Capillary pressure influence on open channels pressure drop

Abstract

The evaluation of the liquid pressure drop in the heat pipe arteries can be improved if the flow behavior inside these channels of very small sizes is better studied. In this paper experimental tests performed with a small square section open channel (side of 4 mm) aiming to visualize and analyze the liquid flow are presented. The channel size is not as small as the usual size used for heat pipe and the characteristic length (width of the groove) is then larger than the water capillary length (about 2.7 mm), thus gravity force being predominant on capillary force. For this reason, although present results will not add specific and newer details about the liquid flow in the arteries (microchannels) of the heat pipes, they in any case extend the already available experimental data concerning the pressure drop evaluation in small open ducts. Actually, the aim of the present series of tests is to easily acquire familiarity and to better observe the phenomena that similarly exist in channels of much smaller size. An aspect due to the capillary pressure generated at the gas–liquid interface (free surface of the open channels) is its influence on the measurement of the liquid pressure drop due to viscous effects. The apparatus and the visualization devices used in these tests allowed to take pictures (images) of the gas–liquid interface (meniscus) at several positions along the channel. Images have been processed using appropriate software called “ImageJ” and the interfacial meniscus radius and liquid height along the channel have been measured. The values of these two parameters allowed the estimation of the liquid viscous pressure drop and the friction factor.