# External Natural Convection from Heated Vertical Plate

(Difference between revisions)
 Revision as of 03:08, 17 June 2010 (view source) (Created page with '===6.2.2 External Natural Convection from Heated Vertical Plate=== For external natural convection near a vertical flat plate as shown in Fig. 6.1, the boundary layer assumption …')← Older edit Revision as of 04:17, 17 June 2010 (view source)Newer edit → Line 5: Line 5: -
$\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y}=0 \qquad \qquad()$
+
$\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y}=0 \qquad \qquad(1)$
- (6.17) + - If one assumes that the fluid is single component so that the natural convection is driven by the density difference induced by the temperature gradient, eq. (6.13) becomes: + If one assumes that the fluid is single component so that the natural convection is driven by the density difference induced by the temperature gradient, eq. (10) from [[Generalized Governing Equations]] becomes: -
$\rho \frac{D\mathbf{V}}{Dt}=\left( -\nabla p+{{\rho }_{\infty }}\mathbf{g} \right)-{{\rho }_{\infty }}\mathbf{g}\beta (T-{{T}_{\infty }})+\nabla \cdot (\mu \nabla \mathbf{V}) \qquad \qquad()$
+
$\rho \frac{D\mathbf{V}}{Dt}=\left( -\nabla p+{{\rho }_{\infty }}\mathbf{g} \right)-{{\rho }_{\infty }}\mathbf{g}\beta (T-{{T}_{\infty }})+\nabla \cdot (\mu \nabla \mathbf{V}) \qquad \qquad(2)$
- (6.18) + Applying the boundary layer assumption and assuming steady flow with constant thermophysical properties, the momentum equation becomes: Applying the boundary layer assumption and assuming steady flow with constant thermophysical properties, the momentum equation becomes: -
$u\frac{\partial u}{\partial x}+v\frac{\partial u}{\partial y}=-\frac{1}{{{\rho }_{\infty }}}\frac{\partial p}{\partial x}-g+g\beta (T-{{T}_{\infty }})+\nu \frac{{{\partial }^{2}}u}{\partial {{y}^{2}}} \qquad \qquad()$
+
$u\frac{\partial u}{\partial x}+v\frac{\partial u}{\partial y}=-\frac{1}{{{\rho }_{\infty }}}\frac{\partial p}{\partial x}-g+g\beta (T-{{T}_{\infty }})+\nu \frac{{{\partial }^{2}}u}{\partial {{y}^{2}}} \qquad \qquad(3)$
- (6.19) + Line 33: Line 30: - Substituting the above two equations into eq. (6.19), the momentum equation becomes: + Substituting the above two equations into eq. (3), the momentum equation becomes: -
$u\frac{\partial u}{\partial x}+v\frac{\partial u}{\partial y}=\nu \frac{{{\partial }^{2}}u}{\partial {{y}^{2}}}+g\beta (T-{{T}_{\infty }}) \qquad \qquad()$
+
$u\frac{\partial u}{\partial x}+v\frac{\partial u}{\partial y}=\nu \frac{{{\partial }^{2}}u}{\partial {{y}^{2}}}+g\beta (T-{{T}_{\infty }}) \qquad \qquad(4)$
- (6.20) + Line 43: Line 39: -
$u\frac{\partial T}{\partial x}+v\frac{\partial T}{\partial y}=\alpha \frac{{{\partial }^{2}}T}{\partial {{y}^{2}}} \qquad \qquad()$
+
$u\frac{\partial T}{\partial x}+v\frac{\partial T}{\partial y}=\alpha \frac{{{\partial }^{2}}T}{\partial {{y}^{2}}} \qquad \qquad(5)$
- (6.21) + Line 50: Line 45: -
$u=v=0,\text{ at }y=0 \qquad \qquad()$
+
$u=v=0,\text{ at }y=0 \qquad \qquad(6)$
- (6.22) + Line 57: Line 51: -
$T={{T}_{w}},\text{ at }y=0 \qquad \qquad()$
+
$T={{T}_{w}},\text{ at }y=0 \qquad \qquad(7)$
- (6.23) + Line 64: Line 57: -
$u=v=0,\text{ }y\to \infty \qquad \qquad()$
+
$u=v=0,\text{ }y\to \infty \qquad \qquad(8)$
- (6.24) + Line 71: Line 63: -
$T={{T}_{\infty }},\text{ }y\to \infty \qquad \qquad()$
+
$T={{T}_{\infty }},\text{ }y\to \infty \qquad \qquad(9)$
- (6.25) + ==References== ==References==

## Contents

### 6.2.2 External Natural Convection from Heated Vertical Plate

For external natural convection near a vertical flat plate as shown in Fig. 6.1, the boundary layer assumption can be applied to simplify the above generalized governing equations. The boundary layer treatment for the case of natural convection is very similar to that for the case of forced convection that was discussed in Chapter 4. The difference between the natural convection problem shown in Fig. 6.1 and forced convection over a flat plate is that the free stream velocity in the outside of the velocity boundary layer in natural convection is zero for natural convection. In addition, the pressure outside the boundary layer is hydrostatic for the case of natural convection, instead of being externally imposed as in the case of forced convection.

For 2-D external convection of an incompressible fluid as shown in Fig. 6.1, the continuity equation becomes

$\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y}=0 \qquad \qquad(1)$

If one assumes that the fluid is single component so that the natural convection is driven by the density difference induced by the temperature gradient, eq. (10) from Generalized Governing Equations becomes:

$\rho \frac{D\mathbf{V}}{Dt}=\left( -\nabla p+{{\rho }_{\infty }}\mathbf{g} \right)-{{\rho }_{\infty }}\mathbf{g}\beta (T-{{T}_{\infty }})+\nabla \cdot (\mu \nabla \mathbf{V}) \qquad \qquad(2)$

Applying the boundary layer assumption and assuming steady flow with constant thermophysical properties, the momentum equation becomes:

$u\frac{\partial u}{\partial x}+v\frac{\partial u}{\partial y}=-\frac{1}{{{\rho }_{\infty }}}\frac{\partial p}{\partial x}-g+g\beta (T-{{T}_{\infty }})+\nu \frac{{{\partial }^{2}}u}{\partial {{y}^{2}}} \qquad \qquad(3)$

The pressure in the boundary layer, p, is independent of y ($\partial p/\partial y=0$) and equals that outside the boundary layer at the same longitudinal position, ${p_{\infty}}$, i.e.,

$\frac{\partial p}{\partial x}=\frac{dp}{dx}=\frac{d{{p}_{\infty }}}{dx}$

The hydrostatic pressure, ${p_{\infty}}$, is dictated by the density and the longitudinal position:

$\frac{d{{p}_{\infty }}}{dx}=-{{\rho }_{\infty }}g$

Substituting the above two equations into eq. (3), the momentum equation becomes:

$u\frac{\partial u}{\partial x}+v\frac{\partial u}{\partial y}=\nu \frac{{{\partial }^{2}}u}{\partial {{y}^{2}}}+g\beta (T-{{T}_{\infty }}) \qquad \qquad(4)$

After applying the boundary layer assumption and assuming the viscous dissipation is negligible, the energy equation becomes:

$u\frac{\partial T}{\partial x}+v\frac{\partial T}{\partial y}=\alpha \frac{{{\partial }^{2}}T}{\partial {{y}^{2}}} \qquad \qquad(5)$

At the heated wall, the non-slip and impermeable conditions yield the following boundary condition for the momentum equation:

$u=v=0,\text{ at }y=0 \qquad \qquad(6)$

The temperature at the heated wall is specified, i.e.,

$T={{T}_{w}},\text{ at }y=0 \qquad \qquad(7)$

Since the quiescent fluid far away from the heated plate is not disturbed by the existence of the heated plate, the velocity at the locations away from the flat plate should be zero:

$u=v=0,\text{ }y\to \infty \qquad \qquad(8)$

Also, the temperature of the fluid outside the thermal boundary layer is not affected by the heated wall:

$T={{T}_{\infty }},\text{ }y\to \infty \qquad \qquad(9)$