Theoretical and experimental studies of heat transfer characteristics of a single-phase natural circulation mini-loop with end heat exchangers

In the present work, the heat transfer and fluid flow characteristics of a single-phase natural circulation loop (SPNCL) with both the heating and the cooling ends are theoretically and experimentally studied. Distilled water is chosen as the working fluid circulating in a rectangular loop that the top and bottom sides are respectively cooled and heated by 245 mm tube-in-tube heat exchangers. The height and diameter of the mini-loop are 250 mm and 4 mm, respectively. Both analytical and experimental results are obtained by varying the heating fluid temperatures from 30 °C to 60 °C but fixing cooling fluid temperature of 10 °C. Based on the experimental data, a Nu ∼ Re correlation is obtained and applied into the one-dimensional mathematical model. A good agreement between the experimental and theoretical results with a new correlation can be observed. Experimental results show that stable flow can be reached for the cases with different Th. The start-up time of natural circulation from quiescent state shortens with the increase of Th. The Reynolds number and heat transfer rate at steady state are proportional to the heating fluid temperature Th. Based on the proposed mathematical model, an optimal ratio of the heater length to the loop height can be reached when the total length and diameter of the mini-loop keep constants.