chilton and colburn j-factor analogy
DESCRIPTION
Chilton and Colburn J-factor analogy. Recall: The equation for heat transfer in the turbulent regime. Sieder -Tate Equation. (for forced convection/ turbulent, Re > 10000 & 0.5 < Pr < 100). If we divide this by . Dimensionless Groups. Chilton and Colburn J-factor analogy. - PowerPoint PPT PresentationTRANSCRIPT
Chilton and Colburn J-factor analogy
Recall: The equation for heat transfer in the turbulent regime
Sieder-Tate Equation ๐๐ข=0.023๐ ๐0.8 ๐๐ 1 /3๐๐ฃ
๐๐ฃ=( ๐๐๐ค )0.14
(for forced convection/ turbulent, Re > 10000 & 0.5 < Pr < 100)
If we divide this by
๐๐๐ข
๐๐ ๐๐ ๐๐=0.023
(๐๐ ๐ )0.8 (๐ ๐๐ )13 ( ๐๐1 )
0.14
๐๐ ๐๐ ๐๐
Dimensionless Groups
Dim. Group Ratio Equation
Prandtl, Pr molecular diffusivity of momentum / molecular diffusivity of heat
Schmidt, Sc momentum diffusivity/ mass diffusivity
Lewis, Le thermal diffusivity/ mass diffusivity
Stanton, St heat transferred/ thermal capacity
Nusselt, Nu convective / conductive heat transfer across the boundary
Chilton and Colburn J-factor analogy
This can be rearranged as
๐๐๐ข
๐๐ ๐๐ ๐๐=0.023
(๐๐ ๐ )0.8 (๐ ๐๐ )13 ( ๐๐1 )
0.14
๐๐ ๐๐ ๐๐
๐ ๐๐ก๐๐๐
23 ( ๐๐1 )
โ 0.14
=0.023๐๐ ๐โ 0.2
๐2 =0.023๐๐ ๐
โ0.2
For the turbulent flow region, an empirical equation relating f and Re
Chilton and Colburn J-factor analogy
๐2
=๐ ๐๐ก๐ ๐๐
23 ( ๐๐1 )
0.14
=0.023๐๐ ๐โ0.2
} rsub { } ๐ฑ ๐ฏ ยฟ
This is called as the J-factor for heat transfer
Chilton and Colburn J-factor analogy
In a similar manner, we can relate the mass transfer and momentum transfer using
๐๐โฒ ๐ท๐ท๐๐
=0.023 (๐๐ ๐ )0.83 (๐๐๐ )0.33
the equation for mass transfer of all liquids and gases
If we divide this by
๐๐โฒ
๐ฃ(๐ ๐๐
23 ) (๐๐ ๐ )0.03=0.023๐๐ ๐
โ 0.2
Chilton and Colburn J-factor analogy
T
๐๐โฒ
๐ฃ(๐ ๐๐
23 )=0.023 ๐๐ ๐
โ0.2
๐๐โฒ
๐ฃ(๐ ๐๐
23 ) (๐๐ ๐ )0.03=0.023๐๐ ๐
โ 0.2
๐๐โฒ
๐ฃ(๐ ๐๐
23 )= ๐
2
Chilton and Colburn J-factor analogy
๐2
=๐๐โฒ
๐ฃ(๐ ๐๐
23 )=0.023๐๐ ๐
โ0.2
This is called as the J-factor for mass transfer
} rsub { } ๐ฑ ๐ซ ยฟ
Chilton and Colburn J-factor analogy
Extends the Reynolds analogy to liquids
f2= hc p๐ ๐ฃ
=๐๐โฒ
๐ฃ
f2= hc p๐ ๐ฃ
(๐ ๐๐
23 )( ๐๐1 )
0.14
=๐๐โฒ
๐ฃ (๐ ๐๐
23 )
Chilton and Colburn J-factor analogy
If we let
f2= hc p๐ ๐ฃ
(๐ ๐๐
23 )=
๐๐โฒ
๐ฃ (๐ ๐๐
23 )
} rsub { } ๐ฑ ๐ฏ ยฟ } rsub { } ๐ฑ ๐ซ ยฟ
๐2 = ๐ฝ ๐ป=J D
Applies to the following ranges:For heat transfer:10,000 < Re < 300,0000.6 < Pr < 100For mass transfer: 2,000 < Re < 300,0000.6 < Sc < 2,500
( ๐๐1 )0.14
=1
Martinelli Analogy
Reynolds Analogy demonstrates similarity of mechanism (the gradients are assumed equal) Pr = 1 and Sc = 1
Chilton-Colburn J-factor Analogy demonstrates numerical similarity(implies that the correlation equations are not faithful statements of the mechanism, but useful in predicting numerical values of coefficients wider range of Pr and Sc
Martinelli Analogy
Martinelli Analogy (heat and momentum transfer) applicable to the entire range of Pr number
Assumptions:1. The T driving forces between the wall and the fluid is small
enough so that ฮผ/ฮผ1 = 12. Well-developed turbulent flow exists within the test section3. Heat flux across the tube wall is constant along the test
section4. Both stress and heat flux are zero at the center of the tube
and increases linearly with radius to a maximum at the wall5. At any point ฮตq = ฮตฯ
Martinelli Analogy
Assumptions:
6. The velocity profile distribution given by Figure 12.5 is valid
Martinelli Analogy
๐๐ด ( ๐๐1 )=โ (๐ผ+๐ผ ๐ก ) (๐ (๐๐๐๐ )
๐๐ )
๐ ๐ฆ( ๐๐1 )=โ(๐๐+๐๐ก)( ๐ (๐ฃ ๐ )๐๐ )
Both equal to zero;For cylindrical geometry
Martinelli Analogy
๐๐ด ( ๐๐1 )=โ (๐ผ+๐ผ ๐ก ) (๐ (๐๐๐๐ )
๐๐ )
๐ ๐ฆ( ๐๐1 )=โ(๐๐+๐๐ก)( ๐ (๐ฃ ๐ )๐๐ )
Both equal to zero;For cylindrical geometry
Integrated and expressed as function of position
Converted in the form
Martinelli Analogy
Martinelli Analogy
Martinelli Analogy (heat and momentum transfer) applicable to the entire range of Pr number
predicts Nu for liquid metals contributes to understanding of the mechanism of heat and momentum transfer
Martinelli Analogy
Martinelli Analogy (heat and momentum transfer) applicable to the entire range of Pr number
predicts Nu for liquid metals contributes to understanding of the mechanism of heat and momentum transfer
Analogies
EXAMPLECompare the value of the Nusselt number, given by the appropriate empirical equation, to that predicted by the Reynolds, Colburn and Martinelli analogies for each of the following substances at Re= 100,000 and f = 0.0046. Consider all substances at 1000F, subject to heating with the tube wall at 1500F.
Example
Sample CalculationFor air,
๐๐๐ข=0.023 (๐๐ ๐ )0.8 (๐ ๐๐ )13 ( ๐๐1 )
0.14
๐๐๐ข=0.023 (100,000 )0.8 (0.71 )13 ( 0.018
0.02 )0.14
๐๐๐ข=20 2(๐๐๐ ๐ก ๐๐๐๐ข๐๐๐ก๐๐ฃ๐๐๐ข๐)
Example
Sample CalculationFor air, by Reynolds analogy
๐ ๐๐ก=๐ ๐๐ข
๐๐ ๐๐ ๐๐= f
2
๐๐๐ข=f2๐๐ ๐๐ ๐๐=( 0.0046
2 ) (105 )(0.71)
๐๐๐ข=16 3.3
Example
Sample CalculationFor air, by Colburn analogy
๐ ๐๐ก=๐ ๐๐ข
๐๐ ๐๐ ๐๐
f2=๐ ๐๐ก (๐ ๐๐
23 )( ๐๐1 )
0.14
๐๐๐ข=๐๐ ๐ (๐ ๐๐ )13 ( ๐2 )( ๐๐1 )
0.14
๐๐๐ข=105 (0.71 )13 ( 0.0046
2 )( 0.0180.02 )
0.14
๐๐๐ข=202
Example
Sample CalculationFor air, by Martinelli analogy
๐๐๐ข=170
FIN