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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: oneregion, tworegion, and multipleregion. The classification depends on the properties of the phase change material (PCM) involved and the initial conditions. For a singlecomponent PCM, melting or solidification occurs at a single temperature. Pure water, for example, melts at a uniform temperature of 0 °C, while pure nOctadecane (<math>{C_{18}}{H_{38}}</math>) melts at 28 °C. For the solidliquid phase change process of a PCM with a single melting point, the solidliquid interface appears as a clearlyobservable sharp border. Initial conditions for the solidliquid phase change process of a singlecomponent PCM determine whether the problem will be classified as a one or tworegion problem.
 
   
  For the melting (or solidification) process, if the initial temperature of the PCM, <math>{T_i}</math>, equals the melting point, <math>{T_m}</math>, 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 ''oneregion problem''. Figure 3.29 shows the temperature distribution of onedimensional, oneregion melting and solidification problems. The surface temperature, <math>{T_0}</math> , is greater than <math>{T_m}</math> for melting and is below <math>{T_m}</math> for solidification.
 
 
 
  [[Image:Chapter3_(3).pngthumb400 pxalt=Oneregion melting and solidification<center> (a) Melting (<math></math>) (b) Solidification (<math></math>)</center> <br> Figure 1: Oneregion melting and solidification]]
 
 
 
  [[Image:Chapter3_(3).pngthumb400 pxalt=Tworegion melting and solidification.<center> (a) Melting (<math></math>) (b) Solidification (<math></math>)</center> <br> Figure 2: Tworegion melting and solidification.]]
 
 
 
  For the melting process, if the initial temperature of the PCM, <math>{T_i}</math>, is below the melting point of the PCM, <math>{T_m}</math>, (or above, for solidification), the temperature distribution of both the liquid and solid phases must be determined; this is called a ''tworegion problem''. Figure 3.30 shows the temperature distribution of onedimensional tworegion melting and solidification problems.
 
 
 
  For a multicomponent PCM, the solidliquid phase change process occurs over a range of temperatures (<math>{T_m}</math>1, <math>{T_{m2}}</math>), instead of a single temperature. The PCM is liquid if its temperature is above <math>{T_{m2}}</math> and solid when its temperature is below <math>{T_{m1}}</math>. Between the solid and liquid phases there is a ''mushy zone'' where the temperature falls within the phase change temperature range (<math>{T_{m1}}</math>, <math>{T_{m2}}</math>). 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 onedimensional 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 solidliquid phase change problems, the location of the solidliquid 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 solidliquid phase change of a PCM with a single melting temperature, the solidliquid 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 onedimensional melting problem illustrated in Fig. 3.30, the solidliquid interface separates the liquid and solid phases. The temperatures of the liquid and solid phases near the interface must equal the
 
 
 
  [[Image:Chapter3_(4).pngthumb400 pxalt=Solidification of a multicomponent PCMFigure 1: Solidification of a multicomponent PCM]]
 
 
 
  temperature of the interface, which is at the melting point, <math>{T_m}.</math> Therefore, the boundary conditions at the interface can be expressed as
 
 
 
  <center><math>{T_\ell }(x,t) = {T_s}(x,t) = {T_m},\quad \quad x = s(t) \qquad \qquad( )</math></center>(3.472)
 
 
 
  where <math>{T_\ell }(x,t)</math> and <math>{T_s}(x,t)</math> 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 <math>10\% </math> when it melts. Therefore, the density of the liquid paraffin, <math>{\rho _\ell },</math> is less than the density of the solid paraffin, <math>{\rho _s}.</math> 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 solidliquid phase change will produce an extra increment of motion in the solidliquid interface. For the melting problem in Fig. 3.30(a), where the liquid phase velocity at <math>x = 0</math> is zero, if the density of the solid is larger than the density of the liquid (i.e., <math>{\rho _s} > {\rho _\ell }</math>) the resulting extra motion of the interface is along the positive direction of the <math>x</math> axis. Assume that the velocity of the solidliquid interface due to phase change is <math>{u_p} = ds/dt,</math> while the extra velocity of the solidliquid interface due to density change is <math>{u_\rho }</math>. The density change must satisfy the conservation of mass at the interface, i.e.,
 
 
 
  <center><math>{\rho _s}({u_p}  {u_\rho }) = {\rho _\ell }{u_p} \qquad \qquad( )</math></center>
 
  (3.473)
 
 
 
  The extra velocity induced by the density change can be obtained by rearranging eq. (3.473) as
 
 
 
  <center><math>{u_\rho } = \frac{{{\rho _s}  {\rho _\ell }}}{{{\rho _s}}}{u_p} \qquad \qquad( )</math></center>
 
  (3.474)
 
 
 
  which is also valid for case <math>{\rho _s} > {\rho _\ell },</math> except the extra velocity becomes negative. Another necessary boundary condition is the energy balance at the solidliquid interface. If the enthalpy of the liquid and solid phases at the melting point are <math>{h_\ell }</math> and <math>{h_s}</math>, the energy balance at the solidliquid interface can be expressed as:
 
 
 
  <center><math>{q''_\ell }  {q''_s} = {\rho _\ell }{u_p}{h_\ell }  {\rho _s}({u_p}  {u_\rho }){h_s}\quad \quad x = s(t) \qquad \qquad( )</math></center>
 
  (3.475)
 
 
 
  where <math>{q''_\ell }</math> and <math>{q''_s}</math> are the heat fluxes in the <math>x</math>direction in the liquid and solid phases, respectively. Substituting eq. (3.474) into eq. (3.475), one obtains
 
 
 
  <center><math>{q''_\ell }  {q''_s} = {\rho _\ell }{h_{s\ell }}\frac{{ds}}{{dt}}\quad \quad x = s(t) \qquad \qquad( )</math></center>
 
  (3.476)
 
 
 
  where <math>{h_{s\ell }} = {h_\ell }  {h_s}</math> 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:
 
 
 
  <center><math>{q''_\ell } = \mathop {\left. {  {k_\ell }\frac{{\partial {T_\ell }(x,t)}}{{\partial x}}} \right}{x = s(t)} \qquad \qquad( )</math></center>
 
  (3.477)
 
 
 
  <center><math>{q''_s} = \mathop {\left. {  {k_s}\frac{{\partial {T_s}(x,t)}}{{\partial x}}} \right}{x = s(t)} \qquad \qquad( )</math></center>
 
  (3.478)
 
 
 
  The energy balance at the solidliquid interface for a melting problem can be obtained by substituting eqs. (3.477) and (3.478) into eq. (3.476), i.e.,
 
 
 
  <center><math>{k_s}\frac{{\partial {T_s}(x,t)}}{{\partial x}}  {k_\ell }\frac{{\partial {T_\ell }(x,t)}}{{\partial x}} = {\rho _\ell }{h_{s\ell }}\frac{{ds(t)}}{{dt}}\quad \quad x = s(t) \qquad \qquad( )</math></center>
 
  (3.479)
 
 
 
  For a solidification process, the energy balance equation at the interface can be obtained by a similar procedure:
 
 
 
  <center><math>{k_s}\frac{{\partial {T_s}(x,t)}}{{\partial x}}  {k_\ell }\frac{{\partial {T_\ell }(x,t)}}{{\partial x}} = {\rho _s}{h_{s\ell }}\frac{{ds(t)}}{{dt}}\quad \quad x = s(t) \qquad \qquad( )</math></center>
 
  (3.480)
 
 
 
  The only difference between eqs. (3.479) or (3.480) is the density on the righthand side of the equation. If the temperature distributions in the liquid and solid phases are known, the location of the solidliquid 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 onedimensional problems only. For multidimensional phase change problems, the boundary conditions at the interface can be expressed as
 
 
 
  <center><math>{T_\ell }(x,y,t) = {T_s}(x,y,t) = {T_m}\quad \quad x = s(y,t) \qquad \qquad( )</math></center>
 
  (3.481)
 
 
 
  <center><math>{k_s}\frac{{\partial {T_s}(x,y,t)}}{{\partial n}}  {k_\ell }\frac{{\partial {T_\ell }(x,y,t)}}{{\partial n}} = {\rho _\ell }{h_{s\ell }}{v_n}\quad \quad x = s(y,t) \qquad \qquad( )</math></center>
 
  (3.482)
 
 
 
  where <math>{\mathbf{n}}</math> is a unit vector along the normal direction of the solidliquid interface, and <math>{v_n}</math> is the solidliquid interface velocity along the ndirection.
 
  It is apparent that eq. (3.482) is not convenient for numerical solution because it contains temperature derivatives along the <math>n</math>direction. Suppose the shape of the solidliquid interface can be expressed as
 
 
 
  <center><math>x = s(y,t)</math></center>
 
 
 
  Equation (3.482) can then become the following form [[#References(see Problem 3.55; Ozisik, 1993)]]:
 
 
 
  <center><math>\left[ {1 + \mathop {\left( {\frac{{\partial s}}{{\partial y}}} \right)}^2 } \right]\left[ {{k_s}\frac{{\partial {T_s}}}{{\partial x}}  {k_\ell }\frac{{\partial {T_\ell }}}{{\partial x}}} \right] = {\rho _\ell }{h_{s\ell }}\frac{{\partial s}}{{\partial t}}\quad \quad x = s(y,t) \qquad \qquad( )</math></center>
 
  (3.483)
 
 
 
  Similarly, for a threedimensional melting problem with an interface described by
 
 
 
  <center><math>z = s(x,y,t)</math></center>
 
 
 
  the energy balance at the interface is
 
 
 
  <center><math>\left[ {1 + \mathop {\left( {\frac{{\partial s}}{{\partial x}}} \right)}^2 + \mathop {\left( {\frac{{\partial s}}{{\partial y}}} \right)}^2 } \right]\left[ {{k_s}\frac{{\partial {T_s}}}{{\partial x}}  {k_\ell }\frac{{\partial {T_\ell }}}{{\partial x}}} \right] = {\rho _\ell }{h_{s\ell }}\frac{{\partial s}}{{\partial t}}\quad \quad z = s(x,y,t) \qquad \qquad( )</math></center>
 
  (3.484)
 
 
 
  For solidification problems, it is necessary to replace the liquid density <math>{\rho _\ell }</math> in eqs. (3.483) and (3.484) with the solidphase density <math>{\rho _s}.</math> The density change in solidliquid 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 <math>{T_i},</math> which exceeds the melting point <math>{T_m},</math> is in a halfspace <math>x > 0.</math> At time <math>t = 0,</math> the temperature at the boundary <math>x = 0</math> is suddenly decreased to a temperature <math>{T_0},</math> which is below the melting point of the PCM. Solidification occurs from the time <math>t = 0.</math> This is a tworegion 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
 
 
 
  <center><math>\frac{{{\partial ^2}{T_1}}}{{\partial {x^2}}} = \frac{1}{{{\alpha _1}}}\frac{{\partial {T_1}}}{{\partial t}}\quad \quad 0 < x < s(t),\quad t > 0 \qquad \qquad( )</math></center>
 
  (3.485)
 
 
 
  <center><math>{T_1}(x,t) = {T_0}\quad \quad x = 0,\quad t > 0 \qquad \qquad( )</math></center>
 
  (3.486)
 
 
 
  For the liquid phase, the governing equations are
 
 
 
  <center><math>\frac{{{\partial ^2}{T_2}}}{{\partial {x^2}}} = \frac{1}{{{\alpha _2}}}\frac{{\partial {T_2}}}{{\partial t}}\quad \quad s(t) < x < \infty ,\quad t > 0 \qquad \qquad( )</math></center>
 
  (3.487)
 
 
 
  <center><math>{T_2}(x,t) \to {T_i}\quad \quad x \to \infty ,\quad t > 0 \qquad \qquad( )</math></center>
 
  (3.488)
 
 
 
  <center><math>{T_2}(x,t) = {T_i}\quad \quad x > 0,\quad t = 0 \qquad \qquad( )</math></center>
 
  (3.489)
 
 
 
  The boundary conditions at the interface are
 
 
 
  <center><math>{T_1}(x,t) = {T_2}(x,t) = {T_m}\quad \quad x = s(t),\quad t > 0 \qquad \qquad( )</math></center>
 
  (3.490)
 
 
 
  <center><math>{k_1}\frac{{\partial {T_1}}}{{\partial x}}  {k_2}\frac{{\partial {T_2}}}{{\partial x}} = \rho {h_{s\ell }}\frac{{ds}}{{dt}}\quad \quad x = s(t),\quad t > 0 \qquad \qquad( )</math></center>
 
  (3.491)
 
 
 
  Before obtaining the solution of the above problem, a scale analysis of the energy balance eq. (3.491) is performed. At the solidliquid interface, the scales of derivatives of solid and liquid temperature are
 
 
 
  <center><math>\frac{{\partial {T_1}}}{{\partial x}} \sim \frac{{{T_m}  {T_0}}}{s}</math></center>
 
 
 
  <center><math>\frac{{\partial {T_2}}}{{\partial x}} \sim \frac{{{T_i}  {T_m}}}{s}</math></center>
 
 
 
  The scale of the interface velocity is
 
 
 
  <center><math>\frac{{ds}}{{dt}} \sim \frac{s}{t}</math></center>
 
 
 
  Substituting the above equations into eq. (3.491), one obtains
 
 
 
  <center><math>{k_1}\frac{{{T_m}  {T_0}}}{s}  {k_2}\frac{{{T_i}  {T_m}}}{s} \sim \rho {h_{s\ell }}\frac{s}{t}</math></center>
 
 
 
  The scale of the location of the solidliquid interface is obtained by rearranging the above equation, i.e.,
 
 
 
  <center><math>\frac{{{s^2}}}{{{\alpha _1}t}} \sim {\rm{Ste}}\left( {1  \frac{{{k_2}}}{{{k_1}}}\frac{{{T_i}  {T_m}}}{{{T_m}  {T_0}}}} \right) \qquad \qquad( )</math></center>
 
  (3.492)
 
 
 
  where
 
 
 
  <center><math>{\rm{Ste}} = \frac{{{c_{p1}}({T_m}  {T_0})}}{{{h_{s\ell }}}} \qquad \qquad( )</math></center>
 
  (3.493)
 
 
 
  is the Stefan number. Named after J. Stefan, a pioneer in discovery of the solidliquid phase change phenomena, the Stefan number is a very important dimensionless variable in solidliquid phase change phenomena, The Stefan number represents the ratio of sensible heat, <math>{c_{{p_1}}}({T_m}  {T_0}),</math> to latent heat, <math>{h_{s\ell }}</math>. 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 <math>{h_{s\ell }}</math> is very high. Therefore, the effect of the sensible heat transfer on the motion of the solidliquid 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 <math>{T_i}  {T_m} \ll {T_m}  {T_0}</math> or <math>{k_2} \ll {k_1}</math>. In that case, eq. (3.492) can be simplified as
 
 
 
  <center><math>\frac{{{s^2}}}{{{\alpha _1}t}} \sim {\rm{Ste}} \qquad \qquad( )</math></center>
 
  (3.494)
 
 
 
  which means that the interfacial velocity increases with increasing <math>\Delta T = \left {{T_m}  {T_0}} \right</math> or decreasing <math>{h_{s\ell }}</math>.
 
 
 
  ===Dimensionless Form of the Governing Equations===
 
 
 
  The governing eqs. (3.485) – (3.491) can be nondimensionalized by introducing the following dimensionless variables:
 
 
 
  <center><math>\left. \begin{array}{l}
 
  \theta = \frac{{{T_m}  T}}{{{T_m}  {T_0}}}\,\,\,\,{\theta _i} = \frac{{{T_m}  {T_i}}}{{{T_m}  {T_0}}}\,\,\,X = \frac{x}{L}\quad S = \frac{s}{L}\quad \tau = \frac{{{\alpha _1}t}}{{{L^2}}} \\
 
  {N_\alpha } = \frac{{{\alpha _2}}}{{{\alpha _1}}}\,\,\,\,\,\,{N_k} = \frac{{{k_2}}}{{{k_1}}}\,\,\,\,{\rm{Ste}} = \frac{{{c_{{p_1}}}({T_m}  {T_0})}}{{{h_{s\ell }}}} \\
 
  \end{array} \right\} \qquad \qquad( )</math></center>
 
  (3.495)
 
 
 
  where <math>L</math> 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:
 
 
 
  <center><math>\frac{{{\partial ^2}{\theta _1}}}{{\partial {X^2}}} = \frac{{\partial {\theta _1}}}{{\partial \tau }}\quad \quad 0 < X < S(\tau ),\quad \tau > 0 \qquad \qquad( )</math></center>
 
  (3.496)
 
 
 
  <center><math>{\theta _1}(X,\tau ) = 1\quad \quad X = 0,\quad \tau > 0 \qquad \qquad( )</math></center>
 
  (3.497)
 
 
 
  <center><math>\frac{{{\partial ^2}{\theta _2}}}{{\partial {X^2}}} = \frac{1}{{{N_\alpha }}}\frac{{\partial {\theta _2}}}{{\partial \tau }}\quad \quad S(\tau ) < X < \infty ,\quad \tau > 0 \qquad \qquad( )</math></center>
 
  (3.498)
 
 
 
  <center><math>{\theta _2}(X,\tau ) \to {\theta _i}\quad \quad X \to \infty ,\quad \tau > 0 \qquad \qquad( )</math></center>
 
  (3.499)
 
 
 
  <center><math>{\theta _2}(X,\tau ) = {\theta _i}\quad \quad X > 0,\quad \tau = 0 \qquad \qquad( )</math></center>
 
  (3.500)
 
 
 
  <center><math>{\theta _1}(X,\tau ) = {\theta _2}(X,\tau ) = 0\quad \quad X = S(\tau ),\quad \tau > 0 \qquad \qquad( )</math></center>
 
  (3.501)
 
 
 
  <center><math>  \frac{{\partial {\theta _1}}}{{\partial X}} + {N_k}\frac{{\partial {\theta _2}}}{{\partial X}} = \frac{1}{{Ste}}\frac{{dS}}{{d\tau }}\quad \quad X = S(\tau ),\quad \tau > 0 \qquad \qquad( )</math></center>
 
  (3.502)
 
 
 
  [[Image:Chapter3_(4).jpgthumb400 pxalt=Dimensionless temperature distribution in the PCMFigure : Dimensionless temperature distribution in the PCM]]
 
 
 
  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 oneregion and tworegion problems will be valid for both melting and solidification problems.
 
 
 
  ===Exact Solution of the OneRegion Problem===
 