72 research outputs found
Cosmic censorship and spherical gravitational collapse with tangential pressure
We study the spherical gravitational collapse of a compact object under the
approximation that the radial pressure is identically zero, and the tangential
pressure is related to the density by a linear equation of state. It turns out
that the Einstein equations can be reduced to the solution of an integral for
the evolution of the area radius. We show that for positive pressure there is a
finite region near the center which necessarily expands outwards, if collapse
begins from rest. This region could be surrounded by an inward moving one which
could collapse to a singularity - any such singularity will necessarily be
covered by a horizon. For negative pressure the entire object collapses
inwards, but any singularities that could arise are not naked. Thus the nature
of the evolution is very different from that of dust, even when the ratio of
pressure to density is infinitesimally small.Comment: 16 pages, Latex file, two figures, uses epsf.st
Interior Weyl-type Solutions of the Einstein-Maxwell Field Equations
Static solutions of the electro-gravitational field equations exhibiting a
functional relationship between the electric and gravitational potentials are
studied. General results for these metrics are presented which extend previous
work of Majumdar. In particular, it is shown that for any solution of the field
equations exhibiting such a Weyl-type relationship, there exists a relationship
between the matter density, the electric field density and the charge density.
It is also found that the Majumdar condition can hold for a bounded perfect
fluid only if the matter pressure vanishes (that is, charged dust). By
restricting to spherically symmetric distributions of charged matter, a number
of exact solutions are presented in closed form which generalise the
Schwarzschild interior solution. Some of these solutions exhibit functional
relations between the electric and gravitational potentials different to the
quadratic one of Weyl. All the non-dust solutions are well-behaved and, by
matching them to the Reissner-Nordstr\"{o}m solution, all of the constants of
integration are identified in terms of the total mass, total charge and radius
of the source. This is done in detail for a number of specific examples. These
are also shown to satisfy the weak and strong energy conditions and many other
regularity and energy conditions that may be required of any physically
reasonable matter distribution.Comment: 21 pages, RevTex, to appear in General Relativity and Gravitatio
Parametric analysis of the factors affecting the efficiency of ground heat exchangers and design application aspects in Cyprus
The objective of this paper is to examine the factors affecting the sizing and positioning of Ground Heat Exchangers (GHEs) in Cyprus. This is achieved through the investigation of the influence of the temperature, thermal conductivity, specific heat and density of the ground as well as pipe diameter on the performance of GHEs using computer software modelling in conjunction with test data. Also, the long term temperature variation of the ground around the boreholes is examined since this affects the positioning of the GHEs. Because of the large number of parameters involved in the design the desired result can be achieved in various ways by considering the specific parameters. The results of the simulations have shown that, generally speaking, the island of Cyprus is suitable for geothermal heat pumps.Research Promotion Foundation (RPF); European Regional Development Fund (ERDF
Adoption of Waste Heat Recovery Technologies: Reviewing the Relevant Barriers and Recommendations on How to Overcome Them
Data Availability: All data generated or analyzed during this study are included in this published article.Copyright © The Author(s) 2022. The wide adoption of heat recovery technologies in industry is hampered by specific “barriers” related to both technical and non-technical issues. This paper attempts to determine these barriers and make recommendations on how to address them. First, a literature review of related material is presented. Among numerous barriers, the main ones identified are (i) lack of information, (ii) lack of technology knowledge, (iii) technology risks, (iv) high initial and running and maintenance costs, (v) lack of financial support and lack of governmental incentives, (vi) size and available space limitations, (vii) lack of available infrastructure, (viii) production constraints and risk of production disruptions, (x) risk of the system negative impact on the company operations, and (xi) policy and regulations restrictions. Then, based on the above, a structured questionnaire on barriers to the adoption of waste heat recovery (WHR) technologies was prepared and issued to a number of industries throughout the European Union. Upon analyzing the questionnaire, an assessment of the importance and negative impact of each of the above-mentioned barriers is made. Subsequently, strategies and recommendations on how to overcome the barriers is reported. These recommendations are hoped to be adopted as far as possible in the packaging, installation, commissioning, and demonstration of new and old WHR technologies.European Union’s Horizon 2020 research and innovation program under Grant agreement No. 680599
Spherical Universes with Anisotropic Pressure
Einstein's equations are solved for spherically symmetric universes composed
of dust with tangential pressure provided by angular momentum, L(R), which
differs from shell to shell. The metric is given in terms of the shell label,
R, and the proper time, tau, experienced by the dust particles. The general
solution contains four arbitrary functions of R - M(R), L(R), E(R) and r(0,R).
The solution is described by quadratures, which are in general elliptic
integrals. It provides a generalization of the Lemaitre-Tolman-Bondi solution.
We present a discussion of the types of solution, and some examples. The
relationship to Einstein clusters and the significance for gravitational
collapse is also discussed.Comment: 24 pages, 11 figures, accepted for publication in Classical and
Quantum Gravit
Methodology for estimating the ground heat absorption rate of Ground Heat Exchangers
In Ground Source Heat Pump systems, the heat exchange rate is an important factor with regard to the initial cost of the system. When the Ground Heat Exchanger (GHE) is installed in a lithology with high thermal properties in the presence of groundwater, the heat exchange rates are larger than in the cases with poor thermal response of the ground and no groundwater. This research, hence, focuses on a methodology of measuring and analyzing the thermal properties of the lithologies encountered in an area, which can be used for the prediction of heat injection rates of a GHE, depending on its characteristics, the installation area ground properties and groundwater flow. A tool was created with the use of FlexPDE software, and a study case was chosen in order to validate the results. Twenty-two, 100 m in depth, boreholes located in Lefkosia (Cyprus) were tested through simulation for their geothermal performance over time. Subsequently the estimated heat load for the boreholes, after 24 h of operation in cooling mode, was used with the help of Geographic Information System software for the compilation of a heat load per meter depth map that can be transferred to the ground by a GHE. A review of similar studies and Geographical Information System applications referring to other countries is also presented and their results are compared to the results of this study. The step by step procedure presented in this paper can be used by engineers handling geothermal projects as a useful guide for sizing GHEs and calculating the heat injection rates of any area
Black holes vs. naked singularities formation in collapsing Einstein's clusters
Non-static, spherically symmetric clusters of counter-rotating particles, of
the type first introduced by Einstein, are analysed here. The initial data
space can be parameterized in terms of three arbitrary functions, namely;
initial density, velocity and angular momentum profiles. The final state of
collapse, black hole or naked singularity, turns out to depend on the order of
the first non-vanishing derivatives of such functions at the centre. The work
extends recent results by Harada, Iguchi and Nakao.Comment: 13 pages, LaTeX format. To appear in Physical Review
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Waste Heat Recovery Technologies Revisited with Emphasis on New Solutions, including Heat Pipes, and Case Studies
Copyright: © 2022 by the authors. Industrial processes are characterized by energy losses, such as heat streams rejected to the environment in the form of exhaust gases or effluents occurring at different temperature levels. Hence, waste heat recovery (WHR) has been a challenge for industries, as it can lead to energy savings, higher energy efficiency, and sustainability. As a consequence, WHR methods and technologies have been used extensively in the European Union (EU) (and worldwide for that matter). The current paper revisits and reviews conventional WHR technologies, their use in all types of industry, and their limitations. Special attention is given to alternative “new” technologies, which are discussed for parameters such as projected energy and cost savings. Finally, an extended review of case studies regarding applications of WHR technologies is presented. The information presented here can also be used to determine target energy performance, as well as capital and installation costs, for increasing the attractiveness of WHR technologies, leading to the widespread adoption by industry.European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 680599
Gravastar Solutions with Continuous Pressures and Equation of State
We study the gravitational vacuum star (gravastar) configuration as proposed
by other authors in a model where the interior de Sitter spacetime segment is
continuously extended to the exterior Schwarzschild spacetime. The multilayered
structure in previous papers is replaced by a continuous stress-energy tensor
at the price of introducing anisotropy in the (fluid) model of the gravastar.
Either with an ansatz for the equation of state connecting the radial and
tangential pressure or with a calculated equation of state with
non-homogeneous energy/fluid density, solutions are obtained which in all
aspects satisfy the conditions expected for an anisotropic gravastar. Certain
energy conditions have been shown to be obeyed and a polytropic equation of
state has been derived. Stability of the solution with respect to possible
axial perturbation is shown to hold.Comment: 19 pages, 9 figures. Latest version contains new and updated
references along with some clarifying remarks in the stability analysi
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