Fluid Mechanics, Heat Transfer and Thermodynamic Issues of Micropipe Flows

Abstract

Once the notion is aspired to examine the momentum and heat transfer characteristics of fluid flow in detail, the concept of energy mechanism is inevitably handled through both 1st and 2nd laws. Since the fundamental engineering phenomenon of internal flow is widely encountered in industrial installations, which may range from operations with nonNewtonian fluids (Yilbas & Pakdemirli, 2005) to heat exchangers (Stewart et al., 2005) and from geothermal district heating systems (Ozgener et al., 2007) even to micropipe systems (Kandlikar et al., 2003), the general scientific and technological frame of thermo-fluid operations has been in the consideration of several researchers. From methodological perspective, at macro level (d≥3 mm), the explicit analytical correlations are capable of characterizing the flow and heat transfer issues of internal laminar flows. However, when the pipe diameter coincides with the micro range (d≤1 mm) the order the pipe diameter and the level of surface roughness result in augmented entropy generation rates, besides give rise to substantial shifts in the velocity and temperature profiles from those of the characteristic recognitions, which as a consequence highlights the necessity in the identification of the so developed energy behaviors and the involved influential parameters related with the design, construction and operation of the object appliance. Through experimental and computational investigations, involved researchers considered both the fluid flow and heat transfer mechanisms of micropipe flows. Kandlikar et al. (2003), for single-phase flow with small hydraulic diameters, studied the effects of surface roughness on pressure drop and heat transfer and concluded that transition to turbulent flows occurs at Reynolds number values much below 2300. Laminar and transitional flows in dimpled tubes were experimentally investigated by Vicente et al. (2002); the onset of transition at a relatively low Reynolds number of 1400 with 10% higher roughness induced friction factors when compared to the smooth tube ones were their primary findings. Engin et al. (2004) reported significant departures in the flow characteristics, from the conventional laminar flow theory, due to wall roughness effects in micropipe flows. The grow of friction coefficient with higher Reynolds number and lower hydraulic diameter were the theoretical and experimental evaluations of Renaud et al. (2008) in trapezoidal micro-channels. Guo & Li (2003) studied the mechanism of surface roughness provoked surface friction and concluded that the early transition from laminar to turbulent flow arose due to the frictional activity. Smooth micro-tubes under adiabatic conditions were experimentally investigated by Parlak et al. (2011); they determined that, as long as the viscous heating effects are taken