bluuebunny/crossref_metadata_2025 huggingface dataset:
Frictional Effects on a Base and a Flow-Trimmed Impeller of a Low Specific Speed Industrial Compressor
Authors: David Tim , Zhang Donghui , Cave Michael
Published: September 2008

Flow-trimming base designs for lower flow applications are widely used in the turbo-machinery industry to minimize development costs. This process requires the use of the same casing as the base design, along with a decrease in the flow path width of the impeller, diffuser, crossover, and return vane. Two stages: a baseline and a flow-trimmed impeller, diffuser and return vane are studied in this paper to review impacts on performance and their underlying causes. To measure the performance changes of the two rigs, a traverse survey for flow angle and total pressure was done at the locations of diffuser inlet, return vane leading edge and stage discharge. It was found that the performance difference between the base impeller and trimmed one is very little. However, diffuser performance of the base rig is better than the flow-trimmed rig. Diffuser data shows frictional losses increased in the flow-trimmed rig. Rig data also suggests that increased friction in the trimmed stage changes the flow angle at the return vane leading edge compared to the base stage. This increases the loss further for the flow-trimmed rig because of resulting negative incidence on the return vane relative to the base rig. Data suggests that while flow-trimming may be a valid design approach for impellers, additional issues arise when downstream stationary components are trimmed.
A New Alternative for Reduction of Secondary Flows in Low Pressure Turbines
Authors: Torre Diego , Va´zquez Rau´l , de la Rosa Blanco Elena , Hodson Howard P.
Published: September 2008

This paper describes a new flow mechanism for the reduction of secondary flows in Low Pressure Turbines using the benefit of contoured endwalls. The extensive application of contoured endwalls in recent years has provided a deeper understanding of the physical phenomenon that governs the reduction of secondary flows. Based on this understanding, the endwall geometry of a linear cascade of solid-thin profiles typical of Low Pressure Turbines has been redesigned. Experimental data are presented for the validation of this new solution. Based on these data, a reduction of 72% in the SKEH and 20% in the mixed-out endwall losses can be obtained. CFD simulations are also presented to illustrate the effect of the new endwall on the secondary flows. Furthermore, an explanation of the flow mechanism that governs the reduction of the SKEH and the losses is given.
Influence of Casing Roughness on the Aerodynamic Structure of Tip Vortices in an Axial Flow Turbine
Authors: Rao Nikhil M. , Gumusel Baris , Kavurmacioglu Levent , Camci Cengiz
Published: September 2008

The aerodynamic influence of casing surface roughness on over-tip-leakage flow was investigated in a large scale, rotating, axial turbine rig. Phase-locked measurements of the absolute total pressure in a cold flow turbine research facility were conducted at the turbine stage exit using a high-frequency-response total pressure probe. Time accurate measurements provided valuable aerodynamic information quantifying the near tip flow modifications imposed by artificially roughened casing inner surface. A partial segment of the turbine casing was roughened by using a roughness layer of two different mean roughness heights. A smooth wall as a baseline case was also investigated by attaching a smooth layer of equivalent thickness to the casing surface. Artificially roughening the casing surface significantly reduced the leakage mass flow rate and the momentum deficit in the core of the tip vortex. The reductions obtained in the tip vortex size and strength influenced the tip-side passage vortex and other typical core flow characteristics in the passage. The influence of casing roughness was studied in a range of tip clearance values.
Investigation of 3D Unsteady Flows in a Two-Stage Shrouded Axial Turbine Using Stereoscopic PIV and FRAP: Part II — Kinematics of Shroud Cavity Flow
Authors: Yun Y. I. , Porreca L. , Kalfas A. I. , Song S. J. , Abhari R. S.
Published: September 2008

This paper presents an experimental study of the behavior of leakage flow across shrouded turbine blades. Stereoscopic particle image velocimetry and fast response aerodynamic probe measurements have been conducted in a low-speed two-stage axial turbine with a partial shroud. The dominant flow feature within the exit cavity is the radially outward motion of the main flow into the shroud cavity. The radial migration of the main flow is induced by flow separation at the trailing edge of the shroud due to a sudden area expansion. The radially outward motion is the strongest at mid-pitch as a result of interactions between vortices formed within the cavity. The main flow entering the exit cavity divides into two streams. One stream moves upstream towards the adjacent seal knife and reenters the main flow stream. The other stream moves downstream due to the interaction with the thin seal leakage flow layer. Closer to the casing wall, the flow interacts with the underturned seal leakage flow and gains swirl. Eventually, axial vorticity is generated due to these complex flow interactions. This vorticity is generated by a vortex tilting mechanism and gives rise to additional secondary flow. Due to these fluid motions combined with a contoured casing wall, three layers (the seal leakage layer, cavity flow layer, and main flow) are formed downstream of the shroud cavity. This result is different from the two-layer structure which is found downstream of conventional shroud cavities. The seal leakage jet formed through the seal clearance still exists at 25.6 percent axial chord downstream of the second rotor. This delay of complete dissipation of the seal leakage jet and its mixing with the cavity flow layer is due to the contoured casing wall. Time-averaged flow downstream of the shroud cavity shows the upstream stator’s influence on the cavity flow. The time-averaged main flow can be viewed as a wake flow induced by the upstream stator whose separation at the shroud trailing edge induces pitchwise non-uniformity of the cavity flow.
Predicting Entropy Generation Rates in Transitional Boundary Layers Based on Intermittency
Authors: Nolan Kevin P. , Walsh Edmond J. , McEligot Donald M. , Volino Ralph J.
Published: September 2008

Prediction of thermodynamic loss in transitional boundary layers is typically based on time averaged data only. This approach effectively ignores the intermittent nature of the transition region. In this work laminar and turbulent conditionally-sampled boundary layer data for zero pressure gradient and accelerating transitional boundary layers have been analyzed to calculate the entropy generation rate in the transition region. By weighting the non-dimensional dissipation coefficient for the laminar conditioned data and turbulent conditioned data with the intermittency factor, the entropy generation rate in the transition region can be determined and compared to the time averaged data and correlations for laminar and turbulent flow. It is demonstrated that this method provides an accurate and detailed picture of the entropy generation rate during transition in contrast with simple time averaging. The data used in this paper have been taken from conditionally-sampled boundary layer measurements available in the literature for favorable pressure gradient flows. Based on these measurements a semi-empirical technique is developed to predict the entropy generation rate in a transitional boundary layer with promising results.
Modular Multi-Stage Axial Compressor Design: A Conceptual Study With an Example
Authors: Attia Magdy S. , Hemerly Christopher
Published: September 2008

Increasingly, companies are becoming more interested in reducing cost. Initially, cost reduction efforts were focused on the manufacturing and service side since the majority of the budget was consumed during that phase. However, recent studies indicate that up to 80% of the life cycle costs (LCC) has been embedded in the engine’s DNA at the end of the development and design phase. This fact is motivating many companies to consider cost cutting initiatives much earlier in the development cycle than before. One concept to aid in such cost reduction is the modular design of expensive and development-intensive components, such as multi-stage axial compressors. With sufficient planning, any potentially negative impact on performance can be addressed and almost entirely eliminated. Conceptually, the compressor is divided into five (5) functional modules. In the successful modular design, the core module size is maximized, while all other modules are held to a minimum. It is the objective of this approach to utilize the core module in all the compressors, thus, maximizing commonality and minimizing all relevant development, design, manufacture, procurement, and service costs. This paper introduces the modular concept with an example; a multi-stage high pressure compressor (HPC) design is carried out to the preliminary meanline phase. The compressor is consequently divided up into its five (5) modules, and a modular upgrade is then developed for a different application using the same core. Discussion is presented as to the advantages and potential limitations.
Flow Physics and Profiling of Recessed Blade Tips: Impact on Performance and Heat Load
Authors: Mischo Bob , Behr Thomas , Abhari Reza S.
Published: September 2008

In axial turbine the tip clearance flow occurring in rotor blade rows is responsible for about one third of the aerodynamic losses in the blade row and in many cases is the limiting factor for the blade lifetime. The tip leakage vortex forms when the leaking fluid crosses the gap between the rotor blade tip and the casing from pressure to suction side and rolls up into a vortex on the blade suction side. The flow through the tip gap is both of high velocity and high temperature, with the heat transfer to the blade from the hot fluid being very high in the blade tip area. In order to avoid blade tip burnout and a failure of the machine, blade tip cooling is commonly used. This paper presents the physical study and an improved design of a recessed blade tip for a highly loaded axial turbine rotor blade with application in high pressure axial turbines in aero engine or power generation. With use of three-dimensional Computational Fluid Dynamics (CFD), the flow field near the tip of the blade for different shapes of the recess cavities is investigated. Through better understanding and control of cavity vortical structures, an improved design is presented and the differences to the generic flat tip blade are highlighted. It is observed that by an appropriate profiling of the recess shape, the total tip heat transfer Nusselt Number was significantly reduced, being 15% lower than the flat tip and 7% lower than the baseline recess shape. Experimental results also showed an overall improvement of 0.2% in the one-and-1/2-stage turbine total efficiency with the improved recess design compared to the flat tip case. The CFD analysis conducted on single rotor row configurations predicted a 0.38% total efficiency increase for the rotor equipped with the new recess design compared to the flat tip rotor.
Experimental and Numerical Study of Real-Gas Flow in a Supersonic ORC Turbine Nozzle
Authors: Turunen-Saaresti Teemu , Tang Jin , van Buijtenen Jos , Larjola Jaakko
Published: September 2008

Using organic matter as the working fluid in small Rankine cycle power plants is beneficial. However, high molecular weight of the fluid and the single-stage design of the turbine lead to a supersonic flow in the turbine. An Organic Rankine Cycle (ORC) plant was designed and tested. Toluene was used as the working fluid and as lubricant. The turbine and the feed pump were placed on the same shaft as the high-speed generator in the designed 175 kW unit. CFD simulations were used in the design process. Toluene is behaving as a real gas in the nozzle. To ensure an accurate simulation, a real gas model of toluene was implemented in an existing Navier-Stokes flow solver. Polynomial and rational regression were used to achieve the functions for the gas properties. The pressure and temperature were measured at the nozzle inlet and outlet. In the CFD simulations the nozzle ring was modelled with and without a temperature probe in order to model the effect of the probe to the flow field and compare the simulated pressure and temperature values against the measurements. The nozzle geometry was also modelled in 2D and 3D in order to see the effect of the 3D in the flow field. There was quite a good agreement between the measured and simulated data. The agreement in the temperature was better than in the pressure. The effect of 3D on the simulation results was minor, which was expected. The simulated flow field revealed that the shock waves developing in the trailing edge of the nozzle were seen in the turbine rotor inlet.
The Influence of Reynolds Number, Mach Number and Incidence Effects on Loss Production in Low Pressure Turbine Airfoils
Authors: Va´zquez Rau´l , Antoranz Antonio , Cadrecha David , Arman˜anzas Leyre
Published: September 2008

This paper presents an experimental study of the flow field in an annular cascade of Low Pressure Turbine airfoils. The influence of Reynolds number, Mach number and incidence on profile and end wall losses have been investigated. The annular cascade consisted of 100 high lift, high aspect ratio, high turning blades that are characteristic of modern LP Turbines. The investigation was carried out for a wide range of Reynolds numbers, extending from 120k to 315k, exit Mach numbers, from 0.5 to 0.9, and incidences from −20 to +14 degrees. Results clearly indicate a significant effect of incidence and Mach number in secondary loss production; however, the Reynolds number shows it much weaker impact. It has also been found that the profile loss production is strongly influenced by both Reynolds and Mach numbers, being the impact of the incidence weaker. Finally, measured data suggest that, in order to properly reproduce the performance of these types of airfoils, annular cascades can be required as far as linear cascades may miss some essential flow features.
A Study on the Mechanism of Tip Leakage Flow Unsteadiness in an Isolated Compressor Rotor
Authors: Zhang Hongwu , Deng Xiangyang , Lin Feng , Chen Jingyi , Huang Weiguang
Published: September 2008

A numerical study of unsteady tip leakage flow in an isolated axial compressor rotor is presented, aiming at clarifying the originating flow mechanism of this unsteady phenomenon. First, CFD simulations utilizing a three-dimensional, time-accurate, Reynolds-averaged Navier-Stokes solver demonstrates that the tip leakage flow pattern, which manifests itself as an interacting cross- and through-flow in the tip region, can become periodically oscillatory in a range of operating conditions. A flow mechanism is then clarified to explain this unsteady flow phenomenon at its onset that this periodic flow oscillation is a result of dynamic balance, as opposed to static balance, between two counter-acting driving “forces”. One such “force” is the aerodynamic loading of the blades, i.e. the pressure difference across the pressure and suction sides of the compressor blades created by the main through flow. Its counter-acting “force” is the unloading of the blades, i.e. the reduction of the pressure difference caused by the tip leakage cross flow that originates from the pressure side, rushes into the suction side through the tip clearance. At operating conditions in which both “forces” are strong and in the same order, their static balance will be broken. While a larger blade loading creates a stronger tip leakage flow, the tip leakage flow tends to diminish itself because its accompanying effect is to unload the blade. Since the weaker tip leakage flow cannot overcome the ability of the main through flow to recover the original aerodynamic loading for the blade, the whole process restarts and periodically oscillatory tip leakage flow forms. Furthermore, a dimensionless analysis shows that the onset of the observed unsteadiness is conditioned by the tip leakage flow, which can or cannot reach the neighboring blade before mixing with the main flow.
Inverse Problem in Aerodynamic Shape Design of Turbomachinery Blades
Authors: Li Yingchen , Yang Dianliang , Feng Zhenping
Published: September 2008

There are various methods for aerodynamic shape design in turbomachinery blades, but at the state of the art the shape design has still been a formidable problem. Optimal shape design based on adjoint method has been developed rapidly in the last decades in aeronautic field with the start of Jameson’s work. As a gradient-based optimization, the adjoint method introduces an adjoint system and the sensitivity derivative is computed by solving a linear adjoint equation, which makes the computational cost almost independent of the number of design variables. Because the adjoint method realizes the quick and exact sensitivity analysis and saves large computational resources, it has been the highlight in aerodynamic shape design of CFD field. Combining the continuous adjoint method with quasi-Newton method, we developed an optimization algorithm for turbomachinery aerodynamic design governed by two-dimensional Euler equations in this paper. The blade shape to be optimized is parameterized by non-uniform B-spline and the computational domain is discreted with H-grids. Then the adjoint equations and their boundary conditions are deduced in detail, both in computational and physical spaces, and are solved numerically by using time-marching finite difference method based on Jameson’s diffusion scheme. With the solved adjoint variables, the final expression of the cost function gradient with respect to the design variables is formulated. Finally, several numerical cases of turbomachinery blade aerodynamic design are presented and analyzed to validate the present optimization algorithm.
Study on Effects of Spontaneous Condensation on Performance of Low Pressure Stages in a Steam Turbine
Authors: Li Liang , Feng Zhenping , Li Guojun
Published: September 2008

The formation of water droplets in low-pressure steam turbine seriously degrades the performance of the turbine. In order to simulate the wet steam flow with spontaneous condensation, an Eulerian/Eulerian model was developed, in which the Navier-Stokes equations for water vapor flow are coupled with two additional equations describing the formation and the distributions of water droplets. The classical condensation theory was used to model the condensation process. With this model, the three dimensional (3D) steady wet steam flow with spontaneous condensation in three low pressure (LP) stages of an industrial steam turbine was numerically investigated and the results were compared with those in superheated flow. The distribution of pressure, the enthalpy drop, the reaction degree, the outflow velocity and flow angle in each wet steam turbine stage obviously change due to the spontaneous condensation in wet steam flow, compare to those in the superheated flow. The re-distribution of flow parameters in condensing flow leads to that the turbine stages run at ‘off-design’ condition actually, which leads to additional efficiency losses besides the well-known non-equilibrium losses.
On Design of Transonic Fan Rotors by 3D Inverse Design Method
Authors: Hu Peixin , Zangeneh Mehrdad , Choo Benjamin , Rahmati Mohammad
Published: September 2008

The application of 3D inverse design to transonic fans can offer designers many advantages in terms of reduction in design time and providing a more direct means of using the insight obtained into flow physics from CFD computations directly in the design process. A number of papers on application of inverse design method to transonic fans have already been reported. However, in order to apply this approach in product design a number of issues need to be addressed. For example, how can the method be used to affect and control the fan rotor characteristics? The robustness of the method and its ability to deal with accurate representation of leading and trailing edges, as well as tip clearance flow. In this paper the further enhancement of the 3D viscous transonic inverse design code TURBOdesign-2 and its application to the re-design of NASA37 and NASA67 rotors will be described. In this inverse design method the blade geometry can be computed by the specification of the blade loading (meridional derivative of rVθ) or the pressure loading. In both cases the blade normal thickness is specified to ensure structural integrity of the design. Improvements to the code include implementation of full approximation storage (FAS) multigrid technique in the solver, which increases the speed of the computation. This method allows the modification of blade thickness and pressure loading by B-splines. In addition improvements have been made in the treatment of proper leading edge geometry. Two well known examples of NASA 67 and NASA 37 rotors are used to provide a step-by-step guide to the application of the method to the design of transonic fan rotors. Improved designs are validated by commercial CFD code CFX.
A Global Preconditioning Method for Low Mach Number Viscous Flows in Rotating Machinery
Authors: Sheng Chunhua , Wang Xiao
Published: September 2008

A preconditioning scheme is applied to a compressible turbomachinery flow solver MSU-TURBO for simulating viscous flows at low Mach number and incompressible region. The Navier-Stokes equations are cast in a non-inertial rotating frame. A constant diagonal preconditioning matrix is applied to the conservative form of the governing equations, which contains a single parameter depending on the reference Mach number and rotational speed of the relative frame. The effect of the rotational speed on preconditioned scheme is numerically investigated for two low speed viscous flows in rotating machinery, a NASA low speed centrifugal compressor (LSCC) and a marine propeller (P5168). Computations are compared against the original MSU-TURBO solutions, and suggestions are provided for computing the low Mach number flows in rotating turbomachinery using the preconditioned TURBO solver.
Upgrade of a 16-Stage Industrial Compressor: Part II — Extension of the Analysis Method to the Design Function and Results
Authors: Attia Magdy S.
Published: September 2008

A retrofit package that includes a slightly larger inlet and new, custom diffusion airfoils (CDA) was designed to replace the 16-stage axial compressor. The method used, and presented here, builds on earlier developments and is an extension of the scheme used to predict the compressor performance (Part I). The use of results from single-row 3D CFD, and their implementation into a streamline curvature (Throughflow) code lead to a better understanding of the compressor performance, which in turn lead to a better model of the compressor. This paper shows how the role of this newly developed model has been modified and adapted to the design environment. The 3D CFD results had previously provided a more accurate representation of deviation and losses, particularly at and near the end walls. The Throughflow code, when re-converged for design purposes, generated a much different solution for the individual streamlines than had been previously calculated using correlation or S1S2 analyses. Consequently, the newly generated boundary conditions for designing the individual stream sections, such as inlet and exit Mach numbers and air angles were also quite different. The designer then embarked on tailoring the individual sections to their respective duties under the guidelines of the newly developed method in true custom diffusion fashion. Iterations were conducted to optimize the section and airfoil shapes taking into consideration 3D effects. The end result was a systematic technique for designing multi-stage axial compressors and generating 3D airfoil shapes. The retrofit compressor upgrade package achieved its performance targets and delivered a measured polytropic efficiency of 93.4%.
Practical Use of Similarity and Scaling Laws for Centrifugal Compressor Design
Authors: Dufour Guillaume , Carbonneau Xavier , Cazalbou Jean-Bernard , Chassaing Patrick
Published: September 2008

This article focuses on the practical use of the similarity principle for centrifugal compressor design, i.e. geometrical scaling of existing impellers to meet new specifications. Basic principles of similarity are first used to derive scaling laws. Then, the analysis of typical specifications and the use of partial similarity (neglecting the Reynolds-number effects) allows the use of the pressure ratio-specific speed diagram so that a compressor can be scaled along its best-efficiency operating line. A practical method is proposed to use these scaling laws in a design context, in order to define the scaling potential of an existing stage as the ensemble of specifications that can be met by scaling. Finally, the scaling potential of an industrial compressor is evaluated and represented as a surface in the 3D space defined by the similarity variables, with the associated efficiency variations.
Effect of Turbulence Intensity and Periodic Unsteady Wake Flow Condition on Boundary Layer Development, Separation, and Re-Attachment Over the Separation Bubble Along the Suction Surface of a Low Pressure Turbine Blade
Authors: O¨ztu¨rk B. , Schobeiri M. T.
Published: September 2008

The paper experimentally investigates the individual and combined effects of periodic unsteady wake flows and freestream turbulence intensity (FSTI) on flow separation along the suction surface of a low pressure turbine blade. The experiments were carried out at a Reynolds number of 110,000 based on the suction surface length and the cascade exit velocity. The experimental matrix includes freestream turbulence intensities of 1.9%, 3.0%, 8.0%, 13.0% and three different unsteady wake frequencies with the steady inlet flow as the reference configuration. Detailed boundary layer measurements are performed along the suction surface of a highly loaded turbine blade with a separation zone. Particular attention is paid to the aerodynamic behavior of the separation zone at different FSTIs at steady and periodic unsteady flow conditions. The objective of the research is (a) to quantify the effect of FSTIs on the dynamics of the separation bubble at steady inlet flow condition, and (b) to investigate the combined effects of FSTI and the unsteady wake flow on the behavior of the separation bubble. The experimental investigations were performed on a large scale, subsonic unsteady turbine cascade research facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL) of Texas A&M University.
Numerical Simulation of a Multistage Radial Inflow Turbine
Authors: Li Yijin , Zheng Qun
Published: September 2008

Aerodynamic performances of a three–stage radial inflow turbine are investigated with numerical simulation. Detailed flow fields, flow patterns, such as secondary flows etc. in the radial turbine impeller are presented and discussed in this paper.
Robust Implicit Multigrid Reynolds-Stress Model Computation of 3-D Turbomachinery Flows
Authors: Gerolymos G. A. , Vallet I.
Published: September 2008

The purpose of this paper is to present a numerical methodology for the computation of complex 3-D turbomachinery flows using advanced multiequation turbulence closures, including full 7-equation Reynolds-stress transport models. A general frame-work describing the turbulence models and possible future improvements is presented. The flow equations are discretized on structured multiblock grids, using an upwind biased (O[Δx3] MUSCL reconstruction) finite-volume scheme. Time-integration uses a local-dual-time-stepping implicit procedure, with internal subiterations. Computational efficiency is achieved by a specific approximate factorization of the implicit subiterations, designed to minimize the computational cost of the turbulence-transport-equations. Convergence is still accelerated using a mean-flow-multigrid full-approximation-scheme method, where multigrid is applied on the mean-flow-variables only. Speed-ups of a factor 3 are obtained using 3 levels of multigrid (fine + 2 coarser grids). Computational examples are presented using several Reynolds-stress model variants (and also a baseline k–ε model), for various turbomachinery configurations, and compared with available experimental measurements.
3D Numerical Analysis of Joule Heating Effect on Electroosmotic Flow in Microchannels
Authors: Monazami Reza , Yazdi Shahrzad , Salehi Mahmoud A.
Published: September 2008

In this paper, a three-dimensional numerical model is developed to analyze the influence of the Joule heating on flow characteristics of an electroosmotic flow through square cross section micro-channels. The governing system of equations consists of three sets of equations: electric potential distribution, flow-field and energy equations. The solution procedure involves three steps. The net charge distribution on the cross section of the micro-channel is computed by solving two-dimensional Poisson-Boltzmann equation using the finite element method. Then, using the computed fluid’s charge distribution, the magnitude of the resulting body force due to interaction of an external electric field with the charged fluid elements is calculated along the micro-channel. Finally, three dimensional coupled Navier-Stokes and energy equations are solved by considering the presence of the electro-kinetic body forces and the volumetric heat generation due to Joule heating for three different external electric field strengths. The results reveal that flow patterns are significantly affected by temperature field distribution caused by Joule heating effect especially for high electric field strength cases.
Cooling of Microelectronics by Shear-Driven Liquid Films
Authors: Kabov Oleg
Published: September 2008

The recent development of microelectronics is closely linked to the problem of thermal regulation. The levels of heat generation in high-speed computer chips are now approaching very high values and they are on the edge of exceeding the capabilities of today’s air-cooling techniques. Thin liquid films may provide very high heat transfer intensity and may be used for cooling of microelectronics. A particularly promising technological solution is a set-up where heat is transferred to a very thin liquid film driven by a forced gas or vapor flow in a micro-channel. However, development such a cooling system requires significant advances in fundamental research, since the stability of joint flow of liquid film and gas is rather complex problem. Flow patterns, heat transfer laws and film rupture mechanisms for shear-driven locally heated liquid film flows remain only partially understood. The paper focuses upon shear-driven liquid film evaporative cooling of high-speed computer chips. The recent progress that has been achieved through conducting theoretical and numerical modeling as well as new experimental data has been discussed.
Effects of Interfacial Interactions Between Fluid and Solid Wall on Microscale Flows
Authors: Liang Xingang
Published: September 2008

This work discusses the interfacial effects on flow and heat transfer at micro/nano scale. Different from bulk cases where interfaces can be simply treated as a boundary, the interfacial effects are not limited to the interface at microscale but extend into a significant, even the whole domain of the flow and heat transfer field when the characteristic size of the domain is close to the mean free path (MFP) of fluid particles. Most of microscale flow phenomena result from interfacial interactions. Any changes in the interactions between the fluid and solid wall particles could affect the flow and heat transfer characteristics, such as flow and temperature profiles, friction coefficient. The interactions depend on many parameters, such as the force between fluid and solid wall particles, microstructure of interfaces. The flow and heat transfer features does not only depend on the fluid itself, but also on the interaction with the solid wall because the interface impact can go deep inside the flow. Same fluid, same channel shape but different wall materials could have different flow characters.
Micro-Scale Analysis, Fabrication and Characterization of Devices in SMAµL IIT Bombay
Authors: Mitra Sushanta K. , Gandhi Prasanna S.
Published: September 2008

Experimental and numerical investigations of liquid flows in the microchannels (50 150 μm) have been carried out for straight and serpentine geometry. CFD-ACE+ is used as numerical tool for analyzing flow through channels with designed roughness elements. μ-PIV is used for characterizing the flow through straight and serpentine sections of the channels. Such laser-based non-intrusive measurement technique is also used to characterize a microfluidic device meant for detection of multiple species. This device is fabricated from a mask using Excimer laser and species detection is achieved by balancing the pressure driven flow with the applied electric field. This device can be used for separation of biological species. In a parallel effort, experimental and numerical investigation of mechanics of affinity cantilevers for biosensor application has been carried out. A new model based on electrostatic repulsion between charged antigens has been proposed. Fabrication of biosensor is carried out on our Excimer laser. A characterization tool, micromap 5010, is used to measure static displacements resulting from bioactivity. A microfabrication facility to fabricate three-dimensional microstructures based on microstereolithography principles has been developed here. This facility will be useful for fabrication and further analysis (using μ-PIV) of flow through complex biological structures. Overall the SMAμL has a solid foundation laid for investigation of complex microchannel flow, heat transfer, biosensors and other MEMS devices.
Hydrodynamic Cavitation and Boiling in Refrigerant (R-123) Flow Inside Microchannels
Authors: Peles Yoav , Schneider Brandon
Published: September 2008

This report investigates the effect that hydrodynamic cavitation has on heat transfer. The fluid medium is refrigerant R-123 flowing through 227 μm hydraulic diameter microchannels. The cavitation is instigated by the inlet orifice. Adiabatic tests were conducted to study the two-phase cavitating flow morphologies and hydrodynamic characteristics of the flow. Diabatic experiments were performed resulting in surface temperatures under heat fluxes up to 213 W/cm2 and mass velocities from 622 kg/m2s to 1368 kg/m2s. Results were compared to non-cavitating flows at the same mass velocities. It was found that the cavitating flows can significantly enhance the heat transfer. The heat transfer coefficient of the cavitating flows was larger than the non-cavitating flows by as much as 84%.
Surface Wettability Effects in Heat and Fluid Flow
Authors: Takata Yasuyuki
Published: September 2008

Effects of surface wettability on liquid-vapor phase change phenomena and single- and two-phase flow in tube have been studied in wide range of contact angles using superhy drophilic (SH) and super-water-repellent (SWR) surfaces. Heat transfer in falling film evaporation on a TiO2-coated SH surface is tremendously enhanced due to very thin stable film. In pool bioling, critical heat flux (CHF) and minimum heat flux (MHF) increase with the decrease in contact angle. Wetting limit temperature of water drop on heated surface increases with the decrease in contact angle. In pool boiling on SWR surface, bubble nucleation and film boiling occur in extremely small superheating. Drag reduction was observed in water flow in tube with SWR coating in laminar flow region, and on the other hand, in two-phase flow pressure drop for the SH wall is smaller than that for the SWR wall.
Integrated Microfluidic-Microoptical Detection Systems Fabricated by Dry Etching of Soda-Lime Glass
Authors: Mazurczyk Radoslaw , Vieillard Julien , Boum Lynda-Lawrence , Bouchard Aude , Chevolot Yann , Cremillieu Pierre , Hannes Benjamin , Krawczyk Stanislas
Published: September 2008

In this paper, a method of fabrication of a microfluidic system, integrated with fluorescence detection optics, is presented. Soda-lime glass is used as a substrate material, in which the waveguiding optics is fabricated by means of an ion exchange process, whereas the microfluidic channel is manufactured utilising reactive ion etching technique. Basic optical characteristics of the device are also reported.
3D Numerical Analysis of Velocity Profiles of PD, EO and Combined PD-EO Flows Through Microchannels
Authors: Yazdi Shahrzad , Monazami Reza , Salehi Mahmoud A.
Published: September 2008

In this paper, a three-dimensional numerical model is developed to analyze flow characteristics of pressure driven, electroosmotic and combined pressure driven-electroosmotic flows through micro-channels. The governing system of equations consists of the electric-field and flow-field equations. The solution procedure involves three steps. The net charge distribution on the cross section of the micro-channel is computed by solving two-dimensional Poisson-Boltzmann equation using the finite element method. Then, using the computed fluid’s charge distribution, the magnitude of the resulting body force due to interaction of an external electric field with the charged fluid is calculated along the micro-channel. Finally, three dimensional Navier-Stokes equations are solved by considering the presence of the electro-kinetic body forces in the flow system for electroosmotic and combined pressure driven electroosmotic flow cases. The results reveal that the flow patterns for combined PD-EO cases are significantly different from the parabolic velocity profile of the laminar pressure-driven flow. The effect of the liquid bulk ionic concentration and the external electric field strength on flow patterns through the square-shaped micro-channels is also investigated over a wide range of external electric field strengths and bulk ionic concentration.
Flow Boiling Study in Mini-Channels
Authors: Madrid Francesc , Caney Nadia , Marty Philippe
Published: September 2008

Mini-channel heat exchangers improve thermal performance in comparison to conventional macro-channel heat exchangers, being highly efficient, compact and requiring low fluid mass. However, classical correlations for two-phase flow in macro-channels fail in predicting the heat transfer coefficient in mini-channels. Therefore, new studies are needed in order to provide better knowledge on flow boiling phenomena in confined spaces. The proposed paper presents an experimental study on two-phase vertical flow boiling in mini-channels. The aim of this work is to determine the heat transfer coefficient and to study the pressure drop in a mini-channel heat exchanger (hydraulic diameter of 840μm) in order to obtain better understanding of the flow boiling mechanisms. A vertical upward flow test section is connected to a primary HFE-7100 circuit. A preheater imposes a given sub-cooled fluid temperature or a given two-phase vapour quality at the inlet. Downstream in the test loop, the fluid is condensed and pumped again into the test section. The pressure drop and the heat transfer coefficient in the test section have been measured for a variety of conditions. Different heat flux, inlet vapour quality and mass flow rate values have been tested. For the heat transfer coefficient, a correlating model is proposed as a function of the superficial velocity. This parameter appears to be much more appropriate than the vapour quality or the mass flow rate for dry-out occurrence prediction. A single critical velocity value has been found.
Numerical Study of the Effect of Surface Tension on Vapor Bubble Growth During Flow Boiling in Microchannels
Authors: Mukherjee Abhijit , Kandlikar Satish G.
Published: September 2008

Microchannel heat sinks typically consist of parallel channels connected through a common header. During flow boiling random temporal and spatial formation of vapor bubbles may lead to reversed flow in certain channels which causing an early CHF condition. Inside the microchannels the liquid surface tension forces is expected to play an important role and impact the vapor bubble growth and corresponding wall heat transfer. In the present study growth of a vapor bubble inside a microchannel during flow boiling is numerically studied by varying the surface tension but keeping the value of contact angle constant. The complete Navier-Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid-vapor interface is captured using the level set technique. The fluid properties used are of water but the surface tension value is varied systematically. The effect of surface tension on bubble growth rate and wall heat transfer is quantified. The results indicate that for the range of parameters investigated surface tension has little influence on bubble growth and wall heat transfer.
Modifications and Extensions to the Annular Flow Model
Authors: Patankar Priyadarshan U. , Puranik Bhalchandra P.
Published: September 2008

Boiling heat transfer to fluid flow in microchannel heat sinks is being looked upon as a promising solution to the problem of cooling microprocessors with large power densities. In the present work, an annular flow model [1] is implemented to investigate the boiling heat transfer and two-phase flow characteristics in microchannel heat sinks. A modification in the model for the deposition mass transfer coefficient is proposed to better compare the existing experimental data [2]. The deposition mass transfer coefficient affects the distribution of liquid in the form of entrained droplets and the liquid film. The liquid film thickness is the most significant parameter in the determination of the heat transfer coefficient. The suggested change ensures consistent results for the behavior of the entrained fraction. We further report pressure drop results obtained using the modified annular flow model and a comparison with existing experimental data. Finally, we present results predicted by the annular flow model for non-uniform heating of a microchannel, in an effort to simulate hot spots on a microprocessor chip. A few preliminary results obtained from the modified model to simulate boiling and two-phase flow in a parallel microchannel device with non-uniform heating are presented.
The Development of an Integrated Fan and Heat Sink Solution for Thermal Management in Low Profile Applications
Authors: Walsh Ed , Grimes Ronan
Published: September 2008

The increasing heat flux densities from portable electronics are leading to new methodologies being implemented to provide thermal management within such devices. Many technologies are under development to transport heat within electronic equipment to allow it to be transported into the surroundings via conduction, natural convection and radiation. Few have considered the approach of implementing a forced convection cooling solution in such devices. This work addresses the potential of a low profile integrated fan and heat sink solution to electronics thermal management issues of the future, particularly focusing upon possible solutions in low profile portable electronics. We investigate two heat sink designs with mini channel features, applicable to low profile applications. The thermal performance of the heat sinks is seen to differ by approximately 40% and highlights the importance of efficient heat sink design at this scale.
Investigating Kinetics of Temperature-Dependent Processes Using Microfluidics
Authors: Laval Philippe , Salmon Jean-Baptiste , Cristobal Galder , Joanicot Mathieu
Published: September 2008

We have developed an original microfluidic system to study fast kinetics of temperature-dependent processes in an emulsion. Using flow focusing geometries, aqueous droplets are continuously formed in an oil flow. These droplets, acting as microreactors (100 nL), contain the solution to be investigated, and are formed at high temperature. They flow in a microchannel to a cooled area through a controlled temperature gradient (typically from 60 to 10°C in a few seconds). Along the microchannel, the distance being equivalent to the time thanks to the use of droplets, the kinetics of the process can be followed (from 10 to 300 s). In particular, the microdevice has been used to study the kinetics of crystal nucleation of a solute dispersed in water after a temperature quench.
The Development of Integrated Microfluidic Chemistry Platforms for Lead Optimisation in the Pharmaceutical Industry
Authors: Fernandez-Suarez Miryam , Garcia-Egido Eduardo , Montembault Mickael , Chapela Maria J. , Wong-Hawkes Stephanie Y. F.
Published: September 2008

During the last decade, GlaxoSmithKline Pharmaceuticals (GSK) has carried out much of the seminal work in the area of micro fluidics and micro flow assay for lead optimisation. It has pioneered and built an in-house micro fluidic system for drug discovery. The huge and diverse advantages of this approach come from its miniaturised nature and its scale, which makes it easily automatable. As a result of its miniaturised nature it allows for greater control over heat and mass transfer, along with lower consumption of reagents (both chemical and biological) and solvents, less waste generation and decreased exposure to potentially toxic materials. But for a pharmaceutical company, the main advantage of this technology is the capability of coupling a fast microfluidic chemistry generator with a modern compatible miniaturised screening technique to generate instant biological information (i.e. the assay results) in “real time” that can be used to refine the chemistry (closing the feedback loop) and therefore allowing for a much faster lead optimisation. We will review some of the efforts within GSK towards this pioneering work in the development of miniaturised chemistry platforms capable of performing multiple functions such as synthesis, separation, quantification and screening.
Experimental Evaluation of Marangoni Shear in the Contact Line Region of an Evaporating 99+ % Pure Octane Meniscus
Authors: Panchamgam Sashidhar S. , Plawsky Joel L. , Wayner Peter C.
Published: September 2008

Image analyzing interferometry was used to study the spreading characteristics of an evaporating meniscus containing octane (purity: 99+ %) on a quartz surface. The thickness and curvature profiles in the contact line region of the meniscus were obtained using a microscopic data analysis procedure. The results obtained for the octane were compared with that of pure pentane under similar operating conditions. Isothermal experimental conditions of the meniscus were used for the in-situ estimation of the retarded dispersion constant. The experimental results for the pure pentane demonstrate that the disjoining pressure (the intermolecular interactions) in the thin film region controls the fluid flow. Also, an imbalance between the disjoining pressure in the thin film region and the capillary pressure in the thicker meniscus region resulted in a creeping evaporating pentane meniscus, which spread over the solid (quartz) surface. On the contrary, for less pure octane, the intermolecular interactions between octane and quartz had a significantly different contribution for fluid flow and hence the octane meniscus of lower purity did not creep over the quartz surface. As a result, we had a stationary, evaporating octane meniscus. Using the experimental data and a simple model for velocity distribution, we evaluated the Marangoni shear in a portion of the stationary, evaporating octane meniscus. An extremely small change in the concentration due to distillation had a significant effect on fluid flow and microscale heat transfer. Also, it was found that non-idealities in small interfacial systems, i.e. the presence of impurities in the working fluid, can have a significant effect on the thickness of the adsorbed film and therefore the spreading characteristics of an almost pure octane meniscus.