Experimental investigation of internally-finned tube geometries on turbulent heat-transfer and fluid flow

Abstract

Experimental investigations of variable property forced convection flow in smooth and internally-finned tubes were conducted. Length-averaged measurements of heat transfer and pressure drop for fully developed turbulent flow using water as the test fluid were obtained in fifteen pairs of finned tubes. Isothermal friction factors, diabatic friction factors, and Nusselt numbers were measured for fin geometries of 8 ≤ N ≤ 54, 0.015 ≤ H ≤ 0.017, and 0 ≤ ᵞ ≤ 45 degrees. For turbulent flow the operating conditions ranged from 7,000 < Re < 70,000, 3.5 < Pr < 5.0, and 0.62 < μb/μw < 1.35. In addition, isothermal friction factors were obtained for the range of 2,000 < Re < 12,000 using ethylene glycol as the test fluid.

For turbulent flow, the finned tube results exhibited substantial increases in both heat transfer and friction factor compared to the smooth tube. There is a general agreement between the friction factor and the Nusselt number data for both heating and cooling situations. Based on an analysis of the data, the finned tube data indicated that the heat transfer is enhanced by 20% to around 220% depending on the fin geometry. The corresponding increase in the pressure drop ranged from 40% to around 170% for these tubes. Furthermore, it appears that the smooth tube variable property correction factors may not be applicable to the finned tubes. The measured data were compared to the works of others, including Carnavos (1980) and Chiou et al. (1995). Improved generalized correlations for Nusselt numbers and friction factors for turbulent flow in internally finned tubes are presented. These correlations are based on the characteristic length scale for longitudinally finned tubes developed by Edwards and Jensen (1994).