Effect of Minichannel Cross Sectional Shape in Condensation

The objective of this work is to present new condensation heat transfer coefficients measured inside a single square cross section minichannel, having a 1.18 m side length, and compare them to the ones measured in a circular minichannel. This subject is particularly interesting since most of the mini and microchannels used in practical applications have non circular cross sections. The test section used in the present work is obtained from a thick wall copper tube which is machined to draw a complex passage for the coolant. This experimental technique allows to measure directly the temperature in the tube wall and in the water channel. The heat flux is determined from the temperature profile of the coolant in the measuring sector. The experimental technique is checked in single phase runs which represent a severe testing condition due to the low values of heat flux. Tests have been performed with R134a at 40°C saturation temperature, at mass velocities ranging between 200 and 800 kg m⁻²s⁻¹. As compared to the heat transfer coefficients measured in a circular minichannel, in the square minichannel the authors find that heat transfer is enhanced at the lowest values of mass velocity; this must be due to the effect of the surface tension. No heat transfer coefficient increase has been found at the highest values of the mass velocity where condensation seems to be shear stress dominated.