1,721,056 research outputs found
A one-material cylindrical model to determine short- and long-term fluid-to-ground response factors of single U-tube borehole heat exchangers
Full text linkA new cylindrical model is proposed, suitable to determine both the short-term and the long-term fluid-to-ground thermal response factor of a single U-tube borehole heat exchanger (BHE). In the model, a BHE is represented by an equivalent cylinder, with the same radius and heat capacity as the BHE. The cylinder is made of a homogeneous material and contains a heat-generating cylindrical surface with an equivalent radius, req, optimized by repeated 2D finite-element simulations. The thermal resistance of the layer between req and the BHE radius equals the BHE thermal resistance. A correlation yielding the optimized values of req is provided
Comparison of isothermal and isoflux g-functions for borehole-heat-exchanger fields
No full textA new numerical approach to determine the g-function of a borehole field with the boundary condition of uniform temperature and time-constant mean heat flux at the surface of the boreholes is presented. The method is employed to compare the g-functions obtained by this boundary condition with those obtained by the usual condition of uniform and constant heat flux, for a single borehole and for a field of six boreholes placed in two lines. Boreholes with length 150 m, diameter 15 cm, buried depth 1.5 m and mutual distance 7.5 m, for the field, are considered. The results show that the difference between the two kinds of g-functions is less than 1.5 % for a single borehole, while it reaches 8.7 % for the borehole field, at high values of time. Finally, we show that the superposition of the effects of the single boreholes yields correctly the g-function of the field in the case of uniform heat flux, but overestimates the g-function in the case of uniform temperature and constant mean heat flux
Tecniche CAD per la simulazione e il progetto di MMIC non lineari: Il contributo italiano
No full textThe evaluation of the effective thermal conductivity of metal-foam loaded phase change materials
Full text linkPhase Change Materials (PCMs) are suitable materials to be included in Latent Thermal Energy Storage Systems (LTESS) to enhance the storage capacity per unit of volume. In order to increase their typical low thermal conductivity, PCMs are often loaded with high-porosity open-cell metal foams. Several correlations have been proposed in the literature to evaluate the effective thermal properties of the composite medium made of PCM and metal foam. However, the values of the effective thermal conductivity (keff) predicted by the different relationships can be very different from each other, with a consequent strong impact on the results of numerical simulations. In this work, a critical overview of the accuracy of the most used literature correlations for the evaluation of the effective thermal conductivity of open-cell metal-foam loaded PCMs is made: the temperature distribution obtained through a numerical model using different correlations is compared with the experimental values measured by testing different commercial paraffins loaded with copper or aluminum foams, subjected to complete melting. Since no correlation proves to yield accurate results for all the composite PCMs tested in this work, a new method for the calculation of the effective thermal conductivity of the PCM-metal foam medium is suggested
Dimensionless fluid-to-ground thermal response of single-line bore fields with isothermal fluid
Full text linkThe design of a borehole-heat-exchanger (BHE) field is usually performed by means of dimensionless functions, called g-functions, that yield the time evolution of the mean temperature of the external surface of the BHEs, Tsm, produced by a time constant heat flux, which is usually considered as uniform. The mean temperature of the fluid, Tfm, is then evaluated by adding to Tsm the product of the heat flux per unit length and the BHE thermal resistance. This method overestimates the difference between Tfm and Tsm in the short term, and overestimates the g-function of the field in the long term. Methods to obtain more accurate results have been proposed, but require difficult and time-consuming numerical computations. In this paper dimensionless fluid-to-ground (ftg) functions that yield directly the time evolution of Tfm, in a time scale from a few minutes to hundreds of years, are
provided for any single-line bore field subjected to a time constant heat flux, composed of up to four BHEs fed in parallel with the same inlet temperature. The ftg-functions are obtained by finite-element simulations implemented in COMSOL Multiphysics, and are reported in two Excel files that, after entering the dimensionless parameters of the BHE field under examination, instantly yield a short-term and a long-term ftg-function perfectly joined at the separation instant. The main novelties of this work are the characterization of each BHE field by a few dimensionless parameters, the improvement of the BHE model presented in Naldi and Zanchini 2020, the accuracy, speed and simplicity of use of the final results. The validations of the simulation codes for a single BHE and for fields of 3 and 4 BHEs, by comparison with analytical solutions, yielded root-mean-square deviations equal to 0.023%, 0.43%, and 0.49% of the mean value, respectively. The validation of the simulation code for two BHEs, performed with an extremely high distance between the BHEs, yielded a root-mean-square deviation equal to 0.054% of the mean value, with respect to the long-term ftg-function obtained for a single BHE
Physical simulation of small-signal rate-dependent anomalies in GaAs MESFET's: a two-dimensional frequency-domain approach
No full textFull-time-scale fluid-to-ground thermal response of a borefield with uniform fluid temperature
Full text linkThe most accurate method for the design and the simulation of a borehole heat exchanger (BHE) field is employing the fluid-to-ground thermal response of the field, namely the mean-fluid-temperature rise produced by a time-constant thermal power supplied to the fluid. Usually, a short-term and a long-term model are applied, with results matched at a selected time instant. In this paper we propose a method to determine the full-time-scale thermal response of a BHE field that employs one numerical model and yields accurate results with a reasonable computation time. Each BHE is modeled as a one-material cylinder with the same radius as the BHE, surrounded by the ground and containing a heat-generating cylindrical surface whose temperature represents that of the fluid. The condition of uniform fluid temperature and time-constant total power supplied to the fluid, necessary for the long-term accuracy, is obtained iteratively, by imposing at the generating surface uniform time-dependent temperatures that converge to the desired condition. A 2 × 2 square BHE field is employed as an example. The method is recommended to obtain the thermal response of a BHE field with uniform fluid temperature, with high accuracy both in the short and in the long term
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