bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Investigation of Transverse Jet in Supersonic Cross-Flow Using CFD Techniques
Authors: Rathore P. S. , Karuppa Raj R. Thundil
Published: July 2018

High speed jets in cross flows are central to fuel injection in supersonic combustion scramjet engines. In supersonic combustion scramjet engines, the sonic under expanded transverse jet of fuel is injected into a supersonic cross flow of air, where efficient mixing of fuel and air is one of the major critical issues. Due to the limited flow residence time inside the combustion chamber, the enhancement of supersonic turbulent mixing of jet fuel and cross-flow air is a critical issue in developing supersonic air-breathing engines. The accurate estimation and detailed physical understanding of the turbulent mixing mechanisms plays an important role in combustor design of scramjet engines. This numerical study aims at understanding the complex physical phenomenon involved in mixing of fuel jet and air and the associated turbulence characteristics, boundary layer capture and flow separation. In the current study the flow field resulting from the transverse injection of fuel jet into cross-flow of air is simulated numerically by solving the appropriate governing equations for a 2-dimensional flow. Numerical simulations are used to study an under-expanded jet injected into a supersonic cross flow. This study examines the flow structure, separation topology and performance characteristics of an under expanded transverse jet issuing normally into supersonic free stream. The influence of the compressibility effect on the shock wave structure and on the vortex system ahead and behind of the jet are studied by solving Favre averaged Navier Stokes (FANS) equations with SST k-ω turbulence model. The influence of the jet Mach number and jet-to-cross-flow pressure ratio on shock wave structure of the flow and jet penetration depth are studied. The simulated numerical results are compared with the experimental data available in the literature. Grid independence study is carried out for all the simulations carried out in the work to have good accurate results. It was found out that wall pressure profile of transverse jet injection for the high jet-to-cross-flow pressure ratio is predicted more accurately by the SST k-ω turbulence model. The jet penetration depth found out to be increasing with the increase in jet-to-cross-flow pressure ratio and fuel jet slot width.
Comparative Study of Flange-to-Seal Contact Couplings With Bolt Relaxation Under Creep Condition
Authors: Mao Jianfeng , Zhang Junhui , Wang Weizhe
Published: July 2018

Future coal-fired steam turbines promise increased efficiency and low emissions. However, this comes at the expense of increased thermal load from higher inlet steam temperatures and pressures leading to severe creep that significantly influences the sealing behavior and high temperature strength of bolted flange-seal couplings. Flanges with different thicknesses were employed for a comparative study. The important stress/creep values in the flanges and U-type seals had been obtained for variations in flange thickness and bolt relaxation whilst maintaining other leading parameters constant. The variation of contact stresses due to creep deformation plays an important role in achieving a leak proof sealing. In this paper, a two-dimensional finite element analysis (FEA) of bolted flange-seal couplings has been carried out by taking the relaxation of bolt stress under full-loading turbine service. The creep strength of flanges and U-type seals are investigated by Cocks-Ashby (C-A) equivalent strain method. The multiaxial state of stresses is considered in this method by using C-A multiaxial coefficient. According to ASME allowable creep limit, the C-A equivalent strains of three flange-seal couplings are evaluated and compared. Furthermore, based on the results of contact stresses, the creep behavior of U-type seals is analyzed varying flange thickness. Finally, analysis shows that the thinner flange-seal coupling has larger long-term contact stress, while the U-type seal with the thicker flange has the least creep strength.
An Advanced Unstructured-Grid Finite-Volume Design System for Gas Turbine Combustion Analysis
Authors: Anand M. S. , Eggels R. , Staufer M. , Zedda M. , Zhu J.
Published: July 2018

A general-purpose combustion Computational Fluid Dynamics (CFD) design analysis tool has been developed. The method is pressure-based and applicable to both incompressible and compressible flows. The unstructured finite-volume approach used can take arbitrary shapes of mesh cells to resolve complicated geometries. Turbulence is simulated either by Reynolds-Averaged Navier-Stokes (RANS) or by Large Eddy Simulation (LES) approaches. Combustion is modeled by various combinations of combustion chemistry and combustion-turbulence models including transport probability density function (PDF) model. A Lagrangian approach is used to simulate fuel spray droplet. The resulting tool has being used in routine combustor simulations for a variety of commercial and military combustor development programs. Application examples presented include simulations of several combustors and comparisons with available rig data.
Effect of Inlet Tangential Port Area on Droplet Size Distribution of Small-Scale Simplex Atomizer
Authors: Selvan Govindaraj Muthu , H. Suryanarayana Rao Muralidhara , Vyas Vinod Kumar , Shanmugasundaram Karthy , Venugopal Narendran , Sankar Karthic
Published: July 2018

An experimental investigation was conducted to study the effects of increased area of inlet tangential ports on the droplet size distribution of small-scale simplex atomizer. The spray characteristics of four different simplex atomizers representing increasing area of inlet tangential ports (diameter range 0.6 mm to 0.9 mm) are examined using water as working fluid. Measurements of droplet size and droplet size distributions of the four different atomizer configurations were carried using Malvern droplet size instrument at various downstream locations from final orifice exit. These measurments has been taken for five different injection pressures of spray. Variation of droplet size and droplet size distribution along the flow direction of spray was examined. The effect of increase in injection pressure on droplet size distribution of the spray was examined. Increase in inlet tangential port area significantly affects droplet size and droplet size distributions of the spray and affects the length of primary breakup region.
Troubleshooting of Vibration Problems in Gas Turbines With Proximity and Seismic Probes
Authors: Gopalakrishnan Padmanabhan , Sharma Pankajkumar
Published: July 2018

Gas Turbines generally have heavy rotors and light casings, and some of them exhibit significant motion between bearing housing and free space. Hence, rotor forces would have higher transmissibility when measured on the bearing housing. This makes casing absolute measurement as bare sufficient for vibration monitoring and protection in Gas Turbines. Most of the Industrial Gas Turbines in field have their acceptable vibration limits defined in velocity units. Typically 1/2 inch per sec 0-pk is referred to as alert set point where as 1 inch per sec 0-pk as danger set point as per ISO-10816-1.

However, such casing based measurements many a times doesn’t yield necessary information on what might be going on in the journal bearings, which are having larger clearances with rotor. This also happens when the Gas Turbine does not exhibit significant motion between bearing housing and free space as said by Maalouf [1]. Some typical situations arise in Gas Turbines which might also need shaft relative vibration measurements using proximity probes. This paper discusses some of the experiences of the author in the field of Gas Turbine vibration diagnosis using shaft relative and casing absolute methodology. It explains how shaft relative measurement adds more value to find out the actual malfunction diagnosis when casing measurement data becomes insufficient in such cases.
Non-Linear Dynamics, Instability and Chaos of Two Spool Aero Gas Turbine Rotor System
Authors: Payyoor Narayanan , Tiwari Mayank , Gupta Kshitij
Published: July 2018

The study provides an insight into the non-linear dynamic analysis of a two spool aero gas turbine rotor system. A nonlinear finite element model of the twin spool system is developed using Timoshenko beam element, considering the elastic deflections of the bearings and resulting nonlinear Hertz contact forces, coupled with the radial clearances present in the bearing. A brief study on the instability is performed on a modified two spool system with non-linearity present only at the inter-shaft bearing. The instability study was performed using shooting technique coupled with fixed point algorithm. An overview on chaotic dynamics of the system is also done on the same system using Wolf algorithm. Effect of bearing clearances, rotor speed and unbalance on the response, instability and chaos has also been discussed.
CFD Modeling to Study the Effect of Spacer Design on Centrifugal Compressor Exit Scroll Performance
Authors: Patri Vrushabhendra V. , Meduri Uday K. , Iurisci Giuseppe , Giachi Marco
Published: July 2018

Present day Centrifugal Compressor (CC) technology trend is moving towards innovative designs for improved performance. Computational Fluid Dynamics (CFD) plays an important role in the assessment and improvement of CC component design. Statoric parts like inlet plenum, diffuser, return channel, and exit scroll play a vital role in determining the overall performance of any compressor.

Exit scrolls (volute) are an integral part of any CC. A volute is essentially a flow collecting duct with its cross-section area increasing around the circumference as more volume of fluid is added. There are different types of volutes used for CC based on the type of application, casing size and the site conditions. The performance of a volute is very important in terms of magnitude of losses, static pressure recovery as well as impact on the upstream impeller. It is essential that the volute is well matched with the impeller and diffuser at the best stage efficiency point.

In addition to that, some obstructions needed for mechanical purposes (spacers, bolts, grooves) result in more losses, decreasing overall compressor efficiency affecting severely the upstream impeller. The purpose of this paper is to describe an extensive CFD analysis that was performed to understand and quantify the negative impact arising due to the presence of spacers, their shapes and circumferential location on the performance of the whole compressor.

The paper is divided into two sections. First section describes the CFD studies to understand performance of a typical scroll with spacers (boat tail spacers) as obstacles and effect of tangential location of spacers. The second section describes the CFD studies to understand the impact of the shape of spacers in a comparative manner.
An Evaluation of ASME’s “Design by Analysis” Guidelines
Authors: Raghavan Kotur S.
Published: July 2018

ASME’s Boiler and Pressure Vessel Codes have a history of over one hundred years. The codes have been evolving over time with continuous revisions, improvements and refinements. A major milestone has been the incorporation of “Design by Analysis (DBA)” guidelines about fifty years back (for instance Sec. VIII, Division 2). These were introduced as it was recognized that the prevailing Design by Rules (Section VIII, Division 1) tended to be somewhat over-conservative. The essence of DBA guidelines consists of evaluating the elastic stresses at critical locations and checking the same against the allowable. The allowable happen to functions of the nature of stress distribution and the nature of load. A given stress could be of membrane, bending or peak category and also be either primary or secondary. At the time of appearance of the DBA guidelines, the state of the art of stress analysis was not well advanced and the finite element method was just getting developed. As of today, however, the finite element method has reached a high level of maturity and is very widely used. The latest edition (2010) has recognized this and it contains modeling and post-processing guidelines applicable to FE analysis. This edition also recommends the use of one of three possible approaches. The first is the elastic analysis and classification and categorization of stresses with guidelines regarding how to deal with two- and three-dimensional situations. The other two options are provided to take care of situations wherein the categorization process may lead to either uncertainty or ambiguity. These involve nonlinear analysis either by way of Limit-Load method or Elastic-Plastic Stress Analysis. In either approach the analyst will look for the loads at which there is an onset of gross plastic flow. In the present paper an attempt is made to evaluate the latest DBA guidelines from design application point of view. The purpose is to assess the limitations of the elastic analysis approach. Studies are undertaken to focus typically on the following aspects: 1. Two dimensional problems involving symmetry or axisymmetry. There are situations in which the “bending” stresses are liable to be misinterpreted. 2. Three dimensional problems with emphasis on the assessment of bending stress as categorization in 3D situations is a real challenge 3. General situations involving the secondary stresses. The allowable stress limit for secondary stress is somewhat arbitrary and perhaps very conservative. The studies tend to suggest that the nonlinear route is to be adopted as it is reliable and accounts for many uncertainties associated with the elastic approach.
Effect of Hole Shape on Film Cooling Effectiveness Over a Flat Plate
Authors: Felix J. , Harshavardhana N. , Giridhara Babu Y. , Rajanna D. , Vinod Kumar N.
Published: July 2018

Film cooling method of hot section components in the gas turbine engines is under continuous optimization for the enhanced cooling performance. In the present study, film cooling performance for a row of different shaped holes like triangular, circular and extended triangular have been considered. The adiabatic film effectiveness and the convective heat transfer coefficients are found experimentally on a flat plate. All the three test models are having holes of 5 mm diameter drilled at 20 mm pitch and inclined at an angle of 22 degrees. At the immediate downstream of these models, a flat plate is attached for finding the effect of these hole configurations. This flat plate is made with the low conductivity substrate and the stainless sheet of 0.2 mm thick is pasted over it in the flow path. The test model along with the flat plate is placed to the side wall of the rectangular duct where the mainstream air is supplied. The setup is made in such a way that the coolant air passed through the holes will create a film over the flat plate downstream. Infra Red camera is used to capture the thermal image of the entire test plate. The flat plate is connected with six thermocouples to have the reference surface temperature to correct the IR thermal image data. K-type thermocouples are used to measure the coolant and mainstream air temperatures. In both the heat transfer coefficient and adiabatic film cooling effectiveness experiments the blowing ratio is varied by 0.5 to 2.0, by keeping the constant mainstream air velocity of 20 m/s at ambient temperature. In the heat transfer coefficient experiments, the flat plate is heated with the constant heat flux conditions. In the adiabatic film cooling experiments, the coolant air is maintained at the temperature of −50°C with the help of liquid nitrogen heat exchanger bath. Results are plotted by taking the adiabatic film cooling effectiveness and convective heat transfer coefficient values from the centerline of holes downstream along the flow direction. From the results, the triangular and extended triangular hole models shown higher heat transfer coefficient and adiabatic film cooling effectiveness than the circular hole model.
NO Prediction in Turbulent Diffusion Flame Using Multiple Unsteady Laminar Flamelet Modeling
Authors: Yadav Rakesh , Rajeshirke Pravin , Nakod Pravin
Published: July 2018

The steady laminar flamelet model (SLFM) [1, 2] has been shown to be reasonably good for the predictions of mean temperature and the major species in turbulent flames [3, 4]. However, the SLFM approach has limitations in the prediction of the slow chemistry phenomena like NO formation [5, 6]. In case of SLFM, the turbulence and chemistry are coupled through a single variable, called scalar dissipation, which is representative of the strain inside the flow. The SLFM model is not able to respond to the steep changes in the scalar dissipation values and generally tend to approach to the equilibrium solution as the strain relaxes [7]. The pollutant like NO is formed in the post flame zones and with a high residence time, where the scalar dissipation diminishes and hence the NO is over predicted using SLFM approach. In order to improve the prediction of slow forming species, a transient history of the scalar dissipation evolution is required. In this work, a multiple unsteady laminar flamelet approach is implemented and used to model the NO formation in two turbulent diffusion flames using detailed chemistry. In this approach, multiple unsteady flamelet equations are solved, where each flamelet is associated with its own scalar dissipation history. The time averaged mean variables are calculated from weighted average contributions from different flamelets.

The unsteady laminar flamelet solution starts with a converged solution obtained from steady laminar flamelet modeling approach. The unsteady flamelet equations are therefore solved as a post processing step with the frozen flow field. The domain averaged scalar dissipation for a flamelet at each time step is obtained by solving a scalar transport equation, which represents the probability of occurrence of the considered flamelet. The present work involves the study of the effect of number of flamelets and also the different methods of probability initialization on the accuracy of NO prediction. The current model predictions are compared with the experimental data. It is seen that the NO predictions improves significantly even with a single unsteady flamelet and further improves marginally with an increase in number of unsteady flamelets.
A Comparative CFD Study on Flamelet Generated Manifold and Steady Laminar Flamelet Modeling for Turbulent Flames
Authors: Nakod Pravin , Yadav Rakesh , Rajeshirke Pravin , Orsino Stefano
Published: July 2018

Laminar Flamelet Model (LFM) [1–2] represents the turbulent flame brush using statistical averaging of laminar flamelets whose structure is not affected by turbulence. The chemical non-equilibrium effects considered in this model are due to local turbulent straining only. In contrast, Flamelet Generated Manifold (FGM) [3] model considers that the scalar evolution, the realized trajectories on the thermo-chemical manifold in a turbulent flame is approximated by the scalar evolution similar to that in a laminar flame. This model does not involve any assumption on flame structure. Therefore, it can be successfully used to model ignition, slow chemistry and quenching effects far away from the equilibrium. In FGM, 1D premixed flamelets are solved in reaction-progress space rather than physical space. This helps better solution convergence for the flamelets over the entire mixture fraction range, especially with large kinetic mechanisms at the flammability limits [4].

In the present work, a systematic comparative study of FGM model with LFM for four different turbulent diffusion/premixed flames is presented. First flame considered in this work is methane-air flame with dilution air at the downstream. Second and third flame considered are jet flames in a coaxial flow of hot combustion products from a lean premixed flame called Cabra lifted H2 and CH4 flames [5–6] where the reacting flow associated with the central jet exhibits similar chemical kinetics, heat transfer and molecular transport as recirculation burners without the complex recirculating fluid mechanic. The fourth flame considered is Sandia flame D [7], a piloted methane-air jet flame. It is observed that the simulation results predicted by FGM model are more physical and accurate compared to LFM in all the flames presented in this work.
Heat Transfer Analysis of a Gas Turbine and Improving Predictability Using Prototype Test Data
Authors: Reddy T. Vishnu Vardhan , De Prosperis Roberto
Published: July 2018

Gas turbines are extremely complex & expensive machines that are subjected to various operating and environmental conditions. Increased customer attention towards higher efficiency is driving the industry to move to higher firing temperatures, which imparts higher thermal & mechanical loads on engine components. This in turn demands higher reliability levels for the engine components to ensure availability and safety of the machine. An effective modeling approach, which can accurately predict the thermal behavior of various components of a gas turbine, can be used to understand the critical regions, locate probable areas of failure, and to improve the overall design.

This paper presents a methodology to perform steady state thermal analysis of gas turbine flange to flange components (excluding the main flow path,) using a 2D FE model. A brief overview of the in-house tools and the modeling approach adopted to capture the complex physics using these tools are discussed. The results from this FE analysis are rigorously validated with data collected from various test campaigns of GE industrial Frame5 gas turbines. Several sensitivity studies were performed to study the impact of the uncertainties and to improve the thermal predictions compared to measurements. Studies using CFD tools and hand calculations were also performed to validate assumptions. This approach gives a thorough understanding of how a vast thermal model of gas turbine is developed and validated using test data, resulting in the improvement of thermal predictions of critical components.
Numerical Studies on Effect of Stepped Tip Clearance Height on the Performance of Single Stage Transonic Axial Flow Compressor
Authors: Batabyal Pritam , Alone Dilipkumar B. , Maharana S. K.
Published: July 2018

This paper presents a numerical case study of various stepped tip clearances and their effect on the performance of a single stage transonic axial flow compressor, using commercially available software ANSYS FLUENT 14.0. A steady state, implicit, three dimensional, pressure based flow solver with SST k-Ω turbulence model has been selected for the numerical study. The stepped tip clearances have been compared with the baseline model of zero tip clearance at 70% and 100 % design speed. It has been observed that the compressor peak stage efficiency and maximum stage pressure ratio decreases as the tip clearances in the rear part are increased. The stall margin also increases with increase in tip clearance compared to the baseline model. An ‘optimum’ value of stepped tip clearance has been obtained giving peak stage compressor performance. The CFD results have been validated with the earlier published experimental data on the same compressor at 70% design speed.
Turbulent Behaviour in a Low Aspect Ratio Dump Combustor at Low Swirl Number
Authors: Yadav N. P. , Kushari Abhijit
Published: July 2018

This paper reports an experimental investigation of a non-reacting flow in a low aspect ratio dump combustor with taper exit. The separated flow reattachment length for this combustor was higher than the length of the combustor. In order to understand the flow behavior inside the combustor, the velocity measurement inside the combustor was carried out by calibrated one-dimensional hot wire probes for the same flow conditions. The turbulent intensity variation inside the combustor without swirl flow was found to be axi-symmetric. The turbulence intensity along the centre line is lower than at other radial locations because of the presence of the potential core. The intensity in the core region increases as one moves along the centerline in the axial direction, which can be attributed to the weakening of the potential core and the increase in the extent of the shear layer. The turbulence behavior significantly changed with the use of a swirler at the inlet of the combustor. The intensity variation inside the combustor is more uniform than the low swirl number SN = 0.23. The higher turbulence kinetic energy and turbulent dissipation rate was observed at this swirl number. Therefore, the effect of swirl is expected to improve the combustion through better mixing and flame stabilization.
High Sampling Rate Measurements of Jet Engine Fuel Control System Transients for Study on the Effect of Modification in Changeover Valve
Authors: Uppal Tarun , Yadu Prakash Kumar , Kannur Sudhakar , Ravindra D. N. , Kumar Ananda
Published: July 2018

Validation of the design is a milestone in development cycle of a system. Actual testing of the component is an ultimate means to carry out the design validation. It is required to perform testing and verify the hardware functionality in order to gain confidence in operational implementation. Data acquisition during the performance acceptance testing is an essential requirement for any design validation by testing. If the system dynamics are changing rapidly the need for high speed measurements becomes essential such as in the case of jet engine fuel control system which is a dynamical system with fast transients of the order of few milliseconds. A better acquisition hardware and processing algorithm helped to reduce the latency in the acquired signals. Further, since there are multiple design test centers for fuel control system development, portability of data acquisition system aided in validating the subsystem with same standard of preparation at all centers prior to system integration.
Gas Turbine Blade Damper: A Design Optimization Study to Mitigate High Resonance Blade Vibration
Authors: Kumar Dilip , Barad Sanjay , Suresh T. N.
Published: July 2018

This paper describes the design optimization study of an under platform damper to mitigate high vibration problem of a gas turbine rotor blade under resonance condition. An existing theoretical model explicitly, Casba friction damper model was used to evaluate the dynamic characteristics of the turbine blade with under platform damper. Turbine blade is approximated as two degrees of spring-damper-mass system, which is dynamically equivalent to real turbine blades for its first two eigen values. Blade tip response predictions were carried out for different damper mass, stiffness and coefficient of friction under simulated rotational speed of the rotor, to arrive at an optimum mass to control the blade tip response.

As a practical application, along with damper mass optimization, shape and mass distribution of the damper is obtained by design trials to ensure good contact between the blade root and damper upper surface. Contact analysis was carried using the ANSYS software. The asymmetric skewed damper geometry posed complications with respect to modelling and optimisation.

In realistic application, with the kind of uncertainties in contact pattern, variation in friction coefficient, geometric tolerances, validation/verification plays a major role in assessing the design. As part of verification of this damper design, a full scale gas turbine engine test program was envisaged and completed. Modified optimum damper was implanted as a design change, engine was instrumented for blade vibration measurement. Non-Intrusive Stress Measurement system was used for measuring blade tip amplitudes from all the blades in the rotor. Test blade tip vibration was analysed and compared against the predications.

This optimised damper configuration has showed significant reduction in blade amplitudes during full-scale gas turbine testing, in comparison to original design proving the efficacy of new modified damper.
Identification of Multiple Fault Parameters in a Rigid-Rotor and Flexible-Bearing-Coupling System: An Experimental Investigation
Authors: Lal Mohit , Tiwari Rajiv
Published: July 2018

Rotating machineries are very common and widely used in the modern industrial world. A breakdown of the rotating machine may result in economic losses and even worse, in the damage of human beings. So it needs accurate and reliable prediction of its dynamic characteristic. The multiple fault identification algorithm developed in [1] identifies the bearing and coupling dynamic parameters along with residual unbalances in a rigid-rotor and flexible-bearing-coupling system has been validated experimentally in the present article. An indigenously developed test rig was used for the experimentation. After estimating multiple fault parameters the accuracy of estimated parameters has been checked by checking the consistency of the estimated parameters and by performing the standard impact test. It has been observed that with more number of measurements consistencies of the estimated parameters are very good.
A Study on Bird Impact Damages on Shrouded Fan Blades of an Aero-Engine
Authors: Prakash Raghu , Channegowda Hithesh , Kaliyaperumal Anandavel
Published: July 2018

Aero-engine fan blades are the critical components that are vulnerable to Foreign Object Damage (FOD) such as bird impact. The thrust loss due to bird impact can affect engine core function, resulting in catastrophic failure. The fan blades should be designed to have adequate resistance to bird impact. The present paper focuses on numerical evaluation of bird impact response on shrouded and un-shrouded rotating set of fan blades using three dimensional computational methodology. The impact response is compared between shrouded and un-shrouded blades in terms of deformation, von-Mises stress, plastic strain, and energy absorbed. Numerical analysis results indicate that the shrouded fan blade absorbs 35 % more energy compared to un-shrouded blade. Deformation damage at impact location of shrouded blade is lesser compared to un-shrouded blade. The maximum plastic strain observed on shrouded blade due to bird impact is also 50 % lesser than the un-shrouded blade. The study suggests that the shrouded fan blades provide better impact resistance characteristics to bird impact compared to un-shrouded fan blades.
Thermal and Electrical Performance of a PV Module Integrated With Microencapsulated Phase Change Material
Authors: Ho C. J. , Tanuwijaya A. O. , Lai Chi-Ming
Published: July 2018

The efficiency in photovoltaic modules decreases as the cell temperature increases. It is necessary to have adequate thermal management mechanism for a photovoltaic module, especially when combined with building facade. This study aims to investigate the thermal and electrical performance of a photovoltaic module integrated with a microencapsulated phase change material layer under temporal variations of daily solar irradiation and exterior ambient temperature, via computational fluid dynamics simulations. The results show that the melting temperature and aspect ratio of the microencapsulated phase change material layer have significant effects on the thermal and electrical performances of photovoltaic modules.
Limits of Parabolic Flow Theory in Microfluidic Particle Separation: A Computational Study
Authors: Pawell Ryan S. , Barber Tracie J. , Inglis David W. , Taylor Robert A.
Published: July 2018

Microfluidic particle separation technologies are useful for enriching rare cell populations for academic and clinical purposes. In order to separate particles based on size, deterministic lateral displacement (DLD) arrays are designed assuming that the flow profile between posts is parabolic or shifted parabolic (depending on post geometry). The design process also assumes the shape of the normalized flow profile is speed-invariant. The work presented here shows flow profile shapes vary, in arrays with circular and triangular posts, from this assumption at practical flow rates (10 < Re < 100). The root-mean-square error (RMSE) of this assumption in the circular post arrays peaked at 0.144. The RMSE in the triangular post array peaked at 0.136. Flow development occurred more rapidly in circular post arrays when compared to triangular post arrays. Additionally, the changes in critical bumping diameter (DCB) the DLD design metric used to calculate the size-based separation threshold were examined for 10 different row shift fractions (FRS). These errors correspond to a DCB that varies as much as 11.7% in the circular post arrays and 15.1% in the triangular post arrays.
CHF Enhancement of Pool Boiling in Graphene Oxide Nanofluid With Chemical Reduction
Authors: Moon Sung Bo , Park Seong Dae , Park Hyung Wook , Bang In Cheol
Published: July 2018

Graphene oxide (GO) can be deposited on a heater surface to produce nanoscale structures that enhance the thermal limit of the heater. GO has shown a very unique feature showing CHF enhancement without wettability improvement in terms of apparent advancing contact angle unlike any other typical nanoparticles. Many studies have analyzed the reason for the enhanced critical heat flux (CHF) of the heater. Three major models have been used for studying the enhanced CHF of the heater in many heat transfer studies: wettability of surface (contact angle), Rayleigh–Taylor instability wavelength, and thermal activity. In this study, scanning electron microscopy (SEM) images and the contact angle were taken to explain the enhanced wettability of a heater surface in a GO nanofluid. GO is composed of carbon and oxygen. This material has good affinity with water because of its polarity. In an electric field, GO reduces into reduced graphene oxide (RGO). This chemical reduction on the surface may be one factor that enhances the CHF. To examine how a GO nanofluid can enhance CHF to more than twice its original magnitude, a wettability model was applied that uses the contact angle of the nichrome wire heater surface after the CHF experiment. The reason why the wettability model could not completely account for the CHF enhancement in a GO nanofluid was also determined. The reduction of GO was considered to explain CHF enhancement.
Fabrication of GaN Films in a Chemical Vapor Deposition Reactor
Authors: Meng J. , Jaluria Y. , Wong S.
Published: July 2018

A three-dimensional numerical study has been carried out on the rotating disk GaN MOCVD process, and it is also coupled with an experimental study on the flow and thermal transport processes in the system. An impingement type reactor, with a rotating base, is considered. The dependence of the thin film growth rate and uniformity on operating conditions such as inflow velocity, rotational speed, and susceptor temperature are investigated in detail. Similarly, the effect of the geometry and configuration of the reactor are studied. The study also considers the effect of thermal and solutal buoyancy on the resulting flow. The flow and the associated transport processes are discussed in detail on the basis of the results obtained to suggest approaches to improve the uniformity of the film, minimize fluid loss and reduce flow recirculation that could affect growth rate and uniformity.
Numerical Simulation of 2D Electrothermal Flow Using Boundary Element Method
Authors: Ren Qinlong , Chan Cho Lik , Arvayo Alberto L.
Published: July 2018

Microfluidics and its applications to Lab-on-a-Chip have attracted a lot of attention. Because of the small length scale, the flow is characterized by a low Re number. The governing equations become linear. Boundary element method (BEM) is a very good option for simulating the fluid flow with high accuracy. In this paper, we present a 2D numerical modeling of the electrothermal flow using BEM.

In electrothermal flow the volumetric force is caused by electric field and temperature gradient. The physics is mathematically modeled by (i) Laplace equation for the electrical potential, (ii) Poisson equation for the heat conduction caused by Joule heating, (iii) continuity and Stokes equation for the low Reynolds number flow. We begin by solving the electrical potential and electric field. The heat conduction is caused by the Joule heating as the heat generation term. Superposition principle is used to solve for the temperature field. The Coulomb and dielectric forces are generated by the electrical field and temperature gradient of the system. We analyze the Stokes flow problem by superposition of fundamental solution for free-space velocity caused by body force and BEM for the corresponding homogeneous Stokes equation. It is well known that a singularity integral arises when the source point approaches the field point. To overcome this problem, we solve the free-space velocity analytically. For the BEM part, we also calculate all the integral terms analytically. With this effort, our solution is more accurate. In addition, we improve the robustness of the matrix system by combining the velocity integral equation with the traction integral equation.

Our purpose is to design a pump for the microfluidics system. Since the system is a long channel, the flow is fully developed in the area far away from the electrodes. With this assumption, the velocity profile is parabolic at the inlet and outlet of the channel. So we can get appropriate boundary conditions for the BEM part of Stokes equation. Consequently, we can simulate the electrothermal flow in an open channel.

In this paper, we will present the formulation and implementation of BEM to model electrothermal flow. Results of electrical potential, temperature field, Joule heating, electrothermal force, and velocity field will be presented.
Analysis and Optimisation of Two-Dimensional Silicon Complex Grating With Different Ridge Heights or Groove Depths for Solar Cells
Authors: Li Pingping , Cheng Qiang , Wu Hao , Song Jinlin , Zhou Huaichun
Published: July 2018

In this study, two kinds of two-dimensional (2D) complex gratings are proposed for a potential application as absorbing surfaces for solar cells in the visible and near-infrared wavelength regions, which are based on the superposition of multiple 2D simple gratings with different ridge heights for convex gratings or different groove depths for concave gratings, respectively. Silicon is selected as the complex grating material because it is common in micro/nanofabrication. Compared with one-dimensional (1D) gratings, the new structures present excellent radiative properties to rays from all directions. Besides, the new gratings can achieve satisfactory performance under both TM and TE waves, which cannot be easily obtained by 1D gratings. Furthermore, these two kinds of 2D complex gratings can both achieve higher absorptance in the whole of the interested spectral range by making full use of the microcavity resonance than 2D simple gratings with the same ridge height or groove depth. Taguchi method is employed as an efficient way of searching for the optimal profiles for the 2D complex gratings. The average spectral absorptance of the optimized structure for the 2D complex convex grating with two different ridge heights is above 0.93 within wavelength region from 0.3 to 1.1 μm for both TM and TE waves under normal incidence, which suggests that the proposed structures can be well suitable for solar absorber applications. The Finite-different time-domain (FDTD) method is used for all numerical calculations to obtain spectral absorptance of different structures.
Rarefied Conditions in the Convective-Diffusive Regimes of a Disc in Natural Convection
Authors: Stafford Jason , Enright Ryan , Kempers Roger
Published: July 2018

A numerical investigation has been conducted on a heated circular disc immersed in quiescent air in an enclosed environment and for rarefied conditions. For moderate to low Rayleigh numbers, laminar natural convection can be described by two regimes and a transitional region which separates the diffusive limit and laminar boundary layer, or convective limit. A practical scenario was simulated which involved lowering Rayleigh number and increasing Knudsen number of the disc simultaneously to examine the convective-diffusive relationship under a rarefied condition. The numerical model used to solve the continuum equations has been extended using a first order slip condition and assessed based on the available experimental data from literature. Although similar convective-diffusive regimes exist, there is a divergence from the classical continuum diffusive limit when operating in rarefied conditions with Knudsen numbers of order 10−3 or higher. The conditions for this second transitional region in heat transport from the surface have been highlighted and have been attributed to the temperature jump condition. This has resulted in recommendations on the limitation of existing analytical heat transfer models when applied to rarefied conditions. The findings also have practical significance to thermal metrology and aerospace industries where buoyancy induced flows and low ambient pressures are frequently experienced.
Experimental and Numerical Evaluation of Small-Scale Cryosurgery Using Ultrafine Cryoprobe
Authors: Okajima Junnosuke , Komiya Atsuki , Maruyama Shigenao
Published: July 2018

Cryosurgery is one of the surgical treatments using a frozen phenomenon in biological tissue. In order to reduce the invasiveness of cryosurgery, the miniaturization of cryoprobe, which is a cooling device for cryosurgery, has been required. The authors have developed a ultrafine cryoprobe for realizing low-invasive cryosurgery by the local freezing. The objective of this study is to evaluate the small-scale cryosurgery using the ultrafine cryoprobe experimentally and numerically.

The ultrafine cryoprobe has a double-tube structure and consists of two stainless microtube. The outer diameter of ultrafine cryoprobe was 550 μm. The inner tube, which has 70 μm in inner diameter, depressurizes the high-pressure liquidized refrigerant. Depressurized refrigerant changes its state to two-phase and passes through the gap between outer and inner tube. The alternative Freon of HFC-23 was used as a refrigerant, which has the boiling point of −82°C at 0.1 MPa.

The cooling performance of this ultrafine cryoprobe was tested by the freezing experiment of the gelated water kept at 37°C. The gelated water at 37°C is a substitute of the biological tissue. As a result of the cooling in 1 minute, the surface temperature of the ultrafine cryoprobe was reached at −35°C and the radius of frozen region was 2 mm.

In order to evaluate the temperature distribution in the frozen region, the numerical simulation was conducted. The two-dimensional axisymmetric bioheat transfer equation with phase change was solved. By using the result from the numerical simulation, the temperature distribution in the frozen region and expected necrosis area is discussed.
Two-Phase Flow and Heat Transfer in Pin-Fin Enhanced Micro-Gaps With Non-Uniform Heating
Authors: Isaacs Steven A. , Joshi Yogendra , Zhang Yue , Bakir Muhannad S. , Kim Yoon Jo
Published: July 2018

In modern microprocessors, thermal management has become one of the main hurdles in continued performance enhancement. Cooling schemes utilizing single phase microfluidics have been investigated extensively for enhanced heat dissipation from microprocessors. However, two-phase fluidic cooling devices are becoming a promising approach, and are less understood. This study aims to examine two-phase flow and heat transfer within a pin-fin enhanced micro-gap. The pin-fin array covered an area of 1cm × 1cm and had a pin diameter, height and pitch of 150μm, 200μm and 225μm, respectively, (aspect ratio of 1.33). Heating from two upstream heaters was considered. The working fluid used was R245fa. The average heat transfer coefficient was evaluated for a range of heat fluxes and flow rates. Flow regime visualization was performed using high-speed imaging. Results indicate a sharp transition to convective flow boiling mechanism. Unique, conically-shaped two-phase wakes are recorded, demonstrating 2D spreading capability of the device. Surface roughness features are also discussed.
Ultrasound Preprocessing to Improve Nanoparticles’ Performance in Enhancing Cryosurgery
Authors: Sun Zi-Qiao , Yang Yang , Deng Zhong-Shan , Liu Jing
Published: July 2018

Nanoparticles mediated cryosurgery was recently established as an efficient way to significantly improve the output of a conventional freezing therapy. To further improve this newly emerging nanomedicine way for ablating target tumor, here the ultrasound preprocessing was proposed for the first time to enhance the freezing strength to a better level on the tissues. In vitro experiments were carried out to evaluate the effects of a single ultrasound preprocessing, a single nanoparticle loading or their combination in enhancing the cryosurgery. Meanwhile, the mechanisms of the ultrasound preprocessing to improve nanoparticles’ mediated freezing capability were also theoretically interpreted. Experimental measurements demonstrate that, the ultrasound preprocessing on the target tissue site injected with nanoparticles not only evidently expended the freezing area, but also helps realize a much lower temperature scale and offers higher freezing rate during the nanocryosurgical process. Two main reasons to contribute to such effects were identified as the enhanced convective heat transfer in micro scale and the varied cellular impermeability caused by the ultrasound. The combined effect of ultrasound preprocessing and nanoparticles would be greater than the sum of their individual effects in mediating the nanocryosurgery. As a convinced approach, the present method opens a new way for the improved freezing ablation on tumor which can possibly be used in future clinics.
Experimental Investigation of a Two-Phase Drainage Process in Porous Media
Authors: Goharzadeh Afshin , Fatt Yap Yit , Song Jake Joong Hoon , Chai John , Vargas Francisco
Published: July 2018

This paper presents an experimental study of two phase water-oil displacement in transparent porous media model made of mini-channels. The working fluid (water or/and oil) was injected into the porous matrix using a syringe pump with a constant flow rate. The influence of gravity and flow rates were studied. The volume fraction was defined and measured during the drainage. Two different experimental configurations, namely vertical and horizontal mini-channel were studied. For the vertical mini-channel, two injections, namely gravity-assisted (top-to-bottom) injection and anti-gravity (bottom-to-top) injection were performed. Experiments using horizontal mini-channel were also performed with the inlet and outlet placed on the same horizontal plane. It was observed that the gravity has significant effect in case of gravity-assisted injection and did not influence the anti-gravity injection or the horizontal injection setups.
Synthesis and Characterization of Solid State Phase Change Material Microcapsules for Thermal Management Applications
Authors: Cao Fangyu , Ye Jing , Yang Bao
Published: July 2018

Polyalcohols such as neopentyl glycol (NPG) undergo solid-state crystal transformations that absorb/release sufficient latent heat. These solid-solid phase change materials (PCM) can be used in practical thermal management applications without concerns about liquid leakage and thermal expansion during phase transition. In this paper, microcapsules of NPG encapsulated in silica shell were successfully synthesized with the use of the emulsion technique. The size of the microcapsules was in the range of 0.2–4 μm, and the thickness of the silica shell was about 30 nm. It was found that the endothermic event of the phase change behavior of these NPG-silica microcapsules was initiated at around 39 °C and the latent heat was about 96.0 J/g. A large supercooling of about 43.3 °C was observed in the pure NPG particles without shell. The supercooling of the NPG microcapsules can be reduced to about 14 °C due to the heterogeneous nucleation sites provided by the silica shell. These NPG microcapsules were added into heat transfer fluid PAO to enhance its heat capacity. The effective heat capacity of the fluids can be increased by 56% by adding 20 wt. % NPG-silica microcapsules.