Heat conduction with phase transition

Solid compounds react to heat by expanding or with phase transitions such as melting. The thermal properties that are most interesting for the designer are the melting point (or glass point), the solid-phase transition temperature, the thermal expansion coefficient, and the heat conductivity. These last two properties determine the thermal shock resistance. ... 

Consider the equation of heat conduction in a solid with the laser energy absorbed on the irradiated surface as a heat source. By judiciously selecting the beam geometry, dwell time, and sample configuration, the problem may be reduced to solvable one- and two-dimensional heat flow analyses. Phase transitions can be included and the temperature distributions that are produced can be calculated. Examples will be selected to provide specific guidance in the choice of lasers and materials. The result of all this will be an idea of the effects that one may produce by laser heating of solids. 

The Stephan problem (problem of the phase transition). Subsequent considerations include two phases with the coefficients of heat conductivity fej(u), k. u) and of heat capacity Cj(r<), c iu), in either of which it is... 

These results stimulated a number of studies, both in industry (Conoco, Esso, Shell Pipeline) and in academia (University of Maryland, M.I.T.). The objective was, primarily, to delineate the mechanism that led to these explosive events. The results of many small-scale experiments, primarily conducted by Shell Pipeline Corporation and M.I.T., led to the hypothesis that the apparent explosion was, in fact, a very rapid vaporization of superheated LNG. Contact of LNG, of an appropriate composition, with water led to the heating of a thin film of the LNG well above its expected boiling temperature. If the temperature reached a value where homogeneous nucleation was possible, then prompt, essentially explosive vaporization resulted. This sequence of events has been termed a rapid phase transition (RPT), although in the earlier literature it was often described by the less appropriate title of vapor explosion. 

Some physical properties of water are shown in Table 7.2. Water has higher melting and boiling temperatures, surface tension, dielectric constant, heat capacity, thermal conductivity and heats of phase transition than similar molecules (Table 7.3). Water has a lower density than would be expected from comparison with the above molecules and has the unusual property of expansion on solidification. The thermal conductivity of ice is approximately four times greater than that of water at the same temperature and is high compared with other non-metallic solids. Likewise, the thermal dif-fusivity of ice is about nine times greater than that of water. 

Different SPM systems were developed to study the thermal properties. Thus a tiny thermocouple can be used to measure the heat flow from the surface and to test the local thermo conductivity of polymer surfaces [161]. Recently, a bimetallic cantilever has been used as temperature sensor to investigate phase transitions of n-alkanes with a heat sensitivity of 500 pj for a sample mass as low as to... 

The hysteresis that may appear depends on the direction or the scan rate of temperature change. This tendency is based on the phase transition or slow diffusion process of sample. Remarkably such hysteresis can even appear where a sample shows crystallization within the measurement temperature range. Therefore the ionic conductivity measurement is usually performed at each temperature after reaching the constant value. On the other hand, ionic conductivity is measured at scan rate of 1° to 2° C min. Therefore, when hysteresis appears during the heating or cooling process, the relationship between the phase transition and the ionic conductivity can be used in the analysis at this scan rate with the DSC measurement. It is better to use a small cell design to avoid the temperature distribution in samples. 

Fast pyrolysis occurs in time of few seconds or less. Therefore, not only chemical reaction kinetics but also heat and mass transfer processes, as well as phase transition phenomena, play important roles. The critical issue is to bring the reacting biomass particle to the optimum process temperature and minimise its exposure to the intermediate (lower) temperatures that favour formation of charcoal. This objective can be achieved by using small particles, thus reducing the time necessary for heat up. This option is used in fluidised bed processes that are described later. Another possibility is to transfer heat very fast only to the particle surface that contacts the heat source. Because of the low thermal conductivity the deeper parts of the particles will be maintained at temperatures lower than necessary for char production. The products that form on the surface are immediately removed exposing that way consecutive biomass layers to the contact with the heat source. This second method is applied in ablative processes that are described later. 

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