1,721,089 research outputs found
BHE field design by superposition of effects in space and time
No full textThe long-term time evolution of the mean surface temperature of the most critical BHE, for BHE fields with arbitrarily given monthly heat loads and no groundwater flow, is studied by means of finite-element computations performed through COMSOL Multiphysics and the superposition of effects. A unit step heat load, with duration of one month, is considered, and its effects are evaluated. Then, the effects of any periodic heat load with a period of one year and given monthly heat loads can be obtained by a weighted sum of the effects of the unit step heat load, properly displaced in time. The result of the computations is a set of dimensionless equations that, properly superimposed, yield the time evolution of the dimensionless temperature at the interface between the most critical BHE and the ground, for a period of 50 years, for several typical configurations of BHE fields
Time evolution of the ground temperature around a set of Borehole Heat Exchangers with unbalanced seasonal loads and no groundwater movement
No full textForced and free flow in a vertical annular duct under nonaxisymmetric conditions
No full textThe combined forced and free flow in a vertical annular duct is studied under the hypothesis of steady-state parallel laminar flow. The Boussinesq approximation is invoked and the viscous dissipation is considered as negligible. The thermal boundary conditions are such that axial symmetry does not occur and temperature does not change in the axial direction. The dimensionless local balance equations are solved analytically by means of the Fourier series expansion method. The analytical expressions of the dimensionless temperature field, of the dimensionless velocity field, and of the Fanning friction factor are obtaine
Temperature distribution in a field of long Borehole Heat Exchangers (BHEs) subjected to a monthly averaged heat flux
No full textA method to evaluate the long-term temperature distribution in a field of long Borehole Heat Exchangers (BHEs) subjected to a monthly averaged heat flux is presented, under the assumption that the effects of the groundwater flow are negligible. Each BHE is considered as a cylindrical heat source with diameter D and length L. For each ratio L/D and dimensionless radial distance r/D, the g-function (time-dependent dimensionless temperature, averaged along the BHE length, due to a uniform and constant heat load) is determined by finite-element simulations and written in polynomial form through accurate interpolations, in a very broad range of values of the dimensionless time. Tables of the coefficients of the polynomial functions are provided. The g-functions are then used to determine the time evolution of the mean dimensionless temperature at the interface BHE-ground, averaged among BHEs, for some BHE fields subjected to a monthly averaged heat load
Validation of a simplified model to determine the long term performance of borehole heat exchanger fields with groundwater advection
No full textLoss of performance of double U-tube borehole heat exchangers due to thermal short-circuiting
No full textThe loss of performance of double U-tube Borehole Heat Exchangers due to the thermal short-circuiting between the incoming and the outgoing fluid is studied by means of 3-D finite element simulations performed through COMSOL Multiphysics. Reference is made to a well-designed BHE with a length of 100 m, in winter working conditions, with two different values of the thermal conductivity of the grout and of the volume flow rate
Effects of flow direction and thermal short-circuiting on the performance of small coaxial ground heat exchangers
No full textThe effects of flow direction and thermal short-circuiting on the performance of small-size coaxial ground heat exchangers, currently used in Northern Italy, are studied by finite-element simulations, performed through 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 flow in the outer annular passage is laminar in winter and turbulent in summer. The distribution of the fluid bulk temperature in the inner circular tube is determined by means of the weak form boundary condition available in COMSOL Multiphysics; the forced-convection heat transfer in the outer annular passage is simulated directly. Two Small Coaxial Ground Heat Exchangers (SCGHEs) with the same length (20 m) 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, on account of the small length, the effect of thermal short-circuiting is not important for SCGHEs, especially if the annulus-in flow direction is employed
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