1,720,985 research outputs found
A new modelling approach for piled and other ground heat exchanger applications
Full text linkPile heat exchangers have an increasing role to play in the delivery of renewable heating and cooling energy. Traditionally the thermal design of ground heat exchangers has relied upon analytical approaches which take a relatively simple approach to the inside of the heat exchanger. This approach is justified while the heat exchanger diameter remains small. However, as larger diameter piled foundations are used as heat exchangers, the transient heat transfer processes operating within the pile become more important. To increase our understanding of these processes and ultimately lead to improved thermal design approaches for pile heat exchangers it is important to examine the heat transfer within the pile in detail. To accomplish this, a new numerical approach has been implemented within the finite element software ABAQUS. Coupling of the convective heat transfer due to fluid flow within the heat transfer pipes and the heat transfer by conduction within the pile concrete is the most important facet of the model. The resulting modelling approach, which is ready to generalise to other geothermal applications and to assess thermo-mechanical couplings, has been validated against a multi-stage thermal response test carried out on a test pile in London Clay
Thermo-mechanical modelling for velocity prediction in catastrophic landslides
No full textThermal pressurisation has been proposed in literature as the key phenomenon to interpret the mechanics of the final collapse of large slope failures. A new thermomechanical model is proposed by improving on an existing one, applicable to large landslides and rockslides consisting of a coherent mass sliding on a thin clayey layer. The considered time window is that of catastrophic acceleration, which starts at incipient failure and ends a few seconds later, when the acquired displacement and velocity are such that the landslide is broken up into pieces. The model takes into account frictional heating, pore pressure build-up and thermoplastic collapse of the soil skeleton, leading to the vanishing of shear resistance and unconstrained acceleration. First, an existing thermo-elasto-plastic constitutive model for clays is discussed, and modified by re-formulating it in a general stress space and taking into account thermal softening. The soil constitutive model is then employed into an existing 1-D landslide model (Vardoulakis 2002), resulting in a set of three equations describing the time evolution of temperature, excess pore pressures within the shearband and slide velocity. The resulting model equations are shown to be well-posed, and then are discretised and integrated numerically to back-analyse the final stage of the case history of Vajont that occurred in Italy in 1963. Finally, a generalisation of this model and its potential applicability to the velocity back-prediction of other well-documented case histories are discussed
Thermal performance of thermoactive continuous flight auger piles
Full text linkFoundation piles are being increasingly equipped with heat exchangers to efficiently harvest shallow geothermal energy. For buildings in urban areas, continuous flight auger (CFA) piles are common owing to their speed, cost-efficiency and low noise levels. To construct a thermoactive CFA pile usually requires separate central installation of the heat exchanger. However, the energy performance of this type of pile has not been investigated systematically, with most studies focused on rotary piles where the heat exchanger is attached to the reinforcing cage. In this work, insights are provided about the main influences on the energy efficiency of thermoactive CFA piles, with a focus on the implications of using CFA construction techniques rather than rotary boring. An innovative three-dimensional numerical model, able to capture the different aspects of transient heat transfer, is employed together with analytical methods to evaluate the transient and steady-state behaviour of energy piles in a number of design situations. Attention is given to understanding the role of possible pipe-to-pipe interaction, which cannot be systematically investigated with standard methods. Finally, practical guidelines on the optimal choice of design parameters to maximise the energy efficiency of CFA piles, without altering the geotechnical arrangements, are provided
Numerical Modelling of Thermo-active Micropiles
No full textEnergy piles (EPs), consisting in piled foundations equipped with heat exchangers, have been extensively studied in recent years, both from the thermo-mechanical response and energy performance points of view. However, most research refers to typical rotary bored, CFA or precast driven, medium diameter piles. Not much attention has been devoted to so-called energy micropiles (EMPs), representing an opportunity to provide at the same time energy and structural retrofitting to existing buildings. Existing studies show that EMPs overall may thermally perform differently to EPs, but they are comparable in terms of specific heat flux. In this work, a 3D FE numerical model is employed to perform a comprehensive parametric study considering design factors that are peculiar to EMPs, to assess the most important parameters to maximize their energy performance. The parameter space is efficiently explored resorting to a statistically-based Taguchi approach. Results show that thermal design of EMPs should not be based on the same criteria as those used for medium-large diameter EPs, since different parameters are dominant in enhancing their energy performance. In particular, the pipes diameter should be maximized in EMPs for its strong influence in results, while being very easy to engineer
Influences on the thermal efficiency of energy piles
Full text linkEnergy piles have recently emerged as a viable alternative to borehole heat exchangers, but their energy efficiency has so far seen little research. In this work, a finite element numerical model is developed for the accurate 3D analysis of transient diffusive and convective heat exchange phenomena taking place in geothermal structures. The model is validated by reproducing both the outcome of a thermal response test carried out on a test pile, and the average response of the linear heat source analytical solution. Then, the model is employed to carry out a parametric analysis to identify the key factors in maximising the pile energy efficiency. It is shown that the most influential design parameter is the number of pipes, which can be more conveniently increased, within a reasonable range, compared to increasing the pile dimensions. The influence of changing pile length, concrete conductivity, pile diameter and concrete cover are also discussed in light of their energetic implications. Counter to engineering intuition, the fluid flow rate does not emerge as important in energy efficiency, provided it is sufficient to ensure turbulent flow. The model presented in this paper can be easily adapted to the detailed study of other types of geothermal structures
Influence of thermomechanics in the catastrophic collapse of planar landslides
Full text linkFrictional heating has long been considered a mechanism responsible for the high velocities and long run-out of some large-scale landslides. In this work a landslide model is presented, applicable to large-scale planar landslides occurring in a coherent fashion. The model accounts for temperature rise in the slip zone due to the heat produced by friction, leading to water expansion, thermoplastic collapse of the soil skeleton, and subsequently to an increase of pore-water pressure. The landslide model, comprising equations that describe heat and pore pressure diffusion and the dynamics of the moving mass, is used to analyse the evolution of the Jiufengershan planar landslide as an example. Further, its parameter space is systematically and efficiently explored using a Taguchi parametric analysis in an attempt to quantify dominant parameters. It is shown that the process of sliding is dominated by the softening properties of the material, as expected, but also by the permeability of the slip zone and the thickness of the sliding mass. It is worth noting that the latter two parameters do not enter traditional stability analyses of uniform slope
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
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