Tatsuhiro Ueda

Heat transfer to falling liquid films and film breakdown—I: Subcooled liquid films

Abstract Experiments are described on heat-transfer coefficient and film breakdown heat flux for subcooled water films flowing downwards on outside of a vertical tube uniformly heated. A distortion parameter of liquid films, which is a ratio of an allowable difference in surface tension to the dynamic pressure of uniform film flow, is derived for each laminar and turbulent film flow. Experimental data show that when the parameter attains a constant value by increasing heat flux, the film breakdown takes place in a thin region of the distorted film. A simple analysis is also presented for evaluating film distortion.

Critical heat flux and droplet entrainment rate in boiling of falling liquid films

Abstract Measurements of critical heat flux and rate of droplet entrainment due to bubble bursting are made with boiling liquid films flowing downwards on the outside surface of a uniformly heated vertical tube. The critical condition occurs first at the exit end of the heating section. The critical heat flux shows three types of characteristics by increasing the liquid film flow rate at the exit end of the heating section. The data are expressed in terms of the film flow rate and the film Weber number at the exit end. The relationship between the droplet entrainment rate and the evaporation rate of the film is also discussed.

Behaviour of liquid films and flooding in counter-current two-phase flow—Part 1. Flow in circular tubes

Abstract Experimental results are presented on the flooding gas velocity in tubes over a wide range of parameters—tube diameter, tube length, liquid flow rate, liquid viscosity and surface tension. The flooding phenomenon is caused by interaction between the waves on the liquid film and the upward gas stream. By measuring variation of the maximum height of the wavy liquid films with an increase of the gas flow rate, the complicated effects of tube length and surface tension on flooding are revealed. The data of the flooding velocity are empirically correlated in termes of nondimensional groups for each tube length.

Measurements of velocity, temperature and velocity fluctuation distributions in falling liquid films

Abstract The velocity, temperature and velocity fluctuation distributions within falling spindle oil films in an inclined rectangular channel were measured using hot-wire techniques and thin thermocouples. The interfacial shear was caused by cocurrent air flow. The results indicate that the liquid films are as a whole much more laminar-like than turbulent in a range of Reynolds numbers (4γ/μ) up to the experimental limit of 6000. Mixing motion occurs in the vicinity of the interface; however, the flow near the wall surface exhibits no sign of such eddy motions, as predicted by the wall law for single phase turbulent flow. Although velocity fluctuation is observed within films with interfacial shear, mean velocity profiles are approximately the same as those obtained by the laminar film prediction.

An investigation of critical heat flux and surface rewet in flow boiling systems

Abstract A high thermal capacity copper test section placed at the end of a heating tube is used for investigating the critical heat flux and heat transfer characteristics during rewetting process for Freon-113 upward flow at mass velocities of 357, 627, 1035 and 1465 kg m−2 s−1. The critical heat flux coincides well with the maximum heat flux obtained by the transient cooling test. The wall superheat at the onset of rewetting changes greatly with the inlet quality, and ranges from 120 to 140°C in the subcooled and low quality region and from 60 to 80°C in the high quality region.

Critical heat flux and exit film flow rate in a flow boiling system

Abstract Critical heat flux and liquid film flow rate at exit end of the test tube are measured with Freon 113 upward flow in a uniformly heated tube. The exit film flow rate at the critical heat flux condition is near to zero in all cases of the exit qualities higher than 50%, however, the exit film flow rate tends to increase as the heat flux is increased and the exit quality decreases less than 50%. A correlation is proposed for the critical condition with high heat fluxes, suggesting that the liquid film separation from the heated surface by vapor generation takes an important part of the critical phenomenon.

Behaviour of liquid films and flooding in counter-current two-phase flow. Part 2. Flow in annuli and rod bundles

Abstract Experiments are described on the gas velocity at the onset of flooding and the maximum height of the wavy liquid film flowing downwards on a rod surface. On the basis of a simple analysis for a large amplitude wave on the liquid film, a flooding condition relating the maximum wave height to the gas velocity at the onset of flooding is derived. The values predicted by this condition show a good agreement with the measured results. An equivalent diameter of the channel is defined for the flooding velocity. Applying this diameter, the present data for annuli and rod bundles are well correlated by the same empirical equation as that for flow in circular tubes presented previously.

Initiation conditions of liquid ascent of the countercurrent two-phase flow in vertical pipes (in the presence of two-phase mixture in the lower portion)

Abstract Experiments on the countercurrent two-phase flow of air and water were conducted using vertical pipes of 10–26 mm in diameter to investigate the initiation conditions of liquid ascent. When a liquid film flowed down to a bubbling two-phase mixture in the lower portion of the pipe, liquid ascent began at much lower gas velocities than usual flooding velocities without the bubbling two-phase mixture. In most cases, liquid ascent occurred in a slug flow state. The initiation conditions of the liquid ascent were analyzed physically by considering the level swell of the two-phase mixture fluctuating around the mean height.

On the performance limit of closed two-phase thermosyphons.

This paper discusses an experiment conducted to investigate the performance limit of closed two-phase thermosiphons, together with visual observations of the flow state in the adiabatic section. The working fluids were R-113, methanol, and water. The flow state at the performance limit conditions was a violently disturbed slug type, in which the vapor plugs held up the liquid slugs periodically to a high level, causing a local circulation of liquid in the adiabatic section. This phenomenon is somewhat different from the flooding observed in open systems. An equation correlating the vapor velocity at the performance limit to the rising velocity of vapor plugs in stagnant liquid columns is proposed. This correlation compares satisfactorily with the performance limit data covering a wide range of parameters.

Two-phase mixture level swell in vertical pipes

Abstract The behavior of the two-phase mixture level was examined in the case of bubbling of a stagnant liquid column in vertical pipes and also in the case of bubbling of a liquid column to which liquid is supplied as a falling film. In a range of low air flow rates where the flow pattern is of bubbly type, the mixture level swell and its fluctuation amplitude were small. However, these values increased sharply as the air flow rate was increased and the flow pattern turned into a slug type. The mean height of the two-phase mixture level and the level fluctuation amplitude were analyzed physically and compared with the experimental results.