Effect of surface tension on condensation in porous media

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The analysis in the preceding subsection is valid for gravity-dominated condensation in porous media (<math>{\rm{Bo}} \gg 1</math>
+
The analysis in the [[gravity dominated film-condensation in a porous medium]] is valid for <math>\text{Bo}\gg 1</math>. When the condensation is gravity-capillary forces dominated (<math>\text{Bo}\sim 1</math>) or capillary force dominated (<math>\text{Bo}\ll 1</math>), there will be a two-phase region that is saturated by a mixture of liquid and vapor. The fraction of liquid in the pore space is defined as saturation:
-
). When the condensation is gravity-capillary forces dominated (<math>{\rm{Bo}} \sim 1</math>
+
 
-
) or capillary force dominated (<math>{\rm{Bo}} \ll 1</math>
+
{| class="wikitable" border="0"
-
), there will be a two-phase region that is saturated by a mixture of liquid and vapor, as shown in Fig. 8.29. The fraction of liquid in the pore space is defined as saturation:
+
|-
-
<center><math>
+
| width="100%" |
-
\gamma _\ell   = \frac{{\varepsilon _\ell }}{\varepsilon }
+
<Center><math>{{\gamma }_{\ell }}=\frac{{{\varepsilon }_{\ell }}}{\varepsilon }</math></center>
-
</math></center>
+
|{{EquationRef|(1)}}
-
(8.386)
+
|}
-
where <math>\varepsilon _\ell </math>
+
 
-
and <math>\varepsilon </math>
+
where <math>{{\varepsilon }_{\ell }}</math> and <math>\varepsilon </math> are volume fraction of the liquid and porosity in the porous media. The effect of surface tension on condensation in porous media was studied by Majumdar and Tien (1990) and their work will be briefly described below.
-
are volume fraction of the liquid and porosity in the porous media. The effect of surface tension on condensation in porous media was studied by Majumdar and Tien (1990) and their work will be briefly described below.
+
 
The continuity equation for the two-phase region is
The continuity equation for the two-phase region is
-
<center><math>
+
 
-
\rho _\ell \left( {\frac{{\partial u_\ell }}{{\partial x}} + \frac{{\partial v_\ell }}{{\partial y_2 }}} \right) + \rho _v \left( {\frac{{\partial u_v }}{{\partial x}} + \frac{{\partial v_v }}{{\partial y_2 }}} \right) = 0
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.387)
+
| width="100%" |
-
where y2 is measured from the interface between liquid and the two-phase region (see Fig. 8.29). The mass fluxes for liquid and vapor are governed by Darcy’s law, as follows:
+
<center><math>{{\rho }_{\ell }}\left( \frac{\partial {{u}_{\ell }}}{\partial x}+\frac{\partial {{v}_{\ell }}}{\partial {{y}_{2}}} \right)+{{\rho }_{v}}\left( \frac{\partial {{u}_{v}}}{\partial x}+\frac{\partial {{v}_{v}}}{\partial {{y}_{2}}} \right)=0</math></center>
-
<center><math>
+
|{{EquationRef|(2)}}
-
\dot m''_\ell   = - \frac{{KK_{r\ell } }}{{\nu _\ell }}\nabla p_\ell
+
|}
-
</math></center>
+
 
-
(8.388)
+
where ''y<sub>2</sub>'' is measured from the interface between liquid and the two-phase region (see Fig. 8.29). The mass fluxes for liquid and vapor are governed by Darcy’s law, as follows:
-
<center><math>
+
 
-
\dot m''_v  = - \frac{{KK_{rv} }}{{\nu _v }}\nabla p_v
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.389)
+
| width="100%" |
-
where <math>K_{r\ell } </math>
+
<center><math>{{{\dot{m}}''}_{\ell }}=-\frac{K{{K}_{r\ell }}}{{{\nu }_{\ell }}}\nabla {{p}_{\ell }}</math></center>
-
and <math>K_{rv} </math>
+
|{{EquationRef|(3)}}
-
are relative permeabilities (dimensionless) for liquid and vapor phases, respectively. Their products with the permeability K represent the permeability for liquid and vapor flow in the porous media.   
+
|}
-
Since the density of the vapor is much lower than that of the liquid, the pressure variation in the liquid phase is very insignificant. Also, the change of capillary pressure is due mainly to change of liquid pressure. Consequently, the vapor flow is negligible compared to the liquid flow, so that eq. (8.387) is reduced to  
+
 
-
<center><math>
+
{| class="wikitable" border="0"
-
\frac{{\partial u_\ell }}{{\partial x}} + \frac{{\partial v_\ell }}{{\partial y_2 }} = 0
+
|-
-
</math></center>
+
| width="100%" |
-
(8.390)
+
<center><math>{{{\dot{m}}''}_{v}}=-\frac{K{{K}_{rv}}}{{{\nu }_{v}}}\nabla {{p}_{v}}</math></center>
 +
|{{EquationRef|(4)}}
 +
|}
 +
 
 +
where <math>{{K}_{r\ell }}</math> and <math>{{K}_{rv}}</math> are relative permeabilities (dimensionless) for liquid and vapor phases, respectively. Their products with the permeability ''K'' represent the permeability for liquid and vapor flow in the porous media.   
 +
 
 +
Since the density of the vapor is much lower than that of the liquid, the pressure variation in the liquid phase is very insignificant. Also, the change of capillary pressure is due mainly to change of liquid pressure. Consequently, the vapor flow is negligible compared to the liquid flow, so that eq. (2) is reduced to  
 +
 
 +
{| class="wikitable" border="0"
 +
|-
 +
| width="100%" |
 +
<center><math>\frac{\partial {{u}_{\ell }}}{\partial x}+\frac{\partial {{v}_{\ell }}}{\partial {{y}_{2}}}=0</math></center>
 +
|{{EquationRef|(5)}}
 +
|}
The velocity components in the x- and y- directions are
The velocity components in the x- and y- directions are
-
<center><math>
+
 
-
u_\ell   = K_{r\ell } u_D  = \frac{{KK_{r\ell } (\rho _\ell   - \rho _v )g}}{{\mu _\ell }}
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.391)
+
| width="100%" |
-
<center><math>
+
<center><math>{{u}_{\ell }}={{K}_{r\ell }}{{u}_{D}}=\frac{K{{K}_{r\ell }}({{\rho }_{\ell }}-{{\rho }_{v}})g}{{{\mu }_{\ell }}}</math></center>
-
v_\ell   = - \frac{{KK_{r\ell } }}{{\mu _\ell }}\frac{{\partial p_\ell }}{{\partial y_2 }} = \frac{{KK_{r\ell } }}{{\mu _\ell }}\frac{{\partial p_{cap} }}{{\partial y_2 }}
+
|{{EquationRef|(6)}}
-
</math></center>
+
|}
-
(8.392)
+
 
-
where <center><math>
+
{| class="wikitable" border="0"
-
u_D  = K(\rho _\ell   - \rho _v )g/\mu _\ell
+
|-
-
</math></center>
+
| width="100%" |
-
is Darcian velocity. The capillary pressure can be written as  
+
<center><math>{{v}_{\ell }}=-\frac{K{{K}_{r\ell }}}{{{\mu }_{\ell }}}\frac{\partial {{p}_{\ell }}}{\partial {{y}_{2}}}=\frac{K{{K}_{r\ell }}}{{{\mu }_{\ell }}}\frac{\partial {{p}_{cap}}}{\partial {{y}_{2}}}</math></center>
-
<center><math>
+
|{{EquationRef|(7)}}
-
p_{cap} = \frac{\sigma }{{\sqrt {K/\varepsilon } }}f(s)
+
|}
-
</math></center>
+
 
-
(8.393)
+
where <math>{{u}_{D}}=K({{\rho }_{\ell }}-{{\rho }_{v}})g/{{\mu }_{\ell }}</math> is Darcian velocity. The capillary pressure can be written as  
-
where f(s) is a Leverett’s function (see Section 4.6.6):
+
 
-
<center><math>
+
{| class="wikitable" border="0"
-
f(s) = 1.417(1 - s) - 2.120(1 - s)^2 + 1.263(1 - s)^3  
+
|-
-
</math></center>
+
| width="100%" |
-
(8.394)
+
<center><math>{{p}_{cap}}=\frac{\sigma }{\sqrt{K/\varepsilon }}f(s)</math></center>
-
and s is dimensionless saturation defined as
+
|{{EquationRef|(8)}}
-
<center><math>
+
|}
-
s = \frac{{\gamma _\ell   - \gamma _{\ell i} }}{{1 - \gamma _{\ell i} }}
+
 
-
</math></center>
+
where ''f''(s) is a Leverett’s function:
-
(8.395)
+
 
-
where <math>\gamma _{\ell i} </math>
+
{| class="wikitable" border="0"
-
is irreducible saturation, below which liquid flow will not occur. The relative permeability in eqs. (8.391) and (8.392) is obtained from
+
|-
-
<center><math>
+
| width="100%" |
-
K_{r\ell } = s^3  
+
<center><big><big><math>f(s)=1.417(1-s)-2.120{{(1-s)}^{2}}+1.263{{(1-s)}^{3}}</math></big></big></center>
-
</math></center>
+
|{{EquationRef|(9)}}
-
(8.396)
+
|}
-
Substituting eqs. (8.391) – (8.396) into eq. (8.390), the following form of the dimensionless continuity equation is obtained:
+
 
-
<center><math>
+
and ''s'' is dimensionless saturation defined as
-
3\frac{{\partial s}}{{\partial x}} + \left<center><math> {\frac{{\sigma /\sqrt {K/\varepsilon } }}{{(\rho _\ell   - \rho _v )g}}} \right</math></center>\left<center><math> {(3f' + sf'')\left( {\frac{{\partial s}}{{\partial x}}} \right)^2 + sf'\frac{{\partial ^2 s}}{{\partial y_2^2 }}} \right</math></center> = 0
+
 
-
</math></center>
+
{| class="wikitable" border="0"
-
(8.397)
+
|-
 +
| width="100%" |
 +
<center><math>s=\frac{{{\gamma }_{\ell }}-{{\gamma }_{\ell i}}}{1-{{\gamma }_{\ell i}}}</math></center>
 +
|{{EquationRef|(10)}}
 +
|}
 +
 
 +
where <math>{{\gamma }_{\ell i}}</math> is irreducible saturation, below which liquid flow will not occur. The relative permeability in eqs. (6) and (7) is obtained from
 +
 
 +
{| class="wikitable" border="0"
 +
|-
 +
| width="100%" |
 +
<center><math>{{K}_{r\ell }}={{s}^{3}}</math></center>
 +
|{{EquationRef|(11)}}
 +
|}
 +
 
 +
Substituting eqs. (6) – (11) into eq. (5), the following form of the dimensionless continuity equation is obtained:
 +
 
 +
{| class="wikitable" border="0"
 +
|-
 +
| width="100%" |
 +
<center><math>3\frac{\partial s}{\partial x}+\left[ \frac{\sigma /\sqrt{K/\varepsilon }}{({{\rho }_{\ell }}-{{\rho }_{v}})g} \right]\left[ (3{f}'+s{f}''){{\left( \frac{\partial s}{\partial x} \right)}^{2}}+s{f}'\frac{{{\partial }^{2}}s}{\partial y_{2}^{2}} \right]=0</math></center>
 +
|{{EquationRef|(12)}}
 +
|}
 +
 
which is subjected to the following boundary conditions:
which is subjected to the following boundary conditions:
-
<center><math>
+
 
-
s = 0\begin{array}{*{20}c}
+
{| class="wikitable" border="0"
-
   , & {x = 0} \\
+
|-
-
\end{array}
+
| width="100%" |
-
</math></center>
+
<center><math>s=0\begin{matrix}
-
(8.398)
+
   , & x=0  \\
-
<center><math>
+
\end{matrix}</math></center>
-
s = 1\begin{array}{*{20}c}
+
|{{EquationRef|(13)}}
-
   , & {y_2  = 0} \\
+
|}
-
\end{array}
+
 
-
</math></center>
+
{| class="wikitable" border="0"
-
(8.399)
+
|-
-
<center><math>
+
| width="100%" |
-
s = 0\begin{array}{*{20}c}
+
<center><math>s=1\begin{matrix}
-
   , & {y_2  \to \infty \\
+
   , & {{y}_{2}}=0  \\
-
\end{array}
+
\end{matrix}</math></center>
-
</math></center>
+
|{{EquationRef|(14)}}
-
(8.400)
+
|}
 +
 
 +
{| class="wikitable" border="0"
 +
|-
 +
| width="100%" |
 +
<center><math>s=0\begin{matrix}
 +
   , & {{y}_{2}}\to \infty   \\
 +
\end{matrix}</math></center>
 +
|{{EquationRef|(15)}}
 +
|}
 +
 
Introducing the following similarity variable:
Introducing the following similarity variable:
-
<center><math>
+
 
-
\eta = y_2 \left<center><math> {\frac{{(\rho _\ell   - \rho _v )g}}{{(\sigma /\sqrt {K/\varepsilon } )x}}} \right</math></center>^2  
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.401)
+
| width="100%" |
-
eqs. (8.397) – (8.400) are transformed to
+
<center><math>\eta ={{y}_{2}}{{\left[ \frac{({{\rho }_{\ell }}-{{\rho }_{v}})g}{(\sigma /\sqrt{K/\varepsilon })x} \right]}^{2}}</math></center>
-
<center><math>
+
|{{EquationRef|(16)}}
-
s'' = \frac{{3\eta s' - 2(3f' + sf'')s'^2 }}{{2sf'}}
+
|}
-
</math></center>
+
 
-
(8.402)
+
eqs. (12) – (15) are transformed to
-
<center><math>
+
 
-
s = 1\begin{array}{*{20}c}
+
{| class="wikitable" border="0"
-
   , & {\eta = 0} \\
+
|-
-
\end{array}
+
| width="100%" |
-
</math></center>
+
<center><math>{s}''=\frac{3\eta {s}'-2(3{f}'+s{f}''){{{{s}'}}^{2}}}{2s{f}'}</math></center>
-
(8.403)
+
|{{EquationRef|(17)}}
-
<center><math>
+
|}
-
s = 0\begin{array}{*{20}c}
+
 
-
   , & {\eta \to \infty \\
+
{| class="wikitable" border="0"
-
\end{array}
+
|-
-
</math></center>
+
| width="100%" |
-
(8.404)
+
<center><math>s=1\begin{matrix}
-
The numerical solution of eq. (8.402) was obtained using the Runge-Kutta method and it is concluded that <math>\eta = 1.296</math>
+
   , & \eta =0  \\
-
can be chosen to determine the thickness of the two-phase region.
+
\end{matrix}</math></center>
 +
|{{EquationRef|(18)}}
 +
|}
 +
 
 +
{| class="wikitable" border="0"
 +
|-
 +
| width="100%" |
 +
<center><math>s=0\begin{matrix}
 +
   , & \eta \to \infty   \\
 +
\end{matrix}</math></center>
 +
|{{EquationRef|(19)}}
 +
|}
 +
 
 +
The numerical solution of eq. (17) was obtained using the Runge-Kutta method and it is concluded that <math>\eta =1.296</math> can be chosen to determine the thickness of the two-phase region.
 +
 
For the liquid region, the model in the preceding subsection overpredicts the heat transfer coefficient because of slip in the velocity at the wall. They presented a new model based on the following assumptions: (a) the boundary layer assumption applies in the liquid film, (2) convection terms in the energy equation are negligible, (3) subcooling in the liquid is negligible, and (4) the fluid properties are constants. The governing equations for the liquid film in dimensionless form are
For the liquid region, the model in the preceding subsection overpredicts the heat transfer coefficient because of slip in the velocity at the wall. They presented a new model based on the following assumptions: (a) the boundary layer assumption applies in the liquid film, (2) convection terms in the energy equation are negligible, (3) subcooling in the liquid is negligible, and (4) the fluid properties are constants. The governing equations for the liquid film in dimensionless form are
-
<center><math>
+
 
-
\frac{{\partial u_\ell ^ + }}{{\partial x^ + }} + \frac{{\partial v_\ell ^ + }}{{\partial y^ + }} = 0
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.405)
+
| width="100%" |
-
<center><math>
+
<center><math>\frac{\partial u_{\ell }^{+}}{\partial {{x}^{+}}}+\frac{\partial v_{\ell }^{+}}{\partial {{y}^{+}}}=0</math></center>
-
\frac{{\partial ^2 u_\ell ^ + }}{{\partial y^{ + 2} }} + 1 - u_\ell ^ +   = 0
+
|{{EquationRef|(20)}}
-
</math></center>
+
|}
-
(8.406)
+
 
-
<center><math>
+
{| class="wikitable" border="0"
-
\frac{{\partial ^2 \theta _\ell }}{{\partial y^{ + 2} }} = 0
+
|-
-
</math></center>
+
| width="100%" |
-
(8.407)
+
<center><math>\frac{{{\partial }^{2}}u_{\ell }^{+}}{\partial {{y}^{+2}}}+1-u_{\ell }^{+}=0</math></center>
 +
|{{EquationRef|(21)}}
 +
|}
 +
 
 +
{| class="wikitable" border="0"
 +
|-
 +
| width="100%" |
 +
<center><math>\frac{{{\partial }^{2}}{{\theta }_{\ell }}}{\partial {{y}^{+2}}}=0</math></center>
 +
|{{EquationRef|(22)}}
 +
|}
 +
 
where the dimensionless variables are defined as
where the dimensionless variables are defined as
-
<center><math>
 
-
u_\ell ^ +  = \frac{{u_\ell  }}{{u_D }}{\rm{,  }}v_\ell ^ +  = \frac{{v_\ell  }}{{u_D }}{\rm{,  }}x^ +  = \frac{x}{{\sqrt K }}{\rm{,  }}y^ +  = \frac{y}{{\sqrt K }}{\rm{,  }}\delta _\ell ^ +  = \frac{{\delta _\ell  }}{{\sqrt K }}{\rm{,  }}\theta  = \frac{{T - T_w }}{{T_{sat}  - T_w }}
 
-
</math></center>
 
-
(8.408)
 
-
Compared with eq. (8.356) in Cheng’s (1981) model, eq. (8.406) allows for nonslip conditions at the wall. The boundary conditions at the wall are
 
-
<center><math>
 
-
u_\ell ^ +  = v_\ell ^ +  = \theta  = 0\begin{array}{*{20}c}
 
-
  , & {y^ +  = 0}  \\
 
-
\end{array}
 
-
</math></center>
 
-
(8.409)
 
-
The boundary conditions at the interface between the liquid film and the two-phase region require that the velocity and shear stress in these two regions match, which makes the solution of the condensation problem very challenging. Majumdar and Tien (1990) proposed three models to handle the boundary condition at the interface between the liquid and the two-phase regions; two of them are discussed below.
 
-
Model 1. At the interface between the liquid and the two-phase region, the shear stress is zero, i.e., <math>\partial u_\ell ^ + /\partial y^ +   = 0</math>
+
{| class="wikitable" border="0"
-
at <math>\tilde y = \delta _\ell ^ + </math>
+
|-
-
, which is the same as in classical Nusselt analysis. The velocity profile in the liquid layer is  
+
| width="100%" |
-
<center><math>
+
<center><math>u_{\ell }^{+}=\frac{{{u}_{\ell }}}{{{u}_{D}}}\text{,  }v_{\ell }^{+}=\frac{{{v}_{\ell }}}{{{u}_{D}}}\text{,  }{{x}^{+}}=\frac{x}{\sqrt{K}}\text{,  }{{y}^{+}}=\frac{y}{\sqrt{K}}\text{,  }\delta _{\ell }^{+}=\frac{{{\delta }_{\ell }}}{\sqrt{K}}\text{,  }\theta =\frac{T-{{T}_{w}}}{{{T}_{sat}}-{{T}_{w}}}</math></center>
-
u_\ell ^ +   = 1 - \cosh y^ +   + \tanh \delta _\ell ^ + \sinh y^ +
+
|{{EquationRef|(23)}}
-
</math></center>
+
|}
-
(8.410)
+
 
 +
Compared with velocity profile in the [[gravity dominated film-condensation in a porous medium]], eq. (21) allows for nonslip conditions at the wall. The boundary conditions at the wall are
 +
 
 +
{| class="wikitable" border="0"
 +
|-
 +
| width="100%" |
 +
<center><math>u_{\ell }^{+}=v_{\ell }^{+}=\theta =0\begin{matrix}
 +
  , & {{y}^{+}}=0  \\
 +
\end{matrix}</math></center>
 +
|{{EquationRef|(24)}}
 +
|}
 +
 
 +
The boundary conditions at the interface between the liquid film and the two-phase region require that the velocity and shear stress in these two regions match, which makes the solution of the condensation problem very challenging. [[#References|Majumdar and Tien (1990)]] proposed three models to handle the boundary condition at the interface between the liquid and the two-phase regions; two of them are discussed below.
 +
 
 +
'''Model 1.''' At the interface between the liquid and the two-phase region, the shear stress is zero, i.e., <math>\partial u_{\ell }^{+}/\partial {{y}^{+}}=0</math> at <math>\tilde{y}=\delta _{\ell }^{+}</math>, which is the same as in classical Nusselt analysis. The velocity profile in the liquid layer is  
 +
 
 +
{| class="wikitable" border="0"
 +
|-
 +
| width="100%" |
 +
<center><math>u_{\ell }^{+}=1-\cosh {{y}^{+}}+\tanh \delta _{\ell }^{+}\sinh {{y}^{+}}</math></center>
 +
|{{EquationRef|(25)}}
 +
|}
 +
 
The liquid layer thickness can be obtained from an energy balance at the interface, and the result is
The liquid layer thickness can be obtained from an energy balance at the interface, and the result is
-
<center><math>
+
 
-
(1 - {\rm{sech}}^2 \delta _\ell ^ + )\frac{{d\delta _\ell ^ + }}{{dx^ + }} + (1 - {\rm{sech}}\delta _\ell ^ + )\frac{{0.373}}{{\sqrt {{\rm{Bo}}x^ + } }} = \frac{{{\rm{Ja}}_\ell }}{{\delta _\ell ^ + {\rm{Ra}}_K }}
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.411)
+
| width="100%" |
 +
<center><math>(1-\text{sec}{{\text{h}}^{2}}\delta _{\ell }^{+})\frac{d\delta _{\ell }^{+}}{d{{x}^{+}}}+(1-\text{sech}\delta _{\ell }^{+})\frac{0.373}{\sqrt{\text{Bo}{{x}^{+}}}}=\frac{\text{J}{{\text{a}}_{\ell }}}{\delta _{\ell }^{+}\text{R}{{\text{a}}_{K}}}</math></center>
 +
|{{EquationRef|(26)}}
 +
|}
 +
 
where  
where  
-
<center><math>
+
 
-
{\rm{Ra}}_K  = \frac{{K^{3/2} (\rho _\ell   - \rho _v )g}}{{\mu _\ell \alpha _e }}
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.412)
+
| width="100%" |
-
is the Rayleigh number based on permeability, and <math>\alpha _e  = k_{eff} /(\rho _\ell c_{p\ell } )</math>
+
<center><math>\text{R}{{\text{a}}_{K}}=\frac{{{K}^{3/2}}({{\rho }_{\ell }}-{{\rho }_{v}})g}{{{\mu }_{\ell }}{{\alpha }_{e}}}</math></center>
-
is effective thermal diffusivity. Analytical solution of eq. (8.411) is not possible and it must be solved numerically.
+
|{{EquationRef|(27)}}
-
+
|}
-
Model 2. This model also employs eq. (8.406) to obtain the velocity in the liquid layer, except that the boundary condition at <math>y^ +   = \delta _\ell ^ + </math>
+
 
-
is changed to <math>u_\ell ^ +   = 1</math>
+
is the Rayleigh number based on permeability, and <math>{{\alpha }_{e}}={{k}_{eff}}/({{\rho }_{\ell }}{{c}_{p\ell }})</math> is effective thermal diffusivity. Analytical solution of eq. (26) is not possible and it must be solved numerically.
-
. Although it is not as rigorous as Model 1, it is an improvement over Cheng (1981) because it uses a nonslip condition at the wall. The velocity profile in the liquid layer is
+
 
-
<center><math>
+
'''Model 2.''' This model also employs eq. (21) to obtain the velocity in the liquid layer, except that the boundary condition at <math>{{y}^{+}}=\delta _{\ell }^{+}</math> is changed to <math>u_{\ell }^{+}=1</math>. Although it is not as rigorous as Model 1, it is an improvement over [[#References|Cheng (1981)]] because it uses a nonslip condition at the wall. The velocity profile in the liquid layer is
-
u_\ell ^ +   = 1 - \cosh y^ +   + \coth \delta _\ell ^ + \sinh y^ +
+
 
-
</math></center>
+
{| class="wikitable" border="0"
-
(8.413)
+
|-
 +
| width="100%" |
 +
<center><math>u_{\ell }^{+}=1-\cosh {{y}^{+}}+\coth \delta _{\ell }^{+}\sinh {{y}^{+}}</math></center>
 +
|{{EquationRef|(28)}}
 +
|}
and the overall energy balance at the interface is
and the overall energy balance at the interface is
-
<center><math>
+
 
-
\left( {1 - \frac{1}{{1 + \cosh \delta _\ell ^ + }}} \right)\frac{{d\delta _\ell ^ + }}{{dx^ + }} + \frac{{0.373}}{{\sqrt {{\rm{Bo}}x^ + } }} = \frac{{{\rm{Ja}}_\ell }}{{\delta _\ell ^ + {\rm{Ra}}_K }}
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.414)
+
| width="100%" |
 +
<center><math>\left( 1-\frac{1}{1+\cosh \delta _{\ell }^{+}} \right)\frac{d\delta _{\ell }^{+}}{d{{x}^{+}}}+\frac{0.373}{\sqrt{\text{Bo}{{x}^{+}}}}=\frac{\text{J}{{\text{a}}_{\ell }}}{\delta _{\ell }^{+}\text{R}{{\text{a}}_{K}}}</math></center>
 +
|{{EquationRef|(29)}}
 +
|}
 +
 
which also needs to be solved numerically.
which also needs to be solved numerically.
-
In the liquid region, eq. (8.407) will yield a linear temperature profile, and the local Nusselt number is
+
In the liquid region, eq. (22) will yield a linear temperature profile, and the local Nusselt number is
-
<center><math>
+
 
-
Nu_x  = \frac{{x^ + }}{{\delta _\ell ^ + }}
+
{| class="wikitable" border="0"
-
</math></center>
+
|-
-
(8.415)
+
| width="100%" |
-
Figure 8.31 shows comparison between the results predicted from Models 1 and 2, together with the experimental results by White and Tien (1987) for constant porosity media. The parameter R in the figure is defined as
+
<center><math>N{{u}_{x}}=\frac{{{x}^{+}}}{\delta _{\ell }^{+}}</math></center>
-
<center><math>
+
|{{EquationRef|(30)}}
-
R = \frac{{{\rm{Ra}}_K }}{{{\rm{Bo}}}} = \frac{{\sigma \sqrt {K\varepsilon } }}{{\mu _\ell  \alpha _e }}
+
|}
-
</math></center>
+
 
-
(8.416)
+
==References==
-
which reflects the ratio of surface tension force and viscous force. It can be seen from Fig. 8.31(a) that for aluminum foam, the agreement between Model 1 and experimental results is very good, but Model 2 significantly overpredicts the heat transfer rates. This is expected because Model 2 is not as rigorous as Model 1. The slight overprediction of Model 1 may be attributed to the fact that shear stress at the interface is neglected. For polyurethane foam, on the other hand, Model 1 also significantly overpredict heat transfer rate. This overprediction is due to the fact that surface tension drag at the liquid region interface is neglected in Model 1. Since R in Fig. 8.31 (b) is one order of magnitude higher than that in Fig. 8.31 (a), it is expected that surface tension plays a more significant role in Fig. 8.31 (b).
+
 
 +
Cheng, P., 1981, “Film Condensation Along an Inclined Surface in a Porous Medium,” International Journal of Heat and Mass Transfer, Vol. 24, pp. 983-990.
 +
 
 +
Faghri, A., and Zhang, Y., 2006, ''Transport Phenomena in Multiphase Systems'', Elsevier, Burlington, MA
 +
 
 +
Majumdar, A., Tien, C.L., 1990, “Effects of Surface Tension on Film Condensation in a Porous Medium,” ASME J. Heat Transfer, Vol. 112, pp. 751-757.
 +
 
 +
White, S.M., and Tien, C.L., 1987, “An Experimental Investigation of Film Condensation in Porous Structures,” presented at the 6th International Heat Pipe Conference, Grenoble, France.

Current revision as of 08:34, 26 July 2010

The analysis in the gravity dominated film-condensation in a porous medium is valid for \text{Bo}\gg 1. When the condensation is gravity-capillary forces dominated (Bo˜1) or capillary force dominated (\text{Bo}\ll 1), there will be a two-phase region that is saturated by a mixture of liquid and vapor. The fraction of liquid in the pore space is defined as saturation:

{{\gamma }_{\ell }}=\frac{{{\varepsilon }_{\ell }}}{\varepsilon }
(1)

where {{\varepsilon }_{\ell }} and \varepsilon are volume fraction of the liquid and porosity in the porous media. The effect of surface tension on condensation in porous media was studied by Majumdar and Tien (1990) and their work will be briefly described below.

The continuity equation for the two-phase region is

{{\rho }_{\ell }}\left( \frac{\partial {{u}_{\ell }}}{\partial x}+\frac{\partial {{v}_{\ell }}}{\partial {{y}_{2}}} \right)+{{\rho }_{v}}\left( \frac{\partial {{u}_{v}}}{\partial x}+\frac{\partial {{v}_{v}}}{\partial {{y}_{2}}} \right)=0
(2)

where y2 is measured from the interface between liquid and the two-phase region (see Fig. 8.29). The mass fluxes for liquid and vapor are governed by Darcy’s law, as follows:

{{{\dot{m}}''}_{\ell }}=-\frac{K{{K}_{r\ell }}}{{{\nu }_{\ell }}}\nabla {{p}_{\ell }}
(3)
{{{\dot{m}}''}_{v}}=-\frac{K{{K}_{rv}}}{{{\nu }_{v}}}\nabla {{p}_{v}}
(4)

where {{K}_{r\ell }} and Krv are relative permeabilities (dimensionless) for liquid and vapor phases, respectively. Their products with the permeability K represent the permeability for liquid and vapor flow in the porous media.

Since the density of the vapor is much lower than that of the liquid, the pressure variation in the liquid phase is very insignificant. Also, the change of capillary pressure is due mainly to change of liquid pressure. Consequently, the vapor flow is negligible compared to the liquid flow, so that eq. (2) is reduced to

\frac{\partial {{u}_{\ell }}}{\partial x}+\frac{\partial {{v}_{\ell }}}{\partial {{y}_{2}}}=0
(5)

The velocity components in the x- and y- directions are

{{u}_{\ell }}={{K}_{r\ell }}{{u}_{D}}=\frac{K{{K}_{r\ell }}({{\rho }_{\ell }}-{{\rho }_{v}})g}{{{\mu }_{\ell }}}
(6)
{{v}_{\ell }}=-\frac{K{{K}_{r\ell }}}{{{\mu }_{\ell }}}\frac{\partial {{p}_{\ell }}}{\partial {{y}_{2}}}=\frac{K{{K}_{r\ell }}}{{{\mu }_{\ell }}}\frac{\partial {{p}_{cap}}}{\partial {{y}_{2}}}
(7)

where {{u}_{D}}=K({{\rho }_{\ell }}-{{\rho }_{v}})g/{{\mu }_{\ell }} is Darcian velocity. The capillary pressure can be written as

{{p}_{cap}}=\frac{\sigma }{\sqrt{K/\varepsilon }}f(s)
(8)

where f(s) is a Leverett’s function:

f(s) = 1.417(1 − s) − 2.120(1 − s)2 + 1.263(1 − s)3
(9)

and s is dimensionless saturation defined as

s=\frac{{{\gamma }_{\ell }}-{{\gamma }_{\ell i}}}{1-{{\gamma }_{\ell i}}}
(10)

where {{\gamma }_{\ell i}} is irreducible saturation, below which liquid flow will not occur. The relative permeability in eqs. (6) and (7) is obtained from

{{K}_{r\ell }}={{s}^{3}}
(11)

Substituting eqs. (6) – (11) into eq. (5), the following form of the dimensionless continuity equation is obtained:

3\frac{\partial s}{\partial x}+\left[ \frac{\sigma /\sqrt{K/\varepsilon }}{({{\rho }_{\ell }}-{{\rho }_{v}})g} \right]\left[ (3{f}'+s{f}''){{\left( \frac{\partial s}{\partial x} \right)}^{2}}+s{f}'\frac{{{\partial }^{2}}s}{\partial y_{2}^{2}} \right]=0
(12)

which is subjected to the following boundary conditions:

s=0\begin{matrix}
   , & x=0  \\
\end{matrix}
(13)
s=1\begin{matrix}
   , & {{y}_{2}}=0  \\
\end{matrix}
(14)
s=0\begin{matrix}
   , & {{y}_{2}}\to \infty   \\
\end{matrix}
(15)

Introducing the following similarity variable:

\eta ={{y}_{2}}{{\left[ \frac{({{\rho }_{\ell }}-{{\rho }_{v}})g}{(\sigma /\sqrt{K/\varepsilon })x} \right]}^{2}}
(16)

eqs. (12) – (15) are transformed to

{s}''=\frac{3\eta {s}'-2(3{f}'+s{f}''){{{{s}'}}^{2}}}{2s{f}'}
(17)
s=1\begin{matrix}
   , & \eta =0  \\
\end{matrix}
(18)
s=0\begin{matrix}
   , & \eta \to \infty   \\
\end{matrix}
(19)

The numerical solution of eq. (17) was obtained using the Runge-Kutta method and it is concluded that η = 1.296 can be chosen to determine the thickness of the two-phase region.

For the liquid region, the model in the preceding subsection overpredicts the heat transfer coefficient because of slip in the velocity at the wall. They presented a new model based on the following assumptions: (a) the boundary layer assumption applies in the liquid film, (2) convection terms in the energy equation are negligible, (3) subcooling in the liquid is negligible, and (4) the fluid properties are constants. The governing equations for the liquid film in dimensionless form are

\frac{\partial u_{\ell }^{+}}{\partial {{x}^{+}}}+\frac{\partial v_{\ell }^{+}}{\partial {{y}^{+}}}=0
(20)
\frac{{{\partial }^{2}}u_{\ell }^{+}}{\partial {{y}^{+2}}}+1-u_{\ell }^{+}=0
(21)
\frac{{{\partial }^{2}}{{\theta }_{\ell }}}{\partial {{y}^{+2}}}=0
(22)

where the dimensionless variables are defined as

u_{\ell }^{+}=\frac{{{u}_{\ell }}}{{{u}_{D}}}\text{,   }v_{\ell }^{+}=\frac{{{v}_{\ell }}}{{{u}_{D}}}\text{,   }{{x}^{+}}=\frac{x}{\sqrt{K}}\text{,   }{{y}^{+}}=\frac{y}{\sqrt{K}}\text{,   }\delta _{\ell }^{+}=\frac{{{\delta }_{\ell }}}{\sqrt{K}}\text{,   }\theta =\frac{T-{{T}_{w}}}{{{T}_{sat}}-{{T}_{w}}}
(23)

Compared with velocity profile in the gravity dominated film-condensation in a porous medium, eq. (21) allows for nonslip conditions at the wall. The boundary conditions at the wall are

u_{\ell }^{+}=v_{\ell }^{+}=\theta =0\begin{matrix}
   , & {{y}^{+}}=0  \\
\end{matrix}
(24)

The boundary conditions at the interface between the liquid film and the two-phase region require that the velocity and shear stress in these two regions match, which makes the solution of the condensation problem very challenging. Majumdar and Tien (1990) proposed three models to handle the boundary condition at the interface between the liquid and the two-phase regions; two of them are discussed below.

Model 1. At the interface between the liquid and the two-phase region, the shear stress is zero, i.e., \partial u_{\ell }^{+}/\partial {{y}^{+}}=0 at \tilde{y}=\delta _{\ell }^{+}, which is the same as in classical Nusselt analysis. The velocity profile in the liquid layer is

u_{\ell }^{+}=1-\cosh {{y}^{+}}+\tanh \delta _{\ell }^{+}\sinh {{y}^{+}}
(25)

The liquid layer thickness can be obtained from an energy balance at the interface, and the result is

(1-\text{sec}{{\text{h}}^{2}}\delta _{\ell }^{+})\frac{d\delta _{\ell }^{+}}{d{{x}^{+}}}+(1-\text{sech}\delta _{\ell }^{+})\frac{0.373}{\sqrt{\text{Bo}{{x}^{+}}}}=\frac{\text{J}{{\text{a}}_{\ell }}}{\delta _{\ell }^{+}\text{R}{{\text{a}}_{K}}}
(26)

where

\text{R}{{\text{a}}_{K}}=\frac{{{K}^{3/2}}({{\rho }_{\ell }}-{{\rho }_{v}})g}{{{\mu }_{\ell }}{{\alpha }_{e}}}
(27)

is the Rayleigh number based on permeability, and {{\alpha }_{e}}={{k}_{eff}}/({{\rho }_{\ell }}{{c}_{p\ell }}) is effective thermal diffusivity. Analytical solution of eq. (26) is not possible and it must be solved numerically.

Model 2. This model also employs eq. (21) to obtain the velocity in the liquid layer, except that the boundary condition at {{y}^{+}}=\delta _{\ell }^{+} is changed to u_{\ell }^{+}=1. Although it is not as rigorous as Model 1, it is an improvement over Cheng (1981) because it uses a nonslip condition at the wall. The velocity profile in the liquid layer is

u_{\ell }^{+}=1-\cosh {{y}^{+}}+\coth \delta _{\ell }^{+}\sinh {{y}^{+}}
(28)

and the overall energy balance at the interface is

\left( 1-\frac{1}{1+\cosh \delta _{\ell }^{+}} \right)\frac{d\delta _{\ell }^{+}}{d{{x}^{+}}}+\frac{0.373}{\sqrt{\text{Bo}{{x}^{+}}}}=\frac{\text{J}{{\text{a}}_{\ell }}}{\delta _{\ell }^{+}\text{R}{{\text{a}}_{K}}}
(29)

which also needs to be solved numerically. In the liquid region, eq. (22) will yield a linear temperature profile, and the local Nusselt number is

N{{u}_{x}}=\frac{{{x}^{+}}}{\delta _{\ell }^{+}}
(30)

References

Cheng, P., 1981, “Film Condensation Along an Inclined Surface in a Porous Medium,” International Journal of Heat and Mass Transfer, Vol. 24, pp. 983-990.

Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA

Majumdar, A., Tien, C.L., 1990, “Effects of Surface Tension on Film Condensation in a Porous Medium,” ASME J. Heat Transfer, Vol. 112, pp. 751-757.

White, S.M., and Tien, C.L., 1987, “An Experimental Investigation of Film Condensation in Porous Structures,” presented at the 6th International Heat Pipe Conference, Grenoble, France.