To explain the negative temperature dependence of heat-transfer coefficient in a column-like boiling regime, where a vapor column is formed just behind a nozzle outlet, simplified model experiments were carried out. Optical observation of R113 liquid behavior floating on a flowing water surface gave the following results. R113 liquid on the water surface alternatively could take three states, such as spheroidal drop, lens and continuous thin film. When the temperature difference between water and saturation temperature of R113 was raised, the appearance time of the spheroidal drop state became longer and the average thickness of the lens or continuous thin film states with shorter appearance time increased. Heat transfer to evaporating R113 liquid was enhanced in this order of three states. These results lead to a lower heat-transfer coefficient at a higher temperature difference.
We present the results of experimental investigation of heat transfer during evaporation of thin films flowing over horizontal tubes. Experiments were conducted using 25 mm o.d. copper tubes heated by internal electrical cartridge heaters so that uniform heat flux was generated on the tube outer wall. Five heated tubes were arrayed on a vertical plane with a pitch of 50 mm. Freon R 11 preheated to the saturation temperature at 0.2 MPa was supplied to the topmost heated tube through feeding devices. Heat transfer characteristics of each heated tubes were clarified in the range of film Reynolds number from 10 to 2000. The topmost tube showed 10 to 20% lower heat transfer coefficients than the other four tubes arrayed below it. Tubes except for the topmost one gave the same heat transfer characteristics. Heat transfer coefficients for those tubes were correlated to within 15% accuracy using an empiricalformula : Num=(Ref-2/3+0.010Ref0.3Pr0.25)1/2 Falling film turned into rivulets at flow rates below a certain value. This film breakdown caused the deterioration of heat transfer. A relationship between heat flux and film flow rate was given for the film breakdown.
In this study, a three-dimensional code was developed to simulate the bubble growth and coalescence in nucleate boiling. The level set method is used to capture the liquid-vapor interface. This method can easily handle complex topology changes in such processes as breakup and coalescence of bubbles. Bubble growth in flow boiling and vertical coalescence of bubbles on a single nucleation site in pool boiling are simulated.
An experimental investigation on the flow pattern and pressure drop was carried out for both an adiabatic and a diabatic two-phase flow in a horizontal tube with pure refrigerants R134a and R123 and their mixtures as testfluids. The measured frictional pressure drop in the adiabatic experiments increased in the S-curve as equilibrium vapor quality was increased. These data were compared with various correlations proposed in the past for the frictional pressure drop. Chisholm 1 ) correlation considerally underpredicted the present data both for pure fluids and their mixtures in the entire mass flux range 150 to 600 kg/m 2 s covered in the measurements, while Friedel 2 ) correlation was found rather well to correlate the frictional pressure drop data among compared correlations. However a detailed examination showed Friedel correlation underpredicted the present data in the stratified and stratified-wavy flow regions at low vapor quality and overpredicted in the annular flow region at high quality. A new two-phase multiplier was developed from a dimensional analysis of the frictional pressure drop data measured in the adiabatic experiment. This new multiplier was found successfully to correlate the frictional pressure drop measured in the diabatic flow boiling experiments of pure refrigerants and their mixtures with a mean deviation of 20%.
Enhancement of heat transfer in evaporation of falling film was studied on horizontal tubes with triangular grooves on the tube circumference. An analysis of laminar film in a vertical triangular groove revealed that grooves are effective in enhancing evaporation heat transfer. To verify the enhancement associated with grooved tubes, an experiment was performed in the range of film Reynolds number 100 to 1000 with saturated refrigerant R11 (CCl3F) at a pressure of 0.2 MPa used as the working fluid. Three grooved tubes were tested and enhanced heat transfer coefficient well beyond the geometrical surface increase, yielding larger enhancement than expected from the analytical prediction and about 4 to 8 times higher heat transfer coefficient than that for plain tube. The observed enhancement is probably attributable to an extension of falling film in the valley of the groove as predicted in the analysis. Moreover an intermittent wetting of the crest portion of grooves caused by wavy falling film and subsequent evaporation of thinner film left behind is contributed largely to the enhancement.
