1,721,004 research outputs found
Effect of surface orientation on nucleate boiling and critical heat flux of dielectric fluids
No full textTemperature response factors at different boundary conditions for modelling the single borehole heat exchanger
No full textDesign and simulation of borehole heat exchangers rely on the solution of the transient conduction equation. The typical approach for predicting the ground temperature variations in the short and long term is to recursively apply basic thermal response factors available as analytical functions or as pre-estimated tabulated values. In this paper a review of the existing response factor models for borehole heat exchangers (BHE) analysis is presented and a numerical model, built in Comsol environment is employed for calculating the temperature distribution in time and space around a single, finite length, vertical cylindrical heat source also taking into account its position with reference to the ground surface (effects of the adiabatic length or "buried depth" D). The temperature values are recast as dimensionless response factors in order to compare them with analytical solutions where available. Furthermore new temperature response factors suitable for describing the single Finite Cylindrical Source (FCS) under different operating modes (i.e. boundary conditions) are generated. Boundary conditions include imposed heat transfer rate, imposed temperature and a combination of both conditions, where spatially uniform temperature at the BHE interface is attained while also keeping constant the applied heat transfer rate
Constant temperature response factors for fast calculation of sparse BHE field g-functions
No full textGround Coupled Heat Pumps (GCHP) are efficient heating systems which mainly rely on the correct design of their ground heat exchangers for attaining the best performance. Such design process is often based on a ground response model which involves the solution of the transient conduction equation. A successful calculation technique for ground response predictions is based on basic thermal response factors also known as g-functions. These solutions can be built according to different boundary conditions and eventually taking into account the variation of the heat rate along the vertical borehole heat exchanger (BHE). The present paper describes a new technique for generating temperature response factors for different BHE field geometries by interrogating a table which provides temperature excess values at different dimensionless distances and times from a single, constant temperature, finite length heat source. In spite this method is not completely suitable for calculating the g-functions of densely packed BHE field in the late period, it is demonstrated that the present procedure is very fast and reliable for calculating the temperature response factors in medium time horizons (e.g. the 10 year deadline of the Ashrae Method) or even in the asymptotic time region of sparse BHE fields
Modelling the ground volume for numerically generating single borehole heat exchanger response factors according to the cylindrical source approach
No full textBorehole heat exchangers (BHE) are the most common solution for ground coupled heat pumps (GCHP).The thermal interactions with ground and the GCHP performance are strictly related to a number of fac-tors, including the ground properties, the BHE field geometry, the sequence and strength of heat loadsrequired by the building. A typical assumption to solve this problem is assuming the ground volumeas a purely conductive medium with constant properties: in such a way the transient Fourier equationcan be solved either analytically or numerically with a reasonable computational effort provided that asimple BHE geometry is considered. According to this approach suitable temperature response factorscan be calculated: they account for constant heat transfer rate conditions and they can be later applied tosuperposition techniques for simulating transient heat loads to/from the ground. In literature a numberof reference geometries and related temperature response factors for BHE modelling are available. Theyinclude the infinite and finite line source and the infinite cylindrical source, at imposed heat transfer rateor even imposed temperature. The aim of this paper is to develop a reliable numerical model for gener-ating temperature response factors for the single BHE according to different geometrical and boundaryconditions. A detailed analysis is carried out to recast the Fourier equation and to efficiently solve it withsuitable numerical parameters. The results are compared with literature analytical data and new insightsare provided to the temperature response factor approach for simulating GCHP systems
Loss 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
Measurement of flow parameters in solid–liquid slugging fluidised beds
No full textImpedance measurements have been applied to solid–liquid suspensions in order to infer the liquid and solid phase distributions in a fluidised slugging bed. The bed is constituted by soda spheres with a particle to column diameter ratio equal to 0.125. The instantaneous liquid fraction has been measured and used to obtain the average solid concentrations inside the different regions along the slug. The signal provided by the impedance probes is processed in terms of probability density functions. The local values of the liquid hold-up can thus be inferred. This study demonstrates the reliability of the presented approach and provides information on solid concentrations in the dense and lean regions of square-nose slugging
suspensions
Territorial mapping for the exploitation of low-enthalpy geothermal resources
No full textAmong the renewable energy sources, a particular interest is due to 'low-enthalpy geothermal resources' that allow an advantageous use of heat pumps for winter heating. A correct exploitation of these resources relies on the existence of a general criterion for the choice of the best locations where GCHPs can be successfully installed from the point of view of the payback period length. In the present paper, a method to compare different sites and choose the suitable ones is proposed by taking into account some quantities of the site under consideration, including the Heating Degree Days and the average temperature of the ground. As an example, this method, which has been setup by considering the available data for all the Italian municipalities, is applied to the provinces of Bologna and Genova
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