Topic: Heat Transfer
It is proposed to coat a 1 mm diameter wire with enamel paint (k=0.1 W/mK) to increase the heat transfer with air. If the air side heat transfer coefficient is 100 W/m^2K, the optimum thickness of enamel paint should be
- A. 0.25 mm
- B. 0.5 mm
- C. 1 mm
- D. 2 mm
Answer: B. 0.5 mm
Explanation: The optimum heat transfer occurs at the critical radius, which is r_c = k / h = 0.1 / 100 = 0.001 m = 1 mm. The radius of the 1 mm diameter wire is 0.5 mm. The optimum thickness of the paint coating is the critical radius minus the wire radius: 1 mm - 0.5 mm = 0.5 mm.