1,720,985 research outputs found
Superposition of the single point source solution to generate temperature response factors for geothermal piles
No full textGeothermal piles are a very promising technique to exploit the low enthalpy resource for ground coupled heat pumps. In fact, they are heat exchangers integrated in the foundation structures of the buildings, with reduced need in term of ground surface availability and diminished drilling costs. Unfortunately, to evaluate the ground thermal response to their presence it is not possible to use classical analytical solutions due to their low aspect ratio and to the relevant effect of the heat capacity of the inner cylindrical volume. In addition, different shapes of the pipe arrangement are possible: helix around the foundation pile or a series of vertical pipes connected through U bends at top and bottom of the cylindrical volume. This study proposes a semi-analytical method to model ground heat exchangers with a great flexibility concerning their shape. The method, called Multiple Point Sources (MPS), applies the spatial superposition of the analytical solution for the Single Point Source. It has been validated by means of the comparison with literature analytical methods and FEM results for helix heat exchangers. Finally, it has been applied to find the temperature response factor for different shapes of heat exchanger in geothermal piles
Thermomechanical behaviour of a solid plate undergoing a rapid, local cooling process
No full textIn the experiments to analyze the dynamical and the thermal behavior of the drop impact onto heated solid surfaces, the various impact regimes are characterized through the surface temperature before the impact. Although such “nominal temperature” may represent a valid and consistent parameter to describe such phenomena, other thermal aspects must be considered to understand deeply the physics of the impact. It is clear for example that the surface effusivity has an important role and determines the instantaneous temperature during the drop spreading. Furthermore if the drops are falling onto the heated surface with a given frequency, the time lag between following impacting droplets must be determined in order to obtain the same given “nominal temperature” at each impact event. A drop interaction onto a heated wall generates a transitory, local cooling of the solid surface, i.e. a transient heat conduction in a finite medium subject to a pulse cooling. The software FlexPDE® is a commercial code based on a finite element method, which include a simple solver with an automatic re-gridding routine. The Fourier equation is solved assigning a time dependent boundary condition on the plate, corresponding to the convection coefficient under the wetted area, both for the monophase and for the phase transition regime. The convection coefficient is evaluated from literature data using a conservative value. The wetted area is estimated directly from own experiments using CCD images of drop impact onto heated surfaces. The temperature evolution inside the plate is determined
Reduced scale experimental modelling of distributed thermal response tests for the estimation of the ground thermal conductivity
No full textThe knowledge of the ground thermal properties, and in particular the ground thermal conductivity is fundamental for the correct sizing of the Ground Coupled Heat Pump (GCHP) plant. The Thermal Response Test (TRT) is the most used experimental technique for estimating the ground thermal conductivity. This paper presents an experimental setup aimed to realise a suitable scale prototype of the real borehole heat exchanger (BHE) and the surrounding ground for reduced scale TRT experiments. The scaled ground volume is realised with a slate block. Numerical analyses were carried out to correctly determine suitable geometric and operational parameters for the present setup. The scaled heat exchanger, inserted into the block, is created with additive technology (3D printer) and equipped with a central electrical heater along its entire depth and with temperature sensors at different radial distances and depths. Present measurements highlight the possibility to reliably perform a TRT experiment and to estimate the slate/ground thermal conductivity with an agreement of about +12% with respect to measurements provided by a standard commercial conductivity meter on proper cylindrical samples of the same material and onto 10 different portions of the slate block
Low-Cost Distributed Thermal Response Test for the Estimation of Thermal Ground and Grout Conductivities in Geothermal Heat Pump Applications
No full textThe design process of a borehole heat exchanger (BHE) requires knowledge of building thermal loads, the expected heat pump's COP and the ground's thermophysical properties. The thermal response test (TRT) is a common experimental technique for estimating the ground's thermal conductivity and borehole thermal resistance. In classic TRT, a constant heat transfer rate is provided above ground to the carrier fluid that circulates continuously inside a pilot BHE. The average fluid temperature is measured, and from its time-dependent evolution, it is possible to infer both the thermal resistance of the BHE and the thermal conductivity of the ground. The present paper investigates the possibility of a new approach for TRT with the continuous injection of heat directly into the BHE's grouting by means of electrical resistance imparted along the entire BHE's length, while local (along the depth) temperature measurements are acquired. This DTRT (distributed TRT) approach has seldom been applied and, in most applications, circulating hot fluid and optical fibers are used to infer depth-related temperatures. The distributed measurements allow the detection of thermal ground anomalies along the heat exchanger and even the presence of aquifer layers. The present paper investigates the new EDDTRT (electric depth-distributed TRT, under patenting) approach based on traditional instruments (e.g., RTD) or one-wire digital sensors. The accuracy of the proposed method is numerically assessed by Comsol Multiphysics simulations. The analysis of the data obtained from the "virtual" EDDTRT confirms the possibility of estimating within 10% accuracy both thermal ground and grout conductivities
Morphology of multiple drop impacts onto heated surfaces
No full textThe paper reports experimental results on multiple drop impacts onto a heated solid surface. Line-arrays made of three water drops of millimetric size were produced by “on demand” drop generator. The impact drop size was kept constant while the impact velocity was varied to obtain Weber numbers ranging between 110 and 660. The influence of drop-drop interaction was evidenced by comparison with experiments performed under the same condition using single drops. The wall temperature was changed from 80°C to 260°C, to cover the various evaporation regimes. Also drop spacing was varied to study the effect of this parameter on the drop-drop interaction. Finally two plates with different values of surface roughness were used to evidence its effects on secondary atomisation and impact morphology. A high resolution CCD camera was used to collect images and many peculiar structures were observed during drop impact and boiling. The emphasis is put on the morphology of the impact process, which strongly depends on wall temperature and on the dynamical effects caused by to drop-drop interaction
Comparison of 10- and 25-year horizon designs for vertical borehole heat exchangers in geothermal heat pump applications
No full textThe precise design of Borehole Heat Exchanger (BHE) fields in ground-coupled heat pump (GCHP) systems is essential for maintaining optimal long-term performance. Conventional sizing methodologies, such as the ASHRAE method, typically account for a 10-year operational horizon. This study seeks to expand the applicability of the ASHRAE-Tp8 method to a 25-year operational period and to compare the design results related to different time horizon strategies. To this aim, the dimensionless temperature penalties have been compared against reference results obtained from g-functions pertaining to actual borehole field geometries. New optimized constants have been derived specifically for the ASHRAE-Tp8 method, facilitating its adaptation to the longer time frame of 25 years. The accuracy and reliability of this enhanced method have been extensively validated (at a 25-year time horizon, the average error is 3.8 % respect to the reference code). The findings of this study underscore the potential errors in total borefield length and borehole depth that could arise when applying the conventional 10-year design methodology to a 25-year horizon. On the other hand, the present study leads to deducing that the drilling costs due to a necessary higher borehole depth for a correct design, proper to a 25-year horizon, relatively increase (overall length can increase up to +16.8 %). The proposed methodology demonstrates wide applicability, providing a robust framework adaptable to various operational time frames
Influenza dell'effusività sull'atomizzazione secondaria di gocce impattanti su superfici riscaldate
No full textLo studio dell’atomizzazione secondaria dovuta all’impatto di gocce su superficie calda fornisce importanti contributi alla comprensione del generale fenomeno di impatto di gocce con transizione di fase. Esiste una considerevole quantità di lavori che evidenziano l’importanza dell’effusività del materiale sul fenomeno complessivo di scambio termico tra goccia e solido, ma vengono in genere trascurati dettagli quali l’atomizzazione secondaria prodotta, in particolare per i casi a numero di Weber non piccolo (>100). Il presente lavoro costituisce un primo studio sull’effetto dell’effusività del materiale sulla morfologia del fenomeno e l'atomizzazione secondaria prodotta dall'impatto di una goccia su superfici calde, nei due principali regimi di ebollizione (nucleata ed a film). Sono state utilizzate due superfici di diverso materiale: una lega di alluminio (AlMg3) e acciaio inossidabile (AISI316) aventi effusività rispettivamente di 20.7 ��__ 3 e 7.19 ��__ 3 kgK-1s-5/2. Sono state effettuate visualizzazioni per mezzo di CCD camera ad alta risoluzione spaziale per i due regimi di ebollizione. L'analisi morfologica basata sul confronto delle immagini della telecamera tra i due materiali utilizzati evidenzia (nel caso di ebollizione nucleata) che al ridursi dell'effusività i tempi caratteristici del fenomeno cambiano ed in particolare l'atomizzazione inizia a tempi superiori mentre quantitativamente il numero di gocce prodotte si riduce. Sono state evidenziati fenomeni di generazione di gocce secondarie prodotte da bolle di una particolare forma (detta a pagoda).Nel caso di ebollizione a film le differenze nei tempi caratteristici del fenomeno con materiali ad effusività superiori non è così marcata come nel caso di ebollizione nucleata ma è comunque presente. È invece riscontrabile, sempre nel regime di ebollizione a film, la formazione di un numero maggiore di gocce secondarie di diametro inferiore rispetto al caso ad alta effusività
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Global characterization of innovative polymeric micro-heat sink
No full textDuring the space missions, the problems related to the thermal conditioning of devices, to the personnel comfort and to the thermomechanical stresses are known and important. Furthermore for a space mission certain priorities are stressed, such as the small dimension and the lightness of thermal equipments. Due to the presence of high temperature gradients, which straightforwardly implies significant heating/cooling powers, these characteristics are sometimes difficult to obtain. Besides, miniaturized heat exchangers will have a terrestrial large industrial diffusion for electronic component cooling, in propulsion and in the power production for microsatellites, spacecrafts and airplanes and in many biomedical applications, for example, in cloth conditioning in harsh environmental conditions. Given the great value of the ratio between exchanging surface and volume of these systems, the specific thermal power per unit volume can reach extremely high values (1000 kW/m2 is a possible target). The low density, high elasticity and mechanical resistance suggest that the development of a new technology for heat sink, based on a sheet of polymeric microtubes (with a overall thickness of some tenths of millimeter) could be of great interest for space and terrestrial applications. A polymeric micro-heat sink was designed and built. Many technical problems were solved and a first prototype is available. The micro-heat sink was configured to cool an electric resistance with nitrogen and helium. An experimental set-up was built and some preliminary tests are running. For gas flow rates ranging from 3 to 26 Nl/min, a maximum heat flux of about 14 kW/m2 has been reached. The heat-sink has a surface of 6cm2 and a weight of 0.25g. The maximum pressure drop reached 3 bar. Different polymers were considered in order to test the heat sink thermal and mechanical resistances
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