Falling Films on Arrays of Horizontal Tubes with R-134a, Part I: Boiling Heat Transfer Results for Four Types of Tubes

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.

Falling-Film Evaporation on Tube Bundle with Plain and Enhanced Tubes—Part I: Experimental Results

“…The values of C and n at 5 ı C are given in Table 4 correlations that are typically from 0.67 to 0.7. The negative values of n for the two enhanced boiling were also found in the study of Roques and Thome [9] and Robinson and Thome [23].…”

“…The bubble growth mechanism on an enhanced surface is different from that on a plain surface, because the liquid is mainly evaporated inside the tunnel for structured surfaces, while evaporation occurs on the microlayer for the plain tube. Chien and Webb [12,13], Moeykens and Pate [5], Roques and Thome [9,15], and Roques et al [14] tested several structured, enhanced surfaces both in pool boiling and falling-film evaporation, but the behavior of structured surfaces is still not well understood. As such, the point has not yet been reached where reliable methods are available to guide the custom design of the enhanced surface geometry for a particular fluid and operating condition.…”

“…Roques and Thome [9] proposed another heat transfer measurement strategy to obtain the local value at the midpoint of each tube using a modified Wilson plot technique combined with measurement of the heating water temperature profile. According to their study, the trend for falling-film evaporation is made up of two distinct regions, as shown in Figure 1-a plateau corresponding to an all-wet nucleate boiling dominated regime, where the heat transfer coefficient varies little with the flow rate (a point of onset of dryout) and a partially wet regime with nucleate boiling in the remaining film with a rapidly decreasing heat transfer coefficient tending toward the vapor phase natural convection coefficient at complete dryout.…”

Experimental study on the onset of local dryout in an evaporating falling film on horizontal plain tubes

“…The data were logged only if steady state conditions were attained, which were characterized by variations in the saturation temperature lower than 0.1 K during a period of 10 min. A more detailed description of the test facility, data reduction methods and measurements accuracies can be found in [18].…”

Heat Transfer and Crisis Phenomena at the Film Flows of Freon Mixture over Vertical Structured Surfaces

“…ACCEPTED MANUSCRIPT 4 Recently the modified surfaces, which can intensify heat transfer significantly and stabilize the film flow, are increasingly used in various heat exchanging apparatuses [8][9][10][11][12][13][14].…”