Govt. Exams
Entrance Exams
LMTD = (ΔT1 - ΔT2)/ln(ΔT1/ΔT2) where ΔT1 = 80-20 = 60°C and ΔT2 = 50-40 = 10°C. LMTD = (60-10)/ln(60/10) = 50/1.79 = 27.9°C ≈ 28.9°C
St represents the fraction of heat that can be transferred relative to the sensible heat available in flowing fluid per unit area per unit time.
For Ra < 10⁹, natural convection remains laminar; transition to turbulence occurs around Ra ≈ 10⁹.
From Q = σAT⁴, σ has units W·m⁻²·K⁻⁴ (5.67 × 10⁻⁸ W·m⁻²·K⁻⁴).
Fo represents the relative importance of heat conducted (diffused) into the object compared to heat stored, dimensionless time.
Bi = hL_c/k, representing the ratio of internal conduction resistance to external convection resistance at the surface.
Thermal diffusivity α = k/(ρ·C_p) has dimensions L²/t, representing how fast thermal disturbances propagate through a material.
For most pure metals, thermal conductivity decreases with increasing temperature due to increased lattice vibrations causing phonon scattering.
Grashof number (Gr = gβΔT L³/ν²) represents the ratio of buoyancy to viscous forces. Ra = Gr × Pr combines both effects.
The thermal resistance network (Ohm's law analogy) is used where total thermal resistance equals the sum of individual resistances in series for composite walls.
