Question 1

Two-stage reciprocating air compressor: work per cycle composition  Consider a two-stage reciprocating air compressor. The total work done per cycle equals what combination of stage works? Choose the correct statement.

Accepted Answer

Concept/Approach Each cycle comprises compression (and associated processes) in the low-pressure (L.P.) stage plus in the high-pressure (H.P.) stage. The indicated work per cycle is the sum of the indicated works of both stages, accounting for intercooling and valve actions. Final Answer Equal to the sum of works in L.P. and H.P. cylinders

Question 2

Compressor capacity at standard conditions (FAD concept)  Statement: The actual volume of air delivered by a compressor, reduced to normal (standard) temperature and pressure conditions, is called the compressor capacity. Decide whether the statemen…

Accepted Answer

Concept/Approach Compressor capacity is commonly specified as free air delivery (FAD), i.e., the volumetric flow rate referred to standard temperature and pressure. Therefore the statement is correct. Final Answer True

Question 3

Application range of rotary compressors: flow rate and pressure level  Rotary (positive-displacement or dynamic) compressors are typically selected to deliver which combination of flow and pressure? Choose the best description.

Accepted Answer

Concept/Approach Rotary machines (roots, screw, centrifugal, axial) are well suited for large flow rates. In typical single-stage practice, they achieve moderate to low pressures compared with multistage reciprocating compressors used for very high pressures. Final Answer Large quantities of air at low pressures

Question 4

Compressor type used in gas turbines: identify the rotary compressor  Gas turbines (especially aircraft and large power units) primarily use which type of rotary compressor? Choose the correct option.

Accepted Answer

Concept/Approach Modern aircraft and heavy-duty gas turbines predominantly use axial-flow compressors for their high mass-flow capability and good efficiency at high pressure ratios via multistaging. (Small/industrial units may use centrifugal stages, but axial flow is the standard for large engines.) Final Answer Axial-flow type

Question 5

How compressor capacity is expressed: choose the standard unit  Compressor capacity is normally stated as a volumetric flow rate at standard (or specified) conditions. Select the correct unit.

Accepted Answer

Concept/Approach Capacity is a volumetric flow quantity, typically reported as m³/min (or m³/s, cfm) referred to standard conditions (FAD basis). Final Answer m³/min

Question 6

Axial-flow compressor operating regime: statement on best performance  Statement: An axial compressor gives its optimum performance at high rotational speeds and large volume flows. Decide whether the statement is correct.

Accepted Answer

Concept/Approach Axial-flow compressors are favored for high mass-flow applications and achieve good efficiency at high blade tip speeds (high rpm), handling large volume flows through multiple stages. Final Answer True

Question 7

Effect of large clearance volume in a reciprocating compressor  If the clearance volume of a reciprocating compressor is large, what is the primary effect on delivered flow? Choose the correct outcome.

Accepted Answer

Concept/Approach A larger clearance volume increases the fraction of residual compressed air that re-expands during the suction stroke, reducing the volumetric efficiency. As a result, the net fresh air inducted (and thus delivered volume flow) decreases. Common pitfall Confusing volumetric efficiency with pressure ratio; clearance volume primarily affects capacity, not the set pressure ratio (which is governed by system resistance and valve behavior). Final Answer Reduced volume flow rate

Question 8

Gas-turbine intercooling: effect on net work and thermal efficiency.  Consider a simple gas turbine where intercooling is applied only between multistage compressors (no reheat and no recuperator). Assume the same compressor pressure ratio, the same…

Accepted Answer

Concept/Approach Intercooling between compressor stages lowers the work of compression, so turbine work minus compressor work (net output) increases. However, the air then enters the combustor at a lower temperature, so for the same turbine inlet temperature more fuel (heat input) is needed and the average temperature of heat addition drops, leading to a lower thermal efficiency for a simple (unrecuperated) cycle. Result Net output ↑, Thermal efficiency ↓. Final Answer increases net output but decreases thermal efficiency.

Question 9

Rotary compressors: suitability for flow rate and pressure level.  Evaluate the statement about application range: rotary compressors are preferred when a large volume flow is needed at relatively low delivery pressure. Decide if this statement is c…

Accepted Answer

Concept/Approach Rotary (centrifugal/axial/rotary vane/roots) compressors are commonly used for large volumetric flow rates at modest pressure ratios, where reciprocating compressors become bulky and pulsating. Hence the statement is correct. Final Answer Agree.

Question 10

Compressor definitions: identify the correct efficiency term.  Definition target: the ratio of free air delivery (at standard condition) per stroke to the piston's swept volume. Select the correct name of this ratio.

Accepted Answer

Given Ratio = (Free air delivered per stroke at NTP)/(Swept volume of piston). Concept This ratio measures how effectively the cylinder volume is filled with air referenced to standard conditions; the standard term is volumetric efficiency. Final Answer volumetric efficiency.

Question 11

Gas turbines compared with reciprocating I.C. engines: select the generally valid advantage Which statement best captures a generally valid advantage of a simple (open-cycle) gas turbine relative to a reciprocating internal combustion engine, withou…

Accepted Answer

Introduction / Context: Choosing between turbomachinery and reciprocating engines involves comparing speed range, torque smoothness, efficiency, size, and maintenance. A fair comparison for fundamentals considers a simple open-cycle gas turbine (no bottoming steam cycle) versus an I.C. engine. Given Data / Assumptions: Simple gas turbine (compressor–combustor–turbine), no combined cycle. Reciprocating engine baseline (diesel or SI) with crankshaft and pistons. Focus on general, widely applicable characteristics. Concept / Approach: Gas turbines are rotary machines with continuous combustion and steady flow through rotating stages. They can operate at very high shaft speeds and deliver nearly uniform torque, since there are no discrete firing strokes producing large torque pulses. Reciprocating engines, by contrast, exhibit cyclic torque and speed variations unless heavily damped by flywheels and multiple cylinders. Step-by-Step Solution: Identify rotary vs. reciprocating kinematics: turbines → continuous flow and torque; engines → intermittent combustion events.Relate to speed capability: turbine rotors commonly run at tens of thousands of rpm.Select the statement that is broadly valid without caveats: high speed and uniform torque. Verification / Alternative check: Torque ripple measurements and shaft-speed ratings in specifications confirm very low pulsation and high allowable speed for turbines compared to reciprocating engines. Why Other Options Are Wrong: Higher thermal efficiency claim (option b/e) is not generally true for simple turbines versus modern diesels; options c and d describe reciprocating drawbacks, not turbine advantages in correct form; turbines typically need lighter foundations due to low vibration. Common Pitfalls: Conflating combined-cycle (which can be very efficient) with simple-cycle gas turbines; the question asks for a general advantage independent of bottoming cycles. Final Answer: Can be driven at very high rotational speed and delivers nearly uniform torque

Question 12

Centrifugal compressor flow: location of highest Mach number leading to shock In a radial-flow (centrifugal) compressor, the flow accelerates through the impeller and then is decelerated in the diffuser. Considering where transonic conditions are mo…

Accepted Answer

Introduction / Context: Centrifugal compressors accelerate air within the impeller and then recover static pressure in the diffuser. At high tip speeds and pressure ratios, the flow leaving the impeller can reach transonic conditions. Understanding where the peak Mach number occurs is essential for diagnosing losses, diffuser surge margin, and shock-related efficiency penalties. Given Data / Assumptions: Machine: centrifugal (radial) compressor with an impeller and a downstream diffuser. Operating at a high loading where local Mach numbers approach unity. Focus: location of peak Mach number that precipitates shocks. Concept / Approach: The impeller imparts kinetic energy to the fluid; thus absolute velocity (and often relative velocity near the blade trailing edge) peaks at the impeller exit. The diffuser immediately receives this high-speed flow and attempts controlled deceleration. If the incoming flow is transonic or supersonic, shocks appear in the diffuser inlet or passages, but the highest Mach number causing those shocks is achieved at the impeller outlet radius. Step-by-Step Solution: Recognise energy addition: impeller raises velocity most strongly toward its outlet radius.Assess Mach distribution: maximum Mach commonly occurs at or just as the flow leaves the impeller (tip region).Infer shock origin: deceleration in the diffuser produces shocks when inlet Mach is too high; however, the peak Mach value that triggers this is reached at impeller exit.Therefore, select the impeller outlet radius. Verification / Alternative check: Design practice limits tip relative Mach and uses vaneless spaces/contoured diffusers to manage shocks. Performance maps often show increased loss near choke, consistent with impeller-exit transonics. Why Other Options Are Wrong: Diffuser inlet/outlet: shocks appear here, but the absolute maximum Mach originates at the impeller exit that feeds the diffuser. Impeller inlet: modern designs keep inlet Mach subsonic to avoid leading-edge shocks. Mid-diffuser hub: by this point, the flow is already decelerating. Common Pitfalls: Confusing the location of shocks with the location of peak Mach. Shocks can sit in the diffuser, yet the highest Mach that causes them is reached at the impeller’s outlet radius. Final Answer: impeller outlet radius

Question 13

Gas turbine intake and compression In a simple open-cycle gas turbine, identify whether the working air is drawn from the surrounding atmosphere and mechanically compressed in a compressor before entering the combustor.

Accepted Answer

Introduction / Context: An open-cycle gas turbine (industrial, aero, or power-generation) operates by ingesting ambient air, compressing it, adding heat by fuel combustion, expanding the hot gases through a turbine, and exhausting them back to atmosphere. The intake and compression stages define the front end of this Brayton-cycle machine. Given Data / Assumptions: Cycle type: open. Air source: ambient atmosphere. Compression: mechanical compressor (axial or centrifugal). Concept / Approach: In the Brayton cycle, the compressor raises the pressure and temperature of the incoming air. The combustor then adds heat at roughly constant pressure, and the turbine expands the gas to produce shaft work. The open-cycle architecture explicitly exchanges mass with the environment, unlike closed cycles where the working fluid circulates internally. Step-by-Step Solution: Identify intake: air is drawn from atmosphere through an inlet and filters.Compression: an axial/centrifugal compressor increases pressure.Combustion and expansion: fuel is added, gases expand in the turbine, and are exhausted to atmosphere.Conclusion: the statement is correct for open-cycle gas turbines. Verification / Alternative check: Standard schematics (intake → compressor → combustor → turbine → exhaust) confirm this sequence across aero and industrial configurations. Why Other Options Are Wrong: Closed-loop claim: describes a closed Brayton cycle, not the common open-cycle plant. Restrictions to turbojets or intercooling are unnecessary; the principle holds without them. Common Pitfalls: Confusing open and closed cycles. A regenerator or intercooler can be added, but neither changes the fact that an open-cycle machine pulls air from the atmosphere and compresses it. Final Answer: True

Question 14

Two-stage reciprocating compressor with perfect intercooling Let p1 be the suction pressure to the low-pressure (LP) cylinder and p3 the delivery pressure from the high-pressure (HP) cylinder. For a single-acting, two-stage reciprocating air compres…

Accepted Answer

Introduction / Context: In multi-stage reciprocating compressors with perfect intercooling, equalising the pressure ratio per stage minimises work. This also sets a design relation between LP and HP cylinder sizes because the same mass flow must pass through both stages at different suction pressures. Given Data / Assumptions: Suction to LP: p1; discharge from HP: p3. Complete intercooling returns the HP suction temperature approximately to the LP suction temperature. Single-acting cylinders, same speed and stroke; volumetric efficiencies taken similar for the sizing relation. Concept / Approach: For equal mass flow m through both stages at equal inlet temperature T, we use m ∝ p * V / (R * T) at suction. Thus p1 * V1 = p2 * V2, where p2 is HP suction pressure. With perfect intercooling and optimal staging, p2 = sqrt(p1 * p3). Cylinder displacement V ∝ D^2 (for fixed stroke and speed), so D1^2 / D2^2 = p2 / p1. Step-by-Step Solution: p2 = sqrt(p1 * p3).Mass continuity at equal T: p1 * V1 = p2 * V2.With V ∝ D^2: D1^2 / D2^2 = p2 / p1.So D1/D2 = sqrt(p2 / p1) = sqrt( sqrt(p1 * p3) / p1 ) = (p3 / p1)^(1/4). Verification / Alternative check: Dimensionally and physically consistent: higher overall pressure ratio (p3/p1) demands a relatively larger LP cylinder to ingest the same mass at the lower pressure. Why Other Options Are Wrong: (p1 p3)^(1/2): ignores the square-root nesting from displacement proportionality. (p1/p3)^(1/4) and (p1 p3)^(1/4): invert or misplace the dependence on p3/p1. (p3/p1)^(1/2): overpredicts the diameter ratio by omitting the square-root from area–diameter relation. Common Pitfalls: Forgetting that cylinder capacity scales with D^2 (at fixed stroke), not with D directly; and overlooking the perfect-intercooling relation p2 = sqrt(p1 p3). Final Answer: D1/D2 = (p3/p1)^(1/4)

Question 15

Roots blower delivery frequency In a rotary positive-displacement compressor using a three-lobed Roots rotor, the trapped air pockets are transferred to the discharge how many times per rotor revolution?

Accepted Answer

Introduction / Context: Roots blowers are timing-gear-driven, positive-displacement machines. The number of lobes on each rotor determines the number of trapped volumes delivered per revolution, affecting pulsation frequency, pressure ripple, and noise. Given Data / Assumptions: Three-lobed Roots rotor configuration. Idealised operation without leak-back considerations. One trapped chamber is discharged per lobe engagement event. Concept / Approach: Each lobe pair forms a pocket that moves from the inlet side to the discharge side. With three lobes, three discrete pockets are conveyed per revolution, producing three delivery pulses per rotor revolution per side (neglecting minor harmonics). Step-by-Step Solution: Count lobes → three lobes create three distinct pockets.Track pocket motion → each pocket is delivered once per revolution.Therefore, the delivery event occurs three times each revolution. Verification / Alternative check: Manufacturer literature lists discharge pulsation frequency proportional to rotor speed multiplied by lobe count. Why Other Options Are Wrong: Two/four/six times: correspond to other lobe configurations or misunderstandings of pocket transfer. Once per revolution: would apply to single-lobe concepts, not standard Roots designs. Common Pitfalls: Confusing lobe count with gear teeth count; only lobe count directly sets the base pulsation order. Final Answer: three times

Question 16

Work saving with intercooling in multi-stage compression For a two-stage reciprocating air compressor, is the saving in compression work maximised with incomplete intercooling or with perfect (complete) intercooling between stages?

Accepted Answer

Introduction / Context: Intercooling in multi-stage compression reduces the temperature of the air between stages, lowering the work input required in the subsequent stage. The question distinguishes between incomplete and perfect intercooling with respect to work minimisation. Given Data / Assumptions: Two compression stages with an intercooler. Same mass flow through both stages. Comparison is between incomplete cooling and perfect cooling back to the initial intake temperature. Concept / Approach: Compression work for a given pressure ratio increases with inlet temperature to the stage. Perfect intercooling restores the air temperature before the second stage to the original intake temperature, thereby minimising the total work. Incomplete intercooling leaves the air hotter than ideal, increasing the second-stage work and the total work. Step-by-Step Solution: Assume equal pressure ratio per stage for minimum work.Write stage work trend: W_stage ∝ T_in when pressure ratio is fixed.Perfect intercooling → T_in,HP = T_initial; incomplete intercooling → T_in,HP > T_initial.Therefore, W_total is minimised with perfect intercooling. Verification / Alternative check: Textbook derivations show the minimum work condition when p2 = sqrt(p1 p3) and the intercooler returns temperature to the initial intake value; any temperature above this increases work. Why Other Options Are Wrong: Saying incomplete intercooling saves more work contradicts the temperature–work relationship. “Same saving” or “independent” ignore the direct dependence on intercooler effectiveness. Aftercooling affects delivery temperature, not the interstage work minimum. Common Pitfalls: Confusing aftercooling (post-compression cooling) with intercooling (between stages), and ignoring the strong effect of inlet temperature on compression work. Final Answer: Disagree — maximum work is saved with perfect intercooling

Question 17

Work exchange signs during compressor processes Consider an ideal reciprocating air compressor. During the suction stroke the cylinder pressure is slightly below ambient, during compression it is high, and during delivery it exceeds receiver pressur…

Accepted Answer

Introduction / Context: Work exchange in compressors depends on the pressure-volume path over a cycle. Understanding whether the piston does work on the gas or the gas does work on the piston in the suction, compression, and delivery phases clarifies the indicator diagram and net work input. Given Data / Assumptions: Slight pressure drop during suction below ambient. Finite delivery pressure above cylinder pressure at valve opening. Conventional sign: positive work is work done by the piston on the gas. Concept / Approach: In suction, the gas pressure in the cylinder is slightly below ambient; the ambient performs work on the gas/piston combination as the volume increases, so the net work from the piston to the gas is negative (i.e., work is done on the piston). During compression, the piston reduces volume against rising pressure, doing positive work on the gas. During delivery, the piston overcomes discharge pressure to push air into the receiver, again doing positive work on the gas. Step-by-Step Solution: Suction: p_cyl < p_amb → gas (via ambient) does work on piston → “work is done on the piston”.Compression: piston compresses gas → work done by piston on gas.Delivery: piston forces gas into receiver at higher pressure → work done by piston on gas.Composite statement matches these signs → Agree. Verification / Alternative check: Indicator diagrams show a small negative area during suction and a larger positive area during compression plus delivery, resulting in positive net work input to the compressor. Why Other Options Are Wrong: Disagree: contradicts the accepted sign convention and observed indicator area. Zero suction drop or “no work in delivery” assumptions are nonphysical. Restriction to multistage operation is unnecessary; the logic holds for single-stage too. Common Pitfalls: Mixing thermodynamic sign conventions (work of gas vs. work on gas) and overlooking the small but real suction work magnitude. Final Answer: Agree

Question 18

Purpose of a receiver in compressed-air installations In a reciprocating or screw compressor plant, the air receiver primarily serves which function with respect to the discharge flow?

Accepted Answer

Introduction / Context: Air receivers (accumulators) are pressure vessels placed downstream of compressors. They provide short-term storage and stabilize the system by reducing pressure pulsations and flow fluctuations typical of positive-displacement machines. Given Data / Assumptions: Receiver located immediately after compressor or aftercooler. System operates with demand variations and pulsating discharge (especially for reciprocating units). Receiver sized appropriately for pressure and capacity. Concept / Approach: The receiver acts as a buffer volume. By adding compliance to the discharge line, it damps pressure waves and smooths the flow delivered to downstream equipment. It also offers residence time for moisture separation and allows load/unload control strategies to function effectively. Step-by-Step Solution: Identify issue: reciprocating compressors produce pulsating discharge.Introduce buffer: receiver volume reduces pulsation amplitude.Result: damped pressure waves and smoother system pressure. Verification / Alternative check: Pressure traces upstream and downstream of a receiver show reduced ripple; standard design guides include receiver sizing to achieve target pressure stability. Why Other Options Are Wrong: “No”: contradicts the primary, well-known function. “Only stores condensate”: receivers do collect condensate but are not limited to that role. Fixed efficiency gain claims are unfounded; receivers do not directly change isentropic efficiency. They do not replace aftercoolers, which address temperature and moisture removal. Common Pitfalls: Confusing the roles of aftercoolers, dryers, and receivers; receivers are chiefly for buffering and stability, with ancillary moisture settling. Final Answer: Yes

Question 19

Aero gas turbine design priorities For aircraft propulsion applications, which general design priorities are most important for a gas turbine engine with respect to mass and frontal area?

Accepted Answer

Introduction / Context: Aircraft engines must meet stringent weight and drag constraints. Low engine mass reduces structural weight and improves aircraft performance. Small frontal area lowers parasite drag and nacelle size, key for high-speed flight efficiency. Given Data / Assumptions: Application: aircraft propulsion. Drag scales with frontal area, other things equal. Structural and performance penalties correlate with engine mass. Concept / Approach: Gas turbines deliver a high power-to-weight ratio compared with many alternatives. Designers also minimise frontal area to reduce drag and improve installation aerodynamics. While thrust-specific fuel consumption is crucial, weight and drag constraints remain fundamental drivers of engine architecture and nacelle integration. Step-by-Step Solution: Identify objectives: power-to-weight and low drag.Map to design: compact core, lightweight materials, and streamlined nacelles.Conclude: low weight and small frontal area are indeed primary requirements. Verification / Alternative check: Aero engine specifications routinely cite mass flow per frontal area (thrust density) and engine dry weight as key metrics; nacelle drag budgeting further motivates small frontal area. Why Other Options Are Wrong: Limiting to turbofans or low subsonic regimes ignores that these priorities apply widely across configurations. Fuel consumption matters, but cannot override weight and drag fundamentals in aircraft design. Common Pitfalls: Focusing solely on thermodynamic efficiency and overlooking installation penalties that strongly influence aircraft performance. Final Answer: Correct

Question 20

Propulsion hardware in jet engines Identify the presence of a propeller in a pure jet engine configuration (turbojet).

Accepted Answer

Introduction / Context: A pure jet engine (turbojet) produces thrust by accelerating a jet of exhaust gases through a nozzle, with no external propeller. Other engine types, such as turboprops, use a turbine to drive a propeller for thrust production. Given Data / Assumptions: Configuration considered: turbojet (not turbofan, turboprop, or propfan). Thrust is predominantly from jet momentum, not propeller disk loading. Concept / Approach: Turbojets comprise inlet → compressor → combustor → turbine → nozzle. The turbine extracts just enough power to drive the compressor; remaining enthalpy accelerates the jet to produce thrust. No propeller is present in a pure turbojet. Step-by-Step Solution: Identify engine class: turbojet.Check thrust device: convergent (or convergent-divergent) nozzle, not a propeller.Conclude: “no propeller.” Verification / Alternative check: Aircraft using pure turbojets (historically many fighters) demonstrate zero propeller hardware; by contrast, turboprops explicitly feature a propeller driven by a free power turbine. Why Other Options Are Wrong: Any option with a propeller describes turboprop/propfan or piston-prop configurations, not a pure jet. Common Pitfalls: Confusing turbofans (which have a ducted fan) with turbojets; a fan is not a propeller in the conventional exposed sense. Final Answer: no propeller

Compressors, Gas Dynamics and Gas Turbines Questions