An investigation into the thermo-fluid behavior of volumetrically heated cavities irradiated from the side

Abstract

Radiation induced natural convection has been extensively researched due to its rife presence in numerous natural flows. Recently, it has seen renewed and resurged interests in context of understanding the photo-thermal energy conversion and its redistribution in direct absorption solar collectors (DASCs). The present work serves to further this objective; in particular, convection induced by direct interaction of the fluid and radiation (irradiated from side) has been critically and comprehensively investigated. In addition to DASCs, the present study has significant applications in various other fields such as nuclear power plants, building design, and laser fusion. A detailed mechanistic theoretical modeling framework has been developed to quantify the impact of various fluid properties and incident flux parameters; viz., Rayleigh number (102 to 108 in the laminar regime), Prandtl number (0.01–1000), optical thickness (0.1–100), convective heat loss from the transmitting wall (controlled in the simulation by varying the modified Biot number, Bi* from 0 to 5), aspect ratio (0.1–10). Furthermore, in addition to magnitude of the incident flux, its distribution (i.e., whether uniform or linearly varying) has also been dealt with. Another distinguishing aspect of the study is the use of the “heatlines” to visualize the flow of energy as it occurs during convection caused due to volumetric heating. Detailed analysis reveals that, while a conduction dominated regime is apparent at Ra ≤ 102, a transition regime occurs at 103 ≤ Ra ≤ 104, and convection is the domineering mechanism when Ra > 104. At higher Rayleigh numbers, the pattern of heatlines begins to resemble that of the streamlines. Nature of the flux distribution at the side wall (along the depth direction) significantly impacts the temperature distribution. Furthermore, there exits an optimum value of optical thickness (for a given Rayleigh number), which ensures minimum heat loss and maximum useful energy gain. Overall, through the present work, we have been able to decipher key design and operational parameters (and quantify their impacts) which dictate the momentum and heat transport in side-irradiated volumetrically heated cavities.