Search results

This paper aims to explore the effectiveness and mechanism of a droplet-laden flow in a microfluidic system.

Numerical approach based on the volume of fluid method is implemented for modelling the forced heat transport in a droplet-laden flow in a microchannel.

The heat transfer effectiveness of droplet-laden flow is found to be obviously superior to that of a single-phase flow because of the circulation stream between the droplets. In addition, the effectiveness will be further increased when an elongated droplet is being laden because the circulation streams within and between the droplets are more pronounced. The elongated droplet size affects the heat transfer characteristics signified by Nusselt number, and there exists an optimum value at a fixed parameter.

This paper attempts to clarify the influence on the heat transfer performance when droplet with various shape and size being laden. This work is done by none before. This research work applies a solid foundation for designing a cooling system in microelectromechanical system.

The paper aims to consider heat transfer in incompressible flow in a rotating flat microchannel with allowance for boundary slip conditions of the first and second order. The novelty of the paper encompasses analytical and numerical solutions of the problem, with the latter based on the lattice Boltzmann method (LBM). The analytical solution of the problem includes relations for the velocity and temperature profiles and for the Nusselt number depending on the rotation rate of the microchannel and slip velocity. It was demonstrated that the velocity profiles at high rotation rates transform from parabolic to M-shaped with a minimum at the channel axis. The temperature profiles tend to become uniform (i.e. almost constant). An increase in the channel rotation rate contributes to the increase in the Nusselt number. An increase in the Prandtl number causes a similar effect. The trend caused by the effect of the second-order slip boundary conditions depends on the closure hypothesis. It is shown that heat transfer in a flat microchannel can be successfully modeled using the LBM methodology, which takes into account the second-order boundary conditions.

The paper is based on the comparisons of an analytical solution and a numerical solution, which employs the lattice Boltzmann method. Both mathematical approaches used the first-order and second-order slip boundary conditions. The results obtained using both methods agree well with each other.

The analytical solution of the problem includes relations for the velocity and temperature profiles and for the Nusselt number depending on the rotation rate of the microchannel and slip velocity. It was demonstrated that the velocity profiles at high rotation rates transform from parabolic to M-shaped with a minimum at the channel axis. The temperature profiles tend to become uniform (i.e. almost constant). The increase in the channel rotation rate contributes to the increase in the Nusselt number. An increase in the Prandtl number causes the similar effect. The trend caused by the effect of the second-order slip boundary conditions depends on the closure hypothesis. It is shown that heat transfer in a flat microchannel can be successfully modeled using the LBM methodology, which considers the second-order boundary conditions.

The novelty of the paper encompasses analytical and numerical solutions of the problem, whereas the latter are based on the LBM.

Wave breaking significantly affects the exchange process between ocean and atmosphere. This paper aims to simulate the upper ocean dynamics under the influence of wave breaking, which may help to figure out the transport of energy by these breakers.

The authors use a breaker-LES model to simulate the oceanic boundary layer in hurricane conditions, in which breakers become the main source of momentum and energy instead of traditional wind stress.

The mean horizontal velocities and energy increase rapidly with wind speed, reflecting that input from atmosphere dominates the coherent structure in the upper ocean. The penetration ability of a breaker limits its effective depth and thus the total turbulent kinetic energy (TKE) decreases sharply near the surface. Langmuir circulation is the main source of TKE in deeper water. The authors compared the dissipation rate (e) in the simulations with two estimates and found that the model tends to the scaling of ε∼z^–3.4 at extreme wind speeds.

The probability distribution of breakers is also discussed based on the balance between the input from atmosphere and output by wave breaking. The authors considered the contribution of micro-scale breakers and revaluated the probability density function. The results show stability in hurricane conditions.

The behaviors of sea surface drag coefficient should be well understood for an accurate hurricane prediction. The speed of wind has been applied to characterize the spray production; however, this could result in inaccurate spray productions compared to the experimental or field data if the influence of wave state is not considered. This paper aims to integrate a new sea spray generation function, described by windsea Reynolds number, into the spray momentum flux formula to calculate the spray momentum.

On the basis of this spray momentum, this study proposes the new formulas of spray stress and drag coefficient when the wind speed is high.

Results of the revised formulas show that wave status had significant effects on the spray stress and sea surface drag coefficient. Also, wave age was found to be an important parameter that affects the drag coefficient. The drag coefficient decreased with the increasing wave age. Comparison between this study’s theoretical and observation values of drag coefficient shows that the study results are close to the measured values.

The research findings can enhance the understanding of the behaviors of sea surface drag for an accurate hurricane prediction.

A new sea spray generation function, described by windsea Reynolds number, is integrated into the spray momentum flux formula to calculate the spray momentum. On the basis of this spray momentum, this study proposes the new formulas of spray stress and drag coefficient when the wind speed is high.

This paper aims to demonstrate the capabilities of a diffuse interface free energy lattice Boltzmann method to perform direct numerical simulations of liquid–liquid dispersions in a well-controlled turbulent environment. The goal of this research study is to develop numerical techniques that can visualize and quantify drop interaction with the turbulent vortices. The obtained information will be used for the development of sub-models of drop breakup for multi-scale simulations.

A pure binary liquid system is considered that is subject to fully developed statistically stationary turbulent flow field in a cubic fully periodic box with the edge size of 300 lattice units. Three turbulent flow fields with varying energy input are examined and their coherent structures are visualized using a normalized Q-criterion. The evolution of the liquid–liquid interface is tracked as a function of time. The detailed explanation of the numerical method is provided with a highlight on a choice of the numerical parameters.

Drop breakup mechanisms differ depending on energy input. Drops break due to interaction with the vortices. Quantification of turbulent structures shows that the size of vortices increases with the decrease of energy input. Drop interacts simultaneously with multiple vortices of the size comparable to or smaller than the drop size. Vortices of the size smaller than the drop size disturb drop interface and pinch off the satellites. Vortices of the size comparable to the drop size tend to elongate the drop and tear it apart producing daughter drops and satellites. Addition of the second phase enhances turbulent dissipation at the high wavenumbers. To obtain physically realistic two-phase energy spectra, the multiple-relaxation-time collision operator should be used.

Detailed information of drop breakup in the turbulent flow field is crucial for the development of drop breakup sub-models that are necessary for multi-scale numerical simulations. The improvement of numerical methods that can provide these data and produce reliable results is important. This work made one step towards a better understanding of how drops interact with the turbulent vortices.