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

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.

Ammonia two-phase flow in a horizontal smooth tube: Flow pattern observations, diabatic and adiabatic frictional pressure drops and assessment of prediction methods

Ricardo Lima, Jesus Moreno Quiben, John Richard Thome

The present study illustrates new experimental two-phase flow pattern observations together with diabatic boiling and adiabatic two-phase frictional pressure drop results for ammonia (R7117) flowing inside a 14-mm internal diameter, smooth horizontal stainless steel tube. The flow pattern observations were made for mass velocities of 50, 100 and 160 kg s(-1) m(-2) and saturation temperatures of -14, -2 and 12 degrees C for vapor qualities ranging from 0.05 to 0.6. The flow patterns observed during the study included: stratified-wavy, slug-stratified-wavy, slug, intermittent and annular. For all the experimental conditions, the flow structure observations were compared against the predictions of the flow pattern map model of Wojtan et al. [L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: part I - a new diabatic two-phase How pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955-2969] and showed very good correspondence. The frictional pressure drop measurements were obtained for vapor qualities from 0.05 to 0.6, saturation temperatures from -14 to 14 degrees C, mass velocities from 50 to 160 kg s(-1) m(-2) and heat fluxes from 12 to 25 kW m(-2). The experimental results show the traditional pressure drop trends: the frictional pressure drop increases with vapor quality and mass velocity. Moreover, the results also show that both diabatic and adiabatic frictional pressure drop values are similar, that is, the boiling process in itself does not affect the frictional pressure drop. The correlations of Friedel [L. Friedel, Improved friction drop correlations for horizontal and vertical two-phase pipe flow, in: European Two-Phase Flow Group Meeting, paper E2, Ispra, Italy, 1979], Lockhart and Martinelli [R.W. Lockhart, R.C. Martinelli, Proposed correlation of data for isothermal two-phase two-component in pipes, Chem. Eng. Process 45 (1949) 39-48] and Muller-Steinhagen and Heck [H. Muller-Steinhagen, K. Heck, A simple friction pressure correlation for two-phase flow in pipes, Chem. Eng. Process 20 (1986) 297-308] predicted only 54%, 52% and 60% of the experimental data within +/- 30%, respectively. The correlation of Gronnerud [R. Gronnerud, Investigation of liquid hold-up, flow-resistance and heat transfer in circulation type of evaporators, part iv: two-phase flow resistance in boiling refrigerans, in: Annexe 1972-1, Bull. de I'Inst. Froid, 1979] predicted 93% of the data and the flow pattern based method of Moreno Quiben and Thome [J. Moreno Quiben, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizontal tubes. Part II: new phenomenological model, Int.J. Heat Fluid Flow 28 (2007) 1060-1072] predicted more than 97% of the experimental data within the same error band, while the latter method captures almost 89% of the data within +/- 20%. (C) 2008 Elsevier Ltd. All rights reserved.

2009

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

INFLUENCE OF THE SATURATION TEMPERATURE ON THE HEAT TRANSFER COEFFICIENT FOR FLOW BOILING INSIDE HORIZONTAL SMOOTH TUBES

Ricardo Lima, John Richard Thome

This article presents experimental results of heat transfer from two different studies [1, 2] for horizontal flow boiling inside smooth copper tubes with different refrigerants. The main goal of this study was to verify the influence of the saturation temperature on the heat transfer coefficient. The experimental tests were carried for refrigerants R-134a, R-22 and R-404A, at saturation temperatures ranging from 5 to 20°C for two tubes of 12.7 and 13.84 mm ID and three heat fluxes of 5, 7.5 and 17.5 kW/m2. The vapor quality ranged from 5 to 90% and the mass velocity was 300 kg/s.m2. Under these conditions the heat transfer coefficient shows an interesting trend: it behaves differently, in function of the saturation temperature, in the nucleate and convective boiling dominated regions. In the nucleate boiling dominated region the heat transfer coefficient increases with the saturation temperature. In the convective boiling dominated region this phenomenon is inverted, that is, the heat transfer coefficient decreases with the saturation temperature. This phenomenon was verified through the concept of the thermal resistance of the liquid thickness. Since the heat transfer coefficient is proportional to the inverse of the thermal resistance of the liquid film, it increases with decreasing saturation temperature. The transition between nucleate and convective boiling dominated regions, denominated by [1] as a local minimum of the heat transfer coefficient, was also clearly observed with all the refrigerants.