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Heat Transfer
Convection
GATE 2016
Hard

Mineral oil fills a narrow gap between horizontal plates (radiative flux negligible, k constant). Exp1 (30°C top/60°C bottom, ΔT=30): flux=39. Exp2 (60°C top/30°C bottom, ΔT=30): flux=26. Exp3 (40°C top/70°C bottom, ΔT=30): flux=42. What is the steady-state flux (W/m²) with 70°C top and 40°C bottom?

  • 26
  • B39
  • C42
  • D63

Explanation

Hot-on-top configurations (Exp2, and the asked case) suppress natural convection entirely (stable stratification), reducing to pure conduction, whose flux depends only on ΔT (constant k) — matching Exp2's value of 26 W/m² for the same ΔT=30, regardless of absolute temperature level.

Source: GATE CH 2016 (2016)

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