ABSTRACT
An uncoupled thermoviscoelastic finite element formulation with an application to Underground Coal Conversion (UCC) is presented. An incremental solution procedure with an implicit time stepping scheme is used under the thermorheologically simple material (TSM) postulate. The presented formulation, for a homogeneous and isotropic medium, is employed in the simulation of the UCC post-burn phase. Temperature, displacement, stress, and failure responses under thermomechanical loading are predicted. The solutions for the selected field problem indicate potential roof collapse, stress relaxation, and modest surface subsidence with a progressive thermal and failure front. The corresponding thermoelastic responses with temperature- dependent elastic properties are also presented and compared with the thermoviscoelastic results.
INTRODUCTION
Underground Coal Conversion (UCC) is a viable avenue for extracting clean energy from coal reserves. The principal challenges associated with UCC deal with a reliable, controlled and energy efficient process with minimum environmental disruption. A review of UCC technology can be found in a recent paper by Davis and Jennings [1]. Representative UCC research and development activities are based on field and laboratory experiments coupled with predictive model investigations. In the structural model simulations, a major portion of the effort is directed toward the analysis of thermal, displacement, stress and failure responses. Structural finite element modeling activities for various UCC cavity configurations have been conducted by Advani et al [2,5], Thompson et al [4], Langland and Trent [5], Sutherland et al [6], and Turner et al [7]. Jegbefume and Thompson [8] have conducted a study on roof collapse and surface subsidence using an isothermal linear viscoelastic model. Recently, Advani et al [9] have investigated finite element models for moving boundary evaluation, thermoelastic and thermo-plastic consolidation,and thermoviscoelastic response characteristics. In this paper, finite element model formulations for thermoelastic and thermoviscoelastic UCC post-burn models are presented and a related field problem is analyzed. The thermorheologically linear concept is adopted for the viscoelastic models and the thermally-induced stress and failure responses are determined. The computed results represented with an emphasis on the progressive thermal front, stress mediated UCC chamber configuration, and surface subsidence evaluations.
GOVERNING EQUATIONS AND SOLUTION METHODOLOGY
The presented finite element model formulations are based on the uncoupled theory for the temperature and thermoelastic or viscoelastic responses. Temperature-dependent physical and mechanical properties are incorporated in the models.
