CONVECTION HEAT TRANSFER

1. CONVECTION HEAT TRANSFER P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi A Controllable Characteristic of fluids……

2. Introduction • Convection involves the transfer of heat by the motion and mixing of "macroscopic" portions of a fluid (that is, the flow of a fluid past a solid boundary). • The term natural convection is used if this motion and mixing is caused by density variations resulting from temperature differences within the fluid. • The term forced convection is used if this motion and mixing is caused by an outside force, such as a pump. • Heat transfer by convection is more difficult to analyze than heat transfer by conduction because no single property of the heat transfer medium, such as thermal conductivity, can be defined to describe the mechanism.

3. Heat transfer by convection varies from situation to situation (upon the fluid flow conditions), and it is frequently coupled with the mode of fluid flow. • In practice, analysis of heat transfer by convection is treated empirically (by direct observation). • Convection heat transfer is treated empirically because of the factors that affect the stagnant film thickness: •   Fluid velocity •   Fluid viscosity •   Heat flux •   Surface roughness • Type of flow (single-phase/two-phase)

4. Convection involves the transfer of heat between a surface at a given temperature (Ts) and fluid at a bulk temperature (Tb). • The exact definition of the bulk temperature (Tb) varies depending on the details of the situation. • For flow adjacent to a hot or cold surface, Tb is the temperature of the fluid "far" from the surface. • For boiling or condensation, Tb is the saturation temperature of the fluid. For flow in a pipe, Tb is the average temperature measured at a particular cross-section of the pipe. • Newton’s law of cooling suggests a basic relationship for heat transfer by convection: h is called as Convection Heat Transfer Coefficient, W/m2K

5. Realization of Newton’s Law Cooling • A general heat transfer surface may not be isothermal !?! • Fluid temperature will vary from inlet to exit !?!?! • The local velocity of flow will also vary from inlet to exit ?!?! • How to use Newton’s Law in a Real life?

6. Local Convection Heat Transfer Consider convection heat transfer as a fluid passes over a surface of arbitrary shape: Apply Newton’s law cooling to a local differential element with length dx. h is called as Local Convection Heat Transfer Coefficient, W/m2K

7. The total heat transfer rate Q is Where, havg is the average convection heat transfer coefficient for the entire surface. where Therefore

8. U2 U1 U1 U2 U2 U U Φ Φ Φ Concept of Solid Fluid Interaction • Perfectly smooth surface (ideal surface) Real surface Specular reflection Diffuse reflection • The convective heat transfer is defined for a combined solid and fluid system. • The fluid packets close to a solid wall attain a zero relative velocity close to the solid wall : Momentum Boundary Layer.

9. The fluid packets close to a solid wall come to thermal equilibrium with the wall. • The fluid particles will exchange maximum possible energy flux with the solid wall. • A Zero temperature difference exists between wall and fluid packets at the wall. • A small layer of fluid particles close the the wall come to Mechanical, Thermal and Chemical Equilibrium With solid wall. • Fundamentally this fluid layer is in Thermodynamic Equilibrium with the solid wall.

10. Heat Transfer in Equilibrium Layer At the wall for fluid layer : At Thermodynamic equilibrium • The thickness of stagnant layer decides the magnitude of normal temperature gradient at the wall. • And hence, the thickness of wall fluid layer decides the magnitude of convective heat transfer coefficient. • Typically, the convective heat transfer coefficient for laminar flow is relatively low compared to the convective heat transfer coefficient for turbulent flow. • This is due to turbulent flow having a thinner stagnant fluid film layer on the heat transfer surface.