•Evaporation of single alcohol droplets trapped in electrodynamic balance.•Evaporative cooling caused water condensation on droplet.•Water-alcohol phase separation found in two modes, distinct ...evaporation behavior.•Coulombic fissions were identified in the two types of phase separation.•Droplet charges increased vapor concentration in air, led to cease in evaporation.
The evaporation dynamics of charged, sub-millimeter droplets of short-chain alcohols, suspended in the air by means of an electrodynamic balance are investigated in this study. Condensation of water from the ambient air onto the evaporating droplet results from evaporative cooling, and droplet evaporation is that of a multi-component system. The non-ideal mixing of condensed water and alcohol leads self-aggregation of the alcohol, which can occur in two distinct modes: enclosed-phase and Janus. The evaporative behavior of each mode is analyzed over a broad range of ambient relative humidity for five alcohols. The charges on the droplets can induce Coulombic fissions, which were observed in the two modes of phase separation. The charges act on the gas phase water dipoles, leading to an increase in vapor concentration in the surroundings of the droplet and an eventual cease of evaporation. These results were consistent in the five types of alcohol (methanol, ethanol, isopropanol, 1-propanol, and 1-butanol) at all values of relative humidity investigated (15% - 51%).
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Effects of droplet parameters are reported using model simulations on the evaporation characteristics of droplets containing insoluble nanoparticles and related to the formation of shells of ...particles. The droplet parameters are initial droplet diameter (do), initial particle mass fraction (Yvo), and the Peclet number (Pe) based on the fluid evaporation rate constant (K) to particle diffusivity in the fluid (Dpl). Time-dependent phenomena of the instantaneous droplet size (d), d2 vs time (for example, following the d2-law or the deviation from it), instantaneous evaporation efficiency (i.e., effective area of evaporation due to particle inclusion on the droplet surface compared to a pure fluid droplet) are reported. Analytical expression of shell parameters at shell formation such as time to shell formation, shell size, and evaporation efficiency are derived using the initial droplet parameters and compared with the computational results. It is found that the evaporation behavior of droplets containing insoluble nanoparticles, including the deviation from d2-law, is independent of do but is strongly influenced by the product of Pe·Yvo.
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Thermal conductivity, viscosity, and stability of nanofluids containing multi-walled carbon nanotubes (MWCNTs) stabilized by cationic chitosan were studied. Chitosan with weight fraction of 0.1%, 0.2
...wt%, and 0.5
wt% was used to disperse stably MWCNTs in water. The measured thermal conductivity showed an enhancement from 2.3% to 13% for nanofluids that contained from 0.5 wt% to 3 wt% MWCNTs (0.24 to 1.43 vol %). These values are significantly higher than those predicted using the Maxwell’s theory. We also observed that the enhancements were independent of the base fluid viscosity. Thus, use of microconvection effect to explain the anomalous thermal conductivity enhancement should be reconsidered. MWCNTs can be used either to enhance or reduce the fluid base viscosity depending on the weight fractions. In the viscosity-reduction case, a reduction up to 20% was measured by this work. In the viscosity-enhancement case, the fluid behaved as a non-Newtonian shear-thinning fluid. By assuming that MWCNT nanofluids behave as a generalized second grade fluid where the viscosity coefficient depends upon the rate of deformation, a theoretical model has been developed. The model was found to describe the fluid behavior very well.
•A model is proposed for the first-stage evaporation/drying for liquid droplets containing small particles.•The d2-law for pure fluid is found to need modification as particles begin to populate the ...droplet surface.•A combination of Peclet number and the initial particle concentration determines the time and the size of shell at the end of first stage.
A model is presented for predicting the evaporation behavior of liquid droplets containing nano-sized insoluble particles. The model is constructed based on, and supported by, the existing experimental result. Of particular interest are the evolution of droplet size, and evaporation rate constant prior to the shell formation (also called the first drying or evaporation stage), when the particles forms a compact shell at the droplet surface. As the droplet evaporation proceeds, increasingly more particles are “included” on its surface, reducing the effective liquid surface area for evaporation causing deviation from the classical d2-law for pure fluid droplet evaporation. Peclet number (Pe), a non-dimensional ratio of particle diffusion time to the droplet lifetime, and initial particle concentration (Yvo) are shown to play a role in the evaporation behavior prior to shell formation. The model predicts that: (1) the deviation from the classical d2-law is increasingly significant with increased Pe and Yvo; (2) the time to shell formation decreases, and shell diameter increases, with increased Pe and initial particle concentration (Yvo). As a consequence, Yvo·Pe<1.8 is necessary for the first stage to exist at all. Within the limiting value, the combination of larger Pe and smaller Yvo produces larger hollow shells with low densities. For Yvo·Pe>1.8 the shell forms instantly and evaporation occurs after liquid diffuses through, and wet, the shell. The effect of contact angle (θ) on the effective liquid surface area for evaporation is also discussed.
•A model is proposed for the effect of internal circulation on evaporation/drying (prior to shell formation by particles at the droplet surface) for liquid droplets containing small ...particles.•Competing effects of particle mobility and internal circulation are analyzed.•The effect of international circulation qualitatively alters the distribution of nano-particles during droplet evaporation and helps explain some morphologies observed in a wide range of applications.
The effect of internal circulation on evaporation of fluid droplets containing nano-sized particles is analytically investigated, where internal circulation is caused by viscous effects at the liquid–gas interface in the convective environment. The competing time scales of liquid diffusion, convection, and particle diffusion are first analyzed as influenced by gas phase velocity, relative viscosities of gas and liquid phases, and the droplet size. The results reveal the importance of internal recirculation for droplets of practical sizes. To demonstrate the role of internal circulation plays on particle distribution and shell formation during the evaporation process, a symmetric Hill vortex and strong circulation are assumed for solving a one-dimensional governing equation, which to yield (i) the particle redistribution during the evaporation process, (ii) the time and the size upon shell formation (i.e., the end of the first-stage evaporation/drying) due to inclusion of particles at the droplet surface, and (iii) internal particle distribution that forecast possible morphologies of particle aggregates once the drying process is complete. These results are found to be dependent upon (a) the relative time scales of liquid diffusion and particle mobility (the effect of Lewis number, Le), and (b) the relative importance or evaporation rate (K) and particle mobility (the effect of Peclet number, Pe). Comparisons with previous results without internal circulation are made, as related to the time for shell formation and possible morphologies. Such comparisons reveal some distinctly different, and surprising, phenomena during and at the end of the first-stage evaporation.
•An integrated model is introduced incorporating simultaneous droplet spreading, wetting interactions, heat transfer, and liquid infiltration with temperature-dependent viscosities in unsaturated ...porous media.•This study shows that CMAS droplets of a size encountered by real gas turbines will lose their initial thermal energy soon after impact on a TBC and have negligible influence on the overall infiltration behavior.•The temperature gradient within the TBC topcoat has a substantial impact on the CMAS infiltration rate. An anisotropic microstructure can significantly affect the lateral spreading and final penetration depth but has an insignificant influence on the initial infiltration rate.•Heterogeneous structures such as bilayer and multilayer structures can limit and delay vertical infiltration by diminishing the vertical pressure gradient and increasing the resistance to fluid flow.
Molten calcium-magnesium-alumino-silicate (CMAS) droplets impact and infiltrate porous thermal barrier coatings (TBCs) of gas turbines, thereby causing loss of strain tolerance and delamination of the ceramic topcoat. To develop efficient mitigation strategies, it is crucial to understand CMAS infiltration dynamics into the porous topcoat. An integrated model is introduced incorporating simultaneous droplet spreading, wetting interactions, heat transfer, and liquid infiltration with temperature-dependent viscosities in unsaturated porous media. The model is applied to CMAS droplet interactions with homogeneous/heterogeneous anisotropic TBC topcoats grown by the electron beam physical vapor deposition (EB-PVD) method. Simulation results show that the droplet height and contact diameter dynamics exhibit three stages - initial-stage fast decrease, slow quasi-linear intermediate decrease, and late-stage fast decrease. The first two stages are dominated by the dynamics of infiltration. The initial temperature of the droplet has insignificant effect on the infiltration dynamics. Rather, the temperature gradient in the topcoat is critical to the infiltration rate. The anisotropy determines the final infiltration diameter and depth. Further, bilayer and multilayer coating structures with alternating fine and coarse columns can delay the infiltration rate and promote lateral spreading during the early stage of the droplet infiltration compared to single-layer structures. The results demonstrate that heterogeneous structures provide a viable approach to mitigate fast infiltration and reduce damage to the TBCs during the early stage of droplet infiltration.
Surface tension of evaporating nanofluid droplets Chen, Ruey-Hung; Phuoc, Tran X.; Martello, Donald
International journal of heat and mass transfer,
05/2011, Letnik:
54, Številka:
11
Journal Article
Recenzirano
Measurements of nanofluid surface tension were made using the pendant droplet method. Three different types of nanoparticles were used – laponite, silver and Fe
2O
3 – with de-ionized water (DW) as ...the base fluid. The reported results focus on the following categories: (1) because some nanoparticles require surfactants to form stable colloids, the individual effects of the surfactant and the particles were investigated; (2) due to evaporation of the pendant droplet, the particle concentration increases, affecting the apparent surface tension; (3) because of the evaporation process, a hysteresis was found where the evaporating droplet can only achieve lower values of surface tension than that of nanofluids at the same prepared concentrations; and (4) the Stefan equation relating the apparent surface tension and heat of evaporation was found to be inapplicable for nanofluids investigated. Comparisons with findings for sessile droplets are also discussed, pointing to additional effects of nanoparticles other than the non-equilibrium evaporation process.
Enhanced boiling heat transfer on binary surfaces Holguin, Ryan; Kota, Krishna; Wootton, Stephen ...
International Journal of Heat and Mass Transfer/International journal of heat and mass transfer,
11/2017, Letnik:
114
Journal Article
Recenzirano
Odprti dostop
•Practicable, robust, scalable binary surfaces were pursued for boiling enhancement.•The current boiling performance limits were pushed for PF-5060 on these surfaces.•Presence of a trapped ...non-boiling liquid in the binary surface enhances boiling.
A novel idea for the improvement of boiling heat transfer is that of a binary surface – where a non-boiling liquid coats sub-surface irregularities and aids in heat transfer to the primary working fluid. The threefold goal of this effort was to: (i) prepare durable, low-cost, scalable binary surfaces for boiling heat transfer enhancement, (ii) conduct pool boiling experiments on these surfaces, and (iii) derive the physical mechanisms perceived as responsible for the observed boiling enhancements. Accordingly, robust binary surfaces were prepared on copper using a facile, scalable bulk micro-manufacturing approach. These surfaces consist of numerous micro-/nano-cavities filled by a non-boiling liquid creating puddles around solid islands. Boiling experiments were carried out using a dielectric liquid, PF-5060, as the primary working fluid (the boiling liquid). It was observed that, compared to a smooth/plain copper surface, a binary copper surface, with water as the NBL, was able to simultaneously enhance the maximum heat flux limit by ∼2.2 times and the average heat transfer coefficient by ∼7.5 times. A maximum heat flux of 35.06W/cm2 was recorded, which is higher than the enhancements reported so far in literature for the pool boiling of PF-5060 on any enhanced surface. It was further found that decreasing the contact angle of the non-boiling liquid on the binary surface enhances both the heat transfer coefficient and the maximum heat flux limit.
Recirculation in underexpanded supersonic jet flow with varying degrees of swirl was experimentally investigated due to its potential for enhanced flame holding in supersonic combustion engines. ...Recirculation caused by swirl is free of solid walls, avoiding the challenge of wall cooling due to high heat fluxes. Schlieren imaging, planar laser Mie scattering, and planar particle image velocimetry were used to observe and quantify shock cell and vortex structure, and where applicable, to map velocity in and around the recirculation zone. Case study showed that different patterns of tangential injection, pressure ratios, and nozzle sizes were more or less favorable to flow recirculation. Various metrics of recirculation and shock structure were correlated with the onset, variability, characteristic length, and characteristic time of recirculation vortex. Statistical analysis of instantaneous velocities indicated that (1) recirculation velocity and recirculation zone width had standard deviations of ~ 30% and ~ 20%, respectively (in part due to unsteady precession of the vortex around the axis of mean flow); and (2) the average velocity within the recirculation zone was seen to be insensitive to the pressure ratio (and hence, to the fully expanded Mach number). The recirculation zone width was slightly increased by doubling the pressure ratio, within the range investigated. Thus, under conditions that most favored recirculation, the net effect of (1) and (2) was the formation of a toroidal vortex structure within which the average velocity was sufficiently slowed (by a factor of 3 relative to the bulk velocity) to measurably enhance flame holding in supersonic combustor applications.
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