Numerical investigation of the influence of leading and sequential bubbles on slug flow boiling within a microchannel

Multiphase CFD simulations are presently employed to investigate the flow boiling of multiple sequential elongated bubbles in a horizontal microchannel. Most of the computational studies published so far explored the features of boiling flows within microchannels by simulating the fluid-dynamics of a single evaporating bubble, but the present work shows that multiple bubble simulations are necessary to capture the essential features of the heat transfer process of a slug flow. In particular, it is shown that leading and sequential bubbles interact thermally and hydrodynamically due to the evaporation process, thus possessing different growth rates, velocities and thicknesses of the thin liquid films trapped between the bubbles interfaces and the channel wall. The evaporation of this thin liquid film is the dominant heat transfer mechanism in the vapor bubble region and the transit of trailing bubbles strongly enhances the time-averaged heat transfer coefficient of the bubble-liquid slug unit, by as much as 60% higher relative to the leading bubble under the operating conditions presently set. Furthermore, the presence of a recirculating vortex just after the tail of the bubble in the liquid slug trapped between the bubbles was found in the simulations, significantly improving the heat transfer between the wall and the bulk liquid, thus maintaining the heat transfer coefficient much higher than otherwise expected in the liquid slug region as well. Finally, a new multiple bubble heat transfer model is proposed to predict the local variation of the heat transfer coefficient, which might prove to be useful to improve the current boiling heat transfer methods, such as the three-zone model of Thome et al. [1,2]. The numerical framework employed to perform this study was the commercial CFD solver ANSYS Fluent 12 with a Volume Of Fluid interface capturing method, which was improved here by implementing external functions, in particular a Height Function method to better estimate the surface tension force and an evaporation model to compute the phase change. (C) 2013 Elsevier Masson SAS. All rights reserved.

Numerical investigation of hydrodynamics and heat transfer of elongated bubbles during flow boiling in a microchannel

Mirco Magnini, John Richard Thome

Flow boiling within microchannels has been explored intensively in the last decade due to their capability to remove high heat fluxes from microelectronic devices. However, the contribution of experiments to the understanding of the local features of the flow is still severely limited by the small scales involved. Instead, multiphase CFD simulations with appropriate modeling of interfacial effects overcome the current limitations in experimental techniques. Presently, numerical simulations of single elongated bubbles in flow boiling conditions within circular microchannels were performed. The numerical framework is the commercial CFD code ANSYS Fluent 12 with a Volume Of Fluid interface capturing method, which was improved here by implementing, as external functions, a Height Function method to better estimate the local capillary effects and an evaporation model to compute the local rates of mass and energy exchange at the interface. A detailed insight on bubble dynamics and local patterns enhancing the wall heat transfer is achievable utilizing this improved solver. The numerical results show that, under operating conditions typical for flow boiling experiments in microchannels, the bubble accelerates downstream following an exponential time-law, in good agreement with theoretical models. Thin-film evaporation is proved to be the dominant heat transfer mechanism in the liquid film region between the wall and the elongated bubble, while transient heat convection is found to strongly enhance the heat transfer performance in the bubble wake in the liquid slug between two bubbles. A transient-heat-conduction-based boiling heat transfer model for the liquid film region, which is an extension of a widely quoted mechanistic model, is proposed here. It provides estimations of the local heat transfer coefficient that are in excellent agreement with simulations and it might be included in next-generation predictive methods.

Elsevier2013

Fundamental study on microlayer dynamics in nucleate boiling

Lubomír Bures

Dynamics of nucleate boiling are strongly affected by the formation and behaviour of the microlayer, a layer of liquid underneath growing bubbles. As a result of its minute thickness, very high heat fluxes occur within the microlayer and its evaporation contributes significantly to the overall heat transfer. Microlayer formation is, however, not guaranteed and the transition from the contact-line to the microlayer regime of nucleate boiling is not fully understood. The difficulty of experimental investigation of the microlayer and the uncertainties surrounding its formation and subsequent evolution motivate the use of Direct Numerical Simulation (DNS) to model its behaviour. In this work, a computational strategy for utilising DNS to model nucleate boiling by resolving explicitly the microlayer is developed. The numerical method is based on the resolution of continuum conservation equations for incompressible two-phase flows in Cartesian and axisymmetric cylindrical coordinates. The phasic interface is tracked by means of the geometric Volume-of-Fluid (VOF) method and the algorithm is applicable both to adiabatic and volatile flows. Online, implicit coupling of the fluid and solid domains for the solution of the conjugate heat-transfer problem is included and closure models for the treatment of the interfacial heat-transfer resistance and the dynamic contact angle are introduced. A rigorous verification and validation exercise is performed to evaluate the efficacy of the numerical algorithm. Subsequently, a theoretical criterion for modelling the transition between contact-line and microlayer regimes is derived, and tested. Very good agreement is found with the reference experimental and simulation data and the predictive power of the criterion is demonstrated with the aid of DNS results. The computational procedure is then validated for simulations of nucleate boiling with resolved microlayer using relevant experimental data recently measured at the Massachusetts Institute of Technology; it is shown that the main observed growth features and surface heat-transfer characteristics are well-reproduced using the overall method. A sensitivity study of the dependence of the initial microlayer thickness on the growth conditions is performed and a universal equation describing the thickness distribution is proposed with liquid properties and bubble expansion rate being the governing parameters. Finally, the computational method is extended to coarse-mesh problems by introducing several reduced-order models and the full bubble-growth cycle from nucleation to detachment is simulated. Good agreement with reference measurements is again achieved and the experimental findings regarding the force balance during nucleate boiling are confirmed.

EPFL2021

Experimental investigation of heat transfer and flow characteristics in various geometries of 2-pass internal cooling passages of gas turbine airfoils

Denis Chanteloup

An experimental investigation of the flow and heat transfer characteristics in internal coolant passages of gas turbine airfoils has been conducted. The PIV method was employed for the flow measurements. A stereoscopic PIV system was used and automated. The stereoscopic method allowed measuring the three velocity components in measurement planes. The system was capable of measuring 100 planes in each configuration. Each measurement plane was composed of 30*30 measurement points. The transient liquid crystal technique was adopted for the heat transfer measurements. Full surface Nusselt number distributions were obtained on all the five outer walls of the test models. The transient TLC technique was adapted in large-scale models of internal cooling channels. The gas temperature evolution in location and time was taken into account in the data processing. CFD simulations have been performed in the measured passage configurations. For the calculations, the unstructured flow solver FLUENT/UNS was employed. A test rig was designed and constructed for the experiments to meet the requirements of the intended investigation. For the present investigation, the test section was a large-scale model of a two-pass coolant passage with a sharp 180° bend. Four different coolant passage configurations were tested: A passage with 45° ribs (baseline configuration). A passage similar to the baseline configuration with extraction holes simulating film-cooling extraction. A passage with film-cooling extraction and a turning vane in the bend region. A passage with ribs 50% bigger than in the baseline configuration. This passage also had a turn region back wall angled at 30° to the incoming flow. The four configurations were tested at three Reynolds numbers (25,000, 50,000, 70,000 based on the hydraulic diameter). The configurations with extraction were tested at three extractions (30%, 40%, 50% of the inlet massflow). For the first time, the PIV and TLC techniques were employed simultaneously for a detailed investigation of the turbulent flow and heat transfer characteristics within internal coolant passages connected with 180° turns. The following conclusions can be drawn: 3D-streamlines were extracted from the flow measurements in the various cooling channels. The streamlines underlined the strong influence of both the ribs and the bend geometry on the creation of the secondary flow motion in the channels. The secondary flow motion dominates the heat transfer in the entire cooling channel. In the fully developed region, the rib-induced vortex governs the heat transfer distribution on the ribbed walls behind the ribs, and also on the sidewalls. In the bend region, the bend corner flow cells (with low streamwise velocity motion) deviate the bend incoming flow. They act as if the sharp bend geometry was modified. This geometry modification is very dependant on the studied configuration. The resulting heat transfer distribution is thus strongly configuration dependant. The regions of high heat transfer in the channels have been linked with high impinging flow regions. High gradients of mean impinging velocity components near the walls induce high heat transfer on the wall. In these regions, the Nusselt number showed a good correlation with the Reynolds normal stress corresponding to the impinging velocity. The configurations with extractions have the best thermal performances at all the tested Reynolds numbers. The variation of the extraction ratio does not modify substantially the thermal performances. CFD tools has benefited from the tremendous progress of computing power. 3D simulations of internal cooling configurations are now possible. CFD predictions have gained accuracy in such highly 3D flow problems. The predictions compared well with the measurements, with worst discrepancies of 25%.

EPFL2002