Henok Hailemariam

Thermo-hydro-mechanical analysis of cement-based sensible heat stores for domestic applications

The thermo-hydro-mechanical behaviour of a water-saturated cement-based heat store for domestic applications has been investigated. Numerical simulations have been employed to locate the critical regions during thermal loading, for which analytical solutions have been derived and validated by numerical simulations. The analytical solutions allow a fast screening of materials and design parameters in relation to the stresses induced by thermomechanical loading. Maximum stresses in the system have been quantified based on the thermomechanical properties of three heat exchanger materials selected by design engineers and of the filling material. Sensitivity analyses indicate that the stress distribution is very sensitive to the thermal expansion coefficients of the involved materials. The results of this study can serve as a guide line for the design of the present and similar heat storage systems. The analytical solution developed is a fast and robust method for the evaluation of stresses around heat exchangers embedded in a solid material and can serve as a tool for design optimisation.

Modelling EM Heating of Porous Media with Lattice Element Method

Electromagnetic (EM) heating of porous media has recently gained much interest in the energy and construction sectors in general, and the recovery of highly viscous bituminous or heavy oils from oil sands and shales in particular. In this study, a new model for solving the coupled heat transfer and EM equations using the lattice element method (LEM), to analyze the spatial and temporal temperature distribution of porous media reservoirs is presented. The new model provides a good basis for simulating the meso-scale behavior of EM heated porous media in view of the phenomenon of selective heating, as the different constituent phases of the porous medium can be modeled as discrete nodal elements which dissipate applied EM energy according to their loss content (polarization), which is difficult to model with continuum based models such as the finite element method (FEM).

Thermal Cyclic Stability Analysis of Porous Heat Storage Materials

Assessing the thermal cyclic stability of energy storage materials is of utmost importance in the design and overall serviceability of sensible heat storage systems. In particular, care should be taken to ensure that the plastic strains accumulated upon short and long term cyclic operations are within the design limits, thus preventing critical failure of the different components of the heat storage system. In this study, the thermal cyclic stability of a commercial cement-based porous heat storage material is analyzed in water-saturated conditions by performing heating/cooling cycles in the temperature range from 20 to 80 °C with a newly developed cyclic thermo-mechanical device. The thermo-mechanical device produces a homogenously linear temperature distribution across the specimen, thus recreating the actual heat flow and distribution within sensible heat storage materials upon heat loading/unloading operations. The cyclic thermal, peak induced and accumulated plastic strains due to charging/discharging operations of the sensible heat storage material are studied for several lower and upper temperature cycle limits (20–40 °C, 20–60 °C, 20–80 °C and 60–80 °C) and dwelling periods (0, 10, 40 and 120 min), and the results are analyzed in terms of the intrinsic porous medium structure and cementation behavior.

Effect of Hydro-mechanical Changes on the Relationship between Thermal Conductivity and Dielectric Permittivity of Soils

Soil thermal properties such as thermal conductivity and resistivity, thermal diffusivity and specific heat capacity are vital in conducting analysis and modeling in various fields of geo-mechanics, agriculture, hydrology and others. Heat and water transfer problems in geo-mechanics are strongly coupled processes, thus producing transient temperature, moisture content, stress and thermal conductivity variations in unsaturated soil conditions. Understanding of this coupled phenomenon requires the accurate estimation of thermal conductivity of soil at different soil hydro-mechanical conditions. The same factors affecting soil thermal conductivity also play a predominant role in soil dielectric permittivity or conductivity, and its variations with hydro-mechanical changes (De Vries, 1963; Tarnawski, 2000; Archie, 1942). In this study, the variation of thermal conductivity and dielectric permittivity of three silty clay soils with applied effective stress under one dimensional loading was theoretically studied. Johansen’s (1975) model of soil thermal conductivity and the advanced Lichtenecker and Rother Model (ALRM) (Wagner et al., 2011) of soil dielectric permittivity were both modified using correlations of soil compressibility suggested by Nishida (1956), to analyze the thermal conductivity and dielectric permittivity of the soils at different hydro-mechanical conditions.