Question 1

Heat conduction through a thick spherical shell: select the correct steady-state formula.  Let inner radius = r1, outer radius = r2, thermal conductivity = k, inner and outer surface temperatures = T1 and T2 respectively (T1 > T2). Choose the correc…

Accepted Answer

Concept/Approach Steady 1-D radial conduction in a sphere gives a thermal resistance Rth = (1/(4πk))·(1/r1 − 1/r2). Hence Q = ΔT/Rth. Step-by-step Rth = (1/(4πk))·(1/r1 − 1/r2)Q = (T1 − T2) / Rth = 4πk (T1 − T2) / (1/r1 − 1/r2)Equivalent form: Q = 4πk r1 r2 (T1 − T2)/(r2 − r1) Option check A matches the standard spherical formula (correct). B is for a cylinder of length L (not a sphere). C assumes planar 1-D conduction with area A (not applicable here). D is an equivalent correct form numerically; however the problem statement requests selection of one expression—A is the canonical spherical resistance form. Final Answer Q = 4πk (T1 − T2) / (1/r1 − 1/r2).

Question 2

Psychrometrics: wet-bulb behavior during sensible heating.  Process: heat moist air with no moisture addition (specific humidity constant). How does the wet-bulb temperature change during this sensible heating process? Choose the correct trend.

Accepted Answer

Given Sensible heating ⇒ dry-bulb temperature rises, humidity ratio remains (approximately) constant. Concept On a psychrometric chart, at constant humidity ratio, moving to higher dry-bulb generally increases enthalpy and wet-bulb temperature. Relative humidity decreases. Conclusions Wet-bulb temperature increases during sensible heating. Final Answer increases.

Question 3

Psychrometrics: identify the variable that decreases during sensible cooling of air.  Process: cool moist air with no dehumidification (humidity ratio essentially constant and above dew point). Select the property that definitely decreases in this p…

Accepted Answer

Given Sensible cooling (no moisture removal) ⇒ humidity ratio (specific humidity) remains ~constant, dry-bulb falls. Property trends Dry-bulb temperature: decreases (by definition of sensible cooling). Specific humidity: ~constant (no moisture added/removed). Relative humidity: increases as air is cooled at fixed moisture content. Wet-bulb temperature: typically decreases as enthalpy falls, but the most fundamental and unambiguous decrease is dry-bulb temperature. Final Answer dry bulb temperature.

Question 4

Relative humidity vs. moistening: evaluate the statement.  Statement: “The relative humidity decreases as air gets wet (more moisture content).” Decide whether the statement is correct or incorrect.

Accepted Answer

Concept/Approach At a given temperature, adding moisture moves the state toward saturation; thus the relative humidity rises, not falls. While RH also depends on temperature, the blanket statement that RH decreases as air gets wetter is incorrect. Final Answer Disagree (statement is incorrect).

Question 5

Electrolux refrigerator: identify the absorption system type.  This domestic absorption refrigerator uses ammonia–water–hydrogen. Complete the classification of the system used.

Accepted Answer

Concept/Approach The Electrolux (Platen–Munters) refrigerator is a three-fluid absorption system: ammonia (refrigerant), water (absorbent), and hydrogen (inert gas to reduce condenser/evaporator partial pressures). The process uses heat as the driving input. Final Answer three-fluid (diffusion) absorption system.

Question 6

Modes of heat transfer: radiation and the need for a medium.  Complete the statement about radiation heat transfer with respect to intervening media. Choose the option that correctly fills the blank.

Accepted Answer

Concept Thermal radiation is electromagnetic energy exchange by emission/absorption of photons and does not require any material medium (it can propagate through vacuum). Conduction and convection, in contrast, require material media. Final Answer does not require a medium.

Question 7

Absorption refrigeration (ideal/Carnot): expression for C.O.P.  Let T1 = generator (heat source) temperature, T2 = condenser/absorber (heat sink) temperature, T3 = evaporator (refrigerated space) temperature, all in absolute units (K). Select the co…

Accepted Answer

Concept/Approach An ideal absorption refrigerator can be modeled as a combination of a heat-driven heat pump (generator/absorber loop operating between T1 and T2) powering a refrigeration cycle (evaporator/condenser between T3 and T2). The maximum (Carnot) C.O.P. is the product of the Carnot C.O.P. of the refrigerator and the efficiency of the heat-to-work conversion analogue. Derivation sketch Carnot refrigerator between T3 (cold) and T2 (sink): COPref = T3/(T2 − T3).Carnot heat engine analogue between T1 (source) and T2 (sink): ηCarnot = (T1 − T2)/T1.For a heat-driven refrigerator, the effective COP (per unit heat at T1) = COPref × ηCarnot = (T3/(T2 − T3)) × ((T1 − T2)/T1). Option check B matches the ideal expression. Other options either omit the driving-temperature factor or misplace temperatures. Final Answer COP = (T3/(T2 − T3)) × ((T1 − T2)/T1).

Question 8

Thermal diffusivity comparison: identify the material with the highest α = k/(ρ·cᵖ).  Options: iron, lead, concrete, and wood. Select the material that typically exhibits the greatest thermal diffusivity in engineering data. Choose the correct optio…

Accepted Answer

Given data and idea Thermal diffusivity: α = k/(ρ·cᵖ) — how quickly a material's temperature equalizes internally. Reasoning with typical properties Iron (steel-like): k ≈ 50–80 W/m·K, ρ ≈ 7.9×10³ kg/m³, cᵖ ≈ 0.45 kJ/kg·K → α ≈ 2.0–2.5×10⁻⁵ m²/s.Lead: k ≈ 35 W/m·K, ρ ≈ 11.34×10³ kg/m³, cᵖ ≈ 0.128 kJ/kg·K → α ≈ 35/(11 340×128) ≈ 2.4×10⁻⁵ m²/s.Concrete and wood have much lower k and larger ρ·cᵖ, hence far smaller α (≈10⁻⁶ to 10⁻⁷ m²/s order). Conclusion Among the listed choices, lead generally exhibits the highest thermal diffusivity.

Question 9

Boundary-layer thickness equality: condition on the Prandtl number (Pr).  When do the thermal and hydrodynamic boundary-layer thicknesses become approximately equal over a surface in external flow? Choose the correct value of Pr that ensures δᵗ ≈ δ.

Accepted Answer

Concept The Prandtl number Pr = ν/α compares momentum and thermal diffusivities. When Pr ≈ 1, momentum and heat diffuse at similar rates, so the thermal (δᵗ) and velocity (δ) boundary layers have comparable thicknesses. Final Answer equal to one.

Question 10

Thermal diffusivity: nature and classification.  Identify the most accurate description of thermal diffusivity (α = k/(ρ·cᵖ)). Select the correct statement.

Accepted Answer

Facts about thermal diffusivity α = k/(ρ·cᵖ) has units of m²/s (not dimensionless).It is a material property and commonly varies with temperature (so it can be temperature-dependent). Option analysis A can be true in practice, but the safest classification is that α is a material property.C is incorrect because α is not dimensionless.D is therefore incorrect because it includes a wrong statement. Final Answer It is a physical property of a substance.

Question 11

Fluid mechanics/heat transfer fundamentals: define the Reynolds number.  Using symbols: ρ = density, V = characteristic velocity, l = characteristic length, μ = dynamic viscosity, ν = kinematic viscosity. Select the correct expression for the Reynol…

Accepted Answer

Definition The Reynolds number quantifies the ratio of inertial to viscous forces: Re = (ρ V l)/μ = V l/ν. Why A is correct Dimensions: [ρ]·[V]·[l]/[μ] → (kg·m⁻³)(m·s⁻¹)(m)/(kg·m⁻¹·s⁻¹) = 1 (dimensionless), as required. Final Answer Re = (ρ V l)/μ.

Question 12

Thermal conductivity of liquid water versus temperature: qualitative trend.  How does the thermal conductivity (k) of water change as temperature rises across the practical engineering range? Choose the most correct qualitative statement.

Accepted Answer

Concept Liquid water exhibits a non-monotonic variation of thermal conductivity with temperature: k can increase over some ranges and decrease over others due to changing molecular interactions. Conclusion The safest, broadly correct statement is may increase or decrease depending upon temperature.

Question 13

Definition of a refrigeration system: heat flow direction in operation.  Select the description that correctly defines what a refrigeration system does in terms of heat transfer between a cold body and a hot body. Choose the correct option.

Accepted Answer

Concept A refrigerator (or heat pump) pumps heat from a colder region to a warmer region by consuming external work (or, in absorption systems, an external heat source). Final Answer removes heat from a low temperature body and delivers it to a high temperature body.

Question 14

Perfect black body: select the correct defining property.  Among the listed statements, identify the defining characteristic of an ideal black body in radiation heat transfer. Choose the correct option.

Accepted Answer

Definition An ideal black body has absorptivity = 1 for all wavelengths and directions; it absorbs all incident radiation. Colour in the visible spectrum is irrelevant to the idealized definition. Final Answer absorbs heat radiations of all wavelengths falling on it.

Question 15

Aircraft refrigeration design: prime design criterion for onboard systems.  When designing an aircraft's refrigeration system, which factor is the foremost consideration from an aeronautical/weight perspective? Choose the correct option.

Accepted Answer

Concept In aviation, weight is critical. While efficiency matters, the mass of the equipment has overriding impact on payload, fuel burn, and performance. Final Answer mass of the refrigeration equipment is low.

Question 16

Psychrometrics terminology: DBT–WBT difference.  What is the name of the difference between the dry-bulb temperature (DBT) and the wet-bulb temperature (WBT) of air? Choose the correct term.

Accepted Answer

Definitions Wet-bulb depression = DBT − WBT.Dew point depression = DBT − DPT. Result Therefore, the correct term for (DBT − WBT) is wet bulb depression.

Question 17

Psychrometric chart geometry: shape of constant wet-bulb temperature lines.  Evaluate the statement: “On a standard psychrometric chart, the wet-bulb temperature lines are curved lines.” Decide whether the statement is true or false.

Accepted Answer

Concept On standard psychrometric charts, lines of constant wet-bulb temperature are drawn as straight oblique lines (nearly coincident with lines of constant enthalpy). Relative humidity lines are the curved family. Final Answer Disagree (false).

Question 18

Heat transfer dimensionless number: write the Nusselt number definition  Define Nusselt number (Nu) in terms of convection coefficient, characteristic length, and thermal conductivity for convective heat transfer. Choose the correct expression.

Accepted Answer

Concept/Approach The Nusselt number is a dimensionless group that compares convective to conductive heat transfer across a boundary layer: Nu = (convective heat transfer / conductive heat transfer at the same temperature gradient). Definition Nu = hL/kwhere h = convection coefficient (W/m²·K), L = characteristic length (m), k = thermal conductivity of the fluid (W/m·K). Unit check (W/m²·K × m) / (W/m·K) = 1 (dimensionless). Common pitfalls Inverting k and h giving kL/h, which has dimensions and is incorrect. Using an area instead of a length in the definition; Nu uses a length scale. Final Answer Nu = hL/k

Question 19

Psychrometrics: choose the correct definition of relative humidity  At a given temperature and pressure, select the expression that correctly defines relative humidity of moist air. Choose the correct statement.

Accepted Answer

Concept/Approach Relative humidity φ is defined as the ratio of the actual water vapour content to the maximum possible (saturation) water vapour content at the same temperature and pressure. Equivalent forms φ = pv / pvs = ρv / ρvs at the same T and p, which corresponds to comparing equal volumes. Why option D is correct Option D states the ratio using equal volumes at the same T and p, matching φ = ρv/ρvs. Why others are incorrect Option A defines absolute humidity (kg/m³). Option B defines humidity ratio/specific humidity (kg/kg dry air). Option C compares per unit mass of dry air; that ratio equals the ratio of humidity ratios, not the standard definition of relative humidity. Final Answer Ratio of actual mass of water vapour in a given volume of moist air to that in the same volume of saturated air at the same T and p

Question 20

Refrigeration cycle energy balance: compare heat rejected and heat absorbed  In a vapor-compression refrigerating machine, how does the condenser heat rejection compare to the evaporator heat absorption? Choose the correct relation.

Accepted Answer

Concept/Approach First law over the cycle gives: QH = QL + Win, where QL is heat absorbed in the evaporator, Win is compressor work, and QH is heat rejected in the condenser. Implication Since Win > 0 for a working refrigerator, QH > QL. Final Answer Heat rejected is greater than heat absorbed

Heat Transfer, Refrigeration and Air Conditioning Questions