bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of Surface-Fluid Electrokinetic Coupling on the Laminar Flow Friction Factor in Microtubes
Authors: Brutin D. , Tadrist L.
Published: March 2009

The present work’s originality lies in the evidence of a non negligible effect of the fluid ions’ and co-ions’ interaction with the wall surface in a microtube. This study is based on the EDL theory (Electrical Double Layer) which is developed here for a circular geometry. High electrical surface potentials are taken into account for the present study; they induce the nonlinearity of the problem’s main equation (Poisson-Boltzmann equation). The electrical field is determined, then the velocity profile and finally the Poiseuille number. We show that even with the EDL effect taken into account, the Poiseuille number does not depend on the mean velocity. Our model agrees with the experimental results for high surface potentials (> 25 mV). This is found by comparing with experiments previously carried out with microtubes ranging from 530 to 50 μm.
Examination of the LBM in Simulation of Microchannel Flow in Transitional Regime
Authors: Shen Ching , Tian Dong-Bo , Xie Chong , Fan Jing
Published: March 2009

Gas flows in micro-electro-mechanical systems (MEMS) owing to the small size of the systems possess a relatively large Knuden number and usually belong to the slip and transitional flow regimes. This paper employs three schemes, namely the direct simulation Monte Carlo (DSMC) method, information preservation (IP) method, and the lattice Boltzmann method (LBM), to simulation micro-channel flows at three Knudsen numbers (Kn) of 0.0194, 0.194 and 0.388. The present LBM results are in agreement with those given by Nie et al. (2002), whereas they significantly differ from the DSMC (and IP) results as Kn increases. This suggests that the present version of LBM is not feasible to simulate the micro-channel flows in transition regime.
Statistical Simulation of Non-Circular Cross Section Poiseuille Flows
Authors: Jiang Jian-Zheng , Shen Ching , Fan Jing
Published: March 2009

This paper investigates the Poiseuille flows for rectangular, regular hexagonal, and semicircular cross sections in transition regime using particle approaches, namely the direct simulation Monte Carlo (DSMC) method and the information preservation (IP) method. The DSMC and IP results compare well with each other, while the IP method is much more computationally efficient than the DSMC method. The mass flow rates given by IP and DSMC are in agreement with the BGK solutions of Hasegawa and Sone. For rectangular cross sections in the wide range of the width-to-height ratio, the simulation results of the mass flow rates and the velocity profiles along the midperpendicular line have been given for both methods to estimate the lateral wall influence. For the physical quantities, such as the mass flow rate, which are influenced by the whole field, the lateral wall influence must be considered even for width-to-height ratio as large as 10. And for the physical parameters, such as the maximum velocity and the velocity profile along the midperpendicular line, the lateral wall influence can be negligible if the width-to-height ratio is bigger than 5.
Falling Films in Micro- and Minigrooves: Heat Transfer and Flow Stability
Authors: Gambaryan-Roisman Tatiana , Stephan Peter
Published: March 2009

Structured (in particular, micro- and minigrooved) wall surfaces may improve numerous industrial processes, including falling film evaporation, thin film evaporation in lean premixed prevaporized combustion technology (LPP), and spray and jet cooling. The advantages of such surfaces include the promotion of ultra-thin film evaporation at the apparent contact lines and the prevention of dry patches on hot surfaces. However, the behavior of thin film flow on structured surfaces has not yet been comprehensively studied. We derive a model describing the heat transfer in liquid film flowing down inclined micro- or minigrooved walls. The derived model accounts for peculiarities of the evaporation process in the vicinity of the liquid-vapor-solid contact line (“micro region”) and their effect on the overall heat transfer rate. It is shown that the effect of the micro region is to increase the overall heat transfer rate at the constant fluid flow rate. A long-wave stability analysis has been performed to quantify the effect of the capillary structure on the film stability properties. Sinusoidal and triangular longitudinal groove shapes have been considered. Two cases have been studied: (i) the film completely covers the wall structure; (ii) the film partly covers the wall structure. It is shown that the longitudinal grooves completely covered by the liquid have a stabilizing effect on the falling film flow. The performed analysis is a step towards modeling the wavy motion of the liquid film on grooved surfaces.
Two-Phase Flow Patterns in a Square Micro-Channel
Authors: Zhao Jian-Fu , Li Bin
Published: March 2009

A new set of experimental data of two-phase air-water flow patterns in a square micro-channel is presented. The channel has a cross-section of 1×1 mm2 and a length of 300 mm. The ranges of the gas and liquid superficial velocities are 0.1–10 m/s and 0.2–7 m/s, respectively. Bubble, bubbleslug transitional, slug, and frothy patterns are observed. The present data are compared with other experimental data reported in the literature, and a good agreement is obtained. It is also compared the present data with those obtained from reduced gravity experiments, in which the Bond number has the same order of magnitude. Some problems associated with the micro-scale modeling of microgravity two-phase flow are also discussed.
Influence of Vertical Return Bend on the Two-Phase Flow Pattern in 3-mm Minichannel
Authors: Chen Ing Youn , Wang Chi-Chung , Huang Pi-Shan , Yang Bing-Chwen , Chang Yu-Juei
Published: March 2009

This study provides a qualitatively visual observation of the two-phase flow patterns for air-water mixtures inside a 3 mm smooth tube with the presence of vertical return bend. The curvature ratio (2R/D) is 3.2 whereas the total mass flux is from 70 to 800 kg/m2 s. The flow can be either entering from the upper of the tube or from the lower tube. However, it is found that there is no great difference between those flow entering at the upper tube and that of the lower tube if the inlet mass flux and vapor quality is the same. For a mass flux of 70 kg/m2 s at a vapor quality to 0.009, as the flow is approaching the return bend, one can observe a fluctuating phenomenon at the tail of the long slug that leads to a liquid ripple around the periphery. When the air slug is trying to penetrate the preceding liquid in the return bend, the shape at the front of the air slug was sharpened. A further increase of the vapor quality to 0.05, the flow after the return bend was temporarily turned from stratified flow into the annular flow. At a higher mass flux of 300 kg/m2 s, unlike those flow pattern at 70 kg/m2 s, the increase of the vapor shear interacts with the centrifugal force and the accumulated liquid within the return bend forces the Taylor bubble to be completely disordered. There is no separating and re-merging phenomenon of the air slug for the slug flow pattern across the return bend even for a very low vapor quality of 0.001. This is quite different from those with larger diameter tube (Chen et al. 2002, Wang et al. 2003b, 2003c).
Experimental Investigation of the Effect of Surface Tension and the Correlation on Void Fraction and Frictional Pressure Drop of an Air-Liquid Two-Phase Flow in a Horizontal Flat Capillary Rectangular Channel
Authors: Ide Hideo , Fukano Tohru
Published: March 2009

Air-liquid two-phase flow in a horizontal flat capillary rectangular channel has been studied to clarify the effects of concentration of surfactant solution on the flow phenomena, such as flow patterns, pressure drop, void fraction and so on. The concentrations of surfactant solution were 0, 10, 50 and 100 ppm and the surface tension of each solution was reduced to about 34mN/m from that of pure water of about 72mN/m. The dimension of the channel used was 10.0 mm × 1.0 mm. The drag reduction by mixing the surfactant was examined in both the single phase flow and the two-phase flow. The experimental data of two-phase frictional pressure drop and holdup were compared with the respective correlations which were previously proposed by the other researchers and the present authors. Finally, we proposed new correlations of two-phase frictional pressure drop and holdup in which the effect of surface tension and the aspect ratio of cross section of channel were taken into account.
Measurement on Liquid Film in Microchannels Using Laser Focus Displacement Meter
Authors: Fukamachi Norihiro , Hazuku Tatsuya , Takamasa Tomoji , Hibiki Takashi , Ishii Mamoru
Published: March 2009

To elucidate details of the fascinating nonlinear phenomena of gas-liquid interfaces in micro- and minichannels, precise spatiotemporal knowledge of the interface in gas-liquid two-phase flows is essential. This paper presents a new method for measuring the interface of a liquid film in microchannels using a laser focus displacement meter (LFD). The purpose of the study was to clarify the effectiveness of the new method for obtaining detailed information concerning interface displacement, especially in the case of a thin liquid film, in micro- and mini-channels. In the test, water and nitrogen gas were used as working fluids. To prevent the tube wall signal from disturbing that of the gas-liquid interface, a fluorocarbon tube with water box was used; the refraction index of this device was the same as that for water. With this method, accurate measurements of the liquid film interface were achieved in real time, with a sensitivity of 0.2 μm and 1 kHz. The error caused by refraction of the laser beam passing through the acrylic water box and fluorocarbon tube was estimated theoretically and experimentally. The formulated theoretical equation can derive the real interface displacement using measured displacement in a fluorocarbon tube of 25 μm to 2.0 mm I.D. A preliminary test using fluorocarbon tubes of 1 and 2 mm I.D. showed that the corrected interface displacement calculated by the equation agreed with real displacement within a 1% margin of error. We made simultaneous measurements of the interface in fluorocarbon tubes of 0.5 and 1 mm I.D. using the LFD and a high-speed video camera with a microscope. These showed that the LFD could measure the interface of a liquid film with high spatial and temporal resolution during annular, slug, and piston flow regimes. The data also clarified the existence of a thin liquid film less than 1 μm in thickness in slug and annular flow regimes.
Two-Phase Boiling Flow in Microchannels: Instabilities Issues and Flow Regime Mapping
Authors: Peles Y.
Published: March 2009

The design, fabrication, and evaluation of forced convection boiling two-phase flow, micro scale heat exchanger are described. The micro heat exchanger consist of a heater, and 16 mm long multiple parallel triangular micro channels, with hydraulic diameter ranging from 50 μm to 200 μm. The system allowed simultaneously visualizing the flow regime, to measure the resistor temperature distribution, the pressure drop, and input power levels. Experiments were conducted using water with mass flow rate of 1–10 g/min and heat fluxes ranging from 10–60 W/cm2 in order to better understand the flow mechanism associated with micro scale forced convection boiling two-phase flow. The pressure drop, temperature, fluctuation and flow regimes map were obtained. The results are consistent with newly reported studies and show that flow fluctuation at that scale is exaggerated with respect to conventional size channels. A unique flow regime was observed and was named “rapid bubble growth”.
Extending the Applicability of the Flow Boiling Correlation to Low Reynolds Number Flows in Microchannels
Authors: Kandlikar Satish , Balasubramanian Prabhu
Published: March 2009

As microchannels are applied in flow boiling applications, it is becoming apparent that the Reynolds number based on all liquid flow could approach values below 100. The earlier work by Kandlikar and Steinke (2002, 2003) provided modifications to the Kandlikar correlation (1990, 1991) by extending the range of the correlation to all-liquid Reynolds numbers in the range 1000–3000. The present work utilizes the newly available data on flow boiling in microchannels that cover the all-liquid flow Reynolds number between 50–500. A new correlation is developed in this range that is able to predict the flow boiling heat transfer coefficient and its trends with quality, heat flux and mass flux accurately within less than 15 percent mean deviation. It is noted that the correlation simply accounts for the change of the flow boiling mechanism without incorporating any additional empirical constants. The heat transfer mechanism during flow boiling at such low Reynolds numbers is altered considerably indicating strong presence of nucleate boiling mode of heat transfer.
Pressure Drop in a Capillary Tube in Boiling Two-Phase Flow
Authors: Kaminaga Fumito , Sumith Baduge , Matsumura Kunihito
Published: March 2009

Two-phase pressure drop is experimentally examined in a flow boiling condition in a tube of diameter 1.45 mm using water in ranges of pressure from 10 to 100 kPa, mass flux from 18 to 152 kg/m2s, heat flux from 13 to 646 kW/m2, and exit quality from 0.02 to 0.77. Also, pressure drop in an adiabatic air-water two-phase flow is measured at atmospheric pressure using the same test section and mass flux ranges of liquid and gas as those in the flow boiling. Decreasing system pressure the pressure drop significantly increases at a given mass flux. Influence of vapor phase on the pressure drop is found to be large both in the adiabatic and the diabatic conditions. The frictional pressure drop correlation for the adiabatic two-phase flow is developed and applied to predict pressure drop in the flow boiling. But it cannot give satisfactory predictions. The Chisholm correlation calculating a two-phase pressure drop multiplier is modified to account the influence of vapor phase in a capillary tube and the modified correlation can predict the pressure drop in the flow boiling within an error of 20%.
An Extra Mode of Enhanced Heat Transfer by Oscillating Bubbles in Minichannels and Microchannels
Authors: Schro¨der J. J. , Alraun S.
Published: March 2009

Experimental investigations on heat transfer in tubular micro- or minichannel arrangements more often report on two-phase flow instabilities, pulsations or oscillations, which result in a remarkable influence on heat transfer efficiency. In order to explain the piston-like oscillations of the steam-plugs and water-slugs (-columns), the authors studied the somehow similar process which occurs in the worldwide known toy steam boat. Experiments have been performed which used a demonstration plant made of glass. By controlled electrical heating, high-speed video, pressure and local temperature measurements, the paths of energy have been disclosed. The results are as surprising as the effect of making gold from sand with respect to an equivalent axial heat-conductivity of the water-filled glass tube. Initiated by these results, an abstracting model is presented that analytically quantifies this new regenerating (oscillating and conducting) heat transfer mode e.g. concerning the combination of a heat recharging tube wall and an oscillating water column in a field of diminishing temperatures between the temperature of the boiler surface and the subcooled bulk water. By introducing these heat transfer details, the steam boat can give an answer, not only on frequency and amplitude of the oscillations, but on the steady state conditions for — or time-dependency of — the location of zero-crossing as well. Experimental results and model calculations are in good agreement and need no fitting factors. This is the base to discuss that process along with its physical parameters and compare it to the above mentioned observations in flow-boilers or pulsating heat pipes etc. which use microchannels or minichannels.
Upflow Boiling and Condensation in Rectangular Minichannels
Authors: Kuznetsov V. V. , Dimov S. V. , Houghton P. A. , Shamirzaev A. S. , Sunder S.
Published: March 2009

When boiling or condensation occurs inside very small and non-circular channels, capillary forces influence two-phase flow patterns, which in turn determine heat transfer coefficients and pressure drop. A better understanding of the underlying phenomena would be beneficial from the perspective of optimizing the design of compact evaporators and condensers. The thrust of this study was to understand the nature of up-flow boiling and condensation heat transfer in channels with a small gap. It consisted of two parts. The first part included observation of two-phase flow patterns with refrigerant R21 in a test section containing plain fins. The shape of the channels formed between fins was close to rectangular. The test section was placed in a closed refrigerant loop, and it was fabricated with a transparent wall to allow observation of the flow. An electrically heated coil was used to introduce liquid and vapor at the needed quality into the test section. Regimes of slug, froth, annular and cell flow patterns were recognized and the areas of flow pattern were determined. The second part included up-flow boiling and condensation heat transfer measurement with refrigerant R21 in a set of vertical mini-channels consisting of plain fins. An aluminum fin pad was bonded to two dividing aluminum sheets by dip brazing. Heat was supplied to the test section from a thermoelectric module, which utilized the Peltier effect. A thick copper plate was placed between the dividing sheet on each side of the fin passage and the respective Peltier module to establish a uniform wall temperature. Heat transfer coefficient measurements were done under forced flow conditions. Data are obtained for mass flow rates of 30 and 50 kg/m2s under both boiling and condensation modes with wall superheats ranging from 1 to 5K. The dependence of heat transfer coefficient from wall superheat was not observed both for boiling and condensing modes. It shows the primary role of evaporation from thin films in a confined space when the mass flux is small. At low vapor quality the boiling heat transfer coefficients are considerably higher than that for condensation. A high heat flux in ultra thin liquid film area near the channel corner or in the vicinity of liquid-vapor-solid contact line (after the film rupture) supports the high total heat transfer coefficient in evaporation mode. In contrast with evaporation mode, at upflow condensation mode the heat transfer coefficient is strongly dependent on vapor quality. At plug flow regime the vapor velocity determines the condensing heat transfer.
Experimental Investigation on Condensation Heat Transfer Coefficient Inside Multi-Port Minichannels
Authors: Cavallini A. , Censi G. , Del Col D. , Doretti L. , Longo G. A. , Rossetto L. , Zilio C.
Published: March 2009

Very little experimental information is available in the open literature about condensation inside minichannels. Most of the experimental work has been carried out by using the Wilson plot technique. This method is simple to implement because it does not require the direct measurement of the tube wall temperature. However it becomes inaccurate when a small thermal resistance is present on the test side as compared to the opposite (cooling) side, which is actually the case with a multichannel tube at high values of the internal heat transfer coefficient. In fact, in a multi-port tube internal webs work as fins, and their efficiency is close to unity; thus the internal heat transfer area is higher than the external one. In this paper a new technique to measure the heat transfer coefficient during condensation inside a multi-port extruded minichannel tube is presented. Some R134a preliminary data is also reported.
Design Analysis of a 3-D, Ultra-High Performance, Scalable, Micro Convective Heat Sink With NPCM
Authors: Tao Y.-X. , Moreno R. , Hao Y.
Published: March 2009

The paper proposes a new design of a scalable, heat sink containing 3-D micro/nano network, utilizing liquid mixed with nano phase change materials (NPCM) and having a high surface-to-volume ratio geometry. The conceptual design is capable of reaching 105 W/cm3 using encapsulated nano-size phase change materials, which would result in an order of magnitude higher cooling capacity than typical microchannel heat sink of the same volume and same pumping power. It is also scalable to submicron range, resulting even higher cooling capacity. An analysis for a working model (10 × 10 × 1 mm) is presented utilizing energy conservation principle and uniform temperature and uniform heat flux boundary conditions. The average phase change heat transfer coefficient is obtained using the numerical model results. A process of micro electrochemical deposition to fabricate the target model is illustrated, and the issues associated with system-level applications are discussed.
Heat Pipe Performance Enhancement for Microelectronics Cooling
Authors: Buffone C. , Buffone L. , Sefiane K. , Lin S. , Bradley M.
Published: March 2009

The heat dissipated by convection from the fins of the heat pipe condenser section is strongly limited by the thermal barrier of the oxide layer formed on their aluminum surface. A lot of work has been done to enhance the heat transfer coefficient of this heat pipe section by changing the fins roughness. The present experimental study demonstrates the enhancement in heat transfer coefficient by applying a more conductive coating on the condenser fins surface. A comparison between a conventional technique consisting of applying a rougher surface and this new technique is performed. Results clearly show the performance of the heat pipe exhibits a better enhancement in the case of a more conductive coating than a rougher one. The orientation of the heat pipe is also investigated to demonstrate the effect of gravity on the enhancement so observed. Hydrodynamics inside the heat pipe is considered to explain the findings.
Experiments About Simple Liquid Flows in Microtubes
Authors: Li Zhan-Hua , Cui Hai-Hang
Published: March 2009

The behavior of micro-scale flow is significant for the performance of Micro-Electro-Mechanical-Systems (MEMS) devices. Some experiments about liquid flow through microtubes with diameters about 3∼20μm are presented here. The liquids used in our experiments include some simple liquids with small molecules, such as non-ion water and several kinds of organic liquids (CCL4, C6H5C2H5 and Isopropanol etc.). The flow rate and the normalized friction coefficients were measured in micro-flow experimental apparatus. The results show that when the driven pressure varies from 0 to 1Mpa, the flow behaviors in 20μm microtube for both polar and non-polar liquids are in agreement with Hagen-Poiseuille law of the classical theory. It means that N-S equation based on continuous medium still acts well in this case. For higher pressure drop from 1 to 30Mpa, in the microtubes with diameter of 3∼10μm, the normalized friction coefficients of organic liquids can’t keep constant with pressure increases. However the non-ion water reveals different trends.
Ink Jet Processing of Metallic Nanoparticle Suspensions for Electronic Circuitry Fabrication
Authors: Szczech John B. , Megaridis Constantine M. , Zhang Jie , Gamota Daniel
Published: March 2009

A novel approach in creating circuit electrodes with features as fine as 100 μm is demonstrated using a single 38 μm diameter orifice, piezoelectrically driven print head to deposit metallic nanoparticle suspensions. The suspensions consist of gold particles of ∼20 nm diameter suspended in toluene solvent. The amount of gold nanoparticles present in the suspension is 30% wt. Inductor and capacitor electrode patterns are deposited onto a glass substrate and thermally processed at 300°C for 15 minutes to drive off the solvent and allow the nanoparticles to sinter, thereby yielding a conductive path with a resistivity of O(10−7) Ω m.
Rapid Design, Fabrication and Optimization of a Single Event Thermo-Pneumatic Microactuation for the Delivery of Minute Amounts of Liquids
Authors: Cardenas-Valencia Andres M. , Fries David P. , Langebrake Larry C. , Benson Robert F.
Published: March 2009

The need for efficient metering control of liquids in small devices has led to a boom in the advent of different micro-fluidic actuation mechanisms. Here we present a brief study on a thermal-pneumatic actuation mechanism for an on-demand delivery of minute amounts of liquids. A closely coupled, iterative design-fabrication strategy is used for optimization of a system in which no freely moving membranes are included. Special consideration was given to the heating device, minimizing the energy consumed. The fabrication method and performance of two types of fabricated resistors are compared herein. The first, a conventional Nickel-Chromium resistor using, lift-off micro-fabrication techniques, was initially tested. The second, a Copper cladded liquid crystal polymer in conjunction with a novel mask-less patterning system was used to produce nonconventional heating micro-ohmic heaters. The heating efficiency was proven to be superior using the latter approach. Various micro-fabricated fluidic devices have been designed as case studies and have been fabricated and integrated using a variety of materials to illustrate the functionality of the approach. The combination of design and fabrication steps, the simplicity of the resistive device, and the materials selected combined, yield a direct path to making fluidic transport devices for micro-analytical and power systems.
Theoretical and Experimental Study of Microchannel Blockage Phenomena
Authors: Yamaguchi Eiichiro , Adrian Ronald J.
Published: March 2009

Microchannel blockage phenomena by hard, spherical particles have been investigated experimentally and theoretically. The study was performed over a range of particle-to-channel diameter ratios of 0.14 < R < 0.65. Two mechanisms have been investigated: orthokinetic flocculation and hydrokinetic arching. Arching appears to be the main mechanism for large, hard particles. In the absence of Brownian motion and inter-particle repulsive forces, other than simple Hertzian contact force, the blockage phenomenon is described by three non-dimensional parameters, N, R and β. The mean total number of particles in the channel having length L is N. Ratio of a diameter of particle (dp) and a diameter of channel (D) is R. Blockage efficiency factor β is determined experimentally. The data shows that a critical value Nc exists as a function of R. N > Nc implies high likelihood of blockage; if N < Nc, blockages were never observed. The critical number decreases dramatically with increasing R. Blockages can occur at surprisingly low values of the volume concentration (φ). The experimental results matches well with the theory for the combinations of straight glass capillary, 76 < D < 156μm, 100mm-length, and spherical polymer particle, 22 < dp < 48μm.
Scaling the Performance of Micro-Fans
Authors: Grimes R. , Walsh E. , Kunz S. , Davies M. , Quin D.
Published: March 2009

Pumping of liquids and gases in micro fluidic systems has been the focus of much attention in recent times. Miniaturisation of traditional rotating pumps such as axial and radial flow designs, has been limited by the fabrication techniques employed. As a result of these limitations, the geometry of the majority of rotating micro pumps has been two-dimensional. This paper addresses issues of scaling in micro axial flow fans. The anticipated primary application will be in cooling compact electronic systems, but the results are applicable to a much wider range of pumping applications. Using novel fabrication techniques a series of geometrically similar three dimensional fans were fabricated, ranging in size from the macro to the micro scale. Experimental techniques are described which will be used for the characterisation of these fans. A scaling analysis is used to show how reduced fan scale causes increased local loss as fan dimensions are reduced to the micro scale. Numerical simulations of flow in the channels between the fan blades were performed to investigate the validity of the scaling theory, the results of which give confidence in the scaling analysis. The fundamental finding of this work is that a reduction in scale is accompanied by a reduction in efficiency and thus fan performance.
Liquid Mixing in Static Micro Mixers With Various Cross Sections
Authors: Kockmann Norbert , Engler Michael , Fo¨ll Claus , Woias Peter
Published: March 2009

Micro mixers are an integral part of several micro fluidic devices like micro reactors or analytical equipment. Due to the small dimensions, laminar flow is expected a priori in those devices while the mass transfer is supposed to be dominated by diffusion. A detailed numerical CFD-study by CFDRC-ACE+ of simple static mixers shows a significant deviation from strictly laminar flow in a wide range of Reynolds numbers Re, channel dimensions, and types of cross sections (square, rectangular, trapezoidal). With increasing flow velocity and Re number the flow starts to form vortexes at the entrance of the mixing channel. The vortexes are symmetrical to the symmetry planes of the mixing channel, both for the rectangular and the trapezoidal cross sections investigated here. With further increasing velocity the flow tends to instabilities, which causes a breakup of the flow symmetry. These instabilities are generally found in T-shape mixers with symmetrical flow conditions, but not always in Y-shape mixers or with asymmetrical flow conditions. Within the laminar flow regime diffusive mass transfer is dominant. In this case the mixing quality at constant channel length becomes worse with increasing velocity. This effect can almost be equalized by the onset of the vortex regime, which enhances the mass transfer by convective transport. This paper shows the mixing quality at a certain length for different geometrical parameters and flow conditions.
Comparison of Heat Conduction and Radiation of Nano-Size Gaps
Authors: Liang Xin-Gang , Han Mao-Hua
Published: March 2009

Gas gaps are common structures in many sensors and MEMS. It is usually regarded that heat conduction plays a dominant role when the gap size reduces and thermal radiation between surfaces is negligible. This work compares the heat dissipated by heat conduction with that by thermal radiation under different temperatures and on various scale levels. It is found that the heat flux by thermal radiation can exceed that by heat conduction. Furthermore, a regime map is plotted to recognize the relative importance of heat conduction and thermal radiation at different gap size and temperature. The impact of the thermal accommodation efficient for gas is also discussed.
Peterhead Power Station: Parallel Repowering Innovative Steam Turbine Enhancement
Authors: Depolt Thomas , Gobrecht Edwin , Musch Gu¨nter
Published: March 2009

In the year 2000 one of Europe’s most flexible power stations was commissioned by the authors’ company. The existing fossil fired power station was modified by a “Parallel Repowering”. With that concept three gas turbines (GT) in combination with three heat recovery steam generators (HRSG) were tied-in additionally to the fired boiler. This concept is compelling especially for large steam power plants and offers more flexibility than “Full Repowering” in matching GTs with the existing steam turbine (ST). The key to maintaining reliability of the repowered unit is the ST modernisation. Plant operability enhancements provide the flexibility of the fired boiler and ST for load following and peaking purposes. The authors’ company was responsible for the complete conversion of the fossil fired power station into a modern combined cycle unit. This comprises the tie-in of new steam pipes, bypass stations and the upgrade of the steam turbine auxiliaries as well as the implementation of a new automation system parallel to the existing one. The “Parallel Repowering” offers a maximum of operation variations: •Conventional (Rakine cycle) mode. •Open cycle mode (only GT). •Combined cycle mode. •Hybrid mode. The non-OEM steam turbine needed to be modified for the combined cycle operation with GTs. The condenser load had to be kept as low as possible because of the existing condenser design. Auxiliary systems like the gland steam system and the drain system had to be modified for all different operating modes. Special design features, like the IP rotor cooling system and the flange heating system, had to be extended to operate under all circumstances. One essential difference to the existing operational mode is the necessity of a steam bypass operation. Existing cold reheat (CRH) piping is of carbon steel, so the ST needs to be started with an isolated HP cylinder. The following modifications for the HP turbine were necessary: •For the isolated HP cylinder operation non-return valves (NRV) were built into the CRH line at the HP turbine exhaust. •The HP cylinder will be automatically isolated by closure of the HP valves and the non-return valves in the CRH line, and the simultaneous opening of the HP vent line. •As no instrumentation was available for a reliable monitoring of the isolated operation, a controlled reverse flow from the CRH to the HP vent line was established. •The HP cylinder evacuation is controlled by a dedicated control logic.
Analysis of Effects on Wet Compression on Surge Margin of a Small Gas Turbine
Authors: Wang Yunhui , Wang Guoxue , Li Shuying , Sun Yufen
Published: March 2009

Using energy transformation method, the characteristic map of the compressor of S1A-02 gas turbine is gained. And based on this work, the effects of wet compression on compressor’s steady-state performance are studied. Simulations show that pressure-ratio increased and surge line moved up-left relative to the original one when wet compression used with the assumption that during compression process the amount of energy transferred is a function of mass flow only, which means no matter wet compression is used or not, the amount of energy transferred is the same for any given mass flow. Experiment results show the same tendency as simulation results.
Gas Turbine Exhaust Energy Margin in a Combined Cycle Power Plant
Authors: Johnston Paul B.
Published: March 2009

Margining gas turbine exhaust energy exposes the EPC (Engineering, Procurement & Construction) contractor to risk when developing overall plant performance guarantees. The objective of this paper is to explain the nature of this risk, recognize its significance and propose ways of mitigation. Sharing risk between the Developer and the contractor should be apportioned to maximize value for the project. Attention is focused on 2 on 1 combined cycle power plants, but the results are relevant for all types of gas turbine based power and cogeneration facilities. Risk mitigation alternatives discussed include both assessment of margins to the bottomline performance and the application of performance corrections at the time of field testing. Allowing for corrections leads to enhanced overall plant performance guarantees.
Centrifugal Compressors as a Viable Method for Fuel Gas Boosting
Authors: Kabir Omar M. , Stewart Keith
Published: March 2009

This paper defines major advantages for using centrifugal compressors as fuel gas boosters for gas turbines. The first aspect of this paper defines the technical differences between centrifugal compressors and reciprocating compressors as the fuel gas booster. The second aspect of this paper defines the uniqueness and technical advantages of state-of-the-art centrifugal compressors, which are applied for this service.
Economical and Environmental Aspects of the Microturbine’s Application in Brazil
Authors: Batista Gomes Eli Eber , Melian Cobas Vladimir Rafael , Rosa do Nascimento Marco Antoˆnio , Silva Lora Electo Eduardo
Published: March 2009

Microturbine generators have shown good perspectives for small scale distributed generation. In Brazil, the Thermal Systems Study Group of Federal School of Engineering of Itajuba´, sponsored by the CEMIG (Electrical Utility of Minas Gerais), is developing a project about experimental evaluation of microturbines systems. The objective of this paper is to evaluate the cost of generating electricity with microturbines and show the emissions range of microturbines operating with natural gas. The cost of the microturbine generators as well as the cost of the electricity and natural gas in Brazil at this moment and the projection for the future were considered.
Effect of H2 Addition on Soot Formation in Fuel-Rich CH4/Air Turbulent Diffusion Flames
Authors: Ochi Takeshi , Arai Norio , Furuhata Tomohiko , Kishi Naoki
Published: March 2009

In this study, the difference of temperature and the component of chemical species on soot formation in CH4/air fuel-rich diffusion flames were investigated. Furthermore, for decreasing soot formation in fuel-rich diffusion flames, we added H2 in CH4, investigated the property of combustion, and compared with methane/air fuel-rich flames. We have paid much attention to the influence of the equivalence ratio of methane (+H2)/air, the swirl strength of combustion air and the concentration of C2H2 on the soot formation. The experimental combustor for CH4(+H2)/air combustion was designed, and the soot resulting from the exhaust gas collected with a silica filter and its weight was measured. The microstructure of the soot particles were analyzed with a Scanning Electron Microscopy (SEM). The temperature profiles in the combustor were measured by thermocouples, and the concentrations of the species O2, CO2, H2, CH4 and C2H2 were determined by a TCD (thermal conductivity detector) gas chromatograph (GC) and FID (flame ionization detector) GC. The soot yields diminished with increasing swirl strength and the C2H2 concentration. When H2 was added to the fuel, combustion was promoted and C2H2 concentration in the exhaust gas was diminished. But using strong swirl, fuel and air were mixed quickly, the effect of H2 addition was decreased.
A Study of Integrity Changes in Structures During Exploitation
Authors: Sebastijanovic Slavko , Opalic Milan , Sebastijanovic Nebojsa
Published: March 2009

Spherical and cylindrical pressure vessels are manufactured as welded structures, where cracks could be initiated/propagated during fabrication, pre-service hydro-test, service itself, service welding repair, or during the service hydro-test. In this case, fracture mechanics approach is necessary. This paper analyzes the stress state around cracks in the bottom head which is one of the main components in a cylindrical pressure vessel (radius of 1056 mm, wall thickness of 92 mm, pressure of 15.5 MPa and temperature of 454°C). Sizes of several detected cracks at the inner surface are determined by the NDE methods. Finite element analysis is performed to determine the stress zones in the vicinity of the most critical crack. Such analysis is done before and after the hydro-test. It will show the influence of the hydro-test on propagation of the existing cracks and fracture behavior of repaired cracks through the stress state analysis. Changes in material properties were analyzed. Results will be used to assess the pressure vessel integrity and estimate its useful life.
Tube-to-Tubesheet Joints: Maximum Tensile Stress and Contact Pressure Due to Thermal Loading and Temperature Cycling
Authors: Allam Mahdi A. , Bazergui Andre , Marchand Luc , Derenne Michel
Published: March 2009

Service reliability and durability of tubular heat exchangers and steam generators are much dependent on the proper response of the tube-to-tubesheet joints to the operating conditions. In this paper a 2-D axisymmetric finite element model is proposed and compared to a 3-D finite element solution for the purpose of predicting the temperature effect on the residual contact pressure and maximum tensile residual stresses of such joints. A parametric study using the finite element results shows that, although thermal loading and temperature cycling have a negligible effect on the maximum tensile residual stresses, the room-temperature initial residual contact pressure may be completely relieved following the initiation of plastic deformation in either the tube or the tubesheet during thermal loading. A comparison between the results of the proposed finite element model and those obtained from the literature shows good agreement. A simplified analytical approach, which may be used for the design of tube-to-tubesheet joints, is also proposed to predict the joint behavior at the operating conditions.
Parametric Studies of Power Plant Performance Monitoring
Authors: Sowjanya Vijiapurapu , Craven Robert , Munukutla Sastry
Published: March 2009

Real-time performance monitoring of coal-fired power plants is becoming very important due to the impending deregulation of the electric power industry. Performance testing is made to be real-time by changing the traditional output loss method to include an estimation of coal composition based on the Continuous Emission Monitoring System (CEMS) data. This paper illustrates the robustness of the calculations by introducing a variance into each of the calculation inputs to access its effect on the final outputs of heatrate, boiler efficiency and coal flow. Though the original study was over five power plants this paper presents results for the two most diverse coals.
Applying New and Improved Technology to an Existing Generator-Drive Steam Turbine
Authors: Mindock Mike , DiOrio Jerry , Golinkin Sam
Published: March 2009

There is growing pressure to reduce the pay back period of major capital expenditures, and in most cases, revamping existing turbomachinery provides more attractive economics than new equipment. Moreover, revamping existing power generation equipment with modern technology mitigates the expenses associated with obtaining new environmental licensing. Today, innovative application of technological advancements in mechanical and aero-thermodynamic design/analysis methods combined with new manufacturing methods is yielding significant performance improvements. This paper details the revamping of a 36 MW generator-drive turbine that resulted in an eleven point improvement in steam rate. Details of the steampath thermodynamic design/ analysis methods, sealing, diaphragm construction, blade design and low-pressure steam path design, etc. are presented.
A Method for Evaluating Market Value of Turbine Gaspath Component Alternatives
Authors: Willett Fred T.
Published: March 2009

An economic model was developed to evaluate gas turbine component alternatives for base load combined cycle operation, cyclic duty simple cycle operation, and peaking duty simple cycle operation. Power plant operator value of alternative replacement first stage buckets for a GE Frame 7EA gas turbine is evaluated. The popularity and large installed base of the 7EA has prompted a number of replacement part offerings, in addition to the replacement parts offered by the OEM. A baseline case is established to represent the current bucket repair and replacement situation. Each of the modes of power plant operation is evaluated from both a long-term financial focus and a short-term financial focus. Long-term focus is characterized by a nine-year evaluation period, while short-term focus is based on first year benefit only. Four factors are considered: part price repair price, output increase, and simple cycle efficiency increase. Natural gas and liquid fuels are considered. Two natural gas prices are used; one liquid fuel price is considered. Peak, off-peak, and spot market electricity prices are considered. Two baseline repair price scenarios are evaluated: 50% of new part price and 10% of new part price. The key conclusions can be summarized as: • A reduced-life part with more frequent repair intervals is undesirable, even if the part price is reduced by over 60% and the cooling flow is reduced by 1% W2. • A short-life, “throw-away” part with no required repairs can achieve parity with the baseline if the price is reduced by 25% or more. The operator with a short-term focus will not differentiate between a “throw-away” part and a full-life part. • In general, increased part life has less value to the power plant operator than price reduction or cooling flow reduction. • Repair price (assumed to be 50% of part price) is a relatively small factor for operators with a long-term focus, and no factor at all for operators with a short-term focus. A lower baseline repair price (10% of part price) will decrease the attractiveness of a “throw-away” part, moving the parity point to a 40% price reduction. • A 0.7% W2 reduction in cooling flow has roughly the same first year benefit, at baseline fuel prices, as a 10–15% bucket price reduction, except to the peak duty operator. The peak duty operator finds no benefit to reduced cooling flow unless electricity can be sold at spot market prices.
An Application of LES for Gas Turbine Combustor
Authors: Yaga Mitsuru , Suzuki Kazutaka , Endo Hajime , Yamamoto Tsuyoshi , Aoki Hideyuki , Miura Takatoshi
Published: March 2009

A three-dimensional turbulence spray combustion simulation in a gas turbine combustor with Large Eddy Simulation is carried out. In this study, we construct a new eddy characteristic time model derived from a large-scale motion to estimate the combustion reaction rate with an eddy dissipation concept (EDC) model, and estimate combustion characteristics (temperature and chemical species distribution) in the gas turbine combustor for the purpose of validating this model. The essence of this model is that eddy characteristic time is estimated by considering Kolmogorov scale at first. From this assumption, eddy dissipation rate is apparent. However it is not solved directly in Large Eddy Simulation. So eddy dissipation rate is estimated by an assumption that turbulence energy generation and dissipation are locally equal (it is the same assumption as Smagorinsky model), and it is substituted in the eddy characteristic time formula. The overall reaction C12H24+18O2 → 12CO2+12H2O, is often used for turbulent combustion simulation for saving calculation time, but cannot consider CO and H2 formation in local fuel-rich region. To solve this problem, we use 3-step global mechanism (C12H24+6O2 → 12CO+12H2, CO+0.5O2 ↔ CO2, H2+0.5O2 ↔ H2O) to calculate turbulent non-premixed flame characteristics coupling with EDC. The calculated CO2 mole fraction distribution is in fairly good agreement with the experimental data. However, the calculated temperature distribution does not agree well with the measured result of temperature because of disturbing heat transport to downstream by dilution air jet. Though few problems are left, it is shown that the combustion simulation using LES with EDC model is effective method to calculate the characteristics of turbulent diffusion flame in furnace such as gas turbine combustor.