Study of natural convection instabilities in closed cavities with non-isothermal walls

Thesis (Ph.D.)--University of Washington, 2020Natural convection inside closed cavities has been investigated extensively over the years owing to its application in several industrial processes. Amongst these, the most commonly studied configuration is that of natural convection in air-filled cavities with isothermal vertical walls maintained at different temperatures, which promote a unicellular base air-flow pattern. In the present work, natural convection of air in annular and rectangular cavities with non-isothermal walls supporting a bi-cellular base air-flow pattern is explored both experimentally and numerically. The experiments and numerical studies described in this work represent a significant departure from the more familiar problem of stability of unicellular convection in closed cavities and are therefore a novel addition to the body of literature available for natural convection systems. Two experimental systems, one annular and one rectangular were constructed to study the stability of natural convection in tall, closed air-filled cavities with non-isothermal walls. The thermal boundary conditions applied on the cavities' vertical walls were such that the temperature of one vertical wall was higher than that of the opposing wall in the top portion of the cavity and lower than it in the bottom portion of the cavity. The Rayleigh number or buoyant potential of the bi-cellular (base) air-flow was successively increased/decreased over the course of several experiments, and the stability of flow inside these cavities was studied via monitoring of air temperatures. At low Rayleigh numbers, a steady flow regime was observed for both annular and rectangular cavities. With an increase in Rayleigh number transitions to 3D asymmetric periodic flows, quasi-periodic flows, intermittent flows, and chaotic flows were observed for the annular cavity. On the other hand, transitions from steady to 2D periodic flow, then to 3D periodic flow and finally to chaotic flow were observed for the rectangular cavity. The observed flow bifurcations, the resultant oscillation frequencies, phases, and amplitudes mapped relative to the wall thermal boundary conditions for both annular and rectangular experimental systems are presented in this work. 2D and 3D CFD models of the two experimental systems were developed using the commercial CFD code Simcenter StarCCM+ in order to numerically predict the experimentally observed unsteady natural convection flows. The experimentally imposed thermal boundary conditions were used as inputs for the CFD models. The accuracy of these CFD models in predicting experimental flows is discussed, and comparisons are made between the prediction capabilities of the 2D and 3D CFD models. Furthermore, the numerical simulations and frequency analysis of the experimental data are able to shed light on the primary instability modes causing the transitions from steady to unsteady flows. These instability mechanisms responsible for bifurcations observed in the annular and rectangular cavity are discussed in detail. This work also includes a benchmarking exercise for the commercial CFD code Simcenter StarCCM+ for problems of natural convection in cavities under both Boussinesq and non-Boussinesq temperature conditions. In addition, numerical simulations to understand the impact of non-Boussinesq temperature conditions and variable thermophysical fluid properties on the natural convection instabilities observed in closed, tall, air-filled rectangular and annular cavities with isothermal walls are performed