bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of Acoustic Velocity Perturbation Measurements Using PIV vs. Two-Microphone Technique
Authors: Balusamy Saravanan , Hochgreb Simone
Published: July 2018

Understanding combustion instabilities requires accurate measurements of the acoustic velocity perturbation into injectors. This is often accomplished via the use of the two microphone technique, as this only requires two pressure transducers. However, measurements of the actual velocities emerging from the injectors are not often taken, leaving questions regarding the assumptions about the acoustic velocity. A comparison of velocity measured at downstream of the injector with that of two-microphone technique can show the accuracy and limitations of two-microphone technique. In this paper, velocity measurements are taken using both particle image velocimetry (PIV) and the two-microphone technique in a high pressure facility designed for aeroengine injector measurements. The flow is excited using an area modulation device installed on the choked end of the combustion chamber, with PIV measurements enabled by optical access downstream of the injector through a quartz tube and windows. Acoustic velocity perturbations at the injector are determined by considering the Fourier transformed pressure fluctuations for two microphones installed at a known distance upstream of the injector. PIV measurements are realized by seeding the air flow with micrometric water particles under 2.5 bar pressure at ambient temperature. Phase locked velocity fields are realized by synchronizing the acquisition of PIV images with the revolution of the acoustic modulator using the pressure signal measured at the face of injector. The mean velocity fluctuation is calculated as the difference between maximum and minimum velocities, normalized by the mean velocity of the unforced case. Those values are compared with the peak-to-peak velocity fluctuation amplitude calculated by the two-microphone technique. Although the ranges of velocity fluctuations for both techniques are similar, the variation of fluctuation with forcing frequencies diverges significantly with frequency. The differences can be attributed to several limitations associated with of both techniques, such as the quality of the signal, the signal/noise ratio, the accuracy of PIV measurements and the assumption of isentropic flow of the particle velocity from the plenum through the injector. We conclude that two-microphone methods can be used as a reference value for the velocity fluctuation in low order applications such as flame transfer functions, but not for drawing conclusions regarding the absolute velocity fluctuations in the injector.
Influence of Pre-History and Leading Edge Contouring on Aero-Performance of a 3D Nozzle Guide Vane
Authors: Saha Ranjan , Mamaev Boris I. , Fridh Jens , Laumert Björn , Fransson Torsten H.
Published: July 2018

Experiments are conducted to investigate the effect of the pre-history in the aerodynamic performance of a three-dimensional nozzle guide vane with a hub leading edge contouring. The performance is determined with two pneumatic probes (5 hole and 3 hole) concentrating mainly on the endwall. The investigated vane is a geometrically similar gas turbine vane for the first stage with a reference exit Mach number of 0.9. Results are compared for the baseline and filleted cases for a wide range of operating exit Mach numbers from 0.5 to 0.9. The presented data includes loading distributions, loss distributions, fields of exit flow angles, velocity vector and vorticity contour, as well as, mass-averaged loss coefficients. The results show an insignificant influence of the leading edge fillet on the performance of the vane. However, the pre-history (inlet condition) affects significantly in the secondary loss. Additionally, an oil visualization technique yields information about the streamlines on the solid vane surface which allows identifying the locations of secondary flow vortices, stagnation line and saddle point.
Inverse Heat Transfer Analysis of Porous Extended Surface Using Simplex Search Method
Authors: Singla Rohit Kumar , Das Ranjan , Bhowmik Arka , Repaka Ramjee
Published: July 2018

This work deals with the application of the Nelder-Mead simplex search method (SSM) to study a porous extended surface. At first, analytical expression for calculating the local temperature field has been derived using an implicit Runge-Kutta method. The heat transfer phenomenon is assumed to be governed by conductive, naturally convective and radiative heat transfer, whereas the diffusion of mass through the porous media is also taken into account. Then, using the SSM, critical parameters such as porosity, permeability, and thermal conductivities of the extended surface have been predicted for satisfying a prescribed temperature field. It is found that many alternative solutions can meet a given thermal requirement, which is proposed to offer the flexibility in selecting the material and regulating the thermal conditions. It is observed that the allowable error in the temperature measurement should be limited within 5%. It is also found that even with few temperature measurement points, very good reconstruction of the thermal field is possible using the SSM.
Effect of Different Turbulence Models on the Numerical Analysis of Axial Flow Turbine Stage of a Typical Turbofan Engine
Authors: Ramana Murthy S. V. , Kishore Kumar S.
Published: July 2018

3D NS analysis is routinely used in the design and analysis of modern turbine stages in the recent days. One of the key parameters which influence the results of the numerical analysis is the turbulence models. Thus the choice of the turbulence models plays an important role in the 3D NS analysis. In addition to the turbulence models, the grid resolution in the boundary layer also considerably influences the results of the analysis. The objective of the work is to study the choice of turbulence models, inlet free stream turbulence intensity, the eddy length scales and values of y+ on the performance of an axial flow turbine stage of a typical turbofan engine by using ANSYS CFX 3D NS code. The Reynolds Averaged Navier – Stokes (RANS) equations are solved. The governing equations are discretized using finite volume method. The solution algorithm is based on an implicit scheme coupled with multi-grid. Analyses are carried out using three different turbulence models namely k-ε, k-ω and SST models. It is observed that there is no appreciable change in the turbine performance parameters for the different turbulence models. However, the k-ω and SST models predict a drop in the turbine efficiency of 0.4 % as compared to other models. The effect of various inlet free stream turbulence intensity levels and length scales on the performance parameters is also presented.
Structural Integrity Validation for Rotor Discs and Shafts in Aero-Engines
Authors: Kumar Sanju , Rao Rashmi , Ananthappa Rajeevalochanam B. , Mogullapally Venkateshwarlu
Published: July 2018

Design of rotating structures in military gas turbine engine is complex and arduous. The aero engine main discs and shafts are designed to achieve maximum operating capabilities under severe loads. The potential failures of primary rotating parts are extremely catastrophic. Hence, ignoring structural integrity aspects, results in engine failures, which is a major concern and motivation for the present paper.

In this paper structural integrity analysis using CAE are discussed for rotating components and various structural validation testing employed are highlighted. The structural integrity study carried out conforms to stipulation of MIL 5007 D/E. The validation carried out includes material characterization, cyclic spin tests, overspeed and burst speed tests. This paper also emphasizes structural integrity studies on engine main shafts, for buckling, maneuvers and blade-off loads.
Numerical Investigation of Outlet Hub Geometry on the Performance of Large Axial Fan
Authors: Yin Chao , Hu Jun
Published: July 2018

The large axial fan’s outlet hub shape changes with its working environment. Usually for saving cost, the hub geometry of fan’s outlet section is roughly processed. There is no geometry specification for the outlet hub section, which makes its structure seriously mismatched with other parts. When a blade which has good performance is fitted into an actual production, it probably couldn’t get a desired result. This shows that the effect of outlet-hub geometry on the performance of large axial fan couldn’t be neglected. To study the effect, four cases of different outlet hub geometries of a large axial fan have been designed and investigated in current paper. Each case has been carefully calculated by using numerical simulation methods. By comparing their aerodynamic characteristics and analyzing flow structures in the tip and hub region of the fan blade, the results show that with the same blade tip clearance, the shape of hub geometry has little effect on the tip flow field. However, the fan efficiency is obviously affected by the hub shape. Straight hub could approve the total pressure of the fan, while shrink hub could reduce shaft power more efficiently. A kind of proper outlet hub geometry could greatly improve the flow performance in the hub region and increase the fan efficiency. So we could choose a proper shape of outlet hub for different working requirements.
Study of the Velocity Flow Field Under Distorted Inflow Conditions for a High Aspect Ratio Low Speed Contra Rotating Fan
Authors: Mistry Chetan S. , Pradeep A. M.
Published: July 2018

In this paper, results from an experimental study on the effect of circumferential inflow distortion on the flow behavior of a high aspect ratio, low speed contra rotating fan, are reported. The total pressure at the inlet is artificially distorted by means of (i) 90° sector mesh having porosity of 0.70, (ii) 90° sector mesh having porosity combination of 0.70 and 1/3rd section at the hub region covered by mesh of porosity 0.64 (hub covered combined distortion) and (iii) 90° sector mesh having porosity combination of 0.7 and 1/3rd section at the tip region covered by mesh of porosity 0.64 (tip covered combined distortion). Detailed flow analyses were conducted under design speed combination and design mass flow rate of 6 kg/s. In order to understand the extent of inlet distortion, the distortion sector was rotated circumferentially at intervals of 15° to cover the entire annulus. Detailed measurements of velocity components and flow angles were carried out at the inlet of the first rotor, between the two rotors and at the exit of the second rotor. The study reveals a few interesting aspects on the effect of circumferential and combined distortion inflow distortion on the flow behavior of a contra-rotating stage. The presence of low porosity screen reduces the axial velocity magnitude and generates higher spread of distortion near the localized region of placement of the screen. The pure circumferential distortion has a greater effect on the performance of both the rotors compared to hub and tip covered combined distortion.
Design and Analysis of a High Pressure Turbine Using Computational Methods for Small Gas Turbine Application
Authors: Kumar Kishor , Prathapanayaka R. , Ramana Murthy S. V. , Kishore Kumar S. , Krishna T. M. Ajay
Published: July 2018

This paper describes the aerodynamic design and analysis of a high-pressure, single-stage axial flow turbine suitable for small gas turbine engine application using computational methods. The specifications of turbine were based on the need of a typical high-pressure compressor and geometric restrictions of small gas turbine engine. Baseline design parameters such as flow coefficient, stage loading coefficient are close to 0.23 and 1.22 respectively with maximum flow expansion in the NGV rows. In the preliminary design mode, the meanline approach is used to generate the turbine flow path and the design point performance is achieved by considering three blade sections at hub, mean and tip using the AMDC+KO+MK+BSM loss models to meet the design constraints. An average exit swirl angle of less than 5 degrees is achieved leading to minimum losses in the stage. Also, NGV and rotor blade numbers were chosen based on the optimum blade solidity. Blade profile is redesigned using the results from blade-to-blade analysis and through-flow analysis based on an enhanced Dawes BTOB3D flow solver. Using PbCFD (Pushbutton CFD) and commercially available CFD software ANSYS-CFX, aero-thermodynamic parameters like pressure ratios, aerodynamic power, and efficiencies are computed and these results are compared with one another. The boundary conditions, convergence criterion, and turbulence model used in CFD computations are set uniform for comparison with 8 per cent turbulence intensity. Grid independence study is performed at design point to optimize the grid density for off-design performance predictions. ANSYS-CFX and PbCFD have predicted higher efficiency of 3.4% and 1.2% respectively with respect to targeted efficiency of 89 per cent.
A Study of Existing Multistage Transonic Axial Compressor Design for Surge Margin Improvement
Authors: Kaur Baljeet , Balsaraf Nitin B. , Pratap Ajay
Published: July 2018

The design of multistage axial flow compressors has been revolutionised in recent years by the development of three dimensional multistage viscous calculations (CFD) and the availability of the computational power to allow these methods to be used extensively in design process. Such a multistage turbomachinery was used to redesign the existing three stage transonic compressor for improved aerodynamic performance in terms of SM limit. The redesign activity of compressor configuration was carried out as surge margin obtained with hardware testing of existing machinery was not sufficient to meet the desired design goals. The higher limit of surge margin in accordance with design specification is required to maintain the successful and stable operation of aircraft engine. As at stall point of compressor, aerodynamic instabilities would be initiated resulting surge or rotating stall which potentially leading to a complete mechanical failure of the compression system as well as of the whole engine. Maximizing the SM of multistage compressors is particularly a complex process especially alongwith achieving higher efficiency. Outlined in this paper are the details of how advanced design techniques were incorporated using traditional 2D and CFD methods into redesign activities for compressor performance improvement. The approach used in this work was to modify compressor annulus flowpath and rotor and stator blade geometries based on output of 2D calculations and 3D N-S analysis for SM enhancement of existing design. While carrying out redesign activities for compressor, the constraints of retaining existing inlet and outlet flow area, axial length as well as design parameters i.e. inlet mass flow, rotational speed and operating point pressure ratio were taken care. The advanced design techniques like 3D blading, wide chord blade performance and many others were studied in detailed manner for incorporating them into compressor redesign procedure. Simultaneous resigned compressor configuration based on in-house design procedure showed improvement of SM by 17% at design speed with maintaining mentioned design constraints. Subsequently, the detailed analysis was also performed at off design speeds to have satisfactory performance.
Experimental Study on the Effect of Freestream Turbulence on the Development of an Inflectional Boundary Layer From the Semi-Circular Leading Edge of a Flat Plate
Authors: Samson A. , Sarkar S.
Published: July 2018

The characteristics of a boundary layer from the semi-circular leading edge of a flat plate has been investigated for two levels of stream turbulence (Tu = 0.5% and 7.7%) in a low-speed wind tunnel. Measurements of velocity and surface pressure were made along with a planar PIV to visualize flow structures for varying turbulence levels at a Reynolds number of 25000 (based on the leading edge diameter). At low stream turbulence the measurements reveal flow undergoes separation in the vicinity of leading-edge with reattachment in the downstream. Velocity spectra illustrates that the separated shear layer is laminar up to 20% of separation length and then the perturbations are amplified in the second half attributing to breakdown and reattachment. It is also evident that the shear layer is inviscidly unstable and the predominant shedding frequency when normalised with respect to the momentum thickness at separation shows a good agreement with previous studies. The bubble length is highly susceptible to change in Tu depicting an attached layer which grows into a fully turbulent profile at high Tu. Here, the spectra for an attached layer depicts a turbulent-like flow with band of frequencies from the beginning.
Parametric Studies on Bolted Flange Thermal Analysis to Improve LCF Life
Authors: Yousuf Mohd. , Nelanti Krishna , Allabanda Rajendra Prasad
Published: July 2018

Low cycle fatigue (LCF) life plays an important role in the design of aircraft engine components, especially that of bolted joints. Bolted joints are of critical importance in aircraft engines as they connect parts and transfer loads. In gas turbine engines, most of the bolted flanges experience high pressure loads & high thermal gradients. LCF life/stress is very sensitive to the thermal gradients encountered in flight cycle (from idle to takeoff & takeoff to landing). Accurate flange leakage modeling & accurate boundary condition estimation are critical to design.

This paper emphasizes on thermal analysis of bolted flange using the finite element method. Thermal analysis results show that, during flight cycle (Accel/Decel), flange leakage is a key contributor to thermal gradients. The flange leakage is majorly influenced by the gap between the flanges. Parametric studies have been carried out to understand the impact of gap between the flanges, flange thickness and flange height on the flange thermal gradients. The results presented in this paper will be helpful to the designer in designing better bolted flange joints with improved LCF life.
Isothermal Fatigue Behaviour of Two Nickelbase Alloys for Turbine Blades of Fighter Aircraft Engines
Authors: Sahoo Benudhar , Satpathy R. K. , Panigrahi S. K.
Published: July 2018

Turbine blades of fighter class aero-engines are subjected to repeated and unsteady throttle excursions during exploitation leading to thermo-mechanical fatigue. Failures of few turbine blades have led to premature withdrawals and major accidents. Hence, there is a need to study the high temperature fatigue behavior of these alloys. This case study deals with the isothermal fatigue behavior at 900°C for two nickel base alloys which are used for manufacturing turbine blades. The materials selected for study are precipitation strengthened wrought nickel base super alloy AP 220BD of Russian origin and western origin nickel base DS cast alloy MAR-M 002.

Tensile and low cycle fatigue (LCF) tests were carried out at 900°C. LCF test was carried out at three strain ranges Δεt, i.e., ±0.3%, ±0.5% and ±0.8% using a triangular waveform of frequency 0.33 Hz with the help of MTS 800 servo hydraulic fatigue testing machine. Fractography and metallography have been done on the fracture surface to study the mode of failure and changes in morphology.

Both AP220BD and MAR M 002 shows initial cyclic softening followed by cyclic hardening at low strain Δεt, i.e., ±0.3%. Stabilisation of hysteresis loop takes place at strain range Δεt ±0.5% for both the alloys. At Δεt/2 = 0.8%, AP220 BD shows cyclic hardening while MAR M 002 shows cyclic softening. MAR M 002 shows higher degree of hardening compared to that of AP220BD. The reduction in LCF life with increase in strain range is exponential for wrought alloy but for DS cast alloy it is approximately linear.

LCF life has been correlated with fatigue damage parameters such as Ostergren energy(σmax Δεp) and plastic strain amplitude Δεp/2. Ostergren energy for MAR-M 002 is found to be significantly less compared to that of AP220BD. The increase in plastic strain range Δεp is significant for wrought alloys. While Ostergren energy is a good indicator of fatigue damage for wrought alloy, plastic strain amplitude Δεp/2 seems to be a better indicator for DS cast alloy.

Fractography reveals inter-granular failure with initiation of cracks at at grain boundary / carbides in wrought alloy. DS cast alloy shows cleavage failure with crack initiation at pore cavities in interdendritic spacing.
Exergetic Analysis of Combined Cycle Power Plant With Single Steam Extraction
Authors: Dev Nikhil , Goyal Gopal Krishan , Attri Rajesh , Kumar Naresh
Published: July 2018

Combined Cycle Power Plant (CCPP) is one of the most efficient systems of energy conversion with different topping and bottoming cycles. One of the acceptable schemes, the combination of Brayton and Rankine Cycle, is analyzed for various design parameters. In the present analysis thermodynamic modelling of a CCPP with single steam extraction from bottoming Rankine Cycle is carried out to study the effect of Inlet Air Temperature (IAT), Cycle Ratio (CR), Turbine Inlet Temperature (TIT), air compressor and gas turbine efficiency on the first and second law efficiency. For parametric analysis computer programming tool Engineering Equation Solver (EES) is used and thermodynamic properties of many fluids and gases are inbuilt function of the software. From the results it is concluded that combustion chamber is the source of highest exergy destruction followed by heat recovery steam generator, gas turbine, air compressor and steam turbine. With increase in TIT, optimum CR is also found to be increased because both the gas turbine efficiency and the gas turbine exhaust temperature are increased for the optimum cycle ratio.
Performance Analysis of Casement Type Vertical Axis Wind Turbine
Authors: Wangikar Sandeep S. , Jagtap Sharad U. , Tarmude Abhijeet B. , Pore Abhishek S. , Shinde Sushil P.
Published: July 2018

Increasing worldwide demand for electricity requires the need for harnessing different kinds of renewable energies like wind energy. An increase in prevalence of vertical axis wind turbine (VAWT) has renewed interest in developing the new configurations of vertical axis wind turbines for better performance. This paper describes the performance analysis of a casement type vertical axis wind turbine (CTVAWT). The model of CTVAWT has been manufactured and tested to predict the performance. The performance analysis of CTVAWT was carried out by varying the control parameters such as wind speed and casement angle. The effect of each control parameter on the response parameters i.e. torque and power have been analyzed (by conducting various experiments of CTVAWT).The torque and power increases with increase in casement angle up to 40 degrees further decrease with increase in casement angle. From this analysis the newly developed CTVAWT is working efficiently at 40 degrees.
Model-Based Crack Identification Using Full-Spectrum
Authors: Chandrasekaran Shravankumar , Tiwari Rajiv
Published: July 2018

This paper illustrates the application of full-spectrum technique for model-based identification of the crack and unbalance multi-fault parameters in cracked rotor systems. The rotor model chosen is a Laval rotor with disc unbalance and transverse surface crack. The crack force model is a switching crack, which has harmonic components exciting the rotor both in the same and reverse directions of the rotor spin. Development of identification algorithm uses linearized equations of motion in frequency domain. Full-spectrum obtains the complex Fourier coefficients of the force as well as the response. Further usage of these coefficients in the identification algorithm estimates the viscous damping, disc eccentricity, and additive crack stiffness as fault parameters. The accuracy of estimates increases on considering measurements at multiple spin speeds. The algorithm tests reasonably robust for various levels of measurement noise and bias errors in system parameters.
Graph Theoretic Analysis of Advance Combined Cycle Power Plants Alternatives With Latest Gas Turbines
Authors: Dev Nikhil , Goyal Gopal Krishan , Attri Rajesh , Kumar Naresh
Published: July 2018

In the present work, graph theory and matrix method is used to analyze some of the heat recovery possibilities with the newly available gas turbine engines. The schemes range from dual pressure heat recovery steam generation systems, to triple pressure systems with reheat in supercritical steam conditions. From the developed methodology, result comes out in the form of a number called as index. A real life operating Combined Cycle Power Plant (CCPP) is a very large and complex system. Efficiency of its components and sub-systems are closely intertwined and insuperable without taking the effect of others. For the development of methodology, CCPP is divided into six sub-systems in such a way that no sub-system is independent. Digraph for the interdependencies of sub-system is organized and converted into matrix form for easy computer processing. The results obtained with present methodology are in line with the results available in literature. The methodology is developed with a view that power plant managers can take early decision for selection, improvements and comparison, amongst the various options available, without having in-depth knowledge of thermodynamics analysis.
Application of Fuzzy Logic Control Strategy for Temperature Control in Friction Stir Welding
Authors: Das R. R. Varun , Kalaichelvi V. , Karthikeyan R.
Published: July 2018

Friction Stir welding is a solid state joining process that utilizes a rotating non-consumable tool to plastically deform and forge together parent metals. Welding can be controlled either by using Force, Temperature and Traverse or Seam Control methods. The presence of numerous parameters and conditional variations in FSW production environment can adversely affect weld quality making extensive automation processes impossible till date. The weld quality of FSW is closely related to the stability of the welding temperature. For such a non-linear complex process conventional control theory is not an appropriate choice. A fuzzy logic controller with a specially chosen triangular membership function has been suggested as an effective alternative approach. The aim of the present work includes dynamic modeling of a friction stir welding process and the use of a suitable Fuzzy tuned Control Strategy for temperature control. The Temperature at stir zone is measured using a K type Thermocouple. It has a sensitivity of 41μV/°C and also a wide variety of probes are available within its −200° C to +1250 °C range. The thermocouple is used by drilling a hole in the shank of the tool and letting it pass through it. The spindle speed is used as an appropriate variable to control temperature variations. The dynamic modeling and simulations were performed using Matlab whereas the variable values were derived during friction stir welding of aluminum.
Thermal Based Optimum Design of Tapered Roller Bearing Through Evolutionary Algorithm
Authors: Tiwari Rajiv , Chandran Rahul
Published: July 2018

In high-speed applications the maximum temperature in bearings are a crucial concern. In some applications the bearing is the prime source of heat, the temperature at which a bearing operates dictates the type and amount of lubricant and the material for the fabrication of the bearing components. In the present work a thermal based optimum design of tapered roller bearings has been presented. Internal geometry of the bearing has been optimized based by evolutionary algorithm. Constraints are geometrical, kinematical, strength and thermal in nature. Optimum designs have been found to have better performance parameters. Artificial bee colony algorithm has been used for the present optimization problem, for solving constrained non-linear optimization formulations. A total of nine design variables corresponding to the bearing geometry and constraint factors have been considered. A convergence study has been carried and optimum designs based on temperature is compared with the optimized values based on dynamic capacity, both using artificial bee colony algorithm. There is an excellent improvement found in the optimized bearing designs based on temperature when compared with the optimized results based on dynamic capacity in respect of the maximum temperature in the bearing with the artificial bee colony algorithm.
Thermal Performance of Water-Based Suspensions of Phase Change Nanocapsules in a Natural Circulation Loop
Authors: Ho C. J. , Lai Chi-Ming
Published: July 2018

Experiments were conducted to investigate the heat transfer characteristics of water-based suspensions of phase change nanocapsules in a natural circulation loop with mini-channel heat sinks and heat sources. A total of 23 and 34 rectangular mini-channels, each with width 0.8 mm, depth 1.2 mm, length 50 mm and hydraulic diameter 0.96 mm, were evenly placed on the copper blocks as the heat source and heat sink, respectively. The adiabatic sections of the circulation loop were constructed using PMMA tubes with an outer diameter of 6 mm and an inner diameter of 4 mm, which were fabricated and assembled to construct a rectangular loop with a height of 630 mm and a width of 220 mm. Using a core material of n-eicosane and a shell of urea-formaldehyde resin, the phase change material nanocapsules of mean particle size 150 nm were fabricated successfully and then dispersed in pure water as the working fluid to form the water-based suspensions with mass fractions of the nanocapsules in the range 0.1–1 wt.%. The results clearly indicate that water-based suspensions of phase change nanocapsules can markedly enhance the heat transfer performance of the natural circulation loop considered.
Optical Properties of Thin Crystalline Silicon Nanostructures for Photovoltaic Applications
Authors: Fang Xing , Zhao Changying , Bao Hua
Published: July 2018

The optical performance of four lattice crystalline silicon nanostructures, i.e., cylinder nanowire arrays, cylinder nanohole arrays, square nanowire arrays and square nanohole arrays is numerically investigated in this paper. The method of rigorous coupled-wave analysis (RCWA), an efficient and accurate computational tool, is used to calculate the optical absorption for the lattice constant from 100 nm to 1500 nm. The results indicate that the lattice constant is the foremost structure parameter to determine the ultimate efficiency, and the ultimate efficiencies are reached at the lattice constant around 600 nm. The optimal filling ratio of square nanowire arrays is the lowest among the four nonostructures, whereas the cylinder nanohole arrays exhibit a broad range of optimal filling ratios. Lower optimal filling ratios implies that the nanostructures cost less raw material while maintain the high ultimate efficiencies. The high ultimate efficiency of all structures can be achieved over a large range of incident angles, even the efficiency will slowly decrease as the incident angle increases.
Reflood Heat Transfer in SiC and Graphene Oxide Coated Tubes
Authors: Kim Kyung Mo , Lee Seung Won , Bang In Cheol
Published: July 2018

Quenching experiments were conducted to investigate the effect of nanoparticle deposition on boiling heat transfer during rapid quenching in long vertical tubes. SiC and graphene oxide (GO) nanoparticles were deposited by boiling 0.01 vol% SiC/water and GO/water nanofluids in the vertical tube for 600 and 900 s to observe the repeatability of the nanoparticle deposition. Reflood tests were performed by passing water through bare tube and nanoparticle-coated tube at a constant flow rate (3 cm/s). Quenching curves (temperature vs. time) and saturated boiling curves were obtained at atmospheric pressure. We observed a more enhanced cooling performance in nanoparticle-coated tubes. The quenching time of tubes coated with SiC nanoparticles for 600 and 900 s were reduced by more than 20 and 25 s, respectively, compared to that of the bare tube. For the tubes boiled with GO nanoparticles for 600 and 900 s, the quenching times decreased by 10 and 12 s, respectively, compared to that of the bare tube. Scanning Electron Microscopy (SEM) images were acquired, and the contact angles were measured to observe the effects of surface structures and wettability on the cooling performance.
Improved Thermal Transfer Efficiency for Planar Solar Thermophotovoltaic Devices
Authors: Bierman David M. , Lenert Andrej , Wang Evelyn N.
Published: July 2018

Solar thermophotovoltaic (STPV) devices provide conversion of solar energy to electrical energy through the use of an intermediate absorber/emitter module, which converts the broad solar spectrum to a tailored spectrum that is emitted towards a photovoltaic cell [1]. While the use of an absorber/emitter device could potentially overcome the Shockley-Queisser limit of photovoltaic conversion [2], it also increases the number of heat loss mechanisms. One of the most prohibitive aspects of STPV conversion is the thermal transfer efficiency, which is a measure of how well solar energy is delivered to the emitter. Although reported thermophotovoltaic efficiencies (thermal to electric) have exceeded 10% [3], [4], previously measured STPV conversion efficiencies are below 1% [5], [6], [7].

In this work, we present the design and characterization of a nanostructured absorber for use in a planar STPV device with a high emitter-to-absorber area ratio. We used a process for spatially-selective growth of vertically aligned multi-walled carbon nanotube (MWCNT) forests on highly reflective, smooth tungsten (W) surfaces. We implemented these MWCNT/W absorbers in a TPV system with a one-dimensional photonic crystal emitter, which was spectrally paired with a low bandgap PV cell. A high fidelity, system-level model of the radiative transfer in the device was experimentally validated and used to optimize the absorber surface geometry. For an operating temperature of approximately 1200 K, we experimentally demonstrated a 100% increase in overall STPV efficiency using a 4 to 1 emitter-to-absorber area ratio (relative to a 1 to 1 area ratio), due to improved thermal transfer efficiency. By further increasing the solar concentration incident on the absorber surface, increased emitter-to-absorber area ratios will improve both thermal transfer and overall efficiencies for these planar devices.
Capillary-Limited Evaporation From Well-Defined Microstructured Surfaces
Authors: Adera Solomon , Raj Rishi , Wang Evelyn N.
Published: July 2018

Thermal management is increasingly becoming a bottleneck for a variety of high power density applications such as integrated circuits, solar cells, microprocessors, and energy conversion devices. The performance and reliability of these devices are usually limited by the rate at which heat can be removed from the device footprint, which averages well above 100 W/cm2 (locally this heat flux can exceed 1000 W/cm2). State-of-the-art air cooling strategies which utilize the sensible heat are insufficient at these large heat fluxes. As a result, novel thermal management solutions such as via thin-film evaporation that utilize the latent heat of vaporization of a fluid are needed. The high latent heat of vaporization associated with typical liquid-vapor phase change phenomena allows significant heat transfer with small temperature rise.

In this work, we demonstrate a promising thermal management approach where square arrays of cylindrical micropillar arrays are used for thin-film evaporation. The microstructures control the liquid film thickness and the associated thermal resistance in addition to maintaining a continuous liquid supply via the capillary pumping mechanism. When the capillary-induced liquid supply mechanism cannot deliver sufficient liquid for phase change heat transfer, the critical heat flux is reached and dryout occurs. This capillary limitation on thin-film evaporation was experimentally investigated by fabricating well-defined silicon micropillar arrays using standard contact photolithography and deep reactive ion etching. A thin film resistive heater and thermal sensors were integrated on the back side of the test sample using e-beam evaporation and acetone lift-off. The experiments were carried out in a controlled environmental chamber maintained at the water saturation pressure of ≈3.5 kPa and ≈25 °C. We demonstrated significantly higher heat dissipation capability in excess of 100 W/cm2. These preliminary results suggest the potential of thin-film evaporation from microstructured surfaces for advanced thermal management applications.
Microchannel Condensation: Comparison of Annular Laminar Flow Theory With Detailed Measurements
Authors: Wang H. S. , Rose J. W.
Published: July 2018

Detailed experimental investigations of condensation in microchannels where local heat flux and surface temperature were measured along the channel are compared with theoretical results for the special case of annular, laminar flow. The theoretical model includes surface tension driven transverse flow towards the corners of the channel as well as shear stress driven streamwise flow in an otherwise Nusselt treatment. The theory has no empirical input. When distributions along the channel of the local vapor and wall temperatures are given, local heat flux and heat-transfer coefficient, as well as local vapor quality, may be calculated. Such detailed experimental data have only recently become available. Strict implementation of the theory requires that the onset of condensation occurs within the channel, i.e. the vapor is saturated or superheated at the inlet. The comparisons show remarkably good agreement with the experimental data for two fluids and covering a wide range of experimental conditions.
Analysis of Entropy Generation for a Power-Law Fluid in Microchannels
Authors: Tan L. Y. , Chen G. M.
Published: July 2018

Entropy generation is tied to the exergy destroyed. Hence, the amount of entropy generation is of primary concern as it is related to unavailable work. Viscous dissipation is a form of heat generation due to work done by viscous forces. Its effect on the velocity and temperature profiles would have affected the entropy generation. In this work, second law analysis is carried out on a microchannel between parallel plates for a power-law fluid. The governing energy equation for a rectangular microchannel is first solved analytically. Analytical expression is obtained for the dimensionless entropy generation and Bejan number. Dimensionless entropy generation due to fluid flow irreversibility and heat transfer irreversibility are also computed and compared. The distribution of entropy generation due to heat transfer irreversibility and fluid friction irreversibility changes as Brinkman number increases. A comparison with a previous literature on a circular pipe for the same Brinkman number reveals that the total dimensionless entropy generation in parallel plate is more than the corresponding value in circular pipe. However, the Bejan number for a parallel plate is lower than a circular pipe.
Light Trapping in Semiconductor Thin Film With Disordered Nanoholes
Authors: Lou Minhan , Bao Hua , Zhao Changying
Published: July 2018

Finite-difference time-domain method is employed to investigate the optical properties of a semiconductor thin film patterned with circular holes. The presence of hole arrays, although reduces the amount of material usage, can greatly enhance the integrated absorption of the thin film. The optimal square hole lattice can enhance the integrated absorption by a factor of 5.6 over a bare thin film. It is also found that disorderness, including non-uniform radius and random position, can further enhance the absorption efficiency and broaden the absorption spectra. The effects of random position and non-uniform radius are found to be quite different: while absorption spectrum for thin film containing randomly positioned holes is almost broadband, the non-uniform hole radius only slightly broadens the absorption peaks of the periodic structures. The absorption enhancement in the disordered structure is attributed to the increased number of guided resonances in the structure. We also show that with carefully designed hole pattern the overall absorption can be further enhanced.
Analysis of Triangular Microchannel Under Forced Convection Heat Transfer Condition for Laminar Flow Condition
Authors: Kamble D. A. , Gawali B. S.
Published: July 2018

Micro-convection is a strategic area in transport phenomena, since it is the basis for a wide range of miniaturized high-performance heat transfer applications. Surface area is one of the important parameter for high flux heat transfer in microchannel performance. This experimental study deals with heat transfer using triangular microchannel having hydraulic diameters of 321μm and 289μm. Experimentation is carried out for triangular microchannel set for different heat input and flow rate condition. Triangular microchannel are manufactured with EDM technology. Testing of microchannel under laminar flow is considered with different tip angle, spacing, and length of microchannels. The different microchannels made up of copper material with 29 microchannel each having three different sets of length of 50 mm, 70 mm and 90 mm respectively. Tip angles for triangular microchannel is varied 50 ° and 60 ° with width of 30 mm each respectively are analyzed numerically. Spacing between triangular microchannels is also varied and 300μm and 400μm are considered for the analysis. Water flow rate is considered laminar flow. The flow rate of water is varied from 0.0167 kg/sec to 0.167 kg/sce to carry away heat. It is observed that as hydraulic diameters increase the heat transfer coefficient decreases. As the heat input to microchannel increases from 10 Watt to 100 Watt the temperature drop across varies from 2° C to 22°C as water flow rate increases. The numerical analysis is done using computer C programming. Experimental result differ from theoretical for temperature drop with variation of 2°C to 5°C. It is also observed that in all triangular microchannels its geometry i.e. tip angle and hydraulic diameter are dominant parameters which influences on rate of heat transfer. With increasing channel depth, increases flow passage area therefore enhances heat transfer sufficiently. From experimentation a Nu number correlation is proposed with considering tip angle, length, spacing of microchannel and other related parameters.
The Local and Average Heat Transfer Characteristic of Confined Jet Array Impingement Boiling of Aqueous Ethylene Glycol Solutions
Authors: Hong F. J. , Zhang C. Y. , He W. , Cheng P. , Chen G.
Published: July 2018

Liquid Jet impingement cooling is deemed as one of the most promising high heat flux cooling technologies. Compared with single phase cooling, two-phase cooling has advantages of more uniform heating surface temperature, lower pressure drop and less mass flow rate. In this paper, a closed-loop experimental setup is built to study confined jet array impingement boiling of 43% mass concentration aqueous ethylene glycol solution. The rectangular heating surface made of thin metal film is 20 mm × 40 mm and with the thickness of 0.03 mm. The in-line jet array has the jet orifice diameter d = 1 mm, the dimensionless jet-to-target spacing H/d = 1, and the dimensionless jet-to-jet spacing S/d = 5. The experiments are performed at atmospheric pressure to explore the effects of jet impingement velocity and liquid subcooling. The tested jet velocity is 0.2, 0.31 and 0.5 m/s respectively, while the inlet subcooling is ranged from 36°C to 96°C. The results showed that wall temperature and even heat transfer mode at different locations of the heating surface are quite different, with the lowest temperature on the heating surface directly under the jets and the highest temperature on the heating surface under the center of four jets where the nucleation boiling incepts earliest and the critical heat flux (CHF) occurs. Increasing subcooling and jet velocity can delay the onset of nucleate boiling and enhance the critical heat flux dramatically. Wall temperature overshooting phenomenon can only be found on the heating surface under the center of four jets when the jet velocity is low and sub-cooling is high.
Numerical Simulation of Drag Anchor Installation by a Large Deformation Finite Element Technique
Authors: Zhao Yanbing , Liu Haixiao
Published: July 2018

Previously published finite element analysis of drag anchors only involved the pullout capacity of the anchor. There are no finite element (FE) simulations of the installation of drag anchors probably because of two restrictions. First, during the anchor installation, the installation line is needed, which is difficult to be simulated in the FE analysis. Second, the anchor installation that involves large deformation of surrounding soils can not be solved using the classical FE method. In the present work, the installation line is constructed by connecting cylindrical units with each other using connector elements. Then it is introduced into the installation of drag anchors, which is simulated by a large deformation finite element analysis using the coupled Eulerian-Lagrangian (CEL) technique. By comparing with theoretical solutions, including the tension and profile of the installation line embedded in soils, and the movement direction, drag force, drag angle and trajectory of the anchor, the FE simulation of the drag anchor installation is well verified. The present study also demonstrates that the CEL technique is effective for simulating the anchor-line-soil interactional problems.
Effect of a Strong Middle Layer on Spudcan Penetration
Authors: Gao Wen , Harrup Tom , Hu Yuxia , White David
Published: July 2018

The rapid penetration of one or more of the foundations of a mobile jack-up rig into the seabed is an ongoing major problem in the offshore industry, with the potential to cause major damage to the structure and endangering any personnel on board. A recent example is the jack-up drilling rig Perro Negro 6 incident happened near the mouth of the Congo river in July 2013 with one of the rig’s crew of 103 reported missing and six others injured. This uncontrollable displacement is due to a form of failure known as punch through failure and commonly occurs on stratified seabed profiles. It has been reported that unexpected punch-through accidents have resulted in both rig damage and lost drilling time at a rate of 1 incident per annum with consequential costs estimated at between US$1 and US$10 million [1].

This paper presents the bearing capacity profiles and associated soil flow mechanisms of a common spudcan foundation penetrating into a three layer soft-stiff-soft clay soil through the use of large deformation finite element (LDFE) analysis. The Remeshing and Interpolation with Small Strain (RITSS) [2, 3] technique was implemented in the software package AFENA [4] to conduct the LDFE analysis. Both soil layer thickness and soil layer strength ratios were varied to study their effect on the spudcan penetration responses. The LDFE results of spudcan penetration into the soft-stiff-soft clay soils were calibrated by existing centrifuge test data. A parametric study was then conducted to study the bearing capacity responses and soil flow mechanisms during spudcan large penetrations by varying the soil layer strength ratio and relative layer thickness to the diameter of spudcan.

It was found that there were three types of bearing responses during continuous penetration of spudcan: (a) when the top soft layer is relatively thin, the spudcan bearing response was similar to that of two layer soils with stiff over soft clays; (b) when the top soil layer thickness is medium, a peak resistance is observed when spudcan penetrates into the middle stiff layer followed by reduction; (c) when the soil layer is thick, the peak resistance occurs when spudcan gets into the bottom soft soil layer. The critical thickness of top soil layer is a function of soil strength ratio and middle stiff soil layer thickness. The bearing response types were also corresponding to the soil cavity formations during spudcan initial penetration.