Heat Transfer byForced Convection

Forced Convection- Motion in the medium is induced by external agency such as pump

or blower or agitator

- A common theory for all such situations can not be framed because of varying degree of complexity involved in different cases

- The narrow zone of fluid very close to the surface is thermal boundary layer (TBL) for heat transfer applications. TBL is different from HBL, as the later is used to calculate friction and drag, however former is used to calculate the rate of heat transfer.

- Important dimensionless groups are Nusselt no., Reynolds no, Prandtl no.

- Forced Convection is dominant if Gr/Re2 <<1.

Correlations for Heat Transfer Coefficient(a)Internal flows

(i) Laminar flow through circular pipeFor fully developed flow, Sieder & Tate equation can be used

Nu = 1.86 (Pe (d/L))1/3 (μ/μw)0.14

(μ/μw) < 10 & 0.7< Pr < 16000 & Re < 2100

(ii) Turbulent flow through circular pipeFor fully developed flow, Dittus-Boelter equation can be used

Nu = 0.023 Re0.8 Prn

n = 0.4, fluid being heated n = 0.3, fluid being cooled

0.7< Pr < 160 & Re > 10000If viscosity change is significant, n = 0.33 and a factor (μ/μw)0.14 should be multiplied

(iii) Non-circular duct Replace pipe diameter by ‘equivalent diameter’ in above expressions

(b) External flows(i) Over flat plate(ii) Outside cylinder(iii) flow past sphere(iv) Across bank of tubes

© Through Packed and fluidized bed

Heat Transfer byNatural Convection