Session L12: Bubble Growth, Heat Transfer and Boiling

Statistical approach for free-rising bubbles in quiescent water under single bubbling regimes

Free-rising bubbles in quiescent water are frequently observed in daily living and industrial applications. Occurrences can go from simple boiling phenomena, liquid metal stirring in metallurgy, or bubble leakage under subsea conditions. The measurements of free-rising air bubble sizes issuing from a Borda hole in quiescent water are investigated in this work by means of a robust statistical description. Single bubbling flows are considered in the range of gaseous Reynolds numbers from 0 to 120. Optical diagnostics for bubble characterization are based on diffused backlight illumination technics. Experimental measurements are held for detached bubbles and attached bubbles to the injector, under noncorrelated (fps<10) and correlated (fps>100) image acquisition, respectively. The good agreement between the volumes of the detached bubble and the maximum bubble attached to the injector validates the statistical approach. The results based on a pdf of bubble diameters show that, depending on the flow rate, mainly three types of bubbles are observed: flow formation, fragmentation, and coalescence.   

Dual-Tracer Laser-Induced Fluorescence Thermometry for Understanding Bubble Growth during Nucleate Boiling on Oriented Surfaces

Nucleate boiling often occurs on surfaces oriented at different angles, and therefore understanding the behavior of bubble growth on various surface orientations is important. This work aims to accurately measure three-dimensional (3D), space- and time-resolved, local liquid temperature distributions surrounding a growing bubble that quantify the heat transfer from the liquid during bubble growth on angled surfaces. The dual tracer laser-induced fluorescence thermometry technique combines high-speed imaging to capture transient 2D temperature distributions within a 0.3 ºC accuracy at a 30 μm resolution. Two fluorescent dyes, fluorescein and sulforhodamine B, emit temperature-sensitive emissions in the region of interest when excited by a laser light sheet. The results show that the temperature close to the heated surface and bubble interface exhibits a radical transient behavior at the time of departure, and the bubble works as a pump to remove heat from the surface with a peak temperature difference of up to 10 °C during bubble growth and departure. The experimental results are compared with data available in the literature to validate the accuracy of the technique. Lastly, the temperature information is used to quantify time-resolved heat fluxes contributing to mass transfer near the growing bubble.    

Boiling in a two-phase closed thermosyphon under horizontal vibration

A two-phase closed thermosyphon is a wickless and passive heat transfer device. In the thermosyphon, working fluid vaporizes in the lower evaporator section, and then the vapor condenses in the upper condenser section and returns to the evaporator section by gravitational force. Thermosyphons can be widely used for industrial applications, including cooling of electrical devices, batteries, and avionics equipment. These applications, however, may be encountered in diverse dynamic circumstances such as translation, rotation, and vibration.  As a first step to make a thermosyphon function properly and improve its heat transfer performance under dynamic conditions, we experimentally investigate boiling patterns of the thermosyphon, when it is subjected to horizontal vibration, by varying the amplitude and frequency of vibration, heat input, and filling ratio. A new parameter which characterizes the intensity of vibration is introduced to determine the effect of horizontal vibration on phase change inside the thermosyphon. When the thermosyphon is subjected to the case of a large vibration-intensity parameter, boiling is suppressed due to the high requirement of superheat. Furthermore, as heat input into the thermosyphon increases, the effects of severe horizontal vibration on boiling suppression are annihilated.   

Understanding the role of wettability on nucleate boiling heat transfer coefficient for large-scale boiling surfaces

Boiling phenomena has an important role in many industrial applications, being a very efficient heat transfer mode.

Although several experiments have been conducted to investigate boiling, its mechanisms and heat transfer characteristics are still not understood completely, at larger scales. Analysing and accessing this phenomenon accurately is still a challenge, due to the complexity associated with non-equilibrium thermodynamics at the liquid-vapour interface and its coupling with the fluid dynamics.

For this purpose, here, we employ advanced parallelised multiphase numerical schemes implemented in our in-house solver, TPLS, using the Diffuse Interface Method to simulate pool boiling. This method enable us to model contact line physics with appropriate boundary conditions by eliminating the stress singularity at the three-phase contact line, allowing us to analyse the effect of substrate wettability on boiling performance.

We validate our simulations against nucleate boiling experiments using FC72 on silicon surfaces and we observed that the hydrophilic substrates enhance the heat transfer coefficient (HTC) promoting the departing conditions in multiple bubble systems intensifying the coolability of the surface. We also perform simulations at increasing site-densities from O(10) nucleation sites (for lab-scale surfaces) to O(100) sites (for pilot-scale surfaces) to O(1000) sites (industrial-scale surfaces). This is important because these simulations enable us to determine the heat transfer coefficient as a function of nucleation site-densities.   

Flow boiling heat transfer with self-rewetting fluids

Self-rewetting fluids offer a promising solution in boiling compared to conventional fluids by delaying surface dry-out regions through their unique surface tension characteristics. However, questions remain on the heat transfer capabilities of these fluids in microchannel flow boiling applications. Studies so far rely primarily on experiments which are limited by the data acquisition and, in particular, key information such as dynamic variation of heat-transfer coefficients as a function of flow-regime transition cannot be captured.

In this study, we perform a comprehensive numerical investigation to explore the flow boiling heat transfer characteristics of water (as the conventional fluid) and a 5% v-v butanol-water mixture (as the self-rewetting fluid) in microchannels. To accurately capture the intricacies of flow boiling at the microscale, we employ the Volume of Fluid (VOF) method along with the modified Schrage evaporation model by Hardt and Wondra (2008).

Our Nusselt number predictions have been validated against our experimental data by Venter et al (2023). Also, the bubble shapes are qualitatively similar to those found in our experiments. Our simulations further analyse heat transfer capabilities and surface dry-out region formation rates as a function of flow-rates. This is important given surface tension effects are more dominant at low Reynolds numbers. This analysis aims to deepen our understanding of the potential of self-rewetting fluids under flow boiling conditions, at low Reynolds numbers.   

Numerical simulation of pool boiling on biphilic surfaces

The success of future space missions necessitates the need for more efficient thermal subsystems. Hence, there is a need for investigation into two-phase cooling approaches for their increased capacity relative to their single-phase counterparts. Pool boiling experiments on biphilic surfaces have demonstrated their superiority in heat removal capability over monophilic surfaces. The non-homogeneity of the contact angle has been shown to delay CHF (Critical Heat Flux). However, experiments are unable to properly elucidate the underlying physical mechanisms. In this study, we have conducted numerical experiments to gain a deeper insight into pool boiling on biphilic surfaces. An in-house multiphase solver is utilized to simulate the pool boiling with the heater surface as the boundary condition. The heater surface is a uniform pattern of square hydrophobic patterns with hydrophilic surface between them. In particular, the hydrophobicity ratio (A* = Hydrophobic area/Total area)  is varied and its effect on the heat flux and thermo-fluid characteristics are analyzed. Three cases of A* = 0.1, 0.25, 0.75 are considered under normal gravity conditions.  The case with A* = 0.25 showed the highest average heat flux. The favorable heat removal characteristics due to the heterogeneity of the contact angle are highlighted and explained. This analysis would serve as a basis for a future study where simulations will be carried out in a low gravity environment.   

Computational Study of Subcooled Pool Boiling: Effects on Heat Flux and Bubble Dynamics

In the past, saturated pool boiling has been the primary focus of research, while subcooled pool boiling has received less attention. As a result, the understanding of subcooled pool boiling physics remains limited. The present work examines the effect of subcooling on both heat flux and bubble dynamics by conducting direct numerical simulations (DNS) of pool boiling using an in-house Navier Stokes solver. Liquid-vapor interface is tracked using level set technique. Ghost-fluid formulation is considered to account for sharp jumps in pressure, velocity, and temperature across multiphase boundaries. Two sets of computations have been conducted using FC72 as a working fluid. In the first set, bulk liquid temperature was varied while the rest of parameters were held constant. In the second set, Stefan's number was maintained constant. All cases show that subcooled pool boiling affects boiling curve, heat flux, and bubble dynamics. Computational results were compared to available empirical correlations. Furthermore, details of the flow field reveal flow structures in the form of mushroom-like vortices due to near wall interactions, while vortex ring patterns were observed further away from the wall. Variation of Stefan number revealed negligible effect.   

A subgrid bubble nucleation model for interface resolving simulations of nucleate boiling

The process of nucleate boiling is a complex thermal hydraulic phenomenon with use in a range of engineering applications from cooling of nuclear fuel rods in light water nuclear reactors, to heat exchangers in sensitive industrial processes. While this problem has been the subject of extensive theoretical and experimental inquiry since the early 1950s, there remain gaps in first principles knowledge and uncertainties in empirical models and correlations. The relatively recent increase in tractability of interface resolving computational fluid dynamics simulations have allowed deeper insights into the conditions and important parameters controlling the surface heat flux, such as effects of micro gravity environments. However, even with interface resolving direct numerical simulations, for investigations of practical scale involving dozens of bubbles or more, resolving the micro scale surface topology and roughness necessary to predict inception of bubbles in space and time remains computationally out of reach. We will present a sub-grid model of bubble nucleation site location and renewal time which account for local surface thermal and fluid conditions that provides a numerically stable and computationally efficient closure to the higher-order thermal hydraulic simulations.   

Enhancement of the subcooled microchannel flow boiling heat transfer by the cylinder-induced vortices

Microchannel flow boiling is gaining great attention due to its high heat dissipation with a small temperature difference, which is particularly beneficial in cooling systems. This work numerically studies the subcooled flow boiling heat transfer in microchannels enhanced by the cylinder-induced vortices. Simulations are conducted by an immersed boundary-lattice Boltzmann method. In this method, the boiling flow is solved by using the pseudopotential multiphase lattice Boltzmann model, the flow-induced vibration of the cylinder is modelled by a mass-spring-damping system, the heat transfer equation is solved by the finite difference method, and the boundary condition at the fluid-cylinder interface is handled by a feedback immersed boundary method. Three groups of simulations are examined: a clear channel, a channel with a stationary cylinder, and a channel with a flow-induced vibrating cylinder. Various parameters are varied, including Reynolds number, heat flux, surface wettability, and blockage ratio. In the case of flow over a fixed cylinder with a blockage ratio of 3.0, the induced vortex achieved an enhancement of about 20% in the rates of flow boiling heat transfer in the intermediate region of heat flux compared to the clear channel. Moreover, in the high heat flux region, there is a substantial improvement in heat transfer, exceeding 23.455%, 22.97%, and 25.881% for Reynolds numbers of 600, 800, and 1000, respectively. The presence of the induced vortex effectively delayed the onset of the dryout condition at the critical heat flux point. Additionally, decreasing the blockage ratio enhances the rates of heat transfer. When the vibrating cylinder is positioned near the wall with a low blockage ratio, notable enhancements in heat transfer in moderate range of heat flux. Therefore, this study provides an efficient solution for flow boiling problems without using any additional external power source for evaporative cooling systems of the high-powered applications.   

Understanding bubble dynamics and related heat transfer characteristics in multiple parallel microchannels with and without inlet restrictions

Flow boiling inside a microchannel heat sinks have been recognized as one of the efficient ways of cooling advanced electronic devices which generate high heat such as micro-electronic chips, micro-electro-mechanical systems (MEMS), and fuel cells. Extensive research has been carried out in the past to understand the bubble dynamics and associated heat transfer characteristics inside a single microchannel using numerical simulations [1,2]. In practical scenarios, however, multiple microchannels arranged in parallel are used for effective cooling. Bubble dynamics in such microchannel heat sinks with multiple parallel channels are considerably different from those in single channels and are relatively poorly understood. In the present work, three-dimensional numerical simulations are performed to understand the bubble dynamics, the interaction between the bubbles nucleating in two different channels, and its effect on wall temperature and heat transfer characteristics. For this purpose, direct numerical simulations are carried out using the volume-of-fluid method with an additional model to handle liquid-vapour phase change. Furthermore, one of the major problems associated with the flow boiling in multiple parallel microchannels is instability in boiling arising due to sudden expansion of vapour bubbles formed in the channels which leads to flow reversal and early critical heat flux. An attempt is made to address this instability by using channels with inlet restrictions in a multiple parallel microchannel setup.

References

 

[1] A. Mukherjee, S.G. Kandlikar. “Numerical simulation of growth of a vapor bubble during flow boiling of water in a microchannel”. Microfluid Nanofluid, 1 (2005), pp. 137–145.

[2] M. Magnini, O.K. Matar. “Numerical study of the impact of the channel shape on microchannel boiling heat transfer”. International Journal of Heat and Mass Transfer, 150 (2020), pp. 119322.   

Acoustic Signatures of Boiling Critical Heat Flux and Interfacial Instabilities

Boiling is critical to the efficient and safe operation of electronics cooling, power generation, refrigeration, water purification, and chemical processing. However, pool and flow boiling are prone to various instabilities, leading to significantly reduced heat transfer coefficients, higher pressure drop, and potential damage to equipment. The interplay of physical mechanisms, such as bubble nucleation, growth and departure, turbulent flows, capillary flows, and thermal expansion-driven flows, has limited our understanding of boiling instabilities. In this study, we use wideband acoustic emission sensing, integrated with high-speed imaging and temperature/pressure measurements, to probe the physical mechanisms that dominate critical heat flux (CHF) and flow instabilities during pool and flow boiling. The wideband AE sensing technique allows for higher sampling rates to capture high-frequency interface oscillations and the onset of CHF and flow regime transitions. The resulting acoustic signatures show significant change in dominant frequency and amplitude at CHF and help to quantify the role of liquid film, turbulent diffusion, and interfacial waves during boiling regime transitions.