temperature. The specimen with the pin fin internal geometry had significantly better heat transfer than the other specimens, but it also had higher pressure drop. For certain conditions of helium flow and heating, the temperature more than doubled from the inlet to the outlet of the specimens, producing large changes in gas velocity, density, viscosity, and thermal conductivity. These changes in properties did not affect the correlations for friction factor and Nusselt number in turbulent flow.
Numerical Simulation of the Fluid Flow and Heat Transfer Processes During Scavenging in a Two-Stroke Engine Under Steady-State Conditions
Authors: de Castro Gouveia M. , dos Reis Parise J. A. , Nieckele A. O.
Published: May 2008

A numerical simulation of the scavenging process in a two-stroke flat-piston model engine has been developed. Air enters the cylinder circumferentially, inducing a three-dimensional turbulent swirling flow. The problem was modeled as a steady-state axisymmetric flow through a cylinder with uniform wall temperature. The steady-state regime was simulated by assuming the piston head fixed at the bottom dead center. The calculation was performed employing the k–ε model of turbulence. A comparison of the results obtained for the flow field with available experimental data showed very good agreement, and a comparison with an available numerical solution revealed superior results. The effects of the Reynolds number, inlet port angles, and engine geometry on the flow and in-cylinder heat transfer characteristics were investigated. The Nusselt number substantially increases with larger Reynolds numbers and a smaller bore-to-stroke ratio. It is shown that the positioning of the exhaust valve(s) is the main parameter to control the scavenging process.
Natural Convection Liquid Cooling of a Substrate-Mounted Protrusion in a Square Enclosure: A Parametric Study
Authors: Sathe S. B. , Joshi Y.
Published: May 2008

The coupled conduction and natural convection transport from a substrate-mounted heat generating protrusion in a liquid-filled square enclosure is numerically examined. The governing steady two-dimensional equations are solved using a finite-difference method for a wide range of Rayleigh numbers, protrusion thermal conductivities and widths, substrate heights, and enclosure boundary conditions. The results presented apply to liquids with 10≤Pr≤1000. It was established that in many situations it may be inappropriate to specify simple boundary conditions on the solid surface and decouple the conduction within the substrate or the protrusion. Higher Rayleigh numbers, protrusion thermal conductivities, and widths enhanced cooling. A variation in the substrate height did not affect the maximum protrusion temperature; however, the flow behavior was considerably altered. An empirical correlation for the maximum protrusion temperature was developed for a wide range of parametric values. The enclosure thermal boundary conditions changed the heat transfer in the solid region to only a small extent. Immersion cooling in common dielectric liquids was shown to be advantageous over air cooling only if the thermal conductivity of the protrusion was larger than that of the liquid.
Natural Convection in a Partitioned Cubic Enclosure
Authors: Karki K. C. , Sathyamurthy P. S. , Patankar S. V.
Published: May 2008

Numerical solutions are obtained for fluid flow and heat transfer in a cubic enclosure with a vertical adiabatic partition. The two zones of the enclosure are connected by a single rectangular opening. The partition is oriented parallel to the isothermal sidewalls, one of which is heated and the other cooled while the remaining walls are adiabatic. Results have been presented for air for the Rayleigh numbers in the range 104−107. The width of the opening is held fixed while the height, relative to the enclosure height, is varied from 0.25 to 0.75. The effects of various parameters on the flow structure and heat transfer are investigated. The results of the three-dimensional simulation have also been compared with those for the corresponding two-dimensional configurations.
Laminar Mixed Convection of Cold Water in a Vertical Annulus With a Heated Rotating Inner Cylinder
Authors: Ho C. J. , Tu F. J.
Published: May 2008

A numerical investigation is made to evaluate the perturbing effect of forced convection due to axial rotation of the inner cylinder on natural convection heat transfer of cold water with density inversion effects in a vertical cylindrical annulus. The mixed convection heat and fluid flow structures in the annulus are found to be strongly affected by the density inversion effects. The centrifugally forced convection can result in significant enhancement of the buoyant convection heat transfer of cold water with the density inversion parameter being equal to 0.4 or 0.5; thus the slow axial rotation of the inner cylinder can be a viable means for heat transfer augmentation of cold water natural convection in a vertical annulus.
Nucleate Boiling Characteristics of R-113 in a Small Tube Bundle
Authors: Marto P. J. , Anderson C. L.
Published: May 2008

Heat transfer measurements were made during nucleate boiling of R-113 from a bundle of 15 electrically heated, smooth copper tubes arranged in an equilateral triangular pitch. The bundle was designed to simulate a portion of a refrigeration system flooded-tube evaporator. The outside diameter of the tubes was 15.9 mm, and the tube pitch was 19.1 mm. Five of the tubes that were oriented in a vertical array on the centerline of the bundle were each instrumented with six wall thermocouples to obtain an average wall temperature and a resultant average heat transfer coefficient. All tests were performed at atmospheric pressure. The majority of the data were obtained with increasing heat flux to study the onset of nucleate boiling and the influence of surface “history” upon boiling heat transfer. Data taken during increasing heat flux showed that incipient boiling was dependent upon the number of tubes in operation. The operation of lower tubes in the bundle decreased the incipient boiling heat flux and wall superheat of the upper tubes, and generally increased the boiling heat transfer coefficients of the upper tubes at low heat fluxes where natural convection effects are important. The boiling data confirmed that the average heat transfer coefficient for a smooth-tube bundle is larger than obtained for a single tube.
Model of the Evaporating Meniscus in a Capillary Tube
Authors: Swanson L. W. , Herdt G. C.
Published: May 2008

A mathematical model describing the evaporating meniscus in a capillary tube has been formulated incorporating the full three-dimensional Young–Laplace equation, Marangoni convection, London–van der Waals dispersion forces, and nonequilibrium interface conditions. The results showed that varying the dimensionless superheat had no apparent effect on the meniscus profile. However, varying the dispersion number produced a noticeable change in the meniscus profile, but only at the microscopic level near the tube wall. No change in the apparent contact angle was observed with changes in the dimensionless superheat or dispersion number. In all cases, the dimensionless mean curvature was asymptotic to a value equal to that for a hemispherical meniscus. The local interfacial mass flux and total mass transfer rate increased dramatically as the dispersion number was increased, suggesting that surface coatings can play an important role in improving or degrading capillary pumping. The model also predicted that the local capillary pressure remains constant and equal to 2σ/rc regardless of changes in the dimensionless superheat and dispersion number. It should be noted that the results in this study are theoretical in nature and require experimental verification.
Film Condensation of R-113 on Staggered Bundles of Horizontal Finned Tubes
Authors: Honda H. , Uchima B. , Nozu S. , Torigoe E. , Imai S.
Published: May 2008

Film condensation of R-113 on staggered bundles of horizontal finned tubes with vertical vapor downflow was experimentally investigated. Two tubes with flat-sided annular fins and four tubes with three-dimensional fins were tested. The condensate flow and heat transfer characteristics were compared with the previous results for in-line bundles of the same test tubes and a staggered bundle of smooth tubes. The decrease in heat transfer due to condensate inundation was most significant for the in-line bundles of the three-dimensional fin tubes, whereas the decrease was very slow for both the staggered and in-line bundles of the flat-sided fin tubes. The predictions of the previous theoretical model for a bundle of flat-sided fin tubes agreed fairly well with the measured data at a low vapor velocity. The highest heat transfer performance was provided by the staggered bundle of flat-sided fin tubes with fin dimensions close to the theoretically determined optimum values.
Surface Radiation Exchange for Two-Dimensional Rectangular Enclosures Using the Discrete-Ordinates Method
Authors: Sa`nchez A. , Smith T. F.
Published: May 2008

The purpose of this study is to develop a model based on the discrete-ordinates method for computing radiant exchange between surfaces separated by a transparent medium and to formulate the model so that arbitrary arrangements of the surfaces can be accommodated. Heat fluxes from the model are compared to those based on the radiosity/irradiation analysis. Three test geometries that include shadowing and irregular geometries are used to validate the model. Heat fluxes from the model are in good agreement with those from the radiosity/irradiation analysis. Effects of geometries, surface emittances, grid patterns, finite-difference weighting factor, and number of discrete angles are reported.
Natural Convection With Radiation in a Cavity With Open Top End
Authors: Lage J. L. , Lim J. S. , Bejan A.
Published: May 2008

The heat transfer by natural convection and radiation in a two-dimensional cavity with one side heated, one side and bottom insulated, and the top open was investigated numerically. The first part of the study focused on natural convection alone, and concluded with closed-form correlations for the heat transfer from each of the side walls. It was found that the fluid rises along both side walls in the boundary layer regime. In the second part of the study, the natural convection correlations were incorporated into a seven-equation model for combined convection and radiation. The procedure needed for calculating the floating temperature of the insulated wall was illustrated numerically. The individual effects of the dimensions of the cavity, the temperature of the heated wall, and the emissivities of the two side walls were also documented numerically.
Coupled Structure and Radiation Analysis of Acetylene/Air Flames
Authors: Gore J. P. , Ip U.-S. , Sivathanu Y. R.
Published: May 2008

A coupled radiation-structure analysis of turbulent, non-premixed, strongly radiating acetylene/air flames is described. The analysis extends the laminar flamelet concept to include the effects of local radiative heat loss/gain. A new method for the calculation of the radiative source term is presented. New measurements of mean and fluctuating emission temperatures and radiation intensities, and previous data concerning flame structure are used to evaluate the predictions. Results show good agreement between measurements and predictions of flame structure similar to past uncoupled calculations. The mean emission temperatures and the mean visible radiation intensities are substantially underpredicted by the uncoupled analysis. The coupled calculations provide reasonable estimates of both quantities.
Heat and Mass Transport From Thermally Degrading Thin Cellulosic Materials in a Microgravity Environment
Authors: Kushida G. , Baum H. R. , Kashiwagi T. , di Blasi C.
Published: May 2008

A theoretical model describing the behavior of a thermally thin cellulosic sheet heated by external thermal radiation in a quiescent microgravity environment is developed. This model describes thermal and oxidative degradation of the sheet and the heat and mass transfer of evolved degradation products from the heated cellulosic surface into the gas phase. At present, gas phase oxidation reactions are not included. Without buoyancy, the dominant vorticity creation mechanism in the bulk of the gas is absent except at the material surface by the requirement of the no-slip condition. The no-slip condition is relaxed, permitting the flow to be represented by a velocity potential. This approximation is permissible due to the combination of a microgravity environment and low Reynolds number associated with slow small-area heating by external radiation. Two calculations are carried out: heating without thermal degradation, and heating with thermal degradation of the sheet with endothermic pyrolysis, exothermic thermal oxidative degradation, and highly exothermic char oxidation. The results show that pyrolysis is the main degradation reaction. Moreover, self-sustained propagation of smoldering for cellulosic materials is very difficult due to the lack of sufficient oxygen supply in a quiescent environment.
The One-Dimensional Analysis of Oscillatory Heat Transfer in a Fin Assembly
Authors: Houghton J. M. , Ingham D. B. , Heggs P. J.
Published: May 2008

Studies of the transient heat transfer within extended surfaces have so far considered the fins in isolation. The isolated fin model is not representative of the physical boundary conditions within an extended surface heat exchanger since it does not account for the thermal effects of the supporting interface. The aim of this study is to extend the work on transient heat transfer within finned surfaces by incorporating the supporting wall in the problem. A mathematical one-dimensional solution for harmonic oscillatory heat transfer in a fin assembly is derived. It is concluded that, unlike steady-state situations, the transient heat transfer in a fin assembly can only be found by considering both the wall and the fins simultaneously.
Finite Element Formulation for Two-Dimensional Inverse Heat Conduction Analysis
Authors: Hsu T. R. , Sun N. S. , Chen G. G. , Gong Z. L.
Published: May 2008

This paper presents a finite element algorithm for two-dimensional nonlinear inverse heat conduction analysis. The proposed method is capable of handling both unknown surface heat flux and unknown surface temperature of solids using temperature histories measured at a few discrete point. The proposed algorithms were used in the study of the thermofracture behavior of leaking pipelines with experimental verifications.
Impingement Cooling of Electronics
Authors: Hollworth B. R. , Durbin M.
Published: May 2008

Experiments were conducted to determine the performance of a system of low-velocity air jets used to cool a simulated electronics package. The test model consisted of a uniform array of rectangular elements mounted to a circuit board. Each element was cooled by a cluster of four jets, and the spent fluid was vented at one end of the channel formed between the circuit board and the plate from which the jets were discharged. Reported are measurements of system pressure drop and convective heat transfer coefficients for elements at various sites within the array. Results indicate that (for the geometry tested) the largest portion of the total pressure drop occurs across the jet orifices. Further, the crossflow of spent air appears to enhance heat transfer for those elements near the exit end of the channel.
Effects of Wall Conduction and Darcy Number on Laminar Mixed Convection in a Horizontal Square Porous Channel
Authors: Hwang G. J. , Chao C. H.
Published: May 2008

This paper investigates the effects of peripheral wall conduction and Darcy number on laminar mixed convection in the fully developed region of a horizontal square porous channel with a uniform heat input. Because of the combined effects on nonaxisymmetric channel configuration, buoyancy-induced secondary flow, and noninfinite wall conduction parameter, the flow and heat transfer characteristics are greatly affected by the peripherally nonuniform wall temperature distribution. A modified Darcy-Forchheimer-Ergun flow model in the porous medium and a finite-difference method are applied. Secondary flow patterns, isotherms, wall temperature distributions, the friction factor, and the Nusselt number are presented for a flow in the Darcian porous medium (Da→0) to a pure fluid flow (Da → ∞) with Kp = 10−4 − 104, Gr = 0 − 1010, and Pr = 0.1, 0.73, 7.2, and 100. Wall temperature distributions and the Nusselt numbers are compared with existing experimental data.
Flame Temperature Measurement by Monitoring an Alkali Emission Doublet Exposed to a Selectively Filtered Background Source
Authors: Pattee H. A. , Peterson R. B.
Published: May 2008

The traditional line reversal method for measuring flame temperatures has been modified to eliminate balancing of the background radiation source and facilitate real-time measurement. Two closely spaced emission lines of a Group IA element are simultaneously monitored and compared to the emission of a known source. The new technique uses optical filtering to reduce the apparent temperature of the source at the wavelength corresponding to one of the transitions. The ratio of the absorption/emission intensities of the two lines is directly related to the flame temperature. Flames as hot as 2800 K can readily be probed using the new technique. Experimental data for cool flat flames compare well with thermocouple measurements.
Transient Structure and Radiation Properties of Strongly Radiating Buoyant Flames
Authors: Sivathanu Y. R. , Gore J. P.
Published: May 2008

Measurements of instantaneous temperature and soot volume fractions based on absorption and emission in highly buoyant turbulent acetylene/air and propylene/air flames are reported. These measurements are used to predict mean, rms, probability density functions, and power spectral densities of spectral radiation intensities along a representative horizontal chord in the flame. The results show the presence of large quantities of relatively cold soot in the vicinity of smaller amounts of hot soot particles. The resulting inhomogeneity in the temperature of soot in the flame leads to negative cross correlations between temperature and soot volume fractions. The treatment of such correlations was found necessary for predicting the observed probability density functions and the power spectral densities of spectral radiation intensities.
Freezing and Thawing Heat Transfer With Water Flow Around Isothermally Cooled Cylinders in Staggered and Aligned Arrangements
Authors: Hirata T. , Matsui H.
Published: May 2008

Ice formation and melting phenomena with water flow around isothermally cooled cylinders arranged in staggered and aligned manners, respectively, are examined. Experiments are performed for transient freezing and melting processes as well as for steady-state conditions. Analytical models are also introduced to examine heat transfer characteristics of the transient processes. It is found that the melting is twice as fast as the freezing for both the staggered and aligned arrangements. Experimental correlations for ice storage efficiency as well as for the ice filling-up rate are proposed. It is shown that the ice filling-up rate is strongly affected by the Reynolds number, cooling temperature, and cylinder pitch perpendicular to water flow.
Heat Transfer Analysis of Local Evaporative Turbulent Falling Liquid Films
Authors: Al-Najem N. M. , Ezuddin K. Y. , Darwish M. A.
Published: May 2008

A theoretical study has been conducted for evaporative heating of turbulent free-falling liquid films inside long vertical tubes. The methodology of the present work is based on splitting the energy equation into homogeneous and nonhomogeneous problems. Solving these simple problems yields a rapidly converging solution, which is convenient for computational purposes. The eigenvalues associated with the homogeneous problem can be computed efficiently, without missing any one of them, by the sign-count algorithm. A new correlation for the local evaporative heat transfer coefficient along the tube length is developed over wide ranges of Reynolds and Prandtl numbers. Furthermore, the average heat transfer coefficient is correlated as a function of Reynolds and Prandtl numbers as well as the interfacial shear stress. A correlation for the heat transfer coefficient in the fully developed region is also presented in terms of Reynolds and Prandtl numbers. Typical numerical results showed excellent agreement of the present approach with the available data in the literature. Moreover, a parametric study is made to illustrate the general effects of various variables on the velocity and temperature profiles.
Free Convection Film Boiling Heat Transfer From a Rotating Surface
Authors: Orozco J. , Francisco H.
Published: May 2008

A boundary layer model of laminar, subcooled, free convection film boiling from a rotating sphere has been developed. The conservation equations for the vapor and liquid were simplified, transformed into ordinary differential equations using an integral approach, and solved numerically. The theoretical variation of vapor film thickness with heater temperature and the resulting boiling fluxes were investigated. An experimental facility was built for the purpose of verifying the validity of the theoretical model and good agreement was found between the model and the experimental data at low rpm. The instability of the vapor film near the minimum heat flux for a rotating surface flux was also investigated.
Forced Convection Condensation of Steam on a Small Bank of Horizontal Tubes
Authors: Michael A. G. , Lee W. C. , Rose J. W.
Published: May 2008

Heat transfer measurements are reported for condensation of steam flowing vertically downward over a small bank of staggered horizontal tubes having 10 rows with 4 and 3 tubes per row. The tubes in each row were connected in series and separately supplied with cooling water. The cooling water flow rate and temperature rise were measured individually for each row and tube-wall temperatures were measured on selected tubes. Data were obtained at slightly above atmospheric pressure and the range of steam approach velocity (based on the cross-sectional area of the duct) was 6 to 23 m/s. A general trend of decreasing heat transfer coefficient with depth in the bank was found. However, superimposed on this was a “saw-tooth” effect with the three-tube rows having higher coefficients than the rows with four tubes. The amplitude of the coefficient variation decreased down the bank and was also less pronounced at lower vapor velocities. When compared with other experimental data for condensation of steam on small staggered banks, the present data exhibit somewhat higher vapor-side, heat transfer coefficients.
Pure Steam Condensation Experiments on Nonisothermal Vertical Plates
Authors: Brouwers H. J. H.
Published: May 2008

Pure steam condensation experiments on flat channel plates have been performed. To provide a range of values of the governing groups NTU and Ad, steam was condensed crosscurrentwise on polyvinylidene fluoride (PVDF) plates and cocurrentwise on brass plates (8.0 × 10−6 ≤ Ad ≤ 8.4 × 10−2, 0 ≤ NTU−1 ≤ 10). The theoretical predictions, based on an analysis of Nusselt-type condensation on nonisothermal plates, agree excellently with the plastic-plate measurements. The brass-plate experiments, however, agree with the theoretical model only in a limited range of NTU and Ad.
An Experimental Investigation of Free Surface Transport, Bifurcation, and Adhesion Phenomena as Related to a Hollow Glass Ampule and a Metallic Conductor
Authors: Chen S. C. , Vafai K.
Published: May 2008

An experimental investigation of free surface transport and subsequent bifurcation and adhesion for a hollow glass ampule is presented in this work. Detailed phenomenological features of the process are displayed and discussed. This experimental investigation, which is generic in nature, provides the much needed phenomenological information on free surface transport, glass processing as related to optical fiber production, and glass-to-metal sealing processes. Detailed images of the actual sealing process provide valuable information on identifying and isolating the key regimes in the process and mapping out process defects, and contribute to a basic understanding of the physical mechanisms involved in the sealing process.
Spectral Emissivity of Optically Anisotropic Solid Media
Authors: Malone C. G. , Choi B. I. , Flik M. I. , Cravalho E. G.
Published: May 2008

This work determines the spectral emissivity of a semi-infinite uniaxial medium in vacuum. If the optic axis is normal to the surface, then, for many materials and wavelengths, such as rutile between 10 and 25 μm, the directional and hemispherical spectral emissivities of the medium can be approximated, with an error of less than 10 percent, as those of an isotropic medium possessing the ordinary optical constants. In contrast, if the optic axis is parallel to the surface, the directional and hemispherical spectral emissivities can be predicted only by accounting for the optical anisotropy of the medium. Measurements of the directional emissivities of rutile crystals conform to the theoretical predictions.
Three-Dimensional Analytical Temperature Field Around the Welding Cavity Produced by a Moving Distributed High-Intensity Beam
Authors: Wei P. S. , Shian M. D.
Published: May 2008

An analytical solution for the three-dimensional temperature field in the liquid and heat-affected zones around a welding cavity produced by a moving distributed low- or high-power-density-beam is provided. The incident energy rate distribution is assumed to be Gaussian and the cavity is idealized by a paraboloid of revolution in workpieces of infinite, semi-infinite, or finite thicknesses. The present study finds that temperature fields can be described by the Laguerre and confluent hypergeometric functions. By satisfying a momentum balance at the cavity base and utilizing a consequence of the second law of thermodynamics, the depth of penetration is uniquely determined. The results show that the predicted depths and temperatures of the cavity agree with available experimental data. Some crucial factors affecting the transition from low- to high-power-density-beam welding are presented.
Effective Thermal Conductivity of a Composite Material: A Numerical Approach
Authors: Veyret D. , Cioulachtjian S. , Tadrist L. , Pantaloni J.
Published: May 2008

The purpose of this paper is to characterize conductive heat transfer through composite, granular, or fibrous materials. A numerical approach was used to determine the effective thermal conductivity of the diphasic media. The media are composed of a solid continuous matrix containing similar particles. The configuration studied was a cubic lattice array with cylindrical inclusions. Behavior laws were established according to particle geometry, concentration, and the type of contact between particles. The effective thermal conductivity of the material was determined using the Laplace equation, as were the temperature and flux fields within the cell. A finite-element formulation was therefore used in this study. Calculation is carried out on two and three-dimensional geometric spaces. The results obtained from this calculation were compared to theoretical results found in prior literature.
Heat Transfer and Pressure Drop Correlations for Twisted-Tape Inserts in Isothermal Tubes: Part I—Laminar Flows
Authors: Manglik R. M. , Bergles A. E.
Published: May 2008

Laminar flow correlations for f and Num are developed based on experimental data for water and ethylene glycol, with tape inserts of three different twist ratios. The uniform wall temperature condition is considered, which typifies practical heat exchangers in the chemical and process industry. These and other available data are analyzed to devise flow regime maps that characterize twisted-tape effects in terms of the dominant enhancement mechanisms. Depending upon flow rates and tape geometry, the enhancement in heat transfer is due to the tube partitioning and flow blockage, longer flow path, and secondary fluid circulation; fin effects are found to be negligible in snug- to loose-fitting tapes. The onset of swirl flow and its intensity is determined by a swirl parameter, Sw=Resw/y, that defines the interaction between viscous, convective inertia, and centrifugal forces. Buoyancy-driven free convection that comes into play at low flow rates with large y and ΔTw is shown to scale as Gr/Sw2≫ 1. These parameters, along with numerical baseline solutions for laminar flows with y = ∞, are incorporated into correlations for f and Num by matching the appropriate asymptotic behavior. The correlations describe the experimental data within ±10 to 15 percent, and their generalized applicability is verified by the comparison of predictions with previously published data.
The Investigation of Developing Flow and Heat Transfer in a Long Converging Duct
Authors: Mutama K. R. , Iacovides H.
Published: May 2008

The hydrodynamic and thermal development of turbulent boundary layers subjected to streamwise acceleration has been investigated. This was accomplished by obtaining detailed flow and temperature measurements over 52 entrance heights of a converging channel (with one wall heated) and with uniform velocity and temperature profiles at entry. Results are presented for three Reynolds numbers of 4000, 16,000, and 21,000 corresponding to acceleration parameter k levels of 2.31×10−6, 5.78×10−7, and 4.47×10−7, respectively. At acceleration parameter levels of 2.3×10−6 the flow behaves in a laminarlike fashion in all respects. Even at an acceleration parameter level of 4.5×10−7 the skin friction coefficients and Stanton number values are well below those encountered in parallel channel flow at corresponding Reynolds numbers. The entry region behavior is further complicated by the combined effects of thin inlet boundary layers and streamwise acceleration. These entry effects are perhaps more obvious in the Stanton number development. The well-defined entry conditions make these detailed measurements suitable for turbulence model validation and refinement.
Improved Hybrid Lumped-Differential Formulation for Double-Pipe Heat Exchanger Analysis
Authors: Scofano Neto F. , Cotta R. M.
Published: May 2008

Based on recent developments in the so-called Coupled Integral Equation Approach for the approximate solution of heat and mass diffusion problems, the classical lumped-differential formulation for double-pipe heat exchangers is extended and significantly improved in terms of overall accuracy. The energy equation for the outer annular channel is formally integrated and temperature gradients approximately taken into account, in contrast to the conventional lumped system analysis. The resulting partial differential equation for the inner stream, involving a more general type of boundary condition, is analytically handled through the generalized integral transform technique. A critical accuracy analysis is then performed in order to demonstrate the improvement over the plain lumped-differential formulation.
Natural Convection Heat Transfer From a Staggered Vertical Plate Array
Authors: Tanda G.
Published: May 2008

An experimental study was performed to evaluate the natural convection heat transfer characteristics of an array of four staggered vertical plates. The thermal input at each plate was the same or differed from plate to plate depending on various heating modes. The effects of the interplate spacing and the plate-to-ambient temperature difference were investigated. The experiments were performed in air. Convective interactions among the plates were identified by examining the per-plate heat transfer coefficients and the local heat transfer coefficients along the vertical sides of plates. Local heat transfer results were obtained by means of the schlieren quantitative technique. Comparison of local heat transfer coefficients along the plate assembly with those of a continuous vertical plate (having the same height) showed enhancements up to a factor of two. Comparison of average heat transfer results with those for a parallel plate channel having the same exchanger size showed only little reductions in heat transfer rate, despite a 28 percent reduction in heat transfer area, with enhancements, in terms of specific heat flux, up to 30 percent.
Orientation Effects on Critical Heat Flux From Discrete, In-Line Heat Sources in a Flow Channel
Authors: Gersey C. O. , Mudawar I.
Published: May 2008

The effects of flow orientation on critical heat flux (CHF) were investigated on a series of nine in-line simulated microelectronic chips in Fluorinert FC-72. The chips were subjected to coolant in upflow, downflow, or horizontal flow with the chips on the top or bottom walls of the channel with respect to gravity. Changes in angle of orientation affected CHF for velocities below 200 cm/s, with some chips reaching CHF at heat fluxes below the pool boiling and flooding-induced CHF values. Increased subcooling was found to dampen this adverse effect of orientation slightly. Critical heat flux was overwhelmingly caused by localized dryout of the chip surface. However, during the low velocity downflow tests, low CHF values were measured because of liquid blockage by vapor counterflow and vapor stagnation in the channel. At the horizontal orientation with downward-facing chips, vapor/liquid stratification also yielded low CHF values. Previously derived correlations for water and long, continuous heaters had limited success in predicting CHF for the present discontinuous heater configuration. Because orientation has a profound effect on the hydrodynamics of two-phase flow and, consequently, on CHF for small inlet velocities, downflow angles should be avoided, or when other constraints force the usage of downflow angles, the inlet liquid velocity should be sufficiently large.
The Effect of Surface Gas Injection on Film Boiling Heat Transfer
Authors: Duignan M. R. , Greene G. A. , Irvine T. F.
Published: May 2008

A database was developed for heat transfer from a horizontal plate with film boiling and gas jetting occurring simultaneously, in a pool of water maintained at its saturation temperature. The effect of passing nitrogen through established film boiling is to increase the heat transfer from that surface. At the highest measured superficial gas velocity, approximately 8.5 cm/s, and for a constant surface temperature, the heat transfer increases by a factor of two over the heat transfer with no enhancement by gas flux. Further, as the superficial gas velocity approaches zero, the data approach the stable film boiling asymptote. A model was developed to correlate the database. The result is an equation that represents better than 90 percent of all the measured data within ±15 percent bounds.
A Correlational Approach to Turbulent Saturated Film Boiling
Authors: Chu T. Y.
Published: May 2008

A correlation method for saturated film boiling is proposed. The correlation is based on the analogy between film boiling and natural convection. As in the case of natural convection, the turbulent film boiling correlation takes the form of a Nusselt number versus the Rayleigh number power law, NuB ∝ RaB1/3. The proposed correlation shows very good agreement with current data for film boiling of water from vertical surfaces. The general applicability of the correlation is established by comparisons with film boiling data from R-113 and cryogenic fluids.
Turbulent Prandtl Number—Where Are We?
Authors: Kays William M.
Published: May 2008

The objective of this paper is to examine critically the presently available experimental data on Turbulent Prandtl Number for the two-dimensional turbulent boundry layer, and for fully developed flow in a circular tube or a flat duct, and attempt to draw some conclusions as to where matters presently stand.
Optimal Sizing of Planar Thermal Spreaders
Authors: Hingorani S. , Fahrner C. J. , Mackowski D. W. , Gooding J. S. , Jaeger R. C.
Published: May 2008

Two-dimensional cylindrical and three-dimensional Cartesian thermal spreaders are studied. One of the surfaces is convectively coupled to a uniform environmental temperature while the opposite surface is subjected to a uniform heat flux distribution over a portion of its boundary. The problem is generalized through the introduction of appropriate dimensionless variables, and analytical solutions for the temperature field are presented for each coordinate system. The solutions depend on the usual geometric and heat transfer groups. It is found that, for a range of realistic Biot numbers and a given ratio of the spreader to heater dimensions, a dimensionless spreader thickness exists for which the temperature of the heater reaches a minimum value. Optimal thickness curves are presented for these ranges.
Thermally Induced Optical Nonlinearity During Transient Heating of Thin Films
Authors: Chen G. , Tien C. L.
Published: May 2008

This work studies the temperature field and the optical response of weakly absorbing thin films with thermally induced optical nonlinearity during picosecond to nanosecond pulsed-laser heating. A one-dimensional model is presented that examines the effects of the temperature dependent optical constants and the nonuniform absorption caused by interference. The energy equation is solved numerically, coupled with the matrix method in optical multilayer theory. Both cadmium sulfide (CdS) thin films and a zinc selenide (ZnSe) interference filter are considered. The computational results compare favorably with available experimental data on the ZnSe interference filter. This study shows that the transient temperature distributions in the films are highly nonuniform. Such nonuniformity yields Airy’s formulae for calculating the thin-film reflectance and transmittance inapplicable. Applications of the work include optical bistability, localized change of the film structure, and measurement of the thermal diffusivity of thin films.
Thermal Diffusivity Measurement of GaAs/AlGaAs Thin-Film Structures
Authors: Chen G. , Tien C. L. , Wu X. , Smith J. S.
Published: May 2008

This work develops a new measurement technique that determines the thermal diffusivity of thin films in both parallel and perpendicular directions, and presents experimental results on the thermal diffusivity of GaAs/AlGaAs-based thin-film structures. In the experiment, a modulated laser source heats up the sample and a fast-response temperature sensor patterned directly on the sample picks up the thermal response. From the phase delay between the heating source and the temperature sensor, the thermal diffusivity in either the parallel or perpendicular direction is obtained depending on the experimental configuration. The experiment is performed on a molecular-beam-epitaxy grown vertical-cavity surface-emitting laser (VCSEL) structure. The substrates of the samples are etched away to eliminate the effects of the interface between the film and the substrate. The results show that the thermal diffusivity of the VCSEL structure is 5–7 times smaller than that of its corresponding bulk media. The experiments also provide evidence on the anisotropy of thermal diffusivity caused solely by the effects of interfaces and boundaries of thin films.
Film-Cooling From Holes With Compound Angle Orientations: Part 2—Results Downstream of a Single Row of Holes With 6d Spanwise Spacing
Authors: Ligrani P. M. , Wigle J. M. , Jackson S. W.
Published: May 2008

Experimental results are presented that describe the development and structure of flow downstream of a single row of film-cooling holes with compound angle orientations. With this configuration, holes are spaced 6d apart in the spanwise direction, inclined at 35 deg with respect to the test surface when projected into the streamwise/normal plane, and inclined at 30 deg with respect to the test surface when projected into the spanwise/normal plane. Results are presented for an injectant to free-stream density ratio near 1.0, and injection blowing ratios from 0.5 to 1.50. Comparisons are made with measurements from two other configurations to determine: (1) the effects of hole angle orientation for constant spanwise hole spacing, and (2) the effects of spanwise hole spacing when the hole angle orientation is maintained constant. Results from the first comparison show that the compound angle injection configuration provides significantly improved film-cooling protection compared to a simple angle configuration for the same spanwise hole spacing, normalized streamwise location x/d, and blowing ratio m, for x/d<30 when m=0.50 and for x/d<60 when m=1.0 and 1.5. At x/d>60, spanwise-averaged adiabatic effectiveness data downstream of the two configurations generally cover about the same range. Results from the second comparison show that spanwise-averaged effectiveness values are 20 to 39 percent higher when 6d spanwise hole spacing is employed compared to 7.8d spanwise hole spacing for the same m and x/d, for x/d<60. When plotted in η/m versus Xl/s coordinates, spanwise-averaged film effectiveness data measured downstream of one and two rows of holes from all injection configurations tested show a reasonable collapse. For m values between 1 and 3, spanwise-averaged iso-energetic Stanton number ratios measured just downstream of one row of holes are lower than results measured just downstream of two rows of holes, which evidences greater mixing and higher turbulence levels when the injectant emerges from two rows of holes.
Experimental Investigation of Heat Transfer and Flow Over Baffles of Different Heights
Authors: Habib M. A. , Mobarak A. M. , Sallak M. A. , Abdel Hadi E. A. , Affify R. I.
Published: May 2008

The phenomenon of flow separation in ducts with segmented baffles has many engineering applications, for example, shell-and-tube heat exchangers with segmented baffles, labyrinth shaft seals, laser curtain seals, air-cooled solar collectors, and internally cooled turbine blades. In the present work, an experimental investigation has been done to study the characteristics of the turbulent flow and heat transfer inside the periodic cell formed between segmented baffles staggered in a rectangular duct. In particular, flow field, pressure loss, and local and average heat transfer coefficients were obtained. The experimental runs were carried out for different values of Reynolds numbers and baffle heights (window cuts) at uniform wall heat flux condition along the top and bottom walls. The results indicate that the pressure loss increases as the baffle height does, for a given flow rate. Also, the local and average heat transfer parameters increase with increasing Reynolds number and baffle height. However, the associated increase in the pressure loss was found to be much higher than the increase in the heat transfer coefficient.
Heat Transfer Predictions With Extended k–ε Turbulence Model in Radial Cooling Ducts Rotating in Orthogonal Mode
Authors: Tekriwal P.
Published: May 2008

Standard and extended k–ε turbulence closure models have been employed for three-dimensional heat transfer calculations for radially outward flow in rectangular and square cooling passages rotating in orthogonal mode. The objective of this modeling effort is to validate the numerical model in an attempt to fill the gap between model predictions and the experimental data for heat transfer in rotating systems. While the trend of heat transfer predictions by the standard k–ε turbulence model is satisfactory, the differences between the data and the predictions are approximately 30 percent or so in the case of high rotation number flow. The extended k–ε turbulence model takes an approach where an extra “source” term based on a second time scale of the turbulent kinetic energy production rate is added to the equation for the dissipation rate of turbulent kinetic energy. This yields a more effective calculation of turbulent kinetic energy as compared to the standard k–ε turbulence model in the case of high rotation number and high density ratio flow. As a result, comparison with the experimental data available in the literature shows that an improvement of up to a significant 15 percent (with respect to data) in the heat transfer coefficient predictions is achieved over the standard k–ε model in the case of high rotation number flow. Comparisons between the results of the standard k–ε model and the extended formulation are made at different rotation numbers, different Reynolds numbers, and varying temperature ratio. The results of the extended k–ε turbulence model are either as good or better than those of the standard k–ε model in all these cases of parametric study. Thus, the extended k–ε turbulence model proves to be more general and reduces the discrepancy between the model predictions and the experimental data for heat transfer in rotating systems.