Module 3. Dimensional analysis and heat transfer

Lesson 16

PROBLEMS ON FREE CONVECTION

Example 16.1 Determine the hourly loss of heat from a bare vertical steam pipe whose diameter d = 100 mm and the length h = 4 m, if the wall temperature t_ω = 170°C and the temperature of the surroundings (air) t_f = 30°C.

First we determine the mean temperature

We then proceed to find the values of physical properties (from Table A-1) on and the dimensionless terms:

From Table 15.1 we find c = 0.54 and n =1/4.

Substituting these values into equation, we get:

Whence

The hourly heat loss

Example 16.2 Determine the equivalent thermal conductivity of a flat air layer δ = 25 mm. The temperature of the hot surface tω1 = 150°C and of the cold surface tω2 = 50°C.

λ_f = kcal/m-hr-°C

v_f = 2.31 . 10-5 m²/s and Pr_f = 0.69

λ_eq = ε_c λ = 2.74 × 0.0276 = 0.0755 kcal/m-hr-^oC