Stack Position Research Articles

Multi-objective optimization of thermo-acoustic devices using teaching-learning-based optimization algorithm

The current article presents three case studies based on a multi-objective optimization approach to optimize the performance of thermo-acoustic devices by obtaining the best possible set of geometrical characteristic parameters. In case study 1, the performance of a thermo-acoustic refrigerator is measured in terms of three objectives namely, acoustic cooling load (ΦC), coefficient of performance, and acoustic power loss (W02). Each objective is assigned a weight to facilitate suitable user-defined significance. The case study 2 aims to optimize a thermoacoustic prime-mover. The influence of stack position and its length, resonator length, plate thickness, and plate spacing are considered as design variables. Two objectives namely, pressure amplitude (P) and frequency (f) are considered as objectives for multi-objective optimization of the thermo-acoustic prime-mover. In case study 3, the performance of a thermo-acoustic engine is measured in terms of five objectives namely, work output (W), viscous loss (Rv), conductive heat loss (Qcond), convective heat loss (Qconv), and radiative heat loss (Qrad). Since the multiple objectives are to be optimized simultaneously, each objective is assigned a weight to facilitate suitable user-defined significance. The multi-objective optimization is carried out by the teaching-learning-based optimization algorithm. The results of application of teaching-learning-based optimization algorithm are compared with the results of General Algebraic Modeling System, response surface methodology, and experimental results. The results of the teaching-learning-based optimization algorithm are found better compared to those given by the other approaches.

Optimization of thermoacoustic engine driven thermoacoustic refrigerator using response surface methodology

Thermoacoustic engines (TAEs) are devices which convert heat energy into useful acoustic work whereas thermoacoustic refrigerators (TARs) convert acoustic work into temperature gradient. These devices work without any moving component. Study presented here comprises of a combination system i.e. thermoacoustic engine driven thermoacoustic refrigerator (TADTAR). This system has no moving component and hence it is easy to fabricate but at the same time it is very challenging to design and construct optimized system with comparable performance. The work presented here aims to apply optimization technique to TADTAR in the form of response surface methodology (RSM). Significance of stack position and stack length for engine stack, stack position and stack length for refrigerator stack are investigated in current work. Results from RSM are compared with results from simulations using Design Environment for Low-amplitude Thermoacoustic Energy conversion (DeltaEC) for compliance.

A reward-based algorithm for the stacking of outbound containers

Abstract: As global trade increases, container transshipment activities increase rapidly. Therefore, high competition among container terminals has emerged. The decision for good stacking positions for containers plays a critical role on the performance of a container terminal, since it influences the productivity of the terminal in a strong sense. In this study, we focus on the minimization of the berthing time of vessels by improving the stacking operations through minimization of non-value-added handling operations of containers, which is called reshuffling, as well as the traveling time of the cranes operating at the storage yard. Most of the containers in the container terminal we focus are outbound, which are transported into the container terminal via external trucks and stored in the stacking yard until they are loaded onto vessels. We propose a reward-based algorithm for the stacking of outbound containers by taking the following four components into consideration; container's distance to the closest RTGC, RTGC's workload, the number of stacked containers at the neighborhood bays, and the current height of the stacks at the storage yard. The inputs of the algorithm include the relevant information of the container to be stacked that are just entered into the terminal gate, current usage information of the storage yard and the current positions of the yard cranes. The proposed stacking strategy is in implementation phase to one of the container terminals in Izmir. The results seem to be promising when compared to the current randomized stacking strategy in the container terminal.

Experimental Investigation of Ceramic Substrates in Standing Wave Thermoacoustic Refrigerator

This work experimentally investigates the performance of ceramic substrates used as stacks in standing wave thermoacoustic coolers. Thermoacoustic technology is proposed in this study as an alternative sustainable solution to current issues with vapour compression refrigerators because of its environmentally friendlier attributes. However, the main hindrance to the expansion of this technology is its current lack of efficiency. Hence, an experimental investigation was conducted in this study. The influence of the geometrical configuration of the stack, described as the heart of the device, is investigated. The device was equipped with different selected low-cost porous materials (ceramic substrates) for performance testing and studies. Porosity, length and position of the ceramic substrates are variables that are considered in order to investigate the performance of the cooler. Eight cordierite honeycomb ceramic substrates with square cross sections and of four different lengths (26mm, 48mm, 70mm and 100mm) were considered. Five different stack positions, measured from the hot ends of the stack to the pressure antinode in increments of 100mm, were investigated. Measurement of temperature difference at steady state was used to determine the performance of a particular configuration. Guidance on the design of this sustainable solution for refrigeration and selection of the best geometrical configuration of ceramic substrates are provided. In addition, clarity on the relation between the geometrical configurations and the frequencies of the sound wave is highlighted.

Comprehensive first-principles study of stable stacking faults in hcp metals

The plastic deformation in hcp metals is complex, with the associated dislocation core structures and properties not well understood on many slip planes in most hcp metals. A first step in establishing the dislocation properties is to examine the stable stacking fault energy and its structure on relevant slip planes. However, this has been perplexing in the hcp structure due to additional in-plane displacements on both sides of the slip plane. Here, density functional theory guided by crystal symmetry analysis is used to study all relevant stable stacking faults in 6 hcp metals (Mg, Ti, Zr, Re, Zn, Cd). Specially, the stable stacking fault energy, position, and structure on the Basal, Prism I and II, Pyramidal I and II planes are determined using all-periodic supercells with full atomic relaxation. All metals show similar stacking fault position and structure as dictated by crystal symmetry, but the associated stacking fault energy, being governed by the atomic bonding, differs significantly among them. Stacking faults on all the slip planes except the Basal plane show substantial out-of-plane displacements while stacking faults on the Prism II, Pyramidal I and II planes show additional in-plane displacements, all extending to multiple atom layers. The in-plane displacements are not captured in the standard computational approach for stacking faults, and significant differences are shown in the energies of such stacking faults between the standard approach and fully-relaxed case. The existence of well-defined stable stacking fault on the Pyramidal planes suggests zonal dislocations are unlikely. Calculations on the equilibrium partial separation further suggests 〈c + a〉 dissociation into three partials on the Pyramidal I plane is unlikely and 〈c〉 dissociation on Prism planes is unlikely to be stable against climb-dissociation onto the Basal planes in these metals.

Effects of the Interlayer Interaction and Electric Field on the Band Gap of Polar Bilayers: A Case Study of Sc2CO2

The two-dimensional Sc2CO2 monolayer has net dipole moments as compared with other functional transition metal carbides. This unique feature may provide potential routes to tune its electronic properties. Herein, the band gap engineering of Sc2CO2 bilayers is investigated by the density functional calculation. We found that the band gap of Sc2CO2 bilayers is not only dependent on the relative stacking position and distance but also on the magnitude and direction of external electric field. The competition between the interlayer van der Waals and electrostatic interactions has been revealed as the dominant mechanism for the band gap modulation of polar bilayers. This works demonstrate that polar bilayers exhibit richer electronic properties than the nonpolar one. The interlayer electric field may be utilized to fabricate novel electronic and optoelectronic devices.

Series Resistance Reduction in Stacked Nanowire FETs for 7-nm CMOS Technology

Vertically stacked nanowire field effect transistors currently dominate the race to become mainstream devices for 7-nm CMOS technology node. However, these devices are likely to suffer from the issue of nanowire stack position dependent drain current. In this paper, we show that the nanowire located at the bottom of the stack is farthest away from the source/drain silicide contacts and suffers from higher series resistance as compared to the nanowires that are higher up in the stack. It is found that upscaling the diameter of lower nanowires with respect to the upper nanowires improved uniformity of the current in each nanowire, but with the drawback of threshold voltage reduction. We propose to increase source/drain trench silicide depth as a more promising solution to this problem over the nanowire diameter scaling, without compromising on power or performance of these devices.

Study on the Magnetic Field Homogeneity of HTS Bulk Magnets Including the Degraded HTS Bulk for Developing the Compact NMR Relaxometry

We have been studying the compact magnet for NMR device that consists of a stacked high-temperature superconducting (HTS) GdBCO bulk annuli trapped by a field cooling method. It was difficult to trap the uniform magnetic field above 4.7 T (200-MHz-class NMR magnet) and field homogeneity under 0.01 ppm/cm 3 at liquid nitrogen temperature (77.4 K) because of the low $Jc$- $B$ characteristics of present HTS bulks. On the other hand, the strength and homogeneity of the magnetic field required for NMR relaxometry device are 1.5 T and 150 ppm/cm 3 , respectively. Therefore, we have been investigating the development of the compact magnet for NMR relaxometry device. In our previous work, we obtained the trapped magnetic field over 1.5 T at 77.4 K using the stacked HTS bulk magnet with 80-mm height, and 150-ppm/cm 3 field homogeneity was obtained using the fabricated field compensation methods. However, the asymmetric problem of the magnetic field uniformity occurs by the different superconducting characteristics of HTS bulks when we produce the compact NMR magnet by stacking a large number of HTS bulks. Therefore, in this study, the field homogeneity of HTS bulk magnet by the arrangement of the degraded HTS bulk was investigated using 3-D FEM analysis. We examined the effects of the stacking position of degraded HTS bulk using various $Jc$- $B$ characteristics.

Compact standing wave thermoacoustic generator for power conversion applications

Abstract In the past decade, a variety of thermoacoustic engines (TAEs) were devised to convert thermal energy to acoustic power. In this paper, we optimized the design of a standing wave thermoacoustic generator that can provide high intensity acoustic pressure and convert it into electrical power output using a low cost alternator. Three prototypes of standing wave thermoacoustic generator (TAG) were designed to optimize the overall efficiency. The first prototype of standing wave TAG could produce an acoustic pressure of 0.9 kPa (153 dB) with an input thermal power of 210 W. Further, the maximum heat to electrical conversion efficiency was 0.045% with an input thermal power of 250 W. However, the performance of this system was not fully optimized. The performance of TAE depends upon various parameters including stack position, stack length and resonator length. Hence, a new second prototype of tunable TAG was developed to tune these critical parameters in order to improve the overall efficiency. A compact third prototype of TAG was successfully built with optimized parameters and has been tested. In the improved design, high intensity acoustic pressure of 2.9 kPa (163.5 dB) was observed for the same 210 W input thermal power. The maximum heat to electrical energy conversion efficiency was 0.084% with an input of 250 W which is 87% higher as compared to the first prototype. The major reason for the lower conversion efficiency is due to the low efficiency of the alternator. In future, high efficiency alternator designs can be employed along with careful impedance matching to obtain higher conversion efficiencies. The results described in this paper demonstrate the potential of developing compact portable acoustic power and electricity generators for decentralized power applications.

Grasp of Correlation between Degradation Factor and Cell Position of PEFC Stack

The cost reduction and the improvement of a life performance are necessary for the dissemination of Polymer Electrolyte Fuel Cell (PEFC). If the degradation of the fuel cell was able to be diagnosed at the early stage, the life performance can be improved by applying appropriate processing to PEFC. We have been proposing the PEFC Performance Diagnose System (PPDS) which diagnoses the state of each polarization (the resistance polarization, the activation polarization and the diffusion polarization) by analyzing transient response with Turuga Electric Corporation since 2002. Since 2014, the application of our PPDS to a PEFC short stack was tried because it was able to diagnose each degradation factor of a PEFC single cell. As a result, the PPDS not only found out a degradation cell in the four cells short stack but also was able to diagnose the degradation factor of this deteriorated cell. Therefore, this paper aims to grasp the correlation between the degradation factor and the cell position of the PEFC Short Stack using our PPDS, and the PPDS was able to find the unit with the deteriorated cell when it diagnoses each unit composed of four cells in the stack. As a result, because we elucidated the correlation between the degradation factor and the cell position of a stack, we can propose the structure of a PEFC stack where deteriorating is not caused easily. Moreover, we can apply our PPDS as a maintenance device for a fuel cell vehicle and a domestic use fuel cell.

Sequential permutation table method for optimization of stacking sequence in composite laminates

This paper presents a new stacking-sequence optimization method for composite laminates using a multicriteria objective function with buckling, strength and continuity constraint subject to in-plane normal compressive loads. The objective function combines a critical buckling load factor function and a critical failure load function into a single function to maximizes in which the relative influence of a stacking sequence and number of ply orientations is reflected. The buckling-load and failure-load factors are sorted from maximum to minimum respectively in a sequential permutation table. A sequence-design strategy and a ply-orientation selection strategy are developed to identify the ply orientation of each stacking position and number of ply orientations of the stacking sequence involving continuity constraints based on the sequential permutation table. A benchmark problem is presented to illustrate the accuracy and effectiveness of the method; the detailed results are compared with various heuristic methods.

Design and analysis of standing wave quarter wavelength thermoacoustic engine

Thermoacoustics have been recent area of investigationsdue to its noticeable advantages like simplicity, structural stability, absence of moving component, high reliability, minimal or no maintenance etc. In addition to these, thermoacoustic devices can use different noble gases and their mixtures as working fluid as well as they can use low grade energy sources such as solar energy, exhaust gas of an automobile, waste heat etc. Incurrent work, design of quarter wavelength standing wave type thermoacoustic engine stack, using the linear thermoacoustic theory, is described. A set of dimensionless parameters has been used toreduce large number of dependent parameters. The design procedure followed here can be a guide fordesign and development of thermoacoustic engines. Various operating parameters such asmean pressure, frequency, driving ratio, hot and cold end temperatures of stack areused to investigate parameters such as length of stack, position of stack, and efficiency.

Investigation of contact pressure distribution over the active area of PEM fuel cell stack

Contact pressure distribution over the active area of proton exchange membrane fuel cell (PEMFC) has significant effects on the performance of PEMFCs. Even clamping pressure over the membrane electrode assembly (MEA) affects contact resistance, characteristics of porous media and sealing task. This paper develops a PEM fuel cell model to study the contact pressure distribution over the membrane electrode assembly using finite element model. At first, the three-dimensional model of a single cell was reduced to a two-dimensional model to decrease the calculation time. After validation of the obtained results via the pressure sensitive films, the effect of some parameters such as thickness and material of the end plates, sealant hardness, number of the stack's cells and position of the cell on the contact pressure distribution over the MEA were investigated. The results reveal that optimizing mentioned parameters leads to design PEM fuel cells with proper contact pressure distribution over the MEA.

Study on the Magnetic Field Homogeneity of Stacked HTS Bulk Magnets Including the Deteriorated HTS Bulk by Crack for Compact NMR Relaxometry

We have been studying the compact magnet for NMR device that consists of a stacked high temperature superconducting (HTS) GdBCO bulk annuli trapped by a field cooling (FC) method. It was difficult to trap the uniform magnetic field above 4.7 T (200 MHz-class NMR magnet) and field homogeneity under 0.01ppm/cm3 at liquid nitrogen temperature (77.4K) because of the low Jc-B characteristics of present HTS bulks. On the other hand, the strength and homogeneity of the magnetic field required for NMR relaxometry device are 1.5 T and 150ppm/cm3 respectively. Therefore, we have been investigating the development of the compact magnet for NMR relaxometry device. In our previous study, we obtained the trapped magnetic field over 1.5 T at 77.4K using the stacked HTS bulk magnet with 80mm height, and 150ppm/cm3 field homogeneity was obtained using the fabricated field compensation methods on inner diameter of 20mm HTS bulk magnet. However, it is expected that the asymmetric problem of the magnetic field uniformity will be occurred by the crack in the HTS bulks with long-term operation. Therefore, in this study, the field homogeneity of HTS bulk magnet by the arrangement of the HTS bulk with crack was investigated using 3-D FEM analysis. We examined the effects of the stacking position of HTS bulk with the crack and the allowable range of degradation of HTS bulk to achieve the target field homogeneity of 150ppm/ cm3.

An Intelligent Storage Determining Method for Inbound Containers in Container Terminals

ABSTRACT Yang, X.; Zhao, N.; Bian, Z.; Chai, J., and Mi, C., 2015. An intelligent storage determining method for inbound containers in container terminals. In this paper, the container Stacking Position Determination Problem (SPDP) for inbound containers in the process of vessel loading and unloading activities is discussed, specifically focusing on the storage space allocation problem (SSAP) in container terminals. The process of positioning inbound containers is investigated and analyzed based on an advanced handling strategy, namely assigning quay cranes in their full utilization with the aim of “Full In & Full Out” that is proposed for internal trucks, where intense loading and unloading activities are considered at the same time. A multi-objective integer programming model (MOIPM) is then put forward based on the idea of real-time strategic planning. For the solution, a genetic algorithm (GA) is employed to address the formulated problem. In addition to the effectiveness and reliability of the propos...

Optimal Design of A Standing Wave Thermoacoustic Refrigerator Using GAMS

Abstract This work proposes a multi-objective optimization approach to model and optimize small scale standing wave thermoacoustic refrigerator (TAR). This study aims to optimize the geometric variables namely the stack position, the stack length, the blockage ratio and the plate spacing involved in designing thermoacoustic refrigerators. Unlike most previous studies, these variables are considered interdependent. System parameters and constraints that capture the underlying thermoacoustic dynamics have been used to define the models. The cooling load, the coefficient of performance and the acoustic power loss have been used to measure the performance of the device. The optimization task is formulated as a three-criterion nonlinear programming problem with discontinuous derivatives (DNLP). A practical example considering three different gases is given to illustrate the approach. This approach has been implemented in the software GAMS (General Algebraic modelling System) and Pareto optimal solutions describing the most preferred geometry for maximum performance of the device are computed using the augmented ɛ-constraint method.

Force and Temperature Dependent Folding of a 2-Base-Pair RNA Kissing Complex

The stability and function of proteins from thermophilic species can vary with temperature in a complex manner. However, in considering nucleic acids, ΔH is typically assumed to be constant when interpreting thermal melting results. Here we use temperature controlled optical tweezers or “thermal tweezers” to mechanically characterize the folding thermodynamics of a 2-base-pair kissing complex from the Moloney murine leukemia virus (MMLV) at temperatures ranging from 22°C to 42°C. At each temperature the folding free energy is directly determined from the reversible mechanical work to unfold the kissing complex. Surprisingly, the folding free energy of this basic RNA tertiary interaction depends non-linearly on temperature, indicating that ΔH changes with temperature. Based on mutational perturbation and molecular dynamics simulations, this non-linearity is attributed to the presence of a multi-step, temperature-sensitive, unfolding pathway. Specifically, at elevated temperatures, the unpaired flanking adenine bases in the hairpin loops are flipped out of their respective stacking positions, exposing the underlying kissing base pairs to water, thus reducing their stability. Our study suggests that at mesophilic temperatures, single-stranded residues can be thermally influenced, which may provide profound insights into temperature-dependent RNA folding and conformational dynamics.

Minimizing the Excitation of Parasitic Modes of Vibration in Slender Power Ultrasonic Devices

The design of slender power ultrasonic devices can often be challenging due to the excitation of parasitic modes of vibration during operation. The excitation of these modes is known to manifest from behaviors such as modal coupling which if not controlled or designed out of the system can, under operational conditions, lead to poor device performance and device failure. However, a report published by the authors has indicted that the excitation of these modes of vibration could be minimized through device design, specifically careful location of the piezoceramic stack. This paper illustrates that it is possible, through piezoceramic stack position, to minimize modal coupling between a parasitic mode and the tuned longitudinal mode of vibration for slender ultrasonic devices.

Evaluation of the Revised Lagrangian Particle Model GRAL Against Wind-Tunnel and Field Observations in the Presence of Obstacles

A revised microscale flow field model has been implemented in the Lagrangian particle model Graz Lagrangian Model (GRAL) for computing flows around obstacles. It is based on the Reynolds-averaged Navier–Stokes equations in three dimensions and the widely used standard $$k - \varepsilon $$ turbulence model. Here we focus on evaluating the model regarding computed concentrations by use of a comprehensive wind-tunnel experiment with numerous combinations of building geometries, stack positions, and locations. In addition, two field experiments carried out in Denmark and in the U.S were used to evaluate the model. Further, two different formulations of the standard deviation of wind component fluctuations $$\sigma _i $$ have also been investigated, but no clear picture could be drawn in this respect. Overall the model is able to capture several of the main features of pollutant dispersion around obstacles, but at least one future model improvement was identified for stack releases within the recirculation zone of buildings. Regulatory applications are the bread-and-butter of most GRAL users nowadays, requiring fast and robust modelling algorithms. Thus, a few simplifications have been introduced to decrease the computational time required. Although predicted concentrations for the two field experiments were found to be in good agreement with observations, shortcomings were identified regarding the extent of computed recirculation zones for the idealized wind-tunnel building geometries, with approaching flows perpendicular to building faces.

Stacking sequence optimization of composite laminates for maximum buckling load using permutation search algorithm

Due to the involvement of large number of design variables, it is still one of the key concerns to build an efficient optimization algorithm for the stacking sequence design of composite laminates with various constraints. In this work, the flexural stiffness parameters are expressed in terms of the ply orientations, which helps to formulate the maximum of buckling load factor as a problem of identifying the optimum ply orientation at stacking positions. Afterwards, we suggest a permutation search (PS) algorithm to reduce the evaluations in stacking sequence optimization of composite laminates. In the first stage, permutation operations are sequentially performed for each permutation position, and in the second stage a repair strategy is adopted for overcoming the violation of constraints while maximizing the value of the objective function. A comparison has been performed between the PS and three genetic algorithm (GAs) methods. It has been demonstrated that the number of process analyses for stacking sequence optimization are greatly reduced by the PS algorithm. The novel PS algorithm combined with the modified repair strategy outperforms the studied GA methods for constrained stacking sequence optimization of composite laminate both in computational performance and finding the optimal objective value with high reliability.