Question 1

Axial-flow compressor aerodynamics: exit flow deviation dependence  In an axial-flow compressor, the deviation of the exit flow angle from the blade angle primarily depends on which geometric/flow parameters? Choose the correct option.

Accepted Answer

Concept/Approach Exit flow angle deviation is an empirical/physics-based function of profile camber and cascade solidity (space–chord ratio). Greater camber and lower solidity (higher space–chord) typically increase deviation due to boundary-layer and turning limits. Why other options fall short Camber alone or space–chord alone are incomplete; incidence is more relevant to loss and stall margin than to the systematic deviation law used in design correlations. Final Answer Blade camber and space–chord ratio

Question 2

Single-acting reciprocating compressor: strokes for suction, compression, and delivery  In a single-acting reciprocating air compressor, over how many piston strokes do suction, compression, and delivery occur? Choose the correct option.

Accepted Answer

Concept/Approach In a single-acting cylinder, suction occurs during one piston stroke (e.g., downward), while compression and delivery occur during the opposite stroke (e.g., upward) as the discharge valve opens once cylinder pressure exceeds delivery pressure. Sequence Stroke 1: Suction.Stroke 2: Compression & Delivery. Final Answer Two strokes

Question 3

Effect of clearance volume on compression work per kg in reciprocating compressors  In a reciprocating air compressor, how does the compression work per kilogram of air vary with clearance volume (other conditions fixed)? Choose the correct option.

Accepted Answer

Concept/Approach For a given pressure ratio and compression law (e.g., polytropic), the work per kg depends on states, not on clearance. Clearance affects volumetric efficiency (mass inducted per cycle), but the indicated work per unit mass compressed is essentially unaffected. Common pitfalls Confusing work per cycle with work per unit mass. Clearance reduces the mass taken in per cycle but does not raise or lower the thermodynamic work per kg for the same pressure ratio. Final Answer Independent of clearance volume

Question 4

Compressor operating pressures: inlet versus discharge  Complete the statement about compressor operation. The inlet pressure is always __________ the discharge pressure. Choose the correct option.

Accepted Answer

Concept/Approach By definition, a compressor raises the fluid pressure, thus the discharge pressure must exceed the inlet pressure during normal operation. Final Answer Less than

Question 5

Typical volumetric efficiency range for reciprocating air compressors  What is the usual range of volumetric efficiency for practical reciprocating air compressors under normal conditions? Choose the correct option.

Accepted Answer

Concept/Approach Volumetric efficiency accounts for clearance, valve losses, heating during suction, and leakage. For well-designed reciprocating compressors, typical values lie in the 70–90% band. Final Answer 70% to 90%

Question 6

Intercooling between stages of air compression: effect on required power  Statement: Intercooling in multistage air compressors saves power when compressing a given volume of air to a given final pressure (compared to no intercooling). Decide whethe…

Accepted Answer

Concept/Approach Intercooling removes the heat of compression between stages, returning the air toward the initial suction temperature before the next stage. This reduces the specific volume entering the next stage and reduces the work required, approaching the minimum work of isothermal compression. Final Answer True

Question 7

Rotary air compressors: typical maximum delivery pressure  What is a representative maximum delivery pressure achievable with rotary air compressors (e.g., roots/rotary screw) in single-stage practice? Choose the correct option.

Accepted Answer

Concept/Approach Rotary positive-displacement compressors (roots, screw) are well suited to moderate pressures and high flow. Typical single-stage delivery pressures are around 10 bar. Higher pressures are usually attained using multi-stage reciprocating machines. Final Answer 10 bar

Question 8

Gas turbines vs. steam turbines: identify the true comparative statements in power-plant practice.  Gas turbine plants are compact and include a compressor and a combustion chamber. Overall (simple-cycle) efficiency is typically lower than that of s…

Accepted Answer

Concept/Approach A simple-cycle gas turbine comprises a compressor, combustor, and turbine on one shaft, yielding a compact plant that is lighter and requires less floor space than a comparable steam plant. However, simple-cycle thermal efficiency is generally lower than steam Rankine plants (unless combined-cycle is used). Why 'all of these' is correct (1) Less space: high power-to-weight and compact layout. (2) Compressor + combustor: defining components of gas turbines. (3) Less efficiency (simple cycle): lower cycle efficiency at moderate TIT and pressure ratios compared to large steam plants. Final Answer all of these.

Question 9

Two-stage reciprocating air compressors: find the incorrect statement (if any).  Consider intercooling, minimum work allocation per stage, and the definition of volumetric efficiency. Determine which statement is wrong; if all are correct, select 'n…

Accepted Answer

Given facts Two-stage compression with perfect intercooling (and equal pressure ratios) minimizes total work. For minimum-work allocation, both stages do equal work, so total work = 2 × (work per stage). Volumetric efficiency is defined as (free air delivery per stroke)/(swept volume). Evaluation A is correct: perfect intercooling gives the least work. B is correct: at the minimum-work condition, work is split equally; hence total is twice that per stage. C is the standard definition of volumetric efficiency. Conclusion Since A, B, and C are correct, there is no wrong statement among them. Final Answer none of the above.

Question 10

Aircraft propellers: identify the correct propulsion description.  Choose the statement that correctly describes how propellers produce thrust for aircraft flight.

Accepted Answer

Concept/Approach An aircraft propeller accelerates surrounding ambient air (the propulsive matter) rearward, generating thrust by changing the momentum of the air around the aircraft. This depends on the presence of air. Option check A describes jet/rocket propulsion, not a propeller. B is correct for propellers: they act on the external air stream. C is false: propellers require air. Final Answer The propulsive matter is caused to flow around the propelled body.

Question 11

Prime movers for rotary compressors: identify feasible drives.  Rotary compressors in industry can be coupled to different prime movers. Select the correct statement about typical drive sources.

Accepted Answer

Concept/Approach Rotary compressors are commonly shaft-driven by electric motors (industrial plants) or by internal-combustion engines (mobile/field applications). Both are practical and widely used. Final Answer either (a) or (b).

Question 12

Peak combustion pressure comparison: gas turbines vs. I.C. engines.  Complete the statement: The maximum combustion pressure in a gas turbine is __________ as compared to that in an internal-combustion (reciprocating) engine. Choose the most accurat…

Accepted Answer

much lower than that in an I.C. engine

Question 13

Jet engines: operating condition for higher propulsive efficiency.  Determine at which operating regime a jet engine attains higher propulsive efficiency relative to low-speed operation. Select the best option.

Accepted Answer

Concept/Approach Propulsive efficiency depends on how closely the jet velocity matches the flight speed. For pure jets, efficiency increases with aircraft speed (up to practical limits), since less kinetic energy is wasted in the exhaust stream for a given thrust. Final Answer high speeds.

Question 14

Open-cycle vs. closed-cycle gas turbines: cycle identity.  Statement to evaluate: An open-cycle gas turbine works on the same thermodynamic cycle as a closed-cycle gas turbine (working-fluid handling differs, but the ideal cycle is identical). Choos…

Accepted Answer

Concept/Approach Both open and closed gas turbines ideally follow the Brayton (Joule) cycle: isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-pressure heat rejection. The difference lies in whether the working fluid is exhausted (open) or recirculated via a cooler (closed). Final Answer Agree.

Question 15

Maximum turbine inlet temperature in gas turbines: select the closest typical value.  Choose the approximate maximum temperature level used in gas-turbine practice (turbine inlet temperature, TIT).

Accepted Answer

Concept/Approach Classical gas-turbine practice cites turbine inlet temperatures on the order of about 1000°C (and higher in modern engines with advanced cooling/materials). The lower listed values are far below practical TIT for efficient operation. Final Answer 1000°C.

Question 16

Jet propulsion: identify the correct thrust mechanism.  Choose the statement that correctly describes how thrust is produced in jet propulsion systems.

Accepted Answer

Concept/Approach Jet propulsion (including turbojets and rockets) produces thrust by high-velocity ejection of propellant/exhaust gases from within the vehicle, generating a reaction force according to momentum conservation. Option check A Correct for jets/rockets. B Describes propellers, not jets. C Not generally true for air-breathing jets (requires air); it is true only for rockets. Final Answer The propulsive matter is ejected from within the propelled body.

Question 17

Aftercoolers in compressed-air systems: primary purpose.  Identify what an aftercooler accomplishes downstream of compression to prepare air for distribution/use.

Accepted Answer

Concept/Approach An aftercooler cools compressed air, condensing water vapour (and some oil vapour), so that liquids can be separated before the air enters receivers/distribution. While it does cool the air, the key functional purpose is to condense and remove moisture and oil. Option check C is the most specific and accurate statement of purpose. D is true but incomplete: cooling is the means, not the end, in air-system conditioning. Final Answer cause moisture and oil vapour to drop out.

Question 18

Closed-cycle gas turbine vs open-cycle gas turbine: compare thermal efficiency  Consider an idealized comparison where overall pressure ratio and turbine inlet temperature limits are similar. Choose how the thermal efficiency of a closed-cycle gas t…

Accepted Answer

Concept/Approach A closed-cycle gas turbine recirculates a chosen working fluid (often helium or nitrogen) and can incorporate highly effective heat exchangers (recuperators and intercoolers) with better control of losses and leakage. For similar pressure ratio and turbine inlet temperature limits, this tends to yield a higher thermal efficiency than a simple open cycle without extensive heat recovery. Reasons Ability to use a working fluid with superior properties (high specific heat, low compressor work). Improved heat recovery (recuperation) and reduced exhaust losses. Better part-load performance due to controlled boundary conditions. Common pitfall Assuming open cycle is always better because of simplicity; in practice, closed cycles are often chosen for efficiency with heat recovery. Final Answer Higher efficiency

Question 19

Two-stage reciprocating compressor with perfect intercooling: find the optimum intermediate pressure  Compression from p₁ to p₃ is performed in two stages with perfect intercooling and negligible pressure losses. What is the optimum intermediate pre…

Accepted Answer

Given data Two-stage reciprocating compressor. Perfect intercooling (air cooled back to initial suction temperature between stages). Same polytropic law in both cylinders; negligible losses. Concept/Approach For minimum total work with equal polytropic index in each stage and perfect intercooling, the pressure ratio should be equally split across the stages. This yields the geometric-mean rule for the intermediate pressure. Derivation snapshot Let overall pressure ratio be r = p₃/p₁. For equal stage ratios: r₁ = r₂ = √r.Therefore, p₂ = p₁ r₁ = p₁ √(p₃/p₁) = √(p₁ p₃). Numerical check If p₁ = 1 bar and p₃ = 9 bar ⇒ p₂ = √(1×9) = 3 bar (equal ratios 3 and 3). Final Answer p₂ = √(p₁ p₃)

Question 20

Compressor efficiencies: identify the ratio IP/BP  For a compressor driven by a prime mover, consider the ratio of indicated power (IP) to the shaft/brake power (BP) supplied to the compressor. What is this efficiency called? Choose the correct term.

Accepted Answer

Concept/Approach Mechanical efficiency of a compressor quantifies how effectively the input shaft (brake) power is converted into indicated power within the cylinder, i.e., ηmech = IP / BP. Notes Volumetric efficiency relates to the mass (or volume) actually inducted vs. swept volume. Isothermal efficiency compares actual compression work to ideal isothermal work. Overall/compressor efficiency aggregates multiple effects; it is not simply IP/BP. Final Answer Mechanical efficiency

Compressors, Gas Dynamics and Gas Turbines Questions