Heat transfer characteristics of water flow through a microtube

Convection heat transfer of vertically downward and horizontal flows was experimentally studied using a microtube, 0.501 and 1.00 mm in inner and outer diameters, respectively, and 398 mm in length. The test section was built in a vacuum chamber maintained approximately at the absolute pressure of 1 kPa. Tests used water as the working fluid with constant heat flux boundary conditions. Reynolds numbers at the tube inlet were between 600 and 2000, with heat fluxes varying from 3.5 to 87 kW/m2. The wall temperatures were measured employing 17 T-type thermocouples, each embedded in a small solder cast attached to the microtube. Two extra thermocouples measured the environment temperature inside the vacuum chamber. At low heat flux, the Nusselt number converged to a range of 4.36 and 4.36 + 8%. However, as the heat flux increased to greater than 26 kW/m2, the Nusselt number started deviating far above 4.36 due to the effect of mixed convection. No difference between vertical and horizontal configurations was observed in heat transfer rate at Reynolds numbers less than 1800 at the tube inlet for all heat fluxes. However, when the Reynolds number was larger than 2000 at the tube inlet, the Nusselt number of vertically downward flow was greater than that of horizontal flow. Furthermore, at the high Reynolds number, the heat transfer rate of the vertical flow monotonously increased toward the tube inlet while the horizontal flow appeared thermally fully developed.