An official website of the United States government
Two-phase thermal management offers cooling performance enhancement by an order of magnitude higher than single-phase flow due to the latent heat associated with phase change. Among the modes of phase-change, boiling can effectively remove massive amounts of heat flux from the surface by employing structured or 3D microporous coatings to significantly enlarge the interfacial surface area for improved heat transfer rate as well as increase the number of potential sites for bubble nucleation and departure. The bubble dynamics during pool boiling are often considered to be essential in predicting heat transfer performance, causing it to be a field of significant interest. While prior investigations seek to modulate the bubble dynamics through either active (e.g., surfactants, electricity) or passive means (e.g., surface wettability, microstructures), the utilization of an ordered microporous architecture to instigate desirable liquid and vapor flow field has been limited. Here, we investigate the bubble dynamics using various spatial patterns of inverse opal channels to induce preferential heat and mass flow site in highly-interconnected microporous media. A fully-coated inverse opal surface demonstrates the intrinsic boiling effects of a uniform microporous coating, which exhibits 156% enhancement in heat transfer coefficient in comparison to the polished silicon surface. The boiling heat transfer performances of spatially-variant inverse opal channels significantly differ based on the pitch spacings between the microporous channels, which dictate the bubble coalescent behaviors and bubble departure characteristics. The elucidated boiling heat transfer performances will provide engineering guidance toward designing optimal two-phase thermal management devices.
more »
« less
A unifying criterion of the boiling crisis
Abstract We reveal and justify, both theoretically and experimentally, the existence of a unifying criterion of the boiling crisis. This criterion emerges from an instability in the near-wall interactions of bubbles, which can be described as a percolation process driven by three fundamental boiling parameters: nucleation site density, average bubble footprint radius and product of average bubble growth time and detachment frequency. Our analysis suggests that the boiling crisis occurs on a well-defined critical surface in the multidimensional space of these parameters. Our experiments confirm the existence of this unifying criterion for a wide variety of boiling surface geometries and textures, two boiling regimes (pool and flow boiling) and two fluids (water and liquid nitrogen). This criterion constitutes a simple mechanistic rule to predict the boiling crisis, also providing a guiding principle for designing boiling surfaces that would maximize the nucleate boiling performance.
more »
« less
- PAR ID:
- 10408471
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Boiling is a high-performance heat dissipation process that is central to electronics cooling and power generation. The past decades have witnessed significantly improved and better-controlled boiling heat transfer using structured surfaces, whereas the physical mechanisms that dominate structure-enhanced boiling remain contested. Experimental characterization of boiling has been challenging due to the high dimensionality, stochasticity, and dynamicity of the boiling process. To tackle these issues, this paper presents a coupled multimodal sensing and data fusion platform to characterize boiling states and heat fluxes and identify the key transport parameters in different boiling stages. Pool boiling tests of water on multi-tier copper structures are performed under both steady-state and transient heat loads, during which multimodal, multidimensional signals are recorded, including temperature profiles, optical imaging, and acoustic signals via contact acoustic emission (AE) sensors, hydrophones immersed in the liquid pool, and condenser microphones outside the boiling chamber. The physics-based analysis is focused on i) extracting dynamic characteristics of boiling from time lags between acoustic-optical-thermal signals, ii) analyzing energy balance between thermal diffusion, bubble growth, and acoustic dissipation, and iii) decoupling the response signals for different physical processes, e.g., low-to-midfrequency range AE induced by thermal expansion of liquids and bubble ebullition. Separate multimodal sensing tests, namely a single-phase liquid test and a single-bubble-dynamics test, are performed to reinforce the analysis, which confirms an AE peak of 1.5 kHz corresponding to bubble ebullition. The data-driven analysis is focused on enabling the early fusion of acoustic and optical signals for improved boiling state and flux predictions. Unlike single-modality analysis or commonly-used late fusion algorithms that concatenate processed signals in dense layers, the current work performs the fusion process in the deep feature domain using a multi-layer perceptron regression model. This early fusion algorithm is shown to lead to more accurate and robust predictions. The coupled multimodal sensing and data fusion platform is promising to enable reliable thermal monitoring and advance the understanding of dominant transport mechanisms during boiling.more » « less
-
Surfaces with micrometer-scale pillars have shown great potential in delaying the boiling crisis and enhancing the critical heat flux (CHF). However, physical mechanisms enabling this enhancement remain unclear. This knowledge gap is due to a lack of diagnostics that allow elucidating how micro-pillars affect thermal transport phenomena on the engineered surface. In this study, for the first time, we are able to measure time-dependent temperature and heat flux distributions on a boiling surface with engineered micro-pillars using infrared thermometry. Using these data, we reveal the presence of an intra-pillar liquid layer, created by the nucleation of bubbles and partially refilled by capillary effects. However, contrarily to conventional wisdom, the energy removed by the evaporation of this liquid cannot explain the observed CHF enhancement. Yet, predicting its dry out is the key to delaying the boiling crisis. We achieve this goal using simple analytic models and demonstrate that this process is driven by conduction effects in the boiling substrates and, importantly, in the intra-pillar liquid layer itself. Importantly, these effects also control the wicking flow rate and its penetration length. The boiling crisis occurs when, by coalescing, the size of the intra-pillar liquid layer becomes too large for the wicking flow to reach its innermost region. Our study reveals and quantifies unidentified physical aspects, key to the performance optimization of boiling surfaces for cooling applications.more » « less
-
Abstract Squeezing bubbles in a tapered microgap has proved to be effective for improving flow stability in flow boiling. A previous study from our research group has successfully demonstrated using tapered microgap for transforming pool boiling into a self-sustained flow boiling-like system for cooling CPU through thermosiphon. To overcome the imaging challenges with nucleating vapor bubbles, the present work investigates the squeezing behaviour of air-injected bubbles between a tapered microgap with taper angles of 5°, 10°, and 15°. The air bubbles are injected at a rate of 3 ml/min, 15ml/min, and 30 ml/min in a pool of water. The bubble squeezing is recorded at 2000fps using a Photron high-speed camera. The experimental analysis compares the displacement and velocity of the advancing and receding bubble interfaces. The analysis found that in certain test cases, multiple bubbles coalesced while exiting the tapered microgap. In all the test cases, the receding interface of the bubble slingshots after detaching pushes the bubble out of the tapered microgap. The result from the current study provides an insight into the bubble flow and squeezing behavior that can be used for optimizing taper microgap geometries to enhance critical heat flux and heat transfer coefficient of two-phase, and air-injected single-phase heat transfer systems.more » « less
-
Nucleation and bubble dynamics on a heater surface contribute to high heat transfer rate in pool boiling. Introducing two-phase flow in narrow channels further improves heat transfer. Use of expanding taper microgap geometry further enhances heat transfer, and proper balancing of taper angles and flow lengths leads to self-sustained flow boiling in tapered microgap geometries. This paper focuses on understanding the underlying enhancement mechanism by studying the bubble behavior as they expand and accelerate in the direction of increased taper. The present study conducts a 2D simulation analysis of bubble growth in tapered microgaps with numerical simulations to identify the effect of the fluid properties and tapered angle in the bubble and fluid dynamics behavior. Ansys-Fluent is customized with user-defined-functions (UDFs) accounting for the interfacial heat and mass transport, including a sharp interface and direct calculation of mass transfer with temperature gradients. The study was conducted using air injection and boiling simulation from the conception to the departure of a bubble. The tapered angles were 5°, 10°, and 15°, with flowrates between 3 ml/min to 30 ml/min, 1 mm air inlet, and at 1 mm distance from the convergent end. The departure time of 10 subsequent bubbles was recorded to check the configuration with the quickest bubble removal. A critical flowrate and surface tension region was established for the escape direction of the bubble. In addition, the numerical simulation considered the tapered microgap with a nucleating bubble at atmospheric conditions with a wall superheats of 5 K. The results show that the bubble growing over the heated surface creates fluid circulations and interfacial conditions that suppress the thermal boundary layer leading to an increased local heat transfer coefficient within a range of 1 mm from the interface.more » « less
