bluuebunny/crossref_metadata_2025 huggingface dataset:
Shape Optimization for Cryogenic Cavitating Flows Past an Isolated Hydrofoil
Authors: De Giorgi Maria Grazia , Congedo Pietro Marco , Rodio Maria Giovanna , Ficarella Antonio
Published: July 2009

The aim of this paper is shape optimization of a cryogenic flow past an isolated hydrofoil in order to reduce the cavitation. The numerical simulation of cavitating flows has been performed by way of the commercially available code Fluent (release 6.3), implementing a cavitation model by using external routines. The model is based on a simplified Rayleigh-Plesset equation, and takes into account both nucleation and thermal effects. This study has been divided in two parts. Firstly the cavitation model has been validated by comparison with experimental data, in particular, water cavitation on a NACA0015 airfoil and hydrogen cavitating flow over an external profile. Secondly, Fluent has been coupled with a multi-objective genetic algorithm (MOGA). Genetic algorithms have proved their interest with respect to gradient-based methods because of their high flexibility, and also because of their ability to find global optima of multi-modal problem. The representation of the design space has been previously investigated through a Design of Experiment (DOE) procedure. A shape optimization of an hydrofoil has been computed in order to minimize the vapor volume in different operating conditions.
Experimental and Numerical Simulation on the Starting Period of a Centrifugal Pump
Authors: Chen Gang , Shao Jie , Wu Yulin , Cao Guangjun
Published: July 2009

The centrifugal pump is applied widely in many fields with significant advance made in the understanding of the key technical phenomena related to its running at constant rotating speed; and more and more studies are concerned about its transient operation, because its design not only has close relation with operation parameters under constant speed but also those under the startup and shutoff period, especial the pressure fluctuation and hydraulic performance. In order to investigate the transient characteristics during the startup period of a centrifugal pump, the experiment is carried out and numerical simulation is made. The experiment was conducted on the POL’s pump test rig, and the rapid data acquiring program was added in order to get the torque, pressure, and rotating speed during the startup period. Based on the torque equations induced in the paper, numerical simulation is made with the Large Eddy Simulation (LES) method and UDFs (User Defined Functions). Comparison of pump heads and rotating speed between simulation and experimental results shows that they are in good agreement, indicating that the dynamic characteristics of the centrifugal pump can be predicted accurate comparatively through simulation with LES method during its startup process. In addition, the pressure distribution of half height of vane on the cross section at different time was obtained, showing that the pressure increases with time going.
Compensating for the Phosphorescent Persistence in Intensified Cameras for Micro-PIV
Authors: Eckstein Adric , Vlachos Pavlos
Published: July 2009

Micro-PIV experiments rely upon the use of a microscope to achieve the higher spatial resolution. However, several optical limitations are introduced at these scales [1–3]. In addition, due to the low illumination levels, micro-PIV experiments require the use of either a cooled CCD camera or an image intensifier to provide increased signal-to-noise ratio. Although CCD cameras offer superior sensitivity and signal to noise ratio, intensified CMOS cameras offer an attractive alternative for performing high frequency measurements. However, intensified cameras are known to introduce artifacts such as added background noise. This study examines these issues and the feasibility of employing such technologies for microPIV through the use of the IDT-X5 intensified CMOS camera, capable of 500 Hz at a resolution of 2352×1728 pixels, with pulse separations as low as 2μs.
Direct Numerical Simulation of the Motion of Particles Larger Than the Kolmogorov Scale in a Homogeneous Isotropic Turbulence
Authors: Corre Cedric , Estivalezes Jean-Luc , Vincent Stephane , Simonin Olivier
Published: July 2009

Predicting interactions between particles and a surrounding viscous fluid is the concern of many environmental and industrial applications. A Direct Numerical Simulation (DNS) of dilute isotropic turbulent particulate flow has been conducted in a periodic box, with 1283 grid points. The objective is to understand the modification of isotropic turbulence due to dispersed solid particles by analyzing the DNS results. Previous numerical simulations have been, for the most part, limited to the point-particle regime. On the opposite, in these simulations, the diameter of the particles is larger than the Kolmogorov length scale. In order to maintain a constant turbulent kinetic energy, a physical forcing scheme is implemented. Thereby, statistics on the characteristics of the particles are more reliable. Furthermore, interactions between particles are treated via a repulsing force, consequently, simulations are four-way coupling. Simulations are performed with a fictitious domain approach and with the penalty method. For solving the velocity-pressure coupling, an augmented Lagrangian optimization algorithm is used. Results present the influence of the particle phase on the turbulence spectrum. Moreover, the comparison with particle-free case is particularly interesting notably about the anisotropy of the flow caused by the presence of the particles.
A Validation Study for the Hydrodynamics of Biomass in a Fluidized Bed
Authors: Deza Mirka , Battaglia Francine , Heindel Theodore J.
Published: July 2009

Computational modeling of fluidized beds can be used to predict operation of biomass gasifiers after extensive validation with experimental data. The present work focused on computational simulations of a fluidized bed using a multifluid Eulerian-Eulerian model to represent the gas and solid phases as interpenetrating continua. Hydrodynamic results from the simulations were quantitatively compared with X-ray flow visualization studies of a similar bed. It was found that the Gidaspow model can accurately predict the hydrodynamics of the biomass in a fluidized bed. The coefficient of restitution of biomass was fairly high and did not affect the hydrodynamics of the bed; however, the model was more sensitive to particle sphericity variation.
Development and Validation of a Binary Collision Detection Algorithm for a Polydispersed Particle Mixture
Authors: Wunsch Dirk , Fede Pascal , Simonin Olivier
Published: July 2009

A time driven Lagrangian particle detection algorithm for a poly-dispersed mixture of particles is developed and validated. The advantage of this algorithm is the possibility to treat a distribution of particle diameters. We are especially interested in the collision treatment of poly-dispersed particle mixtures and coalescence. Particle pairs are found by applying the overlap criteria and additionally another second criterion, which allows a substantial increase of the time step without limiting the algorithm’s accuracy. The algorithm, which is developed here, is validated with test cases by comparison with predictions of the kinetic theory of rarefied gases. They are applied to dry granular flows. The algorithm accuracy is determined with respect to time step criteria, for both dilute and dense systems.
Experimental Studies of Pressure Drop and Flow Instability in Evaporative Micro-Channels
Authors: Lee Hee Joon , Liu Dongyao , Yao Shi-Chune
Published: July 2009

Experiments were conducted on evaporative micro-channel systems of water, containing 48 parallel channels of 353 μm hydraulic diameter. The general correlation of two-phase pressure drop for an initial design purpose of evaporative micro-channel systems reported in [1] has been validated. For the water boiling in micro-channels, flow instability was observed. The instability criterion, proposed by Kandlikar [2], is able to predict the water experimental results. However, further examination of his criterion revealed that it can not predict the results of Brutin and Tadrist’s data of n-pentane. This is because the Bond number of water is 0.01, but 0.33 for n-pentane. As a result, the growing bubble of n-pentane may not cover the whole length of the micro-channel. A general expression of the effective length of squeezed bubbles in micro-channel was established for fluids at a wide range of Bond number. Using this proposed effective length, the Brutin and Tadrist’s experimental instability data can also be predicted satisfactorily.
Numerical Issues Associated With the Full-Zone Model of the Optically Heated Floating-Zone Used in Semiconductor Processing
Authors: Li Han , Houchens Brent C.
Published: July 2009

Recent models of the thermocapillary driven liquid bridge or full-zone (FZ) have highlighted numerical difficulties in the system, associated with the large velocity gradients near the free surface and the geometric singularity at r = 0. High resolution spectral solutions have been developed to account for these issues. These result in complex representations and highly specialized numerical procedures. After a brief review of these methods, a simplified formulation for the FZ model with strong form boundary conditions is proposed and discussed. Comparisons are made using base flows and stability analyses. Existing solutions have overcome the geometric singularity either by moving the grid away from the r = 0 axis, or by maintaining the correct Taylor series expansion in the representation of each dependent variable. The former has the weakness that an important constraint is not applied. The later formulation is rigorous, but results in complex expressions for the governing equations. To decrease the load associated with the mathematical manipulation and numerical implementation of this method, here a more general Chebyshev polynomial representation of the stream function is applied to the axisymmetric base flow. This removes the need to maintain the proper expansion and instead offers a set of equations in strong form by treating the axis as a boundary known from the spatial symmetry of the model. However, this does not guarantee that momentum is conserved at the internal symmetry boundaries. Various applications of the other boundary conditions are also studied. In the most accurate representation, all boundary conditions except the thermocapillary condition are cast in the strong form via orthogonality. These strong equations must be chosen carefully to avoid introducing redundant conditions. However, the result is a mathematically simpler representation that mimics the accuracy of previous methods.
Simulations to Determine Laminar Loss Coefficients for Flow in Circular Ducts With Arbitrary Planar Bifurcation Geometries
Authors: Handy Tim A. , Lemley Evan C. , Papavassiliou Dimitrios V. , Neeman Henry J.
Published: July 2009

The goal of this study was to determine laminar stagnation pressure loss coefficients for circular ducts in which flow encounters a planar bifurcation. Flow conditions and pressure losses in these laminar bifurcations are of interest in microfluidic devices, in porous media, and in other networks of small ducts or pores. Until recently, bifurcation geometries had been studied almost exclusively for turbulent flow, which is often found in fluid supply and drain systems. Recently, pressure loss coefficients from simulations of a few arbitrary bifurcation geometries in two-dimensions have been published — the present study describes the extension of these two-dimensional simulations to three-dimensional circular ducts. The pressure loss coefficients determined in this study are intended to allow realistic simulation of existing laminar flow networks or the design of these networks. This study focused on a single inlet duct with two outlet ducts, which were allowed to vary in diameter, flow fraction, and angle — all relative to the inlet duct. All ducts considered in this study were circular with their axes in a common plane. Laminar stagnation pressure loss coefficients were determined by simulating incompressible flow through 475 different geometries and flow condition combinations. In all cases, the flow was laminar in the inlet and outlet ducts with a Reynolds number of 15 in the inlet duct. Simulations of the dividing flow geometries were done using FLUENT and a custom written computer code, which automated the process of creating the three-dimensional flow geometries. The outputs, pressure and velocity distributions at the inlet and outlets, were averaged over the circular ducts and then used to calculate pressure loss coefficients for each of the geometries and flow fraction scenarios simulated. The results for loss coefficient for the geometries considered ranged from 2.0 to 70. The loss coefficient for any geometry increased significantly as the outlet flow fraction increased. A consistent increase in loss coefficient was also observed as a function of decreasing outlet duct diameter. Less significant variation of the loss coefficient was observed as a function of the angles of the outlet ducts.
Developing and Fully Developed Turbulent Flow in Ribbed Channels
Authors: Cardwell N. D. , Vlachos P. P. , Thole K. A.
Published: July 2009

Modern turbomachines operate at combustion temperatures well beyond the incipient melting point of the turbine’s metal components. Cooling channels within turbine airfoils directly affect component lifecycle in addition to influencing almost all aspects of the overall engine design. However, many aspects regarding flow structure and vortex dynamics within these cooling channels are still unknown. In this study, high fidelity Time Resolved Digital Particle Image Velocimetry (TRDPIV) was used to investigate a ribbed cooling channel. The design consisted of a square channel having square transverse ribs which were staggered on both the top and bottom walls. Rib spacing was matched to the channel height and the rib to channel height ratio was kept constant at 0.13. The Reynolds number range investigated was between 2,500 and 20,000. Flow field measurements were performed at the entrance to and within the developed rib roughened section, corresponding to the 1st and 12th ribs. Overall, the results indicate that large scale coherent vortical structures were generated by the presence of the front rib surface and enclosed wake region between the ribs. Higher values of vortex circulation strength were observed for Re = 2,500 in addition to a more homogeneous distribution of identified coherent structures at the developed section. In addition to providing insight and feedback for a common turbine cooling design, this study also illuminates the vortex distribution for a highly turbulent and complex internal flow.
Surface Skin Friction Measurement and Visualization Based on Compliant Coatings
Authors: Fujimatsu Nobuyoshi , Misu Isao
Published: July 2009

New method and means are proposed to measure the skin friction force on an aerodynamic surface based on elastic deformations of silicon rubber and gel coatings. One of characteristics is soft and easy to deform. Object is coated by elastic polymeric film with known shear modulus. Two images between wind-on and wind-off are taken using the CCD camera. The displacement of coating is calculated from the correlation of two images. There are two ways to calibrate the skin friction force to the displacement of coatings. These methods are tested and compared to quantitative skin friction force measurement. One is based on measuring the characteristics of the polymeric film using visco-elastic measurement system. This way is called as a priori calibration. The other way is accomplished from the relation between the skin friction force measured by the PHF embedded on the object and the displacement around there and is known as In-situ method. Characteristics of the coatings such as the displacement and the time response can be easily controlled by the compounding ratio of hardener. We conduct the theoretical analysis for the elastic polymer film and propose the material properties of the coatings required to measure the surface skin friction in the wide range of flow speed. The skin friction over the plate is measured using this technique in various Reynolds number. The traditional measurement using the hot wire anemometry is conducted to validate this technique. The time averaged measurements of this technique are in good agreement with the traditional results. However, the unsteady characteristics of surface skin friction are not captured by the lack of time resolution of the CCD camera.
Numerical Modeling of Coal/Biomass Co-Firing
Authors: Ghenai C. , Janajreh I.
Published: July 2009

Biomass co-firing within existing infrastructure of pulverized coal utility boilers is viewed as a practical near-term means of encouraging renewable energy while minimizing capital requirements, maintaining the high efficiency of pulverized coal boilers and reducing the emissions. Numerical investigation of coal/biomass co-firing is presented in this study. Co-combustion of biomass and coal is a complex problem that involves gas and particle phases, along with the effect of the turbulence on the chemical reactions. The transport equations for the continuous phase (gas) and discrete phase (spherical particles) are solved respectively in the Eulerian and Lagrangian frame of reference. The mathematical models used for co-pulverized coal/biomass particles combustion consist of models for turbulent flow (RNG k-ε model); gas phase combustion (two mixture fractions/PDF model: one mixture fraction is used for the fuel (char) and the second for the volatiles); particles dispersion by turbulent flow (stochastic tracking model); coal/biomass particles devolatilization (two competing rates Kobayashi model); heterogeneous char reaction (kinetics/diffusion limited rate model); and radiation (P-1 radiation model). The coal used is a Canadian high sulfur bituminous coal. The coal was blended with 5 to 20% wheat straw (thermal basis) for co-firing. The effect of the percentage of biomass blended with coal on the velocity field, temperature distribution, particles trajectories and pollutant emissions at the exit of the furnace is presented in this paper. One important result is the reduction of NO and CO2 emissions when using co-combustion. This reduction depends on the proportion of biomass (wheat straw) blended with coal.
Numerical Investigation for Particles Impacting and Layering for Surface Treatment
Authors: Fataoui Kh. , Pateyron B. , ElGanaoui M.
Published: July 2009

The particles projected by arc plasma are generally of micrometric size (ranging in general between 10 and 100 μm) in conventional projection and their impact velocity ranges from 50 to 350 m/s. characteristic times for the formation of a splat are very short: less than 5 μs for the duration of spreading of the particle melted with a solidification which can start before the end of the spreading stage and continues in general between 0.8 and 10 μs after the impact. This work is devoted to studying numerically this complex coupling. The model is firstly validated in comparison with available results of particles spreading (large size and low impact velocity < 1 m/s).
Modeling for a High-Bandwidth High-Flow Valve Design Based on Ho¨rbiger Plate Operation
Authors: Branson David T. , Wang Fengcai , Johnston D. Nigel , Tilley Derek G. , Bowen Chris R. , Keogh Patrick S.
Published: July 2009

In order to expand the operational capabilities of hydraulically actuated systems the development of new valves to allow of enhanced flow rates and bandwidth performance is required. Previously, the technical challenge in developing such valves was the need for large spool strokes to achieve the desired flow rates. However, this would then hinder the dynamic response of the valve. To increase flow without reducing dynamic performance it is proposed that the use of multiple metering edges is appropriate. This is achievable using the Ho¨rbiger plate valve principle and direct connection to a piezoelectric actuator. This paper examines the design criteria associated with such a valve. Simulations undertaken as part of its construction and design show that improved flow rates can be achieved. Results from these simulations are then included in orifice equations to further predict flow as a function of plate separation. Finally a simulation was undertaken to determine the total forces acting on the valve, and the forces on the piezoactuator were found to be compressive under normal conditions, which should lead to predictable and stable operation.
Numerical Simulation of Cavitating Flow in a Francis Turbine
Authors: Zhou Lingjiu , Wang Zhengwei , Luo Yongyao , Peng Guangjie
Published: July 2009

The 3-D unsteady Reynolds averaged Navier-tokes equations based on the pseudo-homogeneous flow theory and a vapor fraction transport-equation that accounts for non-condensable gas are solved to simulate cavitating flow in a Francis turbine. The calculation results agreed with experiment data reasonably. With the decrease of the Thoma number, the cavity first appears near the centre of the hub. At this stage the flow rate and the efficiency change little. Then the cavity near the centre of the hub grows thick and the cavities also appear on the blade suction side near outlet. With further reduce of the Thoma number the cavitation extends to the whole flow path, which causes flow rate and efficiency decrease rapidly.
The Alteration of Runner and Partial Vanes on a Fixed Blade Propeller Water Turbine Basing on the Full Passage Simulation
Authors: Zhang Lanjin , Wang Zhengwei , He Qingqing , Peng Guangjie
Published: July 2009

Basing on the 3D-steady Navier-Stokes equations with standard k-ε turbulence closure models, non-structure mesh with fitted body coordinate and finite element based finite volume method, the internal flow on the full passage of the 6.5-meters head fixed blade propeller water turbine is analyzed. Numerical results show that the low output is caused by unsuitable full passage. The flow on the stay vanes isn’t uniform and the circumferential velocity of the runner rim is too large, which leads to a high loss in the draft tube. So the runner and partial stay vanes in the concrete spiral casing are redesigned. The output of the full passage with new runner and new partial stay vanes under 6.5-meters head is 295KW larger than the old one with 240KW output, and the efficiency is 81%, which is larger than former 70%. The redesign of runner and stay vanes is successful.
Application of Pressure Regulating Valve in the Hydraulic Transients of Hydropower Plant
Authors: Zhang Jian , Ma Shibo , Hu Jianyong , Yu Xiaodong , Cai Fulin
Published: July 2009

In the operation of the hydropower station, the sudden change of load in the large range often happen, the emergency closing law of turbine guide vane is difficult to control speed-rise and pressure-rise to the extent permitted for the length of pressure pipeline longer and discharge larger. With the development of the surge chamber of alternative technique, the pressure regulating valve is used to reduce the large pressure and speed-rise of the unit in hydraulic transients as the effective measure to saving investments. The layout of water conveyance system and process of hydraulic transients of a certain hydropower in China are analyzed, and the pressure regulating valve is installed in the spiral case inlet to meet the hydraulic regulation calculation guarantee after technical and economic comparison, for the joint action of valve and turbine guide vane, the pipeline water hammer pressure and unit speed-rise are reduced effectively. The pressure regulating valve diameter and opening and closing law in combination with the turbine guide vane are discussed in the power station, as well as the possible problems may be caused by the using of the pressure regulating valve is pointed out in theory. The achievements to scheme decision of adopting the pressure regulating valve to replace the surge chamber in high-head hydropower station with longer pressure water conveyance system can be for reference and provide the certain technological supply.
Computer Simulation of the Hydropower Stations With Long Pressurized Pipelines and Far Transmission Line
Authors: Zhou Jianxu , Bi Chengmin , Hu Ming
Published: July 2009

With the rapid development of automatic technology and computer science in the hydropower stations, the computer simulation system named as simulator is developed quickly and applied widely especially as a modern training tool for field operators. In the project of power transmission from west to east in Southwest China, there are lots of hydropower stations with long pressurized pipelines and far transmission line, so their simulators are more complex to be built correctly to show the characteristic of hydraulic-mechanical-electrical system and its dynamic procedure distinctly in real time. Based on the characteristic analysis of hydraulic system and long transmission system, a new easily-decoupled elastic model of water flow in pressurized pipelines is given and recommended, and its application and the choice of appropriate model order are analyzed considering the effect of power system, furthermore, how to build the model of far transmission line is discussed in detail based on its state equations. The results indicate that, in order to realize the truthfulness and robustness of the simulator, the higher order oscillation mode of water flow in long pressurized pipelines should be introduced and the frequency property of far transmission line should be analyzed.
Characteristic Research of Sonic Nozzles Using Step-Down Procedure in Low Reynolds Number (Keynote Paper)
Authors: Lim Jae-Myoung , Jang Seok , Yun Bok-Hyun , Cho Hae-Man , Park Kyung-Am
Published: July 2009

The aim of this investigation is to develop the minute flow measuring technology to calibrate sonic nozzles at the low flow rate and low pressure without the expensive primary standard system. With this calibrated sonic nozzles, calibration service of flow meters to industry will be available. In this experiment, eight sonic nozzles with the throat diameter of 0.2 mm were manufactured according to the specifications of ISO 9300, and then four out of eight nozzles, which have most close values of the rate of discharge coefficients for a pair of nozzles, were selected. At first, all the four nozzles for the stagnation pressure range of 200∼600 kPa were calibrated in the standard system. Then, for the rest of the whole experiments, the step-down procedure was adopted to reduce the stagnation pressure range of the nozzle, 3∼600 kPa. It was proven that the step-down procedure can be done over and over with small additional uncertainty.
An Exploratory Investigation of Cavitation Inception on the Pressure Side of Propellers
Authors: van Rees Wim M. , van Rijsbergen Martijn X. , Kuiper Gert , van Terwisga Tom J. C.
Published: July 2009

Delayed sheet cavitation inception has occasionally been observed in the MARIN Depressurized Towing Tank (DTT). The problems are specifically related to the pressure side of model ship propellers, and occur despite the application of leading-edge roughness. As a consequence, no cavitation at all or cavitation on parts of the propeller blades is observed, in cases where cavitation in the cavitation tunnel or at full scale is present. In an exploratory investigation, the effect of several parameters that may influence cavitation inception is studied in the DTT. In particular, the influences of Reynolds number, free-stream turbulence and additional gas nuclei are investigated. It is concluded that the presence of sufficient gas nuclei is crucial for sheet cavitation inception, even if leading-edge roughness is applied. With additional nuclei in the propeller inflow, sheet cavitation inception in the DTT is no longer delayed with respect to the cavitation tunnel.
Entropy-Based Loss Coefficients for Turbulent Channel Flow With Heat Transfer
Authors: Naterer G. F. , Adeyinka O. B.
Published: July 2009

This paper presents a new entropy-based metric for local loss coefficients in turbulent flows with heat transfer. Unlike conventional methods of global loss characterization, such as a single valve loss coefficient, this paper examines an entropy-based alternative that tracks local variations of the loss coefficient. The local distribution of entropy production can provide useful information regarding the spatial distribution of mechanical energy losses, which can be then used to systematically optimize thermofluid systems. An application involving turbulent channel flow is specifically investigated. The friction factor is represented in terms of the local entropy production rates. By successfully predicting the local flow irreversibilities with this approach, re-design efforts can be more effectively focused on specific regions of highest entropy production.
Three-Dimensional Natural Convective Flow in a Rectangular Enclosure With a Rectangular Heated Section on One Vertical Wall and With the Other Vertical Walls Cooled
Authors: Oosthuizen Patrick H. , Kalendar Abdulrahim , Simko Thomas M.
Published: July 2009

Three-dimensional natural convective flow in a rectangular enclosure with vertical sidewalls and horizontal top and bottom surfaces has been considered. A heated rectangular element is mounted in the middle of one vertical wall of the enclosure, the remainder of this wall being adiabatic. The remaining vertical walls are cooled to a uniform low temperature. The horizontal top and bottom walls are adiabatic. The flow has been assumed to be steady and laminar. Fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces. Radiation effects have been neglected. The numerical solution was obtained using the governing equations written in terms of dimensionless variables. The enclosure height, H′, was used as the characteristic length scale and the difference between the temperatures of the hot wall section and the cooled walls were used as the characteristic temperature scale. The dimensionless governing equations have been solved using FIDAP, a commercial software package that employs the finite element method. The solution has the following parameters: the Rayleigh number, the Prandtl number, the dimensionless height of the heated wall section compared to the overall enclosure height; the dimensionless width of the heated wall section compared to its height; the dimensionless width of the enclosure between the vertical sidewall on which the heated wall section is mounted and the opposite vertical sidewall, and the dimensionless width of the enclosure between the other two vertical sidewalls. Because of the application being considered, results have only been obtained for Pr = 0.7. Attention has been restricted to the case where the dimensionless width of the enclosure between the vertical sidewall on which the heated wall section is mounted and the opposite vertical sidewall is 0.5 and where the dimensionless width of the enclosure between the other two vertical sidewalls is 1.0. A wide range of the other parameters has been considered particular attention having been given to the effect of the dimensionless width of the heated wall section compared to its height on the mean Nusselt number for the heated wall section.
Direct Simulation of Spray Cooling: Effect of Multiple Droplet Impact and Latent Heat of Vaporization of Coolant Liquid on Heat Transfer
Authors: Sarkar Mita , Selvam R. Panneer , Ponnappan Rengasamy
Published: July 2009

Spray cooling is a way of efficiently removing the heat from a hot surface and considered for high power system such as advanced lasers. The heat transfer phenomenon in spray cooling is complex in nature because it occurs due to conduction, convection and phase change. The numerical model of spray cooling is done by solving the set of incompressible Navier-Stokes equations using finite difference method. Level set method is used to capture the liquid vapor interface in our multiphase flow model. Our previous 2D model which included single droplet impact on single growing vapor bubble is modified to introduce multiple droplets impact on thin liquid film with multiple growing vapor bubbles. Though the previous model was effective so far to predict the spray cooling phenomena and also the parameters for high heat removal, but the actual spray cooling phenomena consists of multiple droplets impact on multiple growing vapor bubbles at different time instances. To understand the spray cooling further and to represent it more realistically the inclusion of multiple droplets and multiple vapor bubbles is essential. In the present work, an investigation on the effect of latent heat of vaporization of coolant is conducted for the case of a thin liquid film of 44 μm in removing the heat and bubble growth when a liquid spray droplet is impacting. The flow and heat transfer details are presented for multiple droplet impacts on thin liquid film with multiple growing vapor bubbles.
Inverse Solution to Steady Two-Dimensional Heat Conduction
Authors: Mosaad Mohamed El-Sayed
Published: July 2009

This paper presents an analytical method for solving an inverse problem of steady, two-dimensional, heat conduction in a solid plate with space-variable temperature and heat flux on one of its boundaries. The method doesn’t need the knowledge of the thermal conditions on the other plate boundaries. Explicit closed-form expressions are derived for calculating the two-dimensional distributions of the temperature and heat flux in the plate for arbitrary over-specified boundary temperature and heat flux profiles. The special problem case of isothermal boundary with a specified heat flux profile, and that of insulated boundary with a specified temperature profile are considered realistic cases for this problem. Validity and exactness of the proposed method is proved via test problems with known exact solutions. Sensitivity of the current technique to errors in the input boundary conditions is examined. The proposed method is considered of a theoretical value for the heat conduction area.
Enhanced Mini-Channel Forced Convection With Encapsulated Phase-Change Particles
Authors: Hassanipour Fatemeh , Lage Jose´
Published: July 2009

In this study we propose a new cooling concept using encapsulated phase-change particles flowing in a parallel-plate mini-channel. This novel concept is inspired by the gas exchange process in alveolar capillaries, where red blood cells (RBCs) flow with blood plasma, yielding very high gas transfer efficiency. Another important characteristic of alveolar capillary blood flow, which is related to the high efficiency of the lungs, is the snug fitting of the RBCs into the capillary. Hence, we conjecture that using particles similar to RBCs, i.e. with diameter similar to the spacing between the parallel plates, is likely to lead to high heat transfer efficiency as well. We report here experimental results of preliminary tests performed with octadecane paraffin (C18H38), a phase-change material, encapsulated in a thin melamine shell, flowing with water through a heated parallel-plate channel test module. We measured the temperature distribution along the heated surface of the channel for various water flow rates, with and without particles, and varying the number of particles. Results are reported in terms of the channel heated surface average temperature and the average Nusselt number.
Numerical Analysis for a Vapor Feed Miniature Direct Methanol Fuel Cell System
Authors: Xiao Bin , Faghri Amir
Published: July 2009

A two-dimensional, multiphase model is presented and analyzed for a vapor feed DMFC system. The DMFC model is based on the multiphase mixture formulation and encompasses all components in the porous regions of a vapor feed DMFC using a single computation domain. The evaporation/condensation phenomenon in the anode flow channel is modeled in a separated way. An iterative numerical scheme is used to solve the governing equations in a coupled manner. Numerical simulations are carried out to explore the transient and polarization characteristics of the DMFC, including methanol crossover through the membrane, temperature evolution, anodic and cathodic overpotentials. The results indicate the anode flow channel for the feeding methanol solution is the key parameter for the DMFC performance. The numerical results are also compared with the experimental data in a good agreement.
One-Dimensional Smoke Movement in Vertical Open Shafts at Steady State: Theoretical Prediction and Experimental Verification
Authors: Sun Xiaoqian , Li Yuanzhou , Hu Longhua , Huo Ran , Chow Wanki , Fong Naikong , Lui Cheheng GiGi
Published: July 2009

Smoke movement in a vertical open shaft was studied by considering heat transfer to side walls and density variation due to temperature rise. Such factors used to be ignored but should be important when studying large scale upward movement of smoke in shafts with large height-to-span ratio. Steady state one-dimensional flow in the vertical direction in the shaft was theoretically described. Boussinesq approximation was not suitable in the model when the smoke temperature was high under bigger fires. The near field plume characteristics at the bottom of shaft were described based on a virtual point source assumed. The mass flow rate at the inlet level of the shaft was then calculated by using the Heskestad model. The predicted temperature rise and upward velocity were shown to both decay exponentially with height and the Nusselt number at steady state, but increased with the Reynolds number in shaft. Experiments with a 1/8 scale model were carried out to study smoke movement for different fires and were used to verify the theoretical analysis. Predicted results agreed satisfactorily with the measured values. The fire size was found to be the most important factor affecting the temperature rise and upward velocity. Buoyancy would be stronger and the hot gas thermally expanded to accelerate their upward movement.
The Effects of Thermocouple Materials and Insulating Mica in an Erodable Surface Thermocouple
Authors: Grech Alan , Sant Tonio , Farrugia Mario
Published: July 2009

A finite-difference model of a surface thermocouple (erodable-ribbon type) of a heat flux sensor was built to analyze the transient response of the thermal junction and the two-dimensional effects created by the insulation between the thermocouple materials and the body material of the sensor. Such transient heat flux sensors have previously been used for measurements in internal combustion engines. It is commonly assumed that the heat transfer within these devices is one-dimensional even though the sensors are constructed from at least two different materials. It is common practice to calculate the transient heat flux using properties of body material and this leads to a substantial error as demonstrated by the model. With these sensors, low thermal capacity thermocouple junctions are formed near the surface by abrasion and response times as low as 30μs have been reported. Experiments were performed on an E type surface thermocouple heated at 11W by means of a copper vapor laser pulsating at 10kHz. Measurement of surface thermocouple temperature was performed at a 100kHz sampling rate. A finite-difference model was used to analyze the response of these sensors to the pulsed laser heating operating at 10 kHz. The insulation between the thermocouples and the body material was mica and the body material was AISI 316 stainless steel. The experimental measurements and simulation results are reported in this work. The analysis and comparison of experimental and simulation results showed that for such thermocouples two-dimensional effects exist due to the presence of mica sheets. The temperature decay between pulses was better matched using thermal properties of mica sheets rather than the thermal properties of the body material. However the body material still dominates the temperature swing of the thermocouple junction.
Measurement of Cooling Curves in Centrifugal Casting of a Ferrous Alloy
Authors: Heringer Romulo , Boccalini Ma´rio , Martorano Marcelo A. , Serantoni Cla´udia R.
Published: July 2009

A sensor was developed to measure the cooling curves inside a ferrous alloy during its solidification as centrifugally cast tubes. The temperature evolution at some points within the alloy is necessary to evaluate the heat transfer through the outer surface of the tube during the centrifugal casting process. Serious difficulties exist in this type of measurement, because of the rotation of the mold and the relatively high temperature at which the ferrous alloy is poured. The sensor consists of sheathed thermocouples positioned by a convenient support internally to the rotating mold, within the metal layer. Although the sensor is subjected to thermal and mechanical stresses during the melt pouring and solidification, it must maintain its mechanical and thermal characteristics to temperatures of the order of the melting point of the ferrous alloy. Therefore, the thermocouple sheaths and support have been made of refractory metals, namely, tantalum and niobium, to resist the high temperature. Moreover, the sensor was designed to have low thermal inertia, allowing its temperature to increase above the liquidus temperature of the alloy before solidification of the surrounding liquid metal. Because the sensor is embedded in the solidified tube after solidification, a special design was necessary to allow stripping the tube out of the mold without disturbing the system.
Experimental and Numerical Investigation of Heat Transfer Characteristics of Liquid Flow With Micro-Encapsulated Phase Change Material
Authors: Sabbah Rami , Yagoobi Jamal , Al Hallaj Said
Published: July 2009

This experimental and numerical study investigates Micro-Encapsulated Phase Change Material (MEPCM) heat transfer characteristics and corresponding pressure drop. To conduct this study, an experimental setup consisting of a steel tube with an inner diameter of 4.3mm, outer diameter of 6.5mm and a length of 1,016mm is selected. A MEPCM mass concentration of 20% slurry with particle diameter ranging between 5–15μm is included in this study. Tube wall temperature profile, fluid inlet, outlet temperatures, the pressure drop across the tube are measured and corresponding Nusselt number are determined for various operating conditions. The experimental results are used to validate the numerical model predictions. The numerical model results show good agreement with the experimental data under various operating conditions. The controlling parameters are identified and their effects on the heat transfer characteristics of micro-channels with MEPCM slurries are evaluated.
Performance of a Flexible Evaporator for Loop Heat Pipe Applications
Authors: Limaye Mukta S. , Klausner James F.
Published: July 2009

A flat and flexible evaporator, which conforms to contoured surfaces, has been developed for loop heat pipe applications. A loop heat pipe (LHP) is a passive, two phase heat transfer device that uses a porous membrane in the evaporator to circulate fluid. A number of flexible membranes have been tested as evaporator wicks that have a length of 12.7 cm and heated area of 50.6 cm2. For cellulose, polyethylene, and blotting paper membranes, maximum heat fluxes of 0.43, 1.5 and 2.9 W/cm2 have been observed, respectively. The maximum heat transfer coefficients measured for these membranes are 551, 876, and 2100 W/m2-K, respectively. The best performance was observed by a membrane made of a fibrous cotton matrix, typically used as gauze. This material has a large pore size and high wettability with water. When tested in a rigid, brass evaporator, the maximum heat flux observed is 5.95 W/cm2, and the maximum heat transfer coefficient is 2865 W/m2-K. A flexible evaporator is fabricated using a heat sealable, flexible barrier pouch, and the cotton matrix membrane is sealed inside. The maximum measured heat flux for the flexible evaporator is 3.2 W/cm2 and maximum measured heat transfer coefficient is 1165 W/m2-K. The observed reduction in heat transfer as compared to the rigid evaporator is due to the poor contact between the evaporator and membrane. It is concluded that for the flexible evaporator membranes considered, the heat transfer mechanism is boiling and the maximum heat flux is limited by the wicking rate of the membrane. For a given membrane, the wicking rate increases with a reduction in the wicking length and decreases with an increasing rate of evaporation. To further improve the performance of the flexible evaporator, it is necessary to ensure efficient vapor removal from the evaporator as well as maintaining good contact between the membrane and the evaporator surface.
A Numerical Investigation on Soot Formation From Laminar Diffusion Flames of Ethylene/Methane Mixture
Authors: Guo Hongsheng , Trottier Stephanie , Johnson Matthew R. , Smallwood Gregory J.
Published: July 2009

The sooting propensity of laminar diffusion flames employing ethylene/methane mixture fuel is investigated by numerical simulation. Detailed gas phase chemistry and moments method are used to describe the chemical reaction process and soot particle dynamics, respectively. The numerical model captures the primary features experimentally observed previously. At constant temperatures of air and fuel mixture, both maximum soot volume fraction and soot yield monotonically decrease with increasing the fraction of carbon from methane in the fuel mixture. However, when the temperatures of air and fuel mixture are preheated so that the adiabatic temperatures of all flames are same, the variation of the maximum soot yield becomes higher than what would be expected from a linear combination of the flames of pure ethylene and pure methane, showing a synergistic phenomenon in soot formation. Further analysis of the details of the numerical results suggests that the synergistic phenomenon is caused by the combined effects of the variations in the concentrations of acetylene (C2H2) and methyl radical (CH3). When the fraction of carbon from methane in fuel mixture increases, the concentration of C2H2 monotonically decreases, whereas that of methyl radical increases, resulting in a synergistic phenomenon in the variation of propargyl (C3H3) radical concentration due to the reactions C2H2 + CH3 = PC3H4 + H and PC3H4 + H = C3H3 + H2. This synergistic phenomenon causes a qualitatively similar variation trend in the concentration of pyrene (A4) owing to the reaction paths C3H3 → A1 (benzene) → A2 (naphthalene) → A3 (phenanthrene) → A4. Consequently, the synergistic effect occurs for soot inception and PAH condensation rates, leading to the synergistic phenomenon in soot yield. The similar synergistic phenomenon is not observed in the variation of peak soot volume fraction, since the maximum surface growth rate monotonically decreases, as the fraction of carbon from methane in fuel mixture increases.
Evaluation of Convective Heat and Mass Transfer in Open Sun and Green House Drying
Authors: Shukla S. K.
Published: July 2009

In this communication, a study on open sun and green house drying has been performed to evaluate the convective heat and mass transfer coefficients as a function of climatic parameters. The hourly data for the rate of moisture removal, crop temperature, relative humidity inside greenhouse and outside greenhouse and ambient air temperature for complete drying have been recorded. The experiments were done in the premises of Allahabad Agricultural Institute-Deemed University, after the crop-harvesting season from February 2006 to April 2006. Using these data, the value of convective heat and mass transfer in open sun drying (OSD) and green house sun drying (GHD) under natural and forced convection mode, have been calculated. Also the empirical relations of convective heat and mass transfer with drying time under natural and forced modes have been developed. On the basis of the present study, it is inferred that the value of convective heat and mass transfer is more in open sun drying than in the green house drying under natural convection mode. However its value increases in forced mode of greenhouse drying as compared to natural mode in the initial stage of drying.