Unified Modeling Suite for Two-Phase Flow, Convective Boiling, and Condensation in Macro- and Microchannels

This paper focuses on the unified modeling suite for annular flow that the authors have developed and continue to develop. The annular flow suite currently includes models to predict the void fraction, the entrained liquid fraction, and the wall shear stress and pressure gradient, and a turbulence model for momentum and heat transport inside the annular liquid film. The turbulence model, in particular, allows prediction of the local average liquid film thicknesses and the local heat transfer coefficients during convective evaporation and condensation. The benefit of a unified modeling suite is that all the included prediction methods are consistently formulated and are proven to work well together, and provide a platform for continued advancement based on the other models in the suite. First the unified suite of methods is presented, illustrating in particular the most recent updates. Then results for the pressure drop and the heat transfer coefficient during convective evaporation and condensation are presented and discussed, covering both water and refrigerants flowing through circular tubes and noncircular multi-microchannel configurations for microelectronics cooling.

Convective boiling heat transfer in a single micro-channel

Lorenzo Consolini

The current industrial need for compact high heat density cooling devices has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes, their heat transfer characteristics are still unclear and require investigation. The present study aims at providing further insight on two-phase single micro-channel heat transfer, through a sensitivity analysis on the effect of the different operational parameters, fluid properties, and channel size on thermal performance. The current database includes results for three different refrigerants, R-134a, R-236fa and R-245fa, two channel diameters, 510 and 790µm, and a multitude of heat fluxes and mass velocities, for a total of over 1800 data points. Two-phase flow instability, which represents one of the factors that has prevented a well-established understanding of the micro-scale phenomenon, has been also investigated, with results showing the potential error it may induce on the estimation of the heat transfer coefficients. Finally, a one-dimensional analysis based on the effect of interfacial shear on the temporal evolution of film thickness is proposed for a heat transfer model for slug flow, which has been shown to be the dominant flow mode at low and intermediate vapor qualities in micro-channel evaporators.

EPFL2008

Boiling on a tube bundle

Francesco Agostini

The complexity of the two-phase flow in a tube bundle presents important problems in the design and understanding of the physical phenomena taking place. The working conditions of an evaporator depend largely on the dynamics of the two-phase flow that in turn influence the heat exchange and the pressure drop of the system. A characterization of the flow dynamics, and possibly the identification of the flow pattern in the tube bundle, in thus expected to lead to a better understanding of the phenomena and to reveal on the mechanisms governing the tube bundle. Therefore, the present study aims at providing further insights into two-phase bundle flow through a new visualization system able to provide for the first time a view of the flow in the core of a tube bundle. In addition, the measurement of the light attenuation of a laser beam through the two-phase flow and measurement of the high frequency pressure fluctuations with a piezo-electric pressure transducer are used to characterize the flow. The design and the validation of this new instrumentation also provided a method for the detection of dry-out in tube bundles. This was achieved by a laser attenuation technique, flow visualization, and estimation of the power spectrum of the pressure fluctuation. The current investigation includes results for two different refrigerants, R134a and R236fa, three saturations temperatures Tsat = 5, 10 and 15 °C, mass velocities ranging from 4 to 40 kg/sm2 in adiabatic and diabatic conditions (several heat fluxes). Measurement of the local heat transfer coefficient and two-phase frictional pressure drop were obtained and utilized to improve the current prediction methods. The heat transfer and pressure drop data were supported by extensive characterization of the two-phase flow, which was to improve the understanding of the two-phase flow occurring in tube bundles.

EPFL2008

Experimental investigation on flow boiling pressure drop and heat transfer of R1233zd(E) in a multi-microchannel evaporator

Navid Borhani, Houxue Huang, John Richard Thome

An experimental study on flow boiling pressure drop and heat transfer of a new environmentally friendly refrigerant R1233zd(E), in a parallel multi-microchannel evaporator was carried out. The silicon microchannels evaporator was 10 mm long and 10 mm wide, having 67 parallel channels, each 100 x 100 mu m(2), separated by a fin with a thickness of 50 mu m. Upstream of each channel, a micro orifice was placed to stabilize the two-phase flow and to obtain good flow distribution. The operating conditions for flow boiling tests were: mass fluxes from 500 to 2750 kg m(-2) S-1, heat fluxes from 6 to 50 W cm(-2), inlet subcooling of 5.8 K, and a nominal outlet saturation temperature of 35 degrees C for stable flow boiling. The test section's backside base temperatures were measured by an infrared (IR) camera. The stable flow boiling data without back flow was selected through flow visualization recorded by a highspeed camera coupled with a microscope. These data were then used to assess the applicability of existing two-phase pressure drop models, and to further develop a new empirical model suitable for the high mass flux operating conditions. This new pressure drop model was used to predict the local fluid temperature for the further heat transfer data identification. The fine-resolution local heat transfer coefficients were obtained by solving the three-dimensional inverse heat conduction problem. The experimental results showed that in the saturated flow boiling region the local heat transfer coefficient first decreased moderately in the very low vapor quality region (x < 0.05), then increased significantly but monotonically along the flow direction. The fine-resolution local heat transfer data at the saturated flow boiling region were compared with two groups of heat transfer correlations. The first one considered the flow boiling mechanism occurring in muliti-microchannels as a combination of nucleate boiling and forced convection boiling, while the other one associated this mechanism to liquid thin film evaporation, thus indicating a controversy. It is found that the flow pattern based model belonging to the second group yielded the best agreement with the experimental data, predicting 92.0% of this new database within 30%. (C) 2016 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2016

Experimental investigation on flow boiling pressure drop and heat transfer of R1233zd(E) in a multi-microchannel evaporator

Navid Borhani, Houxue Huang, John Richard Thome

Pergamon-Elsevier Science Ltd2016