Subcooling, impact

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. 

Three different subcooled impact conditions under which experiments were conducted and reported in the literature are simulated in this study. They are (1) K-heptane droplets (1.5 mm diameter) impacting on the stainless steel surface with We — 43 (Chandra and Avedisian, 1991), (2) 3.8 mm water droplets impacting on the inconel surface at a velocity of 1 m/s (Chen and Hsu, 1995), and (3) 4.0 mm water droplets impacting on the copper surface with We — 25 (Inada et al., 1985). The simulations are conducted on uniform Cartesian meshes (Ax = Ay — Az — A). The mesh size (resolution) is determined by considering the mesh refinement criterion in Section V.A. The mesh sizes in this study are chosen to provide a resolution of CPR =15. 

B. Simulation of Subcooled Droplet Impact on Flat Surface in Leidenfrost Regime... 

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... 

Fig. 14 shows the comparison of the photographs from Chandra and Avedisian (1991) with simulated images of this study for a subcooled 1.5 mm n-heptane droplet impact onto a stainless-steel surface of 200 °C. The impact velocity is 93 cm/s, which gives a Weber number of 43 and a Reynolds number of 2300. The initial temperature of the droplet is room temperature (20 °C). In Fig. 14, it can be seen that the evolution of droplet shapes are well simulated by the computation. In the first 2.5 ms of the impact (frames 1-2), the droplet spreads out right after the impact, and a disk-like shape liquid film is formed on the surface. After the droplet reaches the maximum diameter at about 2.1ms, the liquid film starts to retreat back to its center (frame 2 and 3) due to the surface-tension force induced from the periphery of the droplet. Beyond 6.0 ms, the droplet continues to recoil and forms an upward flow in the center of the... 

The simulations are further conducted under the experimental conditions of Inada et al. (1985). In their experiments, 4.0 mm water droplets impact on a heated platinum surface at a temperature up to 420 °C. The subcooling degree... 

A DVP should not be inserted at the terminal point of a column, but a few trays away from it (362). This minimizes the impact of nonkey components (Fig. 18.2a), of subcooling, and of superheating. This location also improves the sensitivity of the measurement, since composition changes are greater at points further away from the products (Fig. 18.1a). 

Internal reflux control. It was stated earlier (Sec. 16.6, guideline 7) that a major strength of scheme 16.4d is its ability to minimize the impact of disturbances in the cooling medium on the column. A similar capability can be incorporated into the alternative schemes (16.4o-c, e) by adding simple instrumentation to set up an internal reflux controller (Fig. 19.2). This controller automatically adjusts the reflux rate for changes in reflux subcooling. 

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