Flow boiling in horizontal flattened tubes: Part II – Flow boiling heat transfer results and model

Experiments of flow boiling heat transfer were conducted in four horizontal flattened smooth copper tubes of two different heights of 2 and 3 mm. The equivalent diameters of the flattened tubes are 8.6, 7.17, 6.25, and 5.3 mm. The working fluids were R22 and R410A. The test conditions were: mass velocities from 150 to 500 kg/m(2) s, heat fluxes from 6 to 40 kW/m(2) and saturation temperature of 5 degrees C. The experimental heat transfer results are presented and the effects of mass flux, heat flux, and tube diameter on heat transfer are analyzed. Furthermore, the flow pattern based flow boiling heat transfer 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 flow pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955-2969; L. Wojtan, T. Ursenbacker, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part II - Development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes, Int. J. Heat Mass Transfer 48 (2005) 2970-2985], using the equivalent diameters, were compared to the experimental data. The model predicts 71% of the entire database of R22 and R410A +/- 30% overall. The model predicts well the flattened tube heat transfer coefficients for R22 while it does not predicts well those for R410A. Based on several physical considerations, a modified flow boiling heat transfer model was proposed for the flattened tubes on the basis of the Wojtan et al. model and it predicts the flattened tube heat transfer database of R22 and R410A by 85.8% within +/- 30%. The modified model is applied to the reduced pressures up to 0.19. (C) 2009 Elsevier Ltd. All rights reserved.

Flow Boiling Heat Transfer and Pressure Drops of R1234ze(E) in a Silicon Micro-pin Fin Evaporator

Chiara Falsetti, Mirco Magnini, John Richard Thome

The development of newer and more efficient cooling techniques to sustain the increasing power density of high-performance computing systems is becoming one of the major challenges in the development of microelectronics. In this framework, two-phase cooling is a promising solution for dissipating the greater amount of generated heat. In the present study, an experimental investigation of two-phase flow boiling in a micro-pin fin evaporator is performed. The micro-evaporator has a heated area of 1 cm(2) containing 66 rows of cylindrical in-line micro-pin fins with diameter, height, and pitch of, respectively, 50 mu m, 100 mu m, and 91.7 mu m. The working fluid is R1234ze(E) tested over a wide range of conditions: mass fluxes varying from 750 kg/m(2) s to 1750 kg/m(2) s and heat fluxes ranging from 20 W/cm(2) to 44 W/cm(2). The effects of saturation temperature on the heat transfer are investigated by testing three different outlet saturation temperatures: 25 degrees C, 30 degrees C, and 35 degrees C. In order to assess the thermal-hydraulic performance of the current heat sink, the total pressure drops are directly measured, while local values of heat transfer coefficient are evaluated by coupling high-speed flow visualization with infrared temperature measurements. According to the experimental results, the mass flux has the most significant impact on the heat transfer coefficient while heat flux is a less influential parameter. The vapor quality varies in a range between 0 and 0.45. The heat transfer coefficient in the subcooled region reaches a maximum value of about 12 kW/m(2) K, whilst in two-phase flow it goes up to 30 kW/m(2) K.

Asme2017

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

Macro-to-microchannel transition in two-phase flow: Part 2-Flow boiling heat transfer and critical heat flux

Chin Lee Ong, John Richard Thome

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co approximate to 0.99) and 1.03 (Co approximate to 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1]. (C) 2011 Elsevier Inc. All rights reserved.