bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Analysis of Flow in a Lock-Up Clutch of an Automotive Torque Converter
Authors: Ejiri E.
Published: March 2009

The flow through a single-plate lock-up clutch in an automotive torque converter was numerically investigated using a CFD code. The flow was computed under various clearances between the lock-up piston and the cover-converter. The stable location of the lock-up piston was determined so that the thrust acting on the lock-up piston became zero. The computation results showed that the fluid flowed outwards in the narrow channel between the lock-up clutch facing and the cover-converter, whereas the flow reversed inwards near the lock-up piston surface upstream and downstream of the narrow channel in most cases. Along the outer surface of the axial flow channel near the exit, the flow also reversed far upstream of the channel. The computation results well predicted the stable position of the lock-up piston. It is noted that the sharp increase in clearance in the coupling range can be clearly captured with the method.
Multi-Dimensional CFD Analysis of Rotating Blades in a Lawnmower Deck
Authors: Hagen P. A. , Chon W. , Amano R. S.
Published: March 2009

Aerodynamic experimentation and investigation of rotating blades has pioneered the research necessary for innovative lawnmower design. In this study, Computational Fluid Dynamics (CFD) models are generated for single and triple-blade arrangements to analyze their flow patterns and behavior. For the 2-D CFD analysis, blade profiles at several arbitrary radial sections have been selected for flow computations around the blade model. Likewise, the 3-D CFD analysis effectively simulates the flow patterns inside the entire triple-blade mower deck, as well as in single-blade enclosures. The accuracy of the attained CFD solutions was determined through comparison with experimental data. The flow behaviors were observed using both Laser Doppler Velocimetry (LDV) and a high-speed video camera recording at 2000 frames per second. Strain gage and pressure transducer analysis also aided in the correlative effort. It has been observed that both the mower deck configuration and blade profile share equal significance in the resultant flow profiles.
Efficient Stator Design for Automotive Engine Cooling Fan Systems
Authors: Bakir F. , Moreau S.
Published: March 2009

This paper deals with stator efficiency improvement meant for automotive engine cooling fan systems. Four stators designed for a Valeo 380 mm rotor were manufactured and tested on a newly designed Valeo-Lemfi test rig. The following points are presented: • Overall performances of the 380 mm rotor. • Overall performances of the 380 mm rotor combined with a short chord stator. Inefficiency of such a design is shown: Slight deflection carried out by the stator is the cause of the slight gain of efficiency. • Overall performances of the 380 mm rotor combined to three long chord stators: this study confirms the gain of efficiency foreseen previously with the simplified radial equilibrium code VENTAX. • Steady velocities measured 33 mm downstream the various stage configurations: These measurements obtained by using a 5-holes probe show high deflection carried out by the long chord stators.
Science in Design of Fluid Machines
Authors: Yedidiah S.
Published: March 2009

This paper discusses a number of auxiliary approaches to which an engineer may need to resort, in order to arrive at a successful design. This paper deals, specifically, only with the design of fluid machines. Its implications, however, extend to all areas of engineering design.
Experimental and Numerical Investigation of the Performance Instability in a Mixed-Flow Impeller
Authors: Friedrichs Jens , Kosyna Gu¨nter
Published: March 2009

This paper describes the recent investigations and results on performance and NPSH-instabilities of a mixed-flow impeller. Several techniques (experimental and numerical) were combined to explain the specific phenomena in the behaviour of impeller momentum-transfer (energy-transfer) under non-cavitating and cavitating conditions. Subject of the investigations was a shrouded impeller of medium specific speed (nq = 61) which shows a performance instability (hysteresis) described by two stable operating points and a NPSH3%-peak at the same particular flow rate. Blade pressure measurements in the rotating system as well as velocity measurements downstream of the impeller in combination with CFD-results were used to explain the differences in momentum-transfer leading to the unstable performance behaviour.
FlowLab: Computational Fluid Dynamics (CFD) Framework for Undergraduate Education
Authors: LaRoche Richard D. , Hutchings Barbara J.
Published: March 2009

Today, the use of Computational Fluid Dynamics (CFD) software in academia occurs primarily in the context of student projects or research. The potential of CFD as a tool to enhance teaching is largely untapped, despite growing interest in computer tools to assist learning. FlowLab (http://flowlab.fluent.com) is a CFD-based educational software package that will allow students to solve fluid dynamics problems without the long learning curve required by today’s commercial CFD packages. We will provide an update of the FlowLab beta-testing program with over 30 universities worldwide. We outline a process for university collaboration and peer-review procedures in the development of FlowLab exercises for engineering classes in fluid dynamics and heat transfer.
The Effect of the Source Location on the Acoustic Excitation on Laminar-Turbulent Transition on Swept Wing Boundary Layer
Authors: Gu¨lac¸ti B. , Aubrun S. , Seraudie A. , Arnal D.
Published: March 2009

The effect of the source location and the direction of the propagation on the laminar-turbulent transition on swept-wing three-dimensional boundary layers are investigated experimentally. Also the crossflow case is handled in detail. The source for the acoustic excitation is placed in four different locations: in front of the wing, on top of the test section, behind the wing and in front of the wind tunnel. Three different experimental cases (streamwise, crossflow and mixed cases) are examined for each location with two different excitation bands. For the most efficient frequency ranges and the highest sound pressure levels an upstream shift of transition motion between 20%–35% of chord length for streamwise case and between 5%–10% for the crossflow case are observed. While in front of the wing and behind the wing are the most efficient loudspeaker positions in the streamwise case, in the crossflow case the most efficient locations are observed to be in front of the wing and on top of the test section. It is concluded that acoustic sound level plays a more important role in the upstream shift of the transition than the source location and placing the loudspeaker in front of the wind tunnel is not an efficient position. For the crossflow instabilities dominated transition the stationary vortices are clearly seen from the velocity contours obtained by the hot-wire. Secondary instabilities couldn’t be observed in the hot-wire spectra. The surface roughness of the wing that is reduced to 0.25µm does not change the transition location in the crossflow case.
Coherent Structures in Turbulent Shear Flows: The Confluence of Experimental and Numerical Approaches (Keynote Paper)
Authors: Bonnet Jean-Paul , Delville Joel , Glauser M. N.
Published: March 2009

Physics based low dimensional approaches are playing an increasingly important role in our understanding of turbulent flows. They provide an avenue for us to understand the connection between coherent structures and the overall dynamics of the flow field. As such these approaches are fundamental to the implementation of physics based active control methodologies. In this paper we review applications of various low dimensional approaches (including Proper Orthogonal Decomposition (POD), Linear Stochastic Estimation (LSE), Conditional Averages and Wavelets) to turbulent shear layers and connect the results to simulation tools. The applications of all these methods to the 2D shear layer suggest a kind of universal behavior of both the large scale structure extracted and the background turbulence, irrespective of the technique (filtering method) used. A review of the application of POD and LSE to the axisymmetric jet at Reynolds numbers between 100,000 and 800,000 and Mach numbers ranging from very low to 0.6 suggest a universal behavior where the dynamics can be described with relatively low dimensional information (1 POD mode and 5 or 6 Fourier azimuthal modes) over the Reynolds/Mach number range studied. These results provide physical justification for simulation tools such as VLES, LES and SDM since such computational methods involve different levels of low-dimensional modeling.
Flow in the Wake of Surface-Mounted Pyramids
Authors: AbuOmar Mazen , Martinuzzi Robert J.
Published: March 2009

The flow around square-based, wall-mounted pyramids in thin and thick boundary layer was experimentally investigated as a function of the pyramid apex angle, ζ, and, angle of attack, α, based on mean flow surface patterns, pressure and velocity measurements. For thin boundary layers, wake periodicity is observed. For slender pyramids (20° < ζ < 75°), periodic shedding of vortices is observed. The shedding frequency scales with the frontal (projected) width. For broad pyramids, wake periodicity cannot be related to vortex shedding. Vortex shedding appears related to the existence of a double vortex-structure along the side faces of the slender pyramids. For thick boundary layers, no wake periodicity is observed and the mean flow structure resembles that of broad pyramids. In all cases, the separation and reattachment point upstream and downstream of the obstacle scales with the projected frontal area.
LES Simulation of Aerodynamic Drag for Heavy Duty Trailer Trucks
Authors: Nakamura S. , Hively E. M. , Conlisk A. T.
Published: March 2009

This report starts with an overview of aerodynamic drag losses of heavy duty trucks. Preliminary computational results of the large eddy simulation (LES) on the drag coefficients versus the gap widths and that on various yaw angles are presented. Flows around the semi-truck models are visualized.
Computational Hydrodynamics for an Autonomous Underwater Vehicle With Moving Control Surfaces
Authors: Arabshahi Abdollah , Gibeling Howard J.
Published: March 2009

The present study was undertaken to provide information for both design improvement and control system development during various stages of an autonomous underwater vehicle (AUV) development project. The need to establish a predictive capability for the hydrodynamic (control) coefficients for an AUV presented an opportunity to apply a multiblock incompressible Navier-Stokes flow solver which has evolved over many years. The solver utilizes a state-of-the-art implicit, upwind numerical scheme to solve the time-dependent Navier-Stokes equations in a generalized time-dependent curvilinear coordinate system. Domain decomposition is accomplished via a general unstructured multiblock approach. In addition, an efficient grid movement capability is incorporated in the code that will handle the relative motion of a multi-component configuration (e.g. oscillating control surfaces). Numerous simulations were conducted during the course of this work. The computations for vehicle and propulsor design consisted mainly of steady state axisymmetric computations, while for control system development both steady and unsteady (prescribed motion) simulations were conducted. The latter cases focus on the forces and moments on the vehicle that are needed for extraction of control information. A brief overview will be presented on the flow solver. This will be followed by a presentation of the numerical results.
Coupled Thermal and Mechanical Effects in Injected Polymer Pieces: Comparison Between Experimental Measurements and Numerical Prediction of Both Pressure and Thermal Contact Resistances Time Evolution
Authors: Masse´ Hugues , Arquis E´ric , Delaunay Didier
Published: March 2009

During the cooling of an injected polymer, a very strong coupling exists between the thermal and mechanical phenomena. In order to fill in the mold and to compensate the effects of reduction of the thermal shrinkage, the pressure in the molding cavity is maintained to a very high level. The purpose of this paper is to show the essential importance of thermomechanical coupling present during the cooling of a hot polymer in a cold mold via a comparison between experiments and numerical simulation. Experiments show that the polymer-mold contact evolves under the effect of the internal contraction caused by the lowering of temperature. On the one hand, the air gap width, depending on the shape of the part, modifies the Thermal Contact Resistance (TCR) between the mold and the part. On the other hand, shrinkage is divided between the two sides of the part depending on the internal stresses, so that the geometry of the part is also linked to changing temperature. The resolution of the coupled thermal and mechanical problems allows us to predict the shape of the part, the stresses, the temperature field in the polymer and in the surrounding mold, and the air gap widths on both sides.
Assessment of Heat Transfer and Airflow Characteristics in Air-Conditioned Room: 3D Time-Dependent Model
Authors: Kameel Ramiz , Khalil Essam E.
Published: March 2009

Airflow characteristics in ventilated and air-conditioned spaces play an important role to attain comfort and hygiene conditions. This paper utilizes a 3D time-dependent Computational Fluid Dynamics (CFD) model to assess the airflow characteristics in different air-conditioned spaces. It is found that the optimum airside design system can be attained, if the airflow is directed to pass all the enclosure areas before the extraction. Still most of these factors and evaluation indices have the shortage of adequately describe the influence of the recirculation zones on the occupancy zone and also on the fresh supplied air. The model of evaluation should assess the airflow characteristics in any enclosure according to its position in the enclosure and the expected target of it along its pass to the extraction.
Fluid Flow and Heat Transfer Around Two Circular Cylinders in Side-by-Side Arrangement
Authors: Cheng Minter
Published: March 2009

Incompressible flows passing through two circular cylinders in side-by-side arrangement are investigated numerically. The calculations are carried out with pitch ratios from 1.1 to 2.0 at Reynolds number of 1000. The flow and temperature fields, flow interference, and the local and the mean Nusselt numbers are studied in this research. It is observed that for the pitch ratios in the range of 2.0 and 1.5, the emerging jet between cylinders deflects and one wide and one narrow wakes behind the cylinders are formed. The gap flow velocity increases as the pitch ratio decreases and consequently increases the mean Nusselt number of the cylinders. As the pitch ratio decreases and is less than 1.5, the jet deflection is more severe and the gap flow velocity starts to decrease slowly, which results in reducing the mean Nusselt number of the cylinders. Due to the rapid reduction of the narrow wake size, the mean Nusselt number of the cylinder with narrow wake shows an uprising tendency for the decreasing pitch ratio less than 1.2.
Numerical Computation of Reacting Flow in Porous Burners With an Extended CH4-Air Reaction Mechanism
Authors: Tong Timothy , Abou-Ellail Mohsen , Li Yuan , Beshay Karam R.
Published: March 2009

The present paper presents, numerical computations for flow, heat transfer and chemical reactions in an axisymmetric inert porous burner. The porous media re-radiate the heat absorbed from the gaseous combustion products by convection and conduction. In the present work, the porous burner species mass fraction source terms are computed from an ‘extended’ reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e. the porosity is not uniform over the entire domain. Therefore, it has to be included inside the partial derivatives of the transport governing equations. Finite-difference equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to insure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved, iteratively, for U, V, p’ (pressure correction), enthalpy and species mass fractions, utilizing a grid of (60×40) nodes. The sixty grid nodes in the axial direction are needed to resolve the detailed structure of the thin reaction zone inside the porous media. The porous burner uses a premixed CH4-air mixture, while its radiating characteristics are computed numerically, using a four-flux radiation model. Sixteen species are included, namely CH4, CH3, CH2, CH, CH2O, CHO, CO, CO2, O2, O, OH, H2, H, H2O, HO2, H2O2, involving 49 chemical reaction equations. It was found that 900 iterations are sufficient for complete conversion of the computed results with errors less than 0.1%. The computed temperature profiles of the gas and the solid show that, heat is conducted from downstream to the upstream of the reaction zone. Most stable species, such as H2O, CO2, H2, keep increasing inside the reaction zone staying appreciable in the combustion products. However, unstable products, such as HO2, H2O2 and CH3, first increase in the preheating region of the reaction zone, they are then consumed fast in the post-reaction zone of the porous burner. Therefore, it appears that their important function is only to help the chemical reactions continue to their inevitable completion of the more stable combustion products.
Thermal Transport Phenomena Over Slot-Perforated Flat Fins With Heat Sink in Forced Convection Environment
Authors: Torii Shuichi , Yang Wen-Jei
Published: March 2009

A theoretical study is performed to investigate unsteady thermal and fluid flow transport phenomena over flat fins with heat sink, which are placed in a forced convection environment. Emphasis is placed on the effects of Reynolds number and fin pitch on heat transfer performance and velocity and thermal fields. It is found from the study that (i) in the high Reynolds number region, the alternating changes in the fluid flow take place for larger fin pitch, (ii) the alternating flow in the space area between two fins is mutually interacted by the corresponding one from the adjacent in-line plate fines, resulting in an amplification of heat transfer performance, and (iii) heat-transfer performance is intensified with a decrease in the fin pitch, whose trend becomes larger in the higher Reynolds number region considered here.
A Review of the History of the NACA Centrifugal Compressor Program and Arrival at Current Computational Design Procedures
Authors: Hamrick Joseph T.
Published: March 2009

Before and during World War Il the design and development of single stage, high pressure ratio centrifugal compressors was essentially a cut and try exercise. To reach a high pressure without substantial experimentation required multiple stages of impellers and diffusers with pressure ratios in the two to one range. While such arrangements were satisfactory for commercial use where weight was not a major consideration, they were not suitable for jet engines. The centrifugal compressor for the Whittle engine, the first British jet engine, was developed by trial and error with numerous modifications of the hub-shroud profile. The centrifugal compressor section of the National Advisory Committee for Aeronautics (NACA) designed, built and tested three compressor impellers during and after World War Il. They were part of a program designed to evaluate various blade shapes, but encountered such instabilities at the design pressure ratios that the experimental results led to no definitive conclusions. In 1948 the Centrifugal Compressor Section was given the assignment to further investigate the three impellers. The investigation led to the development of a quasi three dimensional design procedure that eliminated the guesswork from the basic design of a centrifugal impeller. Since the 1948 to 1955 time period over which the procedure was developed, the advances in computers have allowed refinements in the original computational methods. It is the objective of this presentation to review the history of the NACA centrifugal compressor program and efforts that have led to the latest developments in computational design procedures.
Limitations of Richardson Extrapolation and Possible Remedies for Estimation of Discretization Error
Authors: Celik I. B. , Li J. , Hu G. , Shaffer C.
Published: March 2009

The origin of oscillatory convergence in finite difference methods is investigated. Fairly simple implicit schemes are used to solve the steady convection diffusion equation with variable coefficients and possible scenarios are shown that exhibit the oscillatory convergence. Also a manufactured solution to difference equations is formulated that exhibit desired oscillatory behavior in gird convergence with varying formal order of accuracy. This model-error equation is used to assess statistically the performance of several methods of extrapolation. Alternative extrapolation schemes such as the deferred extrapolation to limit technique to calculate the coefficients in the Taylor series expansion of the error function are also considered. A new method is proposed that is based on the extrapolation of approximate error and shown to be a viable alternative to the other methods. This paper elucidates the problem of oscillatory convergence and brings a new look into the problem of estimating discretization error by optimizing the information from a minimum number of calculations.
Experimental Study of Stratification in a Liquid Storage Tank With a Porous Manifold
Authors: Brown N. M. , Lai F. C.
Published: March 2009

Experiments were conducted to investigate the effectiveness of a porous manifold in the formation and maintenance of thermal stratification in a liquid storage tank. A thermal storage tank with a capacity of 315 liters and a diameter-to-height ratio of 2 was used for the experiment. The porous manifold used was made from rolling up a nylon screen into the shape of a tube. Stratification was observed at a Richardson number as low as Ri = 0.615. Flow visualization was also performed to confirm the effectiveness of the porous manifold in the promotion and maintenance of stable thermal stratification. From the results of flow visualization, one can conclude that a porous manifold is able to reduce the shear-induced mixing between fluids of different temperature, and thus is able to promote and maintain a stable stratification.
A Numerical Model of Solidification of a Massive Casting From Malleable Cast-Iron
Authors: Kavicka Frantisek , Stetina Josef , Stransky Karel , Dobrovska Jana , Sekanina Bohumil , Heger Jaromir
Published: March 2009

A numerical model of the temperature field associated with solidifying castings aims to achieve two general goals: directed solidification (as the primary condition for a healthy casting), and optimisation of the technology of casting together with the preservation of optimum utility properties of the product. A specific goal of this model is the selection and optimisation of the method of cooling to shorten the solidification time to obtain a spherical graphite structure with good nodular properties, and with a sufficient density of graphite spheres (cells). The speed of cooling during solidification and cooling in the mould is therefore a significant quantity influencing the formation of the structure. The achievement of these goals depends on the ability to analyse and, successively, to control the effect of the main factors which characterise the solidification process or accompany it. The analysis of the quantities is focused on determining the causes of the formation of the heterogenic temperature field during casting, considering the phase and structural changes. It is also focused on the thermokinetics of the formation of shrinkage porosities and cavities and on the prediction of their formation. This leads to the optimisation of the shape and sizes of the risers, the method of insulation, the treatment of the level. The model is applicable to various shapes of castings. The software is capable of analysing the temperature field of the actual casting, as well as the temperature field of the mould and cores, including the dependence of their material. It is also capable of considering non-linearity, i.e. the dependence of the thermophysical properties—namely the material of the casting and mould on the temperature, as well as the dependence of the heat transfer coefficients on the surface and interface temperature. The model is also equipped with an original network generator (pre-processing) as well as graphical output (post-processing).
Phase Change in a Three-Dimensional Rectangular Cavity Under Electromagnetically Simulated Low Gravity: Side Wall Heating
Authors: Veilleux Douglas L. , Gonc¸alves Eduardo , Faghri Mohammad , Asako Yutaka , Charmchi M.
Published: March 2009

In this paper, a low gravity environment has been simulated numerically, via an electromagnetic field, by studying the transport phenomena associated with the melting of an electrically conducting phase change material (gallium) inside a three-dimensional enclosure. Both transverse electric and magnetic fields are used to generate a Lorentz force, which is used to counteract the effects of gravity and thus simulate low gravity. The problem is formulated as one-domain by employing an enthalpy-based transformation of the energy equation. The governing equations are then discretized using a control-volume-based finite difference scheme. The results show that the application of an electromagnetic filed can be used to simulate key melting characteristics found for actual low gravity. However, the resulting three-dimensional flow field in the melted region differs from actual low gravity. The application of an electromagnetic field creates a flow phenomenon not found in actual low gravity or previously seen in two-dimensional problems. While these distortions do exist when an electromagnetic field is applied, their intensity is significantly lower than the distortions are found when only a magnetic filed is applied. Since these distortions are of much lower intensity, it can be said that low gravity can be simulated better by an electromagnetic field.
Spectral-Directional Emittance of CuO at High Temperatures
Authors: Jones Peter D. , Teodorescu George , Overfelt Ruel A.
Published: March 2009

Spectral-directional emittance measurements for cupric oxide (CuO) are presented. The data cover polar angles of zero to 84° from the surface normal, wavelengths between 1.5 and 8 μm, and temperatures between 400 and 700°C. The data were generated using a radiometric, direct emission measurement method. The oxide was grown on a very clear, smooth, and mirror-like copper surface, heated in air at 700°C until emission measurements became constant (270 hours). X-ray diffraction and EDS analyses were performed to characterize the spatial and molecular composition of the copper oxide layer. It is generally found that CuO emittance decreases with increasing polar angle, increases with increasing wavelength, and increases with increasing temperature. Spectral-directional emittance values calculated from the Fresnel equations show good agreement with the measurements up to polar angles of 72°.
Direct Assessment of the Accuracy of Stereo PIV in Turbulent Channel Flow
Authors: Christensen K. T. , Wu Y.
Published: March 2009

Stereo particle-image velocimetry (PIV) has become a widely-used method for studying complex flows because it allows one to acquire instantaneous, three-component velocity data on a planar domain with high spatial resolution. However, the accuracy of such measurements must be carefully evaluated before stereo PIV data can be faithfully used in the development of sophisticated turbulence models, assessment of appropriate computational boundary conditions, and in the validation of advanced computations. To this end, the accuracy of stereo PIV is assessed directly in an actual turbulent environment: two-dimensional turbulent channel flow. This flow is a challenging test of stereo PIV because the turbulent velocity fluctuations are quite small compared to the mean (typically less than ten percent of the mean velocity) and strong velocity gradients exist in the near-wall region. Measurements are made in the streamwise–wall-normal plane along the channel’s spanwise centerline using both stereoscopic and conventional 2-D PIV. A large ensemble of statistically independent velocity realizations are acquired with each method at a friction Reynolds number Reτ = u*h/ν = 934. Single-point statistics are computed from the experimental data and compared to statistics determined from a direct numerical simulation (DNS) of turbulent channel flow at a nearly-identical friction Reynolds number of 940 [5]. Excellent agreement is found in the outer region of the flow (y/h > 0.15, where h is the half-height of the channel). For y/h < 0.15, both the conventional and stereo PIV results differ from the DNS data. These differences are most-likely a manifestation of errors associated with strong velocity gradients and intense turbulent events present in this region of the flow.
Critical Heat Flux for Downward Facing Boiling on a Coated Hemispherical Surface
Authors: Yang J. , Dizon M. B. , Cheung F. B. , Rempe J. L. , Suh K. Y. , Kim S. B.
Published: March 2009

An experimental study was performed to investigate the effect of surface coating on the critical heat flux for downward facing boiling on the outer surface of a hemispherical vessel. Steady-state boiling experiments were conducted in the SBLB (subscale boundary layer boiling) facility using test vessels with metallic microporous coatings to obtain the local boiling curves and the local CHF limits. Similar heat transfer performance was observed for microporous aluminum and microporous copper coatings. When compared to the corresponding data without coatings, the boiling curves for the coated vessels were found to shift upward and to the right. This meant that the CHF limit and minimum film boiling temperatures were located at higher wall superheats. In particular, the microporous coatings were found to enhance the local CHF values appreciably at all angular locations explored in the experiments. Results of the present study showed that the microporous aluminum coating was very durable. Even after many cycles of steady state boiling, the vessel coating remained rather intact, with no apparent changes in color or structure. Although similar heat transfer performance was observed for microporous copper coatings, the latter were found to be much less durable and tended to degrade after several cycles of boiling.
Turbulent Natural Convection Heat Transfer in a Square Enclosure: Turbulence Model Comparisons
Authors: James D. L. , Webb S. W.
Published: March 2009

A CFD study using four engineering turbulence models has been performed to evaluate the convective heat transfer predictions for an air-filled square enclosure with a Rayleigh number of 1.58 × 109. The four turbulence models considered are, as implemented by FLUENT: a k-ε model, an RNG k-ε model, a “realizable” k-ε model, and a k-ω model. All of the turbulence models investigated employ the Boussinesq approximation to evaluate the unknown turbulent viscosity and use the ideal gas equation of state to evaluate the density in the body force term. The performance of all models was similar. Predicted Nusselt number distributions on the hot and cold walls were overpredicted when compared to experimental data. The predicted average Nusselt numbers were nearly 20% higher on the active walls, and 10% higher on the passive, horizontal walls.
Comparison Between Theoretical Values and Simulation Results of Transport Coefficients for the Dissipative Particle Dynamics Method
Authors: Satoh Akira
Published: March 2009

In the present study, we have derived an expression for transport coefficients such as viscosity, from the equation of motion of dissipative particles. In the concrete, we have shown the Fokker-Planck equation in phase space, and macroscopic conservation equations such as the equation of continuity and the equation of momentum conservation. The basic equations of the single-particle and pair distribution functions have been derived using the Fokker-Planck equation. The solutions of these distribution functions have approximately been solved by the perturbation method under the assumption of molecular chaos. The expression of the viscosity due to dissipative forces has been obtained using the approximate solutions of the distribution functions. Also, we have conducted non-equilibrium dynamics simulations to investigate the influence of the parameters, which have appeared in defining the equation of motion in the dissipative particle dynamics method.
Experimental Investigation of Flow Induced Rotor Oscillations in a Centrifugal Sewage Water Pump
Authors: Benra Friedrich-Karl , Dohmen Hans Josef , Schneider Oliver
Published: March 2009

Most of the pumps for sewage transport have a special impeller geometry in order to avoid operational disturbances by clogging. The almost exclusively used single stage machines particularly are equipped with single-blade impellers. With this impeller geometry a strongly uneven pressure field along the perimeter of the pump casing can be expected. The resulting periodically unsteady flow forces affect the impeller and produce strong radial deflections of the pump shaft. In this contribution the experimental investigation of the dynamic behavior of the pump rotor as a consequence of the transient hydrodynamic forces is described. To verify the calculated rotor oscillations measurements were performed at several rotating speeds and at different volume flow rates. The pump which before has been investigated numerical was equipped with several sensors. The deflections of the pump rotor were measured with two proximity sensors. The measurement of the vibration accelerations at the pump casing showed the effects of the transient hydrodynamic stimulation forces. Measurements of the static pressure in the casing allowed a correlation between the rotor oscillations and the pressure fluctuations produced by the single-blade impeller.
Concepts of Spatially Varied Flow
Authors: Graber S. David
Published: March 2009

By presenting relationships for spatially varied flow with combined increasing and decreasing discharge, and relating those relationships to prior work, a clearer exposition of spatially varied flow concepts is provided. The applicability and usefulness of energy (First Law of Thermodynamics) and momentum (Newton’s Second Law) principles to spatially varied flows are addressed. The history of the debate initiated in the nineteenth century over the role of the First Law of Thermodynamics in general and hydrodynamics specifically is discussed, related to current issues regarding spatially varied flows, and hopefully resolved in the context of such flows and more generally.
Difference Formulation and Slope Invariance in Spatially Varied Free Surface Flow
Authors: Graber S. David
Published: March 2009

The relationship for combined increasing and decreasing free surface flow is presented in difference form, and then particularized to spatially increasing and spatially decreasing flows. For spatially decreasing flow, a theoretical difference is noted between the writer’s version and that in a widely-used reference source. By investigating an application for uniformly decreasing, full-pipe flow, it is shown by physical reasoning and comparison of an available analytical solution to the alternate versions of the difference relationship that the writer’s difference relationship is the correct one. Free surface spatially varied flow relationships are specialized to pressure conduits to demonstrate conditions of slope invariance, which has important and somewhat counter-intuitive ramifications for certain practical applications encountered in environmental engineering.
A Rational Explanation of Cavitation Bubbles and Its Application to Two Industrial Applications
Authors: Beck Earl J.
Published: March 2009

Cavitation (steam) bubbles have been the scourge of industrial pumps and turbines, causing both loss of efficiency and destructive erosion. While some aspects of the formation, growth and collapse have been understood for some time, the underlying theory has not been forthcoming until now. In particular, a common observation is not known to have been explained. When a small steam bubble releases from a heated surface at the bottom of a container, it grows as it rises into cooler water. Persons questioned on why this growth occurs over the past 30 years consistently have invoked Boyles’s law which, of course, pertains to a permanent gas. This paper answers the growth question and further describes how, with sufficient nuclei a low-density foam of steam bubbles, frequently referred to as “volume boiling” may be formed and usefully used in two industrial applications. It is believed that many useful applications may result from the understanding revealed here.
Cooling Enhancement of a Heat Source Through Flow Pulsation and Porous Block
Authors: Huang Po-Chuan , Nian Shy-Her , Yang Chao-Fu
Published: March 2009

A numerical study of forced pulsating flow in a parallel-plate channel with a porous-block-attached strip heat source at the bottom wall is presented. Comprehensive time-dependent flow and temperature data are calculated and averaged over a pulsation cycle in a periodic steady state. The results show that the cycle-space averaged Nusselt number for pulsating flow is higher than that for steady flow. The heat transfer enhancement factor increases with particle diameter, and pulsation frequent, but decreases with pulsation amplitude. The method combining flow pulsation with particle-porous heat sink can be considered as an augment heat transfer tool for cooling high-speed electronic devices.
Frost Temperature Relations for Defrosting Sensing System
Authors: Iragorry J. , Tao Y.-X.
Published: March 2009

To develop a better defrosting control system on finned evaporators of a refrigeration system, a study is conducted to better quantify the frosting and defrosting processes by using an infrared thermometer to determine the frost surface temperature. For gradual frost deposition a slow variation in surface emissivity is expected, while defrosting process is characterized by a sudden change of this property. As an indicator for the defrosting initiation control mechanism, the times at which the IR signals stabilize at different conditions (Reynolds number, cold surface temperature and ambient temperature) are reported along with the terminal mass concentration of a defined frosting process. On the other hand, the abrupt variation of surface emissivity indicates the termination of the defrosting process. Removable fins are used to measure frost weight, and a video microscope is used to determine the frost thickness. Defrosting initiation time and durations marked by melting temperature are reported as a function of initial mass concentration and defrosting base surface temperature. The presented results could be used to design a better defrosting control system with better accuracy and energy saving features.
Turbulence at Free Surface in Hydraulic Jumps
Authors: Mouaze D. , Murzyn F. , Chaplin J. R.
Published: March 2009

In the context of recent work by Brocchini & Peregrine [1,2], this paper aims to document free surface profiles, and turbulence length scales in hydraulic jumps with Froude numbers between 1.98 and 4.82. Although information on bubble size, frequency and velocities in hydraulic jumps is available in the literature, there is not much data on the features of the free surface, or on mixing layer thickness. In the present case, measurements at the free surface have been realized with two miniature resistive wire gauges each comprising two parallel 50 micron diameter wires with a separation of 1mm. These instruments were calibrated dynamically over a range of frequencies up to 20 Hz. Furthermore optical probes were used to measure properties of the air phase within the jump, including void fractions (up to 98%). The present results extend the range of Froude numbers for which two-phase measurements in hydraulic jumps are available, and, in most respects, confirm earlier results obtained with different experimental techniques. Length scales at the free surface are deduced from cross-correlation analysis of wire gauge measurements, and are compared with similar data obtained from images of the surface.
Steady State Modeling of a Rotating Heat Pipe With a Composite Wick Structure
Authors: Jankowski T. A. , Waynert J. A. , Prenger F. C. , Razani A.
Published: March 2009

A steady state heat pipe model, capable of calculating temperature and pressure distributions in the working fluid of a rotating heat pipe, is described here. The model can predict the performance of rotating heat pipes with a round cross-section, containing an annular gap composite wick structure. In addition to straight heat pipes, with a longitudinal axis that may or may not coincide with the axis of rotation, the model also allows for simulation of bent heat pipes. Using this model, results are generated for a bent heat pipe proposed for use in cooling rotating machinery. For the bent heat pipe, the condenser and adiabatic sections coincide with the axis of rotation, while the evaporator consists of an off-axis eccentrically rotating component, and a bend that allows for portion of the evaporator to be nearly perpendicular to the axis of rotation. The presence of the composite wick allows for heat pipe operation in both the rotating and stationary operating modes. Model results for the stationary operating mode compare favorably to the steady state heat pipe analysis code HTPIPE [1]. These comparisons for the stationary operating limit are significant, since HTPIPE has been benchmarked against experimental heat pipe data for nearly 30 years. As the rotational speed is increased, the rotation induced forces are used to drive the liquid flow to the evaporator. At high rotation rates, the liquid recedes from the wick, and forms a thin layer against the inside wall of the heat pipe. The results show that when a stable liquid layer is formed against the wall of the pipe, the shear stress opposes the rotation induced forces acting on the liquid, and limits the magnitude of the pressure and temperature rises in the working fluid (from the values predicted using a hydrostatic approximation).
Hydraulic Simulation of a Ramjet Combustion Chamber: An Optical Method for Local Equivalence Ratios Measurements and Comparison With a CFD Code
Authors: Heid G.
Published: March 2009

Hydraulic simulation is a very useful tool to predict global performances of ramjet combustion chambers and optical techniques can give a lot of information concerning the structure of the flow. Until recently, in the special case of combustion chambers, an optical method, called “global colorimetry” was used in order to obtain a global repartition of equivalence ratios and to predict combustion efficiency. Even if this method is powerful and predictive, it only gives global results. On the other hand, CFD codes give local results on a mesh and need local measurements to be validated. That is the reason why a new optical method, called “local colorimetry” has been developed with a two lateral inlet ramjet research chamber specifically designed for being installed on a hydraulic test rig in order to obtain an experimental database to validate numerical codes. Visualizations are shown and the optical method using laser tomography, LIF and digital acquisition and processing is explained. Experimental procedures are detailed and results concerning local air and fuel flow rates, and local equivalence ratios or mixture fuel/air ratios are analyzed. In addition, an ONERA CFED code is presented and the calculated results are compared with experimental data. Then, recent developments and prospects of the method are presented.
Natural Convection in Rectangular Cavities at Various Aspect Ratios and Inclinations With Isothermal Sidewalls and Constant Flux Heat Source on the Bottom Wall
Authors: Sharif M. A. R. , Mohammed Taqui-ur-Rahman
Published: March 2009

Natural convection in rectangular cavities is studied numerically using a finite volume based computational procedure. In many applications, especially for cooling of electronic components, a natural convection configuration is encountered where the heat source is at the bottom surface and the sidewalls are at colder temperature while the top wall can be considered adiabatic. The present study is based on such a configuration where a constant flux heat source is symmetrically embedded at the bottom wall. The non-heated parts of the bottom wall are considered adiabatic. The study includes computations for cavities at various aspect ratios, ranging from 0.5 to 2, and inclination angles of the cavity from 0° to 30°. The Grashof number is varied from 103 to 106. The effects of aspect ratio and inclination angles on the heat transfer process are analysed. Results are presented in the form of streamline and isotherm plots as well as the variation of the average Nusselt number and maximum temperature at the heat source surface under different conditions.
Effect of Substrate Temperature on Splashing of Molten Metal Droplets
Authors: Dhiman Rajeev , Chandra Sanjeev
Published: March 2009

The effect of substrate temperature on the splashing of molten metal droplets was studied experimentally. Uniform-size molten tin droplets (550 μm diameter) were produced using a drop-on-demand generator. To achieve high impact velocities substrates were mounted on the rim of a rotating flywheel and heated using cartridge heaters to vary substrate temperature. Droplets hitting a smooth cold substrate splashed extensively producing many small satellite droplets and leaving on the surface a small, irregular splat with many fingers projecting from its periphery. Droplets hitting a hot substrate did not splash but spread out to form a smooth disc. A new splashing criterion was developed to calculate the substrate temperature at which this transition occurred. It assumes splashing to occur when the solid layer produced as a result of droplet solidification grows equal to the splat thickness and obstructs the spreading liquid.