Subtopic: Steady state heat transfer in conduction, convection and radiation
Question Statement:
Molecular diffusivity (D_AB) of water vapour in air is 2.6×10⁻⁵ m² s⁻¹ over an effective distance of 3 mm. Density and coefficient of viscosity of air are 1.2 kg m⁻³ and 2×10⁻⁵ kg m⁻¹ s⁻¹, respectively. Sherwood Number (N_Sh) for water vapour in air is _________.
Official Answer Key:
1.00 to 1.00
Step-by-Step Solution & Explanation
Sherwood number: N_Sh = k_c L / D_AB.
No convective velocity is given — mass transfer is by pure molecular diffusion across a stagnant film of thickness L = 3 mm.
For pure diffusion, k_c = D_AB/L.
N_Sh = (D_AB/L) × L / D_AB = 1.0.
Sh = 1 is the physical lower limit of the Sherwood number and is exactly the diffusion-only case; the density and viscosity data are distractors.
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