Using an Ersatz Thermosiphon Loop to Model Natural Convection Flows Inside a Shallow Enclosure: Preprint

Natural convection loops (NCL) can occur when extracting energy from thermal storage with immersed heat exchangers. To assist in heat exchanger design and annual performance simulations of such systems, this paper proposes modeling an NCL with a comparatively simple 'ersatz' thermosiphon loop (ETL). In an actual thermosiphon loop, fluid in channels or pipes flows in a closed loop, driven by a netbuoyancy head which is equal to the total pressure drop. In the proposed approach, ersatz flow channels corresponding to the actual NCL flow are first defined, based upon experiment, numerical solution, or other information. The heat transfer and friction coefficients in the simplified ETL model must then be adjusted to fit these known data. The test case analyzed here is a horizontal shallowenclosure with temperature boundary conditions at both ends. A numerical solution is used to calibrate the ETL, and an analytical solution is used to extrapolate to other conditions for testing the ETL model predictions. It is shown that over two orders of magnitude variation in heat transfer, the calibrated ETL model predicts the heat transfer to 8% RMSD.