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1

Comparison of heating curves of a rectangular busbar in different conditions of heat abstraction during the short-circuit heating

Zaręba M.

Bulletin of the Polish Academy of Sciences. Technical Sciences

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2018

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Vol. 66, nr 1

79--85

EN

The paper compares heating curves for a rectangular busbar in the conditions of convective and adiabatic heat transfer during short-circuit heating. Different coefficients of heat transfer from the external surface of the busbar and different busbar cross-sections have been assumed. This has allowed determining the error value occurring when the adiabatic rather than convective boundary condition is presumed at short circuit. The analysis takes into account a change of resistivity in the temperature function. The respective boundary-initial problems have been solved with analytical methods using Green’s function. The calculated results show that no considerable errors occur for long-lasting short circuits with an adiabatic rather than convective boundary condition.

2

Temperature changes in the vicinity of thermally loaded structure embedded in the soil: effect of sand content and saturation degree

Różański A.

Studia Geotechnica et Mechanica

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2017

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Vol. 39, nr 2

61--71

EN

Due to the rapid development of geothermal technologies, the problem of efficient and proper evaluation of soil thermal conductivity becomes extremely important. Factors mostly affecting the soil conductivity are the conductivity of solid phase and the degree of saturation. The former one is mainly affected by the mineral composition, in particular, by the content of quartz whose conductivity is the highest one among all the minerals forming soil skeleton. Organic matter, because of its relatively low conductivity, influences the solid conductivity as well. The problem addressed in the paper is the influence of mentioned factors on temperature changes in the vicinity of thermally loaded structure embedded in the soil medium. Numerical simulations are carried out for different values of soil thermal conductivity resulting from various quartz contents and degrees of saturation. In addition, a weak coupled – heat and water transport – problem is considered.

3

Analiza możliwości detekcji niekontrolowanego dopływu wód gruntowych do głębokiego wykopu

Kasprzak A., Popielski P., Radzicki K.

Przegląd Geologiczny

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2017

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Vol. 65, nr 7

459--466

EN

The paper shows a comprehensive analysis of uncontrolled groundwater flow into a deep excavation. The scheme of works performed to construct the diaphgram walls as well as typical causes ofleakage are discussed. The possible ways of locating the lining leakage before executing the foundation with use of water-level change measurements and the thermal monitoring method are presented. In the presented calculations, the analysis of the transient heat flow and filtration coupled process was used. Numerical calculations in the case of leakage occurrence in the lining or in the foundation bottom as well as comparative calculations for the construction without the leakage were conducted. The temperature changes occurring in the diaphgram wall constructed in the ground as a result of hydration process were calculated. Then, the heat spreading as a result of waterflow through the leaky parts of the construction was analyzed. The article endeds with the conclusions and suggestion of the following work direction.

4

Transient Heat Flow in a One-component Lennard-Jones/spline Fluid : a Non-equilibrium Molecular Dynamics Study

Hafskjold B.

Computational Methods in Science and Technology

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2017

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Vol. 23, No. 3

185--198

EN

A one-component Lennard-Jones/spline fluid at equilibrium was perturbed by a sudden change of the temperature at one of the system’s boundaries. The system’s response was determined by non-equilibrium molecular dynamics (NEMD). The results show that heat was transported by two mechanisms: (1) Heat diffusion and conduction, and (2) energy dissipation associated with the propagation of a pressure (shock) wave. These two processes occur at different time scales, which makes it possible to separate them in one single NEMD run. The system was studied in gas, liquid, and supercritical states with various forms and strengths of the thermal perturbation. Near the heat source, heat was transported according to the transient heat equation. In addition, there was a much faster heat transport, correlated with a pressure wave. This second mechanism was similar to the thermo-mechanical “piston effect” in near-critical fluids and could not be explained by the Joule-Thomson effect. For strong perturbations, the pressure wave travelled faster than the speed of sound, turning it into a shock wave. The system’s local measurable heat flux was found to be consistent with Fourier’s law near the heat source, but not in the wake of the shock. The NEMD results were, however, consistent with the Cattaneo-Vernotte model. The system was found to be in local equilibrium in the transient phase, even with very strong perturbations, except for a low-density gas. For dense systems, we did not find that the local equilibrium assumption used in classical irreversible thermodynamics is inconsistent with the Cattaneo-Vernotte model.

5

Porównanie błędu w ocenie wartości temperatury przewodu cylindrycznego przy adiabatycznym i konwekcyjnym odprowadzaniu ciepła podczas nagrzewania zwarciowego

Zaręba M.

Przegląd Elektrotechniczny

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2015

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R. 91, nr 7

97-100

PL

W pracy oszacowano wartość błędu jaki popełnia się zakładając adiabatyczny warunek brzegowy zamiast konwekcyjnego przy zwarciu. W tym celu porównano krzywe rozgrzewu w warunkach konwekcyjnych i adiabatycznych przy tym samym prądzie zwarcia. Badania przeprowadzono dla okrągłego szynoprzewodu DC. Zakładano przy tym różne współczynniki przejmowania ciepła z powierzchni zewnętrznej jak również rożne jego przekroje. Zagadnienie brzegowego-początkowe nagrzewania adiabatycznego rozwiązano przy wykorzystaniu funkcji Greena. Z kolei zagadnienie przy konwekcyjnym warunku brzegowym rozwiązano wykorzystując zależności wyprowadzone w dostępnej literaturze oraz superpozycję składowych pola.

EN

The value of error, which is made by assuming adiabatic boundary condition instead of a convection condition during short circuit was estimated in the paper. For this purpose the heating up curves between adiabatic conditions and convection were compared for the same shortcircuit current. The round busbar DC was studied. Different coefficients of heat transfer from the outer surface and various cross-sections were assumed. The adiabatic boundary-initial problem was solved by using Green's functions. On the other hand, the convection boundary problem was solved using relations were derived in the available literature and the superposition of the field components.

6

Plano-parallel models of the electrical systems with non-uniform heat exchange on a perimeter Part II. Transient temperature field

Gołębiowski J., Zaręba M.

Przegląd Elektrotechniczny

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2015

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R. 91, nr 10

273-277

EN

In the article a spatial-temporal heating curve and distribution of the local time constant of the plano-parallel system were determined by the method of states superposition. The heat exchange is characterized by three different coefficients of the heat transfer on a perimeter of the system cross-section. The transient component of the thermal field was found solving the boundary-initial problem for the equation of heat conduction by the separation of variables method. The presented solution can be a mathematical model of the transient thermal field in a rectangular DC busway or in a long dielectric capacitative heated. It was proved that the busway can be approximated by the element of lumped parameters. In opposition to that, long dielectric have to be modeled by the element of distributed parameters. The results were verified by the method of finite elements (program NISA v. 16) and presented in a graphic form.

PL

Za pomocą metody superpozycji stanów w artykule wyznaczono przestrzenno-czasową krzywą rozgrzewu i rozkład lokalnej stałej czasowej układu płasko-równoległego. Wymianę ciepła charakteryzują trzy różne współczynniki jego przejmowania na obwodzie przekroju układu. Składową przejściową pola termicznego uzyskano rozwiązując metodą separacji zmiennych zagadnienie brzegowo-początkowe dla równania przewodnictwa cieplnego. Prezentowane rozwiązanie może stanowić model matematyczny nieustalonego pola termicznego w prostokątnym szynoprzewodzie DC lub w długim dielektryku nagrzewanym pojemnościowo. Wykazano, że szynoprzewód może być aproksymowany elementem o parametrach skupionych. W przeciwieństwie do tego, długi dielektryk musi być modelowany elementem o parametrach rozłożonych. Wyniki sprawdzono za pomocą metody elementów skończonych (program NISA v. 16) i przedstawiono w postaci graficznej.

7

Laboratory testing of transient heat flow in concrete

Zając B.

Czasopismo Techniczne. Środowisko

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2015

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Y. 112, iss. 2-Ś

199--205

EN

The article is a description of the laboratory testing of transient heat flow in small-size samples of concrete and analysis of the impact of spacers made of a polyurethane material to the flow and the temperature distributions in the sample. This problem affects the working of concrete structural members exposed to a wide range of temperatures, and the thermal stresses generated in them, especially when the heat flow is analysed by a combination made of a flexible connection. The results can be used in issues related to the durability of concrete elements and their connections.

PL

Przedmiotem artykułu jest opis badania laboratoryjnego nieustalonego przepływu ciepła w małogabarytowych próbkach betonowych oraz analiza wpływu przekładki wykonanej z materiału poliuretanowego na przepływ i rozkłady temperatury w próbce. Zagadnienie to ma wpływ na pracę betonowych elementów konstrukcyjnych poddanych działaniu szerokiego zakresu temperatury oraz generowanych w nich naprężeń termicznych, zwłaszcza gdy analizowany jest przepływ ciepła przez ich połączenie wykonane ze złącza podatnego. Wyniki badań mogą być wykorzystane w zagadnieniach związanych z trwałością elementów betonowych oraz ich połączeń.

8

Application of Duhamel's theorem in the thermal field analysis of a DC cable with two layers of insulation

Gołębiowski J., Zaręba M.

Bulletin of the Polish Academy of Sciences. Technical Sciences

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2009

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Vol. 57, nr 1

87-94

EN

In the paper we presented an asymptotic method of determination of the transient thermal field in a three-zone cable (i.e. in a core and in two layers of insulation). A basis of our method was decomposition of the system. We considered separately the region of the core, which we modeled by a concentrated element of the first order. In turn, we treated the two-zone region of insulation as the system of distributed parameters. In the analysis we applied Duhamel's theorem and the method of separation of variables. Eigenvalues of the boundary-initial problem and coefficients of eigenfunctions we determined numerically. In the result we obtained spatial-temporal heat-up curves of the investigated cable. Our results we presented in a graphic form.

9

One-second density of the short-circuit current in a DC cable with convective cooling

Gołębiowski J., Zaręba M.

Archives of Electrical Engineering

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2006

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Vol. 55, nr 2

179-192

EN

The thermal field of a DC cable under a short-circuit regime has been investigated. A convectional model of cooling of the system was assumed (i.e. Hankel's condition). The respective boundary-initial problem has been decomposed. Field components were determined by means of own analytical-numerical method. The given final temperature and obtained distributions enabled the computation of admissible one-second density of the short-circuit current. From the out analysis it follows that cooling conditions have small influence on the one-second current density. The same refers to the thermal field distribution in a core. It follows from the short duration of a short-circuit and a significant thermal inertia of the cable with a relatively large cross-section of a core. However introduction of Hankel's condition in place of Dirichlet's one changes in a significant way the picture of a field within a thin insulation. It raises the time constant of thermal processes in the cable.

PL

W pracy badano pole termiczne kabla prądu stałego w stanie zwarcia. Założono konwekcyjny model chłodzenia układu (tzn. warunek Hankela). Zdekomponowano odpowiednie zagadnienie brzegowo-początkowe. Składowe pola wyznaczono za pomocą własnej metody analityczno-numerycznej. Dana temperatura końcowa i otrzymane rozkłady umożliwiły obliczenie dopuszczalnej jednosekundowej gęstości prądu zwarcia. Z przeprowadzonej analizy wynika, że warunki chłodzenia mają wpływ na jednosekundowa gęstość. To samo odnosi się do pola termicznego w żyle. Wynika to z małego czasu trwania zwarcia i ze znacznej cieplnej inercji kabla o stosunkowo dużym przekroju żyły. Wprowadzenie warunku Hankela w miejsce Dirichleta zmienia jednak istotnie obraz pola w cienkiej izolacji. Zwiększa również stałą czasową cieplnych procesów w kablu.