Thermal Considerations Research Articles

Thermal considerations for microswimmer trap-and-release using standing surface acoustic waves.

Controlled trapping of cells and microorganisms using substrate acoustic waves (SAWs; conventionally termed surface acoustic waves) has proven useful in numerous biological and biomedical applications owing to the label- and contact-free nature of acoustic confinement. However, excessive heating due to vibration damping and other system losses potentially compromises the biocompatibility of the SAW technique. Herein, we investigate the thermal biocompatibility of polydimethylsiloxane (PDMS)-based SAW and glass-based SAW [that supports a bulk acoustic wave (BAW) in the fluid domain] devices operating at different frequencies and applied voltages. First, we use infrared thermography to produce heat maps of regions of interest (ROI) within the aperture of the SAW transducers for PDMS- and glass-based devices. Motile Chlamydomonas reinhardtii algae cells are then used to test the trapping performance and biocompatibility of these devices. At low input power, the PDMS-based SAW system cannot generate a large enough acoustic trapping force to hold swimming C. reinhardtii cells. At high input power, the temperature of this device rises rapidly, damaging (and possibly killing) the cells. The glass-based SAW/BAW hybrid system, on the other hand, can not only trap swimming C. reinhardtii at low input power, but also exhibits better thermal biocompatibility than the PDMS-based SAW system at high input power. Thus, a glass-based SAW/BAW device creates strong acoustic trapping forces in a biocompatible environment, providing a new solution to safely trap active microswimmers for research involving motile cells and microorganisms.

Geothermal Well Construction: A Step Change in Oil and Gas Technologies

Geothermal energy has been described as an engineering problem that, when solved, provides the clean, reliable, safe, and affordable energy being sought globally. It is highly likely that the engineers who play the biggest role in solving that problem, and the technologies they adapt and advance, will come from oil and gas. There is enough energy in the earth’s crust, just a few miles down, to power all of humanity for ages, according to the US Advanced Research Projects Agency-Energy. The problem is how to tap into it safely, efficiently, and cost effectively. After many years of failure to launch because of technology or cost limitations, new companies and technologies - and smarter ways of leveraging those that already exist - are bringing geothermal out of its doldrums, to the point that it may finally be ready to scale and become a major player in the transition to cleaner energy, according to Jamie Beard, executive director of the Geothermal Entrepreneurship Organization (GEO) at The University of Texas at Austin (UT-Austin). “The cutting-edge technological developments in geothermal are devoted to drilling into ever-deeper, hotter, and harder rock,” she said, “and oil and gas holds the key to cost reduction for all of these concepts.” Eric Van Oort, drilling and well engineering expert, educator, and scientist, agrees. The UT-Austin engineering professor and director of the rig automation and performance improvement in drilling (RAPID) industry consortium, said, “Fifty to seventy-five percent of the cost of geothermal development is tied up in drilling and well construction. To scale it, we have to reduce that cost.” Designing for Extremes Well design for geothermal wells is similar to that for oil and gas wells. The challenges arise from drilling deeper and deeper, into hotter and hotter rock. Heat ranging from 150°C (302°F) to 373°C (703°F) and above can be used to profitably generate electricity. Oil and gas well designs traditionally have not had to contend with these extremes. Thermal considerations are unavoidable in deep geothermal well construction. Temperature and thermal effects, chemical composition of produced fluids, and rate of production or pressure depletion pose significant challenges to well casing and design. Nick Cameron, reservoir characterization manager at BP and leader of the supermajor’s studies into geothermal energy, said his company is using corporate data, geological understanding, and oil and gas expertise and experience to look at where their technology can reduce risk and drive down the cost of development. “Metallurgical understanding of materials and how they handle heat is crucial to these efforts,” he said. “Fortunately, there have been significant advancements in this area in recent years.” Cameron said that much work is also being done into changing the chemical composition of the fluids that flow through the geothermal reservoirs.

ПРОЕКТИРОВАНИЕ КРОНШТЕЙНА КРЕПЛЕНИЯ ОПТИЧЕСКОГО СОЛНЕЧНОГО ДАТЧИКА КОСМИЧЕСКОГО АППАРАТА С ИСПОЛЬЗОВАНИЕМ ТОПОЛОГИЧЕСКОЙ ОПТИМИЗАЦИИ

Object of the research is a bracket of solar sensor mounting of spacecraft. Goal of the research is to reduce mass of the spacecraft bracket subject to strength and thermal resistance considerations. Mass reduction is carried using topology optimization based on SIMP-material model (Solid Isotropic Material with Penalization). Optimization problem statement is compliance(or strain energy) minimization subject to volume constraint. For strength and thermal analysis finite element method is used implemented in Ansys Workbench 18.2 software. As result of topology optimization, a material distribution data inside a design domain is obtained for different volume fractions. Based on that result new CAD-model has been developed using Siemens NX software which taking into account manufacturing constraints and providing manufacturing of the designed part using machining. Structural analysis performed for two load cases corresponds to launch regime and maximum lateral acceleration has shown that optimized structure has sufficient strength. Thermal analysis performed in Steady State Thermal software module has shown that thermal resistance requirement also met. As result of proposed research bracket mass reduction of 2.5 times has been achieved. Directions of future work has been outlined. Improvement of proposed method may consist from incorporating of coupled thermal-structure analysis into topology optimization process.

Repeating fast radio bursts caused by small bodies orbiting a pulsar or a magnetar

Context. Asteroids orbiting into the highly magnetized and highly relativistic wind of a pulsar offer a favorable configuration for repeating fast radio bursts (FRB). The body in direct contact with the wind develops a trail formed of a stationary Alfvén wave, called an Alfvén wing. When an element of wind crosses the Alfvén wing, it sees a rotation of the ambient magnetic field that can cause radio-wave instabilities. In the observer’s reference frame, the waves are collimated in a very narrow range of directions, and they have an extremely high intensity. A previous work, published in 2014, showed that planets orbiting a pulsar can cause FRBs when they pass in our line of sight. We predicted periodic FRBs. Since then, random FRB repeaters have been discovered. Aims. We present an upgrade of this theory with which repeaters can be explained by the interaction of smaller bodies with a pulsar wind. Methods. Considering the properties of relativistic Alfvén wings attached to a body in the pulsar wind, and taking thermal consideration into account, we conducted a parametric study. Results. We find that FRBs, including the Lorimer burst (30 Jy), can be explained by small-size pulsar companions (1 to 10 km) between 0.03 and 1 AU from a highly magnetized millisecond pulsar. Some parameter sets are also compatible with a magnetar. Our model is compatible with the high rotation measure of FRB 121102. The bunched timing of the FRBs is the consequence of a moderate wind turbulence. An asteroid belt composed of fewer than 200 bodies would suffice for the FRB occurrence rate measured with FRB 121102. Conclusions. After this upgrade, this model is compatible with the properties discovered since its first publication in 2014, when repeating FRBs were still unknown. It is based on standard physics and on common astrophysical objects that can be found in any type of galaxy. It requires 1010 times less power than (common) isotropic-emission FRB models.

A Novel Two-Stage Design Framework for Two-Dimensional Spatial Packing of Interconnected Components

AbstractPacking and routing separately are each challenging NP-hard problems. Therefore, solving the coupled packing and routing problem simultaneously will require disruptive methods to better address pressing-related challenges, such as system volume reduction, interconnect length reduction, ensuring non-intersection, and physics (thermal, hydraulic, or electromagnetic) considerations. Here we present a novel two-stage sequential design framework to perform simultaneous physics-based packing and routing optimization. Stage 1 generates interference-free initial layouts that are fed to stage 2 as starting points to perform continuous physics-based optimization. Three distinct strategies for stage 1 have been introduced recently, (1) the force-directed layout method (FDLM), (2) an extension of the shortest path algorithms (SPAs), and (3) a unique geometric topology (UGT) generation algorithm. In stage 2, a gradient-based topology optimization method is used to simultaneously optimize both component locations and interconnect routing paths. In addition to geometric considerations, this method supports optimization based on system behavior by including physics-based objectives and constraints. The proposed framework is demonstrated using three case studies. First, the layout generation methods developed for stage 1 are compared with respect to system performance metrics obtained from stage 2. Second, a multi-objective optimization problem using the epsilon-constraint method is solved to obtain Pareto optimal solutions. Third, an extension to multi-loop systems is demonstrated. In summary, the design automation framework integrates several elements together as a step toward a more comprehensive solution of 3D packing and routing problems with both geometric and physics considerations.

Self-Consistent Electro-Thermal Approach for Terahertz Frequency Multiplier Design

Solid-state sources consisting of cascaded Schottky diode-based frequency multipliers are commonly used to generate power at millimeter and submillimeter wave bands. In order to achieve increasing power at those frequencies, first-stage frequency multipliers have to handle a large amount of input power and generate enough output power to drive higher frequency multiplication stages. To that end, an appropriate electro-thermal design of multiplier circuits can result in improvement of power-handling capabilities without degradation of conversion efficiency. This work proposes an approach for the design of frequency multipliers where both electrical and thermal considerations are self-consistently taken into account along the optimization of the Schottky diode structure and the circuit layout. The proposed methodology is validated with the design and test of a split-block waveguide single-chip tripler circuit with output frequency in the 225–325-GHz band. Good agreement between predicted performance and measured results is obtained for a wide input power range and different ambient temperatures.

EFFECT OF THERMAL INERTIA ON DIESEL ENGINES TRANSIENT PERFORMANCE

Transient operation of turbocharged diesel engines is affected by the thermal inertia of the cylinder parts, intake and exhaust manifolds. Because of thermal inertia the temperature of engine parts at steady operation fluctuates during the operating cycle near their average values in a relatively small range, but during transient operation it takes some time to warm or cool the engine parts. Thermal inertia is expressed in changes in fuel combustion, in-cylinder heat transfer and indicated efficiency of the cycle, and increase of general inertia of gas-turbine supercharging system, which determines the necessity to take into account this phenomenon when modeling unsteady engine operation. The conductance-capacitance model was proposed for online internal combustion engines operating cycle simulation tool Blitz-PRO to consider thermal inertia during engine’s transient process. The idea is to consider the heat capacity of engine parts during the heat transfer process, so they accumulate energy at warming and release it at cooling. Combined with equations of heat transfer and thermal conductivity it enables to calculate the change in the average temperatures during engine transient and consider the changes in the overall heat transfer process. The proposed method was tested by comparing the experimental data, obtained from the dyno test-bench based on modified KamAZ-740.10 diesel engine, and the results of modeling in Blitz-PRO. During the experiment, the instantaneous brake torque of the engine, crankshaft and turbocharger speed, supercharged air pressure and the pressure at the turbine’s inlet as well as the intake air mass flow were automatically measured during engine running. Calculations were executed for two setups: with the thermal inertia consideration and without it. As a result, it was found that the most influenced by thermal inertia is the supercharging system: by the 8th second of transient process the calculated supercharged air pressure without thermal inertia consideration is 19% greater, comparing to experimental data. The turbocharger’s rotor speed, intake air flow are influenced greatly too. Suggested method of thermal inertia assessment helps to provide much more accurate simulation of engine transient operation, especially in terms of turbocharging system behavior as it is shown.

Fabrication process and failure analysis for robust quantum dots in silicon

We present an improved fabrication process for overlapping aluminum gate quantum dot devices on Si/SiGe heterostructures that incorporates low-temperature inter-gate oxidation, thermal annealing of gate oxide, on-chip electrostatic discharge (ESD) protection and an optimized interconnect process for thermal budget considerations. This process reduces gate-to-gate leakage, damage from ESD, dewetting of aluminum and formation of undesired alloys in device interconnects. Additionally, cross-sectional scanning transmission electron microscopy (STEM) images elucidate gate electrode morphology in the active region as device geometry is varied. We show that overlapping aluminum gate layers homogeneously conform to the topology beneath them, independent of gate geometry and identify critical dimensions in the gate geometry where pattern transfer becomes non-ideal, causing device failure.

Courtyard as Tropical Hot Humid Passive Design Strategy: Case Study of Indonesian Contemporary Houses in Surabaya Indonesia

Courtyard in building contributes to indoor thermal environment. Courtyard element is commonly applied as passive cooling strategy in the design of boarding house in Indonesia. Courtyard has a potential aspect of being micro climate-modifier to reduce indoor air temperature during the day. This paper discusses the effects of courtyards on indoor thermal environment in Indonesian contemporary boarding houses using field measurement. This paper focuses on the indoor air temperature reduction of 5 courtyard houses design of the contemporary boarding house in Surabaya, East Java, Indonesia. The field experiment method was used for two physical environmental variables: the air temperature and relative humidity. Each sensor was shaded with a paper cups wrapped with the aluminum foil to prevent the effect of direct thermal radiation. Measurement was taken for approximately 23 days continuously in each building. The results of the measurement exposed that the form and enclosure element is pivotal in its thermal environment design consideration for tropical climate. Meanwhile, utilizing ventilation blocks as the primary enclosure also help reduce air temperature in hot-humid climates. The results showed that the indoor air temperatures in the courtyard and surrounding room were approximately 0.3-1.7°C lower than the outdoor air temperature during daytime. During night-time, indoor air temperatures inside swing to 0.8-1.9°C higher than the outdoor. The results of the thermal environment evaluation revealed that indoor air temperatures can be categorized as a neutral temperature of the measurement period. Therefore, the application of courtyards in contemporary boarding houses have proven as possible means of achieving sufficient cooling effects through full-day ventilation strategy, and showed improved performance when combined with ventilated blocks.

Gate-Controlled Suspended Titanium Nanobridge Supercurrent Transistor.

Under standard conditions, the electrostatic field-effect is negligible in conventional metals and was expected to be completely ineffective also in superconducting metals. This common belief was recently put under question by a family of experiments that displayed full gate-voltage-induced suppression of critical current in superconducting all-metallic gated nanotransistors. To date, the microscopic origin of this phenomenon is under debate, and trivial explanations based on heating effects given by the negligible electron leakage from the gates should be excluded. Here, we demonstrate the control of the supercurrent in fully suspended superconducting nanobridges. Our advanced nanofabrication methods allow us to build suspended superconducting Ti-based supercurrent transistors which show ambipolar and monotonic full suppression of the critical current for gate voltages of VGC ≃ 18 V and for temperatures up to ∼80% of the critical temperature. The suspended device architecture minimizes the electron-phonon interaction between the superconducting nanobridge and the substrate, and therefore, it rules out any possible contribution stemming from charge injection into the insulating substrate. Besides, our finite element method simulations of vacuum electron tunneling from the gate to the bridge and thermal considerations rule out the cold-electron field emission as a possible driving mechanism for the observed phenomenology. Our findings promise a better understanding of the field effect in superconducting metals.

Analysis of the Multi-Steps Package (MSP) for Series-Connected SiC-MOSFETs

In this paper, a multi-step packaging (MSP) concept for series-connected SiC-MOSFETs is analyzed. The parasitic capacitance generated by the dielectric isolation of each device in the stack has a significant impact on the dynamic behavior of SiC devices, which impacts the voltage-sharing performances. The study performed in this work reveals that the parasitic capacitance network introduced by the classical planar packaging unbalances the voltage across the series-connected SiC-MOSFETs. Therefore, a new drain-source parasitic capacitance network configuration provided by the MSP is proposed in order to improve the voltage balancing across the series-connected devices. The concept is introduced and analyzed thanks to equivalent models and time domain simulations. To verify the analysis, the voltage sharing between four series-connected 1.2 kV SiC MOSFETs is tested in a double pulse test setup. The experimental results confirm that the MSP has a better performance than the classical one in terms of voltage sharing. Furthermore, the proposed investigation shows that the MSP increases the middle point dv/dt of the switching cell. Sensitive analysis and thermal management considerations are also discussed in order to clarify the MSP limitations and indicate the ways to optimize the MSP from a thermal point of view.

Thermal and Manufacturing Design Considerations for Silicon-Based Embedded Microchannel Three-Dimensional-Manifold Coolers (EMMC)—Part 3: Addressing Challenges in Laser Micromachining-Based Manufacturing of Three-Dimensional-Manifolded Microcooler Devices

Abstract Laser machining is an inexpensive and fast alternative to conventional microfabrication techniques and has the capability to produce complicated three-dimensional (3D), hierarchical structures. It is especially important while performing rapid prototyping and quick design studies of extreme heat flux cooling devices. One of the major issues plaguing the use of laser micromachining to manufacture commercially usable devices, is the formation of debris during cutting and the difficulty in removing these debris efficiently after the machining process. For silicon substrates, this debris can interfere with surrounding components and cause problems during bonding with other substrates by preventing uniform conformal contact. This study delves deep into the challenges faced and methods to overcome them during laser micromachining-based manufacturing of such complicated 3D-manifolded microcooler structures. Specifically, this work summarizes several postprocess techniques that can be employed for complete debris removal during etching of silicon samples using an Nd/YVO4 ultraviolet (UV) laser, detailing the advantages and drawbacks of each approach. A method that was found to be particularly promising to achieve very smooth surfaces with almost complete debris removal was the use of polydimethylsiloxane (PDMS) as a high-rigidity protective coating. In the process, a novel technique to strip PDMS from silicon surface was also developed. The result of this study is valuable to the microfabrication industry where smooth and clean substrate surfaces are highly desirable and it will significantly improve the process of using UV lasers to create microstructures for commercial applications as well as in a research environment.

Thermal and Manufacturing Design Considerations for Silicon-Based Embedded Microchannel-3D Manifold Coolers (EMMCs): Part 1—Experimental Study of Single-Phase Cooling Performance With R-245fa

Abstract High performance and economically viable cooling solutions must be developed to reduce weight and volume, allowing for a wide-spread utilization of hybrid electric vehicles. The traditional embedded microchannel cooling heat sinks suffer from high pressure drop due to small channel dimensions and long flow paths in two-dimensional (2D) plane. Utilizing direct “embedded cooling” strategy in combination with top access three-dimensional (3D) manifold strategy reduces the pressure drop by nearly an order of magnitude. In addition, it provides more temperature uniformity across large area chips and it is less prone to flow instability in two-phase boiling heat transfer. This study presents the experimental results for single-phase thermofluidic performance of an embedded silicon microchannel cold plate (CP) bonded to a 3D manifold for heat fluxes up to 300 W/cm2 using single-phase R-245fa. The heat exchanger consists of a 5 × 5 mm2 heated area with 25 parallel 75 × 150 μm2 microchannels, where the fluid is distributed by a 3D-manifold with four microconduits of 700 × 250 μm2. Heat is applied to the silicon heat sink using electrical Joule-heating in a metal serpentine bridge and the heated surface temperature is monitored in real-time by infrared (IR) camera and electrical resistance thermometry. The maximum and average temperatures of the chip, pressure drop, thermal resistance, and average heat transfer coefficient (HTC) are reported for flow rates of 0.1, 0.2. 0.3, and 0.37 L/min and heat fluxes from 25 to 300 W/cm2. The proposed embedded microchannels-3D manifold cooler, or EMMC, device is capable of removing 300 W/cm2 at maximum temperature 80 °C with pressure drop of less than 30 kPa, where the flow rate, inlet temperature, and pressures are 0.37 L/min, 25 °C and 350 kPa, respectively. The experimental uncertainties of the test results are estimated, and the uncertainties are the highest for heat fluxes < 50 W/cm2 due to difficulty in precisely measuring the fluid temperature at the inlet and outlet of the microcooler.

Thermal and Manufacturing Design Considerations for Silicon-Based Embedded Microchannel-Three-Dimensional Manifold Coolers—Part 2: Parametric Study of EMMCs for High Heat Flux (∼1 kW/cm2) Power Electronics Cooling

Abstract Thermal management of power electronics modules is one of the limiting factors in the peak power capability of the traction inverter system and overall efficiency of the e-drive. Liquid cooling using embedded microchannels with a three-dimensional (3D)-manifold cooler (EMMC) is a promising technology capable of removing heat fluxes of >1 kW/cm2 at tens of kPa pressure drop. In this work, we utilize computational fluid dynamics (CFD) simulations to conduct a parametric study of selected EMMC designs to improve the thermofluidic performance for a 5 mm × 5 mm heated area with the applied heat flux of 800 W/cm2 using single-phase water as working fluid at inlet temperature of 25 °C. We implemented strategies such as: (i) symmetric distribution of manifold inlet/outlet conduits, (ii) reducing the thickness of cold-plate (CP) substrate, and (iii) increasing fluid–solid interfacial area in CP microchannels, which resulted in a reduction in thermal resistance from 0.1 for baseline design to 0.04 cm2 K/W, while the pressure drop increased from 8 to 37 kPa.

Analytical modeling electrical conduction in resistive-switching memory through current-limiting-friendly combination frameworks

Resistive-switching memory (RSM) is one of the most promising candidates for next-generation edge computing devices due to its excellent device performance. Currently, a number of experimental and modeling studies have been reported to understand the conduction behaviors. However, a complete physical picture that can describe the conduction behavior is still missing. Here, we present a conduction model that not only fully accounts for the rich conduction behaviors of RSM devices by harnessing a combination of electronic and thermal considerations via electron mobility and trap-depth and with excellent accuracy but also provides critical insight for continued design, optimization, and application. A physical model that is able to describe both the conduction and switching behaviors using only a single set of expressions is achieved. The proposed model reveals the role of temperature, mobility of electrons, and depth of traps, and allows accurate prediction of various set and reset processes obtained by an entirely new set of general current-limiting parameters.

Dynamic optimization of low-density polyethylene production in tubular reactor under thermal safety constraint

A commercial low-density polyethylene (LDPE) which is produced by the polymerization process of ethylene in the presence of initiators in a long tubular reactor is the most widely used in polymer industry. The highly exothermic nature of the LDPE polymerization process and the heating-cooling prerequisite in the tubular reactor can lead to various problems, particularly safety in terms of thermal runaway and productivity, i.e., decreasing monomer conversion. Therefore, model-based optimization of an industrial LDPE tubular reactor under thermal safety consideration is required to be implemented. A first principle model for this process is developed and validated using industrial data. Mass and energy balances have been derived from kinetics of LDPE polymerization. Thereafter, an expression of reactor temperature under critical condition is developed and incorporated in the reference model for the thermal safety study. In order to ensure the process is thermally safe and meets the desired product grade, the constrained dynamic optimization is proposed to maximize the conversion of the monomer using orthogonal collocation (OC). The dynamic optimization result shows that the maximum reaction temperature under critical condition constraint can be satisfied by optimizing the reactor jacket. Moreover, it is achieved without jeopardizing the monomer conversion and the product grade.

Thermal State, Thickness, and Composition of the Lithospheric Mantle beneath the Upper Muna Kimberlite Field (Siberian Craton) Constrained by Clinopyroxene Xenocrysts and Comparison with Daldyn and Mirny Fields

To gain better insight into the thermal state and composition of the lithospheric mantle beneath the Upper Muna kimberlite field (Siberian craton), a suite of 323 clinopyroxene xenocrysts and 10 mantle xenoliths from the Komsomolskaya-Magnitnaya (KM) pipe have been studied. We selected 188 clinopyroxene grains suitable for precise pressure (P)-temperature (T) estimation using single-clinopyroxene thermobarometry. The majority of P-T points lie along a narrow, elongated field in P-T space with a cluster of high-T and high-P points above 1300 °C, which deviates from the main P-T trend. The latter points may record a thermal event associated with kimberlite magmatism (a “stepped” or “kinked” geotherm). In order to eliminate these factors, the steady-state mantle paleogeotherm for the KM pipe at the time of initiation of kimberlite magmatism (Late Devonian–Early Carboniferous) was constrained by numerical fitting of P-T points below T = 1200 °C. The obtained mantle paleogeotherm is similar to the one from the nearby Novinka pipe, corresponding to a ~34–35 mW/m2 surface heat flux, 225–230 km lithospheric thickness, and 110–120 thick “diamond window” for the Upper Muna field. Coarse peridotite xenoliths are consistent in their P-T estimates with the steady-state mantle paleogeotherm derived from clinopyroxene xenocrysts, whereas porphyroclastic ones plot within the cluster of high-T and high-P clinopyroxene xenocrysts. Discrimination using Cr2O3 demonstrates that peridotitic clinopyroxene xenocrysts are prevalent (89%) among all studied 323 xenocrysts, suggesting that the Upper Muna mantle is predominantly composed of peridotites. Clinopyroxene-poor or -free peridotitic rocks such as harzburgites and dunites may be evident at depths of 140–180 km in the Upper Muna mantle. Judging solely from the thermal considerations and the thickness of the lithosphere, the KM and Novinka pipes should have excellent diamond potential. However, all pipes in the Upper Muna field have low diamond grades (<0.9, in carats/ton), although the lithosphere thickness is almost similar to the values obtained for the high-grade Udachnaya and Mir pipes from the Daldyn and Mirny fields, respectively. Therefore, other factors have affected the diamond grade of the Upper Muna kimberlite field.

Multi-objective optimization in geometric design of tapered roller bearings based on fatigue, wear and thermal considerations through genetic algorithms

To improve the fatigue, wear and thermal based failures of Tapered Roller Bearings (TRBs) a multi-objective optimization technique has been proposed. Objective functions considered are: the dynamic capacity (Cd) that is related to fatigue life, the elasto-hydrodynamic minimum film thickness (hmin) that is associated to the wear life, and the maximum bearing temperature (Tmax) that is related to the lubricant life. This paper presents a non-linear constrained optimization problem of three objectives with eleven design variables and twenty-eight constraints. The said objectives have been optimized individually (i.e., the single-objective optimization) and concurrently (i.e., the multi-objective optimization) through a multi-objective evolutionary procedure, titled as the Elitist Non-dominated Sorting Genetic Algorithm. A set of standard TRBs have been selected for the optimization. Pareto-optimal fronts (POFs) and Pareto-optimal surfaces (POSs) are obtained for one representative standard TRB. Out of many solutions on the POFs/POSs only the knee-point solution has been shown in a tabular form. Life comparison factors have been calculated based on both the optimized and standard TRBs, and results indicate that the optimized TRBs got enhanced lives than standard bearings. To get the graphical impression of optimized TRBs, a skeleton of radial dimensions of all seven optimized bearings based on various combinations of objectives has been shown for one of the representative standard TRB. In few cases the multi-objective optimization has better convergence as compared to single objective optimization due to its inherent diversity by the principle of dominance. The sensitivity investigation has also been conducted to observe the sensitivity of three objectives with design variables. From the sensitivity analysis data, tolerances have been provided for design variables. These tolerances could be used by the manufacturing industry while producing TRBs.

Transient Studies on Low-Enriched-Uranium Core of Ghana Research Reactor–1 (GHARR-1)

Following the core conversion of Ghana’s miniature neutron source reactor (MNSR) from highly enriched uranium (HEU) to low-enriched uranium (LEU), there has been a change in the fuel composition, fuel, clad, and other reactor core parameters. Since the allowable core power in a nuclear reactor is limited by thermal considerations, this study presents transient analysis of the LEU core of Ghana Research Reactor−1 (GHARR-1). The transient study has been carried out using the Monte Carlo N-Particle code version 5 (MCNP5) and the Program for the Analysis of Reactor Transients (PARET)/Argonne National Laboratory (ANL) computational tools. The behavior of the reactor core at normal and accident conditions of large reactivity insertions was studied. Transient results obtained for accidental large reactivity insertions of 6.71 mk indicated that boiling might occur in the coolant because under such large reactivity insertions, the coolant temperature was close to the saturation temperature of the coolant. The results show that boiling will not occur in the core for other reactivity insertions of 1.94, 2.1, 2.99, 3.87, and 4.0 mk considering that the outlet coolant temperatures obtained are far below the saturation temperature of 100°C at a pressure of 1 atm. The clad and fuel meat temperatures obtained for all the reactivity insertions are far below the melting points of Zircaloy-4 clad material and UO2 fuel. The results of the power profiles obtained show that the reactor is inherently safe even under large reactivity insertion conditions. The results obtained were found to agree well with the available experimental results. Comparison of the results of the LEU core with the previous HEU core has shown that temperature rise in the LEU core is lower than that in the HEU core under reactor transient conditions.

Powering Milliwatts to Megawatts

Switched-mode power converters are used to interface equipments and consumer electronic devices over a wide range of power levels. The switch technology of any power converter is a signature of its power rating. The article discusses different power electronic switch technologies used in modern power supply implementation and evaluates their impact in the system design. From the design prospective, power loss is more important than efficiency. It can be established that as power level of converter goes up, it is indispensable to have higher efficiency due to thermal design considerations. A trend between the switching frequency and its efficiency with respect to the power rating of the converter has been inculcated in the article.