Module 3. Dimensional analysis and heat transfer
Lesson 17
HEAT TRANSFER BY FORCED CONVECTION
17.1 Heat Transfer by Forced Convection
17.1.1 Heat transfer in turbulent flow
In turbulent flow, heat transfer within the fluid is caused mainly by mixing. The process of mixing is of such intensity that the fluid temperature remains practically constant over the cross – section of the core. Sharp variation in the temperature is observed only within the boundary layer. Such distribution of temperature makes free flow i.e., natural convection impossible. This heat transfer is fully explained by forced flow i.e., forced convection factors. Nusselt was the first to conduct a detailed investigation in the conditions of turbulent flow of gases.
For straight smooths tubes and fluids of widely varying main parameters:
Here the Reference temperature
is mean temperature of the fluid and Reference. Dimension is equivalent
diameter =
irrespective
of the fraction of perimeter participating in the process of heat exchange.
Formula is applicable for any shape – round, square, rectangular, triangular or
annular. For gases, Pr ≈ constant and 
the
formula is simplified.
∴ for air Pr ≈ 0.70
17.1.2 Heat transfer in laminar flow
Heat transfer in fluids in laminar flow may be calculated by many different formulas. None of them, however, should be given preference. For this class, the following is recommended.
This formula gives the mean heat transfer co–efficient at 1/d > 50. It is applicable to any fluid. Mean fluid temperature, tf and equivalent diameter are taken as reference temperature and dimension for 1/d > 50, apply correction.
Table 17.1 Correction factor
|
1/d |
1 |
2 |
5 |
10 |
15 |
20 |
30 |
40 |
50 |
|
E1 |
1.90 |
1.70 |
1.44 |
1.28 |
1.18 |
1.13 |
1.105 |
1.02 |
1 |