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Sisman, Altug, Visiting ProfessorORCID iD iconorcid.org/0000-0002-7431-5115
Publications (6 of 6) Show all publications
Aydin, M., Onur, M. & Sisman, A. (2019). A new method for analysis of constant-temperature thermal response tests. Geothermics, 78, 1-8
Open this publication in new window or tab >>A new method for analysis of constant-temperature thermal response tests
2019 (English)In: Geothermics, ISSN 0375-6505, E-ISSN 1879-3576, Vol. 78, p. 1-8Article in journal (Refereed) Published
Abstract [en]

In this study, a new analysis method is proposed for estimating thermal conductivity of a ground by using the constant-temperature thermal response test data. The new method is based on an analytical solution of heat transfer rate per unit borehole length by using the Laplace transformation for constant-temperature thermal response tests. Its advantage is that it allows one to estimate thermal conductivity directly from the slope of the logarithmic time dependency of inverse unit-heat-transfer rate value without making an estimation of volumetric heat capacity. The method has been verified by using a numerical model and applied to different experimental data based on different test temperatures and compared with the classical thermal response test method. The results show that the proposed method reliably and effectively estimates thermal conductivity of ground.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2019
Keywords
Ground source heat pumps, Thermal properties of borehole, Thermal response test
National Category
Energy Engineering
Identifiers
urn:nbn:se:uu:diva-378634 (URN)10.1016/j.geothermics.2018.11.001 (DOI)000458467900001 ()
Available from: 2019-03-07 Created: 2019-03-07 Last updated: 2019-03-07Bibliographically approved
Aydin, A., Oikonomou, T., Bagci, G. B. & Sisman, A. (2019). Discrete and Weyl density of states for photonic dispersion relation. Physica Scripta, 94(10), Article ID 105001.
Open this publication in new window or tab >>Discrete and Weyl density of states for photonic dispersion relation
2019 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 94, no 10, article id 105001Article in journal (Refereed) Published
Abstract [en]

The current density of states (DOS) calculations do not take into account the essential discretenessof the state space, since they rely on the unbounded continuum approximation. Recently, discrete DOS based on the quantum-mechanically allowable minimum energy interval has been introducedfor quadratic dispersion relation. In this work, we consider systems exhibiting photonic (photon-like) dispersion relation and calculate the related density and number of states (NOS). Also, a Weyl's conjecture-based DOS function is calculated for photons and acoustic phonons at low frequency limit,by considering the bounded continuum approach. We show that discrete DOS function reducesto expressions of bounded and unbounded continua in the appropriate limits. The uctuationsin discrete DOS completely disappear under accumulation operators. It's interesting that relativeerrors of NOS and DOS functions with respect to discrete ones have exactly the same character withthe ones of quadratic dispersion relation. Furthermore, the application of discrete and Weyl DOS for the calculation of internal energy of a photon gas is presented and importance of discrete DOSis discussed. It's shown that discrete DOS function given in this work needs to be used wheneverthe low energy levels of a physical system are heavily occupied.

Keywords
density of states, Weyl's conjecture, confined systems
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-380267 (URN)10.1088/1402-4896/ab0bc5 (DOI)000478733700001 ()
Available from: 2019-03-25 Created: 2019-03-25 Last updated: 2019-10-02Bibliographically approved
Gultekin, A., Aydin, M. & Sisman, A. (2019). Effects of arrangement geometry and number of boreholes on thermal interaction coefficient of multi-borehole heat exchangers. Applied Energy, 237, 163-170
Open this publication in new window or tab >>Effects of arrangement geometry and number of boreholes on thermal interaction coefficient of multi-borehole heat exchangers
2019 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 237, p. 163-170Article in journal (Refereed) Published
Abstract [en]

In large-scale ground-source heat pump applications, a large number of borehole heat exchangers are used and performance losses become an important issue due to thermal interactions. Dependency of total performance losses on borehole spacing can analytically be expressed by using thermal interaction coefficient. For a given application field, interaction coefficient depends on number of boreholes (N), aspect ratio of borehole's arrangement geometry and operation time. In this study, functional dependencies of interaction coefficient on N and aspect ratio are investigated by considering different rectangular borehole arrangements. Dependencies of both thermal interaction coefficient and total heat transfer rate on aspect ratio are computationally examined. Also, the effects of number of boreholes and operation time on interaction coefficient are studied. The results showed that the values of both interaction coefficient and performance losses decrease with the decrease of aspect ratio of a borehole field. Aspect ratio dependency of total unit heat transfer rate becomes more evident in case of shorter borehole spacing. Furthermore, a strong dependency of interaction coefficient on N is observed when N is much smaller than a critical value, Nc, although an asymptotic behavior appears and dependency on N becomes negligible for N > Nc. Some empiric expressions are proposed for aspect ratio and N dependency of interaction coefficient as well as Nc. The results and the proposed expressions can be used to make an energy efficient and optimal design of a BHE field by maximizing the total performance while minimizing the field allocation and the thermal losses.

Keywords
Ground source heat pumps, Ground heat exchangers, Thermal interaction coefficient, Borehole field configuration
National Category
Energy Engineering Energy Systems
Identifiers
urn:nbn:se:uu:diva-379336 (URN)10.1016/j.apenergy.2019.01.027 (DOI)000459845100015 ()
Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-03-15Bibliographically approved
Aydin, A. & Sisman, A. (2019). Quantum shape effects and novel thermodynamic behaviors at nanoscale. Physics Letters A, 383(7), 655-665
Open this publication in new window or tab >>Quantum shape effects and novel thermodynamic behaviors at nanoscale
2019 (English)In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 383, no 7, p. 655-665Article in journal (Refereed) Published
Abstract [en]

Thermodynamic properties of confined systems depend on sizes of the confinement domain due to quantum nature of particles. Here we show that shape also enters as a control parameter on thermodynamic state functions. By considering specially designed confinement domains, we demonstrate how shape effects alone modify Helmholtz free energy, entropy and internal energy of a confined system. We propose an overlapped quantum boundary layer method to analytically predict quantum shape effects without even solving Schrödinger equation or invoking any other mathematical tools. Thereby we reduce a thermodynamic problem into a simple geometric one and reveal the profound link between geometry and thermodynamics. We report also a torque due to quantum shape effects. Furthermore, we introduce isoformal, shape preserving, process which opens the possibility of a new generation of thermodynamic cycles operating at nanoscale with unique features.

National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-379501 (URN)10.1016/j.physleta.2019.01.009 (DOI)000459519300011 ()
Available from: 2019-03-18 Created: 2019-03-18 Last updated: 2019-03-25Bibliographically approved
Aydin, A., Fransson, J. & Sisman, A. (2019). Thermosize voltage induced in a ballistic graphene nanoribbon junction. Journal of Applied Physics, 126(10), Article ID 104302.
Open this publication in new window or tab >>Thermosize voltage induced in a ballistic graphene nanoribbon junction
2019 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 126, no 10, article id 104302Article in journal (Refereed) Published
Abstract [en]

A thermoelectric voltage is induced in a junction, constituted of two dissimilar materials under a temperature gradient. Similarly, a thermosize voltage is expected to be induced in a junction made by the same material but having differentsizes, so-called thermosize junction. This is a consequence of dissimilarity in Seebeck coefficients due to differencesin classical and/or quantum size effects in the same materials with different sizes. The studies on thermosize effectsin literature are mainly based on semi-classical models under relaxation time approximation or even simpler localequilibrium ones where only very general ideas and results have been discussed without considering quantum transport approaches and specific materials. To make more realistic predictions for a possible experimental verification, here,we consider ballistic thermosize junctions made by narrow and wide (n-w) pristine graphene nanoribbons with perfectarmchair edges and calculate the electronic contribution to the thermosize voltage, at room temperature, by using the Landauer formalism. The results show that the maximum thermosize voltage can be achieved for semiconducting nanoribbons and it is about an order of magnitude larger than that of metallic nanoribbons. In the semiconducting case, the thermosize voltage forms a characteristic plateau for a finite range of gating conditions. We demonstrate, throughnumerical calculations, that the induced thermosize voltage per temperature difference can be in the scale of mV/K,which is high enough for experimental measurements. Owing to their high and persistent thermosize voltage values,graphene nanoribbons are expected to be good candidate for device applications of thermosize effects.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2019
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-393081 (URN)10.1063/1.5111504 (DOI)000486028500025 ()
Available from: 2019-09-16 Created: 2019-09-16 Last updated: 2019-10-31Bibliographically approved
Firat, C., Sisman, A. & Aydin, A. (2018). Characterization of density oscillations in confined and degenerate Fermi gases. Modern physics letters B, 32(32), Article ID 1850393.
Open this publication in new window or tab >>Characterization of density oscillations in confined and degenerate Fermi gases
2018 (English)In: Modern physics letters B, ISSN 0217-9849, Vol. 32, no 32, article id 1850393Article in journal (Refereed) Published
Abstract [en]

Friedel oscillations appear in density of Fermi gases due to Pauli exclusion principle and translational symmetry breaking nearby a defect or impurity. In confined Fermi gases, this symmetry breaking occurs also near to boundaries. Here, density oscillations of a degenerate and confined Fermi gas are considered and characterized. True nature of density oscillations are represented by analytical formulas for degenerate conditions. Analytical characterization is first done for completely degenerate case, then temperature effects are also incorporated with a finer approximation. Envelope functions defining the upper and lower bounds of these oscillations are determined. It is shown that the errors of obtained expressions are negligible as long as the system is degenerate. Numbers, amplitudes, averages and spatial coordinates of oscillations are also given by analytical expressions. The results may be helpful to efficiently predict and easily calculate the oscillations in density and density-dependent properties of confined electrons at nanoscale.

Keywords
Quantum size effects, Friedel oscillations, nano thermodynamics
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-372454 (URN)10.1142/S0217984918503931 (DOI)000451569500006 ()
Available from: 2019-01-08 Created: 2019-01-08 Last updated: 2019-03-25Bibliographically approved
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