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GATE 2016 ME (ME1) – Question 25

Heat Transfer · Modes of heat transfer, one-dimensional conduction, resistance concept · 1 mark · Multiple choice

A plastic sleeve of outer radius r0 = 1 mm covers a wire (radius r = 0.5 mm) carrying electric current. Thermal conductivity of the plastic is 0.15 W/m-K. The heat transfer coefficient on the outer surface of the sleeve exposed to air is 25 W/m2-K. Due to the addition of the plastic cover, the heat transfer from the wire to the ambient will

  1. increase
  2. remain the same
  3. decrease
  4. be zero

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Show answer and explanation

Correct answer: (A) increase

Explanation

Critical insulation radius for a cylinder is rc=k/h=0.15/25=0.006m=6mm. Both the bare radius0.5mm and covered radius1mm lie below this value, where increasing outer radius reduces total resistance as the increased convection area outweighs added conduction resistance. At fixed wire temperature, heat transfer **increases, A**.