In falling film heat transfer on horizontal tube bundles, liquid flow from tube to tube occurs as a falling jet that can take on different flow modes. At low flow rates, the liquid film falls as discrete droplets. At higher flow rates, these droplets form discretely spaced liquid columns. At still higher flow rates, the film falls as a continuous sheet of liquid. Predicting the flow transitions between these flow modes is an essential step in determining the heat transfer coefficient for the particular flow mode, whether for a single phase process or for falling film condensation or evaporation. Previous studies have centered mostly on falling films on plain tube arrays. The objective of the present study is to extend the investigation to tubes with enhanced surfaces: a low finned tube, an enhanced boiling tube and an enhanced condensation tube. The effect of tube spacing on flow transition has also been investigated. The test fluids were water, glycol and a glycol-water mixture. The adiabatic experimental results show that the flow mode transition thresholds for the enhanced boiling tube are very similar to those of the plain tube while the fin structure of the other two enhanced tubes can significantly shift their transition thresholds.

No abstract

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5 • C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the filmReynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.

In tune with the falling film evaporation heat transfer test results described in Part 1, flow visualization of the boiling process and intertube flow mode transitions from droplet to column and sheet flows have been observed, and the onset of dryout results were obtained. A new empirical approach to describe falling film evaporation with a dominance of nucleate boiling that takes into account the onset of dryout has been proposed and is applicable to plain and enhanced tubes. The method predicts most of the current local measurements for R-134a to within ±20% for conditions without dryout (the desired design condition) and has also been extended to cover conditions with partial dryout. Furthermore, a criterion for predicting the onset of dryout as a function of heat flux has also been proposed for the four types of tubes.

No abstract