An improved dropwise condensation heat transfer model modified from previous models is proposed in this paper. The critical radius for onset of droplet condensation is determined in the preceding ...paper (Part I), leading to a more accurate determination of droplet nucleation density and the coalescence radius in this paper (Part II). Effects of subcooling, contact angle, thickness and thermal conductivity of the coating layer on droplet nucleation density, condensation heat flux, and critical condensation heat transfer rate for onset of droplet condensation are illustrated. The predicted droplet nucleation density and dropwise condensation heat flux are shown in excellent agreement with existing experimental data.
An improved thermodynamic model is developed for the critical radius for droplet condensation on subcooled surfaces having a promoter coating layer. In addition to heat conduction through the ...droplet, this model takes into consideration thermal resistances of the coating layer, the liquid-vapor interface, and curvature depression of the equilibrium interface temperature, which are important in nano-size droplets. The critical radius for droplet condensation is obtained based on the availability criterion, and effects of contact angles, degree of subcooling, thickness and thermal conductivity of the coating layer, and saturated pressure on droplet nucleation radius are illustrated. Furthermore, the critical radius for condensation on a superhydrophilic subcooled surface without a promoter coating layer obtained from this new model recovers to the critical radius of droplet condensation in classical heterogeneous nucleation of droplets.
Pool boiling is the heat-transfer mechanism of many heat exchangers inside ocean nuclear power plants working under the complex marine circumstances. Also, ocean conditions will create a new ...acceleration field other than gravity for the fluid, which induces some unique thermal–hydraulic characteristics. In this study, pool boiling under heaving conditions is numerically simulated using multiple relaxation time phase change lattice Boltzmann method. Firstly, the simulated results under static condition have been validated with recognized empirical equations, such as Rohsenow’s correlation at nucleate boiling, Zuber’s model, and Kandlikar’s model about critical heat flux (CHF). Then, pool boiling patterns, the boiling curve of time-averaged heat flux, transient heat flux, and heaving effects on different pool boiling regions are investigated. The results show that pool boiling curves of time-averaged heat flux between heaving conditions and static conditions with middle superheat degrees are similar. Heat transfer under heaving conditions at low superheat is somewhat enhanced, and it is weakened at high superheat, which leads to a slightly smaller critical heat flux with larger superheat compared with that under static conditions. Moreover, distinct fluctuation of the transient heat flux of pool boiling under heaving conditions is found for all boiling regimes. Furthermore, the heaving condition shows both positive and negative effects on pool boiling heat transfer at high-gravity and low-gravity regions, respectively. Besides, both the larger heaving height and shorter period time bring out more violent heaving motion and make a greater impact on pool boiling heat transfer.
Using the improved double distribution function (DDF) thermal Lattice Boltzmann method (LBM), dropwise condensation of dry saturated vapor at a cold spot on vertical hydrophobic flat plates under ...gravity effects are simulated numerically for the first time. Dynamic behaviors of periodic formation of 3D condensing droplets on a 2D cold spot at a constant subcooled temperature, and their subsequent growth, and movement on hydrophobic surfaces are studied. The size of detached droplet on hydrophobic surfaces with different wettabilities under various gravity forces are simulated, and the results are found in good agreement with existing analytical predictions. The induced transient velocity and temperature fields and the variation of dropwise condensation heat flux with time at the cold spot on hydrophobic surfaces are analyzed. It is found that both the period of droplet condensation and its maximum heat transfer rates decrease as the contact angle of the surface is increased.
In early 2020, non-pharmaceutical interventions (NPIs) were implemented in China to reduce and contain the coronavirus disease 2019 (COVID-19) transmission. These NPIs might have also reduced the ...incidence of hand, foot, and mouth disease (HFMD).
The weekly numbers of HFMD cases and meteorological factors in 31 provincial capital cities and municipalities in mainland China were obtained from Chinese Center for Disease Control and Prevention (CCDC) and National Meteorological Information Center of China from 2016 to 2020. The NPI data were collected from local CDCs. The incidence rate ratios (IRRs) were calculated for the entire year of 2020, and for January–July 2020 and August–December 2020. The expected case numbers were estimated using seasonal autoregressive integrated moving average models. The relationships between kindergarten closures and incidence of HFMD were quantified using a generalized additive model. The estimated associations from all cities were pooled using a multivariate meta-regression model.
Stringent NPIs were widely implemented for COVID-19 control from January to July 2020, and the IRRs for HFMD were less than 1 in all 31 cities, and less than 0·1 for 23 cities. Overall, the proportion of HFMD cases reduced by 52·9% (95% CI: 49·3−55·5%) after the implementation of kindergarten closures in 2020, and this effect was generally consistent across subgroups.
The decrease in HFMD incidence was strongly associated with the NPIs for COVID-19. HFMD epidemic peaks were either absent or delayed, and the final epidemic size was reduced. Kindergarten closure is an intervention to prevent HFMD outbreaks.
This research was supported by the National Natural Science Foundation of China (81973102 & 81773487), Public Health Talents Training Program of Shanghai Municipality (GWV-10.2-XD21), the Shanghai New Three-year Action Plan for Public Health (GWV-10.1-XK16), the Major Project of Scientific and Technical Winter Olympics from National Key Research and Development Program of China (2021YFF0306000), 13th Five-Year National Science and Technology Major Project for Infectious Diseases (2018ZX10725-509) and Key projects of the PLA logistics Scientific research Program (BHJ17J013).
Rolling motion caused by ocean condition will induce more complicated inertial forces with their force directions changing all the time, which results more complex bubble behaviors and unique heat ...transfer characteristics. In this work, pool boiling under rolling condition is numerically simulated using multiple relaxation time phase change lattice Boltzmann method (LBM). Pool boiling patterns, boiling curve of time-averaged heat flux, transient heat flux and rolling effects on different pool boiling regions are investigated. The results show that pool boiling curve of time-averaged heat flux between rolling condition and static condition are not obvious until close to critical heat flux, and 9.3% higher CHF is achieved under rolling condition while worse heat transfer is discovered at film boiling. Moreover, distinct fluctuation of transient heat flux of pool boiling under rolling condition is found for all boiling regimes, and its variation pattern along with the rolling motion and bubble behavior is investigated. Furthermore, tangential inertial force caused by rolling motion has positive influence on heat transfer of pool boiling, while the centrifugal force has negative influence on heat transfer, since it is opposite to the gravity and hence decreases the buoyancy force. Besides, larger rolling amplitude and smaller rolling period will induce larger additional inertial forces, and thus make greater influences on the bubbles’ behavior and pool boiling heat transfer.
Coalescence induced droplets self-propelled jumping on textured superhydrophobic surfaces (SHS) is numerically simulated using multiple–relaxation–time (MRT), and three dimensional (3D) multiphase ...isothermal lattice Boltzmann method. Symmetric boundary conditions and parallel computation with OpenMP algorithm are used to accelerate computational speeds. Simulation results for velocity field show that the downward velocity of the droplet is reverted to upward direction due to the counter action of the wall to the contact base of the droplet during the period of droplet deformation on the texture. For a fixed droplet diameter, the spacing of the microstructure is found to play a key role on jumping velocity of the coalescence droplet, and an optimal spacing of the microstructure exists for a maximum jumping velocity. For a texture with small spacings, the adhesion force due to surface tension is large because of the large contact area which results in a decrease of its jumping velocity. On the other hand, for a texture with large roughness spacings, the lower contour of the droplet will fall into the texture, which will also decrease droplet jumping velocity. Simulation results for jumping velocities are used to explain large differences in measured jumping velocities of small droplets (with radius less than 20μm) on hierarchical textured and nanostructured surfaces in existing experiments.
Based on the newly developed phase-change Lattice Boltzmann method (LBM), the problem of steady laminar film condensation on a hydrophilic vertical flat plate at a subcooled temperature is simulated ...in this paper. The effects of inertia force, interface shear stress and convection terms are fully taken into consideration in the LBM simulation. The condensate film thickness, velocity and temperature distributions, and heat transfer characteristics at steady state conditions are obtained numerically. The simulation results are found in good agreement with the classical analytical solutions. The deviations between the classical solutions and those of LBM simulation become somewhat larger at high Jakob numbers and low Prandtl numbers. The contact angle of the fluid with the surface is found to have very little effects on film condensation. It is shown that the LBM can be applied to condensation problems successfully and accurately for the first time.
The phenomenon of coalescence induced droplets self-propelled jumping on superhydrophobic surfaces (SHS) is numerically simulated in this paper using 2D lattice Boltzmann method (LBM). To overcome ...numerical instability problems of a two-phase flows with a high liquid/vapor density ratio and a high liquid/vapor viscosity ratio, the equation of state is modified according to an existing method and the multiphase relaxation time (MRT) method is adopted. The simulated jumping velocity and jumping height of droplets with different radii are found in good agreement with experimental observations. In addition, the reason why coalescence droplets will jump on superhydrophobic surface with a sufficiently high contact angle is explained based on a qualitative analysis.
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