An electric air heater consists of a horizontal array of thin metal strips that are each 10 mm long in the direction of an airstream that is in parallel flow over the top of the strips. Each strip is 0.2 m wide, and 25 strips are arranged side by side, forming a continuous and smooth surface over which the air flows at 2 m/s. During operation, each strip is maintained at 500°C and the air is at 25°C. What is the rate of convection heat transfer from the first strip? The fifth strip? The tenth strip? All the strips?

see explanation below

Explanation:

Given that,

500°C

= 25°C

d = 0.2m

L = 10mm = 0.01m

U₀ = 2m/s

Calculate average temperature

262.5 + 273

= 535.5K

From properties of air table A-4 corresponding to = 535.5K

k = 43.9 × 10⁻³W/m.k

v = 47.57 × 10⁻⁶ m²/s

A)

Number for the first strips is equal to

Calculating heat transfer coefficient from the first strip

The rate of convection heat transfer from the first strip is

The rate of convection heat transfer from the fifth trip is equal to

Calculating

The rate of convection heat transfer from the tenth strip is

Calculating

Calculating the rate of convection heat transfer from the tenth strip

The rate of convection heat transfer from 25th strip is equal to

Calculating

Calculating

Calculating the rate of convection heat transfer from the tenth strip

heats surface water up