An experimental study of thermal and thermohaline convection in saturated porous media

Abstract

Worldwide increased focus on environment, depletion and over exploitation of fossil fuel resources, and their high inflation rates demand to look for sustainable alternative sources of energy. Geothermal energy is a clean, environment friendly, economical, and sustainable natural source of energy. It is the energy available in the underground reservoirs of hot water known as hydrothermal systems. Moreover, thermal energy is available in general in permeable subsurface rock formations and is continuously produced inside the earth as a result of slow decay of radionuclides. This thesis is an experimental hydrogeological analysis of thermal and thermohaline convection in saturated porous media using a noninvasive temperature measuring technique called infrared thermography. Chapter 1 deals with the background of the topic and its relevance for the society. The structure of the earth, types of geothermal reservoirs and basic modes of heat transfer are addressed. Chapter 2 addresses the basic equations of heat transfer under thermal/salt gradient and fluid flow in homogeneous saturated porous media with macroscopic assumption. In Chapter 3, infrared thermography is applied to obtain surface temperature distributions with in-situ calibration for a better understanding of continuous point heat sources at two different flow rates and two different temperatures. Dimensionless experimental results are compared with dimensionless numerical simulations using the one temperature model equation with and without thermal dispersion effects. Inclusion of thermal dispersive effects in fluid conduction-convection model at variable density and viscosity yields a close approximation of the experimental results. In Chapter 4, a series of experiments is conducted in a rectangular tank filled with either saturated glass beads or coarse sand to investigate the effects of fluid density and viscosity variations induced by temperature. Modeling of these experiments yielded good insight and information about the interplay between fluid density, viscosity and velocity fields. Experiments in glass beads (PM1 and PM2) yielded a good approximation at transverse thermal dispersivity of 2mm and 1mm respectively while in natural porous media PM3 experimental results are approximated at 1mm dispersivity. Chapter 5 discusses 2D laboratory experimental studies of thermohaline convection using infrared thermographic analysis for temperature measurement and measuring EC to determine the dissolved salt concentration in water saturated porous medium. Three different concentrations are used. It aims at investigating the combined effects of thermal and mass induced buoyancy in coarse sand. Dimensionless thermal contours, vertical temperature profiles at known horizontal positions and dimensionless concentration profiles under the temperature influence are compared. These experimental cases are numerically analyzed using Oberbeck-Boussinesq approximation for heat and mass transfer. Chapter 6 deals with 2D laboratory experiments conducted in 3 porous media under stable miscible displacement. Results for stable heat scenarios are compared with the results of Chapter 4 for unstable miscible displacements cases. Stable thermal experiments give less extend of the thermal plume due to stabilizing density and viscosity effects when compared to unstable thermal convection experiments in the same porous media but heated from below. Chapter 6 also deals with two cases for natural convection when heated from below with no external flow.