Transient natural convection in a differentially heated cavity with and without a fin on the sidewall

Transient natural convection in a differentially heated cavity is a typical model of many flow processes in the ocean and atmosphere, and has extensive applications in industrial systems for which it is of significance to enhance or depress the heat transfer through the cavity. Therefore, transient natural convection with and without a fin on the sidewall of a differentially heated cavity is experimentally and numerically investigated in this thesis. One of the main objectives of this study is to investigate the effectiveness of the fin for the purpose of enhancing the heat transfer through the heated sidewall. The shadowgraph technique is employed to visualize transient natural convection in a rectangular cavity, and thermistors are used to measure the time series of the temperatures at different locations adjacent to the hot sidewall. The numerical simulations in this thesis focus on the above experimental model. The SIMPLE scheme based on the finite volume method is adopted to solve the two dimensional governing equations using the commercial FLUENT software package. The present flow visualizations demonstrate that the transition of the natural convection flow in the cavity from the start-up may be classified into three stages: an initial stage, a transitional stage and a quasi-steady stage. The initial stage includes the growth of the thermal boundary layer and the Leading Edge Effect. The correlation of the thickness of the thermal boundary layer with time obtained from the present experiments is consistent with that of a previous scaling analysis. In the transitional stage, the separation and trailing waves of the horizontal intrusion are observed and eventually a double-layer structure of the thermal boundary layer is visualized in shadowgraph images. The traveling waves of the inner layer in shadowgraph images become clear in the quasi-steady stage, and temperature measurements indicate that this is because the amplitude of the traveling waves in the thermal boundary layer increases as the stratification of the fluid in the cavity is enforced. The corresponding numerical simulations validated by the experiments show that the bright strips of the double-layer structure in shadowgraph images correspond to the extrema of the second derivative of the temperature. In fact, the stratification of the fluid in the cavity results in a temperature distribution adjacent to the sidewall with a maximum and a minimum of the second derivative of the temperature, which in turn leads to an opposing thermal diffusion directing to the temperature minimum. The flow visualizations on transient natural convection with a fin on the sidewall of the cavity show that the transition from the start-up is likewise classified into three stages with variations from those without the fin, which also depend on the dimension of the fin. Two fins of different geometric parameters, one small square fin and the other large thin fin, are considered in the experiments. In the initial stage, a lower intrusion front appears underneath the fin and later bypasses the fin. The lower intrusion front bypassing the small square fin reattaches to the downstream thermal boundary layer, but the one bypassing the large thin fin moves upwards and strikes the intrusion under the ceiling. Although there are some variations of the transition of the thermal boundary layer flows between the cases with and without a fin, the double-layer structure of the thermal boundary layer appears in all cases in the quasi-steady stage. In particular, the presence of the large thin fin may cause the separation and vortex shedding of the thermal flow around the fin, which trigger the instability of the downstream thermal boundary layer. The numerical simulations of transient natural convection with a fin on the sidewall of the cavity are consistent with the above-mentioned experimental results. The numerical results confirm that the transient strong convection flows such as the lower intrusion front and the oscillations of the thermal flow around the fin may enhance the heat transfer through the sidewall. Furthermore, the numerical results demonstrate that the enhancement of the heat transfer is dependent on the parameters of the fin, such as the dimension and position of the fin and the number of fins, with a maximum enhancement of 33% observed in the present simulations. In summary, both numerical and experimental results show that the fin may significantly change the transient natural convection flows and improve the heat transfer through the cavity if properly configured. This research has great potential for industrial applications in which the enhancement of heat transfer is desirable.