The two-phase downflow of liquid nitrogen

in a vertical rectangular channel

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

Thermophysics Institute of RAS SB

1, Academician Lavrentyev prosp., Novosibirsk, 630090, Russia

pavl@itp.nsc.ruљљљљљљљљљ pecherkin@itp.nsc.ru

S.Sunder, P.Houghton

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

Experimental results for the liquid nitrogen film flow under gravity in a vertical rectangular channel with a narrow gap are presented. Reynolds number for liquid was changed from 200 to 2000. Reynolds number for vapor was changed from 0 to 20000. The flow pattern was observed in an optical cryostat. Pictures and video records were made. A film thickness was measured on wide and narrow sides of the channel by means of the capacity method. The amplitude, shape, and frequency of waves as well as the average film thickness were determined. Observations and measurements of the film thickness demonstrated that the film thickness on the wide side decreased in comparison with Nusselt theory because some amount of liquid overflowed to the corners of the channel and the narrow sides. The interface was covered by a complex system of waves whose structure depends on liquid and vapor flow rates. For low vapor velocities (less than 10 m/s), the effect of vapor resulted in slight alterations of velocity, shape, and frequency of the waves propagation. According to visual observation a qualitative change occurred at Re>15000-17000. The film thickness became uniform over the perimeter, waves transformed into fine ripples, and liquid drops appeared in the center of the channel, i.e., we could observe the transition to the dispersed-annular flow like for the two-phase flow in the round tube for high void fraction. The use of spectral analysis allows us to determine the main wave characteristics on the interface, and their connection with flow regime.The data on a thickness of the liquid nitrogen film, its distribution over the perimeter of a channel, and wavy characteristics of the film are original. They are important for the modeling of heat transfer processes in cryogenic systems and installations.

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. Until there will be no a common classification of microchannel sizes in literature, let's use the terms from some known papers. According to this, microchannels with sizes from 100 mm to 1 mm are called mesochannles, the channels of 1-6 mm are compact heat exchangers, and channels of > 6 mm are considered as the conventional heat exchangers. 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. Thus, an increase in heat transfer coefficients was observed in some papers at condensation of R-12 in a rectangular channel with microfins due to the surface tension drainage force. 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.

By the present, several papers on the flow regimes and pressure losses of the two-phase flows in rectangular channels are published. The regime maps are plotted for the upward flow like it was done for the 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. It was shown before that the direction of a gas flow above a water film falling down the inner wall of a round tube significantly changes the wave profile, effects amplitude and band of frequencies, which make waves unstable. At a co-current flow, the minimal film thickness decreases, and oscillations with higher frequencies appear on the wave surface. Results on the effect of liquid and gas flow rate characteristics in narrow vertical slots on the regimes of two-phase flow and wave characteristics are presented also. Results of statistic studies on the wave characteristics at a film flow of various liquids over vertical smooth surfaces are presented before. To measure the local wave characteristics at a film flow of water, the method of electric conductivity was used. For the first time, wave characteristics at a film flow of liquid nitrogen were obtained in some papers. The capacitance method was used for measurements of the local thickness of a liquid nitrogen film.

The current work deals with a study of the liquid nitrogen downflow in a vertical rectangular channel of 2.6´7.1 mm². Compact heat exchangers of such dimensions are commonly used in cryogenic and low temperature industrial applications, including air separation, processing of liquefied natural gas, manufacture of hydrogen and helium, etc. 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.

Experimental data on characteristics of the wavy film flow of liquid nitrogen was obtained for a co-current vapor flow in a narrow rectangular channel. Distribution profiles for the thickness of liquid nitrogen film over the channel perimeter were obtained at various regime parameters using photography. It is shown that at low vapor velocities, the values of liquid film thickness on the wide and narrow sides of the channel differs significantly with a rise in the irrigation degree at the channel inlet. With a growth of liquid Reynolds number, the average film thickness on the narrow side can be three times as high as the average liquid thickness on the wide side. With an increase of the film thickness, large waves on the narrow side of the channel stimulate development of high-amplitude oscillations of instantaneous film thickness on the wide side of rectangular channel. With a rise of vapor flow rate at the co-current flow, characteristics of the amplitude-wave spectrum of the liquid film thickness on the wide and narrow sides of the channel become closer. Simultaneously, leveling of the time-averaged thickness of liquid nitrogen film occurs over the perimeter of rectangular channel.