Heat transfer characteristics of water flowing through micro-tube heat exchanger with variable fluid properties

The heat transfer characteristics of laminar single-phase forced convective water flow through a micro-tube heat exchanger are numerically investigated in this paper. Two-dimensional simulations are performed to find the effects of variable fluid properties on heat transfer for hydrodynamically and thermally developed flow. The effects of variable fluid properties on convective heat transfer coefficient (h) and Nusselt number (Nu) are significant for micro-convective flow. It is noted that the variation in temperature-dependent thermal conductivity [k(T)] greatly enhances the h as compared to the variation in temperature-dependent viscosity [mu(T)], although water viscosity-temperature sensitivity (S mu T) is greater than that of thermal conductivity-temperature sensitivity (SkT). The effects of variation in wall heat flux (q '' w) and inlet temperature on heat transfer are investigated for variable fluid properties. It is noted that the Nu declines with an augment in q '' w for temperature-dependent density variation [.(T)]. The Nu increases with an increase in q '' w for mu(T) and k(T) variations. The results show that the Nu decreases with an increase in inlet temperature for variable fluid properties. The undevelopment and redevelopment of the flow are observed due to mu(T) variation. Additionally, the effects of wall heat flux, inlet temperature and inlet velocity on the variation of Nu/Pr1/3 with Re are examined for mu(T) variation