52 research outputs found
An Enhanced VOF Method Coupled with Heat Transfer and Phase Change to Characterise Bubble Detachment in Saturated Pool Boiling
No full textThe present numerical investigation identifies quantitative effects of fundamental controlling parameters on the detachment characteristics of isolated bubbles in cases of pool boiling in the nucleate boiling regime. For this purpose, an improved Volume of Fluid (VOF) approach, developed previously in the general framework of OpenFOAM Computational Fluid Dynamics (CFD) Toolbox, is further coupled with heat transfer and phase change. The predictions of the model are quantitatively verified against an existing analytical solution and experimental data in the literature. Following the model validation, four different series of parametric numerical experiments are performed, exploring the effect of the initial thermal boundary layer (ITBL) thickness for the case of saturated pool boiling of R113 as well as the effects of the surface wettability, wall superheat and gravity level for the cases of R113, R22 and R134a refrigerants. It is confirmed that the ITBL is a very important parameter in the bubble growth and detachment process. Furthermore, for all of the examined working fluids the bubble detachment characteristics seem to be significantly affected by the triple-line contact angle (i.e., the wettability of the heated plate) for equilibrium contact angles higher than 45°. As expected, the simulations revealed that the heated wall superheat is very influential on the bubble growth and detachment process. Finally, besides the novelty of the numerical approach, a last finding is the fact that the effect of the gravity level variation in the bubble detachment time and the volume diminishes with the increase of the ambient pressure
Break-up Mechanisms and Conditions for Vapour Slugs Within Mini-Channels
No full textIn the present investigation an enhanced Volume Of Fluid (VOF) based numerical simulation framework is applied for the conduction of parametric numerical simulations, aiming to investigate observed break-up phenomena of vapour slugs, within circular mini-channel branches of a hybrid thermosyphon / pulsating heat pipe device, during microgravity experiments. The simulation results identify three prevailing break-up regimes. The effect of fundamental controlling parameters in the resulting break-up characteristics is also examined. An entrainment of a liquid droplet at the trailing edge of the vapour slug, that is responsible for its subsequent “full” break-up, is identified from the simulations. Moreover, it is quite interesting that the value of the applied heat flux, does not seem to influence the break-up regime and its main characteristics
Accelerating Taylor bubbles within circular capillary channels:Break-up mechanisms and regimes
No full textIn the present paper, an enhanced Volume of Fluid model is applied for the conduction of parametric numerical simulations, to investigate break-up phenomena of accelerating, elongated, vapour bubbles, within circular mini-channels. The effect of fundamental controlling parameters in the resulting break-up characteristics is investigated. Four different series of parametric numerical simulations of isolated vapour bubbles within mini-channels are performed, examining the effects of the imposed pressure difference between the inlet and the outlet of the channel, the surface tension, the effect of the applied heat flux as well as the initial liquid film thickness between the bubbles and the channel, on the developed vapour/liquid interface dynamics. The overall dimensionless number ranges examined are 6.76 < We < 1474.7, 0.007 < Ca < 0.14, 694 < Re < 12541 and by introducing a modified Froude number in order to account for the flow acceleration, 1 < Fr* < 21.86. These dimensionless number ranges are selected in order to overlap with experimental observations in zero-gravity Pulsating Heat Pipe experiments that constitute the motivation for the present numerical investigation. The proposed simulation results identify three prevailing regimes. A “full break-up” regime, a “partial break-up” regime and a “no break-up” regime. The entrainment of liquid droplets at the trailing edge of the vapour slugs is in most cases responsible for their subsequent “full break-up”, into a leading and a trailing bubble, as it is identified from the numerical simulations. Moreover, the applied heat flux does not influence the resulting break-up regimes. Finally, these identified break-up regimes, are grouped together into a well-defined flow map with respect to the We and Fr* numbers.</p
Effect of Channel Aspect Ratio on Flow Boiling Characteristics within Rectangular Micro-passages
No full textA numerical investigation on the effect of channel aspect ratio on a single bubble growth during saturated flow boiling conditions within square microchannels, is conducted in the present paper. The open-source toolbox OpenFOAM is applied for the simulations, utilising a user-enhanced Volume OF Fluid (VOF) solver. The solver enhancements include a treatment for spurious velocities dampening, the implementation of an improved dynamic contact angle sub-model for accurate prediction of wettability effects as well as the implementation of a phase-change model in the fluid domain, accounting for conjugate heat-transfer with a solid domain. It is shown that the variation of the aspect ratio of a microchannel has a significant effect in the local heat transfer coefficient, due to an enhancement of the single-phase heat transfer in combination with the alteration of the underpinned bubble growth dynamics, which result in different contributions of contact line versus liquid film evaporation
Numerical investigation of liquid film instabilities and evaporation in confined oscillating slug-plug flows
No full textAn enhanced volume of fluid (VOF)-based numerical simulation framework that accounts for conjugate heat transfer between solid and two-phase flow regions and phase-change due to boiling/condensation, is utilised in order to investigate the effect of flow oscillation amplitude and frequency on the liquid film evaporation and instability formation in slug-plug flows within heated channels, in saturated flow boiling conditions. Various series of parametric numerical simulations are performed, for different values of flow oscillation amplitude and frequency for a variety of working fluids. For one of the working fluids two different channel diameters are also tested. The oscillations in each case are induced by applying an oscillating pressure boundary condition at the inlet of the channel, keeping the pressure constant at the outlet, after an initial period of constant pressure drop between the inlet and the outlet. Capillary ridges that are initiated at the liquid film, in the vicinity of the leading edge of the considered vapour slugs, are identified as a result of the imposed oscillations, which are translated in the form of capillary waves towards the rear end of the bubbles. It is shown that the formation frequency as well as the geometric characteristics of the generated ridges, are directly related to the corresponding frequency and amplitude of the induced flow oscillations. Furthermore, it is shown that in the initial stages of the bubble fate after the application of the oscillations liquid film evaporation is enhanced with the increase of the oscillation amplitude while it degrades as the frequency of the oscillation becomes higher. However, for large oscillation amplitudes and channel diameters, liquid jets penetrate into the elongated bubbles leading in a lot of cases to bubble break-up.</p
Wettability Effect On Flow Boiling Characteristics Within Micro- passages
No full textA numerical investigation on the effect of wettability characteristics on a single bubble growth during saturated flow boiling conditions within a microchannel, is conducted in the present paper. The numerical simulations are conducted with the open-source toolbox OpenFOAM, utilising a user-enhanced Volume OF Fluid (VOF) solver. The proposed solver enhancements involve a treatment for spurious velocities dampening (a well-known defect of VOF methods), an improved dynamic contact angle treatment to accurately account for wettability effects as well as the implementation of a phase-change model in the fluid domain, accounting for conjugate heattransfer with a solid domain. The predictions of the simulations show that the local Nusselt number (Nu) is more depended on wettability characteristics for low heat fluxes, and less dependent on higher heat fluxes. In more detail, it seems that the local, instantaneous heat transfer coefficient is higher for super-hydrophilic cases in comparison to hydrophilic. However, as the applied heat flux increases, hydrophilic and super-hydrophilic cases show a similar heat transfer enhancement with respect to the single-phase heat transfer in the considered micro-channel. Finally, superhydrophobic cases, show lower heat transfer performance with respect to the single-phase case.This is due to the fact that a vapour blanket is rapidly formed immediately after the nucleation, acting as an insulator of the heated solid surface
The effect of surface wettability on flow boiling characteristics within microchannels
No full textThe process of flow boiling within micro-passages plays a very important role in many industrial appli- cations. However, there is still a lack of understanding of the effect of an important controlling param- eter: surface wettability. In this paper, an advanced numerical investigation on the effect of wettability characteristics on single and multiple bubble growth during saturated flow boiling conditions within a microchannel is performed. The 3D numerical simulations are conducted with the open-source Computa- tional Fluid Dynamics (CFD) toolbox OpenFOAM, utilising a custom user-enhanced Volume Of Fluid (VOF) solver. The proposed solver enhancements involve an appropriate treatment for spurious velocities damp- ening, an improved dynamic contact angle treatment, as well as the implementation of a phase-change model in the fluid domain also accounting for Conjugate Heat Transfer (CHT) with the solid domain. In total, three sets of simulations of hydrophilic and hydrophobic surfaces with constant heat and mass flux were performed. In the first set, a single bubble seed was patched close to the inlet of the microchan- nel and the Heat Transfer Coefficient (HTC) along the channel interface was measured until the nose of the bubble reaches the outlet. The bubble growth and transport process within the channel were anal- ysed, with a minor effect of the wettability characteristics on the HTC observed. In the second set of simulations, multiple recurring nucleation events at the same position were simulated; observing that in such more realistic cases the effect of wettability in the HTC was more profound. Finally, simulations with multiple nucleation sites and recurring nucleation events were conducted to analyse cases closer to reality. These results show indeed that surface wettability plays a significant role on the HTC, with the hydrophilic and hydrophobic cases performing approximately 43.9% and 17.8% higher respectively, com- pared to the single-phase reference simulations. Additionally, it is found that the dominant heat transfer mechanisms for the hydrophilic and hydrophobic surface are liquid film evaporation and contact line evaporation, respectively, and that for the proposed simulation parameters liquid film evaporation can be considered as a more efficient heat transfer mechanism compared to contact line evaporation
Numerical Simulation of Droplet Breakup when Impacting a Narrow Gap
No full textWhen a droplet impacts a pore with sufficiently high velocity the droplet breakups into liquid patterns both above the surface and inside the pore. In the present work, Computational Fluid Dynamics (CFD) simulations are carried out, considering the results obtained by an experimental analysis of droplets impacting on a single narrow gap, to study the factors that control the resulting droplet breakup. The single pore has the form of a slit with a width of either 100 or 150 microns across, and is several times longer than the impacting drop diameter. A droplet with a diameter of 2 mm impacts the gap at either 0.5 or 1.5 m/s. Both the experiments and the numerical simulations show that the droplet remains intact at 0.5 m/s but on the contrary cleaves into two halves at 1.5 m/s. A VOF-based numerical simulation framework that has been previously implemented in OpenFOAM and has been validated against droplet impacts on surfaces with different wettabilities, is utilised to reproduce these experimental runs. Experimental measurements are unable to capture the pressure and velocity fields that develop within the drop at the various stages of impact, however detailed pressure and velocity fields are predicted by the numerical simulation. From the overall analysis of the numerical predictions, characteristic pressure gradients within the droplet are revealed. Furthermore, the volume of the droplet that penetrates into the gaps with respect to time is quantified in detail utilising the numerical simulation results, revealing that the impact velocity does not significantly affect the early stages of the droplet penetration into the considered narrow gaps, while the gap width has a considerable effect in the droplet penetration rate from the early stages of the considered droplet impacts
Numerical Investigation of isolated bubble growth and detachment in cases of pool boiling with different wettability characteristics:Implementation of a dynamic contact angle treatment in OpenFOAM
No full textThe effect of hydraulic diameter on flow boiling within single rectangular microchannels and comparison of heat sink configuration of a single and multiple microchannels
No full textPhase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh, whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW⁄m2 and 150 to 2400 kg⁄m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance RRl compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the RRl by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms.</p
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