Selçuk Erol

Experimental and Analytical Evaluations of Freezing Effects in Borehole Heat Exchangers

The paper addresses the evaluation of the integrity and the durability of borehole heat exchangers when submitted to freezing temperature. Three grouting materials are investigated experimentally and the obtained results demonstrate that the freezing impact strongly depends on the thermal, hydraulic and mechanical properties of the grout. Radial cracks are obtained due to the induced ice pressure that generates tensile effective stress in the grout. The prediction obtained from a thermo-mechanical analytical solution correlates very well with the experimental observations. It is demonstrated that the cracks are induced by an excess of tensile tangential stress around the pipe.

An Analytical Model of Porosity–Permeability for Porous and Fractured Media

The classic Kozeny–Carman equation (KC) uses parameters that are empirically based or not readily measureable for predicting the permeability of unfractured porous media. Numerous published KC modifications share this disadvantage, which potentially limits the range of conditions under which the equations are applicable. It is not straightforward to formulate non-empirical general approaches due to the challenges of representing complex pore and fracture networks. Fractal-based expressions are increasingly popular in this regard, but have not yet been applied accurately and without empirical constants to estimating rock permeability. This study introduces a general non-empirical analytical KC-type expression for predicting matrix and fracture permeability during single-phase flow. It uses fractal methods to characterize geometric factors such as pore connectivity, non-uniform grain or crystal size distribution, pore arrangement, and fracture distribution in relation to pore distribution. Advances include (i) modification of the fractal approach used by Yu and coworkers for industrial applications to formulate KC-type expressions that are consistent with pore size observations on rocks. (ii) Consideration of cross-flow between pores that adhere to a fractal size distribution. (iii) Extension of the classic KC equation to fractured media absent empirical constants, a particular contribution of the study. Predictions based on the novel expression correspond well to measured matrix and fracture permeability data from natural sandstone and carbonate rocks, although the currently available dataset for fractures is sparse. The correspondence between model calculation results and matrix data is better than for existing models.