Heat transfer and flow visualization of falling film condensation on tube arrays with plain and enhanced surfaces

The tubes in shell-and-tube condensers, widely used in refrigeration and chemical process industries, are subjected to condensate inundation from the neighboring tubes. The aim of the present investigation is to study the effect of condensate inundation on the thermal performance of an vertical array of horizontal tubes with plain and enhanced surfaces. The experimental approach is split in two parts: measurement of the heat transfer coefficients and visualization of the flow patterns of the condensate falling between the tubes. Refrigerant R-134a was condensed at a saturation temperature of 304K on tube arrays with up to ten tubes at pitches of 25.5, 28.6, and 44.5mm. Four commercially available copper tubes with a nominal diameter of 19.05mm and 544mm in length were tested: a plain tube, a 26 fpi / 1024 fpm low finned tube (Turbo-Chil) and two tubes with three-dimensional enhanced surface structures (Turbo-CSL and Gewa-C). Measurements were performed at three nominal heat flux levels up to 60kW/m2 with liquid overfeed corresponding to film Reynolds numbers up to 3000. The test section offers full visual access to study the flow patterns of the condensate. Furthermore, the large experimental database is unique in that true local heat transfer coefficients were measured as opposed to tube length averaged values in previous studies. With little liquid inundation the tubes with 3D enhanced surface structures outperform the low finned tube. Increasing liquid inundation deteriorates the thermal performance of the 3D enhanced tubes, while it has nearly no affect on the low finned tube, resulting in a higher heat transfer coefficient for the low finned tube at high film Reynolds numbers. Large differences in condensate flow patterns were observed. For the 3D enhanced tubes the ideal flow modes (droplet, column and sheet mode) were observed, while the flow was very unstable for the other two types of tubes. For the 3D enhanced tubes oscillations occurred in the sheet mode at film Reynolds numbers such that liquid left the array of tubes sideways. A heat transfer model for an array of 3D enhanced tubes based on these visual observations was proposed, including the effects of liquid lost by the sideways slinging phenomenon. The measurements were predicted within a mean error of 3% and a standard deviation of 13% by this model.

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

Contribution to the heat transfer analysis of natural and substitute refrigerants evaporated in a horizontal tube

Olivier Zürcher

The substitution of CFC refrigerants in refrigeration systems, heat pumps and organic Rankine cycles, requests a good knowledge of the heat transfer and pressure drop properties of substitute fluids. A contribution to this international effort is proposed with the study of two hydrofluorocarbon refrigerants (HFC-134a and the zeotropic mixture HFC-407C) and the study of the natural refrigerant ammonia. The HFCs have been experimentally tested on the first test rig developed in the Laboratory of Industrial Energy Systems (LENI) which was substantially modified to cope with ammonia, both in terms of safety requirements and operational conditions. The experimental test section is composed of two concentric tubes, with in tube evaporation of the refrigerant inside the inner tube of counter-current Annular water heating. A new database of the local heat transfer coefficients for the refrigerants HFC-134a, HFC-407C and ammonia together with pressure drop measurements has been collected and used to define and calibrate a new and more general heat transfer model. The two HFC refrigerants have been tested on the test rig developed by Kattan [36]; due to the chemical and the thermophysical properties of ammonia, a new test rig has been designed for the ammonia tests and a new calculation procedure based on the mean temperature measurement on the tube wall has been used. The experimental database covers evaporation tests on plain and on microfin tubes, and with nominal oil concentrations varying from 0 to 5 [%] (by wt.). Detailed modeling is concentrated on the tests on plain tube without oil. Each of the three refrigerants has been evaporated at 4[°C]. The other experimental conditions for the HFCs refrigerants were an inside tube diameter of 10.92[mm], a heat flux range of 2 – 5 [kW/m2] and three mass velocities of G = 100, 200, 300 [kg/(m2s)] visualized as Stratified-Wavy flows (G = 100[kg/(m2s)]), and mainly Annular for the higher mass velocities (G = 200, 300[kg/(m2s)]). The other global conditions for ammonia were an internal tube diameter of 14.00 [mm], a heat flux range of 5 – 70[kW/m2] and eleven mass velocities of G = 10,20,30,40,45,50,55,60,80,120,140[kg/(m2s)], corresponding to Stratified, Stratified-Wavy, Intermittent and Annular flow patterns; complementary tests, varying the mass velocity at constant vapor qualities (x = 20,50,80[%]), have been made in particular to better characterize boundaries between flow patterns. The flow patterns are visualized through glass sections at both ends of the 3.1 [m] long test tube. An extensive review of void fraction models with sensitivity analysis on the actual available flow pattern map has been made. From the 17 correlations reviewed, the models of Taitel-Dukler [71] and Rouhani [79] have been used. Other improvements to the threshold criteria of the Stratified-Wavy to Annular transition, lead to an accurate diabatic map to predict the flow patterns and their transition for very different families of fluids like HFC's and ammonia. A new approach in the prediction of two-phase flow heat transfer has been proposed through the study of each flow pattern separately, according to a new criterion defining the onset of nucleate boiling as a function of the critical convective heat transfer coefficient representative of the location where nucleate boiling might occur. A function based on a pseudo Biot number allows, from the mean heat flux around the tube periphery, to base the model on two different mean heat fluxes applied respectively to the parts of the perimeter in contact with the liquid and the vapor in stratified types of flow. Considering pure convective heat transfer, or mixed convective and nucleate heat transfer, this division allows the use of a common criterion to be applied to each flow pattern. The two-phase flow heat transfer coefficient has finally been obtained as a weighted mean function of the vapor and the liquid heat transfer coefficient with respect to their contact surface with the heated tube. Based on the database of refrigerants HFC-134a and ammonia, the standard deviation of the new heat transfer model is of σ = 27.9[%]. Even if the database showed that the flow conditions were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points were purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database. Due to the precision of the new flow pattern map and the effectiveness of the onset on nucleate boiling criterion, this new heat transfer model accurately predicts the heat transfer conditions during evaporation. Finally, a new method to model separated flows is proposed, based on partial hydraulic diameters and a mean interface velocity.

EPFL2000

Visualization of R-134a Flowing on Tube Arrays with Plain and Enhanced Surfaces under Adiabatic and Condensing Conditions

Daniel Gstöhl, John Richard Thome

Experimental studies were performed to investigate the flow patterns of liquid films of R-134a on vertical arrays of horizontal tubes under adiabatic test conditions at room temperature and during condensation at a saturation temperature of 304 K. Three arrays with tube pitches of 25.5, 28.6, and 44.5 mm and up to ten tubes of four commercially available copper tubes with a nominal diameter of 19.05 mm and 544 mm in length were tested: a plain tube, a 26 fpi/1024 fpm low finned tube (Turbo-Chil), and two tubes with three-dimensional enhanced surface structures (Turbo-CSL and Gewa-C). Flow pattern observations and transitions were obtained and compared to existing flow pattern maps. The effect of the flow pattern on heat transfer was also investigated. The ideal flow modes (droplet, column, and sheet) could strictly be observed on the tubes with 3D enhanced surface structures. On the low-finned tube and the plain tube, the flow was very unstable, causing a part of liquid to leave the array sideways. For the 3D enhanced tubes, a difference in the formation of the sheets could be seen. The Gewa-C tube tended to form small sheets, while the Turbo-CSL formed large sheets at low film Reynolds numbers. A cyclic forward and backward motion of the sheet was observed that increased in amplitude with an increasing film Reynolds number. At high film Reynolds numbers, part of the liquid left the array of tubes sideways due to this oscillation, the effect of which on heat transfer was found to be more significant than the effect of the flow modes themselves.