# Homogeneous model

(Difference between revisions)
 Revision as of 18:28, 7 July 2010 (view source)← Older edit Current revision as of 04:04, 19 July 2010 (view source) (5 intermediate revisions not shown) Line 1: Line 1: - The multi-fluid model presented above is obtained by performing phase averaging as defined in eqs. (2.316) and (2.317). If spatial averaging is performed for all phases within a multiphase control volume, the homogeneous (or mixture) model can be obtained. The relationship between volume averaging and phase averaging is given in eq. (2.319), which indicates that the homogeneous model can be obtained by summing the individual phase equations of the multi-fluid model. + The multi-fluid model presented above is obtained by performing [[Averaging approaches|phase averaging]] as defined in - ====Continuity Equation==== +
${{\left\langle {{\Phi }_{k}} \right\rangle }^{k}}=\frac{1}{\Delta {{V}_{k}}}\int_{\Delta {{V}_{k}}}{{{\Phi }_{k}}dV}$ and $\left\langle {{\Phi }_{k}} \right\rangle =\frac{1}{\Delta V}\int_{\Delta {{V}_{k}}}{{{\Phi }_{k}}dV}$
- The continuity equation for phase k in the multifluid model is expressed by eq. (2.332). Summing the continuity equations for all $\Pi$ phases together, one obtains + If spatial averaging is performed for all phases within a multiphase control volume, the homogeneous (or mixture) model can be obtained. The relationship between volume averaging and [[Averaging approaches|phase averaging]] is given in - + -
$\frac{\partial }{\partial t}\left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}} \right)+\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}=\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}}}\qquad \qquad(1)$
+
$\left\langle \Phi \right\rangle =\sum\limits_{k=1}^{\Pi }{\left\langle {{\Phi }_{k}} \right\rangle }=\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\Phi }_{k}} \right\rangle }^{k}}}$ - (2.385) +
+ which indicates that the homogeneous model can be obtained by summing the individual phase equations of the multi-fluid model. - The right-hand side of equation (1) must be zero because the total mass of all phases produced by phase change must equal the total mass of all phases consumed by phase change. Considering this fact and eq. (2.330), the continuity equation becomes + ==Continuity Equation== + + The continuity equation for phase k in the [[multi-fluid model]] model is expressed as + +
$\frac{\partial }{\partial t}\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}} \right)+\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}} \right)=\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}}$
+ + Summing the continuity equations for all $\Pi$ phases together, one obtains + +
$\frac{\partial }{\partial t}\left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}} \right)+\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}=\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}}}\qquad \qquad(1)$
+ The right-hand side of equation (1) must be zero because the total mass of all phases produced by phase change must equal the total mass of all phases consumed by phase change. Considering this fact and $\left\langle {{\rho }_{k}} \right\rangle ={{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}$ (see [[multi-fluid model]]), the continuity equation becomes
$\frac{\partial \left\langle \rho \right\rangle }{\partial t}+\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}=0\qquad \qquad(2)$
$\frac{\partial \left\langle \rho \right\rangle }{\partial t}+\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}=0\qquad \qquad(2)$
- (2.386) - The bulk velocity of the multiphase mixture is the mass-averaged velocity of all the individual phases: The bulk velocity of the multiphase mixture is the mass-averaged velocity of all the individual phases: -
$\mathbf{\tilde{V}}=\frac{1}{\left\langle \rho \right\rangle }\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}\qquad \qquad(3)$
$\mathbf{\tilde{V}}=\frac{1}{\left\langle \rho \right\rangle }\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}\qquad \qquad(3)$
- (2.387) + - + - + Substituting eq. (3) into eq. (2), the final form of the continuity equation for a multiphase mixture is Substituting eq. (3) into eq. (2), the final form of the continuity equation for a multiphase mixture is -
$\frac{\partial \left\langle \rho \right\rangle }{\partial t}+\nabla \cdot \left( \left\langle \rho \right\rangle \mathbf{\tilde{V}} \right)=0\qquad \qquad(4)$
$\frac{\partial \left\langle \rho \right\rangle }{\partial t}+\nabla \cdot \left( \left\langle \rho \right\rangle \mathbf{\tilde{V}} \right)=0\qquad \qquad(4)$
- (2.388) - It can be seen that eq. (4) has the same form as the local continuity equation (2.51), except that the volume-averaged density and velocity are used in eq. (4), where $\left\langle \rho \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}.$ It can be seen that eq. (4) has the same form as the local continuity equation (2.51), except that the volume-averaged density and velocity are used in eq. (4), where $\left\langle \rho \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}.$ + ==Momentum Equation== - ====Momentum Equation==== + The momentum equation for phase k in the [[multi-fluid model]] is expressed as - The momentum equation for phase k in the multi-fluid model is expressed in eq. (2.334). By adding together the momentum equations for all $\Pi$ phases, one obtains +
\begin{align} + & \frac{\partial }{\partial t}\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}} \right)+\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{\mathbf{V}}_{k}} \right\rangle }^{k}} \right) \\ + & =\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{{\mathbf{{\tau }'}}}_{k}} \right\rangle }^{k}} \right)+{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\mathbf{X}}_{k}}+\sum\limits_{j=1(j\ne k)}^{\Pi }{\left( \left\langle {{\mathbf{F}}_{jk}} \right\rangle +\left\langle {{{{\dot{m}}'''}}_{jk}} \right\rangle {{\left\langle {{\mathbf{V}}_{k,I}} \right\rangle }^{k}} \right)} \\ + \end{align}
+ + By adding together the momentum equations for all $\Pi$ phases, one obtains -
\begin{align} [itex]\begin{align} & \frac{\partial }{\partial t}\left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right) \\ & \frac{\partial }{\partial t}\left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right) \\ & =\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{{\mathbf{{\tau }'}}}_{k}} \right\rangle }^{k}}}+\sum\limits_{k=1}^{\Pi }{\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}} \right)}\mathbf{X}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{\left( \left\langle {{\mathbf{F}}_{jk}} \right\rangle +\left\langle {{{{\dot{m}}'''}}_{jk}} \right\rangle {{\left\langle {{\mathbf{V}}_{k,I}} \right\rangle }^{k}} \right)}} \\ & =\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{{\mathbf{{\tau }'}}}_{k}} \right\rangle }^{k}}}+\sum\limits_{k=1}^{\Pi }{\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}} \right)}\mathbf{X}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{\left( \left\langle {{\mathbf{F}}_{jk}} \right\rangle +\left\langle {{{{\dot{m}}'''}}_{jk}} \right\rangle {{\left\langle {{\mathbf{V}}_{k,I}} \right\rangle }^{k}} \right)}} \\ \end{align}\qquad \qquad(5)
- (2.389) + - + - + The stress tensor of the multiphase mixture is The stress tensor of the multiphase mixture is -
$\left\langle {\mathbf{{\tau }'}} \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\mathbf{\tau }}_{k}} \right\rangle }^{k}}}=-\left\langle p \right\rangle \mathbf{I}+\mu \left[ \nabla \mathbf{\tilde{V}}+\nabla {{{\mathbf{\tilde{V}}}}_{{}}}^{T} \right]-\frac{2}{3}\mu (\nabla \cdot \mathbf{\tilde{V}})\mathbf{I}\qquad \qquad(6)$
$\left\langle {\mathbf{{\tau }'}} \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\mathbf{\tau }}_{k}} \right\rangle }^{k}}}=-\left\langle p \right\rangle \mathbf{I}+\mu \left[ \nabla \mathbf{\tilde{V}}+\nabla {{{\mathbf{\tilde{V}}}}_{{}}}^{T} \right]-\frac{2}{3}\mu (\nabla \cdot \mathbf{\tilde{V}})\mathbf{I}\qquad \qquad(6)$
- (2.390) - The summation of all interphase forces must be zero since $\left\langle {{\mathbf{F}}_{jk}} \right\rangle =-\left\langle {{\mathbf{F}}_{kj}} \right\rangle$, i.e., The summation of all interphase forces must be zero since $\left\langle {{\mathbf{F}}_{jk}} \right\rangle =-\left\langle {{\mathbf{F}}_{kj}} \right\rangle$, i.e., +
$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{}}\left\langle {{\mathbf{F}}_{jk}} \right\rangle }=0\qquad \qquad(7)$
-
$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{}}\left\langle {{\mathbf{F}}_{jk}} \right\rangle }=0\qquad \qquad(7)$
- (2.391) - - Considering eqs. (3), (6) and (7), the momentum equation becomes Considering eqs. (3), (6) and (7), the momentum equation becomes -
$\frac{\partial }{\partial t}\left( \left\langle \rho \right\rangle \mathbf{\tilde{V}} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right)=\nabla \cdot \left\langle {\mathbf{{\tau }'}} \right\rangle +\left\langle \rho \right\rangle \mathbf{X}+{{\mathbf{{\dot{M}}'''}}_{I}}\qquad \qquad(8)$
$\frac{\partial }{\partial t}\left( \left\langle \rho \right\rangle \mathbf{\tilde{V}} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right)=\nabla \cdot \left\langle {\mathbf{{\tau }'}} \right\rangle +\left\langle \rho \right\rangle \mathbf{X}+{{\mathbf{{\dot{M}}'''}}_{I}}\qquad \qquad(8)$
- (2.392) + - + - + where where +
${{\mathbf{{\dot{M}}'''}}_{I}}=\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{}}\left\langle {{{{\dot{m}}'''}}_{jk}} \right\rangle }{{\left\langle {{\mathbf{V}}_{k,I}} \right\rangle }^{k}}\qquad \qquad(9)$
-
${{\mathbf{{\dot{M}}'''}}_{I}}=\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{}}\left\langle {{{{\dot{m}}'''}}_{jk}} \right\rangle }{{\left\langle {{\mathbf{V}}_{k,I}} \right\rangle }^{k}}\qquad \qquad(9)$
+ Equation (9) represents the momentum production rate due to interaction between different phases along their separating interfaces. It must be specified according to the combination of phases in the multiphase system that is under consideration. - (2.393) + - + - + - Equation (9) represents the momentum production rate due to interaction between different phases along their separating interfaces. It must be specified according to the combination of phases in the multiphase system that is under consideration. + - ====Energy Equation==== + ==Energy Equation== - By summing the energy equations for all $\Pi$ phases in the multifluid model, eq. (2.341), one obtains + By summing the energy equations for all $\Pi$ phases in the [[multi-fluid model]]: +
+ \begin{align} + & \frac{\partial }{\partial t}\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{h}_{k}} \right\rangle }^{k}} \right)+\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{h}_{k}} \right\rangle }^{k}} \right)=-\nabla \cdot \left\langle {{{\mathbf{{q}''}}}_{k}} \right\rangle +\left\langle {{{{q}'''}}_{k}} \right\rangle \\ + & +{{\varepsilon }_{k}}\frac{D{{\left\langle {{p}_{k}} \right\rangle }^{k}}}{Dt}+\nabla \left\langle {{\mathbf{V}}_{k}} \right\rangle :\left\langle {{\mathbf{\tau }}_{k}} \right\rangle +\sum\limits_{j=1(j\ne k)}^{\Pi }{\left[ \left\langle {{{{q}'''}}_{jk}} \right\rangle +{{{{\dot{m}}'''}}_{jk}}{{\left\langle {{h}_{k,I}} \right\rangle }^{k}} \right]} \\ + \end{align}
+ one obtains
\begin{align} [itex]\begin{align} Line 85: Line 83: & +\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}\frac{D{{\left\langle {{p}_{k}} \right\rangle }^{k}}}{Dt}}+\sum\limits_{k=1}^{\Pi }{\nabla {{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}:{{\varepsilon }_{k}}{{\left\langle {{\mathbf{\tau }}_{k}} \right\rangle }^{k}}}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{\left\langle {{{{q}'''}}_{jk}} \right\rangle }}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}{{\left\langle {{h}_{k,I}} \right\rangle }^{k}}}} \\ & +\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}\frac{D{{\left\langle {{p}_{k}} \right\rangle }^{k}}}{Dt}}+\sum\limits_{k=1}^{\Pi }{\nabla {{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}:{{\varepsilon }_{k}}{{\left\langle {{\mathbf{\tau }}_{k}} \right\rangle }^{k}}}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{\left\langle {{{{q}'''}}_{jk}} \right\rangle }}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}{{\left\langle {{h}_{k,I}} \right\rangle }^{k}}}} \\ \end{align}\qquad \qquad(10)
- (2.394) The mass average enthalpy of the multiphase mixture is The mass average enthalpy of the multiphase mixture is -
$\tilde{h}=\frac{1}{\left\langle \rho \right\rangle }\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{h}_{k}} \right\rangle }^{k}}}\qquad \qquad(11)$
$\tilde{h}=\frac{1}{\left\langle \rho \right\rangle }\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{h}_{k}} \right\rangle }^{k}}}\qquad \qquad(11)$
- (2.395) + - + - + The fifth term on the right-hand side of eq. (10) is for summation of all interphase heat transfer and it must be zero. The last term on the right-hand side of eq. (10) accounts for contribution of interphase phase change energy flux due to phase change; it can be defined as The fifth term on the right-hand side of eq. (10) is for summation of all interphase heat transfer and it must be zero. The last term on the right-hand side of eq. (10) accounts for contribution of interphase phase change energy flux due to phase change; it can be defined as +
$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}{{\left\langle {{h}_{k,I}} \right\rangle }^{k}}}}={{{q}'''}_{I}}\qquad \qquad(12)$
-
$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}{{\left\langle {{h}_{k,I}} \right\rangle }^{k}}}}={{{q}'''}_{I}}\qquad \qquad(12)$
- (2.396) - - It is usually not zero although $\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{\dot{m}}'''}}}=0$. It is usually not zero although $\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{\dot{m}}'''}}}=0$. Considering eqs. (11) and (12), the energy equation (10) becomes Considering eqs. (11) and (12), the energy equation (10) becomes +
$\frac{\partial }{\partial t}\left( \left\langle \rho \right\rangle \tilde{h} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{h}_{k}} \right\rangle }^{k}}} \right)=-\nabla \cdot \left\langle {\mathbf{{q}''}} \right\rangle +\frac{D\left\langle p \right\rangle }{Dt}+\left\langle {{q}'''} \right\rangle +\nabla \mathbf{\tilde{V}}:\left\langle \mathbf{\tau } \right\rangle +{{{q}'''}_{I}}\qquad \qquad(13)$
-
$\frac{\partial }{\partial t}\left( \left\langle \rho \right\rangle \tilde{h} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{h}_{k}} \right\rangle }^{k}}} \right)=-\nabla \cdot \left\langle {\mathbf{{q}''}} \right\rangle +\frac{D\left\langle p \right\rangle }{Dt}+\left\langle {{q}'''} \right\rangle +\nabla \mathbf{\tilde{V}}:\left\langle \mathbf{\tau } \right\rangle +{{{q}'''}_{I}}\qquad \qquad(13)$
(2.397) + ==Species== - ====Species==== + Summing the equations for conservation of species mass for [[multi-fluid model]], - Summing the equations for conservation of species mass, eq. (2.349), for all phases yields +
$\frac{\partial \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}} \right)}{\partial t}+\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}} \right)=-\nabla \cdot \left\langle {{\mathbf{J}}_{k,i}} \right\rangle +\left\langle {{{{\dot{m}}'''}}_{k,i}} \right\rangle +\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk,i}}}$
+ + for all phases yields
\begin{align} [itex]\begin{align} Line 116: Line 110: & =-\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{\left\langle {{\mathbf{J}}_{k,i}} \right\rangle } \right)+\sum\limits_{k=1}^{\Pi }{\left\langle {{{{\dot{m}}'''}}_{k,i}} \right\rangle }+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk,i}}}} \\ & =-\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{\left\langle {{\mathbf{J}}_{k,i}} \right\rangle } \right)+\sum\limits_{k=1}^{\Pi }{\left\langle {{{{\dot{m}}'''}}_{k,i}} \right\rangle }+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk,i}}}} \\ \end{align}\qquad \qquad(14)
- (2.398) - - By applying eq. (2.330) to the mass density of the ${i^{th}}$ component, one obtains - + By applying $\left\langle {{\rho }_{k}} \right\rangle ={{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}$ (see [[multi-fluid model]]) to the mass density of the ${i^{th}}$ component, one obtains +
$\left\langle {{\rho }_{i}} \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}}}\qquad \qquad(15)$
$\left\langle {{\rho }_{i}} \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}}}\qquad \qquad(15)$
- (2.399) - In accordance with the conservation of mass, the mass source (or sink) of the ${i^{th}}$ component due to phase change in all phases must add up to zero, i.e., In accordance with the conservation of mass, the mass source (or sink) of the ${i^{th}}$ component due to phase change in all phases must add up to zero, i.e., -
$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk,i}}}}=0\qquad \qquad(16)$
$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk,i}}}}=0\qquad \qquad(16)$
- (2.400) + - + - + Substituting eqs. (15) and (16) into eq. (14), and using the mass-averaged velocity defined in eq. (3), the conservation of species mass becomes Substituting eqs. (15) and (16) into eq. (14), and using the mass-averaged velocity defined in eq. (3), the conservation of species mass becomes -
$\frac{\partial \left\langle {{\rho }_{i}} \right\rangle }{\partial t}+\nabla \cdot \left\langle {{\rho }_{i}} \right\rangle \mathbf{\tilde{V}}=-\nabla \cdot {{\mathbf{J}}_{i}}+{{{\dot{m}}'''}_{i}}\qquad \qquad(17)$
$\frac{\partial \left\langle {{\rho }_{i}} \right\rangle }{\partial t}+\nabla \cdot \left\langle {{\rho }_{i}} \right\rangle \mathbf{\tilde{V}}=-\nabla \cdot {{\mathbf{J}}_{i}}+{{{\dot{m}}'''}_{i}}\qquad \qquad(17)$
- (2.401) + + ==References== - ==References== + Faghri, A., and Zhang, Y., 2006, ''Transport Phenomena in Multiphase Systems'', Elsevier, Burlington, MA + + Faghri, A., Zhang, Y., and Howell, J. R., 2010, ''Advanced  Heat and Mass Transfer'', Global Digital Press, Columbia, MO. ==Further Reading== ==Further Reading== ==External Links== ==External Links==

## Current revision as of 04:04, 19 July 2010

The multi-fluid model presented above is obtained by performing phase averaging as defined in

${{\left\langle {{\Phi }_{k}} \right\rangle }^{k}}=\frac{1}{\Delta {{V}_{k}}}\int_{\Delta {{V}_{k}}}{{{\Phi }_{k}}dV}$ and $\left\langle {{\Phi }_{k}} \right\rangle =\frac{1}{\Delta V}\int_{\Delta {{V}_{k}}}{{{\Phi }_{k}}dV}$

If spatial averaging is performed for all phases within a multiphase control volume, the homogeneous (or mixture) model can be obtained. The relationship between volume averaging and phase averaging is given in

$\left\langle \Phi \right\rangle =\sum\limits_{k=1}^{\Pi }{\left\langle {{\Phi }_{k}} \right\rangle }=\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\Phi }_{k}} \right\rangle }^{k}}}$

which indicates that the homogeneous model can be obtained by summing the individual phase equations of the multi-fluid model.

## Continuity Equation

The continuity equation for phase k in the multi-fluid model model is expressed as

$\frac{\partial }{\partial t}\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}} \right)+\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}} \right)=\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}}$

Summing the continuity equations for all Π phases together, one obtains

$\frac{\partial }{\partial t}\left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}} \right)+\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}=\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}}}\qquad \qquad(1)$

The right-hand side of equation (1) must be zero because the total mass of all phases produced by phase change must equal the total mass of all phases consumed by phase change. Considering this fact and $\left\langle {{\rho }_{k}} \right\rangle ={{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}$ (see multi-fluid model), the continuity equation becomes

$\frac{\partial \left\langle \rho \right\rangle }{\partial t}+\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}=0\qquad \qquad(2)$

The bulk velocity of the multiphase mixture is the mass-averaged velocity of all the individual phases:

$\mathbf{\tilde{V}}=\frac{1}{\left\langle \rho \right\rangle }\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}}\qquad \qquad(3)$

Substituting eq. (3) into eq. (2), the final form of the continuity equation for a multiphase mixture is

$\frac{\partial \left\langle \rho \right\rangle }{\partial t}+\nabla \cdot \left( \left\langle \rho \right\rangle \mathbf{\tilde{V}} \right)=0\qquad \qquad(4)$

It can be seen that eq. (4) has the same form as the local continuity equation (2.51), except that the volume-averaged density and velocity are used in eq. (4), where $\left\langle \rho \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}.$

## Momentum Equation

The momentum equation for phase k in the multi-fluid model is expressed as

\begin{align} & \frac{\partial }{\partial t}\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}} \right)+\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{\mathbf{V}}_{k}} \right\rangle }^{k}} \right) \\ & =\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{{\mathbf{{\tau }'}}}_{k}} \right\rangle }^{k}} \right)+{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\mathbf{X}}_{k}}+\sum\limits_{j=1(j\ne k)}^{\Pi }{\left( \left\langle {{\mathbf{F}}_{jk}} \right\rangle +\left\langle {{{{\dot{m}}'''}}_{jk}} \right\rangle {{\left\langle {{\mathbf{V}}_{k,I}} \right\rangle }^{k}} \right)} \\ \end{align}

By adding together the momentum equations for all Π phases, one obtains

\begin{align} & \frac{\partial }{\partial t}\left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right) \\ & =\nabla \cdot \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{{\mathbf{{\tau }'}}}_{k}} \right\rangle }^{k}}}+\sum\limits_{k=1}^{\Pi }{\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}} \right)}\mathbf{X}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{\left( \left\langle {{\mathbf{F}}_{jk}} \right\rangle +\left\langle {{{{\dot{m}}'''}}_{jk}} \right\rangle {{\left\langle {{\mathbf{V}}_{k,I}} \right\rangle }^{k}} \right)}} \\ \end{align}\qquad \qquad(5)

The stress tensor of the multiphase mixture is

$\left\langle {\mathbf{{\tau }'}} \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\mathbf{\tau }}_{k}} \right\rangle }^{k}}}=-\left\langle p \right\rangle \mathbf{I}+\mu \left[ \nabla \mathbf{\tilde{V}}+\nabla {{{\mathbf{\tilde{V}}}}_{{}}}^{T} \right]-\frac{2}{3}\mu (\nabla \cdot \mathbf{\tilde{V}})\mathbf{I}\qquad \qquad(6)$

The summation of all interphase forces must be zero since $\left\langle {{\mathbf{F}}_{jk}} \right\rangle =-\left\langle {{\mathbf{F}}_{kj}} \right\rangle$, i.e.,

$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{}}\left\langle {{\mathbf{F}}_{jk}} \right\rangle }=0\qquad \qquad(7)$

Considering eqs. (3), (6) and (7), the momentum equation becomes

$\frac{\partial }{\partial t}\left( \left\langle \rho \right\rangle \mathbf{\tilde{V}} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right)=\nabla \cdot \left\langle {\mathbf{{\tau }'}} \right\rangle +\left\langle \rho \right\rangle \mathbf{X}+{{\mathbf{{\dot{M}}'''}}_{I}}\qquad \qquad(8)$

where

${{\mathbf{{\dot{M}}'''}}_{I}}=\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{}}\left\langle {{{{\dot{m}}'''}}_{jk}} \right\rangle }{{\left\langle {{\mathbf{V}}_{k,I}} \right\rangle }^{k}}\qquad \qquad(9)$

Equation (9) represents the momentum production rate due to interaction between different phases along their separating interfaces. It must be specified according to the combination of phases in the multiphase system that is under consideration.

## Energy Equation

By summing the energy equations for all Π phases in the multi-fluid model:

\begin{align} & \frac{\partial }{\partial t}\left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{h}_{k}} \right\rangle }^{k}} \right)+\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{h}_{k}} \right\rangle }^{k}} \right)=-\nabla \cdot \left\langle {{{\mathbf{{q}''}}}_{k}} \right\rangle +\left\langle {{{{q}'''}}_{k}} \right\rangle \\ & +{{\varepsilon }_{k}}\frac{D{{\left\langle {{p}_{k}} \right\rangle }^{k}}}{Dt}+\nabla \left\langle {{\mathbf{V}}_{k}} \right\rangle :\left\langle {{\mathbf{\tau }}_{k}} \right\rangle +\sum\limits_{j=1(j\ne k)}^{\Pi }{\left[ \left\langle {{{{q}'''}}_{jk}} \right\rangle +{{{{\dot{m}}'''}}_{jk}}{{\left\langle {{h}_{k,I}} \right\rangle }^{k}} \right]} \\ \end{align}

one obtains

\begin{align} & \frac{\partial }{\partial t}\left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{h}_{k}} \right\rangle }^{k}}} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{h}_{k}} \right\rangle }^{k}}} \right)=-\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{\left\langle {{{\mathbf{{q}''}}}_{k}} \right\rangle } \right)+\sum\limits_{k=1}^{\Pi }{\left\langle {{{{q}'''}}_{k}} \right\rangle } \\ & +\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}\frac{D{{\left\langle {{p}_{k}} \right\rangle }^{k}}}{Dt}}+\sum\limits_{k=1}^{\Pi }{\nabla {{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}:{{\varepsilon }_{k}}{{\left\langle {{\mathbf{\tau }}_{k}} \right\rangle }^{k}}}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{\left\langle {{{{q}'''}}_{jk}} \right\rangle }}+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}{{\left\langle {{h}_{k,I}} \right\rangle }^{k}}}} \\ \end{align}\qquad \qquad(10)

The mass average enthalpy of the multiphase mixture is

$\tilde{h}=\frac{1}{\left\langle \rho \right\rangle }\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{h}_{k}} \right\rangle }^{k}}}\qquad \qquad(11)$

The fifth term on the right-hand side of eq. (10) is for summation of all interphase heat transfer and it must be zero. The last term on the right-hand side of eq. (10) accounts for contribution of interphase phase change energy flux due to phase change; it can be defined as

$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk}}{{\left\langle {{h}_{k,I}} \right\rangle }^{k}}}}={{{q}'''}_{I}}\qquad \qquad(12)$

It is usually not zero although $\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{\dot{m}}'''}}}=0$.

Considering eqs. (11) and (12), the energy equation (10) becomes

$\frac{\partial }{\partial t}\left( \left\langle \rho \right\rangle \tilde{h} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}}{{h}_{k}} \right\rangle }^{k}}} \right)=-\nabla \cdot \left\langle {\mathbf{{q}''}} \right\rangle +\frac{D\left\langle p \right\rangle }{Dt}+\left\langle {{q}'''} \right\rangle +\nabla \mathbf{\tilde{V}}:\left\langle \mathbf{\tau } \right\rangle +{{{q}'''}_{I}}\qquad \qquad(13)$

## Species

Summing the equations for conservation of species mass for multi-fluid model,

$\frac{\partial \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}} \right)}{\partial t}+\nabla \cdot \left( {{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}} \right)=-\nabla \cdot \left\langle {{\mathbf{J}}_{k,i}} \right\rangle +\left\langle {{{{\dot{m}}'''}}_{k,i}} \right\rangle +\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk,i}}}$

for all phases yields

\begin{align} & \frac{\partial }{\partial t}\left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}}} \right)+\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}}{{\left\langle {{\mathbf{V}}_{k}} \right\rangle }^{k}}} \right) \\ & =-\nabla \cdot \left( \sum\limits_{k=1}^{\Pi }{\left\langle {{\mathbf{J}}_{k,i}} \right\rangle } \right)+\sum\limits_{k=1}^{\Pi }{\left\langle {{{{\dot{m}}'''}}_{k,i}} \right\rangle }+\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk,i}}}} \\ \end{align}\qquad \qquad(14)

By applying $\left\langle {{\rho }_{k}} \right\rangle ={{\varepsilon }_{k}}{{\left\langle {{\rho }_{k}} \right\rangle }^{k}}$ (see multi-fluid model) to the mass density of the ith component, one obtains

$\left\langle {{\rho }_{i}} \right\rangle =\sum\limits_{k=1}^{\Pi }{{{\varepsilon }_{k}}{{\left\langle {{\rho }_{k,i}} \right\rangle }^{k}}}\qquad \qquad(15)$

In accordance with the conservation of mass, the mass source (or sink) of the ith component due to phase change in all phases must add up to zero, i.e.,

$\sum\limits_{k=1}^{\Pi }{\sum\limits_{j=1(j\ne k)}^{\Pi }{{{{{\dot{m}}'''}}_{jk,i}}}}=0\qquad \qquad(16)$

Substituting eqs. (15) and (16) into eq. (14), and using the mass-averaged velocity defined in eq. (3), the conservation of species mass becomes

$\frac{\partial \left\langle {{\rho }_{i}} \right\rangle }{\partial t}+\nabla \cdot \left\langle {{\rho }_{i}} \right\rangle \mathbf{\tilde{V}}=-\nabla \cdot {{\mathbf{J}}_{i}}+{{{\dot{m}}'''}_{i}}\qquad \qquad(17)$

## References

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

Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.