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

An experimental investigation of laminar flow and hear transfer in int ernally finned tubes was performed. Length-averaged measurements of he at transfer and pressure drop for thermally developing flow were condu cted for both heating and cooling situations as well as for low and hi gh heat flux cases using ethylene glycol as the test fluid. The heat t ransfer tests were performed with fluid-to-fluid heating or cooling wh ich closely approximates constant wall temperature boundary conditions . Isothermal friction factors, diabatic friction factors, and Nusselt numbers were measured for fin geometry ranges of 8 less than or equal to N less than or equal to 54, 0.015 less than or equal to H less than or equal to 0.17, and 0 less than or equal to gamma less than or equa l to 45 degrees and operating condition ranges of 150 < Re < 2,000, 50 < Pr < 185, 0.3 < mu(b)/mu(w) < 3.6, and 3 x 10(5) < Ra < 8 x 10(6). The length-to-inside tube diameter ratios for the tubes tested were ar ound 120. The maximum heat transfer enhancement relative to a smooth t ube was obtained for tubes with fewer number of tall fins with strong free convection effects and was around 75% at the expense of 50% incre ase in pressure drop penalty over the smooth tube value. Overall, the micro-finned tubes and the tubes with fewer number of tall fins were f ound to be ineffective in laminar flow with small to moderate free con vection effects as little or no heat transfer enhancement was obtained at the expense of a fairly large pressure drop penalty. The results a lso indicated that the fin geometry has little effect on the heat tran sfer, particularly for micro-finned tubes. The effect of free convecti on on the pressure drop was marginal but its influence on the heat tra nsfer was found to be substantial.