Overall Heat-Transfer Coefficient
Overall Heat-Transfer Coefficient
Heat crossing a wall meets convection, conduction and convection in series; their resistances combine into one overall coefficient.
The overall coefficient is U = 1/(1/h_1 + L/k + 1/h_2). where h_1, h_2 are the surface coefficients, L the thickness and k the conductivity.
Inputs
h_1 := 1000 W/(m^2·K)=1000 kg/K/s^3
L := 0.01 m=10 mm
k := 50 W/(m·K)=50 kg·m/K/s^3
h_2 := 25 W/(m^2·K)=25 kg/K/s^3
Add the series resistances and invert.
U := 1/((1/h_1) + (L/k) + (1/h_2))=24.27 kg/K/s^3
Results
The smallest coefficient (largest resistance) dominates — here the air-side film — so improving it gives the biggest gain in heat transfer.
Assumptions & limitations
- Steady one-dimensional flow.
- Series resistances.
- Constant coefficients.