IRIS Università degli Studi di Bolognahttps://cris.unibo.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Sat, 10 Apr 2021 22:56:32 GMT2021-04-10T22:56:32Z10801- Planning the energy retrofitting of a residential building in Bologna, Italyhttp://hdl.handle.net/11585/575847Titolo: Planning the energy retrofitting of a residential building in Bologna, Italy
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/11585/5758472014-01-01T00:00:00Z
- Planned energy-efficient retrofitting of a residential building in Italyhttp://hdl.handle.net/11585/575836.1Titolo: Planned energy-efficient retrofitting of a residential building in Italy
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/11585/575836.12015-01-01T00:00:00Z
- Mixed convection with variable viscosity in a vertical annulus
with uniform wall temperatureshttp://hdl.handle.net/11585/62180Titolo: Mixed convection with variable viscosity in a vertical annulus
with uniform wall temperatures
Abstract: The steady and laminar mixed convection with a temperature-dependent viscosity in a vertical annular duct with uniform wall temperatures is studied analytically. The flow is considered as purely axial and the fluid density is assumed to be a linear function of temperature. Analytical expressions of the dimensionless velocity distribution, of the dimensionless pressure drop and of the Fanning friction factors are determined. The importance of choosing the mean fluid temperature as the reference temperature in the definition of the difference between the pressure and the hydrostatic pressure is pointed out. The results show that the combined effects of buoyancy forces and of a variable fluid viscosity on the cross-section-averaged Fanning friction factor may be important, and that negative values of this
quantity may occur.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/621802008-01-01T00:00:00Z
- Finite-Element Evaluation of Thermal Response Tests Performed on U-Tube Borehole Heat Exchangershttp://hdl.handle.net/11585/71539Titolo: Finite-Element Evaluation of Thermal Response Tests Performed on U-Tube Borehole Heat Exchangers
Abstract: The results of two thermal response tests recently performed on two vertical borehole heat exchangers (BHEs) are presented. The BHEs are located in North Italy; they have the same cross section and a depth of 100 m and 120 m respectively.
The evaluation of the thermal properties of the ground and the grout is performed by a finite-element simulation method, developed through the software package COMSOL Multiphysics 3.4. The problem is sketched as two dimensional and the true
geometry of the BHE cross section is considered. The convective thermal resistance is reproduced through an effective (reduced)
conductivity of the tube walls. Water is considered as a solid in which a uniform heat generation occurs. The heat capacity of the
water contained in the circuit is represented through a time dependent effective density of the water contained in the BHE. The
method allows to reproduce accurately the time evolution of the mean temperature of water, even in the initial part of the heating
process, and to obtain a reliable evaluation of the thermal properties of both ground and grout.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/715392008-01-01T00:00:00Z
- Finite Element Simulation of Coaxial Borehole Heat Exchangershttp://hdl.handle.net/11585/77399Titolo: Finite Element Simulation of Coaxial Borehole Heat Exchangers
Abstract: Finite-element simulations of Coaxial Borehole Heat Exchangers (CBHEs) are performed by means of the software package COMSOL Multiphysics 3.4 (© Comsol, Inc.). The real 2-D axisymmetric unsteady heat conduction and convection problem is considered, both for winter and for summer working conditions. The distribution of the fluid bulk temperature in the inner circular tube is determined by means of the weak form boundary condition; the forced-convection heat transfer in the outer annular passage is simulated directly. The geometrical parameters of a CBHE usually employed in Italy, with a length of 100 m, are employed. Three different materials of the inner circular tube are examined: PE 100, PPR 80, and an ideal material with a vanishing thermal conductivity. Two different grounds and three different water flow rates are considered. The results point out that the effects of thermal short-circuiting and of flow rate on the performance of long CBHEs are important.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11585/773992009-01-01T00:00:00Z
- Rigorous Axiomatic Definition of Entropy Valid Also for Non-Equilibrium Stateshttp://hdl.handle.net/11585/62182Titolo: Rigorous Axiomatic Definition of Entropy Valid Also for Non-Equilibrium States
Abstract: We present a rigorous logical scheme for the definition of entropy, based on operative definitions of all the concepts employed, with all assumptions declared explicitly. Our treatment is an equivalent variation of the general definition of entropy given in E.P. Gyftopoulos and G.P. Beretta, Thermodynamics. Foundations and Applications, Dover, Mineola, 2005. However, here we outline the minimal set of definitions and assumptions required to construct the same definition by the most direct and essential sequence of logical steps.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/621822008-01-01T00:00:00Z
- A rigorous definition of mass in special relativityhttp://hdl.handle.net/11585/62181Titolo: A rigorous definition of mass in special relativity
Abstract: The axiomatic definition of mass in classical mechanics, outlined
by Mach in the second half of 19th century and improved by several authors, is simplified and extended to the theory of special relativity. According to the extended definition presented here, the mass of a relativistic particle is independent of its velocity and coincides with the rest mass, i.e. with the mass defined in classical
mechanics. Then, force is defined as the product of mass and acceleration, both in the classical and in the relativistic framework.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11585/621812008-01-01T00:00:00Z
- Comparison of isothermal and isoflux g-functions for borehole-heat-exchanger fieldshttp://hdl.handle.net/11585/701358.2Titolo: Comparison of isothermal and isoflux g-functions for borehole-heat-exchanger fields
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/11585/701358.22019-01-01T00:00:00Z
- Exergy of pure chemical fuelshttp://hdl.handle.net/11585/56990Titolo: Exergy of pure chemical fuels
Abstract: Expressions of the molar exergy and of the molar flow exergy of a pure chemical fuel are deduced rigorously from the basic principles of thermodynamics; they show that exergy and flow exergy coincide when the temperature and the pressure of the fuel are equal to those of the environment. A simple method to evaluate the Gibbs free energy of reaction under non standard conditions is presented. The dependence of the molar exergy on the temperature and on the relative humidity of the atmosphere is investigated. For hydrocarbons, hydrogen and carbon monoxide, the coefficients of polynomial interpolations of the molar exergy as a function of the relative humidity are given in a table, for three temperature values. Linear interpolation can be used to determine the molar exergy of these fuels at different temperatures.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11585/569902007-01-01T00:00:00Z
- Effects of flow direction and thermal short-circuiting on the performance of coaxial ground heat exchangershttp://hdl.handle.net/11585/75826Titolo: Effects of flow direction and thermal short-circuiting on the performance of coaxial ground heat exchangers
Abstract: The effects of flow direction and thermal short-circuiting on the performance of coaxial ground heat exchangers are studied by finite-element simulations, performed through COMSOL Multiphysics 3.4 (©Comsol, Inc.). The real 2-D axisymmetric unsteady heat conduction and convection problem is considered. The distribution of the fluid bulk temperature in the inner circular tube is determined by means of the “weak boundary form” boundary condition available in COMSOL Multiphysics; the laminar convective heat transfer in the outer annular passage is simulated directly. Two Coaxial Ground Heat Exchangers (CGHEs) with the same length but different cross sections are examined; moreover, two values of the ground thermal conductivity, as well as two materials for the inner tube wall are considered. The results point out that the annulus-in flow direction (fluid inlet in the outer annular passage) is more efficient than the center-in flow direction (fluid inlet in the inner circular tube) and that the effect of short-circuiting is not very important, if the annulus-in flow direction is employed. Indeed, with this inlet configuration the energy loss for thermal short-circuiting is less than 1% for time intervals longer than 1 hour.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11585/758262009-01-01T00:00:00Z