1,721,016 research outputs found
Experimental validation of a steady periodic analytical model for Breathing Walls
Full text linkThe Breathing Wall behaviour under variable boundary conditions is described by an analytical model based on a one-dimensional porous domain crossed by air and subject to third type steady periodic boundary conditions. To the best of the authors’ knowledge, its experimental validation is not provided in literature. In this work, a new model is derived considering Dirichlet steady periodic boundary conditions. The model is experimentally validated testing a 1 m2 no-fines concrete sample in the Dual Air Vented Thermal Box apparatus, specially improved to replicate dynamic thermal conditions. The experiments show that increasing the air flow velocity across the Breathing Wall from 0 to 12 mm/s enhances thermal coupling between the two environments, namely reduces the wall thermal capacity, with a decrease in the penetration time from 4.3 h to 3 h. The model shows a very good agreement with experimental data when predicting temperature distribution across the domain, with error averages and standard deviations within the thermocouple accuracy after calibration, assumed to be 0.15 ∘C. The lesser yet good agreement concerning conduction heat flux density is explained in terms of accuracy in the measurement of the boundary conditions and critical issues in the heat flow measure itself (i.e. probe thermal resistance, thermal contact, emissivity mismatch)
A numerical model to simulate the dynamic performance of Breathing Walls
Full text linkA one-dimensional Finite Difference Model for Breathing Wall components under time dependent Dirichlet boundary conditions is presented. The algorithm undergoes a comprehensive validation against a dynamic analytical model, under either sinusoidal and generically periodic boundary conditions, adopting different airflow velocities and in relation to capacitive and resistive materials alternatively. It is found that the accurate prediction of the temperature profile inside the wall is influenced primarily by the timestep, whose optimal value can be identified through a preliminary frequency analysis of the boundary conditions. Moreover, for a better prediction of the surface heat flow density, and especially in insulating materials, refining the space grid below 1 mm is recommended, as well as the adoption of a 3-point numerical scheme. The numerical model is finally tested against experimental data on a porous concrete wall, showing that numerical errors may compare to other sources of uncertainties, regarding materials properties and boundary conditions
Energy performance of ground heat exchangers embedded in diaphragm walls: Field observations and optimization by numerical modelling
Full text linkGround immersed structures thermally activated by embedded heat exchangers represent a solution for building climatization, that combines efficiency, sustainability and cost saving. However, the performance of thermally activated diaphragm walls is influenced by key factors that still require insights, such as the layout of the exchanger pipe, the ratio between exposed and fully immersed parts of the wall, and the variable thermal condition at the excavation side. In this paper, these aspects are investigated first with reference to a full scale monitored diaphragm wall. From the field observations a finite element model is set up, validated by sensitivity analyses and calibrated on the monitoring data. The model is then used to attempt an optimization of the exchanger pipe layout. For given structure, ground conditions, thermal inputs and properties, the energy performance can be improved by limiting the thermal interference between pipe branches circulating fluid at different temperatures, and by taking advantage of the fully immersed part of the wall, on both faces in direct contact with the soil. A suggestion is given for enhanced pipe layouts that meet these requirements and lead to up to a 15.8% increase of exchanged heat rate for the studied case
Measuring a Breathing Wall's effectiveness and dynamic behaviour
Full text linkBreathing Walls are building structures based on porous materials crossed by an airflow, which act both as building envelopes and ventilation system components. In climates where both heating and cooling are needed, a pro-flux configuration (heat and air mass both flowing in the same direction) might be alternated with a contra-flux configuration (heat and air mass flowing in opposite directions) during the year or even on a day. Understanding and modelling the Breathing Walls' stationary and dynamic behaviour is thus fundamental, in order to optimize their design and to fully exploit their energy-saving potential. In this experimental study, a small-scale no-fines concrete Breathing Wall was investigated. The steady-state contra-flux tests performed in a Dual Air-Vented Thermal Box laboratory apparatus were used to derive the heat recovery efficiency of the sample as a function of the cross airflow velocity. The effectiveness of this technology was then evaluated in a virtual case study. An optimal airflow velocity across the Breathing Wall was found, leading to energy savings between 9% and 14%. Dynamic tests were performed assuming a sinusoidal variation of the operative temperature on one side of the sample. They showed how airflow velocity affected the Breathing Wall inertia and dynamic behaviour
Numerical validation of a simplified design procedure for calculating the heating load in buildings with Breathing Wall components
Full text linkBreathing Walls (BWs) can provide significant building energy saving in winter conditions, but the present standard methodology for heating load calculation fails to consider this technology, thus limiting its application. In this paper, a procedure to include BWs in the EN 12831-1:2017 is then proposed. The methodology is tested against a numerical calculation of the heating load based on the coupling between the Building Energy Simulation (BES) engine TRNSYS and a Matlab Finite Difference Model (FDM) addressing heat and mass transfer across the BW. The very good agreement demonstrates that the BW can be syntethized by two key parameters, namely the effective thermal transmittance at the interior surface and the thermal recovery efficiency
A laboratory apparatus to study thermal response test in the presence of groundwater flow
Full text linkThe standard approach to Thermal Response Test, based on conduction heat transfer in the ground, turns out to be unsuccessful under significant groundwater flow. The applicability of the Moving Infinite Line Source model to interpret the TRT in this case still needs to be proved. In order to study the TRT in the presence of a groundwater flow, an original laboratory apparatus has been developed. The Sand Box design is based on a heat transfer similitude between the real scale TRT problem and the laboratory scale one. The Sand Box sizes (1,2 m x 0.6 m x 1.0 m) are then set in order to keep the boundaries unaffected by the heat source during the TRT. The U-pipe heat exchanger is reproduced through a two-cables electrical resistance 1 m long. A hydraulic loop with a peristaltic pump allows to obtain a Darcy velocity across the sandy soil up to 6,7510-5 m/s. The measurement system consists in several thermocouples in the porous medium and in a flow meter. The TRT results at null groundwater velocity allow to derive a reference thermal conductivity. The first tests with groundwater flow show the suitability of the apparatus and allow to derive some preliminary considerations
Microwave Characterization of Trapping Effects in 100-nm GaN-on-Si HEMT Technology
No full textTrapping effects of a state-of-the-art 100-nm GaN-on-Si high-electron mobility transistor (HEMT) process for radio-frequency (RF) applications are characterized for the first time. Considering an operation with high peak-to-average power ratio (PAPR) signals, pulsed-RF measurements give a more direct understanding of the dynamic trap behavior than the third-order intermodulation products (IM3). The experimental data are used for estimating the time constants describing the transients in the presence of signals with different PAPRs
Validazione sperimentale di un modello per facciate a “doppia pelle” in ventilazione meccanica
No full textGoing 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
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