Nucleate boiling

Résumé

In fluid thermodynamics, nucleate boiling is a type of boiling that takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the heat flux is below the critical heat flux. For water, as shown in the graph below, nucleate boiling occurs when the surface temperature is higher than the saturation temperature (TS) by between . The critical heat flux is the peak on the curve between nucleate boiling and transition boiling. The heat transfer from surface to liquid is greater than that in film boiling. Nucleate boiling is common in electric kettles and is responsible for the noise that occurs before boiling occurs. It also occurs in water boilers where water is rapidly heated. Mechanism Two different regimes may be distinguished in the nucleate boiling range. When the temperature difference is between approximately above TS, isolated bubbles form at nucleation sites and separate from the sur

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https://en.wikipedia.org/wiki/Nucleate_boiling

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In the study of heat transfer, critical heat flux (CHF) is the heat flux at which boiling ceases to be an effective form of transferring heat from a solid surface to a liquid. Description

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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

Chargement

Pulsating heat pipes (PHPs) represent a promising solution for passive on-chip, two-phase cooling of micro-electronics, providing advantages such as a simple construction and operation in any gravitational orientation. Unfortunately, the unique coupling of thermodynamics, hydrodynamics and heat transfer responsible for their operation has so far eluded comprehensive description or accurate prediction. The complexity of the self-sustained two-phase flow in PHPs presents many challenges to the understanding on the physical phenomena taking place. It is important to evaluate the heat and mass transfer mechanisms occurring during their operation in order to better describe their performance as a function of the operating conditions. In the present study, a new facility at the Laboratory of Heat and Mass Transfer (LTCM) was built to allow the synchronized thermal and visual investigation of a Closed Loop Pulsating Heat Pipe (CLPHP). A single-turn channel CLPHP was investigated using R245fa as the working fluid. The tests were carried out at filling ratios from 10 to 90 % and heat inputs from 2 to 60 W, for vertical and inclined orientation. Flow visualization was attained via the transparent front side of the test section which provided full optical access to the flow inside of the CLPHP channels. A novel time-strip image processing technique was applied to the high speed videos to extract qualitative details of the flow regimes and quantitative flow data concerning the liquid/vapor interface dynamics. Local temperature oscillations were also measured and their frequency spectra further helped in characterizing the self-sustained two-phase flow. Thermal resistance measurements were used to qualitatively and quantitatively assess the effect of the flow dynamics on the system thermal performance, which are presented as operational maps. The dynamics governing the PHP operation during oscillating and circulating flows and their effects on the system thermal performance were investigated and insight gained. Four distinct flow regimes and their thermal and flow-dynamics characteristics were identified, suggesting the strong coupling between the two-phase flow pattern and the system thermal behavior. Thin film evaporation was observed to be the most dominant thermal mechanism while heat transfer into the oscillating liquid slug was of second importance, together with localized nucleate boiling. Moreover, the net forces acting on the system could be identified through the novel time-strip technique, revealing new details on the mechanisms producing self-sustained two-phase flow oscillation and circulation, and the two-phase flow pattern transition. The role of gravity for the operation of this single-turn CLPHP was also assessed.

EPFL2014

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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

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Fundamental study on microlayer dynamics in nucleate boiling

Lubomír Bures

EPFL2021

EPFL2014