Heat transfer by convection

Class Lecture Notes

1. Heat Transfer by Convection

2. What is Convection? Convection is a mode of heat transfer between a solid (or liquid) surface and its adjacent liquid or gas that is in bulk motion. It involves combined effect of conduction and fluid motion. Newton’s Law of Cooling   is the Convection Heat Transfer Coefficient  

3. Types of Convection Forced Convection: - when the fluid is forced to flow over the surface by external means such as a fan, pump or wind. Natural (or Free) Convection: - when fluid flow is caused by buoyancy forces that are induced by density differences due to variation of temperature of the fluid.

4. The Nusselt Number It is a dimensionless number. It is also known as Dimensionless Convective heat Transfer Coefficient. It represents the enhancement of heat transfer due to bulk fluid motion over a surface with respect to the heat transfer by conduction.  

5. The Flow Regimes

6. The Reynold Number It is a dimensionless number. It is the ratio of Inertia Forces to the Viscous Forces in a flowing fluid. The value of the Reynold’s Number indicates whether the flow is laminar or turbulent. The value of Reynold’s Number above which the flow corresponds to turbulent flow is called Critical Reynold’s Number. The value of Critical Reynold’s Number depends on the geometry and flow conditions. Typical value of Critical Reynold’s Number over a flat plate =     is the Kinematic Viscosity of the fluid , Where is the dynamic (or shear) viscosity .  

7. The Boundary Layer

8. The Prandtle Number It is a dimensionless number. It is the ratio of Momentum Diffusivity (Kinematic Viscosity) to Thermal Diffusivity. It is the relative thickness of velocity and thermal boundary layers.  

9. Relationship between Dimensionless Numbers   For Forced Convection over a flat plate due to flow parallel to the plate:   a) Laminar Flow:   a) Turbulent Flow:

10. Numerical Exercise 1. A flat plate is supplied with a constant heat flux of 100 [W/m 2 ] from bottom. The plate has 0.5m 2 area and is kept in a room with ambient temperature 20°C. If it has a steady state temperature of 32°C what is the heat transfer coefficient?        

11. Numerical Exercise 2. The air has the following properties at 27C: Density = 1.18 [kg.m-3], Dynamic Viscosity = 1.85X10 -5 [kg.m -1 .s -1 ], Specific Heat capacity, [kJ.kg -1 .K -1 ]. If the thermal conductivity of air at that temperature is [W.m -1 .K -1 ], what is the average Nusselt Number over a flat plate of dimension 10 cm X 10 cm when air is flowing parallel to its surface with a speed 1 [ms -1 ]?            

12. Numerical Exercise 1. Find out the thermal resistance per unit length of a plastic insulating tube having internal radius and external radius . The thermal conductivity of the plastic is .