Latent heat melting

Keywords PCM phase change latent heat melting heat storage cold storage corrosion phase separation incongruent melting subcooling nucleator products. 

Thermal and physical properties of the material to be heated—specific heat, latent heat, melting and boiling points, thermal conductivity, flash point, viscosity, density, and evolution of dissolved gases/vapours generated during heating ... 

Hydrocarbons NoofC atoms Latent heat of adsorption (AH) in J/g Latent heat Melting of emission point (-AH) in J/g Crystallization References temperature (T, =0 ... 

Conduction with Change of Phase A special type of transient problem (the Stefan problem) involves conduction of heat in a material when freezing or melting occurs. The liquid-solid interface moves with time, and in addition to conduction, latent heat is either generated or absorbed at the interface. Various problems of this type are discussed by Bankoff [in Drew et al. (eds.). Advances in Chemical Engineering, vol. 5, Academic, New York, 1964]. 

This is of exactly the same form as eqn. (5.10) and AH is simply the "latent heat of melting" that generations of schoolchildren have measured in school physics labs." We now take some water at a temperature T < T, . We know that this will have a definite tendency to freeze, so Wf is positive. To calculate Wf we have Wf=- A A, and H = U + pV to give us... 

The solidification speed of salol is about 2.3 mm mim at 10°C. Using eqn. (6.15) estimate the energy barrier q that must be crossed by molecules moving from liquid sites to solid sites. The melting point of salol is 43°C and its latent heat of fusion is 3.2 x 10 ° J molecule F Assume that the molecular diameter is about 1 nm. 

AH = latent heat of solidification T, = absolute melting temperature T = actual temperature (absolute). 

In principle the heat required to bring the material up to its processing temperature may be calculated in the case of amorphous polymers by multiplying the mass of the material (IP) by the specific heat s) and the difference between the required melt temperature and ambient temperature (AT). In the case of crystalline polymers it is also necessary to add the product of mass times latent heat of melting of crystalline structures (L). Thus if the density of the material is D then the enthalpy or heat required ( ) to raise volume V to its processing temperature will be given by ... 

The term latent heat is also pertinent to our discussions. The process of changing from solid to gas is referred to as sublimation from solid to liquid, as melting and from liquid to vapor, as vaporization. The amount of heat required to produce such a change of phase is called latent heat. If water is boiled in an open container at a pressure of 1 atmosphere, its temperature does not rise above 100° C (212° F), no matter how much heat is added. The heat that is absorbed without changing the temperature is latent heat it is not lost, but is expended in changing the water to steam. 

In this section we discuss the basic mechanisms of pattern formation in growth processes under the influence of a diffusion field. For simphcity we consider the sohdification of a pure material from the undercooled melt, where the latent heat L is emitted from the solidification front. Since heat diffusion is a slow and rate-limiting process, we may assume that the interface kinetics is fast enough to achieve local equihbrium at the phase boundary. Strictly speaking, we assume an infinitely fast kinetic coefficient. 

To be specific, we consider the two-dimensional growth of a pure substance from its undercooled melt in about its simplest form, where the growth is controlled by the diffusion of the latent heat of freezing. It obeys the diffusion equation and appropriate boundary conditions [95]... 

Here U = T — T )Cp/L is the appropriately rescaled temperature field T measured from the imposed temperature of the undercooled melt far away from the interface. The indices L and 5 refer to the liquid and solid, respectively, and the specific heat Cp and the thermal diffusion constant D are considered to be the same in both phases. L is the latent heat, and n is the normal to the interface. In terms of these parameters,... 

For example, in the case of the reversible isothermal transformation of ice to water at the melting point (273 K), the heat gained by the ice will be the latent heat of fusion (A//f = 6(X)6 J mol" ) and a corresponding quantity of heat will be lost by the surrounding, and... 

Variations in cooling load can be provided from the latent heat of melting of ice or a frozen eutectic. Ice can be formed by allowing it... 

The theorem also applies to a heterogeneous system, such as a liquid in presence of its saturated vapour, or in presence of the solid. In the former case, vapour is liquefied by compression and gives out its latent heat. Under isothermal conditions this would escape as fast as produced, but if the heat is compelled to remain in the system, it raises the temperature and thereby increases the pressure. If, on the other hand, a mixture of ice and water is compressed, ice melts and the mass is cooled by abstraction of heat. If heat is allowed to enter from outside, so as to restore the original temperature, more ice melts, and the pressure falls by reason of the contraction. 

Example 1.—If the specific heats of the solid and liquid forms are linear functions of temperature, show that the melting-point is determined by dividing the latent heat of fusion by the difference between the specific heats of the solid and liquid forms at the melting-point (cf. Taramann, Kryst. and Schmelz., p. 42). 

In this description the temperature field has been taken to be linear in the coordinate y and to be independent of the shape of the melt/crystal interface. This is a good assumption for systems with equal thermal conductivities in melt and crystal and negligible convective heat transport and latent heat release. Extensions of the model that include determination of the temperature field are discussed in the original analysis of Mullins and Sekerka (17) and in other papers (18,19). 

Latent heat of fusion at melting point 16.7cal/g... 

The nature of the heat of fusion AHu deserves particular attention, for it represents the heat required to melt one mole of crystalline units it does not refer to the latent heat AJT required to melt such crystallinity as may occur in a given semicrystalline polymer. The depression of the melting point Tm, already defined as the maximum temperature at which crystalline regions may coexist with amorphous poly-... 

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