bluuebunny/crossref_metadata_2025 huggingface dataset:
Random Walk Behaviour of Malaysia Share Return in Different Economic Circumstance
Authors: Marsani Muhammad Fadhil , Shabri Ani
Published: December 2019

This journal renders the random walk behaviour of the Malaysian daily share return, through tests of efficient market hypothesis (EMH) based on three different financial periods, namely growth, financial crisis, and recovery period. This review also covers the behaviour of extreme return for weekly and monthly series generated from Block maxima-minima method. Autocorrelation Function test (ACF) and Ljung-Box test had been employed to measure average correlation between observations, while Augmented Dickey-Fuller (ADF), Phillips-Perron (PP), Kwiatkowski Phillips Schmidt Shin (KPSS) test had been used to scan the unit root and the stationarity. Multiple variance ratio tests had also been conducted to examine the random walk behaviour. Serial correlation test indicated that the movement of daily return during the financial crisis period was weak-form efficiency. The unit root and stationary tests suggested that each daily series was stationary, but trend stationary for extreme cases. Variance ratio tests indicated that the return during the recovery period was weak-form inefficiency due to the short lag autocorrelation in series.
Simulating Dengue: Comparison of Observed and Predicted Cases from Generic Reaction-Diffusion Model for Transmission of Mosquito-Borne Diseases
Authors: Kon Cynthia Mui Lian , Labadin Jane
Published: December 2019

Dengue is a mosquito-borne disease caused by virus and found mostly in urban and semi-urban areas, in many regions of the world. Female Aedes mosquitoes, which usually bite during daytime, spread the disease. This flu-like disease may progress to severe dengue and cause fatality. A generic reaction-diffusion model for transmission of mosquito-borne diseases was proposed and formulated. The motivation is to explore the ability of the generic model to reproduce observed dengue cases in Borneo, Malaysia. Dengue prevalence in four districts in Borneo namely Kuching, Sibu, Bintulu and Miri are compared with simulations results obtained from the temporal and spatio-temporal generic model respectively. Random diffusion of human and mosquito populations are taken into account in the spatio-temporal model. It is found that temporal simulations closely resemble the general behavior of actual prevalence in the three locations except for Bintulu. The recovery rate in Bintulu district is found to be the lowest among the districts, suggesting a different dengue serotype may be present. From observation, the temporal generic model underestimates the recovery rate in comparison to the spatio-temporal generic model.
IDENTIFYING KEY FACTORS OF COMMUTER TRAIN SERVICE QUALITY:AN EMPIRICAL ANALYSIS FOR DHAKA CITY
Authors: Nowshin Mowrin Anika , Hadiuzzaman Md. , Barua Saurav , Rahman Md. Mizanur
Published: December 2019

Commuter train is a viable alternative to road transport to ease the traffic congestion which requires appropriate planning by concerned authorities. The research is aimed to assess passengers’ perception about commuter train service running in areas near Dhaka city. An Adaptive Neuro Fuzzy Inference System (ANFIS) model has been developed to evaluate service quality (SQ) of commuter train. Field survey data has been conducted among 802 respondents who were the regular user of commuter train and 12 attributes have been selected for model development. ANFIS was developed by the training and then tested by 80% and 20% of the total sample respectively. After that, model performance has been evaluated by (i) Confusion Matrix (ii) Root Mean Square Error (RMSE) and attributes are ranked based on their relative importance. The proposed ANFIS model has 61.50% accuracy in training and 47.80% accuracy in testing.  From the results, it is found that 'Bogie condition', 'Cleanliness', ‘Female harassment’, 'Behavior of staff' and 'Toilet facility' are the most significant attributes. This indicates that some necessary measures should be taken immediately to recover the effects of these attributes to improve the SQ of commuter train.
Pressure Drop and Flow Development in the Entrance Region of Microchannels With Second-Order Velocity Slip Condition and the Requirement for Development Length
Authors: Ray Baibhab , Durst Franz , Ray Subhashis
Published: December 2019

In this investigation, Lfd* and Δp in the entrance region of circular and parallel plate microchannels have been determined for 10−2≤Re≤104 and 10−4≤Kn≤0.2, employing the second-order velocity slip condition at the wall with C1=1 and 0≤C2≤0.5. Results indicate that although local velocity slip at the wall is always higher than that for the fully developed section, local wall shear stress for higher Kn and C2 could be lower than its fully developed value, which is also more prominent for lower Re. Therefore, depending upon the operating condition, K(x) and Kfd could assume negative values, implying that pressure gradient in the developing region could even be less than that in the fully developed section. It has been further observed that both Lfd* and Kfd are characterized by the low and the high Re asymptotes, using which extremely accurate correlations have been proposed for both geometries.
Partial Finite-Time Stabilization of Perturbed Nonlinear Systems Based on the Novel Concept of Nonsingular Terminal Sliding Mode Method
Authors: Razmjooei Hamid , Shafiei Mohammad Hossein
Published: December 2019

In this article, a new technique to design a robust controller to achieve finite-time partial stabilization for a class of nonlinear perturbed systems is proposed. Indeed the system is partially stabilized in a finite time, based on the novel concept of the nonsingular terminal sliding mode (TSM) control method. In the first step, the nonlinear dynamical system is divided into two subsystems based on their required stability properties of the system's states (where finite-time stability is only desired for the first subsystem). Then, using a partial diffeomorphism map to transform the first subsystem into the normal form, the control law is designed. Indeed, by introducing this new concept of the TSM method, robust finite-time stability of only a part of the system's state is guaranteed. Subsequently, simulation results demonstrate the effectiveness of the proposed method, and the results are compared with the existing methods.
Investigating the Effects of Engine Operating Conditions on the Cyclic Variability of a Commercial Flex-Fuel Engine
Authors: Um Min Allah Fazal , Rufino Caio Henrique , Gallo Waldyr Luiz Ribeiro , Zabeu Clayton Barcelos
Published: December 2019

The flex-fuel engines are quite capable of running on gasohol and hydrous ethanol. However, the in-cylinder cyclic variations, which are inherently present in spark-ignition (SI) engines, affect the performance of these engines. Therefore, a comprehensive analysis is required to evaluate the effects of in-cylinder cyclic variations of a flex-fuel engine. The experiments were carried out by using Brazilian commercial Gasohol E27 (mixture of 27% anhydrous ethanol in gasoline) and hydrous ethanol E95h (5% water by volume in ethanol) as fuels for a commercial flex-fuel spark ignition engine. A comparison between the cyclic variations of gasohol and hydrous ethanol is presented in this paper. Moreover, the effects of engine operating parameters (i.e., engine speed, engine load and relative air fuel ratio) on cyclic variations are also investigated. The acquired data of in-cylinder pressure and combustion durations are evaluated by carrying out a statistical analysis. The coefficient of variation for indicated mean effective pressure (IMEP) did not exceed the limit of 5% for all tested conditions. Higher cyclic variability of maximum in-cylinder pressure is observed for gasohol fuel and higher engine speeds. The variability of in-cylinder combustion is also evaluated with the help of different combustion stages, which are characterized by corresponding crank positions of 10%, 50% and 90% mass fractions burned.
Classifying IC Engine Swirl Fields by Complex Moments
Authors: Teh Kwee-Yan , Ge Penghui , Zhao Fengnian , Hung David L. S.
Published: December 2019

Engine in-cylinder flow varies from cycle to cycle, which contributes to variation of the mixing and combustion processes between fuel and air. Such flow field cyclic variability at the macroscopic scale is distinct from random fluctuations at the microscopic scale about the ensemble mean velocity field due to turbulence. At the extreme, the mean velocity field may bear no resemblance to any instantaneous flow field within the ensemble. Rather, these instantaneous fields may appear multimodal. Yet previous attempts to define and identify the flow modes were either qualitative (by visual inspection), or based on strict point-by-point velocity difference between two flow fields. The former approach is clearly subjective; the latter does not accommodate translational and rotational variations of in-cylinder flow patterns relative to a flow mode. Such spatial variations, in location and orientation, of the flow patterns can be quantified by the technique of complex moment normalization. The algebraic properties of complex moments are also intimately related to the geometric and physical properties of two-dimensional/two-component flow fields. In this paper, we take the normalized moments as flow field attributes for further cluster analysis. This analysis approach is demonstrated using a set of in-cylinder flow fields obtained by high-speed particle image velocimetry on a swirl plane of a research optical engine operating under low intake swirl setting. The resulting classification of the flow fields into several clusters (flow modes) are discussed, and the potential and limitations of the analysis approach are appraised.
Effects of Valve Strokes on Spray Shapes for GDI Injector
Authors: Ishii Eiji , Yoshimura Kazuki , Hosaka Tomoyuki
Published: December 2019

The effects of valve strokes on the spray shapes of a GDI injector with six holes were studied. Two kinds of strokes that change the shapes to be either cone-shaped or bell-shaped were selected to investigate the flow characteristics that caused the shapes to change. Fuel-spray behaviors were first observed with an experimental setup; strokes of 100 percent (full stroke) and 26 percent were selected assuming the multiple injection of fuel. The 26 percent stoke was selected as a representative example of narrow stroke. To investigate the flow characteristics that caused the cone and bell shapes, computational fluid simulation was applied to study the fluid flows around the holes and sprays in the air region. It was found that the stroke of 26 percent formed a narrow space upstream of the hole inlets, so the velocity component in the radial direction at the hole inlets increased. The velocity component in the radial direction also increased at the hole outlets, so each spray plume became wider and the spaces between the plumes narrower. Due to the narrower spaces, the mixture of the plumes caused the spray to become bell-shaped. The velocity component in the radial direction at the hole outlets was an important factor that determined the spray shape in the air region.
Systematic Multi-Index Validations of SIDI Engine Flow Field LES Computations Using Crank Angle-Resolved PIV Measurements
Authors: Liu Mengqi , Zhao Fengnian , Li Xuesong , Xu Min , Yue Zongyu , Som Sibendu , Hung David L. S.
Published: December 2019

In-cylinder flow fields make significant impacts on the fuel atomization, fuel mixture formation, and combustion process in spark ignition direct injection (SIDI) engines. In recent years, model-based simulation approaches are preferred in regard to investigating the transient in-cylinder flow field characteristics. Most commonly, the simulation models are validated using single representative flow field at a typical crank angle measured by particle image velocimetry (PIV). However, it provides only limited knowledge about the flow field which is highly three-dimensional and of transient nature. In this study, crank angle-resolved PIV measurements are conducted on three distinct planes inside the cylinder to capture the transient process of flow field characteristics which vary with the crank angle. These three planes consist of one tumble plane through the spark plug tip, one tumble plane along two intake ports, and one swirl plane at 30 mm below the cylinder head. Large eddy simulation (LES) is employed for the numerical computations using the CONVERGE codes. On the basis of large datasets for both temporal and spatial domains, a multi-index systematic validation approach is conducted. Crank angle-resolved calculations of global indices and local indices are implemented using the flow fields velocity data obtained from both PIV and LES on select planes. Global indices reveal the trends in similarities of different crank angle degrees and locations, while local indices give the detail comparison results. In summary, with the systematic multi-index validation approach, the crucial crank angle degrees and locations for model verification will be detected. Furthermore, the corresponding critical flow features are analyzed. Practical guideline of flow field validation is proposed.
Evaluation of Effective Active Site on Pd Methane Oxidation Catalyst in Exhaust Gas of Lean Burn Gas Engine
Authors: Nitta Yoshifuru , Yamasaki Yudai
Published: December 2019

Lean-burn gas engines have recently attracted attentions in the maritime industry, because they can reduce NOx, SOx and CO2 emissions. However, since methane (CH4) is the main component of natural gas, the slipped methane which is the unburned methane emitted from the lean-burn gas engines likely contributes to global warming. It is thus important to make progress on exhaust aftertreatment technologies for lean-burn gas engines. A Palladium (Pd) catalyst for CH4 oxidation is expected to provide a countermeasure for slipped methane, because it can activate at lower exhaust gas temperature. However, a deactivation in higher water (H2O) concentration should be overcome, because H2O inhibits CH4 oxidation.

This study was performed investigates the effects of exhaust gas temperature or gas composition on active Pd catalyst sites to clarify CH4 oxidation performance in the exhaust gas of lean-burn gas engines. The authors developed the method of estimating effective active sites for the Pd catalyst at various exhaust gas temperature. The estimation method is based on the assumption that active sites used for CH4 oxidation process can be shared with the active sites used for Carbon mono-oxide (CO) oxidation. The molecular of chemisorbed CO on the active sites of the Pd catalyst can provide effective active sites for CH4 oxidation process. To clarify the effects of exhaust gas temperature and compositions on active Pd catalyst sites, the authors developed an experimental system for the new estimation method. This paper introduces experimental results and verifications of the new method, showing that chemisorbed CO volume on a Pd/Al2O3 catalyst is increased with increasing Pd loading in 250–450 °C, simulated as a typical exhaust gas temperature range of lean-burn gas engines. The results provide a part of the criteria for the application of Pd catalysts to the reduction of slipped methane in exhaust gas of lean-burn gas engines.
Observation of Flame Propagation in a Heavy-Duty Natural-Gas Lean Burn Spark-Ignition Engine
Authors: Liu Jinlong , Dumitrescu Cosmin E.
Published: December 2019

Natural gas (NG) is an alternative combustible fuel for the transportation sectors due to its clean combustion, small carbon footprint, and, with recent breakthroughs in drilling technologies, increased availability and low cost. Currently, NG is better suited for spark-ignited (SI), as a gasoline replacement in conventional SI engines or as a diesel replacement in diesel engines converted to SI operation. However, the knowledge on the fundamentals of NG flame propagation at conditions representative of modern engines (e.g., at higher compression ratios and/or lean mixtures) is limited. Flame propagation inside an engine can be achieved by replacing the original piston with a see-through one. This study visualized flame activities inside the combustion chamber of an optically-accessible heavy-duty diesel engine retrofitted to NG SI operation to increase the understanding of combustion processes inside such converted engines. Recordings of flame luminosity throughout the combustion period at lean-burn operating conditions indicated that the fully-developed turbulent flame formed from several smaller-scale kernels. These small kernels varied with shapes and locations due to different flow motion around the spark location (including the effect of spark electrodes on the local flow separation), different local temperature, or different energy released in these regions. In addition, the turbulent flame was heavily wrinkled during propagation, despite it was grown from a relatively-circular kernel. Moreover, the intake swirl accelerated the flame propagation process while rotating the turbulent flame during its development. Furthermore, the flame propagation speed reduced dramatically when entering the squish region, while the direction from which the flame first touched the bowl edge changed with individual cycles. The results can help the CFD community to better develop RANS and/or LES simulations of such engines under lean-burn operating conditions.
Investigation of Gasoline Compression Ignition for Combustion Control
Authors: Ravaglioli V. , Ponti F. , De Cesare M.
Published: December 2019

Future emission regulations for Internal Combustion Engines require increasingly stringent reductions of engine-out emissions, especially NOx and particulate matter, together with the continuous improvement of engine efficiency. In the current scenario, even though compression-ignited engines are still considered the most efficient and reliable technology for automotive applications, the use of Diesel-like fuels has become a critical issue, since it is usually not compatible with the required emissions reduction. A large amount of research and experimentation is being carried out to investigate the combined use of compression-ignited engines and gasoline-like fuels, which proved to be very promising, especially in case the fuel is directly-injected in the combustion chamber at high pressure. This work investigates the combustion process occurring in a light-duty compression-ignited engine while directly injecting only gasoline. A specific experimental setup has been designed to guarantee combustion stability over the whole operating range, that is achieved controlling boost pressure and temperature together with all the injection parameters of the multi-jet pattern. The analysis of the experimental data clearly highlights how the variation of the control parameters affect the ignition process of small amounts of directly injected gasoline and the maximum achievable efficiency. In particular, the analysis of the sensitivity to the injection parameters allows identifying an ignition delay model and the key control parameters that might be varied to guarantee a robust control of combustion phasing within the cycle.
Spectral Microscopy Imaging System for High-Resolution and High-Speed Imaging of Fuel Sprays
Authors: Maassen Kenneth F. , Poursadegh Farzad , Genzale Caroline L.
Published: December 2019

Modern high-efficiency engines utilize direct injection for charge preparation at extremely high pressures. At these conditions the scales of atomization become challenging to measure, as primary breakup occurs on the micrometer and nanosecond scales. As such, fuel sprays at these conditions have proven difficult to study via direct imaging. While high-speed cameras now exist that can shutter at tens to hundreds of nanoseconds, and long-range microscopes can be coupled to these cameras to provide high resolution images, the resolving power of these systems is typically limited by pixel size and field of view. The large pixel sizes make the realization of the diffraction-limited optical resolution quite challenging. On the other hand, limited data throughput under high repetition rate operation limits the field of view (FOV) due to reduced sensor area. Therefore, a novel measurement technique is critical to study fuel spray formation at engine-relevant conditions.

In this work, we demonstrate a new high-resolution imaging technique, Spectral Microscopy, which aims to realize diffraction-limited imaging at effective framerates sufficient for capturing primary breakup in engine-relevant sprays. A spectral microscopy system utilizing a consumer-grade DSLR allows for significantly wider FOV with improved resolving power compared to high-speed cameras. Temporal shuttering is accomplished via separate and independently triggered back illumination sources, with wavelengths selected to overlap with the detection bands of the camera sensor’s RGB filter array. The RGB detection channels act as filters to capture independently timed red, green, and blue light pulses, enabling the capture of a three consecutive images at effective framerates exceeding 20 Million fps. To optimize system performance, a backlit illumination system is designed to maximize light throughput, a multi-lens setup is created, and an image processing algorithm is demonstrated that formulates a three-frame image from the camera sensor. The system capabilities are then demonstrated by imaging engine relevant diesel sprays. The spectral microscopy system detailed in this paper allows for micron-scale feature recognition at framerates exceeding 20 Million fps, thus expanding the capability for experimental research on primary breakup in fuel sprays for modern direct-injection engines.
Experimental Study of the Effect of Start of Injection and Blend Ratio on Single Fuel Reformate RCCI
Authors: Hariharan Deivanayagam , Gainey Brian , Yan Ziming , Mamalis Sotirios , Lawler Benjamin
Published: December 2019

A new concept of single-fuel RCCI has been proposed through the catalytic partial oxidation reformation of diesel fuel. The reformed fuel mixture is then used as the low reactivity fuel and diesel itself is used as the high reactivity fuel. In this paper, two selected reformates mixture from the reformation of diesel were selected for further analysis. Each reformate fuel mixture contained a significant fraction of inert gases (89% and 81%). The effects of the difference in the molar concentrations of the reformate mixtures were studied by experimenting with diesel as the direct injected fuel in RCCI over a varying start of injection timings and different blend ratios (i.e., the fraction of low and high reactivities fuels).

The reformate mixture with the lower inert gas concentration had earlier combustion phasing and shorter combustion duration at any given diesel start of injection timing. The higher reactivity separation between reformate mixture and diesel, compared with gasoline and diesel, causes the combustion phasing of reformate-diesel RCCI to be more sensitive to the start of injection timing. The maximum combustion efficiency was found at a CA50 before TDC, whereas the maximum thermal efficiency occurs at a CA50 after TDC. The range of energy-based blend ratios in which reformate-diesel RCCI is possible is between 25% and 45%, limited by ringing intensity (RI) at the low limit of blend ratios, and COV of IMEP and combustion efficiency at the high limit. Intake boosting becomes necessary due to the oxygen deficiency caused by the low energy density of the reformate mixtures as it displaces intake air.
The Challenges of Turbocharger Matching for North American Freight Locomotives
Authors: Lavertu Thomas , Hart Matthew , Homison Christopher , Vaidya Preeti
Published: December 2019

Engine development is centered on developing a solution for best performance while meeting emissions and operational requirements. This will lead to a tradeoff between engine efficiency and emissions across a wide range of load and ambient operating points. Proper airflow to the engine through turbocharger matching is critical to ensure efficient operation and to meet emissions. This study addresses the challenges of turbocharger matching for vehicle advanced emissions control using a North American freight locomotive application as an example. The airflow trends in moving across the various operating points will be shown along with the impact on both the turbocharger and engine performance. First, the airflow trends across the locomotive load set points will be discussed along with the performance and emissions tradeoffs to meet required airflows. Results on the impact on turbocharger performance such as speed will be shown along with the engine efficiency and emissions implications. Next, the ambient operating requirements for a locomotive will be reviewed and the impact on turbocharger matching. Locomotives operate in a wide range of ambient conditions, including altitudes up to 3,050 meters and across ambient air temperatures ranging from −40 °C to well over 38 °C (including higher temperature operation). This thermal swing provides stress on the turbocharger to efficiently deliver the necessary airflow across all conditions. Trends in turbocharger performance will be reviewed and discussed across this range of ambient conditions. In addition, challenges unique to locomotive applications, such as unventilated tunnel operation and vibrational loading, will be reviewed. Finally, potential for advanced technologies such as variable geometry turbines and their applicability to locomotive operation will be discussed.
Experimental Investigations of Marine Diesel Engine Performance Against Dynamic Back Pressure at Varying Sea-States due to Underwater Exhaust Systems
Authors: Sapra Harsh D. , Singh Jaswinder , Dijkstra Chris , De Vos Peter , Visser Klaas
Published: December 2019

Underwater exhaust systems are employed on board ships to allow zero direct emissions to the atmosphere with the possibility of drag reduction via exhaust gas lubrication. However, underwater expulsion of exhaust gases imparts high and dynamic back pressure, which can fluctuate in amplitude and time period as a ship operates in varying sea-states depending on its geographical location and weather conditions. Therefore, this research aims to experimentally investigate the performance of a marine diesel engine against varying amplitudes and time periods of dynamic back pressure at different sea-states due to underwater exhaust systems.

In this study, a turbocharged, marine diesel engine was tested at different loads along the propeller curve against dynamic back pressure waves produced by controlling an electronic butterfly valve placed in the exhaust line after the turbine outlet. Engine performance was investigated against single and multiple back pressure waves of varying amplitudes and wave periods based on real sea-state conditions and wave data.

We found that the adverse effects of dynamic back pressure on engine performance were less severe than those found against static back pressure. Governor control and turbocharger dynamics play an important role in keeping the fuel penalty and thermal loading low against dynamic back pressure. Therefore, a marine engine may be able to handle much higher levels of dynamic back pressures when operating with underwater exhaust systems in higher sea-states.
Operating Limits of Spark-Assisted Compression Ignition Combustion Under Boosted Ultra-EGR Dilute Conditions in a Negative Valve Overlap Engine
Authors: Triantopoulos Vassilis , Martz Jason B. , Sterniak Jeff , Lavoie George , Assanis Dennis N. , Bohac Stanislav V.
Published: December 2019

Spark-assisted compression ignition (SACI) is a low temperature combustion mode that can offer thermal efficiency improvements and lower nitrogen oxide emissions compared to conventional spark-ignited combustion. However, the SACI operating range is often limited due to excessive pressure rise rates driven by rapid heat release rates. Well-controlled experiments were performed to investigate the SACI operating limits under previously unexplored boosted, stoichiometric, EGR dilute conditions, where low temperature combustion engines promise high thermodynamic efficiencies. At higher intake boost, the SACI high load limit shifted towards lower fuel-to-charge equivalence ratio mixtures, creating a larger gap between the conventional spark-ignition EGR dilution limit and the boosted SACI operating limits. Combustion phasing retard was very effective at reducing maximum pressure rise rate levels until the stability limit, primarily due to slower end-gas burn rates. Gross fuel conversion efficiency improvements up to 10% were observed by using intake boost for either load expansion or dilution extension. Changes in engine speed necessitated changes in unburned gas temperature to match autoignition timing, but were shown to have negligible impact on the heat release profile on a crank angle basis. Lower engine speeds were favorable for load expansion, as time-based peak pressure rise rates scaled with engine speed.
Thermal Profiling of a Small Operational Data Center
Authors: Türkmen İsmail , Mercan Cem Ahmet , Erden Hamza Salih
Published: December 2019

This study highlights experimental challenges of setting up a computational fluid dynamics (CFD) model of a 180-m2 small-size high-performance computing (HPC) data center (DC) in a university campus and the validation procedure leading to a reasonably accurate CFD model for the investigation of the thermal environment. Experimental results based on temperature, airflow, pressure, and power measurements help to confirm and identify inefficient design and operational practices as well as problematic areas regarding the thermal management, define the boundary conditions and validate CFD models. Leaky floors, uneven placement of computing equipment and perforated tiles preventing separation of hot and cold air, low-temperature operation, excessive cooling capacity and fan power were identified sources of energy inefficiency in the DC.

CFD model predictions were gradually improved by using experimental measurements in various boundary conditions and detailed geometrical representation of large leakage openings. The performance indicator of the CFD model during the validation process was the temperature prediction error at the rack inlets and exits. After all improvements, the CFD model can make estimations with an error less than 1°C (RMSE < 1.0) at the rack inlet and less than 5°C (RMSE < 5.0) at the rack outlet. The validated CFD model tests the feasibility of various energy efficiency measures. These measures are in the form of operational or design changes in line with the best practices. Impact of leakage between the raised floor and the room, reduced airflow rate, cold-aisle and hot-aisle separation, workload consolidation, and higher temperature operation were among the phenomena tested by using the validated CFD model. The estimated power usage effectiveness (PUE) value reduced from 1.95 to 1.40 with the implemented energy efficiency measures.
Demonstration of a Compliant Micro-Spring Array As a Thermal Interface Material for Pluggable Optoelectronic Transceiver Modules
Authors: Cui Jin , Pan Liang , Weibel Justin A.
Published: December 2019

Pluggable optoelectronic transceiver modules are widely used in the fiber-optic communication infrastructure. It is essential to mitigate thermal contact resistance between the high-power optical module and its riding heat sink in order to maintain the required operation temperature. The pluggable nature of the modules requires dry contact thermal interfaces that permit repeated insertion–disconnect cycles under low compression pressures (∼10–100 kPa). Conventional wet thermal interface materials (TIM), such as greases, or those that require high compression pressures, are not suitable for pluggable operation. Here we demonstrate the use of compliant micro-structured TIM to enhance the thermal contact conductance between an optical module and its riding heat sink under a low compression pressure (20 kPa). The metallized and polymer-coated structures are able to accommodate the surface nonflatness and microscale roughness of the mating surface while maintaining a high effective thermal conductance across the thickness. This dry contact TIM is demonstrated to maintain reliable thermal performance after 100 plug-in and plug-out cycles while under compression.
Thermo-Mechanical Degradation of Thermal Interface Materials: Accelerated Test Development and Reliability Analysis
Authors: Pense Dustin , Carlton Hayden , Huitink David
Published: December 2019

Thermal interface materials (TIMs) comprise an important role in the thermal management of a myriad of electronic devices, and their ability to ensure both enhanced thermal conductance and reliable adhesion at thermal interfaces is paramount to the reliability of electronics packaging. Like most aspects of a typical electronics package, on/off cycles undergone during a device’s operation induce thermo-mechanical stresses that can negatively affect the integrity of the package. Due to the inherently low adhesive strength and structural integrity of polymer TIMs, they present a likely point of failure when succumbed to these interfacial stresses. Methods for quantifying TIM degradation during mechanical cycling have been quite infrequent in literature; an accelerated and repeatable method for measuring the thermal reliability of TIMs would prove to be beneficial. Herein, we present a methodology to quantify the thermal reliability of TIMs during mechanical cycling using a custom-built steady-state thermal conductivity tester. Additionally, an optical technique was utilized to observe void formation, pump-out, and dry-out behavior during cycling, in order to correlate the thermal performance with physical behaviors of the TIM under cyclic stress. After an initial long-term static test, cyclic testing was found to degrade the thermal performance of the TIM through increasing its interfacial resistance. Optical qualitative measurements revealed the breakdown of the TIM structure at the interface, which indicated the formation of voids due to TIM degradation. Applying this testing method for future TIM development could help in optimizing TIM structure for particular package applications.
Electrothermal Modeling and Analysis of Gallium Oxide Power Switching Devices
Authors: Kotecha Ramchandra M. , Zakutayev Andriy , Metzger Wyatt K. , Paret Paul , Moreno Gilberto , Kekelia Bidzina , Bennion Kevin , Mather Barry , Narumanchi Sreekant , Kim Samuel , Graham Samuel
Published: December 2019

Gallium oxide is an emerging wide band-gap material that has the potential to penetrate the power electronics market in the near future. In this paper, a finite-element gallium oxide semiconductor model is presented that can predict the electrical and thermal characteristics of the device. The finite element model of the two-dimensional device architecture is developed inside the Sentaurus environment. A vertical FinFET device architecture is employed to assess the device’s behavior and its static and dynamic characteristics. Enhancement-mode device operation is realized with this type of device architecture without the need for any selective area doping. The dynamic thermal behavior of the device is characterized through its short-circuit behavior. Based on the device static and dynamic behavior, it is envisioned that reliable vertical transistors can be fabricated for the power electronics applications.
Degradation Modeling and Reliability Assessment of Capacitors
Authors: Gupta Anunay , Yadav Om Prakash , Roy Arighna , DeVoto Douglas , Major Joshua
Published: December 2019

The degradation of capacitors under accelerated stress conditions occur in a monotonic and non-linear fashion. Several efforts have been made to model the degradation behavior of capacitor considering either physics-of-failure models or statistical models and subsequently estimate its reliability and lifetime parameters. But most of these models fail to reflect the physical properties of the degradation path, which varies according to several intrinsic and extrinsic factors. These factors introduce random and temporal uncertainty among the population of capacitors. The gamma stochastic process can model both type of uncertainties among the population of capacitors. In this paper, we model the capacitor degradation by non-homogeneous gamma stochastic process in which both the model parameters (shape and scale) are dependent on stress variables. The model parameters are estimated using the maximum likelihood estimation approach.
A Numerical Investigation of Additive Manufactured Foam Structures for Single Phase Hotspot Thermal Management
Authors: Broughton Justin , Joshi Yogendra
Published: December 2019

Wide band-gap (WBG) power devices have been investigated for their higher power densities, efficiencies, and operating temperatures. However, implementation of WBG devices may cause increased volumetric heat generation despite efficiency increases due to a significant decrease in volume. To fully take advantage of the benefits offered by these devices, the packaging, including cooling infrastructure, must also be addressed. Additive manufactured (AM) structures, based on unit cells seen in stochastic foams, have been investigated as an improvement in thermal management for electric vehicle power electronics. Metal foams have a high specific surface area, high thermal conductivity, and low relative density, which makes them excellent for heat transfer applications as demonstrated extensively in prior literature. Additionally, additive manufacturing offers the ability to print directly on a substrate which eliminates a thermal interface layer and make complex structures for little to no increase in cost. Designed foams were examined specifically for their ability to locally control relevant parameters to tailor the heat transfer performance. The design of the AM foam samples, as well as discussion of relevant parameters for proper characterization with respect to thermohydraulic performance, were discussed. Two geometries — one with local, spanwise densification with decreased pore size and another with uniform density and pore size throughout — were investigated for thermal management of several discrete heaters. Pore-scale models with decreased computational domains were used to obtain closure terms for volume-averaged (VA) modeling. The VA models were then used for full-scale modeling of five chip heaters cooled with the designed metal foams.
Effects of Test Temperature and Prior Aging on the Cyclic Stress-Strain Behavior of Lead Free Solders
Authors: Haq Mohammad Ashraful , Hoque Mohd Aminul , Suhling Jeffrey C. , Lall Pradeep
Published: December 2019

In temperature changing environments, solder joints often experience fatigue failure due to cyclic mechanical stresses and strains induced by mismatches in the coefficients of thermal expansion. These stresses and strains lead to damage accumulation and contribute to the crack initiation, crack propagation, and eventually to failure. In this study, we have investigated the cyclic stress-strain behavior of SAC305 and SAC_Q reflowed lead free solders that occur at various testing temperatures and with various prior aging conditions. Lead free solder uniaxial test specimens with circular cross-section have been prepared using vacuum suction method and then were aged for 0 to 20 days at 125 °C. The samples were then subjected to cyclic stress-strain loading using a Micro-mechanical tester at different testing temperatures from T = 25 C to T = 100 C. The evolution of hysteresis loops with duration of prior aging was characterized by measuring the strain energy density dissipated per cycle (loop area), peak stress, and plastic strain range. It was observed that aging degrades the mechanical fatigue properties due to microstructural coarsening. At elevated temperatures, a drop in the loop area and peak stress and an increase in the plastic strain range for both lead free reflowed solder materials were obtained. In addition, SAC_Q samples had a higher loop area and peak stress compared to SAC305.
Investigation of the Effects of High Temperature Aging on Mechanical Behavior and Microstructural Changes in Lead Free Solders
Authors: Wu Jing , Alam Mohammad S. , Suhling Jeffrey C. , Lall Pradeep
Published: December 2019

Aging effects are common in lead free solder joints within electronic assemblies that are exposed to isothermal environments for extended periods. Such exposures lead to evolution of the solder microstructure, which results in changes in the mechanical properties and creep behavior of the solder joints. In our recent investigations, we have been utilizing Scanning Electron Microscopy (SEM) to better understand aging induced degradations. The microstructural evolutions were observed in SAC305 and SAC_Q (SAC+Bi) alloys exposed to isothermal conditions at T = 125 °C for several different regions from several different joints. The microstructures in several fixed regions of interest were recorded after predetermined time intervals of aging, which were 1 hour (up to 24 hours) and 10 hours (up to 150 hours) for the short term aging samples; and 250 hours (up to 2500 hours) for the long term aging samples. Using the recorded images and imaging processing software, the area and diameter of each IMC particle was tracked during the aging process.

As expected, the quantitative analysis of the evolving SAC_Q microstructure showed that the particles coalesced during aging leading to a decrease in the number of particles. This caused an increase in the average diameter of the particles of slightly more than 100% for long term aging of 2500 hours. For SAC305, the average particle diameter was found to increase at three times the rate (increase of 200% after 2500 hours of aging). Thus, coarsening of IMC particles was greatly mitigated in the SAC_Q alloy relative to that observed in SAC305. Immediately after reflow solidification, Bismuth rich phases were present in the SAC_Q joints. During aging at T = 125 °C, the bismuth was observed to quickly go into solution both within the beta-Sn dendrites and in the intermetallic rich regions between dendrites. This resulted in solid solution strengthening of the lead free solder. It was also found that the aging-induced presence of bismuth in solution within the beta-Sn matrix provided an increased resistance to the Ostwald ripening diffusion process that coarsens the Ag3Sn IMC particles. The combination of these two effects in the SAC+Bi alloy lead to greatly improved resistance to aging induced effects relative to the SAC305 solder alloy. Finally, we have compared the time dependent evolution of microstructure with the degradation in strength during aging for of the two solder alloys, and good correlations were observed.
Effect of Process Parameters on Aerosol Jet Printing of Multi-Layer Circuitry
Authors: Lall Pradeep , Goyal Kartik , Kothari Nakul , Leever Ben , Miller Scott
Published: December 2019

Printing technologies such as Aerosol Jet provides the freedom of miniaturizing interconnects and producing fine pitch components. Aerosol Jet, a direct printing technique replaces the traditional steps of manufacturing a printed circuit board such as lithography or etching, which are quite expensive, and further allowing the circuits to be fabricated onto all kinds of substrates. Wide impact areas range from healthcare to wearables to future automotive applications. The aerosol jet printer from Optomec utilized in this study, consists of two types of atomizers depending on ink viscosity. One is Ultrasonic Atomizer which supports ink with viscosity range of 1–5cP, and another is Pneumatic Atomizer with large range of suitable viscosity 1–1000cP. This paper focuses on utilizing the aerosol jet printing using both the atomizers to develop process parameters to be able to successfully print bi-material, multi-layer circuitry. The insulating material between two conductive lines used in the paper is of very high viscosity of 350cP, suitable for Pneumatic atomizer and Silver Nano-particle ink with the viscosity suitable for Ultrasonic atomizer as a conductive ink. A statistical modeling approach is presented to predict the attributes such as micro-via diameter before starting the print process, enabling us to pre-adjust the dimensions in CAD for the desired output. Process parameters to obtain a fine print with good electrical properties and better dimensional accuracy are developed. Importance of pre-cleaning the substrate is discussed, in addition to the printing process efficiency gauged as a function of process capability index and process capability ratio.
Acceleration Factors for Flexible Electronics in Wearable Applications From Actual Human Body Measurements
Authors: Lall Pradeep , Thomas Tony , Yadav Vikas , Narangaparambil Jinesh , Liu Wei
Published: December 2019

The increase in use of flexible electronics in wearable applications has prompted in analyzing the movement characteristics of human body under various day to day actions. The flexible electronics that are attached on the human body were tested for reliability under various conditions of human activity such as walking, jumping, squats, lunges and bicep curls. The human body motion data during these different actions were measured using a set of ten Vicon cameras to measure the position, velocity and accelerations of a standard full body sensor location of a human body. The reliability model presented in this study uses the angle variations of each joint in the human body for all the five human activities listed above. Statistical analysis on the variation of each joint angles were tested with hypothesis testing strategies with different subjects and with different human body actions as well. Acceleration factor modelling on the reliability of the electronics were carried out using test data of flexible electronics subjected to bending, twisting, stretching and folding experiments. These experiments were conducted on flexible electronics till failure with in-situ resistance measurements to monitor the changes in the board during each of these experiments. The experimental measurements of the boards were combined with the human body motion data to model the acceleration factor for each of these tests.
Experimental Analysis for Optimization of Thermal Performance of a Server in Single Phase Immersion Cooling
Authors: Shinde Pravin A. , Bansode Pratik V. , Saini Satyam , Kasukurthy Rajesh , Chauhan Tushar , Shah Jimil M. , Agonafer Dereje
Published: December 2019

Liquid immersion cooling of servers in synthetic dielectric fluids is an emerging technology which offers significant cooling energy savings and increased power densities for data centers. A noteworthy advantage of using immersion cooling is high heat dissipation capacity which is roughly 1200 times greater than air. Other advantages of dielectric fluid immersion cooling include high rack density, better server performance, even temperature profile, reduction in noise etc. The enhanced thermal properties of oil lead to the considerable savings in both upfront and operating cost over traditional methods. In this study, a server is completely submerged in a synthetic dielectric fluid. Experiments are conducted to observe the effects of varying the volumetric flow rate and oil inlet temperature on thermal performance and power consumption of the server. Various parameters like total server power consumption, the temperature of all heat generating components like Central Processing Unit (CPU), Dual in Line Memory Module (DIMM), input/output hub (IOH) chip, Platform Controller Hub (PCH), Network Interface Controller (NIC) are measured at steady state. Since this is an air-cooled server, the results obtained from the experiments will help in proposing better heat removal strategies like heat sink optimization, better ducting and server architecture. Assessment has been made on the effect of thermal shadowing caused by the two CPUs on the nearby components like DIMMs and PCH.