1,721,154 research outputs found
A fast method for evaluating the performance of complex arrangements of borehole heat exchangers
No full textGround coupled heat pumps are systems combining a heat pump with a ground heat exchanger for building heating and cooling purposes. Borehole heat exchangers (BHE) are the most frequently adopted solution for ground-assisted heat pump applications, and in most installations, they represent the most
important cost item. For this reasons, a careful design analysis is needed to either assure long-time performance or reduce the payback period, both parameters related to overall BHE length. The required approach for such a problem is a transient analysis of ground response to different heat loads at different
time scales. A computationally efficient way to tackle the problem is the recursive calculation of a basic thermal response factor for given different heat pulses representing the building energy demand. In this article, a review of the existing response factor models for BHE analysis is performed, and the finite line
source model is employed to develop and test new simplified solutions suitable for fast spatial and temporal superposition and for generating thermal response factors for arbitrary BHE arrangements
Experimental heat transfer and pressure losses of drag reducing polymer solution in a long water loop with a plate heat exchanger
No full textCERN Report, 199
Aspetti termodinamici e termocinetici nella conversione dell'energia per via magnetoidrodinamica a metallo liquido
No full textTesi di Dottorato, Rom
Design and Performance of a Conductance Probe for Measuring the Liquid Fraction in Two-Phase Gas-Liquid Flows
No full textThe performance of impedance probes for measuring the conductance of gas–liquid mixtures in horizontal pipes is here studied by means of the numerical solution of Laplace problem. In particular the work is aimed at optimising the probe geometry in order to improve the probe response both in terms of linearity and in terms of spatial resolution to step changes in phase distribution.
Starting from basic shapes employed in literature (ring and half-ring electrodes), the adopted approach allowed new probe geometries (characterised by non-uniform electrode distance and/or electrode width) to be found. The analysis is performed with reference to annular, stratified and dispersed distributions and the results are compared with available theoretical models. Measurements have been carried out to verify the predicted sensor response and to ascertain the effect of the particle size on the probe response under dispersed flow conditions. The analysis demonstrates the behaviour of different probe arrangements with respect to different flow patterns and shows the possibility to obtain optimised geometries matching the desired features of linear response and of enhanced spatial resolution
Dinamica termica del terreno e dimensionamentodegli impianti di calore a pompa di calore geotermica
No full textNel presente articolo si intendono descrivere
i principi fisici di base che regolano il processo
di scambio termico tra il terreno ed un sistema di scambiatori interrati nelle applicazioni a pompa di calore a circuito chiuso (Ground Coupled Heat Pumps, GCHP). Questa breve analisi
riguarderà gli scambiatori interrati verticali (Borehole Heat Exchangers, BHE), operanti in assenza di significativa circolazione d’acqua nel suolo, condizione per la quale il meccanismo di trasmissione del calore preminente è la conduzione termica
Gas-Liquid Distribution in the Developing Region of Horizontal Intermittent Flows
No full textExperiments have been carried out to analyze the evolution of the air/water flow structure along a horizontal 60 mm i.d. straight pipe. Plug and slug flow regimes were observed. In order to investigate the local structure of the flow, dynamic void fraction measurements have been performed by means of ring impedance probes. From dynamic void fraction measurements the mean void fraction values, the probability distributions of void, the liquid film height in the gas cavities and the slug frequencies have been evaluated. The analysis of the results showed that the flow structure deeply modifies along the pipe and that minor effects have to be ascribed to gas injection mode. The examination of the probability distribution function enabled the identification of the operating conditions at which the transition occurs, thus making it possible to propose a new objective criterion of identificatio
Correct design of vertical borehole heat exchanger systems through the improvement of the ASHRAE method
No full textIn this article, the ASHRAE method for borehole heat exchanger field design is deeply discussed in order to trace its theory and physical meaning. The aim of the article is corroborate previous studies in order to demonstrate that the current version of the method is not suitable for a reliable sizing of the borehole field since the criteria for calculating the temperature penalty term fail to large extent from a correct estimation. Based on the above findings (demonstrated by a comprehensive comparison with reference results pertaining a vast number of borefield configurations), a new version of the temperature penalty calculation procedure is presented that is able to maintain the ASHRAE formalism and simplicity while enhancing the accuracy of the design results
The temperature penalty approach to the design of borehole heat exchangers for heat pump applications
No full textBorehole heat exchangers (BHEs) are the most frequently adopted solution for ground coupled heat pump applications. In most installations, BHEs also represent the most important cost item, and a careful design analysis is needed to either assure long time performance or reduce the payback period, both parameters related to overall BHE length. The most efficient way, from a computational point of view, to predict the temperature evolution in time and space of a ground volume in contact with a system of BHE, is the recursive calculation of a basic thermal response factor, evaluated at different time steps and for given different heat pulses representing the building energy demand. Among the literature models, the Ashrae standard is the most simple method based on the above approach and it does not require, in principle, a dedicated computer code to solve the BHE sizing problem. In this paper a review of the existing response factor models for BHE analysis is performed and a new description of the Ashrae method is
provided. In particular the real meaning of the temperature penalty parameter, fundamental in the Ashrae standard calculation, is clearly explained and a direct method for calculating the long-term effective ground resistance is given. The method is also able to take into account the geometrical disposition of multiple BHE at given overall length
A simple model to evaluate direct contact heat transfer and flow characteristics in annular two-phase flow
No full textCalculations are performed to study the behavior of air-water mixtures in vertical flows employing a simple model for the description of mass, momentum, and energy transfer in annular gas-liquid flows. The model is applied to various flow conditions including thermal nonequilibrium and variable cross section ducts.
Theoretical results agree reasonably well with experimental data regarding well-known isothermal flows. A contribution is provided to the understanding of the two-phase mixtures interactions for gas-liquid internal heat transfer in two-phase, annular, nonisothermal flows
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