# Numerical Solution of 1-D Steady Conduction

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## 3.5 Melting and Solidification

### 3.5.1 Introduction

Melting and solidification find application in the geophysical sciences; industrial forming operations such as casting and laser drilling; latent heat energy storage systems; and food and pharmaceutical processing. Any manmade metal products must undergo liquid forms at some point during manufacturing processes and solidify to form intermediate or final products. Melting and solidification processes can be classified as one of three types: one-region, two-region, and multiple-region. The classification depends on the properties of the phase change material (PCM) involved and the initial conditions. For a single-component PCM, melting or solidification occurs at a single temperature. Pure water, for example, melts at a uniform temperature of 0 °C, while pure n-Octadecane (*C*_{18}*H*_{38}) melts at 28 °C. For the solid-liquid phase change process of a PCM with a single melting point, the solid-liquid interface appears as a clearly-observable sharp border. Initial conditions for the solid-liquid phase change process of a single-component PCM determine whether the problem will be classified as a one- or two-region problem.

For the melting (or solidification) process, if the initial temperature of the PCM, *T*_{i}, equals the melting point, *T*_{m}, the temperature in the solid (liquid) phase remains uniformly equal to the melting point throughout the process. In this case, only the temperature distribution in the liquid (solid) phase needs to be determined. Thus, the temperature of only one phase is unknown and the problem is called a *one-region problem*. Figure 3.29 shows the temperature distribution of one-dimensional, one-region melting and solidification problems. The surface temperature, *T*_{0} , is greater than *T*_{m} for melting and is below *T*_{m} for solidification.

For the melting process, if the initial temperature of the PCM, *T*_{i}, is below the melting point of the PCM, *T*_{m}, (or above, for solidification), the temperature distribution of both the liquid and solid phases must be determined; this is called a *two-region problem*. Figure 3.30 shows the temperature distribution of one-dimensional two-region melting and solidification problems.

For a multi-component PCM, the solid-liquid phase change process occurs over a range of temperatures (*T*_{m}1, *T*_{m2}), instead of a single temperature. The PCM is liquid if its temperature is above *T*_{m2} and solid when its temperature is below *T*_{m1}. Between the solid and liquid phases there is a *mushy zone* where the temperature falls within the phase change temperature range (*T*_{m1}, *T*_{m2}). Successful solution of phase change problems involving these substances requires determination of the temperature distribution in the liquid, solid, and mushy zones; therefore, these are referred to as *multiregion problems*. The temperature distribution of one-dimensional solidification in a multicomponent PCM is shown in Fig. 3.31, where it can be seen that the solution requires tracking of two interfaces.

In solid-liquid phase change problems, the location of the solid-liquid interface is unknown before the final solution is obtained and this presents a special difficulty. Since the interface also moves during melting or solidification, such problems are referred to as *moving boundary problems* and always have time as an independent variable.

For a solid-liquid phase change of a PCM with a single melting temperature, the solid-liquid interface clearly delineates the liquid and solid phases. The boundary conditions at this interface must be specified in order to solve the problem. As shown for the one-dimensional melting problem illustrated in Fig. 3.30, the solid-liquid interface separates the liquid and solid phases. The temperatures of the liquid and solid phases near the interface must equal the

temperature of the interface, which is at the melting point, *T*_{m}. Therefore, the boundary conditions at the interface can be expressed as

where and *T*_{s}(*x*,*t*) are the temperatures of the liquid and solid phases, respectively. The density of the PCM usually differs between the liquid and solid phases; therefore, density change always accompanies the phase change process. The solid PCM is usually denser than the liquid PCM, except for a few substances such as water and gallium. For example, the volume of paraffin, a very useful PCM for energy storage systems, expands about when it melts. Therefore, the density of the liquid paraffin, is less than the density of the solid paraffin, ρ_{s}. When water freezes, however, its volume increases, so the density of ice is less than that of liquid water.
The density change that occurs during solid-liquid phase change will produce an extra increment of motion in the solid-liquid interface. For the melting problem in Fig. 3.30(a), where the liquid phase velocity at *x* = 0 is zero, if the density of the solid is larger than the density of the liquid (i.e., ) the resulting extra motion of the interface is along the positive direction of the *x*- axis. Assume that the velocity of the solid-liquid interface due to phase change is *u*_{p} = *d**s* / *d**t*, while the extra velocity of the solid-liquid interface due to density change is *u*_{ρ}. The density change must satisfy the conservation of mass at the interface, i.e.,

(3.473)

The extra velocity induced by the density change can be obtained by rearranging eq. (3.473) as

(3.474)

which is also valid for case except the extra velocity becomes negative. Another necessary boundary condition is the energy balance at the solid-liquid interface. If the enthalpy of the liquid and solid phases at the melting point are and *h*_{s}, the energy balance at the solid-liquid interface can be expressed as:

(3.475)

where and *q*''_{s} are the heat fluxes in the *x*-direction in the liquid and solid phases, respectively. Substituting eq. (3.474) into eq. (3.475), one obtains

(3.476)

where is the latent heat of melting. If convection in the liquid phase can be neglected and heat conduction is the only heat transfer mechanism in both the liquid and solid phases, the heat flux in both phases can be determined by Fourier’s law of conduction:

(3.477)

(3.478)

The energy balance at the solid-liquid interface for a melting problem can be obtained by substituting eqs. (3.477) and (3.478) into eq. (3.476), i.e.,

(3.479)

For a solidification process, the energy balance equation at the interface can be obtained by a similar procedure:

(3.480)

The only difference between eqs. (3.479) or (3.480) is the density on the right-hand side of the equation. If the temperature distributions in the liquid and solid phases are known, the location of the solid-liquid interface can be obtained by solving eqs. (3.479) or (3.480). It should be noted that density change causes advection in the liquid phase, which further complicates the problem. The above boundary conditions are valid for one-dimensional problems only. For multi-dimensional phase change problems, the boundary conditions at the interface can be expressed as

(3.481)

(3.482)

where is a unit vector along the normal direction of the solid-liquid interface, and *v*_{n} is the solid-liquid interface velocity along the n-direction.
It is apparent that eq. (3.482) is not convenient for numerical solution because it contains temperature derivatives along the *n*-direction. Suppose the shape of the solid-liquid interface can be expressed as

*x*=

*s*(

*y*,

*t*)

Equation (3.482) can then become the following form (see Problem 3.55; Ozisik, 1993):

(3.483)

Similarly, for a three-dimensional melting problem with an interface described by

*z*=

*s*(

*x*,

*y*,

*t*)

the energy balance at the interface is

(3.484)

For solidification problems, it is necessary to replace the liquid density in eqs. (3.483) and (3.484) with the solid-phase density ρ_{s}. The density change in solid-liquid phase change is often neglected in the literature in order to eliminate the additional interface motion discussed earlier. This section presents a number of prototypical problems of melting and solidification; its goal is to establish the physics of this form of phase change and to demonstrate the variety of tools available for its solution.

## 3.5.2 Exact Solution

### Governing Equations of the Solidification Problem

The physical model of the solidification problem to be investigated in this subsection is shown in Fig. 3.30(b), where a liquid PCM with a uniform initial temperature *T*_{i}, which exceeds the melting point *T*_{m}, is in a half-space *x* > 0. At time *t* = 0, the temperature at the boundary *x* = 0 is suddenly decreased to a temperature *T*_{0}, which is below the melting point of the PCM. Solidification occurs from the time *t* = 0. This is a two-region solidification problem as the temperatures of both the liquid and solid phases are unknown and must be determined. It is assumed that the densities of the PCM for both phases are the same. Natural convection in the liquid phase is neglected, and therefore the heat transfer mechanism in both phases is pure conduction.
The temperature in the solid phase must satisfy

(3.485)

(3.486)

For the liquid phase, the governing equations are

(3.487)

(3.488)

(3.489)

The boundary conditions at the interface are

(3.490)

(3.491)

Before obtaining the solution of the above problem, a scale analysis of the energy balance eq. (3.491) is performed. At the solid-liquid interface, the scales of derivatives of solid and liquid temperature are

The scale of the interface velocity is

Substituting the above equations into eq. (3.491), one obtains

The scale of the location of the solid-liquid interface is obtained by rearranging the above equation, i.e.,

(3.492)

where

(3.493)

is the Stefan number. Named after J. Stefan, a pioneer in discovery of the solid-liquid phase change phenomena, the Stefan number is a very important dimensionless variable in solid-liquid phase change phenomena, The Stefan number represents the ratio of sensible heat, to latent heat, . For a latent heat thermal energy storage system, the Stefan number is usually very small because the temperature difference in such a system is small, while the latent heat is very high. Therefore, the effect of the sensible heat transfer on the motion of the solid-liquid interface is very weak, and various approximate solutions to the phase change problem can be introduced without incurring significant error. It can be seen from eq. (3.492) that the effect of heat conduction in the liquid phase can be neglected if or . In that case, eq. (3.492) can be simplified as

(3.494)

which means that the interfacial velocity increases with increasing or decreasing .

### Dimensionless Form of the Governing Equations

The governing eqs. (3.485) – (3.491) can be nondimensionalized by introducing the following dimensionless variables:

(3.495)

where *L* is a characteristic length of the problem and can be determined by the nature of the problem or requirement of the solution procedure. The dimensionless governing equations are as follows:

(3.496)

(3.497)

(3.498)

(3.499)

(3.500)

(3.501)

(3.502)

Dimensionless temperature distribution in a PCM can be qualitively illustrated by Fig. 3.32. It can be seen that the dimensionless temperature distribution is similar to that of a melting problem. Equations (3.496) – (3.502) are also valid for melting problems, provided that the subscripts “1” and “2” represent liquid and solid, respectively. The following solutions of one-region and two-region problems will be valid for both melting and solidification problems.