bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of a One-Dimensional Dynamic Gas Turbine Secondary Air System Model—Part I: Tool Components Development and Validation
Authors: Gallar L. , Calcagni C. , Pachidis V. , Pilidis P.
Published: February 2010

The Secondary Air System (SAS) of a jet engine is an open system: there is a bleed off take from the compressor — at the lowest possible pressure compatible with the sink where the flow is to be discharged —, the air then travels through the internal cavities of the engine cooling down the compressor and turbine discs and sealing and cooling bearing chambers. Eventually, the air is discharged at the turbine rims preventing the air of the main gas path from entering the internal turbomachinery cavities that would damage the turbine assembly. Ultimately the system is also primarily responsible for determining the endloads exerted on the turbine discs. The amount of air bled from the main gas path, although necessary, impairs the engine performance because it is purged from the main engine cycle. In order to quantify and minimise its pernicious effect, the usual practice is to model the engine SAS in steady state conditions with 1D network solvers where the net nodes represent the various components of the system. For usual engine transients it is sufficient to analyse the system performance with a quasi steady approach because the time constant of the air system is insignificant compared with the turbomachinery characteristic time. Nonetheless, the rapid changes that occur during slam accelerations or failure scenarios — particularly shaft failure events — call for a different approach to calculate the endloads fluctuations. However, to the author’s knowledge, there is not such an approach to predict the transient response of the system available in the literature hitherto. The aim of the present research is to develop a dynamic model for gas turbine secondary air systems capable of tackling the sudden changes in the flow properties that occur within the system in the aforementioned cases. The whole system is initially broken down into a series of chambers of a finite volume connected by pipes that are initially modelled in isolation and then interconnected. The resultant tool constitutes a baseline onto which further improvements and modifications will be implemented in subsequent works. This first part of the paper explains the mathematical apparatus behind the model of the two main components of the SAS — chambers and pipes — isolated. Then the assumptions made and the limitations that arise as a result are described thoroughly. Finally, the computational results obtained are successfully compared against experimental data available in the public literature for validation.
The Role of Diffusion and Oxidation Kinetics in a Coating Life Prediction Code: Application to Components
Authors: Rinaldi Claudia , Mandelli Marco
Published: February 2010

Under the umbrella of an Italian R&D programme, furnace-cyclic tests of thermal barrier coating (TBC) specimens were carried out to study the coating/substrate interactions and to determine a quantitative description of the degradation as induced by diffusion and oxidation at the interface of TBC and the bond coat (BC). On the basis of these experiments a coating life prediction code was developed, which utilized two alternative approaches for the oxidation kinetics of BC: “phenomenological” oxidation law and “free kinetics” oxidation behaviour. In this paper, the capability is shown of the latter methodology to predict more reliably the oxide thickness, the formation of non alumina oxide and formation of Kirkendall voids. The model was tested for MCrAlY and MCrAlYRe bond coats deposited by different thermal spray processes on various alloys. Examples of predictions are shown together with some comparisons with the experimental findings on the exposed specimens or ex-service components. Finally the oxidation kinetics is discussed with regards to the influence of the presence of TBC on the oxidation process in BC, taking into consideration that alumina and Zirconia–Yttria properties and their interactions. The inverse problem solution (IPS) method was applied together with the developed code to estimate the local mean operating temperature on the component surface. Some examples of code application to ex-service components are also shown.
Development and Validation of a Thickened Flame Modeling Approach for Large Eddy Simulation of Premixed Combustion
Authors: Strakey Peter A. , Eggenspieler Gilles
Published: February 2010

The development of a dynamic Thickened Flame (TF) turbulence chemistry interaction model is presented based on a novel approach to determine the sub-filter flame wrinkling efficiency. The basic premise of the TF model is to artificially decrease the reaction rates and increase the species and thermal diffusivities by the same amount which thickens the flame to a scale that can be resolved on the LES grid while still recovering the laminar flame speed. The TF modeling approach adopted here uses local reaction rates and gradients of product species to thicken the flame to a scale large enough to be resolved by the LES grid. The thickening factor, which is a function of the local grid size and laminar flame thickness, is only applied in the flame region and is commonly referred to as dynamic thickening. Spatial filtering of the velocity field is used to determine the efficiency function by accounting for turbulent kinetic energy between the grid-scale and the thickened flame scale. The TF model was implemented into the commercial CFD code FLUENT. Validation of the approach is conducted by comparing model results to experimental data collected in a lab-scale burner. The burner is based on an enclosed, scaled-down version of the Low Swirl Injector (LSI) developed at Lawrence Berkeley National Laboratory. A perfectly premixed lean methane-air flame was studied as well as the cold-flow characteristics of the combustor. Planar Laser Induced Fluorescence (PLIF) of the hydroxyl molecule was collected for the combusting condition as well as velocity field data using Particle Image Velocimetry (PIV). Thermal imaging of the quartz liner surface temperature was also conducted to validate the thermal wall boundary conditions applied in the LES calculations.
Flow Dynamics and Film Cooling Effectiveness on a Non-Axisymmetric Contour Endwall in a Two-Dimensional Cascade Passage
Authors: Mahmood Gazi I. , Gustafson Ross , Acharya Sumanta
Published: February 2010

The measured flow field and temperature field near a three-dimensional asymmetric contour endwall employed in a linear blade cascade are presented with and without film-cooling flow on the endwall. Flow field temperature and Nusselt number distributions along the asymmetric endwall with wall heating and no film-cooling flow are also reported to show local high heat transfer region on the endwall and justify the locations of the coolant holes. Adiabatic film-cooling effectiveness along the endwall is then measured to indicate the local effects of the coolant jets. The near endwall flow field and temperature field provide the coolant flow behavior and the interaction of coolant jets with the boundary layer flow. Thus, the local film-cooling effectiveness can be explained with the coolant jet trajectories. The measurements are obtained at the Reynolds number of 2.30×105 based on blade actual chord and inlet velocity, coolant-to-free stream temperature ratio of 0.93, and coolant-to-free stream density ratio of 1.06. The cascade employs the hub side blade section and passage geometry of the first stage rotor of GE-E3 turbine engine. The contour endwall profile is employed on the bottom endwall only in the cascade. The blowing ratio of the film-cooling flow varies from 1.0 to 2.4 from 71 discrete cylindrical holes located in the contour endwall. The three-dimensional profile of the endwall varies in height in both the pitchwise and axial directions. The flow field is quantified with the streamwise vorticity and turbulent intensity, pitchwise static pressure difference, flow yaw angle, and pitchwise velocity. Both the flow field and temperature data indicate that the coolant jets cover more distance in the pitchwise and axial direction in the passage as the blowing ratio increases. Thus, the local and average film-cooling effectiveness increase with the blowing ratio.
Ultra Low NOx Using Rich Catalytic/Lean-Burn Catalytic Pilots: Gas Turbine Engine Test
Authors: Baird Benjamin , Etemad Shahrokh , Karim Hasan , Alavandi Sandeep , Pfefferle William C.
Published: February 2010

Lean premixed (LP) combustion is limited to a minimum NOx of 5–9 ppm due to flame stability. To improve flame stability and lower NOx, an advanced Rich Catalytic Lean-burn technology was developed and integrated to replace only the injector pilot with catalyst modules. This system focuses on replacing the highest temperature zone of the LP combustion system with a catalytic pilot (which can stabilize the main flame at reduced equivalence ratio) to reduce overall NOx emissions. This concept synergistically combines the best features of catalytic and aerodynamically stabilized combustion technologies. During Taurus 70 (T70) engine testing of a set of injectors equipped with catalyst modules for natural gas applications, emissions at baseload conditions were NOx of ∼ 2.5 ppmv and CO less than 2 ppm, both corrected to 15% O2. Low single digit emissions were achieved from 50% to 100% load. Combustor acoustics were low (below 0.1 psi/690 Pa) during testing. Reactor catalyst temperature was within the design limit and no-preburner was required. The initial emissions results are promising providing an alternative to conventional LP technology for less than 5 ppm NOx systems. Further development work is needed to establish engine integration and long term catalyst durability before advancing to production.
Nondestructive Detection of Delamination in Thermal Barrier Coatings Using Ultrasonic Technique
Authors: Chen Roger H. L. , Zhang Binwei , Alvin Mary Anne
Published: February 2010

Nondestructive testing using an acousto-ultrasonic technique has been utilized to detect the change of material properties and provide early warning of failure of thermal barrier coating (TBC) systems. Testing was performed on Rene´ N5 and Haynes 230 coupons with an applied NETL-bond coat, as well as on coupons containing both an applied MCrAlY bond coat and 7-YSZ top coat. The coupons were subjected to either cyclic or isothermal testing at 1100°C. Ultrasonic testing was performed before and after thermal testing using piezoelectric sensors with dry contact on the surface of the coatings. Proof-of-concept test results indicated that changes in the properties of the ∼ 40 μ.m bond coat can be detected using the proposed technique. Waveforms generated via Pitch/Catch indicated minor changes within the bond coat applied to Rene´ N5 substrate after 400∼500 hours of cyclic oxidation at 1100°C. In contrast, marked differences in waveforms and travel time reflected significant crack formation and spallation of the bond coat from the Haynes 230 substrate. Finite element analysis (FEA) simulation of the wave propagation on a simplified TBC system with nonlinear effects was conducted. FEA results clearly show detection of a small embedded void incorporated to simulate delamination. Comparisons between experimental measurements and finite element simulations were used to estimate the material properties of the coatings and the substrate.
Interpretation of Flow Reactor Based Ignition Delay Measurements
Authors: Beerer David , McDonell Vincent , Samuelsen Scott , Angello Leonard
Published: February 2010

Compositional variation of global gas supplies is becoming a growing concern. Both the range and rate-of-change of this variation is expected to increase as global markets for Liquefied Natural Gas (LNG) continue to expand. Greater fuel composition variation poses increased operational risk to gas turbine engines employing lean premixed combustion systems. Information on ignition delay at high pressure and intermediate temperatures is valuable for lean premixed gas turbine design. In order to avoid autoignition of the fuel/air mixture within the premixer, the ignition delay time must be greater than the residence time. Evaluating the residence time is not a straight forward task because of the complex aerodynamics due to recirculation zones, separation regions, and boundary layers effects which may create regions where the local residence times may be longer than the bulk or average residence time. Additionally, reliable experiments on ignition delay at gas turbine conditions are difficult to conduct. Devices for testing include shock tubes, rapid compression machine and flow reactors. In a flow reactor ignition delay data are commonly determined by measuring the distance from the fuel injector to the reaction front (L) and dividing it by the bulk or average flow velocity (U) under steady flow conditions to obtain a bulk residence time which is assumed to be equal to the ignition delay time. However this method is susceptible to the same boundary layer effects or recirculation zones found in premixers. An alternative method for obtaining ignition delay data in a flow reactor is presented herein, where ignition delay times are obtained by measuring the time difference between fuel injection and ignition using high speed instrumentation. Ignition delay times for methane, ethane and propane at gas turbine conditions were in the range of 40–500 ms. The results obtained show excellent agreement with recently proposed chemical mechanisms for hydrocarbons at low temperature/high pressure conditions.
Thermal Design of a High Temperature Bayonet Tube Heat Exchanger With Inner and Outer Fins
Authors: Ma T. , Xie G. N. , Wang Q. W.
Published: February 2010

In this paper, a new metallic high temperature bayonet tube heat exchanger with inner and outer fins is proposed. The prime motivation for the presented study is to design a heat exchanger used in the ultra high temperature environment, such as Externally Fired Combined Cycle and syngas production processes. Previous research has shown that bayonet-element is a suitable structure in such a cruel condition, but the heat transfer efficiency is not high. Therefore, in order to improve the heat transfer performance and reduce the cost, a new bayonet tube heat exchanger with fins being employed outside and inside outer tubes of bayonet-elements has been proposed and designed. An improved Log-Mean Temperature Difference method is used for the thermal design process. Meanwhile, two other identical structure heat exchanger are designed for comparison: the one made of only one material, the other without fins. These designs are performed under identical mass flow, inlet temperature, operating pressure, specified allowable fractions of total pressure drop and geometrical size except for fins or materials. The results indicate that the new type heat exchanger has a great potential to increase the heat transfer performance and reduce the cost.
Actuation and Control of a Moveable Geometry System for a Full-Scale, Multi-Stage Centrifugal Compressor Test Vehicle
Authors: Gilarranz R. Jose´ L. , Lombardi Louis M. , Keesler Jarrod E. , Maier William C. , Koch Jay M. , Sorokes James M.
Published: February 2010

In recent years, several papers have been written regarding the use of moveable geometry systems to enable the rotation of otherwise stationary vanes used in centrifugal compressor research test vehicles. These systems typically are installed in single stage rigs or are placed at the inlet of the first stage of multi-stage centrifugal compressor test vehicles. This paper describes the capabilities of a state-of-the-art test vehicle that was developed by the Original Equipment Manufacturer (OEM) as a result of the OEM’s ongoing Research and Development Program aimed towards the implementation of novel and advanced technologies during the development of high-performance centrifugal compressors. The test vehicle is equipped with a variety of internal instrumentation that allow the collection of detailed aero/thermodynamic inter-stage performance data that is used to evaluate the behavior of the machine. The design of the unit also incorporates moveable vanes at the inlet guides upstream of each impeller, at each diffuser inlet and at the inlet of each return channel. The moveable geometry components allow infinite tuning of these components in a multistage environment, which allows the optimization of the aerodynamic performance of the stages based on design and/or off-design operating requirements of the process. The variable geometry system also allows the vanes to be positioned in such a way as to maximize the operating range of the compressor. The incorporation of adjustable vanes into the test vehicle allowed the OEM to significantly reduce the test cycle time, while maximizing the test data that was obtained from a single build. The positioning of the moveable vanes is controlled by a PC-based system that has been integrated into the OEM’s data acquisition system. This paper presents the work executed during the specification, design and implementation of the moveable geometry control system that was developed for the test vehicle. It covers topics such as the selection of the actuators and control hardware, as well as the integration of the actuators with the moveable vanes and other test unit components. Also discussed are the specification and development of the control software and the techniques, hardware and procedures used for the calibration of the moveable geometry system. The calibration was required to accurately determine the transfer function between the actuator movement and the actual rotation of the vanes. The paper also discusses the use of 5-hole pressure probes during the actual test to measure the flow direction upstream of the moveable vanes and how this information was used to achieve the test objectives. Finally, sample test data is presented to illustrate the impact that the moveable geometry system had over the performance of the compressor stages.
3-D Unsteady Simulation of a Modern High Pressure Turbine Stage Using Phase Lag Periodicity: Analysis of Flow and Heat Transfer
Authors: Shyam Vikram , Ameri Ali , Luk Daniel F. , Chen Jen-Ping
Published: February 2010

Unsteady 3-D RANS simulations have been performed on a highly loaded transonic turbine stage and results are compared to steady calculations as well as experiment. A low Reynolds number k-ε turbulence model is employed to provide closure for the RANS system. A phase-lag boundary condition is used in the periodic direction. This allows the unsteady simulation to be performed by using only one blade from each of the two rows. The objective of this paper is to study the effect of unsteadiness on rotor heat transfer and to glean any insight into unsteady flow physics. The role of the stator wake passing on the pressure distribution at the leading edge is also studied. The simulated heat transfer and pressure results agreed favorably with experiment. The time-averaged heat transfer predicted by the unsteady simulation is higher than the heat transfer predicted by the steady simulation everywhere except at the leading edge. The shock structure formed due to stator-rotor interaction was analyzed. Heat transfer and pressure at the hub and casing were also studied. Thermal segregation was observed that leads to the heat transfer patterns predicted by steady and unsteady simulations to be different.
Finite-Volume Solution of the P3 Equations of Radiative Transfer and Coupling to Reacting Flow Calculations
Authors: Ravishankar Mahesh , Mazumder Sandip , Kumar Ankan
Published: February 2010

The method of spherical harmonics (or PN) is a popular method for approximate solution of the radiative transfer equation (RTE) in participating media. A rigorous conservative finite-volume (FV) procedure is presented for discretization of the P3 equations of radiative transfer in two-dimensional geometry—a set of four coupled second-order partial differential equations. The FV procedure, presented here, is applicable to any arbitrary unstructured mesh topology. The resulting coupled set of discrete algebraic equations are solved implicitly using a coupled solver that involves decomposition of the computational domain into groups of geometrically contiguous cells using the Binary Spatial Partitioning algorithm, followed by fully implicit coupled solution within each cell group using a pre-conditioned Generalized Minimum Residual (GMRES) solver. The RTE solver is first verified by comparing predicted results with published Monte Carlo (MC) results for a benchmark problem. For completeness, results using the P1 approximation are also presented. As expected, results agree well with MC results for large/intermediate optical thicknesses, and the discrepancy between MC and P3 results increase as the optical thickness is decreased. The P3 approximation is found to be more accurate than the P1 approximation for optically thick cases. Finally, the new RTE solver is coupled to a reacting flow code and demonstrated for a laminar flame calculation using an unstructured mesh. It is found that the solution of the 4 P3 equations requires 14.5% additional CPU time, while the solution of the single P1 equation requires 9.3% additional CPU time over the 10 equations that are solved for the reacting flow calculations.
A Genetic Algorithm Based Multi-Objective Thermal Design Optimization of Liquid Cooled Offset Strip Fin Heat Sinks
Authors: Ndao Sidy , Peles Yoav , Jensen Michael K.
Published: February 2010

A genetic algorithm based multi-objective thermal design optimization of liquid cooled offset strip fin heat sinks is presented. Using water and HFE-7000 as coolants, Matlab’s genetic algorithm and direct search toolbox functions were utilized to determine the optimal thermal design of the offset strip fin heat sink based on the total thermal resistance and power consumption under constant pressure drop. For a relatively small fin length, the total thermal resistance decreases with increasing fin length and aspect ratio α. For larger fin lengths, the total thermal resistance increases with increasing fin length whereas the power consumption continuously increases with increasing fin length and aspect ratio α for a given pressure drop. A plot of the Pareto front indicates a trade-off between the total thermal resistance and pumping power consumption.
Nanomaterial Enabled Laser Transfer of Temperature Sensitive Organic Light Emitting Diode Materials
Authors: Ko Seung Hwan , Pan Heng , Misra Nipun , Grigoropoulos Costas P.
Published: February 2010

Organic material direct writing is demonstrated based on nanomaterial enabled laser transfer (NELT). Through utilization of proper nanoparticle size and type, and the laser wavelength choice, a single laser pulse could transfer well defined and arbitrarily shaped tris-(8-hydroxyquinoline)Al patterns ranging from several microns to millimeter size. The unique properties of nanomaterials allow laser induced forward transfer at irradiation energies and temperatures lower than commonly used. The technique may be well suited for the mass production of temperature sensitive devices.
Conservative Interpretation of Nonconservative Discrete Ordinates Radiative Intensity Distribution
Authors: Kim Kihwan , Kang Seok Hun , Song Tae-Ho
Published: February 2010

The discrete ordinates interpolation method (DOIM) and the finite element discrete ordinates method (FEDOM) show good accuracy and versatility for calculation of radiative intensity. However, these methods are nonconservative since the intensity is computed only at grid points without considering control volume. When these methods are to be used together with a finite volume-based code for fluid flow and transport analysis, intensity at the center of control volume or surface, whichever is missing, needs to be calculated, and the control volume photon balance should be evaluated. For this reason, the method of satisfying control volume photon balance, without sacrificing the accuracy, is critically discussed first. Based on this rationale, the supplementary DOIM (SDOIM) is proposed to calculate the missing intensity. In addition, the integration method of RTE (IMRTE), used in DOM or FVM to satisfy the control volume photon balance, and linear interpolation method (LIM) are also examined to compare with the SDOIM. The accuracy, physical reliability and smoothness of the intensity obtained by using the three methods are carefully analyzed. Application of the SDOIM shows reliable results which are accurate and free from physically unrealistic intensity distribution.
Radiation Characteristics and Turbulence-Radiation Interactions in Sooting Turbulent Jet Flames
Authors: Mehta Ranjan S. , Modest Michael F. , Haworth Daniel C.
Published: February 2010

The transported PDF method coupled with a detailed gas-phase chemistry, soot model and radiative transfer equation solver is applied to various turbulent jet flames with Reynolds numbers varying from ∼ 6700 to 15100. Two ethylene–air flames and four flames with a blend of methane–ethylene and enhanced oxygen concentration are simulated. A Lagrangian particle Monte Carlo method is used to solve the transported joint probability density function (PDF) equations, as it can accommodate the high dimensionality of the problem with relative ease. Detailed kinetics are used to accurately model the gas-phase chemistry coupled with a detailed soot model. Radiation is calculated using a particle-based photon Monte Carlo method, which is coupled with the PDF method and the soot model to accurately account for both emission and absorption turbulence–radiation interactions (TRI), using line-by-line databases for radiative properties of CO2 and H2O; soot radiative properties are also modeled as nongray. Turbulence–radiation interactions can have a strong effect on the net radiative heat loss from sooting flames. For a given temperature, species and soot distribution, TRI increases emission from the flames by 30–60%. Absorption also increases, but primarily due to the increase in emission. The net heat loss from the flame increases by 45–90% when accounting for TRI. This ixs much higher than the corresponding increase due to TRI in nonsooting flames. Absorption TRI was found to be negligible in the laboratory scale sooting flames with soot levels on the order of a few ppm, but may be important in larger industrial scale flames.
Thermodynamic Analysis of Heat and Fluid Flow in a Microchannel With Internal Fins
Authors: Narayanaswamy Ramesh , Chandratilleke Tilak T. , Foong Andrew J. L.
Published: February 2010

Efficient cooling techniques are one of the critical design requirements for maintaining reliable operational characteristics of modern, miniaturised high performance electronic components. Microchannel heat sinks form an integral part of most devices used for thermal management in electronic equipment cooling. A microchannel of square cross section, having internal longitudinal fins is considered for analysis. A numerical study is carried out to investigate the fluid flow and heat transfer characteristics. Three-dimensional numerical simulations are performed on the microchannel in the presence of a hydrodynamically developed, thermally developing laminar flow. Further on, a thermodynamic analysis is carried out, for a range of fin heights and thermophysical parameters, in order to obtain the irreversibilities due to heat transfer and fluid flow within the microchannel. An optimum fin height, corresponding to the thermodynamically optimum conditions that minimize the entropy generation rates has been obtained. The effect of the presence of internal fins on the entropy generated due to heat transfer, fluid friction, and the total entropy generation is also provided.
Absortivity Correlation for CO2 at Elevated Pressures by Employing Tree Regression Analysis and Logistic Dose Functions
Authors: Leo´n Y. M. , Tremante A. , Malpica F.
Published: February 2010

Gas turbine inlet temperatures have reached, for special applications, values near to 1700K, as a result, radiative effects must be considered in heat transfer studies. In consequence, optical properties estimation has to be faced, which has increased the complexity of the correlation currently used. An absortivity correlation for CO2 at low resolution spectra, available from literature, for high pressure by employing a tree regression analysis and logistic dose function approach is presented therein. The model applied for the spectral range of 2μm to 15μm and for pressures in the range of 0.5 atm to 10 atm and for temperatures in the range of 294K to 1390K. Associated error is compared with Hottel emissivity and high resolution line spectra data base (HITRAN).
Theoretical Study on Transient Hot-Strip Method by Numerical Analysis
Authors: Wei Gaosheng , Du Xiaoze , Zhang Xinxin , Yu Fan
Published: February 2010

This paper presented the effects of finite dimensions of the sample and non-zero heat capacity of the strip on thermal conductivity determination with the transient hot-strip (THS) method. Through numerical analysis of temperature field within the system composed of the samples and the strip, the temperature transients at the strip surface were obtained to calculate the thermal conductivities of materials, which were compared to the exact values. The effect of heat losses out of the external surfaces of the sample and the heat capacity of the strip on thermal conductivity determination were then analyzed comprehensively. It is shown that the sample finite dimensions have great effect on thermal conductivity determination, especially on the materials with relatively higher thermal diffusivities, and the measured thermal conductivity always lower than the exact value due to the lower convective heat transfer coefficient out of the external surfaces of the sample. The measurement error is estimated less than 2.2 percent for the material with thermal diffusivity less than 4.0×10−6 m2/s with the sample dimensions of 120 mm × 60 mm (width × thickness) and the fitting time interval of 20–450s. The non-zero heat capacity of the strip has great effect on thermal conductivity determinations of the materials with relatively lower thermal diffusivities. The measurement error is estimated less than 5 percent for the material with thermal diffusivity larger than 0.8×10−7 m2/s with Cr20Ni80 alloy as the strip.
Accounting for Finite Flash Duration in Diffusivity Experiments
Authors: McMasters Robert L. , Dinwiddie Ralph B.
Published: February 2010

The laser flash method, as a means of measuring thermal diffusivity, is well established, and several manufacturers produce equipment for performing these types of experiments. Most analysis methods used for interpreting the data from these experiments assume one-dimensional transient conduction, with insulated surfaces during the time subsequent to the flash. More recently, models of greater sophistication employing nonlinear regression have been applied to flash diffusivity experiments. These models assume an instantaneous flash and are highly accurate for most samples of moderate diffusivity and sample thickness. As samples become thinner and more highly conductive, the duration of the experiments becomes very short. Since the duration of the flash is typically on the order of several milliseconds, the assumption that this period of time is instantaneous becomes less valid for very short experiments. A model accounting for the duration of the flash is applied to three samples of stainless steel of varying thicknesses and analyzed with two different mathematical models. One model accounts for the finite duration of the flash and the other does not. The model accounting for the flash duration generates results that are much more consistent between samples than the model assuming an instantaneous flash. Moreover, the conformance of the mathematical model accounting for flash duration is much closer to the measured data than the model which assumes an instantaneous flash. As part of the finite flash duration model, the length of the flash is estimated by nonlinear regression, optimizing the conformance of the model to the measured data. Additionally, the starting time of the flash is treated as a parameter and is determined simultaneously with flash duration, thermal diffusivity and flash intensity. Statistical methods are also used for showing the validity of the added level of sophistication of the more advanced mathematical model.
An Experimental Study of Boiling Heat Transfer on Mesh-Covered Fins
Authors: Chien Liang-Han , Huang H.-L.
Published: February 2010

This study investigates a new enhanced boiling surface, which is made by wrapping wire mesh on finned tubes. Pool boiling performance of the new enhanced tubes has been tested in Refrigerant-134a at 5, 10, 20, and 26.67°C saturation temperatures. Brass or copper mesh of 80, 100, or 120 meshes per inch was wrapped on finned tubes of 42 or 60 FPI (fins per inch). The fin heights were either 0.2 mm or 0.4 mm. The test results show that the mesh covered fin tubes significantly enhanced the boiling performance by forming many pores of proper sizes on the surface and sustaining vapor in the tunnels formed by the mesh and fins. The preferred mesh size decreases with decreasing heat flux. The mesh covered on 60FPI fin tube having 0.4 mm fin height and 100 mesh per inch yields the best boiling performance. It enhances the boiling heat transfer coefficient by 7∼8 folds at 5°C as compared with the smooth tube.
SLMB Modeling of the Full Spectrum Cumulative k-Distribution of H2O
Authors: Andre´ Fre´de´ric , Vaillon Rodolphe
Published: February 2010

Radiative heat transfer is significant in many applications involving energy exchanges in gaseous media, such as combustion in engines or furnaces, atmospheric heat balances,.. The Line-By-Line (LBL) approach is the most reliable technique to determine the radiative properties of gases but is rarely used in radiative transfer simulations due to the associated prohibitive computational requirements. Approximate models are favored for such calculations. A comprehensive description of these existing methodologies can be found in Refs. [1, 2].
CFD Investigation Into the Flow Field in a Four Row Staggered Plate Fin-and-Tube Heat Exchanger Experiencing Gross Fin-Side Flow Maldistribution Using the K-ω Turbulence Model
Authors: Hoffmann-Vocke J. , Neale J. , Walmsley M.
Published: February 2010

The flow field in a section of a four-row staggered plate fin-and-tube heat exchanger with gross inlet flow maldistribution has been investigated using CFD modelling. The commercial CFD code Fluent 6.3 has been used to carry out 3D unsteady flow modelling using the low Reynolds number variation of the standard k-ω turbulence model. A significant amount of flow dispersion is shown to occur upstream of the heat exchanger inlet resulting in a 37% reduction in inlet velocity before the flow enters the heat exchanger. Flow dispersion as the fluid passes through the heat exchanger results in a further 28% reduction in the average flow velocity. Significant sections of the heat exchanger experience angled flow resulting in a flow pattern commonly seen in inline tube arrangements. The lower friction factor of the inline flow regime results in a higher mass flux when compared to areas exhibiting the staggered flow regime. This variation results in off-centre velocity peaks entering and leaving the heat exchanger. On average the non-uniform flow cases resulted in a 29.4% increase in pressure drop across the heat exchanger system when compared to uniform flow conditions. Average heat transfer coefficients for the non-uniform flow conditions showed a 20.3% increase over uniform flow conditions of equal mass flow.
Analysis of Flame Radiative Heat Transfer Using Large Eddy Simulation
Authors: Yan Zhenghua , Sunden Bengt , Delichatsios Michael A.
Published: February 2010

A series of comprehensive large eddy simulations of non-premixed turbulent hydrocarbon flames of different sizes in a typical fire scenario have been carried out to compute the flame radiative heat transfer. In the simulation, considerations include the modelling of sub-grid turbulence, turbulent combustion, soot formation, thermal radiation and interactive heat transfer inside solid walls, etc. The instantaneous thermal radiation was calculated using the discrete transfer method with the radiation property evaluated by both an approximated fast narrow band model and an integral model. Simulation was validated against experimental data. Flame radiation heat transfer was compared for flames of different sizes. The effect of thermal radiation property evaluation model on calculation of radiation and the role of thermal radiation in total heat transfer are analyzed.
Determination of Optimum Design Parameters of Horizontal Parallel Pipe and Vertical U-Tube Ground Heat Exchangers
Authors: Demir Hakan , Koyun Ahmet , Atayılmaz S¸. O¨zgu¨r
Published: February 2010

The most important part of a ground source heat pump (GSHP) is the ground heat exchanger (GHE) that consists of pipes buried in the soil and is used for transferring heat between the soil and the heat exchanger of the ground source heat pump. Soil composition, thermal properties and water content affect the length of ground heat exchanger. Another parameter affects the size of the ground heat exchanger is the shape. There are two basic ground heat exchanger configurations: vertical U-tube and horizontal parallel pipe. There are plenty of works on ground source heat pumps and ground heat exchangers in the literature. Most of the works on ground heat exchangers are based on the heat transfer in the soil and temperature distribution around the coil. Some of the works for thermo-economic optimization of thermal systems are based on thermodynamic cycles. This study covers comparative thermo-economical analysis of horizontal parallel pipe and vertical u-tube ground heat exchangers. An objective function has been defined based on heating capacity, investment and energy consumption costs of ground heat exchanger. Investment and energy consumption costs were taken into account as total cost in the objective function. The effects of the soil thermal conductivity, number of pipes, thermal capacity of ground heat exchanger, pipe diameter and the burial depth on the objective function were examined. The main disadvantage of U-tube ground heat exchanger is higher borehole cost that makes installation cost higher than parallel pipe ground heat exchanger. To make reference functions equal for both type of ground heat exchangers, the borehole cost must be under 20 $/m (now 55 $/m) for a given heating or cooling capacity. The performance of ground heat exchangers depends on the soil characteristics especially the soil thermal conductivity.
Infrared Species Limited Data Tomography Through Tikhonov Reconstruction
Authors: Daun K. J.
Published: February 2010

Optical tomography based on infrared laser light absorption is a promising way to measure spatially- and temporally-resolved fuel concentrations within combustion devices, but limited optical access often restricts the number of laser beams and their arrangement. This paper demonstrates how Tikhonov regularization based on a Laplacian smoothing matrix can solve the resulting limited data tomography problem, and provides superior reconstructions for Gaussian phantoms compared to modified Landweber reconstruction.
A k-Distribution-Based Spectral Module for Radiation Calculations in Multi-Phase Mixtures
Authors: Pal Gopalendu , Wang Anquan , Modest Michael F.
Published: February 2010

k-distribution-based approaches are promising models for radiation calculations in strongly nongray participating media. Advanced k-distribution methods were found to achieve close-to benchmark line-by-line (LBL) accuracy for strongly inhomogeneous multi-phase media accompanied by several orders of magnitude smaller computational cost. In this paper, a k-distribution-based portable spectral module is developed, incorporating several state-of-the-art k-distribution methods along with compact and high-accuracy databases of k-distributions. The module construction is flexible — the user can choose among various k-distribution methods with their relevant k-distribution databases, to carry out accurate radiation calculations. The spectral module is portable, such that it can be coupled to any flow solver code with its own grid structure, discretization scheme, and solver libraries. This open source code module is made available for free for all noncommercial purposes. This article outlines in detail the design and the use of the spectral module. The k-distribution methods included in the module are briefly described with a discussion of their advantages, disadvantages and their domain of applicability. Examples are provided for various sample radiation calculations in multi-phase mixtures using the new spectral module and the results are compared with LBL calculations.
Saturation Boiling of PF-5060 Dielectric Liquids on Micro-Porous Copper Dendrites Surfaces
Authors: El-Genk Mohamed S. , Ali Amir F.
Published: February 2010

Experiments are performed to investigate saturation boiling of degassed PF-5060 dielectric liquid on micro-porous copper dendrites surface layers deposited on 10 × 10 mm Cu substrates. The electrochemically deposited surface layers are of different thickness (50, 70, and 220 μm), average pore size (15, 30 and 80 μm) and volume porosity (96, 94.3, and 93.6%). The thickest layer, deposited using 3.0 A/cm2 for 25s, gives the best results: the saturation CHF of 25.27 W/cm2 occurs at a surface superheat of only 2.9 K and the maximum nucleate boiling heat transfer coefficient, hMNB, near the end of the fully developed nucleate boiling region is 8.76 W/cm2.K. In addition, nucleate boiling begins at surface temperature slightly above saturation (< 0.5 K) with no temperature excursion. The temperature excursions before initiating boiling on the 70 μm and 50 μm thick Cu nano-dendrites surface layers are small (3.7 K and 6 K), corresponding to surface temperatures of ∼ 55.1 °C and 57.4 °C, respectively. These temperatures are much lower than recommended (85 °C) for reliable operation of most silicon electronics and CPUs.
Near Critical Heat Flux From Small Substrates Under Controlled Spray Cooling
Authors: Escobar-Vargas Sergio , Gonzalez Jorge E. , Ruiz Orlando , Bash Cullen , Sharma Ratnesh , Fabris Drazen
Published: February 2010

The increasing power density on electronic components has resulted in temperature problems related to the generation of hot spots and the need to remove high heat flux in small areas. This work is aimed at the cooling of small surfaces (1 mm × 1.2 mm) by using a monodisperse spray from thermal ink jet (TIJ) atomizers. Heat fluxes near the critical heat flux (CHF) are obtained for different conditions of cooling mass flow rate, droplet deposition, and number of active droplet jets. Experimental results at quasiequilibrium show the heat flux scales to the cooling mass flow rate. It is observed that two simultaneously activated jets result in slightly smaller heat flux compared to a single jet of droplets for the same mass flow rate. Droplet momentum and spreading or splashing, as determined by a combination of Weber number and Reynolds number effect via K = We1/2Re1/4, may impact the efficiency of the delivery of the cooling mass flow. Current experimental results at K = 24.5 and K = 52.2 for the copper surface temperatures ranging 110 – 120 °C indicate there is little influence of the splashing on the heat dissipation. System heat losses are measured experimentally and compared to a numerical and analytical solution to estimate the actual heat dissipated by the droplet change of phase.
Experimental Investigation of Nitrogen Flow in Long Microtubes
Authors: Li Chengwen , Jia Li , Zhang Tiantian , Li Xing
Published: February 2010

Nitrogen flow characteristics in Polyetheretherketone microtubes with inner diameters (D) ranging from 0.255mm to 0.553mm were experimentally investigated. It is indicated that most of the experimental points in laminar region are coincided with the conventional theoretical predicted value, but several plots caused by instrumental errors are lower than predicted values at small Re. In turbulent region, the friction factors for D = 0.255mm microtubes with L = 0.800m and 1.591m are slightly lower than conventional values; the experimental data for D = 0.553mm microtube with L = 0.800m is lower than that in D = 0.255mm pipes. The entrance effect obviously influences friction factor even if the L/D of microtubes is more than 60, where it can always be neglected in macro-scale. Due to the enhancement of compressibility effect as diameter decrease (Kn increase), friction constant is larger in smaller-size microtubes. The transition Reynolds number in current experiment (except for L = 0.200mm and D = 0.553mm) ranges from1600–2000, while a little early transition phenomenon is found in L = 0.200m, D = 0.553mm tube.
Effect of Shroud and Baffle on Heat Transfer in a Solar Thermal Storage Tank
Authors: Boetcher S. K. S. , Kulacki F. A.
Published: February 2010

Enhancing heat transfer during the charge and discharge of solar thermal storage tanks is an ongoing technical challenge. The types of thermal storage systems considered in the present study comprise an immersed heat exchanger at the top of a solar thermal storage fluid. The discharge process of a thermal store with specified dimensions is numerically simulated over a range of Rayleigh numbers, 105 < RaD <107. The immersed heat exchanger is modeled as a two-dimensional isothermal cylinder which is situated near the top of a water-filled tank with adiabatic walls. An adiabatic shroud whose shape is parametrically varied is placed around the cylinder. In addition, the shroud is connected to an adiabatic baffle situated beneath the cylinder. Nusselt numbers are calculated for different shroud shapes at different Rayleigh numbers. Results show that the shroud is effective in increasing the heat transfer rate. Optimal shroud and baffle geometries are presented as well as qualitative flow results.
An Experimental Setup for IR Gas Radiative Property Measurements at Temperatures Up to 6000K: Application to CO2 and CO2-N2 Mixtures
Authors: Depraz S. , Soufiani A. , Perrin M. Y. , Babou Y.
Published: February 2010

An experimental setup is developed to measure gaseous radiative properties at high temperature and is first applied to CO2 and CO2-N2 mixtures. The gas mixture is heated by a microwave discharge at atmospheric pressure. In a first step, a characterization of the plasma is performed through analysis of the emission spectra in the visible and near UV ranges. This characterization is performed very close to the exit of the plasma cavity where possible departure from local equilibrium is most likely to be observed. The gas flow is shown to be stable and close to local thermodynamic equilibrium at temperatures up to 6000K. In a second step, a Fourier transform spectrometer (BOMEM DA8) with a resolution of up to 4.10−3 cm−1 is associated with the microwave plasma source to perform emission measurements in the infrared range in order to study the infrared properties of CO2. These measurements are carried out farther away from the exit where the gas mixture is at lower temperatures with a significant concentration of CO2. Temperature measurements are derived from Abel inverted CO overtone emission spectra (Δv = 2) and, as preliminary results, CO2 emission measurements in the 2.7 μm range are compared to calculations.