Buoyancy Thermocapillary Convection of Volatile Fluids in Confined and Sealed Geometries Chapter 1. Introduction Chapter 2. Mathematical model Chapter 3. Convection at atmospheric conditions Chapter 4. Convection under pure vapor Chapter 5. Convection at reduced pressures Chapter 6. Linear stability analysis Chapter 7. Conclusions and recommendations. This thesis represents the first systematic description of the two phase flow problem. Two phase flows of volatile fluids in confined geometries driven by an applied temperature gradient play an important role in a range of applications including thermal management such as heat pipes thermosyphons capillary pumped loops and other evaporative cooling devices. Previously this problem has been addressed using a piecemeal approach that relied heavily on correlations and unproven assumptions and the science and technology behind heat pipes have barely evolved in recent decades. The model introduced in this thesis however presents a comprehensive physically based description of both the liquid and the gas phase. The model has been implemented numerically and successfully validated against the available experimental data and the numerical results are used to determine the key physical processes that control the heat and mass flow and describe the flow stability. One of the key contributions of this thesis work is the description of the role of noncondensables such as air on transport. In particular it is shown that many of the assumptions used by current engineering models of evaporative cooling devices are based on experiments conducted at atmospheric pressures and these assumptions break down partially or completely when most of the noncondensables are removed requiring a new modeling approach presented in the thesis. Moreover Numerical solutions are used to motivate and justify a simplified analytical description of transport in both the liquid and the gas layer which can be used to describe flow stability and determine the critical Marangoni number and wavelength describing the onset of the convective pattern. As a result the results presented in the thesis should be of interest both to engineers working in heat transfer and researchers interested in fluid dynamics and pattern formation.