Revisiting classical concepts of Linear Elastic Fracture Mechanics - Part I: The closing ‘mathematical’ crack in an infinite plate and the respective Stress Intensity Factors

This is the first part of a short three-paper series, aiming to revisit some classical concepts of Linear Elastic Fracture Mechanics. The motive of this first paper is to highlight some controversial issues, related to the un­natu­ral overlapping of the lips of a ‘mathematical’ crack in an in­fin­­­ite plate load­ed by specific combinations of principal stresses at in­finity (predicted by the clas­si­c­al solu­tion of the respective first fundamental problem), and the closely as­so­ciated issue of negative mode-I Stress Intensity Factor. The problem is con­­­front­ed by superimposing to the first funda­mental problem of Lin­ear Elastic Frac­ture Mechanics for an in­fin­ite cracked plate (with stress-free crack lips) an ‘in­­verse’ mixed fund­amental problem. This superposition provides naturally ac­­­­­­­­ceptable stress and displacement fields, prohibiting overlapping of the lips (by means of contact stresses generated along the crack lips, which force the over­lapped lips back to naturally accepted position) and, also, non-negative mode-I Stress Intensity Factors. The solu­tions of this first paper form the basis for the next two papers of the series, dealing with the respective prob­lems in fi­­n­ite do­­mains (recall, for example, the cracked Brazil­ian disc con­fig­u­ra­tion) weak­­ened by artificial notches (rather than ‘math­e­mat­ical’ cracks), by far more interesting for practical engineer­ing ap­pli­­ca­tions

Thermodynamic and economic assessments of a hybrid PVT-ORC combined heating and power system for swimming pools

T he thermodynamic and economic performance of a solar combined heat and power (S - CHP) system based on an array of hybrid photovoltaic - thermal (PVT) collectors and an organic Rankine cycle (ORC) engine is considered for the provision of heating and power to swimming pool facilities . Priority is given to meet ing the thermal demand of the swimming pool , in order to ensure a comfortable condition for swimmers in cold er weather conditions, while excess thermal output from the collector s at high er temperatures is converted to electricity by the ORC engine in warm er weather conditions. The thermodynamic performance of this system and its dynamic characteristics are analysed on the basis of a transient thermodynamic model. Various heat losses and gains are considered in accordance to environmental and user - rela ted factors for both indoor and outdoor swimming pools. A case study is then performed for the swimming pool at the University Sport Centre (USC) of Bari, Italy. The r esults show that employing a zeotropic mixture of R245fa/ R227ea (30/70%) as the ORC working fluid allows such an ORC system to generate ~50% more power than when using pure R236ea due to the better temperature match of the cycle to the low - temperature hot - water heat source from the output of the PVT collectors . Apart from generating electricity, the ORC engine also alleviates PVT collector overheating , and reduc es the required size of the hot - water storage tank. With an installation of 2000 m 2 of PVT collectors, e nergetic analyses indicate that the proposed S - CHP system can cover 84 - 9 6 % of the thermal demand of the swimming pool during the warm summer months and 61 % of its annual ly integrated total thermal demand. In addition, the system produces a combined (from the collectors and ORC engine) of 328 MWh of electricity per year, corresponding to 36% of the total electricity demand of the USC , with ~4% coming from the ORC engine . The analysis suggests a minimum payback time of 12. 7 years with a n optimized tank volume of 125 m

Direct numerical simulation of the autoignition of a hydrogen plume in a turbulent coflow of hot air

The autoignition of an axisymmetric nitrogen-diluted hydrogen plume in a turbulent coflowing stream of high-temperature air was investigated in a laboratory-scale set-up using three-dimensional numerical simulations with detailed chemistry and transport. The plume was formed by releasing the fuel from an injector with bulk velocity equal to that of the surrounding air coflow. In the ‘random spots' regime, autoignition appeared randomly in space and time in the form of scattered localized spots from which post-ignition flamelets propagated outwards in the presence of strong advection. Autoignition spots were found to occur at a favourable mixture fraction close to the most reactive mixture fraction calculated a priori from considerations of homogeneous mixtures based on inert mixing of the fuel and oxidizer streams. The value of the favourable mixture fraction evolved in the domain subject to the effect of the scalar dissipation rate. The hydroperoxyl radical appeared as a precursor to the build-up of the radical pool and the ensuing thermal runaway at the autoignition spots. Subsequently, flamelets propagated in all directions with complex dynamics, without anchoring or forming a continuous flame sheet. These observations, as well as the frequency of and scatter in appearance of the spots, are in good agreement with experiments in a similar set-up. In agreement with experimental observations, an increase in turbulence intensity resulted in a downstream shift of autoignition. An attempt is made to understand the key processes that control the mean axial and radial locations of the spots, and are responsible for the observed scatter. The advection of the most reactive mixture through the domain, and hence the history of evolution of the developing radical pools were considered to this effec

The critical influence of some “tiny” geometrical details on the stress field in a Brazilian Disc with a central notch of finite width and length

The role of some geometrical characteristics of the notches ma­chined in circular discs, in order to determine the mode-I fracture tough­ness of brittle materials, is discussed. The study is implemented both analyti­cally and numerically. For the analytic study advantage is taken of a recently intro­duced solution for the stress- and displacement-fields developed in a finite disc with a central notch of finite width and length and rounded corners. The vari­ation of the stresses along strategic loci and the deformation of the peri­me­ter of the notch obtained analytically are used for the calibration/validation of a flexible nu­mer­ical model, which is then used for a parametric investiga­tion of the role of geometrical features of the notched disc (thickness of the disc, length and width of the notch, radius of the rounded corners of the notch). It is con­cluded that the role of the width of the notch is of critical im­port­ance. Both the ana­lytic and the numerical studies indicate definitely that ignoring the ac­curate geo­metric shape of the notch leads to erroneous results concerning the actual stress field around the crown of the notch. Therefore, it is possible that misleading values of the fracture toughness of the material of the disc may be obtained

Diffusion-absorption refrigeration cycle simulations in gPROMS using SAFT-γ Mie

Diffusion-absorption refrigeration (DAR) is a clean thermally-powered refrigeration technology that can readily be activated by low- to medium-grade renewable heat. There is an ongoing interest in identifying or designing new working fluids for performance improvement, particularly in solar applications with non-concentrating solar collectors providing heat at temperatures < 150 °C. In this work, the state-of-the-art statistical associating fluid theory (SAFT) is adopted for predicting the thermodynamic properties of suitable DAR working fluids. A first-law thermodynamic analysis is performed in the software environment gPROMS for a DAR cycle using ammonia as the refrigerant, water as the absorbent and hydrogen as the auxiliary gas. The simulation results show good agreement with experimental data generated in a prototype DAR system with a nominal cooling capacity of 100 W. In particular, at a charge pressure of 17 bar and when delivering cooling at 5 °C, the model results agree with experimental COP data to within ± 7 % over a range of heat inputs from 150 to 500 W. The maximum coefficient of performance (COP) is estimated to be 0.24 at a heat input of 250 W. The group-contribution SAFT-γ Mie equation of state is of particular interest as it offers good agreement with experimental data and provides flexibility in extending the model to test different working fluids with a high degree of fidelity. A methodology is also presented that allows the DAR thermodynamic analysis and working-fluid modelling to be integrated into a more general technology optimisation framework

Revisiting classical concepts of Linear Elastic Fracture Mechanics - Part I: The closing ‘mathematical’ crack in an infinite plate and the respective Stress Intensity Factors

This is the first part of a short three-paper series, aiming to revisit some classical concepts of Linear Elastic Fracture Mechanics. The motive of this first paper is to highlight some controversial issues, related to the un­natu­ral overlapping of the lips of a ‘mathematical’ crack in an in­fin­­­ite plate load­ed by specific combinations of principal stresses at in­finity (predicted by the clas­si­c­al solu­tion of the respective first fundamental problem), and the closely as­so­ciated issue of negative mode-I Stress Intensity Factor. The problem is con­­­front­ed by superimposing to the first funda­mental problem of Lin­ear Elastic Frac­ture Mechanics for an in­fin­ite cracked plate (with stress-free crack lips) an ‘in­­verse’ mixed fund­amental problem. This superposition provides naturally ac­­­­­­­­ceptable stress and displacement fields, prohibiting overlapping of the lips (by means of contact stresses generated along the crack lips, which force the over­lapped lips back to naturally accepted position) and, also, non-negative mode-I Stress Intensity Factors. The solu­tions of this first paper form the basis for the next two papers of the series, dealing with the respective prob­lems in fi­­n­ite do­­mains (recall, for example, the cracked Brazil­ian disc con­fig­u­ra­tion) weak­­ened by artificial notches (rather than ‘math­e­mat­ical’ cracks), by far more interesting for practical engineer­ing ap­pli­­ca­tions

Thermoeconomic assessment of a spectral-splitting hybrid PVT system in dairy farms for combined heat and power

We investigate the thermoeconomic potential of a solar-combined heat and power (S-CHP) system based on concentrating, spectral-splitting hybrid photovoltaic-thermal (PVT) collectors for the provision of electricity, steam and hot water for processing milk products in dairy applications. Transient simulations are conducted by using a system model with real-time demand and weather data as inputs, taking account of the spectrum-selective features of the PV cells as well as key heat transfer mechanisms that determine the electrical and thermal performance of the PVT collector. Economic performance is also assessed by considering the investment and savings enabled by the reduced electrical and fuel consumption. The results show that incorporating spectral beam-splitting technology into hybrid PVT collectors can be effective in maintaining the PV cells at low temperatures, while at the same time supplying thermal outputs (fluid streams) at temperatures significantly higher than then cell temperatures for steam generation and/or hot water provision. Based on a 15, 000-m2 installed area, it is found that 80% of the thermal demand for steam generation and 60% of the hot water demand can be satisfied by the PVT S-CHP system, along with a net electrical output amounting to 60% of the demand. Economic and environmental assessments show that the system has an excellent decarbonisation potential (1, 500 tCO2/year) and is economically viable if the investment cost of the spectrum splitter is lower than 0.85 of the cost of the parabolic concentrator (i.e., &lt;2, 150 €/m2 spectrum splitter) in this application

Multimode Interference 3 dB Splitters in Hollow Core Metallic Waveguides for Low-Loss THz Wave Transmission

A multimode interference half-power splitter on hollow core polysterene-coated metallic waveguide is demonstrated in this study. The modal properties, propagation length, optical field evolution, and dispersion of the aforementioned device have been investigated using the full vectorial finite-element-based modal analysis and beam propagation method, as well as the finite-difference time-domain approach

Low-loss multimode interference couplers for terahertz waves

The terahertz (THz) frequency region of the electromagnetic spectrum is located between the traditional microwave spectrum and the optical frequencies, and offers a significant scientific and technological potential in many fields, such as in sensing, in imaging and in spectroscopy. Waveguiding in this intermediate spectral region is a major challenge. Amongst the various THz waveguides suggested, metal-clad plasmonic waveguides and specifically hollow core structures, coated with insulating material are the most promising low-loss waveguides used in both active and passive devices. Optical power splitters are important components in the design of optoelectronic systems and optical communication networks such as Mach-Zehnder Interferometric switches, polarization splitter and polarization scramblers. Several designs for the implementation of the 3dB power splitters have been proposed in the past, such as the directional coupler-based approach, the Y-junction-based devices and the MMI-based approach. In the present paper a novel MMI-based 3dB THz wave splitter is implemented using Gold/polystyrene (PS) coated hollow glass rectangular waveguides. The H-field FEM based full-vector formulation is used here to calculate the complex propagation characteristics of the waveguide structure and the finite element beam propagation method (FE-BPM) and finite difference time domain (FDTD) approach to demonstrate the performance of the proposed 3dB splitter