ASRB NET — Farm Machinery & Power / Land & Water Management Engineering / Agricultural Structures & Process Engineering · India · Agricultural Scientists Recruitment Board (ICAR)
Original practice questions written to the published syllabus · not past papersConduction, convection, radiation, heat exchangers and diffusion — the physics of drying, cooling, freezing and cold-store walls.
In the exams: GATE AG Section 7 (heat exchangers and composite walls each asked in 7 of 8 papers 2019–26); ICAR AICE subject 41 Unit 2; NCEES PE processing
ASRB NET — Agricultural Structures & Process Engineering
Conceptual MCQs across the whole subject with emphasis on cold-store insulation, heat-exchanger types used in dairies, drying theory and the physical meaning of dimensionless groups.
How our questions for ASRB NET spread across the units of Heat & Mass Transfer. Difficulty: 27 easy, 20 medium, 1 hard.
| Composite walls and critical radius of insulation | 8 | |
| Simultaneous heat and mass transfer in drying | 8 | |
| Convection and dimensionless numbers (Nu, Re, Pr) | 4 | |
| Radiation: Stefan-Boltzmann law and view factors | 4 | |
| Transient conduction: lumped capacity and Heisler charts | 4 | |
| Convective mass transfer | 4 | |
| Steady conduction through plane walls and cylinders | 3 | |
| Extended surfaces (fins) | 3 | |
| Condensation and boiling | 3 | |
| Diffusive mass transfer and Fick's law | 3 | |
| Heat exchangers: LMTD method | 2 | |
| Heat exchangers: effectiveness-NTU method | 2 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
A spherical storage vessel has an inner radius of 0.5 m and is covered with 0.1 m of insulation (k = 0.04 W/m·K). The insulation inner surface is at −5 °C and its outer surface at 25 °C. The steady heat gain through the insulation is
D. 45.2 W
Spherical shell resistance R = (r2 − r1)/(4πk r1 r2) = 0.1/(4π × 0.04 × 0.5 × 0.6) = 0.6631 K/W, so Q = 30/0.6631 = 45.2 W. Using the plane-wall formula with the inner area gives 37.7 W and with the outer area 54.3 W; dropping r1 r2 from the denominator gives 151 W.
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
A cold-store wall of 150 m² consists of 230 mm brick (k = 0.7 W/m·K) and 100 mm insulation (k = 0.025 W/m·K). The outside air is at 40 °C with h = 20 W/m²·K and the inside air at 2 °C with h = 10 W/m²·K. The steady heat gain through the wall is closest to
B. 1,270 W
Resistance per m²: 1/20 + 0.23/0.7 + 0.10/0.025 + 1/10 = 0.05 + 0.3286 + 4.0 + 0.1 = 4.4786 m²·K/W. Flux q = 38/4.4786 = 8.485 W/m², and Q = 8.485 × 150 = 1,270 W. Ignoring the air films gives 1,320 W; leaving out the insulation gives 11,900 W; 8.5 W is the flux per square metre, not the total.
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
A small spherical vessel is insulated with a material of k = 0.05 W/m·K and loses heat to air with h = 5 W/m²·K. The critical radius of insulation for this sphere is
C. 20 mm
For a sphere r_c = 2k/h = 2 × 0.05/5 = 0.02 m = 20 mm. 10 mm is the cylinder value k/h; 100 is h/k, the inverted ratio in m⁻¹. 40 mm doubles the sphere value (4k/h).
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
A small body obeying the lumped-capacitance model is plunged into a fluid. After a time equal to one time constant, its excess temperature (T − T∞) has fallen to about what fraction of the initial excess?
B. 0.368
θ/θ_i = exp(−t/τ) = e⁻¹ = 0.368 at t = τ. 0.632 is the fraction of the change already completed; 0.5 is the half-time (t = 0.693τ); 0.135 is reached after two time constants.
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
Adding fins to a heat-exchange surface is most worthwhile when the surface transfers heat to
C. a gas with a low convection coefficient
Fin effectiveness √(kP/(hA_c)) is large when h is small. Fins are therefore used on the air side of radiators, condensers and engine cylinders. With boiling, condensation or liquid metals, h is so high that the fin's own conduction resistance limits it and effectiveness is poor.
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
As the excess temperature of a heated surface in a pool of saturated water is increased, the boiling regimes follow the order
A. natural convection → nucleate → transition → film boiling
At small ΔT_e heat moves by natural convection; bubbles then form at nucleation sites (nucleate boiling) up to the critical heat flux; beyond it, unstable vapour patches (transition boiling) reduce the flux to a minimum at the Leidenfrost point; above that, a stable vapour film covers the surface (film boiling).
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
The typical order of magnitude of binary diffusion coefficients at ordinary temperature is
A. gases about 10⁻⁵ m²/s, liquids about 10⁻⁹ m²/s, solids much smaller
Gas-phase diffusivities are around 10⁻⁵ m²/s (water vapour in air about 2.5 × 10⁻⁵), liquid-phase around 10⁻⁹ m²/s, and diffusion in solids and food matrices is slower still. This four-orders-of-magnitude gap is why internal diffusion controls drying in the falling-rate period.
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
The Sherwood number, Sh = h_m L/D_AB, is the mass-transfer analogue of the
B. Nusselt number
Sh is the dimensionless convective mass-transfer coefficient, as Nu is for heat. The Schmidt number Sc = ν/D is the analogue of Pr; the mass-transfer Biot number h_m L/D_solid uses the diffusivity inside the solid.
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
Grain with a moisture content of 20% (wet basis) has a dry-basis moisture content of
A. 25%
M_db = M_wb/(1 − M_wb) = 0.20/0.80 = 0.25 = 25%. 16.7% is the reverse conversion, M_wb = M_db/(1 + M_db), applied to 20%; 80% is the dry-matter fraction.
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
If internal diffusion controls the falling-rate drying of fruit slices, halving the slice thickness (all else unchanged) changes the time to reach a given moisture ratio by a factor of about
C. 1/4
In the diffusion solution MR depends on D_eff t/L², so for a fixed MR, t ∝ L². Halving L gives (1/2)² = 1/4 of the time. The factor 1/2 would apply if drying were controlled by surface area per mass under external control, as in the constant-rate period.
Further reading: Incropera et al., Fundamentals of Heat and Mass Transfer
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