Experimental flow boiling heat transfer in a small polyimide channel

New experimental flow boiling heat transfer results averaged along a very low thermal conductivity polyimide channel are presented. The experimental setup consists of a horizontal, 285 mm-long, 2.689 mm inner diameter polyimide tube, heated by a counter-current, external water flow. R245fa is the working fluid, at mass fluxes ranging from 100 to 500 kg m(-2) s(-1), heat fluxes from 15 to 55 kW m(-2), and three saturation temperatures: 35, 41 and 47 degrees C. Heat transfer coefficient results are presented averaged over the whole polyimide channel for mean vapor qualities ranging from 0.05 to 0.80. High-speed flow visualization was possible downstream of the polyimide tube. Parametric analysis of the results revealed a strong dependence of the heat transfer coefficients on mass flux and vapor quality, and mild influence of the heat flux and the saturation temperature. The experimental database is compared with results obtained for metallic tubes of comparable size in similar experimental facilities. Last, experimental results are contrasted with several heat transfer methods, of which a convective model for annular flow well predicts the heat transfer coefficient values and convective boiling trends. (C) 2016 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

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

Ricardo Lima, Jesus Moreno Quiben, John Richard Thome

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.

2009

Flow boiling in horizontal smooth tubes: New heat transfer results for R-134a at three saturation temperatures

Ricardo Lima, Jesus Moreno Quiben, John Richard Thome

The present study presents new flow boiling heat transfer results of R-134a flowing inside a 13.84 mm internal diameter, smooth horizontal copper tube. The heat transfer measurements were made over a wide range of test conditions: saturation temperatures of 5, 15 and 20 C, (corresponding to reduced pressures of 0.08, 0.12 and 0.14), vapor qualities ranged from 0.01 to 0.99, mass velocities of 300 and 500 kg/m(2) s, and heat fluxes of 7.5 and 17.5 kW/m(2). The experimental results clearly show that a local minimum heat transfer coefficient systematically occurs within slug flow pattern or near the slug-to-intermittent flow pattern transition. The vapor quality x(min) at which the local minimum occurs seems to be primarily sensitive to mass velocity and heat flux. Thus, it is influenced by the competition between nucleate and convective boiling mechanisms that control macroscale flow boiling. The experimental results were compared to four types of predictive methods: (a) strictly convective, (b) superposition, (c) strictly empirical and (d) flow pattern based. Generally, all the methods tend to underpredict the experimental data and the higher errors occur in two particular regions: low and high vapor qualities. These vapor qualities correspond to slug and annular patterns, respectively. For slug flow, methods that require the identification of nucleate boiling related regions tend to predict the heat transfer coefficient accurately. This emphasizes that for slug flows, heat transfer is not a simple juxtaposition of nucleate and convective boiling contributions, but that the integration of these two heat transfer mechanisms is also a function of flow parameters. The comparisons between experimental and predicted data show that the best overall results are obtained with superposition and flow pattern based methods. (C) 2008 Elsevier Ltd. All rights reserved.

2009

Flow boiling in horizontal smooth tubes: New heat transfer results for R-134a at three saturation temperatures

Ricardo Lima, Jesus Moreno Quiben, John Richard Thome

2009