bluuebunny/crossref_metadata_2025 huggingface dataset:
Scaling of the Turbulent Natural Convection Flow in a Heated Square Cavity
Authors: Henkes R. A. W. M. , Hoogendoorn C. J.
Published: May 2008

By numerically solving the Reynolds equations for air and water in a square cavity, with differentially heated vertical walls, at Rayleigh numbers up to 1020 the scalings of the turbulent natural convection flow are derived. Turbulence is modeled by the standard k–ε model and by the low-Reynolds-number k–ε models of Chien and of Jones and Launder. Both the scalings with respect to the Rayleigh number (based on the cavity size H) and with respect to the local height (y/H) are considered. The scalings are derived for the inner layer, outer layer, and core region. The Rayleigh number scalings are almost the same as the scalings for the natural convection boundary layer along a hot vertical plate. The scalings found are almost independent of the k–ε model used.
A Two-Fluid Model for Reacting Turbulent Two-Phase Flows
Authors: Chan S. H. , Abou-Ellail M. M. M.
Published: May 2008

A reacting two-fluid model, based on the solution of separate transport equations for reacting gas-liquid two-phase flow, is presented. New time-mean transport equations for two-phase mixture fraction f and its variance g are derived. The new two-fluid transport equations for f and g are useful for two-phase reacting flows in which phases strongly interact. They are applicable to both submerged and nonsubmerged combustion. A pdf approach to the reaction process is adopted. The mixture fraction pdf assumes the shape of a beta function while the instantaneous thermochemical properties are computed from an equilibrium model. The proposed two-fluid model is verified by predicting turbulent flow structures of an n-pentane spray flame and a nonreacting bubbly jet flow for which experimental data exist. Good agreement is found between the predictions and the corresponding experimental data.
Experimental and Numerical Study of Microwave Thawing Heat Transfer for Food Materials
Authors: Zeng Xin , Faghri A.
Published: May 2008

A combined experimental and numerical study of microwave thawing of a food analog material is reported. The objective of this study is to determine thawing histories and temperature profiles for cylindrically shaped samples. Microwave thawing experiments of samples with different aspect ratios were performed to collect time-temperature data for various power levels. These measured thawing times were utilized in a comparative analysis with the numerical predictions. The heat transfer phenomena due to the interaction between phase change and heat generation was analyzed. A two-dimensional mathematical model was developed to deal with the complicated thawing process, which includes the frozen, mushy, and thawed phases, and the evaporation of water. The present study is anticipated to form a fundamental understanding of the interactions between microwave irradiation and phase-change heat transfer.
Heat Transfer Measurement and Analysis for Sintered Porous Channels
Authors: Hwang G. J. , Chao C. H.
Published: May 2008

This paper presents the results of heat transfer measurement and analysis for two 5×5×1 cm porous channels. The channels were made of sintered bronze beads with two different mean diameters, dp=0.72 and 1.59 mm. The local wall temperature distribution, inlet and outlet pressures and temperatures, and heat transfer coefficients were measured for heat flux of 0.8, 1.6, 2.4, and 3.2 W/cm2 with air velocity ranging from 0.16 to 5 m/s and inlet air pressure of 1~3 atm. The measurement covers the data in both thermal entrance and thermally fully developed regions. The local Nusselt numbers were correlated in the fully developed region. The fully developed Nusselt numbers were analyzed theoretically by using a non-Darcy, two-equation flow model. Heat transfer between the solid and fluid phases was modeled by a relation of the form hloc=ARen. A wall function was introduced to model the transverse thermal dispersion process for the wall effect on the lateral mixing of fluid. The predicted fully developed Nusselt numbers are in good agreement with the measured values.
Use of the Kelvin-Clapeyron Equation to Model an Evaporating Curved Microfilm
Authors: DasGupta S. , Kim I. Y. , Wayner P. C.
Published: May 2008

A Kelvin–Clapeyron change-of-phase heat transfer model is used to evaluate experimental data for an evaporating meniscus. The details of the evaporating process near the contact line are obtained. The heat flux and the heat transfer coefficient are a function of the film thickness profile, which is a measure of both the intermolecular stress field in the contact line region and the resistance to conduction. The results indicate that a stationary meniscus with a high evaporative flux is possible. At equilibrium, the augmented Young–Laplace equation accurately predicts the meniscus slope. The interfacial slope is a function of the heat flux.
An Experimental and Analytical Investigation of Ice Formation From a Circular Water Jet Impinging on a Horizontal Cold Surface
Authors: Moallemi M. K. , Naraghi M. N.
Published: May 2008

The transient freezing of water impinging vertically on a subzero disk through a circular jet is studied experimentally to determine the interaction of the fluid flow and the solidification process. Experiments are performed over a range of the jet Reynolds number (1600 < Rei < 3500) based on the average velocity and radius of the falling jet at the impingement point. For this range of Reynolds numbers, that corresponds to tube Reynolds numbers less than 1100, and in the absence of solidification, the thin liquid film is characterized by a smooth circular hydraulic jump whose diameter is measured and correlated with the jet Reynolds number. The solidification process is initiated away from the jet (i.e., outside of the hydraulic jump) and moves inward toward the jet. The formation and growth of ice on the cold surface affect the flow field over the surface. This effect manifests itself in the form of a rapid reduction of the hydraulic jump diameter accompanied by instability in its position until its complete disappearance. The effect of fluid flow on the solidification process is found to be a small reduction in the nucleation temperature. The ice layer profiles at different times for different values of jet Reynolds number, and Stefan numbers of the surface and jet are also measured and reported. An approximate model is developed for the calculation of the transient crust growth by neglecting the interaction between the flow and solidification. The predicted solid crust profiles are compared with the measured ones, and the extent of the flow-freezing interactions is discussed. The approximate model is also used for a parametric study of the problem for a constant temperature surface.
Challenges in Microscale Conductive and Radiative Heat Transfer
Authors: Tien C. L. , Chen G.
Published: May 2008

This work addresses challenges in the emerging field of microlength scale radiative and conductive heat transfer in solids and recommends specific directions of future research. Microlength scale heat transfer involves thermal energy transport processes in which heat carrier characteristic lengths become comparable to each other or the characteristic device dimension. Identification of these characteristic lengths leads to the definition of different microscale heat transfer regimes. A review of the theoretical bases describing heat transfer in each regime is followed by a discussion of the obstacles confronted in current research. Engineering challenges are illustrated with the applications of microscale heat transfer in cryogenic systems, material processing, and electronic, optical, and optoelectronic devices. The experimental difficulties discussed have hampered the development of microscale heat transfer research and deserve great efforts to overcome them.
Transient Effects of Uncertainties on the Sensitivities of Temperatures and Heat Fluxes Using Stochastic Finite Elements
Authors: Fadale T. D. , Emery A. F.
Published: May 2008

This paper addresses the question in the design of experiments of where to place sensors for optimal sensitivity and the post-experiment determination of which sensors yield relevant data. The authors in their previous works have described the spatial dependence of the response sensitivities and the importance of conducting a sensitivity analysis for a better understanding of the system response. This paper describes the formulation of the method for a transient analysis and its application to thermal problems. The results have been verified using the Monte Carlo sampling technique to simulate the variations in the parameters. The results show that there are not only optimal locations to maximize the sensitivities of the responses, but also optimal times of measurement. Sample test cases are used to demonstrate the effects of time of measurement and placement of sensors on the accuracy of the measured temperatures.
An Experimental Evaluation of the Effective Thermal Conductivities of Packed Beds at High Temperatures
Authors: Nasr K. , Viskanta R. , Ramadhyani S.
Published: May 2008

Combined conduction and radiation heat transfer in packed beds of spherical particles was investigated. Three different packing materials (alumina, aluminum, and glass) of various particle diameters (2.5 to 13.5 mm) were tested. Internal bed temperature profiles and corresponding effective thermal conductivities were measured under steady-state conditions for a temperature range between 350 K and 1300 K. The effects of particle diameter and local bed temperature were examined. It was found that higher effective thermal conductivities were obtained with larger particles and higher thermal conductivity packing materials. The measured values for the effective thermal conductivity were compared against the predictions of two commonly used models, the Kunii–Smith and the Zehner–Bauer–Schlu¨nder models. Both models performed well at high temperatures but were found to overpredict the effective thermal conductivity at low temperatures. An attempt was made to quantify the relative contributions of conduction and radiation. Applying the diffusion approximation, the radiative conductivity was formulated, normalized, and compared with the findings of other investigators.
A Near-Wall Eddy Conductivity Model for Fluids With Different Prandtl Numbers
Authors: So R. M. C. , Sommer T. P.
Published: May 2008

Near-wall turbulence models for the velocity and temperature fields based on the transport equations for the Reynolds stresses, the dissipation rate of turbulent kinetic energy, and the temperature variance and its dissipation rate are formulated for flows with widely different Prandtl numbers. Conventional high-Reynolds-number models are used to close these equations and modifications are proposed to render them asymptotically correct near a wall compared to the behavior of the corresponding exact equations. Thus formulated, two additional constants are introduced into the definition of the eddy conductivity. These constants are found to be parametric in the Prandtl number. The near-wall models are used to calculate flows with different wall thermal boundary conditions covering a wide range of Reynolds numbers and Prandtl numbers. The calculated Nusselt number variations with Prandtl number are in good agreement with established formulae at two different Reynolds numbers. Furthermore, the mean profiles, turbulence statistics, heat flux, temperature variance, and the dissipation rates of turbulent kinetic energy and temperature variance are compared with measurements and direct numerical simulation data. These comparisons show that correct near-wall asymptotic behavior is recovered for the calculated turbulence statistics and the calculations are in good agreement with measurements over the range of Prandtl numbers investigated.
Heat Transfer Modeling and the Assumption of Zero Wall Temperature Fluctuations
Authors: Sommer T. P. , So R. M. C. , Zhang H. S.
Published: May 2008

At present, it is not clear how the fluctuating temperature at the wall can be properly specified for near-wall turbulent heat-flux models. The conventional approach is to assume zero fluctuating temperature or zero gradient for the temperature variance at the wall. These are idealized specifications and the latter condition could lead to an ill-posed problem for fully developed pipe and channel flows. In this paper, the validity and extent of the zero fluctuating wall temperature condition for heat transfer calculations are examined. The approach taken is to assume Taylor series expansions in the wall normal coordinate for the fluctuating quantities that are general enough to account for both zero and nonzero temperature fluctuations at the wall and to develop a near-wall turbulence model allowing finite values of the wall temperature variance. As for the wall temperature variance boundary condition, it is estimated by solving the coupled heat transfer problem between the fluid and the solid wall. The eddy thermal conductivity is calculated from the temperature variance and its dissipation rate. Heat transfer calculations assuming both zero and nonzero fluctuating wall temperature reveal that the zero fluctuating wall temperature assumption is quite valid for the mean field and the associated integral heat transfer properties. The effects of nonzero fluctuating wall temperature on the fluctuating field are limited only to a small region near the wall for most fluid/solid combinations considered.
Heat Transfer in Coaxial Jet Mixing With Swirled Inner Jet
Authors: Dellenback P. A. , Sanger J. L. , Metzger D. E.
Published: May 2008

Convective heat transfer data are presented for coaxial jet mixing in a constant-diameter tube. The inner jet diameter was approximately twice the annular gap dimension. Water, with a nominal inlet Prandtl number of 6, was used as the working fluid. For the inner jet, Reynolds numbers of 30,000 and 100,000 were examined and the swirl number was varied from zero to one. Annular flow rates were characterized by a ratio of annular-to-inner jet axial momentum, which was varied from 0 to 8.3. In all cases the annular jet was unswirled. Plots of local Nusselt numbers show minima and maxima corresponding to the separation and reattachment associated with wall-bounded recirculation. As inner jet swirl strength increased from zero to its maximum value, the location of peak Nusselt number shifted upstream. Local Nusselt numbers achieved magnitudes as high as 9.7 times fully developed values for cases with high swirl and low annular flow rate. As the annular jet’s flow rate was increased, the heat transfer enhancement decreased while the near-wall recirculation zones were stretched and shifted downstream, until at sufficiently high values of the momentum flux ratio, the zones were no longer in evidence from the heat transfer data.
The Effect of Embedded Longitudinal Vortex Arrays on Turbulent Boundary Layer Heat Transfer
Authors: Pauley W. R. , Eaton J. K.
Published: May 2008

Heat transfer and fluid mechanics data were obtained for a turbulent boundary layer with arrays of embedded streamwise vortices containing both counterrotating and corotating vortex pairs. The data show that these arrays can cause both large local variations in the heat transfer rate and significant net heat transfer augmentation over large areas. Close proximity of other vortices strongly affects the development of the vortex arrays by modifying the trajectory that they follow. The vortices in turn produce strong distortion of the normal two-dimensional boundary layer structure, which is due to their secondary flow. When one vortex convects another toward the wall, a strong boundary layer distortion occurs. The heat transfer is elevated where the secondary flow is directed toward the wall and reduced where the secondary flow is directed away from the wall. When adjacent vortices lift their neighbor away from the wall, minimal modification of the heat transfer results. The primary influence of grouping multiple vortex pairs into arrays is the development of stable patterns of vortices. These stable vortex patterns produce vortices that interact with the boundary layer and strongly modify the heat transfer far downstream, even where the vortices have decayed in strength.
Comparison of Wing-Type Vortex Generators for Heat Transfer Enhancement in Channel Flows
Authors: Tiggelbeck St. , Mitra N. K. , Fiebig M.
Published: May 2008

Longitudinal vortices can be generated in a channel flow by punching or mounting small triangular or rectangular pieces on the channel wall. Depending on their forms, these vortex generators (VG) are called delta wing, rectangular wing, pair of delta winglets, and pair of rectangular winglets. The heat transfer enhancement and the flow losses incurred by these four basic forms of VGs have been measured and compared in the Reynolds number range of 2000 to 9000 and for angles of attack between 30 and 90 deg. Local heat transfer coefficients on the wall have been measured by liquid crystal thermography. Results show that winglets perform better than wings and a pair of delta winglets can enhance heat transfer by 46 percent at Re=2000 to 120 percent at Re=8000 over the heat transfer on a plate.
Augmented Heat Transfer in a Rectangular Channel With Permeable Ribs Mounted on the Wall
Authors: Hwang Jenn-Jiang , Liou Tong-Miin
Published: May 2008

Turbulent heat transfer and friction in a rectangular channel with perforated ribs arranged on one of the principal walls are investigated experimentally. The effects of rib open-area ratio, rib pitch-to-height ratio, rib height-to-channel hydraulic diameter ratio, and flow Reynolds number are examined. To facilitate comparison, measurements for conventional solid-type ribs are also conducted. Laser holographic interferometry is employed to determine the rib permeability and measure the heat transfer coefficients of the ribbed wall. Results show that ribs with appropriately high open-area ratio at high Reynolds number range are permeable, and the critical Reynolds number of initiation of flow permeability decreases with increasing rib open-area ratio. By examining the local heat transfer coefficient distributions, it is found that permeable ribbed geometry has an advantage of obviating the possibility of hot spots. In addition, the permeable ribbed geometry provides a higher thermal performance than the solid-type ribbed one, and the best thermal performance occurs when the rib open-area ratio is 0.44. Compact heat transfer and friction correlations are also developed for channels with permeable ribs.
Unsteady Wake Effect on Film Effectiveness and Heat Transfer Coefficient From a Turbine Blade With One Row of Air and CO2 Film Injection
Authors: Ou S. , Han J. C.
Published: May 2008

The effect of unsteady wake and film injection on heat transfer coefficients and film effectiveness from a gas turbine blade was found experimentally. A spoked wheel type wake generator produced the unsteady flow. Experiments were done with a five airfoil linear cascades in a low-speed wind tunnel at a chord Reynolds number of 3 × 105, two wake Strouhal numbers of 0.1 and 0.3, and a no-wake case. A model turbine blade injected air or CO2 through one row of film holes each on the pressure and suction surfaces. The results show that the large-density injectant (CO2) causes higher heat transfer coefficients on the suction surface and lower heat transfer coefficients on the pressure surface. At the higher blowing ratios of 1.0 and 1.5, the film effectiveness increases with increasing injectant-to-mainstream density ratio at a given Strouhal number. However, the density ratio effect on film effectiveness is reversed at the lowest blowing ratio of 0.5. Higher wake Strouhal numbers enhance the heat transfer coefficients but reduce film effectiveness for both density ratio injectants at all three blowing ratios. The effect of the wake Strouhal number on the heat transfer coefficients on the suction surface is greater than that on the pressure surface.
Heat Transfer and Surface Renewal Dynamics in Gas-Fluidized Beds
Authors: Pence D. V. , Beasley D. E. , Figliola R. S.
Published: May 2008

Local instantaneous heat transfer between a submerged horizontal cylinder and a gas-fluidized bed operating in the bubble-flow regime was measured and the resulting signals analyzed. Unique to this investigation is the division of particle convective heat transfer into transient and steady-state contact dynamics through analysis of instantaneous heat transfer signals. Transient particle convection results from stationary particles in contact with the heat transfer surface and yields a heat transfer rate that decays exponentially in time. Steady-state particle convection results from active particle mixing at the heat transfer surface and results in a relatively constant heat transfer rate during emulsion phase contact. The average time of contact for each phase is assessed in this study. Signals were acquired using a constant-temperature platinum film heat flux sensor. Instantaneous heat transfer signals were obtained for various particle sizes by varying the angular position of the heat transfer probe and the fluidization velocity. Individual occurrences of emulsion phase heat transfer that are steady-state in nature are characterized by contact times significantly higher than both the mean transient and mean emulsion phase contact times under the same operating conditions. Transient and steady-state contact times are found to vary with angular position, particle size, and fluidizing velocity. Due to the extremely short transient contact times observed under these fluidization conditions, mean transient heat transfer coefficients are approximately equal to the mean steady-state heat transfer coefficients.
Transient Heat and Moisture Transfer in Thin Silica Gel Beds
Authors: Kafui K. D.
Published: May 2008

A numerical model has been developed to simulate the transport of heat and moisture in thin beds of microporous silica gel, assuming surface and pore volume diffusion of moisture and conduction of heat as the dominant intraparticle transport mechanisms in the desiccant. Predictions from the model have been compared with the results of single-blow tests for packed beds of silica gel from the literature and for a matrix of silica gel-coated parallel passages from this study. The model has been used to explore the significance of intraparticle diffusion resistance to heat and moisture transport in packed beds and parallel passage matrices of silica gel for different desiccant sizes and air velocities.
Convection-Diffusion Controlled Laminar Micro Flames
Authors: Ban H. , Venkatesh S. , Saito K.
Published: May 2008

Small laminar diffusion flames (flame height ≃2–3 mm) established by a fuel jet issuing into a quiescent medium are investigated. It was found that for these flames buoyancy effects disappeared as the flame size decreased (Fr≫1), and diffusive transport of the fuel was comparable to the convective transport of the fuel. The effect of buoyancy on these flames was studied by examining the flame shape for horizontally oriented burners. A phenomenological model was developed (based on experimentally determined flame shapes) to compare diffusion and convection transport effects. Finally, the flame shapes were theoretically determined by solving the conservation equations using similarity methods. It was seen that when the axial diffusion (in momentum and species equations) terms are included in the conservation equations, the calculated flame shape is in better agreement (as compared to without the axial diffusion term) with the experimentally measured flame shape.
Optical Properties of Soot in Buoyant Laminar Diffusion Flames
Authors: Ko¨ylu¨ U¨. O¨. , Faeth G. M.
Published: May 2008

The structure and optical properties of soot were studied in the fuel-rich (underfire) region of buoyant laminar diffusion flames of ethylene and acetylene burning in coflowing air. The objective was to evaluate scattering predictions based on the Rayleigh–Debye–Gans (RDG) approximation for polydisperse fractal aggregates of spherical primary soot particles having constant diameters, for conditions where the Guinier (small angle) regime, and the transition between the Guinier and the power-law (large-angle) regimes, were dominant, in order to supplement earlier work for conditions where the power-law regime was dominant. Soot structure was measured using thermophoretic sampling and analysis by transmission electron microscopy (TEM) to yield primary particle diameters, distributions of the number of primary particles per aggregate, and the aggregate mass fractal dimensions. Soot optical property measurements included vv, hh, hv, and vh differential scattering cross sections, total scattering cross sections, and the albedo at 514.5 nm, as well as several soot structure parameters inferred from these measurements using the approximate theory. The approximate RDG theory generally provided an acceptable basis to treat the optical properties of the present soot aggregates over a range of conditions spanning the Guinier and power-law regimes. Other scattering approximations were less satisfactory with performance progressively becoming less satisfactory in the order: RDG polydisperse fractal aggregate scattering using a single mean squared radius of gyration (from the Guinier regime), Mie scattering for an equivalent sphere, and Rayleigh scattering—the last underestimating differential scattering levels by a factor of roughly 100 for the present test conditions.
K-Distribution Analysis of Gas Radiation With Nongray, Emitting, Absorbing, and Anisotropic Scattering Particles
Authors: Tang K. C. , Brewster M. Q.
Published: May 2008

The K-distribution technique is presented for predicting nongray gas radiation in the presence of particle scattering. This technique transforms the otherwise formidable spectral integration problem from the frequency domain to the K or gas absorption coefficient domain. This transformation is made possible by the negligible variation of blackbody intensity and particle radiative properties with frequency within a gas band. Application of the K-distribution technique to an isothermal emission problem is made. Results are compared with those from the Monte-Carlo method, These results show that, with the K-distribution technique, the detailed absorption line structure within a gas band as well as the scattering effect due to particles can be simulated with great numerical accuracy and computational efficiency. Essentially, line-by-line accuracy is achieved without line-by-line spectral integration.
Thermal Modeling of Absolute Cryogenic Radiometers
Authors: Zhang Z. M. , Datla R. U. , Lorentz S. R. , Tang H. C.
Published: May 2008

This work consists of a detailed thermal modeling of two different radiometers operated at cryogenic temperatures. Both employ a temperature sensor and an electrical-substitution technique to determine the absolute radiant power entering the aperture of a receiver. Their sensing elements are different: One is a germanium resistance thermometer, and the other is a superconducting kinetic-inductance thermometer. The finite element method is used to predict the transient and steady-state temperature distribution in the receiver. The nonequivalence between the radiant power and the electrical power due to the temperature gradient in the receiver is shown to be small and is minimized by placing the thermometer near the thermal impedance. In the radiometer with a germanium resistance thermometer, the random noise dominates the uncertainty for small incident powers and limits the ultimate sensitivity. At high power levels, the measurement accuracy is limited by the uncertainty of the absorptance of the cavity. Recommendations are given based on the modeling for future improvement of the dynamic response of both radiometers.
Leidenfrost Evaporation of Liquid Nitrogen Droplets
Authors: Chandra S. , Aziz S. D.
Published: May 2008

The evaporation of a single droplet of liquid nitrogen, levitated during film boiling above a solid, impervious surface, was studied experimentally. The droplet initial diameter (1.9 mm), surface temperature (~20°C), ambient temperature (~20°C), and ambient pressure (~0.1 MPa) were held constant. The principal parameters varied were the surface material (copper or glass), and roughness (0.35 to 50 μm). Measurements were made of the droplet diameter evolution and the surface temperature variation during droplet impact. Predictions from existing models of droplets in Leidenfrost evaporation agree well with measurements of the droplet evaporation rate. The droplet lifetime was found to be slightly longer on the glass surface than it was on the copper surface, corresponding to the greater cooling of the glass surface during droplet impact. The droplet evaporation rate was unchanged by small increases in surface roughness. However, ridges on the surface with a height of the same magnitude as the thickness of the vapor film under the drop caused vapor bubble nucleation in the droplet, and significantly reduced the droplet evaporation time.
Velocity Characteristics of a Confined Highly-Turbulent Swirling Flow Near a Swirl Plate (Data Bank Contribution)
Authors: Shi R. X. , Chehroudi B.
Published: May 2008

Axial and tangential components of the velocity vector are measured using a Laser Doppler Velocimeter (LDV) system in a confined highly turbulent isothermal swirling flow near a swirl plate. The flow has essential features of swirl-stabilized flame combustors. Throughout this study, a constant “nominal” swirl number of 0.36 is generated by air jets from a set of slots in a swirl plate. A low-speed coflowing air, referred to as dilution air, is uniformly distributed around the swirling flow by use of an annular-shaped honeycomb. Three different swirling air flow rates with a fixed dilution flow rate are studied and results are discussed. Detailed mean axial and tangential velocity profiles at several axial locations show that the size and the strength of the central recirculation zone are strongly dependent on the swirling air flow rate. Increasing the swirl air flow rate increases both the radial extent and the axial length of the central recirculation zone. Mean total and reversed air flow rates are calculated by integrating the mean axial velocity profiles. In the setup used in this study and up to the axial positions investigated, the reversed flow rate as a percent of the total flow rate seems to be linearly proportional to the reversed-flow zone area, being independent of the swirl air flow rate at a fixed nominal swirl number value. As swirl air flow rate is increased, the root mean square (rms) of the axial and tangential velocity fluctuations increase monotonically at almost all radial positions except sufficiently away from the swirl plate and near the chamber axis. Several velocity biasing correction methods are reviewed. A simple velocity biasing correction scheme is applied in this study to investigate its effect on the conclusions reached in the study.
A Simple Method for Estimating Velocity Distributions in Swirling Flows
Authors: Kinnear D. , Davidson P. A.
Published: May 2008

We describe the important structural features of swirling recirculating flows induced by a rotating boundary. A knowledge of this structure has allowed us to match the core flow to the boundary layer using a momentum-integral technique. In particular, we derive a single integral-differential equation, valid for any shape of container, which predicts the distribution of swirl, secondary recirculation, and wall shear stress. This momentum-integral approach has been applied to three cases: flow between parallel disks; flow in a cone; and flow in a hemisphere. The results compare favorably with published experimental data, and with computed numerical results. Our momentum-integral approach complements numerical solution methods. For simple geometries all the important information can, in principle, be derived using the momentum-integral approach, and this is particularly useful for establishing the scaling laws. In more complex geometries a numerical approach may be more appropriate. However, even in such cases, the scaling laws derived using the momentum-integral analysis are still useful as they allow extrapolation of a single computation to a wide range of high Reynolds number flows.
A Numerical Study of Confined Turbulent Jets
Authors: Zhu J. , Shih T.-H.
Published: May 2008

A numerical investigation is reported of turbulent incompressible jets confined in two ducts, one cylindrical and the other conical with a 5 deg divergence. In each case, three Craya-Curtet numbers are considered which correspond, respectively, to flow situations with no, moderate, and strong recirculation. Turbulence closure is achieved by using the K-ε model and a recently proposed realizable Reynolds stress algebraic equation model. Calculations are carried out with a finite-volume procedure. Second-order accurate differencing schemes and sufficiently fine grids are used to ensure numerical accuracy. The calculated results are compared with experimental data. It is shown that the numerical methods presented are capable of capturing the essential flow features observed in the experiments and that the realizable Reynolds stress algebraic equation model performs better than the K-ε model for this class of flows.
Predictions of the Structure of Turbulent, Moderately Underexpanded Jets
Authors: Cumber P. S. , Fairweather M. , Falle S. A. E. G. , Giddings J. R.
Published: May 2008

A mathematical model capable of predicting the structure of turbulent, underexpanded jets is described. The model is based on solutions of the fluid flow equations obtained using a second-order accurate, finite-volume integration scheme coupled to an adaptive grid algorithm. Turbulence within these jets is modelled using a k-ε approach coupled to the compressible dissipation rate model of Sarkar et al. (1991a). Comparison of model predictions and experimental data, reported in the literature, on a number of moderately underexpanded jets demonstrate significant improvements over results derived using the standard k-ε approach, and the adequacy of the compressibility corrected turbulence model for predicting such jets.
On the Dynamics of Flow Structure Development in an Excited Plane Jet
Authors: Hsiao Fei-Bin , Huang Jiann-Min
Published: May 2008

Coherent structure dynamics in the developing region of a plane jet under acoustic excitation is studied experimentally by means of hot-wire measurements and smoke-wire flow visualization. The mean and fluctuation velocity properties, such as mean and fluctuation intensities in both streamwise and transverse directions, turbulent shear stress variations, and the turbulent energy production and energy convection phenomena, are carefully investigated. The results indicate that flow property behavior is closely related to vortex formation and the merging processes, which are the dominant mechanisms governing flow behavior in the developing region of a plane jet.
Surface Disturbance Evolution and the Splattering of Turbulent Liquid Jets
Authors: Bhunia Sourav K. , Lienhard John H.
Published: May 2008

In this study, we investigate the growth of surface disturbances on turbulent liquid jets in air and its relation to the amount of splattering when the jet strikes a target. A laser-based optical technique is used to measure the instantaneous local amplitude of fluctuations on jets produced by fully-turbulent tube nozzles. Measurements were made over the portion of the jet between 0.2 and 45 nozzle diameters downstream of the nozzle. Jets of water, an isopropanol-water mixture, and water with a surfactant were studied. The local rms amplitude of surface disturbances scales with the jet Weber number and the dimensionless distance from the target. The measurements show a nonexponential growth of the rms amplitude as the liquid moves downstream. Power spectra of the disturbance amplitudes show broadband turbulent disturbances to be dominant over any single wavenumber Rayleigh-type instability. The measured rms amplitude of roughness on the jet surface correlates well with the fraction of impinging liquid splattered, as hypothesized in previous models of splattering. A mathematical model of the free-surface/turbulence interaction is also given. The spectrum of surface disturbances is calculated based on the pressure spectrum of isotropic, homogeneous turbulence. Both the theoretical model and the experiments show that the high-wavenumber portion of the spectrum decays as k−19/3 owing to the damping effect of capillary forces on the turbulent pressure spectrum that drives surface roughening.
The Wave-Thinning and Breakup of Liquid Sheets
Authors: Spielbauer T. M. , Aidun C. K.
Published: May 2008

In this study, the radially thinning liquid sheets formed by a simple splash plate nozzle were investigated. These sheets, observed using high-speed video and 16 mm photography, were found to breakup by way of a localized rupture mechanism. The sheet is thinned by geometric distortion due to the radial expansion of the sheet, and by stretching caused by the presence of large amplitude sinuous waves. The downstream sheet thickness in the wave-thinned region was calculated from experimentally measured perforation growth rates. These thicknesses were found to be about 12 percent of the predicted value for an undisturbed sheet. Trends in the downstream position at which significant thinning due to sinuous waves is predicted to occur agree with trends in experimentally measured rates of hole formation.
Development of Prototype Pump Using a Vibrating Pipe With a Valve
Authors: Hashimoto Hiroyuki , Hiyama Hirokuni , Sato Rokuro
Published: May 2008

Tests were conducted on a prototype pump which has an extremely simple structure and excellent controllability. Its structural and hydrodynamic features are different from those of previous conventional reciprocating pumps. The pump structure constitutes a leak-proof short vibrating pipe equipped with a nonreturn valve on the edge of its outlet. The authors developed a prototype pump which uses a 25 mm diameter vibration pipe and an electromagnetic excitation device. The pump performance, intentionally changed by adjusting the coil voltage or the coil current frequency, featured a maximum pressure of approximately 1.0 bars, a maximum flow rate of approximately 40 liters per minute, and a maximum efficiency of approximately 30 percent. Results of both a one-year test run, using water as the working medium, and a three-month durability test, using concentrated nitric acid as the working fluid, assuming application as a chemical pump, indicated favorable results.
On the Compressible Flow Losses Through Abrupt Enlargements and Contractions
Authors: Marjanovic´ Predrag , Djordjevic´ Vladan
Published: May 2008

The well-known structure of incompressible flow through abrupt enlargements and contractions is applied to the subsonic compressible flow through the same area change. Using the basic system of equations for 1-D model of flow, both cases are solved for adiabatic and isothermal conditions. The changes for all flow parameters (M, v, p, p0, T, T0, s) are obtained analytically and shown graphically. The results are compared with the available experimental data.
Experimental Study of Interactions of Shock Wave With Free-Stream Turbulence
Authors: Honkan A. , Watkins C. B. , Andreopoulos J.
Published: May 2008

Phenomena related to turbulence interactions with shock waves have been studied in detail. The present investigation is focused on interactions of a normal shock wave with homogeneous/grid-generated turbulence. When a shock wave formed in a shock-tube is passed through a grid, the induced flow behind the shock has the features of a compressible flow with free-stream turbulence. The decaying turbulence is subjected to an interaction with the reflected shock traveling in the opposite direction. Data were sampled simultaneously from four channels of high frequency response pressure transducers and dual hot-wires probes. A cold-wire was used to provide instantaneous total temperature measurements while a single hot-wire provided instantaneous mass flux measurements. Amplification of velocity and temperature fluctuations and dissipative length scales has been found in all experiments. Velocity fluctuations of large eddies are amplified more than the fluctuations of small eddies. The dissipative length scale, however, of the large eddies is amplified less than the length scale of the small eddies.
Effect of Selective Modification of Turbulence on Two-Equation Models for Particle-Laden Turbulent Flows
Authors: Squires K. D. , Eaton J. K.
Published: May 2008

The effect of selective modification of turbulence by particles on K–ε models for dilute two-phase flows has been evaluated using data from direct numerical simulation of particle-laden isotropic turbulence. Simulation results were obtained using as many as 643 grid points and up to 106 particles. The ratio of the particle time constant to large-eddy turbulence time scale varied from 0.14 to 1.50 and particle mass loadings of 0.0, 0.1, 0.5, and 1.0 were used in the simulations. Simulation results demonstrate that the balance between enstrophy production by turbulent vortex stretching and viscous destruction is disrupted by momentum exchange with the particle cloud. Selective modification of the turbulence by lighter particles results in a significant attenuation of enstrophy production by turbulent vortex stretching. The decrease in enstrophy production causes the model constant Cε2 to increase for large mass loading. Heavy particles are found to act as a sink of enstrophy for all mass loadings used in the simulations. Preferential concentration of lighter particles by turbulence, however, can generate vorticity fluctuations, especially at higher mass loadings. For these cases conventional modeling of the destruction of dissipation by particles term in the ε equation requires that Cε3 be negative.
Detailed Measurements of Vertical Annular Two-Phase Flow—Part I: Drop Velocities and Sizes
Authors: Azzopardi B. J. , Teixeira J. C. F.
Published: May 2008

Phase anemometry and laser diffraction techniques have been employed to measure drop sizes in annular two-phase flow. The former technique also provides drop velocities. When converted to the same basis, the drop size distributions from the two techniques are in agreement. Drop velocities were 20 percent below the corresponding local velocities for the gas. Standard deviations of the drop velocities were 10 to 65 percent higher than those for the gas.
Detailed Measurements of Vertical Annular Two-Phase Flow—Part II: Gas Core Turbulence
Authors: Azzopardi B. J. , Teixeira J. C. F.
Published: May 2008

Laser anemometry techniques have been employed to measure gas velocities in annular two-phase flow. Mean velocities and Reynold’s stresses in the gas appear to be similar to those in corresponding rough pipes. However, turbulence intensities are higher than the values corresponding to flow over a wall with a roughness equivalent to the film interface.
Flow Patterns of Falling Liquid Film Observed Near an Obstacle
Authors: Katoh Kenji , Fujita Hideomi , Takaya Masato
Published: May 2008

The purpose of this study is to investigate the flow patterns of a falling liquid film as observed near an obstacle on a flow channel. In this report, a bend on the channel was chosen as the obstacle and the behavior of a ring-shaped swelling of the liquid film, which appears near the obstacle at a low flow rate, is considered. The Weber number at which the swelling shifts from upstream to downstream of the bend is theoretically estimated. The experimental results for the swelling behavior agree well with those of the above theory. Moreover, in order to investigate the characteristics of the stationary wave observed in the upstream side of the swelling, the wavelength and the damping rate of the amplitude were measured by the needle contact method and agree with theory.
Ventilated Flow Between Corotating Disks With Large Obstructions in a Fixed Cylindrical Enclosure (Data Bank Contribution)
Authors: Gor D. , Humphrey J. A. C. , Greif R.
Published: May 2008

Time-resolved laser-Doppler velocimeter measurements of the circumferential velocity component were obtained for the flow between the center pair of four disks of common radius R2 corotating at angular velocity Ω in a fixed, cylindrical enclosure. Mean and rms profiles of this velocity component were obtained for two disk rotation speeds (300 and 3600 rpm), two relatively thick tapered obstructions (long and short) placed radially inward midway between each pair of disks, and three ventilation conditions (unventilated, blowing, and sucking) resulting from an imposed inter-disk radial throughflow. The profiles were determined at four circumferential locations downstream of the respective obstructions; radially along the midplane, and axially at selected radial locations. The profiles for the unventilated flow case show that the circumferential component of motion signficantly accelerates near the hub, in the region between the tip of the obstruction and the rotating hub. Elsewhere, this component of motion is significantly decelerated. The presence of ventilation, whether directed radially outward or inward, significantly affects the flow field only in the region immediately around the hub, and far downstream of the obstruction where it increases both the mean and rms velocities. Analysis of the time records suggests that the observed increases in the rms values are due to the circumferentially periodic nature of the radial ventilation condition. These observations are, for the most part, independent of the disk speed of rotation and the length of the obstructions. A comparison of present unventilated flow results with the corresponding results of Usry et al. (1993), who used much thinner obstructions, reveals the extent to which increasing the obstruction blockage ratio induces larger levels of flow unsteadiness.
Measurements of the Turbulence Structure Downstream of a Tube Bundle at High Reynolds Numbers
Authors: Karnik U.
Published: May 2008

Mean and turbulent velocity profiles are measured in natural gas at a static pressure of approximately 5200 kPa and a Reynolds number of around 7 × 106. These measurements are obtained by conventional hot film anemometry suitably modified for use in a natural gas environment. Measurements are taken in a 102.26 mm nominal diameter pipe following a development length of around 76D. The mean velocity profile and the turbulence intensities at this location are typical for a fully developed pipe flow. Further, measurements downstream of a 19 tube bundle flow conditioner are also presented. The tube bundle is traversed downstream of a 90 degree, long radius (r = 1.5D) elbow and the measurements are taken at 19D downstream of the elbow exit. Measurements include those of the axial mean and turbulent velocities and the integral length scales. It is found that the decay of turbulence is slower and the magnitude of the length scale is smaller in comparison to measurements at lower Reynolds numbers. Orifice meter comparisons, performed in a 19D test section (meter run), confirm earlier findings that turbulence is one of the factors that affects orifice meter accuracy.
A New Correlation for Pressure Drop in Arrays of Rectangular Blocks in Air-Cooled Electronic Units
Authors: Molki M. , Faghri M. , Ozbay O.
Published: May 2008

An experimental work is carried out to investigate the pressure drop and to visualize the flow field in the entrance region of an array of rectangular modules attached to the lower wall of a duct. The modules are positioned in an in-line arrangement to simulate the geometry often encountered in the cooling passages of electronic units. The investigation is performed in both laminar and turbulent regions with Reynolds number ranging from 400 to 15,000. The geometric parameters range from H/L = 0.125 to 1.5 and S/L = 0.125 to 0.5, while the value of B/L is fixed to 0.5. The flow visualization revealed a highly separated region on the first module of the array. The pressure drops are correlated for the range of Re, H/L, and S/L employed in this work.