Superheated state, boiling

A given phase may persist beyond the point at which transition to another phase should properly occur. On the T5 isotherm, for instance, it is possible to compress the vapour in clean conditions beyond point V, as shown by the dots, without condensation occurring. This vapour is supercooled, and Pv > Pl- When disturbed, the supercooled vapour condenses at once. In the same way, clean liquid may be superheated without boiling, in which case Pl > Pv Supercooled or superheated phases are metastable. They appear to be stable, but are thermodynamically unstable, since another state of lower chemical potential exists. 

To start with, let us consider a particular case of metastable states - a liquid superheated with respect to the liquid-vapor equilibrium temperature. For simplicity let us take a pure liquid at positive pressures, see Fig. 1. The region of superheated states is limited from below by the binodal Ts(p) and from above by the experimental line of attainable superheat, or, in other words, the line of spontaneous boiling-up T (p Cxp) of the liquid. An understandable limitation is imposed on the volume of superheated sample V and the time period Cxp of experiment. Naturally, the experimental time should be shorter than the life time t of the metastable state. 

In many situations, similar violent boiling can take place with much smaller unstable superheated states than those required for homogeneous nucleate boiling, e.g. for LNG and LPG with superheats of only 1 or 2 K. These events are termed quasi-homogeneous nucleate (QHN) boiling events, and take place in the absence of solid surfaces with nucleation sites [5]. 

In the adequate case (1.0 or 2.0 mL tetralin), the catalyst appeared to be wet differently from dry sand-bath or suspension states. As in the case of decalin dehydrogenation under the superheated liquid-film conditions, the catalyst temperature is higher than the boiling point, exhibiting a temperature gradient, and the substrate liquid is limited in amount to... 

Note Catalyst = platinum nanoparticles supported on granular activated carbon (Pt/C, 5 wt-metal%), 0.30 g charged amount of decalin and tetralin = 1.0 mL (superheated liquid-film state). Reaction conditions = boiling and refluxing by heating at 210°C and 240°C and cooling at 5°C. 

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. 

Superheated liquids are liquids which exist at temperatures above their equilibrium boiling point at the system pressure. These liquids are metastable in a thermodynamic sense, i.e., they are stable with respect to small perturbations on the system, but if the perturbation is sufficiently large, superheated liquids will partially vaporize and form a final, more stable state, usually consisting of vapor and residual liquid. 

Draw an isobaric heating process on a T s diagram for a nonazeotropic mixture from a compressed liquid state to a superheated vapor state. Does the temperature remain the same in the boiling region ... 

For the case of sublimable secondary EM, as Belyaev indicates, variation of the aggregate state occurs at a boiling temperature which corresponds to the external pressure. Indeed, the evaporation rate from the free surface of a superheated fluid is extraordinarily high, and superheating of the fluid is practically impossible. Dilution of the vapors by reaction products diffusing from the combustion zone even lowers the temperature of the liquid surface somewhat compared with the boiling temperature. 

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