1. HW #4 /Tutorial # 4 WRF Chapter 18; WWWR Chapter 19 ID Chapter 6
To be discussed on Feb. 7, 2017.
By either volunteer or class list.
Correction
Question WWWR # 19.19

2. Convective Heat Transfer

3. Fundamental Considerations In Convective Heat Transfer
Two main classifications of convective heat transfer
These have to do with the driving force causing fluid to flow
Natural or free convection
Fluid motion results from heat transfer
Fluid heated/ cooled -> density change/ buoyancy effect -> natural circulation in which affected fluid moves of its own accord past the solid surface
- fluid it replaces is similarly affected by the energy transfer - process is repeated

4. Forced convection
Fluid circulation is produced by external agency (fan or pump)
Analytical Methods
Dimensional Analysis
Analogy between Energy and Momentum Exchange

5. Significant Parameters In Convective Heat Transfer
Both have same dimensions L2/t; thus their ratio must be dimensionless
This ratio, that of molecular diffusivity of momentum to the molecular diffusivity of heat, is designed the Prandtl number
Pr º = n a
mcp k

6. Prandtl number
observed to be a combination of fluid properties;
thus Pr itself may be thought of as a property.
Primarily a function of temperature s

7. A ratio of conductive thermal resistance to the convective thermal resistance of the fluid
Nusselt number
Nu º hL k
Where the thermal conductivity of the fluid as opposed to that of the solid, which was the case in the evaluation of the Biot modulus.

8. Dimensional Analysis of Convective Energy Transfer Forced Convection

11. Dimensional Analysis for Forced Convection

13. Natural Convection

16. Courtesy Contribution by ChBE Year Representative, 2004.
Dimensional Analysis for
Natural Convection

18. Energy and Momentum Transfer Analogies

20. er 8) 9) s on The Colburn analogy expression is Cf St Pr 2/3 = 2
(19-37) 8) 9) s on er

21. Example 1
Water at 50oF enters a heat-exchanger tube having an inside diameter of 1 in and a length of 10 ft. The water flows at 20 gal/min. For a constant wall temperature of 210oF estimate the

24. = (50+130)/2
=(90+210)/2 = 150
Film temperature = (water mean bulk temperature + pipe wall temperature)/2
Mean bulk temperature of water = (inlet + outlet)/2

26. Second iteration is required since if |TL – 130| > 3oF?