PERFORMANCE AND MODELING OF THERMOSIPHON HEAT-EXCHANGERS FOR SOLAR WATER-HEATERS

Determining the performance of indirect solar heating systems that use thermosyphon heat exchangers requires knowledge of how thermosyphon p ow rate and heat exchanger performance vary with. operating conditions . In this paper measured performance of a two-pass, tube-in-shell, dou ble-wall heat exchanger is discussed in terms of modeling issues. Ther mosyphon heat exchangers may operate in the developing, mixed convecti on regime where natural convection effects can significantly influence overall heat transfer and friction coefficients. Existing,models whic h assume the thermal and hydraulic behaviors of thermosyphon heal exch angers are only functions of the thermosyphon and collector flow rates may not be suitable for all heat exchanger types. For example, the ov erall heat-transfer coefficient-area product for the two-pass, tube-in -shell hear exchanger is best expressed as a function of Reynolds, Gra shof, and Prandtl numbers on the thermosyphon side of the hear exchang er It is proposed that annual simulations of solar water heaters with thermosyphon heat exchangers use this type of relationship to characte rize heat transfer in the heat exchanger.