Subcooling Temperature Research Articles

High jump of impinged droplets before Leidenfrost state.

Unlike the traditionally reported Leidenfrost droplet which only floats on a thin film of vapor, we observe a prominent jump of the impinged droplets in the transition from the contact boiling to the Leidenfrost state. The vapor generation between the droplet and the substrate is vigorous enough to propel the spreading droplet pancake to an anomalous height. The maximum repellent height can be treated as an index of the total transferred energy. Counterintuitively, a stronger vaporization and a higher jump can be generated in the conditions normally considered to be unfavorable to heat transfer, such as a lower substrate temperature, a lower droplet impact velocity, a higher droplet temperature, or a lower thermal conductivity of the deposition on the substrate. Since the total transferred energy is the accumulation of the instantaneous heat flux during the droplet contacting with the substrate, it can be deduced that a longer contact time period is secured in the case of a lower instantaneous heat flux. The inference is supported by our experimental observations. Moreover, the phase diagrams describe the characteristics of the high repellency under different substrate temperatures, droplet subcooling temperatures, and Weber numbers. It allows us to manipulate the droplet jump for the relative applications.

Influence of different parameters of preparing self-assembled monolayers on copper surfaces in the dropwise condensation heat transfer: an experimental study

Dropwise condensation (DWC) is one of the phase change heat transfer regimes, which due to its high capacity has received much attention from investigators. To produce DWC, it is necessary to have hydrophobic surfaces. Low surface energy coatings have been usually used to produce hydrophobic surfaces. These coatings due to their low thermal conductivities must have the minimum thickness. Therefore, self-assembled monolayers coatings like stearic acid or 1-octadecanethiol coatings (with thicknesses as high as a few nanometers) have been used in the literature. To create hydrophobic coatings by these materials, the surfaces are immersed in hydrophobic solutions with particular concentrations and in a specific time. There is not any general investigation on the effects of the time and concentration of these coatings on DWC in the literature. In this study, first, a proper apparatus for DWC experiments is designed and fabricated. Then, the effects of concentration and immersing time of the two mentioned materials on DWC phenomena have been studied. Between four coating times and solution concentrations of 1-octadecanethiol, concentration of 0.025 M at coating time of 30 min increases DWC heat transfer coefficient by about 4–7 times with respect to filmwise condensation in different subcooling temperatures. Concentration of 0.0025 M at coating time of 1 hour of stearic acid also increases the heat transfer coefficient by about 3–4 times. As well as the 1-octadecanethiol coating has a more uniform structure and higher heat transfer coefficient with respect to stearic acid coating.

Analysis and comparison on condensation performance of core tubes in air-cooling condenser

A numerical simulation was presented to investigate condensation performance of core tubes in air-cooled condenser (ACC). The simulation had been performed in three iso-sectional tubes varying pattern from flat, oval to round. The local heat transfer coefficient (HTC) under the changes of tube inclination angle (β), vapor saturated temperature (Ts), vapor-inlet velocity (U), and sub-cool temperature (ΔTcool) was illustrated to verify the influence of these factors on heat performance of each individual tube. Meanwhile, the comparison on overall heat-flow characteristics among three tubes was also carried out. Regardless of the tube pattern, it was shown that a large β in 0–30°, a high Ts in 316–342 K, and a great U in 103–195 m/s effectively enhanced the local HTC, but a high ΔTcool in 2–8 K reduced it. A nearly twofold increase in Nusselt number was achieved by the round tube compared to flat and oval tubes in a negligible raise of friction coefficient, meanwhile the flat and oval tubes had very close value of performance evaluation criteria. This study highlighted the tube pattern effect to thermal-hydraulic development on the vapor-side of an ACC finned tube system.

MoSe2 nanosheets embedded in mesoporous carbon as anode materials for sodium ion batteries

MoSe2 nanosheets embedded in a mesoporous carbon matrix is synthesized through a facile route, where molybdenum source is recrystallized forming nanocrystals and further react with selenium. To obtain fine nanocrystals, freeze-drying technique is used to creating a large subcooled temperature. With low MoSe2 content of ~ 54 wt%, the mesoporous MoSe2/C composite shows an initial capacity of 450 mA h g−1 and a reversible capacity of 276 mA h g−1 after 200 cycles.

Theoretical investigation of two-phase flow instability between parallel channels of natural circulation in rolling motion

In present work, the two-phase flow instability between rectangular parallel channels of natural circulation under static and rolling conditions was coupled studied theoretically. Models including two-phase flow instability, natural circulation system components, and the additional force were established in combination based on the homogenous model. A computational program was written in FORTRAN language which was solved by Gear multi-value method by using control volume integrating method. The program was validated with experiments, and the results matched well with the experiment data. The marginal stability boundary (MSB) maps under different parameters were obtained by using nondimensional numbers Nsub and Npch. The influence of different kinds of pressure drop, inlet subcooling temperature of heating channels, system pressure, valve resistance, venturi flowmeters resistance, structure height, rolling condition, and the interaction effect of natural circulation and two-phase flow instability between rectangular parallel channels were analyzed. The results show that with the increase in system pressure and venturi flowmeters resistance, the system stability of natural circulation is enhanced. The influence of inlet subcooling temperature is nonlinear. The increase in valve resistance leads to the instability of system. The increase in structure height does not change system stability significantly. The rise in rolling angle and period both reduce the system stability.

NUMERICAL SIMULATION OF SUBCOOLED BOILING INSIDE THE NARROW RECTANGULAR CHANNEL OF FUEL PLATE FOR DOWNWARD FLOW

In this research, considering the structural parameters and operating conditions for NP reactors, a numerical simulation of downward sub-cooled boiling flow in a vertical rectangular channel under atmospheric pressure was carried out by VOF model of the FLUENT. The Lee model of FLUENT was used to realize the phase transition process of water liquid and water vapor. And CSF (Continuum Surface Force) model was adopted to handle surface tension. The wall super-heat at the Onset of Boiling (ONB) was calculated under different conditions. The simulation results were consistent with experimental data. The impact of different parameters on ONB was analyzed which showed that the distance from the entrance to ONB obviously increases with increasing inlet velocity and subcooled temperature. The increase of heat flux improved the wall super-heat at ONB, but no significant effect of inlet subcooled temperature and inlet velocity was found. In addition, the heat transfer coefficient along the direction of flow was calculated and analyzed. The results showed that the heat transfer coefficient increased significantly with increasing heat flux. However, it was not a simple monotonic relationship between heat transfer coefficient and the inlet velociety or inlet temperature. Moreover, the ONB and the heat transfer coefficient of present work were compared with the case of single-heated.

HEAT PUMP SYSTEM EFFICIENCY COMPARISON OF DIFFERENT RENEWABLE ENERGY SOURCES – A FAMILY HOUSE CASE STUDY IN ZAGREB CITY AREA

The wider Zagreb city area, the capital city of the Republic of Croatia, has signifi cant potential for major implementation of geothermal heat pump systems in not just the residential sector, but in the commercial sector as well. Geothermal heat pumps often cumulatively describe diff erent earth energy heat exchanger installations, from groundwater up to shallow ground horizontal and borehole heat exchanger installations. The Zagreb area is especially favourable for both hydrogeological and thermogeological parameters, which allows for the implementation of diff erent designs for residential and commercial heating and cooling. An analysis of comparative energy and economic advantages of heating and cooling with heat pump systems (air, ground or groundwater) was made for the capital city of Zagreb. Since heat pump systems in residential areas are always a higher- class investment, there is often a need for year- round heating since projects increasingly incorporate outdoor swimming pools. Up till now, many authors investigated a comparison of winter heating energy costs for diff erent heat pump systems. However, outdoor swimming pools sometimes consume as much heating energy during the spring- summer-autumn seasons, as do houses in the autumn- winter-spring seasons. Since air temperatures during the spring-summer seasons are much more favourable than sub-cooled ground or groundwater temperatures, air-source heat pumps when used during the whole year for heating, could off set an economically unfavourable coeffi cient of performance (COP) during the coldest days of winter. On a real residential home, currently in the construction phase, techno-economic analysis was performed comparing the seasonal performance factor (SPF) for different heat pump systems during year-round heating.

Enhanced flow boiling in silicon nanowire-coated manifold microchannels

An experimental study was carried out to investigate the heat transfer, pressure drop and flow instability characteristics associated with the flow pattern of deionized water during two-phase boiling in a silicon-based manifold microchannel heat sink coated with silicon nanowires (SiNWs) compared to a plain-wall device. The manifold microchannel device featured parallel transverse microchannels etched on a silicon substrate and longitudinal microchannels etched on a glass cover plate. Silicon nanowires were generated on the bottom and the sidewalls of the silicon microchannels. A closed-loop experimental system was constructed to demonstrate thermal and hydraulic performance. Experimental results were presented with mass fluxes ranging from 250 to 1250 kg/m2 s and subcooled inlet temperatures from 15 K to 65 K. Results for the SiNWs device showed an approximate 20% improvement in heat flux rejection compared to the plain-wall device under the same wall superheat conditions. A subcooled inlet temperature of 65 K associated with a mass flux of 1250 kg/m2 s is shown to be capable of dissipating an effective heat flux of 431.3 W/cm2 with a wall superheat of about 85 K. Overall, the SiNW coatings proved positive effects on enhancing the flow boiling heat transfer with slower pressure drop increase, meanwhile the three-dimensional manifold microchannel design is revealed to effectively mitigate flow instability during the entire single and two-phase flow regions. This indicates great potential in utilizing three-dimensional flows by integrating SiNWs surface structures in high heat flux cooling applications.

Unconventional Gas-Dehydration-System Failure Resulting in a Gas-Hydrate Blockage

Summary In this paper, we present findings of how liquid-hydrocarbon carry-over from production separators could form solid contaminants that later became a root cause for an unexpected failure of the triethylene glycol (TEG) gas-dehydration unit to deliver dry export gas. This incident was later followed by an unanticipated capillary-aided gas-hydrate blockage inside the gas-export line during a planned-maintenance shutdown. After a detailed operational review before and after a gas-hydrate blockage incident, all sections of the TEG-dehydration tower unit were opened and inspected for (1) any presence of solid contamination and blockages and (2) an evaluation of the presence of potential equipment damage inside the dehydration unit. A compositional analysis with a gas-chromatography/flame-ionization detector (GC/FID), X-ray powder diffraction (XRD), and energy-dispersive X-ray spectrometry (EDS) was conducted on samples collected from the unit to understand its nature. Finally, using a customized gas-hydrate rocking-cell apparatus, gas-hydrate growth was observed under an 8°C subcooling temperature. From vessel inspection, it was found that a liquid-hydrocarbon carry-over results in the accumulation of high-chain hydrocarbon deposits (C19–C36) with some mineral scales inside the unit, even when filtration units are present. These deposits form when liquid carry-over passes through the reboiler unit, which gradually blocks the TEG-distribution spray nozzles, and reduces the efficiency of gas dehydration significantly. Later, with only 20.5% of the spray nozzles in working condition, the differential pressure (dP) across the distribution bar had exceeded its maximal operating limit, causing the distributor bar (VRGF) to split open along the weldment area. Hence, this resulted in total functional failure to continuously deliver dry gas and the formation of gas-hydrate blockage inside the gas-export pipeline. On the basis of the detailed operational review, field testing, compositional analysis, and hydrate-growth observations, the sequence of events that caused a catastrophic capillary-aided gas-hydrate blockage under a shut-in condition even in the presence of a hydrate-risk-management program is presented in this paper.

Energetic and exergetic investigation of a novel refrigeration system utilizing ejector integrated subcooling using different refrigerants

Increase in coefficient of performance of refrigeration systems is eminently important for various applications such as household refrigerator, tumble dryer, and more in practical applications. A new ejector subcooling system is described and investigated numerically without using an extra pump or compressor. After condenser, some part of the liquid (by-pass refrigerant) is expanded so that its temperature could be lower than the intended subcooling temperature. By using this by-pass refrigerant, the main flow of the refrigerant is subcooled. After evaporation, the by-pass refrigerant is expanded using an ejector. Seven different refrigerants are considered as working fluids (namely; R32, R1234yf, R290, R134a, R717, R600a and R245fa). R1234yf, R290, R600a and R717 are considered as low global warming potential refrigerants in European Union regulation. For each refrigerant, the values of coefficient of performance, relative increment in coefficient of performance and exergy efficiencies are calculated and demonstrated graphically for different condenser temperatures between 30°C/70°C and evaporator temperatures between −20°C/10°C. Results show that the best performance is obtained for R1234yf with an increment of about 20% in coefficient of performance and 18% in exergy efficiency. These efficiencies can be higher or lower approximately by 3% using ejector with higher or lower component efficiencies.

Subcooled flow boiling on hydrophilic and super-hydrophilic surfaces in microchannel under different orientations

An experimental investigation of subcooled flow boiling in the rectangular microchannel with hydraulic diameter of 0.94 mm (5.01 mm × 0.52 mm) for different orientations on hydrophilic and super-hydrophilic surfaces is conducted utilizing deionized water as the working fluid. The contact angles of the bare silicon surface and the nano-silica surface prepared through the plasma enhanced chemical vapor deposition method are 65° ± 3° and less than 5° respectively. With an inlet subcooling temperature of 10 °C, the mass fluxes and heat fluxes are tested in the range of 200–500 kg/(m2 s) and 4–25 W/cm2 respectively, while the orientation angles are 0° (bottom-heated horizontal flow), 90° (vertical upflow), 180° (top-heated horizontal flow) and 270° (vertical downflow). Significant appearances at the onset of nucleate boiling (ONB) accompanied with bubble nucleation are observed in the boiling curves. An earlier occurrence of critical heat flux (CHF) is presented for vertical downflow. With the direction of buoyancy opposite to the fluid inflow, the buoyancy force impedes the departure and movement of bubbles against the inertia, causing easier bubble coalescence and elongation as well as prolonged dry-out, which deteriorates heat transfer. At low mass fluxes the pressure drop for the case of vertical downflow is the highest among others, whereas at high mass fluxes the bottom-heated horizontal flow exhibits the maximum pressure drop. Most intense pressure drop fluctuation is obtained for vertical downflow, indicating the most severe flow instability under this orientation. The effect of orientation is weakened with the increment of mass flux due to the dominance of inertia force. At the mass flux of 200 kg/(m2 s), the super-hydrophilic surface delays CHF without increased pressure drop penalty in the vertical orientation, while in the horizontal orientation the effect of surface wettability seems more significant.

Numerical simulation of vapor volume fraction in a vertical channel under low-pressure conditions

This numerical study involved investigating void behavior under low-pressure subcooled flow boiling by using an Eulerian approach (two-fluid model). In the simulation, a vertical pipe with a length of 0.15 m and diameter of 0.01229 m was considered. Different levels of uniform wall heat flux, mass flux, and inlet subcooling temperature were applied although a constant pressure of 1.65 bar was used for all the simulations. A sensitivity study of the empirical coefficients used to access the predictive capacity of the existing mass transfer models was conducted. Thus, the k-epsilon model was used for the turbulence of the fluid. The axial vapor volume fraction profile, liquid temperature is compared at the operating pressure. Furthermore, the most sensitive flow characteristics of the channel were identified. The results indicated that the predictions of numerical phase evolution relative to the experimental observations were in good qualitative agreement with those obtained in extant studies. Additionally, the changes in drag coefficients were helpful in precisely predicting the void fraction. A commercial CFD solver was used for the implementation of the model.

Tetramethyl ammonium chloride as dual functional inhibitor for methane and carbon dioxide hydrates

This experimental work evaluates the dual functional (thermodynamic and kinetic) behaviour of quaternary ammonium salt namely tetramethyl ammonium chloride (TMACl) for both methane (CH4) and carbon dioxide (CO2) hydrates. The Hydrate Liquid Vapour Equilibrium (HLwVE) data for CH4 and CO2 are evaluated in the presence and absence of aqueous TMACl solutions (1, 5 and 10 wt%) through T-cycle method at different temperature and pressure conditions. The inhibition impact, i.e., average suppression temperature (ΔŦ) and Hydrate dissociation enthalpies (ΔHdiss) are also reported for the considered systems for both gases. The study is further extended to evaluate the formation kinetics of CH4 and CO2 hydrated in the presence and absence of aqueous TMACl systems. The impact of TMACl is reported via the induction time, the initial formation rate and a gas uptake at different subcooling temperatures for both gases. Furthermore, the kinetic inhibition performance of TMACl is also assessed via the relative inhibition performance (RIP) compared with literature systems for both CH4 and CO2 hydrates. Additionally, an electrolyte based thermodynamic model proposed by Dickens and Quinby-Hunt, (1997) is applied to predict the HLwVE temperatures of both gases (CH4 and CO2) in the presence of TMACl. The obtained thermodynamic and kinetic results indicate that TMACl efficiently acts as a potential dual functional hydrate inhibitor for CH4 and CO2 gases. Therefore, the outcome of its efficacy, this study will envision the use of TMACl for both CH4 and CO2 hydrate systems and can potentially be applied in offshore flow assurance strategies.

Experimental evaluation of RAFT-based Poly(N-isopropylacrylamide) (PNIPAM) kinetic hydrate inhibitors

As the oil and gas industry produces hydrocarbons from deeper waters and colder regions the issue of hydrate formation becomes more serious. As a result, hydrate inhibition has focused on kinetic hydrate inhibitors (KHI) and anti-agglomerants (AA) as an alternative to the existing approaches which involves injecting vast quantities of thermodynamic inhibitors. In this research, we evaluated the effect of different architectures (linear and branched) of poly(N-isopropylacrylamide) (PNIPAM) polymers synthesized using reversible addition−fragmentation chain-transfer (RAFT) polymerization. Unlike non-reversible deactivation radical polymerisation (RDRP) synthetic routes this generates accurately controlled KHI candidates with target molecular weight, narrow molecular weight distributions and controlled architecture, so that the effect on hydrate inhibition can be more accurately assessed. The RAFT-based polymers (linear and branched) were compared to a commercially available linear PNIPAM synthesized via non-RDRP radical polymerization and control groups (pure water, PVP, and Luvicap). The hydrate experiments were performed in a high pressure autoclave with continuous cooling under different cooling rates (0.25 K/min, 0.033 K/min, and 0.017 K/min). In addition, a cold restart was simulated using constant subcooling. The results regarding subcooling temperature, onset time, and hydrate fraction with resistance-to-flow were compared to known KHIs. These revealed that a linear PNIPAM-MacroRAFT polymer delayed the hydrate nucleation with similar performance to known KHIs (eg., PVP and Luvicap). However, a branched polymer showed the best performance in terms of hydrate fraction and resistance-to-flow among all of the systems tested in this study. These data provide valuable information regarding linear and branched PNIPAM-MacroRAFT polymers by demonstrating their ability to delay hydrate formation but also in preventing hydrate agglomeration. These findings confirm that polymer architecture can effect hydrate inhibition.

Thermodynamic stability and formation kinetics of CHClF2 hydrates in the presence of NiCl2

A hydrate-based technology is suggested for the desalination of industrial effluents, especially electroplating effluents including NiCl2. To decrease the required energy for operating a hydrate-based desalination (HBD) process, chlorodifluoromethane (CHClF2) was used as a guest for hydrate formation at relatively mild conditions. The phase equilibria of CHClF2 hydrates formed in aqueous NiCl2 solutions were investigated to identify the effect of NiCl2 on the thermodynamic stability of CHClF2 hydrates. The dissociation phase boundary of CHClF2 hydrates in the presence of NiCl2 was shifted to higher pressures and lower temperatures as the salinity of aqueous solutions increased. To confirm the feasibility of NiCl2 separation from wastewaters via the HBD process, X-ray diffraction and Raman spectroscopic measurements were performed. The results showed that the Ni2+ and Cl− ions did not participate in the hydrate framework, which revealed that the HBD process could be used to remove NiCl2 from effluents. The formation kinetics of CHClF2 hydrates were analyzed by a new kinetic model adopting transient time-dependent apparent rate constants, which significantly depends on the sub-cooling temperature and the concentration of NiCl2 in the aqueous solutions. Thermodynamic approximations regarding the Ni2+ and Cl− ion exclusion from the clathrate framework and the full occupancy of CHClF2 molecules in the large cages of structure I clathrate hydrate were confirmed by comparing the calculated Ni2+ concentrations in the liquid phase in equilibrium with CHClF2 clathrate hydrates with the experimental measurements. These results provide good information for separating metal ions from wastewater streams by HBD processes.

CFD investigation of sub-cooled boiling flow using a mechanistic wall heat partitioning approach with Wet-Steam properties

In this paper, the mechanistic wall heat partitioning approach was used to capture the complex heat and mass transfer in sub-cooled boiling flows. In order to accommodate the changes of local variables to be relevant to the physical properties of sub-cooled fluids, the Wet-Steam (IAPWS-IF97) is used as the working fluid. Currently, the approach is evaluated based on the bubble sliding along the wall before lifting-off, which is usually found in the flow boiling situations. In the simulation, the closure mechanistic models, including the fractal analysis, the force balance and the mechanistic frequency, were coupled with the Eulerian–Eulerian two-fluid framework, while the Shear Stress Transport model was used as a turbulent modelling closure. The Multiple Size Group model was introduced to handle the bubble interactions and predict the bubble size distribution. Moreover, the effect of adopting the sub-cooled liquid properties into the modelling was investigated and compared with the experiments over a wide range of flow conditions. Specifically, the predicted void fraction and the sub-cooling temperature near the heated wall were precisely compared with the cases of using the constant-property liquid. Overall, the satisfactory agreements were found between the experiments and the predictions of the liquid temperature, void fraction, interfacial area concentration, Sauter mean diameter and bubble and liquid velocities with the exception of the case of high heat and mass fluxes. To enhance the current prediction accuracy for a situation of having a high superheating temperature, more bubble interactions on the boiling wall, such as merging of the bubbles while sliding, need to be considered. Furthermore, to assess the model capability, this mechanistic approach will be introduced to elucidate the sub-cooled boiling flow in situations of using different fluids in the near future.

Experimental study on the Onset of Nucleate Boiling in a narrow rectangular channel under transversely non-uniform and uniform heating

Onset of Nucleate Boiling (ONB) in a narrow rectangular channel was experimentally studied under transversely non-uniform and uniform heating conditions. The experiment was performed under various mass fluxes and inlet subcooling conditions. The behavior of the bubbles was recorded using a high-speed camera, and was analyzed using an image processing technique. In the case of uniform heat flux, the wall temperature was uniformly distributed in the transverse direction. In the case of non-uniform heat flux, the wall temperature had the lowest value in the middle of the heated surface, and increased along the transverse direction toward the edges. At the same mass flux and inlet subcooling temperature, the thermal power at the ONB under non-uniform heat flux was lower than that under uniform heat flux. However, the local heat flux and wall temperature at the ONB were similar at both heating conditions. In the subcooled boiling region, it was found that the boiling heat transfer under non-uniform heat flux was lower than that under uniform heat flux. Additionally, the slope of the wall temperature-thermal power curve after the ONB was higher in the non-uniform heat flux case. Using the results elicited from image processing, the evaporation and quenching heat transfers were evaluated to analyze the differences in boiling heat transfers. The boiling heat transfer was less affected by evaporation and quenching in the case of non-uniform heat flux.

Experimental Investigation on Critical Heat Flux for Water Flowing in a Vertical Uniformly Heated Rifled Tube under Near-critical Pressures

This study experimentally investigated the critical heat flux (CHF) of departure from nucleate boiling (DNB) of water flowing under near-critical pressures in a 2 m-long vertical upward rifled tube with the size of ϕ35 × 5.67 mm. Operating conditions included pressures of 18–21 MPa, mass fluxes of 475–1000 kg·m−2·s−1, inlet subcooling temperatures of 3–5°C, and wall heat fluxes of 40–960 kW·m−2. Tube wall temperature distribution and heat transfer performance in different test conditions were obtained. The effects of the operating parameters on CHF were analyzed. Four heat transfer coefficient correlations were evaluated against our experimental data for further investigation of the two-phase heat transfer characteristics. A heat transfer correlation based on Martinelli number utilized in two-phase region and two empirical correlations used to predict the CHF in rifled tube at near-critical pressures were proposed. Meanwhile, experimental CHF data in rifled tube were compared with six widely used correlations and a CHF look-up table. The CHF enhancement effect in rifled tube is obvious as compared with the CHF data in smooth tube. Results show that DNB occurs at low vapor quality and subcooled region in the rifled tube at near-critical pressures. The increase in pressure leads to the early occurrence of DNB and the decrease in CHF, whereas the increase in mass flux delays the occurrence of DNB and results in the increase in CHF. DNB presents a tendency to move toward the inlet of the rifled tube. At individual operating conditions, DNB and dryout coexist at different sections of the rifled tube.

Forced Convective and Subcooled Flow Boiling Heat Transfer to Water–Al2O3 Microfluid in an Annular Heat Exchanger

An experimental study has been conducted on subcooled nucleate flow boiling heat transfer of water–Al2O3 particulate liquid inside a vertical annulus. The experimental apparatus provides the particular conditions to investigate the influence of heat flux, flow rate, inlet temperature, and particle concentration on the heat transfer coefficient in both single-phase convection and two-phase nucleate boiling regimes. Operating conditions are subcooling temperature (30–50 °C), volume flow rate (2–4 l/min), heat flux (8–110 kW/m2), and particle concentration (0.01–0.03 vol%). The experimental data is in good agreement with well-known Chen model with the accuracy of 10.84%.

Multiscale Dynamic Growth and Energy Transport of Droplets during Condensation.

Condensation is an important physical process and has direct relevance for a range of engineering applications, including heat transfer, antifrosting, and self-cleaning. Understanding the mechanism of droplet growth during condensation is an important aspect, but past works have not typically considered the dynamics of the multiscale process. In this paper, we developed a dynamic growth model, which considers the continuous and multiscale nature of the droplet growth process from several nanometers to hundreds of microns. This model couples the transient phase change heat transfer and two-phase flow both inside and outside the droplet. Accordingly, the energy transport is distinct from the classical pure conduction model. We show that convection near the liquid-vapor interface and inside the droplets plays an increasingly important role as droplets grow and finally dominates the energy transport process. Driven by strong convection, the droplets mix well and the discrete layers of temperature observed in the pure conduction model disappear at the microscale. This model that considers convection can lead to over 4 times higher predicted overall heat transfer than that obtained with the pure conduction model. The interfacial mass flow through the liquid-vapor interface is the dominant factor responsible for the strong convection. We studied the critical radius where convection starts to have a significant influence on droplet growth under different subcooling temperatures and contact angles. Droplets have smaller critical radii under larger subcooling temperatures or larger contact angles, ranging from 0.5 to 20 μm. This work identifies the modes of energy transport in condensation at different scales, which not only enhances our fundamental understanding of individual droplet growth but provides design guidelines for various dropwise and jumping-droplet condensation research.