Departure Diameter Research Articles

CFD study on film boiling features of cryogenic fluid influenced by heat structures and gravity levels

Understanding bubble behaviors and thermal characteristics under various working conditions is of great significance to the efficient utilization of cryogenic propellants. In the present study, simulation models of pool film boiling at horizontal plate and cylinder surface are proposed with Volume of Fluid (VOF) method combined with Lee’s phase change model. Based on the physical and thermal performance, the gravity levels, physical properties, and heater structure dimensions are investigated respectively, which could satisfy the common application conditions of cryogenic propellants. The results show that the movement of bubbles at gas–liquid interface is critical to the pool film boiling heat transfer, and also the heat transfer mechanism affects the bubble motion behaviors. The critical wavelength and the most dangerous wavelength are significant length scales for the bubble behaviors in pool film boiling. When the cylindrical heater size is larger than the most dangerous wavelength, the bubble behaviors as well as the heat flux appear the similar characteristics with that at the plate heater surface. Moreover, the critical wavelength is approximately two times of the bubble departure diameter in the present cryogenic liquid boiling events, and the parameters, including liquid–vapor density difference, surface tension, and gas thermal conductivity, could affect the boiling heat flux significantly. In addition, under 0.03g condition, the bubble departure diameter could reach 27 mm and the boiling heat flux decreases to 800 W/m2, which are significantly different from that in normal gravity. In general, the present CFD model could execute a film boiling study accurately, and a series of detailed results on the bubble behaviors as well as the heat flux could be revealed.

IR measurements of heat transfer coefficients and nucleation parameters during saturated nucleate boiling of water–glycerin mixtures

Heat transfer coefficients and nucleation parameters were experimentally measured during saturated nucleate boiling of water–glycerin mixtures on a thin smooth titanium foil. Infrared thermometry was employed to study boiling of water–glycerin mixtures at atmospheric pressure in water mass fraction range from 100 % down to 60 % and heat flux range from 0 to 200 kW m−2. Heat transfer coefficients were found to deteriorate with an increase of concentration of glycerin in the boiling mixture. The deterioration was more pronounced for mixtures with lower water mass fraction and for higher heat fluxes. Bubble departure diameters were observed to be weakly dependent on heat flux and independent of composition of the boiling mixture. Nucleation frequencies were found to increase with heat flux and to be inversely proportional to water mass fraction of the boiling mixture. Thermal energy transferred during nucleation of individual bubbles was independent of both heat flux and mixture composition. Values and trends of evaluated parameters suggest that for investigated mixtures and boiling conditions, heat transfer into the liquid phase is more important relative to the transport of latent heat by nucleating bubbles.

How Coalescing Bubbles Depart from a Wall.

Bubble evolution plays a fundamental role in boiling and gas-evolving electrochemical systems. One key stage is bubble departure, which is traditionally considered to be buoyancy-driven. However, conventional understanding cannot provide the full physical picture, especially for departure events with small bubble sizes commonly observed in water splitting and high heat flux boiling experiments. Here, we report a new regime of bubble departure owing to the coalescence of two bubbles, where the departure diameter can be much smaller than the conventional buoyancy limit. We show the significant reduction of the bubble base area due to the dynamics of the three-phase contact line during coalescence, which promotes bubble departure. More importantly, combining buoyancy-driven and coalescence-induced bubble departure modes, we demonstrate a unified relationship between the departure diameter and nucleation site density. By elucidating how coalescing bubbles depart from a wall, our work provides design guidelines for energy systems which can largely benefit from efficient bubble departure.

Visualized-experimental investigation on a mini-diameter loop thermosyphon with a wide range of filling ratios

The mini-diameter loop thermosyphon, which has an excellent heat transfer performance with simple, compact structure and pump-free advantages, can effectively solve the problem of heat transfer under high heat load in a confined space. The mini-diameter loop thermosyphon exhibits different gas-liquid transport and heat transfer patterns than the conventional-diameter one due to various gas-liquid phase transformation properties and a strong friction force in the mini-diameter channel. Furthermore, data on the visual analysis of gas-liquid transport and heat transfer properties of the mini-diameter loop thermosyphon under various filling ratios is still lacking. In this paper, a visible experimental research on a mini-diameter loop thermosyphon (3 mm) in a wide filling ratio range(44% -97%) is undertaken. The maximal heat transfer capacity increases initially and subsequently falls with the filling ratio, according to the testing data. At a low filling ratio, the flow pattern changes as slug flow - churn flow - annular flow with the heat load increase, leading to a low loop thermal resistance, and dry out occurs at 185 W heat load under 50% filling ratio. The gas-liquid flow shifts to slug flow-geyser boiling-bubble flow at high filling ratios (73% and 82%), and the bubble departure diameter steadily decreases with increasing heat load. Consequently, the heat transfer performance (thermal resistance) has been enhanced, and the maximum heat transfer capacity has been increased to 280 W. Single-phase flow is plainly seen in the loop when the filling ratio reaches 91% or even 97%, thus reduced heat transfer capacity. Therefore, the high-filling-ratio mini-diameter loop thermosyphon is better for high-load heat dissipation. However, a low thermal resistance shows at the low filling ratio, but the heat load is also low. As a result, a loop thermosyphon with a low filling ratio should be selected for the low heat load situation.

Wetting States and Departure Diameters of Bubbles on Micro-/Nanostructured Surfaces.

Wetting states for droplets have been extensively investigated in the past. As the counter phase of the droplets, bubbles' wetting states have rarely been systematically explored. The wetting state of a bubble is closely related to its departure diameter, which plays significant roles in bubble-generated processes in boiling heat transfer and gas-evolving reactions. Based on the principle of minimum surface energy, we explicitly define three equilibrium wetting states (hemi-wicking state, Wenzel state, and Cassie-Baxter state) for bubbles on micro-/nanostructured surfaces in this paper. We analyze the three-phase contact line profiles for bubbles under these wetting states and propose theoretical models for predicting departure diameters of hemi-wicking-state bubble and Wenzel-state bubble on micro-/nanostructured surfaces. We identify competing effects of bubble departure in Wenzel state: the augmentation of contact line length due to the roughness, which would delay bubble departure, and the decrease of contact line length due to the reduced apparent contact angle, which would facilitate bubble departure. We demonstrate that hemi-wicking-state bubble exhibits a much smaller departure diameter on the textured surfaces. These findings are supported by numerical simulations by the three-dimensional (3D) multiple-relaxation-time lattice Boltzmann method. It is found that the length of the outermost contact lines instead of all contact lines determines the departure diameter of hemi-wicking-state bubble based on bubble detachment processes captured by our 3D numerical simulations. This work offers an avenue for the accurate prediction and control of bubble departure behaviors from micro-/nanostructured surfaces, and therefore can guide optimal designs of micro-/nanostructured surfaces in a variety of applications in boiling, desalination, and hydrogen production by electrolysis.

Bubble dynamics of R-123 and R-134a on pore/sub-tunnel surfaces

Structured enhanced surfaces are widely used to promote nucleate boiling heat transfer in refrigeration and process industries. Despite the frequent usage of the enhanced surfaces, there appears a significant lack of understanding on the boiling mechanism or predictive models. To enhance the understanding, the bubble dynamic data, which are the building blocks of the predictive models, are of necessity. This study continues that by Mehdi et al. [15], who reported R-134a bubble dynamic data on a pored surface. In this study, the tests were extended to R-123. The nine samples included those having 0.1–0.3 mm pore diameters and those having 0.75–3.0 mm pore pitches. The results showed that, R-123 yielded larger bubble departure diameters and more nucleation site densities than R-134a. The reason was attributed to the larger surface tension and liquid thermal conductivity of R-123 over R-134a. For both refrigerants, the trends of bubble dynamics according to the pore geometry were almost identical. The bubble departure diameter and the bubble generation frequency increased as the pore diameter increased. The nucleation site density increased as the pore pitch decreased. An improved model was developed extending that of Chien and Webb [13]. The model reasonably predicted the heat transfer coefficients as well as the bubble dynamic data of the present surfaces (95% of the heat transfer coefficients within ± 40%).

Assessment of wall heat flux partitioning model for two-phase CFD

The Eulerian-Eulerian two-phase flow (EETF) model coupled with the Wall Heat Flux Partitioning (WHFP) model is slowly becoming an effective tool for two-phase hydrothermal analysis in nuclear applications, with prediction accuracy highly dependent on sub-models for three bubble parameters, namely nucleation site density (N), bubble departure diameter (Dw) and bubble departure frequency (f). In this study, the preformance of different sub-models combinations is evaluated under a wide range of conditions, especially focusing on the prediction of void fraction. The four most commonly used or investigator-recommended combinations are identified by a literature review. A large database of flow boiling experiments covering 52 sets of experiments is developed for the assessment, rather than the popular case-by-case adjustment for a limited number of experiments. One of the combinations is distinguished to be the optimal combination, with an overall 44.2% mean absolute error (MAE). A nearly fixed relationship between void fraction and steam quality is predicted by this combination. It suggests that there is an ‘optimal’ range of operating conditions for which the combination can accurately predict the results. Based on the current assessment, for water, this range is between 17 and 97 K for subcooling and a mass flux larger than 770 kg/(m2s). However, the prediction of bubble diameter and gas velocity are not accurate, since the Kurul and Podowski model simply assumes the bubble diameter as the function of liquid temperature. Models considering bubble coalescence and breakup are desired to predict the bubble diameter and gas velocity more accurately. It is also shown that the performance of combinations, which are genreally considered to be applicable to a broader range, is not satisfactory. Experimental measurements of bubble departure diameter and nucleation site density under a wider range of conditions are desired to evaluate those sub-models.

Experimental investigation and new correlation for the bubble departure diameter of methane and ethane binary mixtures during nucleate pool boiling

Methane and ethane are the main components of liquefied natural gas (LNG) and mixed-refrigerant liquefaction cycles. The pool boiling characteristics and bubble dynamic parameters of methane/ethane mixtures are important for the design and optimization of LNG systems. In this study, visual experiments were carried out to investigate the pool boiling of methane, ethane and their mixtures on a horizontal flat surface. Heat transfer characteristics and bubble departure diameters were acquired at 0.3 MPa for different concentrations of the mixtures. The bubble departure diameter increases with the increase in superheat temperature and dimensionless Jacob number for all concentrations of the mixture. The non-linear variation with concentrations of the bubble departure diameter can be observed due to the effects of mixtures. The bubble departure diameters for mixtures were predicted by twelve existing correlations, with Cole's correlation having the lowest mean absolute relative deviation (MARD) of 46.28%, with only 38.78% points within ±30% deviation. Therefore, a new model for mixtures was developed based on force equilibrium analysis in this study. Marangoni force is taken into account in the proposed model, as well as the effects of mass transfer on the bubble growth rate in mixtures. The MARD of the new model is 18.78%, which is better than all the twelve existing correlations.

Separate effect of oxidation on the subcooled flow boiling performance of Zircaloy-4 at atmospheric pressure

In this paper, we present the results of an experimental investigation aimed at elucidating the separate effect of oxidation on the subcooled flow boiling performance (i.e., heat transfer coefficient and critical heat flux) of Zircaloy-4. We also present the results of exploratory tests which we conducted in preparation to this investigation, which demonstrate that even nominally identical unoxidized samples (i.e., with the same exact roughness and wettability) can have very different boiling dynamics and boiling performance under the same exact operating conditions. Such differences can be even larger than those between unoxidized and oxidized samples. Thus, to carefully separate the effect of oxidation, as part of this work, we developed a special testing and characterization protocol. We conducted boiling tests on the same exact Zircaloy-4 sample, from unoxidized conditions throughout its pre-transition, transition and post-transition oxidation stages. All the flow boiling tests were run at atmospheric pressure with degassed and deionized water at 1 bar, with a subcooling of 10 K and a flow rate of 1000 kg/m2/s. Together with boiling experiments, we characterized the surface morphology at the micro- and nano-scale in order to clarify how the oxidation-induced changes of surface properties affect the performance and the dynamics of the boiling process. Crucially, to eliminate random effects, we conducted this characterization on the same exact spot of the boiling surface at all stages of oxidation. Our results reveal that, differently from pool boiling studies in the literature, the oxidation of the surface has little or no impact on the dynamics (i.e., on nucleation site density, bubble growth time and departure frequency, and bubble size) and the performance of subcooled flow boiling, except for the post-transition oxide. This oxide has smaller bubble departure diameter and growth time, higher nucleation site density, and a higher critical heat flux limit. This behavior seems to be triggered by micron-scale cracks on the oxidized surface, which make the surface wick liquid.

Numerical study of interfacial dynamics in flow boiling of R134a inside smooth and structured tubes

A numerical study of flow boiling has been performed with R134a refrigerant flowing inside a horizontal tube. Flow pattern has been captured using VOF methodology in OpenFoam software up to mass flux of 150 kg/m2s. The various stages of bubble dynamics like nucleation, growth, merging and departure has been observed for ranges of heat and mass fluxes. The phase fraction variations during flow boiling have also been observed at two different saturation temperatures and the differences in interfacial features have been reported. An attempt has been made to estimate the number of nucleation sites and the bubble departure diameter for R134a in the case of flow boiling inside the smooth tube. With the help of numerical simulations, the study has also been made by providing artificial nucleation sites in the form of indentations/cavities inside the tube. An increase in nucleation sites has been observed for the dimpled tube as compared to the plain tube and the corresponding change in bubble departure phenomenon has been reported. The heat transfer coefficients at different heat flux, mass flux, saturation temperature and tube configurations have been evaluated to link observed interfacial phenomenon with enhancement factor.

Nanotexturing-enhanced heat transfer and interfacial evaporation for energy-efficient solar-thermal water desalination

Seawater desalination based on nano-interface-enhanced evaporation is one of the promising energy-effective solutions to alleviate global clean water scarcity. However, the heat and mass transfer mechanisms from the absorber surface to the surrounding liquid, which is critically significant to improve the evaporation performance in practical applications, are still unclear. Here, we develop an all-carbon hierarchical architecture via engineering of vertically oriented graphenes on a graphite felt monolith, and simultaneously, gain new insights into the effects of the surface morphology on the heat and mass transfer at the absorber/liquid interface by manipulating the graphene texturing from microscale to nanoscale. In the visualization experiments, compared with the microstructured and smooth surfaces, a higher-density bubble distribution with a smaller departure diameter on the nanostructured surface is observed, leading to the significant enhancement of heat transfer through the micro-convection and transient conduction mechanisms. Specifically, the heat transfer coefficient value of the nanostructured surface is more than 1.5 times that of the microstructured surface, thus achieving an increment of ∼5% in freshwater productivity.

A New Method for Estimating Bubble Diameter at Different Gravity Levels for Nucleate Pool Boiling

Abstract Nucleate boiling has significant applications in earth gravity (in industrial cooling applications) and microgravity conditions (in space exploration, specifically in making space applications more compact). However, the effect of gravity on the growth rate and bubble size is not yet well understood. We perform numerical simulations of nucleate boiling using an adaptive moment-of-fluid (MoF) method for a single vapor bubble (water or Perfluoro-n-hexane) in saturated liquid for different gravity levels. Results concerning the growth rate of the bubble, specifically the departure diameter and departure time, have been provided. The MoF method has been first validated by comparing results with a theoretical solution of vapor bubble growth in superheated liquid without any heat-transfer from the wall. Next, bubble growth rate, bubble shape, and heat transfer results under earth gravity, reduced gravity, and microgravity conditions are reported, and they are in good agreement with experiments. Finally, a new method is proposed for estimating the bubble diameter at different gravity levels. This method is based on an analysis of empirical data at different gravity values and using power-series curve fitting to obtain a generalized bubble growth curve irrespective of the gravity value. This method is shown to provide a good estimate of the bubble diameter for a specific gravity value and time.

Investigation of subcooled boiling wall closures at high pressure using a two-phase CFD code

This study validates the applicability of the CUPID code for simulating subcooled wall boiling under high-pressure conditions against number of DEBORA tests. In addition, a new numerical technique in which the interfacial momentum non-drag forces are calculated at the cell faces rather than the center is presented. This method reduced the numerical instability often triggered by calculating these terms at the cell center. Simulation results showed good agreement against the experimental data except for the bubble sizes in the bulk. Thus, a new model to calculate the Sauter mean diameter is proposed. Next, the effect of the relationship between the bubble departure diameter (Ddep) and the nucleation site density (N) on the performance of the Wall Heat Flux Partitioning (WHFP) model is investigated. Three correlations for Ddep and two for N are grouped into six combinations. Results by the different combinations show that despite the significant difference in the calculated Ddep, most combinations reasonably predict vapor distribution and liquid temperature. Analysis of the axial propagations of wall boiling parameters shows that the N term stabilizes the inconsistences in Ddep values by following a behavior reflective of Ddep to keep the total energy balance. Moreover, ratio of the heat flux components vary widely along the flow depending on the combinations. These results suggest that separate validation of Ddep correlations may be insufficient since its performance relies on the accompanying N correlations.

Enhanced pool boiling heat transfer by metallic nanoporous surfaces under low pressure

The nano-structure enhanced pooling boiling at standard atmosphere has been fully studied in literature but it is rarely investigated under low pressure. However, the boiling behaviors and bubble dynamics of nano-structures highly depend on the working pressure. This study investigated the enhanced pool boiling (under low pressure) of a nanoporous copper surface (NCS) fabricated by the hot-dip galvanizing & dealloying. The boiling curves and bubble characteristics of the NCS under low-pressure boiling conditions (25 kPa and 65 kPa) were evaluated by a test set up with high-speed visualization. To validate the present experimental plain-wall HTC under low pressure, three predicted models (Labuntsov, modified Rohsenow, and Stephan-Abdelsalam) were applied to obtain the theoretical HTCs for comparison. Besides, the optimization of the galvanization time has been conducted. Experimental results showed that the NCS significantly improved the pool boiling heat transfer and mitigated the negative influence of sub-atmospheric conditions. The enhanced mechanisms are the augmented nucleate site density, decreased bubble departure diameter, and increased bubbles generation frequency induced by the promoted hydrophilicity and rough morphology of the NCS. Among the three analyzed samples, the NCS-500 °C-3 min surface yielded the lowest wall superheat at the onset of boiling (ONB) and the highest heat transfer coefficient (HTC). In addition, the nanoporous surface leads to higher HTC enhancement upon the plain surface at a lower pressure. Specifically, the HTC enhancement percentage was increased from 71.6% (at 65 kPa) to 139% (at 25 kPa) as the pressure decreased.

Boiling at subatmospheric pressures on hydrophobic surface: Bubble dynamics and heat transfer

It is well–known that the physical and chemical properties of a heat exchange surface, especially wettability, significantly affect the pool boiling performance. Nevertheless, the influence of wetting properties on the evolution of vapor bubbles, the heat transfer rate and the crisis phenomena development, especially at subatmospheric pressures, remains a poorly studied research area. The aim of current research is to investigate the evolution of two-phase flows and heat transfer rate during water boiling on a hydrophobic surface within the pressure range 11.3–102.8 kPa. The experiments were carried out with the use of a specially designed transparent heated substrate with sprayable hydrophobic coating, and integrated high-speed experimental techniques including visualization and IR thermography. It was revealed that the regularities of the bubble evolution with pressure reduction during boiling on hydrophobic surfaces significantly differ from hydrophilic surfaces. In particular, it was shown that during boiling on a hydrophobic surface the bubble departure diameter does not change and bubble emission frequency increases significantly with the reduction of pressure, which is inconsistent with the trends observed during boiling on a hydrophilic surface. Moreover, the bottom side visualization allowed us to analyze the evolution of the void fraction and bubble site density, the sizes of the dry spots under the sessile bubbles during boiling on the hydrophobic surface with pressure change. It was shown that superheats corresponding to the onset of nucleate boiling and heat transfer rate on the hydrophobic surface don't depend on pressure, while the heat transfer enhancement was observed at low heat fluxes in comparison with hydrophilic surface.

Bubble dynamics of R-134a/POE and R-123/MO mixture on enhanced surfaces having pores on sub-tunnels

Structured enhanced surfaces are widely used to promote the nucleate boiling heat transfer in a refrigeration system, where a compressor oil circulates. However, there is a significant lack of understanding of the mechanism or predictive models, especially on the boiling of refrigerant/oil mixtures on enhanced surfaces. In this study, as a building block, the bubble dynamic data as well as the heat transfer coefficients were obtained for the refrigerant/oil mixtures on pored surfaces. The test mixtures included R-134a/polylester (POE) and R-123/mineral oil (MO). The results showed that the oil degraded the size of the departing bubbles. The degradation was more severe for R-123/MO than R-134a/POE, and it increased as the saturation temperature increased. The sequence of degradation among samples generally followed the reverse order to the magnitude of the heat transfer coefficient of the pure refrigerant. Similarly, the oil degraded the frequency. The degradation was, in general, in reverse order to the degradation of the bubble departure diameter. The oil also degraded the nucleation site density. As for the heat transfer coefficient, the degradation was more significant at a higher saturation temperature, and for R-134a/POE than for R-123/MO. The changes of bubble dynamic parameters as well as the thermo-physical properties of the refrigerants were shown to be responsible. A heat transfer prediction model was developed based on the bubble dynamic parameters.

Numerical simulation of single bubble growth and heat transfer considering multi-parameter influence during nucleate pool boiling of water

Nucleate boiling is an essential heat exchange method in industry. Due to the complicated mechanisms of the boiling process, prediction of its heat transfer performance has always been challenging. Studying the growth and heat transfer characteristics of a single bubble is of great significance for understanding the internal mechanisms of nucleate boiling. In this paper, single bubble growth in saturated pool boiling was numerically studied. The microlayer evaporation at the bubble base was calculated and added to the simulation via UDFs. The effects of contact angle, wall superheat, and microlayer initial thickness distribution were investigated. The results show that a thinner microlayer has a greater evaporation rate, resulting in an increased bubble growth rate and departure diameter. The bubble growth rate, departure time, and departure diameter increase with the increase in the contact angle and wall superheat. As the wall superheat increases, there is a competition between microlayer evaporation and phase interface evaporation, which ultimately leads to an extremum of the total contribution of microlayer evaporation to bubble growth. The heat transferred by microlayer evaporation accounts for at least 67.3% and up to 83.6% of the bubble's latent heat at departure, indicating that microlayer evaporation has a significant impact on bubble growth.

CNN-based visual analysis to study local boiling characteristics

Visual analysis allows an estimate of different local boiling characteristics including bubble growth rate, departure diameters and frequencies of nucleation, nucleation site density and evolution of bubbles and dry spots in time. At the same time, visual determination of the presented characteristics in case of big amounts of data requires the development of the appropriate software which will allow not only determination of bubble location, but also an estimate of their sizes based on high-speed video. The presented problem can be solved by using the instance segmentation approach based on a convolutional neural network. In the presented work Mask R-CNN network architecture was used for estimation of the local boiling characteristics.

Enhancement of pool boiling heat transfer using 3D-printed groove structure

Groove structures were fabricated by selective laser melting (SLM) technique to enhance saturated boiling heat transfer at atmospheric pressure. Prepared groove structures have different groove lengths and heights, and the printing material used is AlSi10Mg. Pool boiling experiments were explored with deionized water as the working fluid, and the heat transfer performance of different groove structures was compared based on smooth surface. It was found that the groove structure can significantly increase the boiling heat transfer coefficient (HTC) and critical heat flux (CHF) compared with the smooth surface. With the increase of groove length, the CHF of the groove structure gradually increases until it reaches the maximum at the groove length of 2.2 mm, which corresponded to the bubble departure diameter. Maximum CHF and HTC improvements were 3.1 times and 2 times, respectively. The HTC and CHF of groove structure decrease with the increase of groove height, which is due to the increase of liquid and vapor flow resistance in the groove. Difference of HTC of samples with the same structure size and different height declines with the increase of groove length. Groove structure facilitates gas-liquid flow and separation, thereby enhancing boiling heat transfer. The semi-melted powder bonding on the surface of the 3D-printed skeleton and the groove structure work together to improve the boiling heat transfer performance.

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

In this study, pool boiling heat transfer of de-ionized water was experimentally studied on a scored copper surface at a heat-flux range of 0 - 60 W/cm2. Bubble dynamics in an isolated bubble region were carefully investigated, including bubble departure diameters, bubble departure frequencies, and active nucleation site densities. The bubble dynamics were compared with available models, indicating the suitable models regarding the present experimental results. Then, based on the bubble dynamics, a mechanistic heat transfer model, developed in our previous studies, was employed to predict the present boiling curve. In the mechanistic model, heat fluxes from natural convection, transient heat conduction, and microlayer evaporation were incorporated.