THE TWO-PHASE COCURRENT DOWNFLOW OF LIQUID NITROGEN IN A VERTICAL RECTANGULAR CHANNEL

Pecherkin N.I., Pavlenko A.N., Chekhovich V.Yu., Zhukov V.E., Serov A.F., Nazarov A.D.

Institute of Thermophysics SB RAS, Russia, Novosibirsk

Houghton P.A., Sunder S.

Air Products and Chemicals, Inc, USA , Allentown, PA

Channels of a noncircular cross-section are used to cool the equipment in power engineering, microelectronics, nuclear technology, air and spacecrafts, and in many other applications. They cover a wide range of sizes from units and tens of micrometers to several millimeters. The smaller the size of channel, the more peculiarities and differences in regularities of heat transfer and pressure drop are observed in comparison with the flow in channels of an ordinary size. This is true both for the flow of single-phase liquids and the processes of boiling and condensation. Since a considerable surface tension arises in the points with a small curvature radius, the flow in rectangular channels may have some peculiarities in comparison with the flow in round tubes.

The liquid downflow in a rectangular channel with a co-current vapor flow or without it differs from the upward flow by the fact that at low flow rates, liquid flows as a film over the walls of the channel. If a distance between walls of the channel is compatible with a capillary constant, for the downward flow, liquid moves from the wide side of the channel to its corners.

The current work deals with a study of the liquid nitrogen downflow in a vertical rectangular channel of 2.6´7.1 mm². The flow was studied with a co-current vapor flow and without it. Visualization, photo- and video recording of the flow regimes at the walls and over its cross-section were made as well as the measurements of the film thickness at the narrow and wide sides of the channel.

The studies were carried out in an optical cryostat with four observation windows. The test section walls were made from organic glass. To observe the process and make pictures of the interface in the cross-section, there were glass windows in a top of the entrance chamber and in the bottom of the exit chamber of the test section. To measure the film thickness at the wide and narrow sides of the channel, two coaxial capascitive probes were installed. Experiments were carried out at the atmospheric pressure. A range of Reynolds number for the liquid phase was from 200 to 2000, and for vapor it was from 0 to 20000. Vapor velocities from 0 to 15 m/s corresponded to the above Reynolds numbers. Reynolds numbers were determined by superficial liquid and vapor velocities and equivalent hydraulic diameter.

For low liquid flow rates without vapor flow at a wide side of the channel, liquid flows as a film with almost smooth surface, where the waves of insignificant amplitude and frequency occur. At the narrow sides of the channel, film thickness is very small. A considerable part of liquid flows in the corners of the channel.

Profiles of the wavy surface and amplitude-wave spectrums show two separate flows at the narrow and wide sides of the channel with various wave characteristics. These flows have their characteristic wavelength and average thickness of the film. Therefore, local liquid Reynolds numbers, calculated through the reduced liquid flow rate may be quite different for the narrow and wide sides of the channel.

The average film thickness at the wide side gradually increases with an increase of Reynolds number. For high Reynolds numbers, the average film thickness at the narrow side is 2-3 times higher than that at the wide side. This increase is caused by the process of liquid accumulation in the corners of the channel under the action of surface tension. According to the comparison of the film thickness at the wide side with calculation by Nusselt formula for a smooth plate, the real film thickness decreases almost twice in contrast to the calculated value due to the liquid flow to the narrow side.

The significant vapor effect on the flow structure in the channel begins at the flow rates, corresponding to Reynolds numbers of above 10000. At the narrow side the wave amplitude is high even for the low vapor flow rates. With an increase of the vapor flow rate, separate waves of high amplitude appear at the narrow side. For high vapor velocities waves at the wide and narrow sides interact with each other, roll waves of high amplitude appear, and sometimes they can fill the whole cross-section of the channel. The transverse wave dimension decreases along the wide side and their frequency increases. Waves with higher amplitude promote liquid flow from one side to another and equalize the average film thickness over the perimeter. The film thickness is smoothed out over the perimeter only under the action of the high-speed vapor flow during transition to the dispersed-annular flow.