bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluctuations in isometric muscle force can be described by one linear projection of low‐frequency components of motor unit discharge rates
Authors: Negro Francesco , Holobar Aleš , Farina Dario
Published: October 2009

The aim of the study was to investigate the relation between linear transformations of motor unit discharge rates and muscle force. Intramuscular (wire electrodes) and high‐density surface EMG (13 × 5 electrode grid) were recorded from the abductor digiti minimi muscle of eight healthy men during 60 s contractions at 5%, 7.5% and 10% of the maximal force. Spike trains of a total of 222 motor units were identified from the EMG recordings with decomposition algorithms. Principal component analysis of the smoothed motor unit discharge rates indicated that one component (first common component, FCC) described 44.2 ± 7.5% of the total variability of the smoothed discharge rates when computed over the entire contraction interval and 64.3 ± 10.2% of the variability when computed over 5 s intervals. When the FCC was computed from four or more motor units per contraction, it correlated with the force produced by the muscle (62.7 ± 10.1%) by a greater degree (P < 0.001) than the smoothed discharge rates of individual motor units (41.4 ± 7.8%). The correlation between FCC and the force signal increased up to 71.8 ± 13.1% when the duration and the shape of the smoothing window for discharge rates were similar to the average motor unit twitch force. Moreover, the coefficients of variation (CoV) for the force and for the FCC signal were correlated in all subjects (R2 range = 0.14–0.56; P < 0.05) whereas the CoV for force was correlated to the interspike interval variability in only one subject (R2= 0.12; P < 0.05). Similar results were further obtained from measures on the tibialis anterior muscle of an additional eight subjects during contractions at forces up to 20% of the maximal force (e.g. FCC explained 59.8 ± 11.0% of variability of the smoothed discharge rates). In conclusion, one signal captures most of the underlying variability of the low‐frequency components of motor unit discharge rates and explains large part of the fluctuations in the motor output during isometric contractions.
Excitation of locus coeruleus noradrenergic neurons by thyrotropin‐releasing hormone
Authors: Ishibashi Hitoshi , Nakahata Yoshihisa , Eto Kei , Nabekura Junichi
Published: October 2009

Locus coeruleus (LC) noradrenergic neurons are implicated in a variety of functions including the regulation of vigilance and the modulation of sensory processing. Thyrotropin‐releasing hormone (TRH) is an endogenous neuropeptide that induces a variety of behavioural changes including arousal and antinociception. In the present study, we explored whether the activity of LC noradrenergic neurons is modulated by TRH. Using current‐clamp recording from isolated rat LC neurons, we found that TRH increased the firing rate of spontaneous action potentials. The TRH action was mimicked by TRH analogues including taltirelin and TRH‐gly. In voltage‐clamp recording at a holding potential of −50 mV, TRH produced an inward current associated with a decrease in the membrane K+conductance. This current was inhibited by the TRH receptor antagonist chlordiazepoxide. Following inhibition of the pH‐sensitive K+conductance by extracellular acidification, the TRH response was fully inhibited. The TRH‐induced current was also inhibited by the phospholipase C (PLC) inhibitor U‐73122, but not by the protein kinase C inhibitor chelerythrine nor by chelation of intracellular Ca2+by BAPTA. The recovery from the facilitatory action of TRH on the spike frequency was markedly inhibited by a high concentration of wortmannin. These results suggest that TRH activates LC noradrenergic neurons by decreasing an acid‐sensitive K+conductance via PLC‐mediated hydrolysis of phosphatidylinositol 4,5‐bisphosphate. The present findings demonstrate that TRH activates LC neurons and characterize the underlying signalling mechanisms. The action of TRH on LC neurons may influence a variety of CNS functions related to the noradrenergic system which include arousal and analgesia.
A Monte Carlo Method of Solving Heat Conduction Problems
Authors: Fraley S. K. , Hoffman T. J. , Stevens P. N.
Published: October 2009

A new approach in the use of Monte Carlo to solve heat conduction problems is developed using a transport equation approximation to the heat conduction equation. A variety of problems is analyzed with this method and their solutions are compared to those obtained with analytical techniques. This Monte Carlo approach appears to be limited to the calculation of temperatures at specific points rather than temperature distributions. The method is applicable to the solution of multimedia problems with no inherent limitations as to the geometric complexity of the problem.
Transient Heat Transfer between a Plate and a Fluid whose Temperature Varies Periodically with Time
Authors: Sucec J.
Published: October 2009

Using the method of complex temperature in conjunction with the Laplace transformation, an exact analytical solution is found for the transient, conjugate, forced convection problem consisting of a plate, whose base is insulated, interacting with a fluid, moving in a steady slug fashion, whose temperature, at points far from the plate, varies sinusoidally with time. Simple quasi-steady results are derived for comparison. Also presented is a method for determining the qualitative conditions under which one might expect a quasi-steady analysis to be valid in a general problem.
Periodic Streamwise Variations of Heat Transfer Coefficients for Inline and Staggered Arrays of Circular Jets with Crossflow of Spent Air
Authors: Florschuetz L. W. , Berry R. A. , Metzger D. E.
Published: October 2009

Heat transfer characteristics were measured for inline and staggered arrays of circular jets impinging on a surface parallel to the jet orifice plate. The impinging flow was constrained to exit in a single direction along the channel formed by the jet plate and the heat transfer surface. In this configuration the air discharged from upstream transverse rows of jet holes imposes a crossflow of increasing magnitude on the succeeding downstream jet rows. Streamwise heat transfer coefficient profiles were determined for a streamwise resolution of one-third the streamwise hole spacing, utilizing a specially constructed test surface. These profiles are characterized by significant periodic variations. The downstream amplitudes are diminished by the increasing crossflow magnitude, but can persist for at least ten rows of holes. Results were obtained for streamwise hole spacings of 5, 10, and 15 hole diameters; transverse hole spacings of 4, 6, and 8 diameters; and channel heights of 1, 2, and 3 diameters. The number of transverse hole rows was fixed at ten for all configurations. The characteristics of the periodic variations are presented and discussed as a function of the geometric parameters, including the effect of hole pattern.
An Experimental Study of Heat Transfer During Forced Convection over a Rectangular Body
Authors: Test F. L. , Lessmann R. C.
Published: October 2009

An experimental investigation has been performed to determine the constant surface temperature heat transfer behavior on the upper surface of a rectangular model with a chord length of 20.3 cm (8 in.) and an aspect ratio of 6/1. Data were obtained for angles of attack from 0 to 50 deg and freestream velocities of 9.1, 15.2, and 21.3 m/s (30, 50 and 70 ft/s). Separation existed on a portion of the upper surface for angles between 0 and 20 deg with the flow being turbulent after reattachment. Above 30 deg the flow was always laminar with the stagnation point on the upper surface. The heat transfer results in the laminar case were strongly influenced by freestream disturbances.
The Numerical Prediction of Developing Turbulent Flow and Heat Transfer in a Square Duct
Authors: Emery A. F. , Neighbors P. K. , Gessner F. B.
Published: October 2009

Velocity and temperature profiles were computed for developing turbulent flow in a square duct with constant wall temperature, constant wall heat flux or asymmetric heating. The computations utilized an explicit numerical differencing scheme and an algebraic closure model based upon a three-dimensional mixing length. The computed local and fully-developed shear stresses and heat transfer are shown to be in good agreement with measured data and with predictions using the k–ε closure model.
Radiation Properties for Polydispersions: Application to Coal
Authors: Buckius R. O. , Hwang D. C.
Published: October 2009

The extinction and absorption coefficients and the asymmetry factor for polydispersions of absorbing spherical particles are analyzed. The results are based upon Mie’s theory for single spherical particles and particle size distributions found in practical systems. Dimensinnless spectral radiation properties are shown to be independent of the explicit size distribution and functions only of the average radii and the index of refraction. The Planck and Rosseland mean coefficients are also presented and the dependence on temperature is explicitly denoted for a large practical temperature range. The results for coal with optical properties which are wavelength dependent indicate the usefulness of the dimensionless and mean properties.
Heat Transfer Characteristics of a Spark-Ignition Engine
Authors: Alkidas A. C.
Published: October 2009

Transient heat flux measurements were obtained at four positions on the cylinder head of a four-stroke single-cylinder spark-ignition engine. Tests were performed for both fired and motored operation of the engine. The primary engine operational variable was engine speed. The results showed that the heat flux varies considerably with position of measurement. At fired conditions, the initial high rate of increase of heat flux at each position of measurement correlated with the calculated time of arrival of the flame at that position. Finally, as expected, the peak heat flux was found to increase with increased engine speed.
The Segmented Oxidizing Monolith Catalytic Converter: Theory and Performance
Authors: Wendland D. W.
Published: October 2009

In an oxidizing monolithic catalytic converter, mass transfer rates to the catalyst sites are greatest in the flow passage entrance regions. Segmenting the monolith, or slicing it normal to the flow direction and separating the pieces, increases conversion efficiency by forcing the species boundary layers to repeatedly redevelop. Models are presented which apply the heat transfer/mass transfer analogy to a developing flow correlation to provide an upper limit to conversion in a general monolith. The models predict that segmenting a small monolith into four pieces decreases hydrocarbon and carbon monoxide residuals by as much as 45 percent and 65 percent, respectively. Engine testing demonstrated 33 and 47 percent residual decreases.
Optimal Area Allocation in Multistage Heat Exchanger Systems
Authors: Ait-Ali M. A. , Wilde D. J.
Published: October 2009

In constrained optimization, valuable analytical insight can be gained by focusing attention on the terms of a problem before obtaining any solution particular to the numerical values of the given parameters. The necessary optimality conditions derived from the first semi-log derivatives may give a general result useful as a design rule for all similar problems. This approach is illustrated in the determination of optimal area allocation among the stages of two different multistage heat exchanger systems. This area allocation minimizes the total heat transfer area of the system with respect to: (1) interstage temperature for a given overall temperature change of the process stream; (2) “base area,” newly defined here as the ratio of capacity rate to the overall heat transfer coefficient.
Heat Transfer Characteristics of an Obliquely Impinging Circular Jet
Authors: Sparrow E. M. , Lovell B. J.
Published: October 2009

Measurements of local heat (mass) transfer coefficients were made on a surface on which a circular jet impinges at an oblique angle. The angle of inclination of the jet relative to the surface was varied from 90 deg (normal impingement) to 30 deg. The Reynolds number and the distance between the jet orifice and the impingement plate were also varied parametrically. To facilitate the experiments, the naphthalene sublimation technique was employed, and the resulting mass transfer coefficients were converted to heat transfer coefficients by the well-established analogy between the two processes. It was found that the point of maximum mass transfer is displaced from the geometrical impingement point, with the extent of the displacement increasing with greater jet inclination. The local coefficients on the uphill side of the maximum point drop off more rapidly than do those on the downhill side, thus creating an imbalance in the cooling/heating capabilities on the two sides. Neither the maximum transfer coefficient nor the surface-averaged transfer coefficient are highly sensitive to the inclination of the jet; during the course of the experiments, the largest inclination-induced decreases in these quantities were in the 15 to 20 percent range.
Convective Instability of Rotating Radiating Fluids
Authors: Onyegegbu S. O.
Published: October 2009

The Benard problem of a rotating radiating nongray fluid is examined analytically for both stationary and oscillatory convection. Radiation is modeled by the Milne-Eddington approximation. The time derivatives are retained in the disturbance equations and both types of instability were studied using the same approximating functions. Results presented show that, in the presence of rotation, radiation increases the zone in which overstability is the preferred mode of instability.
Influence of Streamwise Molecular Heat Conduction on the Heat Transfer Coefficient for Liquid Metals in Turbulent Flow between Parallel Plates
Authors: Faggiani S. , Gori Fabio
Published: October 2009

The energy equation for mechanically fully developed turbulent flow between parallel plates is numerically solved taking into account the term of streamwise heat conduction. Solutions are presented for Re = 7060 and 73620, and for Pr ranging from 0.001 to 0.1. Generally the influence of the streamwise molecular conduction results in a remarkable decrease of the Nusselt number in the thermal inlet.
Heat Transfer by Conduction and Radiation in a One-Dimensional Absorbing, Emitting and Anisotropically-Scattering Medium
Authors: Yuen W. W. , Wong L. W.
Published: October 2009

Heat transfer by simultaneous conduction and radiation in an absorbing, emitting and anisotropically-scattering material is investigated theoretically. Consideration is given to a one-dimensional system bounded by two parallel gray, diffuse and isothermal walls. Assuming a physical model of linear-anisotropic scattering, the resulting integral-differential equation is solved by a successive approximation technique similar to the method of undetermined parameters. The solution method is demonstrated to be relatively simple and yields solution converging qucikly to the exact results. Results show that for the present one-dimensional system, the common approach of treating the total heat transfer as a simple addition of separate independent contributions from conduction and radiation is quite inaccurate for certain cases. This approach is thus ineffective in illustrating the general effect of scattering. Both the scattering albedo and the forward-backward scattering parameters are shown to have some interesting effects on the total heat transfer and the medium’s temperature. The magnitude of these effects depends on the surface emissivity of the two boundaries.
Numerical Anisotropic Heat Conduction Solutions Using Boundary-Fitted Coordinate Systems
Authors: McWhorter J. C. , Sadd M. H.
Published: October 2009

A numerical method is presented and applied to the solution of two-dimensional steady-state, anisotropic heat conduction problems. The solution method employs the boundary-fitted coordinate system technique which transforms the region under study onto a fixed rectangular domain where computations are done easily. Consequently, the procedure eliminates the shape of the boundary as a complicating factor. The method is quite general and can handle any multiply-connected, arbitrary-shaped region. Two examples, a solid disk and an eccentric annulus, are worked out to demonstrate the utility of the method. Isotherm results check with previously published solutions.
A Methodology of Predicting Cavity Geometry Based on Scanned Surface Temperature Data—Prescribed Surface Temperature at the Cavity Side
Authors: Hsieh C. K. , Su K. C.
Published: October 2009

The scanned surface temperature data from a body are used to predict the cavity lying underneath the surface. The basic system under investigation is a plane wall having a rectangular cavity at the back surface. The front surface dissipates heat by convection; this is also the surface whose temperature is scanned. For a prescribed surface temperature specified on the cavity side, a numerical solution is found convenient to predict the cavity top and the approximate location of the cavity wall. A recheck of the cavity wall position calls for matching the recalculated surface temperature with the measured temperature. The data are found to be well behaved to the extent that an interpolation is possible when the mesh size chosen happens to miss the wall position. The methodology can also be extended to prediction of holes in a three-dimensional body.
Transient Solidification in Flow into a Rod Bundle
Authors: Epstein M. , Stachyra L. J. , Lambert G. A.
Published: October 2009

Transient solidification of a liquid in axial flow into and through a cold rod bundle was investigated. Experiments are described in which the amount of liquid displaced through the rod bundle before freezing shut as a function of pressure drop was measured. Simple relations for the liquid mass displaced into or through the rod bundle were derived by combining the notion of a time-dependent hydraulic diameter with an available approximate method for treating transient solidification in a tube flow. The relations are shown to be approximately consistent with the experimental data.
Experimental Study of Laminar Heat Transfer to In-Tube Flow of Non-Newtonian Fluids
Authors: Joshi S. D. , Bergles A. E.
Published: October 2009

An experimental study is reported of heat transfer to laminar flow of two water-methocel pseudoplastic (power law) solutions in a circular tube subjected to a uniform wall heat flux. The object of this study was to evaluate the effects of non-Newtonian behavior and temperature-dependent consistency on heat transfer. The experimental Nusselt numbers are compared with numerical predictions and experimental data. Two correlations are recommended according to the temperature-dependence of the rheological characteristics.
Fluid-to-Fluid Conjugate Heat Transfer for a Vertical Pipe—Internal Forced Convection and External Natural Convection
Authors: Sparrow E. M. , Faghri M.
Published: October 2009

An analysis is made of the interactive heat transfer problem involving forced convection flow in a vertical pipe and natural convection boundary layer flow external to the pipe. Both flows are laminar. Solutions of the conservation equations for mass, momentum, and energy were obtained numerically by an iterative scheme which deals successively with the internal and external flows. Remarkably rapid convergence was achieved by adopting a procedure whereby information is transferred between the two flows via heat transfer coefficients rather than via the wall or bulk temperatures or the heat flux. Results are presented for the axial distributions of the internal and external Nusselt numbers, of the wall temperature, and of the bulk temperature of the internal flow—all as a function of three parameters. It was found that at any (dimensionless) axial station, the pipe Nusselt number is insensitive to the parameters and is bounded between the values for uniform wall temperature and uniform wall heat flux. On the other hand, the external natural convection Nusselt number is highly sensitive to the parameters and departs substantially from the standard uniform wall temperature results.
Heat Transfer Coefficients on the Downstream Face of an Abrupt Enlargement or Inlet Constriction in a Pipe
Authors: Sparrow E. M. , O’Brien J. E.
Published: October 2009

Measurements were made of the local and average heat transfer coefficients on the downstream face of an enlargement step in a pipe. Two flow configurations were investigated: (a) an abrupt enlargement from a smaller diameter pipe to a larger diameter pipe and (b) partial constriction of a pipe inlet by a large baffle plate. Air was the working fluid. The transfer coefficients were determined by means of the naphthalene sublimation technique; axial pressure distributions were also measured. The highest values of the local transfer coefficient were found to occur on the portion of the enlargement face adjacent to the aperture through which the flow enters the enlarged space. On the other hand, the lowest coefficients occur in the corner where the enlargement face meets the wall of the enlarged pipe. The radial distributions of the transfer coefficient on the enlargement face vary with the Reynolds number. With regard to average transfer coefficients, higher values (by at least 50 percent) are attained for the constricted inlet than for the abrupt enlargement. The average coefficients for the enlargement face are much higher (by a factor of two or three) than those on the wall of the enlarged pipe for fully developed flow conditions.
Flashing Inception in Flowing Liquids
Authors: Jones O. C.
Published: October 2009

The inception of net vaporization in flashing flows is examined. It is suggested that the flashing inception can be expressed as two additive effects. One is due to the static decompression which is a function of the initial temperature and also of the expansion rate. The other effect which is a function of Reynolds number and flashing index, is due to the turbulent fluctuations of the flowing liquid. It is shown that by taking a three standard deviation band on the turbulent velocity fluctuations, an adequate representation of the inverse mass flux effect on flashing inception for existing data is obtained. The turbulence effects are combined with the correlation of Alamgir and Lienhard to provide predictive methods recommended for the case where both static and convective decompression effects exist.
Dynamic Model of Enhanced Boiling Heat Transfer on Porous Surfaces—Part I: Experimental Investigation
Authors: Nakayama W. , Daikoku T. , Kuwahara H. , Nakajima T.
Published: October 2009

Enhancement of nucleate boiling heat transfer has been studied with the structured surfaces composed of interconnected internal cavities in the form of tunnels and small pores connecting the pool liquid and the tunnels. The boiling curves of R-11, water and nitrogen show 80 to 90 percent reduction of wall superheat required to transfer the same heat flux as that on plain surfaces, when the pore diameter is set around 0.1 mm. The experimental data on bubble formation showed a significant contribution of latent heat transport to the enhancement. A visualization study made with a transparent structured model suggested that the liquid suction into the tunnel is triggered by the bubble growth at active pores and subsequent evaporation inside the tunnel plays a vital role in driving the bubble formation cycle. This observation led to a conception of the dynamic model expounded in Part II.
Dynamic Model of Enhanced Boiling Heat Transfer on Porous Surfaces—Part II: Analytical Modeling
Authors: Nakayama W. , Daikoku T. , Kuwahara H. , Nakajima T.
Published: October 2009

Based on the experimental results reported in Part I, an analytical model of the dynamic cycle of bubble formation is proposed. The cycle consists of a waiting period and a bubble growth period; in the former the pressure in the tunnel is increased due to evaporation from internally held meniscuses and in the latter a certain amount of pool liquid is sucked in the tunnel to be subsequently evaporated. The equations are formulated with the adoption of a moderate number of empirical constants. Their solutions give the predictions of latent heat flux due to internal evaporation, population density of active sites, and frequency of bubble formation. The population density is then used to estimate convective heat flux on the outer surface from the empirical correlation established in Part I. The analytical predictions are compared with the experimental data of Part I. The results are encouraging and indicate the course of future study to implement the model.
On the Prediction of the Minimum Pool Boiling Heat Flux
Authors: Lienhard J. H. , Dhir V. K.
Published: October 2009

A criterion is offered for the collapse of film boiling in a saturated liquid at the minimum heat flux. The criterion says the vapor film collapse occurs when insufficient vapor is generated to sustain the growing wave after it reaches a constant rate of increase of amplitude. This criterion yields an accurate prediction for horizontal flat plates and cylinders. The prediction requires the use of empirical generalizations about the configuration of film boiling, which are also developed here.
Dropwise Condensation on Surfaces Having Different Thermal Conductivities
Authors: Stylianou S. A. , Rose J. W.
Published: October 2009

Heat flux and vapor-to-surface temperature difference measurements have been made for dropwise condensation of steam on copper, bronze, copper-plated bronze, and ptfe-coated copper surfaces. Except in the case of the ptfe surface the promoter used was dioctadecyl disulphide. Tests were carried out at various pressures ranging from atmospheric down to about 12 kPa. Care was taken to insure that the apparatus was leak-tight and a local venting technique was used to obviate the effects of traces of non-condensing gases. The heat flux and surface temperature were obtained from temperatures indicated by fine-wire thermocouples accurately located at different distances from the condensing surface. In order to randomize the error in the extrapolated surface temperature for the bronze plate, the thermocouples were repeatedly removed and re-installed. Tests were carried out using a total of six different thermocouple installations. The results for the copper and bronze surfaces were in close agreement with each other and with earlier results for dioctadecyl disulphide-promoted copper surfaces. For the ptfe-coated copper surface, when the ptfe layer resistance was accounted for on the basis of simple one-dimensional steady conduction, the residual vapor-to-(ptfe) surface temperature differences were found to be very close to those obtained for the dioctadecyl disulphide-promoted copper surface.
Nonlinear Stability of Film Condensation
Authors: U¨nsal M. , Thomas W. C.
Published: October 2009

Perturbation methods are used to investigate the nonlinear stability of a vertical condensate film adjacent to quiescent saturated vapor. The analysis presented leads to closed-form expressions for the wave amplification rate and the wave velocity. Application of the results to steam condensation show that supercritically unstable infinitesimal disturbances reach finite equilibrium amplitudes while subcritically stable finite-amplitude wave motion is not possible. The nondimensional parameters which affect the supercritically-stable finite-amplitude wave motion are identified and the effects on the stability of the condensate film are discussed.
Combined Heat and Mass Transfer in Mixed Convection over a Horizontal Flat Plate
Authors: Chen T. S. , Strobel F. A.
Published: October 2009

The combined effects of buoyancy forces from thermal and species diffusion on the heat and mass transfer characteristics are analyzed for laminar boundary layer flow over a horizontal flat plate. The analysis is restricted to processes with low concentration levels such that the interfacial velocities due to mass diffusion and the diffusion-thermo/thermo-diffusion effects can be neglected. Numerical results for friction factor, Nusselt number, and Sherwood number are presented for gases having a Prandtl number of 0.7, with Schmidt numbers ranging from 0.6 to 2.0. In general, it is found that, for the thermally assisting flow, the surface heat and mass transfer rates as well as the wall shear stress increase with increasing thermal buoyancy force. These quantities are further enhanced when the buoyancy force from species diffusion assists the thermal buoyancy force, but are reduced when the two buoyancy forces oppose each other. While a higher heat transfer rate is found to be associated with a lower Schmidt number, a higher mass transfer rate occurs at a higher Schmidt number.
The Onset of Longitudinal Vortices in Mixed Convective Flow over an Inclined Surface in a Porous Medium
Authors: Hsu C. T. , Cheng P.
Published: October 2009

The conditions marking the onset of vortex instability in mixed convective flow over an inclined surface in a saturated porous medium are investigated by means of a linear stability analysis. The basic state is assumed to be the steady two-dimensional boundary layer flow. The three-dimensional perturbation equations are simplified on the basis of a scaling argument whereby most of the streamwise derivatives of the disturbances are found to be negligible. For vortex disturbances, the resulting simplified equations in terms of the amplitude are solved approximately by the local similarity method. The eigenvalue problem is solved numerically for the cases of (1) an inclined surface at constant wall temperature with free stream velocity at zero angle of incidence with the inclined surface and (2) an inclined surface with constant heat flux with free stream velocity at 45 deg with respect to the inclined surface. Both aiding and opposing external flows are considered. The critical parameters and the critical wave numbers of disturbances for the two cases are obtained. It is found that the effect of the external flow is to suppress the growth of vortex disturbances in both aiding and opposing flows. At the same value of the mixed convection parameter, the opposing flow is found to be more unstable than the aiding flow.
Turbulent Heat Transfer and Fluid Flow in an Unsymmetrically Heated Triangular Duct
Authors: Altemani C. A. C. , Sparrow E. M.
Published: October 2009

Experiments were performed to determine entrance-region and fully developed heat transfer characteristics for turbulent airflow in an unsymmetrically heated equilateral triangular duct; friction factors were also measured. Two of the walls were heated while the third was not directly heated. The resulting thermal boundary conditions consisted of uniform heating per unit axial length and circumferentially uniform temperature on the heated walls. Special techniques were employed to minimize extraneous heat losses, and numerical finite-difference solutions played an important role in both the design of the apparatus and in the data reduction. The thermal entrance lengths required to attain thermally developed conditions were found to increase markedly with the Reynolds number and were generally greater than those for conventional pipe flows—a behavior which can be attributed to the unsymmetric heating. The fully developed Nusselt numbers were compared with circular tube correlations from the literature, from which it was shown that the hydraulic diameter is not fully sufficient to rationalize the circular and noncircular duct results. However, excellent Nusselt number predictions were obtained by employing the Petukhov-Popou correlation in conjunction with the measured friction factors for the triangular duct. This approach may have general applicability for predicting noncircular duct heat transfer. The friction factor results also affirmed the inadequacies of the hydraulic diameter but supported a general noncircular duct correlation available in the literature.
Analysis of Laminar Heat Transfer in Internally Finned Tubes with Uniform Outside Wall Temperature
Authors: Soliman H. M. , Chau T. S. , Trupp A. C.
Published: October 2009

An analysis is presented for fully developed laminar convective heat transfer in tubes with internal longitudinal fins and uniform outside wall temperature. The governing momentum and energy equations were solved numerically, with the influence of fin conductance accounted for by a single parameter. The distributions of fin temperature, fluid temperature and local heat flux (both at the fin and unfinned surfaces) are presented. These are shown to be strongly dependent on finned tube geometry and, in some cases, on the fin conductance parameter as well. Based on average heat transfer per unit area, the various fins proved more effective than the unfinned surfaces. Values for overall Nusselt number indicated significant heat transfer enhancement over smooth tube conditions.
Natural Convection Heat Transfer from the Upper Plate of a Colinear, Separated Pair of Vertical Plates
Authors: Sparrow E. M. , Faghri M.
Published: October 2009

The effect of a buoyant boundary layer spawned by a heated vertical plate on the natural convection heat transfer from an upper colinear vertical plate has been determined analytically. The interplate spacing was varied parametrically, as were the relative temperatures and relative lengths of the two plates; the Prandtl number was equal to 0.7 for all cases. Heat transfer at the upper plate was found to be affected both by the preheating and by the finite velocity imparted to the fluid by the first plate, respectively tending to degrade and to enhance the heat transfer. The upper-plate heat transfer was compared to that of an otherwise identical vertical plate, but with the lower plate absent. When the temperatures of the upper and lower plates are the same, the overall upper-plate heat transfer is less than that of its single-plate counterpart for small interplate spacings, with the opposite relationship at larger spacings. If the temperature of the upper plate is substantially below that of the lower plate, the overall heat transfer is degraded. On the other hand, heat transfer enhancement generally occurs when the upper plate is relatively hot. In general, the heat transfer from relatively short upper plates is very sensitive to the presence of the lower plate, with a lessening sensitivity with increasing plate length. The computed temperature and velocity profiles demonstrated that near the leading edge of the upper plate, a new boundary layer develops within the already existing boundary layer spawned by the first plate.
Natural Convection on Both Sides of a Vertical Wall Separating Fluids at Different Temperatures
Authors: Anderson R. , Bejan A.
Published: October 2009

This paper describes an analytical study of laminar natural convection on both sides of a vertical conducting wall of finite height separating two semi-infinite fluid reservoirs of different temperatures. The countercurrent boundary layer flow formed on the two sides is illustrated via representative streamlines, temperature and heat flux distributions. The net heat transfer between reservoirs is reported for the general case in which the wall thermal resistance is not negligible relative to the overall reservoir-to-reservoir thermal resistance.
An Experimental Study of Natural Convection in Trapezoidal Enclosures
Authors: Iyican L. , Witte L. C. , Bayazitogˇlu Y.
Published: October 2009

Experimental data for natural convection of air in an inclined trapezoidal enclosure are reported for a Rayleigh number range of ∼ 2 × 103 to ∼ 5 × 107. The small side of the trapezoid was electrically heated while the opposing large side was cooled to a uniform temperature. The effect of tilt angle from 0 to 90 deg (from horizontal) was investigated at 15 deg increments. Data were also obtained for 180 deg (hot surface facing down). A comparison of the data to an analysis using a two-dimensional circulation pattern showed reasonable agreement in the Rayleigh number-tilt angle range where two-dimensional circulation could be expected. The experimental data are correlated by an equation of the form, Nu = C Ran, over a wide Rayleigh number range. The data exhibit a local minimum in the Nusselt number-tilt angle curve between 90 and 0 deg in a manner similar to that observed in inclined rectangular channels.
A Mechanistic Model for Countercurrent Steam-Water Flow
Authors: Segev A. , Collier R. P.
Published: October 2009

A one-dimensional model has been developed to describe the flow behavior of a liquid film draining down a heated wall, in the presence of countercurrent vapor flow. The effects of nonequilibrium void generation at the walls and condensation of countercurrent vapor on the penetrating liquid film interface, as well as on the bypassed liquid, have been considered. The momentum exchange is described by a correlation, assumed to apply at the entrance region, where the amount of liquid film flow is limited by the net vapor upflow. Theoretical predictions were compared with over 600 test results, obtained in 1/15- and 2/15-scale models of a PWR downcomer, over a wide range of test parameters. The comparisons exhibit a fairly good agreement.
A Self-Sustained Oscillatory Flow Phenomenon in Two-Phase Condensing Flow Systems
Authors: Bhatt B. L. , Wedekind G. L.
Published: October 2009

This paper presents the results of an experimental and theoretical investigation of an unstable flow phenomenon that leads to sustained limit-cycle type of oscillations of large amplitude, and which under certain conditions, can involve flow reversals. This unstable behavior normally exists for conditions of low outlet throttling. Upstream compressible volume and downstream inertia appear to be the dominant energy storage mechanisms for the self-sustained oscillations. A linearized analysis based on the system mean void fraction model successfully predicts the experimentally observed stability boundary.
Nucleate Boiling Performance of Refrigerants and Refrigerant-Oil Mixtures
Authors: Chongrungreong S. , Sauer H. J.
Published: October 2009

Current and previous studies by the authors and others have shown shown that the carryover of oil in refrigeration systems can have a significant influence on the boiling performance in the evaporator of refrigeration systems. This investigation was conducted primarily to develop a general correlation equation for predicting the heat transfer coefficient for refrigerants and refrigerant-oil mixtures under pool boiling conditions. Experimental results were obtained to establish the validity of the correlation equation.
Visible Radiation Transfer in a Black Ink Suspension
Authors: Wagner T. R. , Incropera F. P. , Houf W. G.
Published: October 2009

Measurements of the directional and spatial distribution of the radiation intensity have been made for an irradiated India ink suspension, and the results have been used to determine radiation fluxes associated with the forward and backward directions. The inherent radiative properties of the ink have also been measured and used with appropriate theories (the discrete ordinate and three-flux methods) to predict the radiation field within the suspension. For the prescribed laboratory conditions, the radiation is concentrated within a cone of half angle θ ≈ 0.85 rad throughout the suspension, and for this region good agreement is obtained between the intensity measurements and predictions based on the discrete ordinate method. Good agreement is also obtained between radiation fluxes measured for the forward direction and predictions based on both the discrete ordinate and three-flux methods. Agreement between the predicted and measured intensities for θ ≳ 0.85 rad, as well as agreement between predicted and measured radiation fluxes for the backward direction, is less satisfactory. Differences are attributed to uncertainties in the radiation intensity and phase function measurements.
A Source Function Expansion in Radiative Transfer
Authors: O¨zis¸ik M. N. , Sutton W. H.
Published: October 2009

The radiative heat transfer problem for an isotropically scattering slab with specularly reflecting boundaries is reduced to the solution of a set of algebraic equations by expanding the source function in Legendre polynomials in the space variable in the integral form of the equation of radiative transfer. The lowest order S-1 analysis requires very little computer time for calculations, is easy to apply and yields results which are sufficiently accurate. For an absorbing, emitting, isotropically scattering medium with small and intermediate optical thickness (i.e., τ = 2), which is of great interest in engineering applications, and for which the P-1 and P-3 solutions of the P-N method are not sufficiently accurate, the S-1 solution yields highly accurate results. In the case of a slab with diffusely reflecting boundaries, the problem is split up into a set of simpler problems each of which is solved with the source function expansion technique as a special case of the general problem considered.
Laminar, Mixed-Convection, Boundary-Layer, Nongray-Radiative, Diffusion Flames
Authors: Liu C. N. , Shih T. M.
Published: October 2009

The two-dimensional, laminar, mixed-mode convection (free and forced), boundary-layer, radiative, diffusion flame has been analytically investigated with special attention directed toward the nongray nature of the combustion products. The solution procedures include Shvab-Zeldovich approximation and von Mises transformation for the physical transport equations and the finite difference method for the computation of the transformed equations. The local radiative fluxes for nongray, gray and radiatively-ideal gases are prescribed in detail. Twelve parameters that govern the flame characteristics have been identified. Setting appropriate parameters to zero yields several limiting solutions which are compared with the reported results. In addition to extensive results of distributions in the field variables and fluxes, the ratio of the radiation loss to the combustion energy evolved was computed and a radiatively-ideal gas having behaviors similar to the nongray gases was identified.
Experiments on Heat Exchanger Solidity for Coal-Fired Fluidized Bed Applications
Authors: Miller G. , Zakkay V.
Published: October 2009

The efficient extraction of a high-temperature working fluid from a coal-fired fluidized bed combustor depends, to a great extent, on the design of the immersed heat exchanger. Of special importance is the solidity of the cooling tubes immersed in the bed. The interaction between increasing solidity and the consequent degradation of proper fluidization and circulation is being studied at the New York University fluidized bed combustion facility. In a preliminary set of experiments it was found that under certain conditions, the solidity of heat exchanger in the bed may be significantly increased, giving designers an additional variable parameter.
Buoyancy-Induced Flow Due to Isolated Thermal Sources on a Vertical Surface
Authors: Jaluria Y.
Published: October 2009

The natural convection flow due to multiple isolated heated elements located on a vertical adiabatic surface has been studied analytically. The problem, which is of particular relevance to electronic circuitry cooling and to the question of locating sources in manufacturing systems, is considered for a Prandtl number of 0.7, which applies for air. Of particular interest were the temperature and velocity fields that arise and the dependence of these on the heat input and on the distance between the heated elements. The flow is treated as a boundary layer problem and the governing equations are solved numerically. The results obtained indicate the general nature of the flow and the dependence of the heat-transfer coefficient for an element, located in the wake of another, on the energy input and location. The downstream variation of the surface temperature, the velocity level, and the resulting velocity and temperature profiles are studied in detail. The results obtained are also compared with those for a single source, bringing out several interesting features.
Effects of Mass Flux, Flow Quality, Thermal and Surface Properties of Materials on Rewet of Dispersed Flow Film Boiling
Authors: Iloeje O. C. , Plummer D. N. , Rohsenow W. M. , Griffith P.
Published: October 2009

The effects of mass flux, flow quality, material thermal properties, surface roughness, and surface oxidation on the rewet-wall superheat for dispersed vertical flow were experimentally investigated. The mass fluxes tested were 40.7, 81.4, 169.5 and 271.3 kg/s-m2. Flow qualities varied from 10–90 percent. The test materials were copper, aluminum, and inconel-600. Overall dimensions of the test pieces were 10.16 mm i.d., 25.4 mm o.d., and 25.4 mm long. A smooth inside surface was prepared for each material, with roughness amplitudes of approximately 0.5 μm. Two inconel test pieces had roughnesses of 15–20 μm and 66 μm, respectively. Another inconel piece was oxidized to a thin oxide layer of approximately 1.3 μm. The test fluid was liquid nitrogen. The results indicated that the rewet wall superheat increased with mass flux. The rewet wall superheat decreased with increasing flow quality, with the rate of decrease being more rapid at higher mass fluxes and higher qualities. Increases in wall roughnesses, and the presence of surface oxidation, increased the rewet superheat. The effect of an oxide layer increased with increasing mass flux and decreasing quality, and was thought to result from the decrease in contact angle between the liquid and the oxidized surface. Differences in wall thermal properties were not very significant up to a mass velocity of 81.4 kg/s-m2. Above this mass flux, the copper data did not clearly show the increasing Tmin with increasing mass flux, while the inconel data did. A comparison of Tmin for several materials with different thermal properties therefore was not made above mass flux of 81.4 kg/s-m2.
Inward Melting in a Vertical Tube Which Allows Free Expansion of the Phase-Change Medium
Authors: Sparrow E. M. , Broadbent J. A.
Published: October 2009

Experiments on the melting of a phase-change medium in a vertical tube yielded quantitative results both for the heat transfer and the timewise evolution of the melting front. The upper surface of the phase-change medium was bounded by an insulated air space, which accommodated the volume changes which accompany the melting process. Numerical solutions based on a pure conduction model were also performed for comparison purposes. It was found that the rate of melting and the heat transfer are significantly affected by fluid motions in the liquid melt induced by the volume change and by natural convection, with the former being significant only at early times. For melting initiated with the solid at the phase-change temperature, the experimentally determined values of the energy transfer associated with the melting process were about 50 percent higher than those predicted by the conduction model. Furthermore, the measured values of the energy stored in the liquid melt were about twice the conduction prediction. A compact dimensionless correlation of the experimental results was achieved using the Fourier, Stefan, and Grashof numbers. Initial subcooling of the solid substantially decreased the rate of melting, with corresponding decreases in the energy transfers for melting and sensible heat storage.
Transient Freezing of a Liquid in a Convectively Cooled Tube
Authors: Sadeghipour M. S. , O¨zis¸ik M. N. , Mulligan J. C.
Published: October 2009

The transient freezing of a liquid in laminar flow inside a circular tube subjected to convection heat transfer on the external surface is investigated analytically. The effects of Biot number, fluid inlet temperature, and the external ambient temperature on the length of this freeze-free zone are illustrated. The variation of the thickness of the freeze layer as a function of time and position along the tube is determined.