A flat-plate solar collector has a black absorber plate maintained at 80°C. The ambient temperature is 20°C and the heat loss coefficient (overall U value including radiation and convection) is 8 W/(m²·K). For a collector area of 4 m², what is the heat loss rate to the surroundings?
Answer: C
Q_loss = U × A × ΔT = 8 × 4 × (80-20) = 8 × 4 × 60 = 1920 W. The overall U value already accounts for both convective and radiative losses.
Q.42Easy
What is the SI unit of thermal conductivity?
Answer: D
Thermal conductivity is expressed as W/(m·K) in SI units, which is equivalent to J/(s·m·K) since 1 W = 1 J/s.
Q.43Easy
In a parallel flow heat exchanger, the temperature approach is minimum at which end?
Answer: C
In parallel flow, both fluids flow in the same direction. The smallest temperature difference (approach) occurs at the outlet end where the hot fluid has cooled and cold fluid has heated.
Q.44Easy
The Biot number (Bi = hL_c/k) is used to determine which regime of unsteady-state heat conduction?
Answer: D
Biot number classifies transient conduction regimes: Bi << 1 indicates uniform internal temperature (lumped model valid), while Bi >> 1 indicates significant spatial temperature gradients requiring solution of differential equations.
Q.45Easy
For laminar flow inside circular tubes, the Nusselt number (Nu) is approximately constant at what value?
Answer: B
For fully developed laminar flow in a circular tube with constant wall temperature (thermal entry), Nu ≈ 4.36. For constant heat flux, Nu ≈ 3.66. The question refers to the thermally developed region.
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Q.46Easy
A flat plate at 100°C is cooled by air at 25°C. The convection heat transfer coefficient is 50 W/(m²·K). Calculate heat flux if plate area is 2 m².
A condensing steam at 100°C releases latent heat to water inside tubes. This is an example of which type of phase-change heat transfer?
Answer: C
Condensation is phase change from vapor to liquid, releasing latent heat. This occurs when steam condenses on cooler tube surfaces. Pool boiling and convective boiling involve liquid-to-vapor phase change.
Q.48Easy
Which of the following is the SI unit of kinematic viscosity?
Answer: A
Kinematic viscosity is dynamic viscosity divided by density. Its SI unit is m²/s (or Stokes in CGS, where 1 Stoke = 10⁻⁴ m²/s).
Q.49Easy
A fluid flows through a pipe of diameter 0.1 m with velocity 2 m/s. If the kinematic viscosity is 1×10⁻⁶ m²/s, calculate the Reynolds number.
Answer: A
Re = (ρVD)/μ = VD/ν. Re = (2 × 0.1)/(1×10⁻⁶) = 0.2/(1×10⁻⁶) = 2×10⁵. Flow is turbulent.
Q.50Easy
Which equation relates pressure drop to velocity in laminar flow through pipes?
Answer: A
Hagen-Poiseuille equation (ΔP = 32μLV/D²) is specific for laminar flow. Darcy-Weisbach is general for both laminar and turbulent flows.
Q.51Easy
A centrifugal pump operates at 1500 RPM. If the impeller diameter is 0.3 m, what is the tip speed?
Answer: A
Tip speed = πDN/60 = π × 0.3 × 601500 = 23.56 m/s (where N is in RPM).
Q.52Easy
In the Bernoulli equation, which term represents the pressure energy per unit volume?
Answer: C
The Bernoulli equation is P + ½ρV² + ρgz = constant. P itself is pressure energy per unit volume (not P/ρ which is pressure head).
Q.53Easy
A fluid with dynamic viscosity 0.5 Pa·s and density 800 kg/m³ flows in a pipe. Its kinematic viscosity is:
Answer: A
ν = μ/ρ = 0.8005 = 6.25×10⁻⁴ m²/s.
Q.54Easy
For a Newtonian fluid, shear stress (τ) is related to shear rate (dV/dy) by:
Answer: A
Newton's law of viscosity: τ = μ(dV/dy). Option B is power law (non-Newtonian), C is Bingham plastic model.
Q.55Easy
In fluid mechanics, which dimensionless number represents the ratio of inertial forces to viscous forces?
Answer: A
Reynolds number Re = (Inertial forces)/(Viscous forces) = ρVD/μ. It characterizes flow regime (laminar vs turbulent).
Q.56Easy
Which pumping principle is used in a gear pump?
Answer: B
Gear pumps are positive displacement pumps. They move fixed volumes of fluid per rotation. Dynamic principle applies to centrifugal pumps.
Q.57Easy
A fluid with dynamic viscosity of 0.8 Pa·s and density of 900 kg/m³ flows through a circular pipe. What is the kinematic viscosity of this fluid?
For laminar flow of a viscous fluid between two parallel plates, the velocity profile is:
Answer: B
For laminar flow between parallel plates (Poiseuille flow), the velocity profile is parabolic due to the balance between pressure gradient and viscous forces.
Q.59Easy
Which dimensionless number is used to characterize the ratio of inertial forces to viscous forces in fluid flow?
Answer: B
Reynolds number (Re = ρVD/μ) represents the ratio of inertial to viscous forces and determines the flow regime (laminar, transitional, or turbulent).
Q.60Easy
Which of the following pump types operates on the principle of positive displacement with rotating screws?
Answer: C
Screw pumps (Archimedean screw or progressive cavity pumps) use rotating screws for positive displacement. They are used for high viscosity fluids.
