Contents I. INTRODUCTION
Single-phase liquid flow in internally enhanced tubes is becoming more important in commercial HVAC applications, where enhanced tube bundles are used in flooded evaporators and shell-side condensers to increase heat transfer. This enables water chillers to reach high efficiency, which helps mitigate global warming concerns of HVAC systems. One kind of internally enhanced tube is the micro-fin tube. Jensen and Vlakancic [1] defined the micro-fin tube to have a height less than 0.03Di (i.e. 2e/Di<0.06), where Di, is the inside diameter and e is the fin height. Basically, such kind of tube is widely used in high flow rate applications because the heat transfer enhancement in high flow rates (turbulent region) is more pronounced than that in the low flow rates (laminar region). Khanpara et al. [2] for turbulent heat transfer in micro-fin tubes reported an increase of 30 to 100% with Reynolds numbers between 5,000 and 11,000. Brognaux et al. [3] indicated that there was a 65 to 95% increase in heat transfer for the micro-fin tube over the smooth tube. However, there is also a 35 to 80% increase in the isothermal pressure drop. The work of [1] indicated that the micro-fins increased heat transfer ranging from 20 to 220% in the turbulent flow region. However, there was a penalty due to friction factor increase ranging from 40 to 140%. Webb et al. [4] calculated the “efficiency index”, defined as the ratio of the heat transfer and the friction factor of enhanced tube to those variables for the plain tube, to vary from 0.98 to 1.18 for the seven different micro-fin tubes with Reynolds numbers between 20,000 and 80,000. For the laminar flow, several researchers [[3], [5]–[7]] concluded that the heat transfer and pressure drop were not greatly affected by micro-fins. Trupp and Haine [8] indicated that the secondary flow inside the tube with longitudinal fins was insignificant in the laminar flow and the thermal entry length was shown not to be relevant to the fin zeometrv.
