Saturated droplet impact

A. Simulation of Saturated Droplet Impact on Flat Surface in the Leidenfrost Regime... 

Ge and Fan (2005) developed a 3-D numerical model based on the level-set method and finite-volume technique to simulate the saturated droplet impact on a superheated flat surface. A 2-D vapor-flow model was coupled with the heat-transfer model to account for the vapor-flow dynamics caused by the Leidenfrost evaporation. The droplet is assumed to be spherical before the collision and the liquid is assumed to be incompressible. 

Groendes and Mesler (1982) studied the saturated film boiling impacts of a 4.7 mm water droplet on a quartz surface of 460 °C. The fluctuation of the surface temperature was detected using a fast-response thermometer. The maximal temperature drop of the solid surface during a droplet impact was reported to be about 20 °C. Considering the lower thermal diffusivity of quartz, this temperature drop implies a low heat-transfer rate on the surface. Biance et al. (2003) studied the steady-state evaporation of the water droplet on a superheated surface and found that for the nonwetting contact condition, the droplet size cannot exceed the capillary length. 

During the subcooled droplet impact, the droplet temperature will undergo significant changes due to heat transfer from the hot surface. As the liquid properties such as density p (T), viscosity /q(7), and surface tension a(T) vary with the local temperature T, the local liquid properties can be quantified once the local temperature can be accounted for. The droplet temperature is simulated by the following heat-transfer model and vapor-layer model. Since the liquid temperature changes from its initial temperature (usually room temperature) to the saturated temperature of the liquid during the impact, the linear... 

Depending on the surface temperature, diverse heat transfer mechanisms may develop when a droplet impacts onto a heated surface, which can be described by the classical boiling curve of a droplet gently deposited on a heated surface (see Fig. 8.2 (1) film evaporation (Tw < where Tsat is the liquid saturation temperature), (2) bubble boiling (Tgat — Nukiyama)) ( ) transition (TNuj yama — 7 r < TLeidenfrost) and (4) film boiling (Tw > TLeidenfrost)-... 

Condensation scrubbing is a relatively recent development in wet scrubber technology. Most conventional scrubbers rely on the mechanisms of impaction and diffusion to achieve contact between the PM and liquid droplets. In a condensation scrubber, the PM act as condensation nuclei for the formation of droplets. Generally, condensation scrubbing depends on first establishing saturation conditions in the gas stream. Once saturation is achieved, steam is injected into the gas stream. The steam creates a condition of supersaturation and leads to condensation of water on the fine PM in the gas stream. The large condensed droplets are then removed by one of several conventional devices, such as a high efficiency mist eliminator. 

The simulation shown in Fig. 10 is an impact of a saturated water droplet of 2.3 mm in diameter onto a surface of 400°C with an impact velocity of 65 cm/s, corresponding to a Weber number of 15. This simulation and all others presented in this study are conducted on uniform meshes (Ax — Ay — Az = A). The mesh resolution of the simulation shown in Fig. 10 was 0.08 mm in grid size, although different resolutions are also tested and the results are compared in Figs. 11 and 12. The average time-step in this case is around 5 ps. It takes 4000 iterations to simulate a real time of 20 ms of the impact process. The simulation... 

In subcooled impact, the initial droplet temperature is lower than the saturated temperature of the liquid of the droplet, thus the transient heat transfer inside the droplet needs to be considered. Since the thickness of the vapor layer may be comparable with the mean free path of the gas molecules in the subcooled impact, the kinetic slip treatment of the boundary condition needs to be applied at the liquid-vapor and vapor-solid interface to modify the continuum system. 

Let us consider now the case of a specific ionic polysaccharide. The unique properties of complexes of the cationic chitosan with non-ionic sorbitan esters provides an interesting example. Grant and co-workers (2006) have established that mixtures of chitosan and surfactant form emulsion-like solutions and/or creams, where the surfactant component is present as droplets or micelle-like particles and the chitosan solution acts as the system s continuous phase. It was established that the length and the degree of saturation of the surfactant hydrocarbon chain have a significant impact on the development of the chitosan-surfactant complexes. Moreover, an optimal distance between the chitosan s protonated amine groups is required for effective interactions to occur between the polysaccharide and the sorbitan esters. 

As envisioned previously, ESI response is concentration-dependent. A linear response versus concentration is up to the maximum concentration of about 10 M. When analyte concentration exceeds this limit, the ESI response levels off. This is because ESI intensity is proportional to the surface concentration of an ion. At about lO M, the droplet surface is completely saturated and higher concentration will not increase the total number of surface charges available for ion formation [25]. This will impact the high concentration end of quantitation using LC/ESl-MS. For the low concentration end, the detection limit depends on the sensitivity of LC/MS system, including efficient ion transfer/detection and removal of chemical noise in the system. 

As shown in Ref. 43 the deposited volume can be deduced from the force curves measured during the deposition process. In the case of femtoliter droplets (diameter in the 1 p.m range], a sharp increase of droplet size is observed for short times (t < 2 s typically), followed by a saturation regime where prolonged contact time has no obvious impact on the liquid flow. This dynamics indicates a nondiffusive process for the liquid transfer in striking a contrast to molecule transfer in DPN and confirms the fundamental difference... 

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