Abstract
Extensive numerical analysis was performed for the unsteady state, natural convection in the annular cylinders. The cylinder’s boundaries were thermally insulated, except the inner surface. The fluid (water) in the cylinder initially was assumed at a cold temperature while the inner surface was subjected to a high temperature. The time history for the heat transfer by diffusion and advection was studied. The time needed for fully charging the storage tank and rate of heat transfer was calculated. The predicted results were compared with the pure heat diffusion process and with a steady-state convection system. Therefore, CFD simulations were performed for natural convection in the storage tank. The main objective of this study was to establish correlations for the rate of heat transfer as a function of time and other controlling parameters. The correlation is needed in designing a thermal energy storage system for domestic and industrial heating processes. One of the drawbacks of the conventional thermal storage systems is the slow charging and discharging, where the heat transfer is mainly diffusion dominated. To overcome such a problem, a system was designed based on the natural convective heat transfer mechanism. Therefore, the heat transfer and fluid flow in a cylindrical storage tank were simulated for a range of Rayleigh numbers (104 to 108) and radius ratio. It was found that a convection-operated storage tank reduces the thermal charging process time drastically compared with the thermally diffusion charging process. The rate of reduction in the charging time mainly depends on the rate of heating and geometric parameter of the tank. To the best of the authors’ knowledge, the work is novel.
Highlights
Thermal storage systems are an essential part of domestic and commercial water heating systems, especially solar heating systems, for water heating and power generation
The steady and unsteady states of natural convection in enclosures have been extensively studied by Hussein et al [1]
Very limited data are available on the natural convection in annular geometries, unsteady state convection with thermally insulated boundaries
Summary
Thermal storage systems are an essential part of domestic and commercial water heating systems, especially solar heating systems, for water heating and power generation. Papanicolaou and Belessiotis [3] studied transient natural convection in a cylindrical enclosure filled with water at high Ra numbers (1010 to 1015 ) They considered and compared the prediction of different turbulent flow models. The results showed a significant effect of Ra number and aspect ratio on the flow and on the thermal fields, which are expected. The results indicated that the variation in the Ra number and height ratio have a major influence on the flow structure and isotherm patterns They noticed two flow regimes for lower Ra number values and the dominance of conduction heat transfer. The authors provided a continuous, local, and curvature dependent Nusselt number correlation for the laminar, transition, and turbulent regimes for natural convection from vertically heated cylinders.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
