Falling Films on Horizontal Tubes with R-134a, Part II: Flow Visualization, Onset of Dryout and Heat Transfer Predictions

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.

EXPERIMENTAL STUDY ON FLOW BOILING HEAT TRANSFER OF MULTIPORT TUBES WITH R245fa AND R1234ze(E)

Bruno Agostini, John Richard Thome, Farzad Vakili Farahani

This article presents an experimental study of upward flow boiling heat transfer in multiport tubes. Usually, multiport tubes are utilized horizontally in automotive and air-conditioning systems while here there is interest in vertical flow as part of a two-phase thermosyphon electronics cooling system. Experiments were carried out in a flat aluminum extruded multiport tube, which was composed of 7 parallel rectangular channels (1.1mm × 2.1mm) with a hydraulic diameter of 1.4 mm. Two refrigerants, R245fa and the new refrigerant R1234ze, the latter a new environmentally safe refrigerant proposed as a potential replacement for R134a, were tested. A new hot water heating technique that accounts for either uniform or non-uniform local heat flux distribution along the channel, particularly for exploring high vapor qualities into the dryout region, was developed to obtain and reduce the data. Accordingly, heat transfer coefficients were measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. Effects of heat flux, mass flux, vapor quality, and saturation temperature on flow boiling heat transfer in multiport tubes were considered. Finally, the experimental results were compared with some well-known correlations to evaluate the capabilities of existing prediction methods. The analysis completed so far shows that the three-zone model for slug flows works well for that subset of test results, utilizing the apparent surface roughness in place of the dryout thickness in the model, as has been previously done for silicon, stainless steel and copper microchannels with measured surface roughnesses. Notably, the non-dryout data were well predicted by the Cooper nucleate pool boiling correlation as in many other studies, although there is no proof that nucleate boiling is taking place except near the point of inception of boiling.

2012

Falling film evaporation on a single tube and on a tube bundle

Jean-François Roques

This study is a contribution for the improvement of horizontal falling film heat exchangers. This type of evaporator has the potential to be widely used in the petrochemical industry, for the sea water desalination, or in the large refrigeration systems. An experimental test facility has been constructed in order to study the evaporation of the liquid refrigerant R-134a flowing on a vertical array of horizontal copper tubes with a length of 0.55 meter and a diameter of 19.1mm. Special care has been taken to achieve uniform liquid distribution along the top tube. Four types of tubes were tested including a plain tube, two tubes with mechanically enhanced boiling surfaces (Turbo-BII HP and Gewa-B) and a porous coated tube (High-Flux). Tube arrays with three different tube pitches were tested. Measurements were performed at three different nominal heat flux levels over a wide range of liquid overfeed. For comparison, heat transfer coefficients in pool boiling conditions were measured. The experimental parameters are the following: the liquid film Reynolds numbers from 0 to 3000, heat fluxes between 20 and 60kW/m2 and tube pitches from 22.3 and 25.5mm. The heat transfer coefficients have been measured locally at the mid point of each of the ten tubes. The study shows that there are two types of fundamental behavior characterized by a particular film Reynolds number: Over a threshold value of this Reynolds number, the nucleate boiling in the film governs the heat transfer and this heat transfer coefficient is independent of the Reynolds number. Below this threshold value, the heat transfer coefficients drop drastically due to the formation of local dry patches. The heat transfer coefficient during falling film evaporation was found to be approximately 1.3 times larger than the heat transfer coefficient during pool boiling and this multiplier was found to be dependant on the heat flux and tube spacing but not the flow mode. Finally, the understanding of the physical phenomena governing the falling film evaporation of liquid refrigerants has been improved. Furthermore, a method for predicting the film Reynolds number at the onset of dry patch formation has been developed and a heat transfer correlation for boiling iii falling films proposed. These represent significant improvements for the design of falling film evaporators.

EPFL2004

Local Heat Transfer Coefficient Prediction of Substitute Refrigerants with Smooth or Enhanced Surfaces.

Daniel Favrat, John Richard Thome, Olivier Zürcher

Lack of precision and inconsistencies in existing correlations, an attempt was successfully made to develop a new and more mechanistically based correlation (Kattan, 1996 [1]) for boiling inside plain tubes. This new general correlation is original in that it takes into account the effect of two phase flow patterns on evaporative heat transfer. As a first step, the Steiner (1993, [4]) flow map was rewieved and a proposal was made to better account for the transition boundaries between stratified-wavy and annular flow in particular. The importance of the heat flux on the transition between annular flow and annular with partial dryout was identified and original propositions were developed for predicting its occurence, which allows a significantly more accurate prediction of the heat transfer coefficients at high vapor qualities in the boiling process. An extension of the new plain tube correlation is proposed for microfin tubes using enhancement factors which are calculated empirically for the effects of the fins on convective and nucleate boiling contributions to heat transfer.