COREN Engineering Practice Exam — Mechanical Engineering Specialisation (Nigeria)

In a steam power plant operating on the ideal Rankine cycle, what is the effect of superheating the steam to a higher temperature before it enters the turbine?

• AIt increases the moisture content of the steam at the turbine exit.
• BIt increases the net work output and the thermal efficiency of the cycle while decreasing…
• CIt reduces the heat input required in the boiler.
• DIt increases the pressure drop across the condenser.

Which of the following processes represents the polytropic index n = 0 in the general thermodynamic process equation pV^n = C?

• AIsobaric process
• BIsochoric process
• CIsothermal process
• DIsentropic process

How does regenerative feed heating improve the thermal efficiency of a steam power plant operating on the Rankine cycle?

• ABy reducing the maximum cycle temperature in the boiler
• BBy increasing the heat rejected in the condenser
• CBy preheating the feed water using steam extracted from the turbine, thereby reducing the…
• DBy increasing the pressure drop across the turbine stages

In a combined cycle gas turbine (CCGT) power plant, what is the primary function of the Heat Recovery Steam Generator (HRSG)?

• ATo compress the air before it enters the combustion chamber of the gas turbine
• BTo condense the exhaust steam from the steam turbine using cooling water
• CTo capture the high-temperature exhaust heat from the gas turbine and use it to generate…
• DTo reheat the steam between the high-pressure and low-pressure stages of the steam turbine

Under Fourier's law of heat conduction, what does the negative sign in the equation q = -k A (dT/dx) physically represent?

• AThat thermal conductivity decreases with temperature
• BThat heat flows in the direction of decreasing temperature (from hot to cold)
• CThat the temperature gradient is always positive
• DThat the rate of heat transfer is negative in a steady state

What dimensionless parameter represents the ratio of convective heat transfer to conductive heat transfer across a fluid boundary layer?

• APrandtl number (Pr)
• BReynolds number (Re)
• CNusselt number (Nu)
• DGrashof number (Gr)

In radiative heat transfer, what is the definition of a 'gray body'?

• AA body that absorbs all incident radiation regardless of wavelength or direction
• BA body whose monochromatic emissivity is constant and independent of wavelength
• CA body that reflects all incident thermal radiation
• DA body that emits radiation only in the visible spectrum

For a counter-flow heat exchanger, why is the Log Mean Temperature Difference (LMTD) generally preferred over the arithmetic mean temperature difference?

• ALMTD is always easier to compute manually.
• BThe temperature profile of the fluids along the heat exchanger is non-linear (exponential…
• CArithmetic mean is only valid for parallel-flow heat exchangers.
• DLMTD accounts for heat loss to the surrounding environment.

What is the thermodynamic definition of the thermal efficiency of an internal combustion engine?

• AThe ratio of fuel energy input to the volume of exhaust gas
• BThe ratio of the net work output of the engine to the total heat input from fuel combusti…
• CThe ratio of indicated mean effective pressure to the brake power
• DThe ratio of actual air-fuel ratio to the stoichiometric air-fuel ratio

How does increasing the compression ratio affect the performance and operation of a spark-ignition (Otto cycle) internal combustion engine?

• AIt decreases thermal efficiency and reduces the risk of engine knock.
• BIt increases thermal efficiency but increases the risk of engine knock (auto-ignition of…
• CIt decreases fuel consumption by lowering combustion temperatures.
• DIt increases exhaust gas temperatures and reduces nitrogen oxide (NOx) emissions.

What is the primary thermodynamic difference between the Otto cycle and the Diesel cycle?

• AThe Otto cycle has constant-volume heat addition, while the Diesel cycle has constant-pre…
• BThe Otto cycle operates at higher compression ratios than the Diesel cycle.
• CThe Otto cycle adds heat at constant pressure, while the Diesel cycle adds heat at consta…
• DThe Diesel cycle does not reject heat to the environment.

What is the function of an economizer in a modern thermal power plant boiler?

• ATo remove particulate matter from the flue gases before release
• BTo preheat the combustion air before it enters the furnace using flue gas heat
• CTo heat the boiler feedwater using waste heat from the flue gases before the water enters…
• DTo superheat the saturated steam using high-temperature radiant heat

In a steam boiler, what is the critical difference between a water-tube boiler and a fire-tube boiler?

• AWater-tube boilers contain flue gases inside the tubes, while fire-tube boilers contain w…
• BWater-tube boilers contain water/steam inside the tubes with hot flue gases flowing aroun…
• CFire-tube boilers can operate at much higher pressures and steam generation rates than wa…
• DWater-tube boilers are generally safer because they contain a larger volume of water in t…

Why is the actual efficiency of a real gas turbine cycle (Brayton cycle) lower than the ideal Brayton cycle efficiency?

• ADue to irreversibilities such as friction in the turbine and compressor, and pressure dro…
• BBecause the combustion process in a real engine is isothermal
• CDue to the absence of a regenerator in the ideal cycle
• DBecause real turbines operate at higher pressure ratios than ideal turbines

What is the significance of the critical radius of insulation for a cylindrical pipe?

• AIt is the minimum thickness of insulation required to prevent freezing of the fluid.
• BIt is the outer radius of insulation at which the rate of heat transfer is maximized; add…
• CIt is the radius where thermal conductivity of the insulation matches the convective heat…
• DIt is the maximum radius of insulation that can fit within standard support brackets.

Which thermodynamic property represents the portion of heat energy that is unavailable to perform useful work in a closed system?

• AEnthalpy
• BInternal energy
• CEntropy multiplied by surrounding temperature (T_0 * S)
• DGibbs free energy

In a steam surface condenser, what is the primary purpose of maintaining a high vacuum?

• ATo prevent air leakage into the condenser shell
• BTo maximize the expansion of steam through the turbine, thereby increasing turbine work a…
• CTo increase the condensation temperature of the steam
• DTo reduce the cooling water flow rate required for condensation

What is the thermodynamic cycle of a gas turbine engine with intercooling, reheating, and regeneration?

• AStirling cycle
• BEricsson cycle
• CModified Rankine cycle
• DRegenerative Brayton cycle with multistage compression and expansion

During combustion in an internal combustion engine, what does 'knocking' or 'detonation' physically represent?

• AThe rapid mechanical wear of the piston rings hitting the cylinder wall
• BThe uncontrolled auto-ignition of the unburned gas mixture (end-gas) ahead of the normal…
• CThe spark plug firing multiple times during a single power stroke
• DThe exhaust valve opening before combustion is completed

Which type of heat transfer mechanism is dominant in the furnace water-wall tubes of a pulverized coal-fired utility boiler?

• AForced convection
• BNatural convection
• CThermal radiation
• DSolid conduction

Under what conditions is Bernoulli's equation, p/rho + V^2/2 + gz = constant, strictly valid along a streamline?

• AViscous, compressible, unsteady, and rotational flow
• BInviscid, incompressible, steady, and irrotational flow
• CViscous, incompressible, steady, and rotational flow
• DInviscid, compressible, unsteady, and irrotational flow

How does the head loss in a pipe transition from laminar to turbulent flow according to the Darcy-Weisbach equation?

• AIn laminar flow, head loss is proportional to velocity; in turbulent flow, it is approxim…
• BIn laminar flow, head loss is proportional to velocity squared; in turbulent flow, it is…
• CHead loss is independent of velocity in both flow regimes.
• DIn laminar flow, head loss is constant; in turbulent flow, it increases linearly with vel…

What is the physical meaning of Net Positive Suction Head Required (NPSHr) for a centrifugal pump?

• AIt is the minimum pressure required at the pump discharge to prevent backflow.
• BIt is the minimum suction pressure (head) required at the pump impeller inlet to prevent…
• CIt is the head generated by the pump at its best efficiency point (BEP).
• DIt is the pressure drop across the pump casing due to friction.

According to the centrifugal pump affinity laws, how do the flow rate (Q) and head (H) scale with pump shaft speed (N) for a constant impeller diameter?

• AQ scales with N^2, and H scales with N
• BQ scales with N, and H scales with N^2
• CQ scales with N^3, and H scales with N^2
• DBoth Q and H scale linearly with N

What is the primary operational cause of cavitation in hydraulic turbines and pumps?

• AThe local fluid pressure dropping below the vapor pressure of the liquid, causing vapor c…
• BHigh concentrations of dissolved air in the water supply
• CFrictional heating of the fluid passing through the impeller
• DCorrosion of the impeller blades due to acidic water

What type of hydraulic turbine is a Pelton wheel, and under what head and flow conditions is it best suited?

• AReaction turbine; suited for low head and high flow rate
• BImpulse turbine; suited for high head and low flow rate
• CReaction turbine; suited for high head and low flow rate
• DImpulse turbine; suited for low head and high flow rate

In turbomachinery, what does the dimensionless parameter 'specific speed' (N_s) indicate about a pump or turbine?

• AThe mechanical rotational speed limit before rotor failure
• BThe geometric shape and type of the impeller (radial, mixed, or axial flow)
• CThe ratio of viscous forces to pressure forces on the blades
• DThe volumetric efficiency of the pump sealing rings

What is the function of a draft tube in a reaction turbine (like a Francis or Kaplan turbine)?

• ATo accelerate the water before it enters the turbine runner
• BTo direct the exhaust water away from the generator room
• CTo recover the kinetic energy of the water leaving the runner by decelerating it, thereby…
• DTo regulate the flow rate of water entering the turbine casing

How does a centrifugal compressor develop pressure in the working fluid?

• ABy trapping the fluid in positive displacement chambers and reducing their volume
• BBy accelerating the fluid to high velocity using a rotating impeller, and then converting…
• CBy passing the fluid through a series of converging stationary nozzles
• DBy heating the fluid in a combustion chamber

What is 'surging' in the context of centrifugal compressor operation?

• AA mechanical vibration caused by imbalance in the compressor shaft
• BA severe aerodynamic instability characterized by complete flow reversal and pressure flu…
• CThe rapid accumulation of liquid droplets on the compressor blades
• DAn increase in motor current due to overloading at high flow rates

Which parameter is used to classify fluid flow as laminar or turbulent, and what physical forces does it compare?

• AMach number; compares inertial forces to speed of sound
• BFroude number; compares inertial forces to gravitational forces
• CReynolds number (Re); compares inertial forces to viscous forces
• DWeber number; compares inertial forces to surface tension

What is the boundary layer thickness defined as in fluid mechanics?

• AThe distance from the wall where the velocity reaches 50% of the free-stream velocity
• BThe distance from the solid boundary where the local fluid velocity reaches 99% of the fr…
• CThe physical thickness of the viscous sublayer where flow is laminar
• DThe distance where shear stress becomes infinite

In pipe network analysis, what does the Moody chart relate to determine the Darcy friction factor?

• AReynolds number and relative roughness of the pipe
• BFluid density and pipe length
• CFriction factor and pressure drop only
• DDynamic viscosity and pipe diameter

What is the difference between major losses and minor losses in a piping system?

• AMajor losses are due to pipe fittings, while minor losses are due to pipe friction.
• BMajor losses are due to friction along the straight runs of pipe, while minor losses are…
• CMajor losses are only present in turbulent flow, while minor losses occur in laminar flow.
• DMajor losses are measured in pressure, while minor losses are measured in height.

Why is a multi-stage centrifugal pump used instead of a single-stage pump for certain applications?

• ATo handle fluids containing large abrasive solids
• BTo achieve a higher discharge pressure (head) by passing the fluid through multiple impel…
• CTo increase the total volumetric flow rate of the pumping system
• DTo eliminate the need for a mechanical shaft seal

According to the ASME code for transmission shaft design, which combination of loading is typically considered to calculate the design shaft diameter?

• APure tension and compression
• BCombined bending and torsional moments, with shock and fatigue factors applied
• CRadial and axial bearing loads only
• DHydrostatic pressure and thermal expansion

In the design of rolling contact bearings, what does the L10 (or rating) life represent?

• AThe maximum speed at which 10% of bearings will fail within 1 hour
• BThe life (in millions of revolutions) that 90% of a group of identical bearings will reac…
• CThe average life expectancy of the bearing under normal lubrication conditions
• DThe time in years before the bearing seals must be replaced

What does the Lewis equation, W_t = sigma * b * p_c * y, calculate in the design of spur gears?

• AThe scoring limit of the gear teeth based on contact pressure
• BThe bending strength of the gear tooth, treating it as a cantilever beam
• CThe dynamic load factor due to pitch errors
• DThe wear load capacity based on surface fatigue

How does a helical gear differ from a spur gear in terms of load transmission and noise characteristics?

• AHelical gears have straight teeth parallel to the axis, making them noisier than spur gea…
• BHelical gears have teeth cut at an angle (helix angle) to the shaft axis, allowing gradua…
• CHelical gears are less expensive to manufacture and do not generate axial forces.
• DHelical gears can only connect intersecting shafts.

What is the primary function of a flywheel in an engine or machine mechanism?

• ATo increase the average rotational speed of the shaft
• BTo store energy during periods of excess energy generation and release it during periods…
• CTo balance the reciprocating forces of the piston
• DTo multiply the torque output of the crankshaft

In mechanics of machines, what is the 'critical speed' or 'whirling speed' of a rotating shaft?

• AThe speed at which the shaft material yields due to centrifugal stress
• BThe rotational speed at which the shaft becomes dynamically unstable, causing severe bend…
• CThe speed where the lubrication film in the bearings breaks down
• DThe maximum speed before torsional fatigue occurs

What is the purpose of an epicyclic (planetary) gear train compared to a simple gear train?

• ATo connect shafts that are perpendicular to each other
• BTo achieve high gear ratios in a compact space with coaxial input and output shafts
• CTo eliminate the need for lubrication
• DTo reduce the manufacturing cost of the gears

In a spring-mass-damper system, what does the term 'critically damped' mean?

• AThe system vibrates with increasing amplitude over time.
• BThe damping is so high that the system never returns to its equilibrium position.
• CIt is the minimum amount of damping that returns the system to equilibrium without oscill…
• DThe damping ratio (zeta) is greater than 1.0.

In Finite Element Analysis (FEA), what is the purpose of performing a 'mesh convergence study'?

• ATo verify that the material properties are isotropic
• BTo ensure that the results of the simulation are independent of the size of the elements…
• CTo accelerate the solver execution time
• DTo determine the fatigue limits of the model

Which type of modeling representation in CAD defines both the geometric boundaries and the internal volume of a physical object?

• AWireframe modeling
• BSurface modeling
• CSolid modeling
• D2D orthographic projection

In machine design, what is the primary purpose of a key and keyway interface between a shaft and a pulley?

• ATo prevent axial movement of the pulley along the shaft
• BTo transmit torque from the shaft to the pulley (or vice-versa) while preventing relative…
• CTo reduce friction between the shaft and the pulley hub
• DTo balance the rotational weight of the shaft assembly