bluuebunny/crossref_metadata_2025_split huggingface dataset:
Phenomenological Modeling of Combustion in Pre-Chamber and the Pilot Flame for Natural Gas Engines
Authors: Liu Long , Wen Xia , Xiong Qian , Ma Xiuzhen 
Published: December 2019

Abstract
With energy shortages and increasing environmental problems, natural gas, as a clean energy, has the advantages of cheap price and large reserves and has become one of the main alternative fuels for marine diesel engines. For large bore natural gas engines, pre-chamber spark plug ignition can be used to increase engine efficiency. The engine mainly relies on the flame ejected from the pre-chamber to ignite the mixture of natural gas and air in the main combustion chamber. The ignition flame in the main combustion chamber is the main factor affecting the combustion process. Although the pre-chamber natural gas engines have been extensively studied, the characteristics of combustion in the pre-chamber and the development of ignition flame in the main combustion chamber have not been fully understood. In this study, a two-zone phenomenological combustion model of pre-chamber spark-ignition natural gas engines is established based on the exchange of mass and energy of the gas flow process in the pre-chamber and the main combustion chamber. The basic characteristics of the developed model are: a spherical flame surface is used to describe the combustion state in the pre-chamber, and according to the turbulent jet theory, the influence of turbulence on the state of the pilot flame is considered based on the Reynolds number. According to the phenomenological model, the time when the flame starts to be injected from the pre-chamber to the main combustion chamber, and the parameters such as the length of the pilot flame are analyzed. The model was verified by experimental data, and the results showed that the calculated values were in good agreement with the experimental values. It provides an effective tool for mastering the law of flame development and supporting the optimization of combustion efficiency.
A Novel Active Optimization Approach for Rapid and Efficient Design Space Exploration Using Ensemble Machine Learning
Authors: Owoyele Opeoluwa , Pal Pinaki 
Published: December 2019

Abstract
In this work, a novel design optimization technique based on active learning, which involves dynamic exploration and exploitation of the design space of interest using an ensemble of machine learning algorithms, is presented. In this approach, a hybrid methodology incorporating an explorative weak learner (regularized basis function model) which fits high-level information about the response surface, and an exploitative strong learner (based on committee machine) that fits finer details around promising regions identified by the weak learner, is employed. For each design iteration, an aristocratic approach is used to select a set of nominees, where points that meet a threshold merit value as predicted by the weak learner are selected to be evaluated using expensive function evaluation. In addition to these points, the global optimum as predicted by the strong learner is also evaluated to enable rapid convergence to the actual global optimum once the most promising region has been identified by the optimizer. This methodology is first tested by applying it to the optimization of a two-dimensional multi-modal surface. The performance of the new active learning approach is compared with traditional global optimization methods, namely micro-genetic algorithm (μGA) and particle swarm optimization (PSO). It is demonstrated that the new optimizer is able to reach the global optimum much faster, with a significantly fewer number of function evaluations. Subsequently, the new optimizer is also applied to a complex internal combustion (IC) engine combustion optimization case with nine control parameters related to fuel injection, initial thermodynamic conditions, and in-cylinder flow. It is again found that the new approach significantly lowers the number of function evaluations that are needed to reach the optimum design configuration (by up to 80%) when compared to particle swarm and genetic algorithm-based optimization techniques.
Thermochemical and Sensible Energy Recuperation Using Thermally-Integrated Reactor and Diesel-Ammonia Dual Fueling Strategy
Authors: Kane Seamus P. , Zarling Darrick , Northrop William F. 
Published: December 2019

Abstract
Anhydrous ammonia produced using wind power on farms can be a renewable alternative to conventional fertilizers and to fossil fuels used in engine-powered equipment. Although it has been shown that ammonia can be used in dual fuel modes in diesel engines, its inherently low flame speed results in poor combustion efficiency and thus reduces allowable diesel fuel replacement ratios. In this work, a novel method using a thermochemical recuperation (TCR) reactor system to partially decompose ammonia into hydrogen and nitrogen over a catalyst was demonstrated in diesel engine powered tractor. In the experiments, a John Deere 6400 agricultural tractor powered by a non-EPA tier-certified 4045TL diesel engine was operated in dual-fuel mode using anhydrous ammonia as the secondary fuel. Liquid ammonia from a tank was vaporized and heated using a series of heat exchangers and partially decomposed to hydrogen gas before being fumigated into the intake manifold. The catalytic TCR reactor utilized both exhaust waste heat and unburned hydrocarbon heating value to drive the ammonia decomposition process. Engine emissions and performance data were collected across a standard 8-mode test. The engine was operated using diesel only and in dual fuel mode with up to 42% replacement of diesel with ammonia on a lower heating value basis. Engine loading was accomplished using a power takeoff (PTO) dynamometer. Measured brake thermal efficiency was improved by up to 5.0% using thermochemical recuperation, and brake specific CO2 emissions were reduced by up to 44% over diesel-only rates.
Characteristics of Cycle-to-Cycle Combustion Variability at Partial-Burn Limited and Misfire Limited Spark Timing Under Highly Diluted Conditions
Authors: Maldonado Bryan , Stefanopoulou Anna , Scarcelli Riccardo , Som Sibendu 
Published: December 2019

Abstract
Diluted combustion with exhaust gas recirculation (EGR) has been widely employed to improve the fuel economy of spark ignition engines. The combustion kinetics, however, are affected and the flame propagation speed is decreased. In order to compensate for this adverse effect, the spark timing needs to be recalibrated to achieve maximum brake torque (MBT). At high levels of EGR dilution, the spark timing is constrained by two ignition limits: 1) the partial-burn limit where the spark timing is retarded from MBT and 2) the misfire limit where the spark timing is too advanced. This work uses a probabilistic framework to capture the differences between both ignition limits. In particular, it introduces the concept of a nominal indicated mean effective pressure (IMEP) distribution based on the stochastic properties of the cycle-to-cycle variability (CCV) at nominal stable conditions. By defining a nominal band where fully burned cycles occur with high probability, we introduce a cycle classification method that can be used to 1) determine the level of randomness of misfire and partial-burn events, and 2) measure CCV. The new CCV metric based on the density of the nominal band is compared with the traditional coefficient of variation of IMEP (CoVIMEP). It is shown that the nominal band concept, together with the CoVIMEP, can help to discern between partial-burn limited and misfire limited conditions. Furthermore, the Kullback-Leibler divergence is used to demonstrate that the IMEP distribution is significantly different between nominal and partial-burn/misfire limited conditions. Experiments are carried at various EGR levels and spark timings while recoding in-cylinder pressure at steady state. Although the emphasis of this work is to characterize the differences of both ignition limits from a probabilistic point of view, similarities between partial-burn cycles at either limiting conditions are also discussed.
CFD Simulation of a Premixed Spark Injection Hydrogen Engine
Authors: Lee Benjamin , Ge Haiwen , Parameswaran Siva , Zhao Peng 
Published: December 2019

Abstract
Hydrogen has great potential as an alternative fuel for internal combustion engines (ICE) because of its high energy density by mass, high specific heat ratio, free of CO2 and PM emission, and ultra-low NOx emission. In the present paper, a premixed spark-ignition engine fueled by hydrogen was simulated. 3D combustion CFD was employed to simulate and optimize the engine combustion. Parametric studies of equivalence ratio were conducted to identify the appropriate range for practical operation. It was found that higher equivalence ratio in H2ICE is not feasible since it exhibits very high pressure rise rate and strong pressure oscillations. It is due to its much fast flame speed and detonabiltiy. Frequency analysis was conducted on the predicted pressure trace. The dominant frequencies were compared with the first four drum mode. It was found that the dominant resonance frequency corresponds to the drum mode of (0,1). This is due to the fact that the pressure wave originates from the center of combustion chamber. With an equivalence ratio of 0.4, the engine produces similar pressure trace as the baseline gasoline engine, while its close-cycle work is less than half of the gasoline engine, due to low equivalence ratio and hydrogen’s low energy density per unit volume. NOx emission is two order of magnitude lower than the stoichiometric gasoline engine. However, for lean burn PFI H2ICE, N2O emission is much higher than stoichiometric gasoline engine, which needs special attention in the future. Spark timing was swept for the equivalence ratio 0.4 case to further optimize the engine. The mechanisms of NO and N2O formation at different operating conditions are compared and discussed. It was found that NO emission is sensitive to spark timing, while N2O emission is not. The engine-out N2O emission is about one order of magnitude lower than the engine-out NO emission.
Assessment of Turbulent Combustion Models for Simulating Pre-Chamber Ignition in a Natural Gas Engine
Authors: Kim Joohan , Scarcelli Riccardo , Som Sibendu , Shah Ashish , Biruduganti Munidhar S. , Longman Douglas E. 
Published: December 2019

Abstract
Lean combustion in an internal combustion engine is a promising strategy to increase thermal efficiency by leveraging a more favorable specific heat ratio of the fresh mixture and simultaneously suppressing the heat losses to the cylinder wall. However, unstable ignition events and slow flame propagation at fuel-lean condition lead to high cycle-to-cycle variability and hence limit the high-efficiency engine operating range. Pre-chamber ignition is considered an effective concept to extend the lean operating limit, by providing spatially distributed ignition with multiple turbulent flame-jets and enabling faster combustion rate compared to the conventional spark ignition approach. From a numerical modeling perspective, to date, still the science base and available simulation tools are inadequate for understanding and predicting the combustion processes in pre-chamber ignited engines. In this paper, conceptually different RANS combustion models widely adopted in the engine modeling community were used to simulate the ignition and combustion processes in a medium-duty natural gas engine with a pre-chamber spark-ignition system. A flamelet-based turbulent combustion model, i.e., G-equation, and a multi-zone well-stirred reactor model were employed for the multi-dimensional study. Simulation results were compared with experimental data in terms of in-cylinder pressure and heat release rate. Finally, the analysis of the performance of the two models is carried out to highlight the strengths and limitations of the two formulations respectively.
Numerical Study of Advanced Compression Ignition and Combustion in a Gasoline Direct Injection Engine
Authors: Kim Sayop , Kim Joohan , Shah Ashish , Pal Pinaki , Scarcelli Riccardo , Rockstroh Toby , Som Sibendu , Wu Yunchao , Lu Tianfeng 
Published: December 2019

Abstract
Multi-mode combustion strategies may provide a considerable thermal efficiency improvement targeted at part-load operating conditions for light-duty spark-ignition (SI) engines. The extension from boosted SI mode at high loads to advanced compression ignition (ACI) mode at low loads can be achieved by increasing compression ratio and utilizing intake air heating. In order to enable an accurate control of intake charge condition for ACI control and rapid mode-switches, it is essential to gain fundamental insight into the autoignition process with regard to thermal and fuel-air mixture stratification in the combustion chamber. In this work, a computational fluid dynamics (CFD) study is carried out to reveal some degrees of correlation between the mixture and thermal stratifications induced by the cylinder wall temperature and combustion-phasing across varying engine load conditions.
The computational analysis begins with a calibrated simulation setup best matching the engine experiments, and subsequently evaluates the baseline setup for an extended range of engine load conditions for two excess-air ratio cases. The present study emphasizes the dominance of thermal stratifications for autoignition process due to wall temperature effects depending on the engine load conditions of interest. In addition, this study also aims to provide insight into the impact of mixture stratification on cyclic variability, especially at colder wall engine conditions (e.g., cold start). Observations herein provide highlight of the propensity of autoignition in correlation with mixture reactivity space.
Dynamic Control of Airflow Balance in Data Centers
Authors: Linder Stephen Paul , Van Gilder Jim , Zhang Yan , Barrett Enda 
Published: December 2019

Abstract
Efficient cooling of data center infrastructure is an important way to reduce total energy consumption. Containment, with separation of hot and cold airflows has allowed significant increase in efficiencies. However, balancing the airflow, so that IT equipment in an aisle only receives the cooling airflow that that aisle needs is still often not done. We propose a new architecture where IT racks are clustered together with shared hot aisles ducted to a common ceiling plenum. Each aisle has an actively controlled damper used to balance the airflow to the cooling infrastructure. Using a differential air pressure sensor in each aisle and an algorithm designed to balance the flow network, we minimize the cooling airflow and maximize cooling efficiency.
Evaporation Rate Measurement at Multiple Scales Using Temperature-Sensitive Fluorescence Dyes
Authors: Suh Youngjoon , Lin Cheng-Hui , Gowda Hamsa , Won Yoonjin 
Published: December 2019

Abstract
As modern electronics continuously exceed their performance limits, there is an urgent need to develop new cooling devices that balance the increasing power demands. To meet this need, cutting-edge cooling devices often utilize microscale structures that facilitate two-phase heat transfer. However, it has been difficult to understand how microstructures trigger enhanced evaporation performances through traditional experimental methods due to low spatial resolution. The previous methods can only provide coarse interpretations on how physical properties such as permeability, thermal conduction, and effective surface areas interact at the microscale to effectively dissipate heat. This motivates researchers to develop new methods to observe and analyze local evaporation phenomena at the microscale.
Herein, we present techniques to characterize submicron to macroscale evaporative phenomena of microscale structures using micro laser induced fluorescence (μLIF). We corroborate the use of unsealed temperature-sensitive dyes by systematically investigating their effects on temperature, concentration, and liquid thickness on the fluorescence intensity. Considering these factors, we analyze the evaporative performances of microstructures using two approaches. The first approach characterizes local or overall evaporation rates by measuring the solution drying time. The second method employs an intensity-to-temperature calibration curve to convert temperature-sensitive fluorescence signals to surface temperatures. Then, submicron-level evaporation rates are calculated by employing a species transport equation for vapor at the liquid-vapor interface. Using these methods, we reveal that capillary-assisted liquid feeding dominates evaporation phenomena on microstructured surfaces. This study will enable engineers to decompose the key thermofluidic parameters contributing to the evaporative performance of microscale structures.
Metallic PCM’s Microstructural Stability Under Repetitive Melting/Solidification Cycles
Authors: Báez Rafael , González Luis E. , Quintero Pedro O. , Boteler Lauren M. 
Published: December 2019

Abstract
Metallic phase change materials (mPCMs) have been demonstrated as potential passive cooling solution for pulse power applications. The possibility of integrating a metallic PCM directly on top of a heat source, reducing the thermal resistance between the device and the cooling system, could result in a significant improvement in thermal management for transient applications. However, many thermo-physical properties of these alloys are still unknown, furthermore their microstructural stability with repetitive melting/solidification cycles is not warrant. In this work we provide a series of potential mPCMs for thermal management of electronics operating on a wide range of application temperatures, followed by an experimental investigation of microstructural and thermo-physical stability of these materials under repetitive melting solidification cycles. Results of the effect of cyclic thermal loading of theses alloys on the melting behavior and latent heat of fusion is discussed.
Thermal stability of 51.0 wt.% In-32.5 wt.%Bi-16.5 wt.%Sn and 50wt.%Bi-26.7wt%Pb-13.3wt%Sn-10wt%Cd alloys, as potential mid temperature mPCM, have been evaluated. The results suggest that these mPCM maintain their thermo-physical stability over large periods of thermal cycles.
Thermoreflectance Imaging of Electromigration in Aluminum Interconnects at Different Ambient Temperatures
Authors: Alajlouni Sami , Maize Kerry , Bermel Peter , Shakouri Ali 
Published: December 2019

Abstract
Electromigration (EM) is the process of displacing atoms in metals due to current flow leading to interconnect failures in electronic circuits. As electronics feature sizes continue to shrink, EM is becoming an increasingly serious reliability concern. EM, in aluminum (Al) interconnects, has been studied previously, but typically on the device level using Black’s law [1], without emphasis on the localized heating and defect generation around the failure site. To better understand the local EM process, thermoreflectance (TR) thermal imaging technique is used to obtain temperature profiles with submicron resolution [2]. We show that a simple lifetime prediction using Black’s law is not possible for a micro Al wire having a patterned constriction. The wire fails at two distinct failure locations depending on the level of current excitation. Moreover, the lifetime dependence on ambient temperature was studied. Each failure location had its own extracted activation energy. Our findings suggest that Black’s law may be extended to local features. They also show the potential for the design of local features in extending the lifetime of metallic interconnects. In summary, the temperature profile with submicron spatial resolution offers a unique opportunity to better understand the different mechanisms contributing to EM failures which can be used to design highly reliable interconnects.
System-Level Thermal Management and Reliability of Automotive Electronics: Goals and Opportunities in the Next Generation of Electric and Hybrid Electric Vehicles
Authors: Nafis Bakhtiyar Mohammad , Iradukunda Ange-Christian , Al Razi Imam , Huitink David R. , Peng Yarui 
Published: December 2019

Abstract
The reliable operation of electronic equipment is strongly related to the thermal and mechanical conditions it is exposed to during operation. In order to ensure a long lifetime of components, it is imperative that any electronic packaging design takes into careful consideration the appropriateness of various thermal management schemes and the application-specific requirements in order to keep temperatures within certain limits. The exact requirement varies with the application, and electronic packaging designs for automotive applications are at particular risk of failure due to the naturally harsh conditions it is exposed to. Electronic devices in vehicles have to be able to operate and survive at much higher temperatures than their consumer counterparts. While that has always been an issue, the rise of electric and hybrid electric vehicles (EVs and HEVs), combined with a desire to fit as much as possible into the smallest form factor, the challenge of removing enough heat from electronic devices in automotive vehicles is constantly evolving. This paper closely examines the new challenges in thermal management in various driving environments and aims to classify each existing cooling methods in terms of their performance. Drive schedules used by the Environmental Protection Agency (EPA) for emission and fuel economy testing are taken as examples of different realistic driving scenarios and their predicted thermal profiles are evaluated against various cooling methods, both active, passive or a combination of the two (hybrid). Particular focus is placed upon emerging solutions regarded to hold great potential, such as phase change materials (PCMs). Phase change materials have been regarded for some time as a means of transferring heat quickly away from the region with the electronic components. Phase change materials are widely regarded as a possible means of carrying out cooling in large scales from small areas, considering their advantages such as high latent heat of fusion, high specific heat, controllable temperature stability and small volume change during phase change, etc. They have already been utilized as a method of passive cooling in electronics in various ways, such as in heat spreaders and finned heat sinks. The applications, however, have been mostly for system-on-chip handheld devices, and their adoption in automotive power electronics, such as those used in traction inverters, has been much slower. A brief discussion is made on some of the potential areas of application and challenges relating to more widespread adoption of PCMs. Merits of some of the existing PCM based solutions for automotive electronics applications are also discussed, as are their drawbacks and modifications.