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

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.

Boiling on a Tube Bundle

Eugene van Rooyen

The complexity of two-phase flow boiling on a tube bundle presents many challenges to the understanding of the physical phenomena taking place. It is important to quantify these numerous heat flow mechanisms in order to better describe the performance of tube bundles as a function of the operational conditions. In the present study, the bundle boiling facility at the Laboratory of Heat and Mass Transfer (LTCM) was modified to obtain high-speed videos to characterise the two-phase regimes and some bubble dynamics of the boiling process. It was then used to measure heat transfer on single tubes and in bundle boiling conditions. Pressure drop measurements were also made during adiabatic and diabatic bundle conditions. New enhanced boiling tubes from Wolverine Tube Inc. (Turbo-B5) and the Wieland-Werke AG (Gewa-B5) were investigated using R134a and R236fa as test fluids. The tests were carried out at saturation temperatures Tsat of 5°C and 15°C, mass flow rates from 4 to 35 kg/m2s and heat fluxes from 15 to 70 kW/m2, typical of actual operating conditions. The flow pattern investigation was conducted using visual observations from a borescope inserted in the middle of the bundle. Measurements of the light attenuation of a laser beam through the intertube two-phase flow and local pressure fluctuations with piezo-electric pressure transducers were also taken to further help in characterising the complex flow. Pressure drop measurements and data reduction procedures were revised and used to develop new, improved frictional pressure drop prediction methods for adiabatic and diabatic two-phase conditions. The physical phenomena governing the enhanced tube evaporation process and their effects on the performance of tube bundles were investigated and insight gained. A new method based on a theoretical analysis of thin film evaporation was used to propose a new correlating parameter. A large new database of local heat transfer coefficients were obtained and then utilised to generate improved prediction methods for pool boiling and bundle boiling, including a method for predicting the onset of dryout.

EPFL2011

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