Bubble dynamics and mechanistic boiling heat transfer prediction on a scored copper surface

Nucleate pool boiling heat transfer of acetone and HFE7200 on copper surfaces with nanoparticle coatings

A critical review on bubble dynamics parameters influencing boiling heat transfer

Analysis of vapor bubble diameter, departure frequency and dynamics in a single cavity

Understanding the detailed dynamics of bubble ebullition cycle is central to effectively exploit coolant phase change in subcooled flow boiling. In the present study, the bubble growth rate and the bubble departure mechanism in subcooled flow boiling conditions are investigated. The bubble growth rate is estimated by employing an energy balance model, which ensures that the applied wall heat flux contribution to components, such as (i) microlayer evaporation, (ii) conduction through superheated layer region, and (iii) condensation heat transfer, are accounted. The bubble departure diameter and the type of departure (i.e., sliding or lift-off) are predicted using a simple force balance model on the vapor bubble. The implemented models are thoroughly validated against the benchmark experimental data. Furthermore, the influence of operating conditions, viz., pressure (1–3 bar), mass flux (200–1000 kgm−2s−1), heat flux (200–500 kWm−2), and the degree of subcooling (20–30 K) on the bubble dynamics, is investigated for subcooled flow boiling conditions. Based on the parameters considered, a flow regime map is developed to identify the bubble departure type and diameter for a given set of operating conditions. It was noticed that the bubble departure diameter was maximum at low pressure (1 bar), low mass flux (200 kgm−2s−1), high heat flux (500 kWm−2), and low degree of subcooling (20 °C). For all the values of pressure and degree of subcooling, the sliding mode of departure was noticed at low and high mass flux values, whereas bubble departs by lift-off for moderate values of mass flux.

Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

Saturated flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip

Lattice Boltzmann study of bubble dynamics and heat transfer on a hybrid rough surface with a cavity-pillar structure

Experiments to understand bubble base evaporation mechanisms and heat transfer on nano-coated surfaces of varying wettability under nucleate pool boiling regime

Two-phase slug flow exhibits intrinsically stochastic and statistically periodic flow behaviors. Understanding the mechanisms of flow and heat transfer of upward two-phase slug flow is important. This paper develops a mechanistic model of flow and heat transfer for upward two-phase slug flow in vertical pipes. First, a hydrodynamic model of two-phase slug flow in regular-sized channels is developed based on the hypothesis of the slug unit cell. Each slug unit cell is hypothesized to be composed of a liquid slug and a Taylor bubble region. Second, a mechanistic heat transfer model is derived based on the hydrodynamic model. The overall heat transfer coefficient is integrated by using the local heat transfer coefficients of liquid slug and the Taylor bubble region. Third, the proposed mechanistic model is validated by the experimental void fraction, pressure drop, and two-phase heat transfer coefficient from different sources. Finally, the effect of flow geometry and parameters—such as superficial gas and liquid velocities, void fraction, slug frequency, pressure drop, and the ratio of slug length to unit cell length—on heat transfer in two-phase slug flow is comprehensively investigated. The enhancement in heat transfer in two-phase slug flow compared with that of single-phase flow can be attributed to an increase in the turbulence of the liquid due to the injection of air and a reduction in the thermal boundary layer owing to the frequent alternation between liquid slug and the Taylor bubble.

We develop a neural network model capable of predicting the margin to the boiling crisis (i.e., the departure from nucleate boiling ratio, DNBR) from high-resolution infrared measurements of the bubble dynamics on surfaces with different morphologies and wettability (or wickability). We use a feature ranking algorithm, i.e., minimum redundancy maximum relevance, to elucidate the importance of fundamental boiling parameters, i.e., nucleation site density, bubble departure frequency, growth time, and footprint radius, in predicting the boiling crisis. We conclude that these parameters are all necessary and equally important. This result has profound implications, as it undermines the general validity of many observations and mechanistic models that attempt to predict the critical heat flux (CHF) by describing how a single boiling parameter changes with the heat flux or from one surface to another. Notably, the neural network model can predict the DNBR on CHF-enhancing surfaces of different wickability without using any input information related to the surface properties. This result suggests that, at least on the considered surfaces, surface wickability enhances the CHF by modifying the bubble dynamics, i.e., the aforesaid boiling parameters, rather than acting as an additional heat removal mechanism.

Saturated flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct

Boiling heat transfer and bubble dynamics in microgravity

The generation and departure of bubbles are closely related to the boiling heat transfer efficiency. To investigate the bubble boiling characteristics on a heated micro-wire, platinum wires with diameter of 0.1 and 0.2 mm were used as the experimental material. The bubble dynamics over a horizontal heated wire and an inclined heated wire were investigated by a high-speed camera and an electronic microscope. The influences of heat flux and subcooled condition on the bubble departure diameter and the departure frequency were analyzed. The results showed that three kinds of bubble motion track were observed over the heated micro-wires under subcooled condition, including jet flow, sweeping, and rotation. The multi-jet flow phenomenon could be seen over the bubble on the heated 0.2 mm wire under the conditions of heat flux of 1.7 × 105 W m−2 and mainstream temperature of 36 °C. As the diameter increased, the bubbles grown on the lower side of the heated wire would gradually rotate over the wire. There were three phenomena observed on the 0.1 mm wire when two bubbles collided under conditions of 2.9 × 105 W m−2 heat flux and 34 °C mainstream temperature. For an inclined micro-wire with the inclined angle of 25°, the movement of the bubble would only move along the wire to the tip and eventually break away. When the mainstream temperature was 40 °C, the bubbles departure diameter and the frequency increased with the increase in heat flux. When the heat flux was 3.0 × 105 W m−2, the bubble departure diameter and frequency increased with the increase in temperature.

Among the different ways of achieving heat transfer enhancement in pool boiling, modification of surface roughness and the associated change in the surface wettability are two widely studied methods. It is well established that microstructure can play an important role in varying surface roughness and improving interfacial wettability. In the present study, pool boiling heat transfer on a number of micro grooved and flat (brass and copper) surfaces with water as the working fluid is investigated experimentally for enhancement of critical heat flux. The micro structured sample surfaces feature parallel grooves having dimensions of tens of microns and are fabricated using a mechanical micromachining process. Classical pool boiling experiments are conducted for three different values of heat flux (75 kw/m2, 100 kw/m2, 125 kw/m2). The bubble nucleation process is visualized and the bubble departure diameter is measured with the help of a high speed camera. It is observed that the nucleation site density and departing bubble diameter increases, while the bubble departure frequency decreases with an increasing heat flux for both kind of surfaces and materials. Comparison of the results among the flat and micro-grooved surfaces reveals that the boiling on micro-grooved surfaces results in higher heat transfer coefficient, more nucleation sites, lower bubble departure frequency and larger bubble diameter under similar operating conditions.Among the different ways of achieving heat transfer enhancement in pool boiling, modification of surface roughness and the associated change in the surface wettability are two widely studied methods. It is well established that microstructure can play an important role in varying surface roughness and improving interfacial wettability. In the present study, pool boiling heat transfer on a number of micro grooved and flat (brass and copper) surfaces with water as the working fluid is investigated experimentally for enhancement of critical heat flux. The micro structured sample surfaces feature parallel grooves having dimensions of tens of microns and are fabricated using a mechanical micromachining process. Classical pool boiling experiments are conducted for three different values of heat flux (75 kw/m2, 100 kw/m2, 125 kw/m2). The bubble nucleation process is visualized and the bubble departure diameter is measured with the help of a high speed camera. It is observed that the nucleation site density and d...

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.