Heat Transfer in FullyDeveloped Internal Turbulent Flow
From ThermalFluidsPedia
(5 intermediate revisions not shown)  
Line 1:  Line 1:  
  Heat transfer in fullydeveloped turbulent flow in a circular tube subject to constant heat flux (<math>{{{q}''}_{w}}=</math> const) will be considered in this  +  Heat transfer in fullydeveloped turbulent flow in a circular tube subject to constant heat flux (<math>{{{q}''}_{w}}=</math> const) will be considered in this article <ref name="ON1999">Oosthuizen, P.H., and Naylor, D., 1999, Introduction to Convective Heat Transfer Analysis, WCB/McGrawHill, New York.</ref><ref>Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.</ref>. When the turbulent flow in the tube is fully developed, we have <math>\bar{v}=0</math> and the energy equation becomes 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 17:  Line 17:  
{{EquationRef(2)}}  {{EquationRef(2)}}  
}  }  
  where <math>{{\bar{T}}_{c}}</math> is the timeaveraged temperature at the centerline of the tube, and  +  where <math>{{\bar{T}}_{c}}</math> is the timeaveraged temperature at the centerline of the tube, and ''T<sub>w</sub>'' is the wall temperature. Thus, <math>({{T}_{w}}\bar{T})/({{T}_{w}}{{\bar{T}}_{c}})</math> is a function of ''r'' only, i.e., 
  <math>({{T}_{w}}\bar{T})/({{T}_{w}}{{\bar{T}}_{c}})</math> is a function of r only, i.e.,  +  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 27:  Line 26:  
{{EquationRef(3)}}  {{EquationRef(3)}}  
}  }  
  where f is independent from x. Differentiating  +  where ''f'' is independent from ''x''. Differentiating eq. (2) yields 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 45:  Line 44:  
{{EquationRef(5)}}  {{EquationRef(5)}}  
}  }  
  Substituting eq. (  +  Substituting eq. (3) into eq. (5), one obtains: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 63:  Line 62:  
{{EquationRef(7)}}  {{EquationRef(7)}}  
}  }  
  Therefore, eq. (  +  Therefore, eq. (4) becomes: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 90:  Line 89:  
{{EquationRef(10)}}  {{EquationRef(10)}}  
}  }  
  Since <math>{{{q}''}_{w}}=</math> const, it follows from eq. (  +  Since <math>{{{q}''}_{w}}=</math> const, it follows from eq. (9) that <math>({{T}_{w}}{{\bar{T}}_{m}})= Const</math>, i.e., 
  <math>({{T}_{w}}{{\bar{T}}_{m}})=</math>  +  
  +  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 101:  Line 98:  
{{EquationRef(11)}}  {{EquationRef(11)}}  
}  }  
  Combining eqs. (  +  Combining eqs. (7), (8) and (11), the following relationships are obtained: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 110:  Line 107:  
{{EquationRef(12)}}  {{EquationRef(12)}}  
}  }  
  The timeaveraged mean temperature,  +  The timeaveraged mean temperature, <math>{{\bar{T}}_{m}}</math>, changes with ''x'' as the result of heat transfer from the tube wall. The rate of mean temperature change can be obtained as follows: 
  <math>{{\bar{T}}_{m}}</math>  +  
  , changes with x as the result of heat transfer from the tube wall.  +  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 121:  Line 116:  
{{EquationRef(13)}}  {{EquationRef(13)}}  
}  }  
  Substituting eq. (  +  Substituting eq. (12) into eq. (1), the energy equation becomes: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 130:  Line 125:  
{{EquationRef(14)}}  {{EquationRef(14)}}  
}  }  
  where <math>y={{r}_{0}}r</math> is the distance measured from the tube wall. Equation (  +  where <math>y={{r}_{0}}r</math> is the distance measured from the tube wall. Equation (14) is subject to the following two boundary conditions: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 147:  Line 142:  
{{EquationRef(16)}}  {{EquationRef(16)}}  
}  }  
  Integrating eq. (  +  Integrating eq. (14) in the interval of (''r<sub>0</sub>'', ''r'') and considering eq. (15), we have: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 174:  Line 169:  
{{EquationRef(19)}}  {{EquationRef(19)}}  
}  }  
  Integrating eq. (  +  Integrating eq. (18) in the interval of (0, ''y'') and considering eq. (16), one obtains: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 183:  Line 178:  
{{EquationRef(20)}}  {{EquationRef(20)}}  
}  }  
  If the profiles of axial velocity and the thermal eddy diffusivity are known, eq. (  +  If the profiles of axial velocity and the thermal eddy diffusivity are known, eq. (20) can be used to obtain the correlation for internal forced convection heat transfer. With the exception of the very thin viscous sublayer, the velocity profile in the most part of the tube is fairly flat. Therefore, it is assumed that the timeaveraged velocity, <math>\bar{u}</math>, in eq. (19) can be replaced by <math>{{\bar{u}}_{m}}</math>, and ''I''(''y'') becomes: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 192:  Line 187:  
{{EquationRef(21)}}  {{EquationRef(21)}}  
}  }  
  Substituting eqs. (  +  Substituting eqs. (21) and (13) into eq. (20) yields: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 210:  Line 205:  
{{EquationRef(23)}}  {{EquationRef(23)}}  
}  }  
  where y+ is defined  +  where ''y''+ is defined as <math>{{y}^{+}}=\frac{y{{u}_{\tau }}}{\nu }</math>. 
  To consider heat transfer in an internal turbulent flow, the entire turbulent boundary layer is divided into three regions: (1) inner region (<math>{{y}^{+}}<5</math>), (2) buffer region (<math>5\le {{y}^{+}}\le 30</math>), and (3) outer region (<math>{{y}^{+}}>30</math>). In the inner region <math>{{\varepsilon }_{M}}={{\varepsilon }_{H}}=0</math> and eq. (  +  
+  To consider heat transfer in an internal turbulent flow, the entire turbulent boundary layer is divided into three regions: (1) inner region (<math>{{y}^{+}}<5</math>), (2) buffer region (<math>5\le {{y}^{+}}\le 30</math>), and (3) outer region (<math>{{y}^{+}}>30</math>). In the inner region <math>{{\varepsilon }_{M}}={{\varepsilon }_{H}}=0</math> and eq. (23) becomes  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 229:  Line 225:  
{{EquationRef(25)}}  {{EquationRef(25)}}  
}  }  
  The temperature at the boundary between the inner and buffer regions (<math>{{y}^{+}}=5</math>),  +  The temperature at the boundary between the inner and buffer regions (<math>{{y}^{+}}=5</math>), <math>{{\bar{T}}_{s}}</math>, can be obtained from eq. (25) as 
  <math>{{\bar{T}}_{s}}</math>  +  
  +  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 249:  Line 243:  
{{EquationRef(27)}}  {{EquationRef(27)}}  
}  }  
  Substituting eq. (  +  Substituting eq. (27) into eq. (23) and assuming the turbulent Prandtl number <math>{{\Pr }^{t}}=1</math>, the following expression is obtained: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 258:  Line 252:  
{{EquationRef(28)}}  {{EquationRef(28)}}  
}  }  
  Since the buffer region is also very thin,  +  
  <math>1y/{{r}_{0}}</math>  +  Since the buffer region is also very thin, <math>1y/{{r}_{0}}</math> in eq. (28) is effectively equal to 1. Defining <math>{{T}^{+}}={(\bar{T}{{T}_{w}})}/{\left( \frac{{{{{q}''}}_{w}}}{\rho {{c}_{p}}}\sqrt{\frac{\rho }{{{\tau }_{w}}}} \right)}\;</math> and Integrating eq. (28) yields 
  +  
  <math>{{T}^{+}}={(\bar{T}{{T}_{w}})}/{\left( \frac{{{{{q}''}}_{w}}}{\rho {{c}_{p}}}\sqrt{\frac{\rho }{{{\tau }_{w}}}} \right)}\;</math>  +  
  +  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 280:  Line 271:  
{{EquationRef(30)}}  {{EquationRef(30)}}  
}  }  
  The temperature at the top of the buffer region where <math>{{y}^{+}}=30</math>,  +  The temperature at the top of the buffer region where <math>{{y}^{+}}=30</math>, <math>{{\bar{T}}_{b}}</math>, becomes 
  <math>{{\bar{T}}_{b}}</math>  +  
  , becomes  +  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 291:  Line 280:  
{{EquationRef(31)}}  {{EquationRef(31)}}  
}  }  
  For the outer region where <math>{{\varepsilon }_{M}}\gg \nu \text{ and }{{\varepsilon }_{H}}\gg \alpha </math>, eq. (  +  For the outer region where <math>{{\varepsilon }_{M}}\gg \nu \text{ and }{{\varepsilon }_{H}}\gg \alpha </math>, eq. (23) becomes 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 301:  Line 290:  
}  }  
where the turbulent Prandtl number is assumed to be equal to 1.  where the turbulent Prandtl number is assumed to be equal to 1.  
  It is assumed that the Nikuradse equation  +  
+  It is assumed that the Nikuradse equation is valid in the outer region and the velocity gradient in this region becomes:  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 310:  Line 300:  
{{EquationRef(33)}}  {{EquationRef(33)}}  
}  }  
  The expression of apparent shear stress in this region  +  The expression of apparent shear stress in this region, can be nondimensionalized as: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 319:  Line 309:  
{{EquationRef(34)}}  {{EquationRef(34)}}  
}  }  
  Substituting  +  Substituting 
+  <center><math>{{\tau }_{app}}={{\tau }_{w}}\cdot \left( \frac{r}{{{r}_{o}}} \right)={{\tau }_{w}}\cdot \left( 1\frac{y}{{{r}_{o}}} \right)</math></center>  
+  
+  and eq. (33) into eq. (34), the eddy diffusivity in the outer region is obtained as:  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 328:  Line 321:  
{{EquationRef(35)}}  {{EquationRef(35)}}  
}  }  
  Substituting eq. (  +  Substituting eq. (35) into eq. (32), the temperature distribution in this region becomes: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 346:  Line 339:  
{{EquationRef(37)}}  {{EquationRef(37)}}  
}  }  
  The temperature at the center of the tube, <math>{{\bar{T}}_{c}}</math>, can be obtained by letting  +  The temperature at the center of the tube, <math>{{\bar{T}}_{c}}</math>, can be obtained by letting <math>{{y}^{+}}=y_{c}^{+}</math> in eq. (36), i.e. 
  <math>{{y}^{+}}=y_{c}^{+}</math>  +  
  +  
  +  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
    
Line 357:  Line 348:  
{{EquationRef(38)}}  {{EquationRef(38)}}  
}  }  
  The overall temperature change from the wall to the center of the tube can be obtained by adding eqs. (  +  The overall temperature change from the wall to the center of the tube can be obtained by adding eqs. (26), (31) and (38): 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 366:  Line 357:  
{{EquationRef(39)}}  {{EquationRef(39)}}  
}  }  
  It follows from the definition of friction factor,  +  It follows from the definition of friction factor, <math>{{c}_{f}}=\frac{{{\tau }_{w}}}{\rho \bar{u}_{m}^{2}/2}</math>, that 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 375:  Line 366:  
{{EquationRef(40)}}  {{EquationRef(40)}}  
}  }  
  Substituting eq. (  +  Substituting eq. (40) into eq. (39) and considering the definition of Reynolds number, <math>{{\operatorname{Re}}_{D}}={{\bar{u}}_{m}}D/\nu </math>, eq. (39) becomes: 
  +  
+  { class="wikitable" border="0"  
+    
+   width="100%" <center>  
<math>{{T}_{w}}{{\bar{T}}_{c}}=\left( \frac{{{{{q}''}}_{w}}}{\rho {{c}_{p}}{{{\bar{u}}}_{m}}} \right)\sqrt{\frac{2}{{{c}_{f}}}}\left[ 2.5\ln \left( \frac{{{\operatorname{Re}}_{D}}}{60}\sqrt{\frac{{{c}_{f}}}{2}} \right)+5\ln (5\Pr +1)+5\Pr \right]</math>  <math>{{T}_{w}}{{\bar{T}}_{c}}=\left( \frac{{{{{q}''}}_{w}}}{\rho {{c}_{p}}{{{\bar{u}}}_{m}}} \right)\sqrt{\frac{2}{{{c}_{f}}}}\left[ 2.5\ln \left( \frac{{{\operatorname{Re}}_{D}}}{60}\sqrt{\frac{{{c}_{f}}}{2}} \right)+5\ln (5\Pr +1)+5\Pr \right]</math>  
  +  </center>  
  In order to obtain the heat transfer coefficient, <math>h={{{q}''}_{w}}/({{T}_{w}}{{\bar{T}}_{m}})</math>, the temperature difference <math>{{T}_{w}}{{\bar{T}}_{m}}</math> must be obtained. If the velocity profile can be approximated by  +  {{EquationRef(41)}} 
+  }  
+  
+  In order to obtain the heat transfer coefficient, <math>h={{{q}''}_{w}}/({{T}_{w}}{{\bar{T}}_{m}})</math>, the temperature difference <math>{{T}_{w}}{{\bar{T}}_{m}}</math> must be obtained. If the velocity profile can be approximated by <math>\frac{{\bar{u}}}{{{{\bar{u}}}_{c}}}={{\left( \frac{y}{{{r}_{o}}} \right)}^{1/7}}</math>, and the temperature and velocity can also be approximated by the oneseventh law, i.e.,  
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 386:  Line 383:  
<math>\frac{{{T}_{w}}\bar{T}}{{{T}_{w}}{{{\bar{T}}}_{c}}}={{\left( \frac{y}{{{r}_{o}}} \right)}^{1/7}}{{,}_{_{\text{ }}}}\frac{{\bar{u}}}{{{{\bar{u}}}_{c}}}={{\left( \frac{y}{{{r}_{o}}} \right)}^{1/7}}</math>  <math>\frac{{{T}_{w}}\bar{T}}{{{T}_{w}}{{{\bar{T}}}_{c}}}={{\left( \frac{y}{{{r}_{o}}} \right)}^{1/7}}{{,}_{_{\text{ }}}}\frac{{\bar{u}}}{{{{\bar{u}}}_{c}}}={{\left( \frac{y}{{{r}_{o}}} \right)}^{1/7}}</math>  
</center>  </center>  
  {{EquationRef(  +  {{EquationRef(42)}} 
}  }  
Line 396:  Line 393:  
<math>{{T}_{w}}{{\bar{T}}_{m}}=\frac{\int_{0}^{{{r}_{o}}}{\bar{u}({{T}_{w}}\bar{T})2\pi rdr}}{\int_{0}^{{{r}_{o}}}{\bar{u}2\pi rdr}}=\frac{5}{6}({{T}_{w}}{{\bar{T}}_{c}})</math>  <math>{{T}_{w}}{{\bar{T}}_{m}}=\frac{\int_{0}^{{{r}_{o}}}{\bar{u}({{T}_{w}}\bar{T})2\pi rdr}}{\int_{0}^{{{r}_{o}}}{\bar{u}2\pi rdr}}=\frac{5}{6}({{T}_{w}}{{\bar{T}}_{c}})</math>  
</center>  </center>  
  {{EquationRef(  +  {{EquationRef(43)}} 
}  }  
  Substituting eq. (  +  Substituting eq. (41) into eq. (43) results in: 
{ class="wikitable" border="0"  { class="wikitable" border="0"  
Line 405:  Line 402:  
<math>{{T}_{w}}{{\bar{T}}_{m}}=\frac{5}{6}\left( \frac{{{{{q}''}}_{w}}}{\rho {{c}_{p}}{{{\bar{u}}}_{m}}} \right)\sqrt{\frac{2}{{{c}_{f}}}}\left[ 2.5\ln \left( \frac{{{\operatorname{Re}}_{D}}}{60}\sqrt{\frac{{{c}_{f}}}{2}} \right)+5\ln (5\Pr +1)+5\Pr \right]</math>  <math>{{T}_{w}}{{\bar{T}}_{m}}=\frac{5}{6}\left( \frac{{{{{q}''}}_{w}}}{\rho {{c}_{p}}{{{\bar{u}}}_{m}}} \right)\sqrt{\frac{2}{{{c}_{f}}}}\left[ 2.5\ln \left( \frac{{{\operatorname{Re}}_{D}}}{60}\sqrt{\frac{{{c}_{f}}}{2}} \right)+5\ln (5\Pr +1)+5\Pr \right]</math>  
</center>  </center>  
  {{EquationRef(  +  {{EquationRef(44)}} 
}  }  
which can be rearranged to the following empirical correlation  which can be rearranged to the following empirical correlation  
Line 414:  Line 411:  
<math>\text{N}{{\text{u}}_{D}}=\frac{{{\operatorname{Re}}_{D}}\Pr \sqrt{\frac{{{c}_{f}}}{2}}}{\frac{5}{6}\left[ 2.5\ln \left( \frac{{{\operatorname{Re}}_{D}}}{60}\sqrt{\frac{{{c}_{f}}}{2}} \right)+5\ln (5\Pr +1)+5\Pr \right]}</math>  <math>\text{N}{{\text{u}}_{D}}=\frac{{{\operatorname{Re}}_{D}}\Pr \sqrt{\frac{{{c}_{f}}}{2}}}{\frac{5}{6}\left[ 2.5\ln \left( \frac{{{\operatorname{Re}}_{D}}}{60}\sqrt{\frac{{{c}_{f}}}{2}} \right)+5\ln (5\Pr +1)+5\Pr \right]}</math>  
</center>  </center>  
  {{EquationRef(  +  {{EquationRef(45)}} 
}  }  
  which can be used together with appropriate friction coefficient  +  which can be used together with appropriate friction coefficient to obtain the Nusselt number. 
+  
+  ==References==  
+  {{Reflist}} 
Current revision as of 20:21, 23 July 2010
Heat transfer in fullydeveloped turbulent flow in a circular tube subject to constant heat flux (q''_{w} = const) will be considered in this article ^{[1]}^{[2]}. When the turbulent flow in the tube is fully developed, we have and the energy equation becomes

After the turbulent flow is hydrodynamically and thermally fully developed, the timeaveraged temperature profile is no longer a function of axial distance from the inlet, i.e.,

where is the timeaveraged temperature at the centerline of the tube, and T_{w} is the wall temperature. Thus, is a function of r only, i.e.,

where f is independent from x. Differentiating eq. (2) yields

At the wall, the contribution of eddy diffusivity on the heat transfer is negligible, and the heat flux at the wall becomes

Substituting eq. (3) into eq. (5), one obtains:

Since the heat flux is constant, q''_{w} = const, it follows that , i.e.,

Therefore, eq. (4) becomes:

For fully developed flow, the local heat transfer coefficient is:

where is the timeaveraged mean temperature defined as:

Since q''_{w} = const, it follows from eq. (9) that , i.e.,

Combining eqs. (7), (8) and (11), the following relationships are obtained:

The timeaveraged mean temperature, , changes with x as the result of heat transfer from the tube wall. The rate of mean temperature change can be obtained as follows:

Substituting eq. (12) into eq. (1), the energy equation becomes:

where y = r_{0} − r is the distance measured from the tube wall. Equation (14) is subject to the following two boundary conditions:
(axisymmetric condition) 

Integrating eq. (14) in the interval of (r_{0}, r) and considering eq. (15), we have:

which can be rearranged to

where

Integrating eq. (18) in the interval of (0, y) and considering eq. (16), one obtains:

If the profiles of axial velocity and the thermal eddy diffusivity are known, eq. (20) can be used to obtain the correlation for internal forced convection heat transfer. With the exception of the very thin viscous sublayer, the velocity profile in the most part of the tube is fairly flat. Therefore, it is assumed that the timeaveraged velocity, , in eq. (19) can be replaced by , and I(y) becomes:

Substituting eqs. (21) and (13) into eq. (20) yields:

which can be rewritten in terms of wall coordinate

where y+ is defined as .
To consider heat transfer in an internal turbulent flow, the entire turbulent boundary layer is divided into three regions: (1) inner region (y^{ + } < 5), (2) buffer region (), and (3) outer region (y^{ + } > 30). In the inner region and eq. (23) becomes

Since the inner region is very thin, and 1 − y / r_{0} is effectively equal to 1. Therefore, the temperature profile in the inner region becomes:

The temperature at the boundary between the inner and buffer regions (y^{ + } = 5), , can be obtained from eq. (25) as

In the buffer region where , the eddy diffusivity in the buffer region is:

Substituting eq. (27) into eq. (23) and assuming the turbulent Prandtl number , the following expression is obtained:

Since the buffer region is also very thin, 1 − y / r_{0} in eq. (28) is effectively equal to 1. Defining and Integrating eq. (28) yields

i.e.,

The temperature at the top of the buffer region where y^{ + } = 30, , becomes

For the outer region where , eq. (23) becomes

where the turbulent Prandtl number is assumed to be equal to 1.
It is assumed that the Nikuradse equation is valid in the outer region and the velocity gradient in this region becomes:

The expression of apparent shear stress in this region, can be nondimensionalized as:

Substituting
and eq. (33) into eq. (34), the eddy diffusivity in the outer region is obtained as:

Substituting eq. (35) into eq. (32), the temperature distribution in this region becomes:

which is valid from y^{ + } = 30 to the center of the tube where y_{c} = r_{0} or

The temperature at the center of the tube, , can be obtained by letting in eq. (36), i.e.

The overall temperature change from the wall to the center of the tube can be obtained by adding eqs. (26), (31) and (38):

It follows from the definition of friction factor, , that

Substituting eq. (40) into eq. (39) and considering the definition of Reynolds number, , eq. (39) becomes:

In order to obtain the heat transfer coefficient, , the temperature difference must be obtained. If the velocity profile can be approximated by , and the temperature and velocity can also be approximated by the oneseventh law, i.e.,

it follows that

Substituting eq. (41) into eq. (43) results in:

which can be rearranged to the following empirical correlation

which can be used together with appropriate friction coefficient to obtain the Nusselt number.