NUMERICAL INVESTIGATION OF LAMINAR-FLOW AND HEAT-TRANSFER IN INTERNALLY FINNED TUBES

A numerical investigation of laminar mixed convection developing flow and heat transfer with variable viscosity in internally finned tubes w ith helical fins and constant wall temperature boundary condition was performed. The numerical model was validated by comparison with experi mental data and was able to predict experimentally measured friction f actors and Nusselt numbers with an accuracy of +/- 10% for a wide rang e of fin geometries and operating conditions. A parametric study was c onducted for fin geometry ranges of 8 less than or equal to N less tha n or equal to 54, 0.03 less than or equal to H less than or equal to 0 .1, and 0 less than or equal to gamma less than or equal to 45 degrees , where N is the number of fins, H is the non-dimensional fin height, and gamma is the fin helix angle. The ranges of operating condition we re 10(2) less than or equal to Re-in less than or equal to 10(3), 50 l ess than or equal to Pr-in less than or equal to 1,000, 0 less than or equal to Ra-in and -30 less than or equal to Delta T less than or equ al to 30 K, where Delta T is the wall-to-inlet temperature difference. The model provided local information that supplemented length-average d experimental results, particularly revealing that coring (higher vel ocities in the core of the tube and retarded velocities in the inter-f in region) is responsible for poor heat transfer performance of the tu bes with large number of fins. The magnitude of this coring was exagge rated as the number of fins was increased but was about the same for b oth heating and cooling situations. The length-averaged results indica ted that large heat transfer enhancement can be obtained for short non -dimensional lengths.