Applicability of Uniform Heat Flux Nusselt Number Correlations to Thermosyphon Heat Exchangers for Solar Water Heaters

[+] Author and Article Information
S. Dahl, J. Davidson

Mechanical Engineering Department, University of Minnesota, Minneapolis, MN

J. Sol. Energy Eng 121(2), 85-90 (May 01, 1999) (6 pages) doi:10.1115/1.2888151 History: Received September 01, 1998; Revised March 01, 1999; Online February 14, 2008


Nusselt numbers are measured in three counterflow tube-in-shell heat exchangers with flow rates and temperatures representative of thermosyphon operation in solar water heating systems. Mixed convection heat transfer correlations for these tube-in-shell heat exchangers were previously developed in Dahl and Davidson (1998) from data obtained in carefully controlled experiments with uniform heat flux at the tube walls. The data presented in this paper confirm that the uniform heat flux correlations apply under morerealistic conditions. Water flows in the shell and 50 percent ethylene glycol circulates in the tubes. Actual Nusselt numbers are within 15 percent of the values predicted for a constant heat flux boundary condition. The data reconfirm the importance of mixed convection in determining heat transfer rates. Under most operating conditions, natural convection heat transfer accounts for more than half of the total heat transfer rate.

Copyright © 1999 by The American Society of Mechanical Engineers
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