Question 1

Intake conditions: what is the velocity of air entering a rocket engine compared with that entering a conventional aircraft engine intake?

Accepted Answer

Introduction / Context: Aircraft gas-turbine engines ingest atmospheric air through an intake. Rocket engines, in contrast, carry their own oxidizer and do not rely on atmospheric air for combustion. This fundamental difference affects intake flow conditions and engine operation in vacuum. Given Data / Assumptions: Conventional aircraft engine: uses air-breathing cycle, requires inlet duct and ram air. Rocket engine: closed propellant system (fuel + oxidizer) with no atmospheric intake. Comparison is about the velocity of “air entering” the engine. Concept / Approach: Since a rocket does not ingest atmospheric air, the mass flow of air into a rocket is zero. Therefore, the velocity of air entering is effectively zero, in sharp contrast to an aircraft engine where air enters the inlet with a finite velocity equal to flight speed relative to the engine plus inlet-induced velocities. Step-by-Step Solution: Identify medium: aircraft engine → air-breathing; rocket → self-oxidizing.For rockets, atmospheric air flow into the engine is absent.Hence, the air-entry velocity quantity is zero by definition. Verification / Alternative check: Rocket performance equations (Tsiolkovsky) and nozzle mass flow are based on propellant tank feed, not ambient air ingestion, confirming no air inflow. Why Other Options Are Wrong: “Less”, “More”, or “Same” presume air actually enters the rocket, which it does not. “Undefined” is not needed; the physically correct answer for air entering is zero. Common Pitfalls: Conflating exhaust jet entrainment of ambient gases with engine intake; entrainment may occur externally but is not engine air ingestion. Final Answer: Zero

Question 2

In aircraft gas-turbine (jet) propulsion, a very large excess of air is supplied for cooling and complete combustion. For typical operating conditions, what overall air–fuel ratio (by mass) is most representative for a turbojet/turbofan combustion c…

Accepted Answer

Introduction / Context: In gas-turbine (jet) engines, the combustor uses much more air than the chemically required amount. The question assesses fundamental understanding of why overall air–fuel ratio (AFR) in jet engines is very high compared to the stoichiometric value for hydrocarbon fuels (about 15:1 by mass) and asks for a representative numerical value used in practice. Given Data / Assumptions: Overall AFR refers to total mass of air entering the combustor compared to total fuel, not just the primary zone. Stoichiometric AFR for typical aviation kerosene is around 14.7–15:1, but excess air is routed for dilution and liner cooling. We are looking for a typical range used at cruise or rated conditions, not a transient or idle point. Concept / Approach: Jet engines require high turbine inlet temperature limits and uniform exit temperature profiles. To protect hardware and ensure complete combustion with low emissions, only part of the air participates in initial burning; the rest is added downstream for dilution and wall cooling. Therefore, overall AFR is many times the stoichiometric AFR, commonly 50:1 to 80:1, with about 60:1 being a widely cited representative figure for design estimates. Step-by-Step Solution: 1) Stoichiometric AFR for kerosene ≈ 15:1 (for reference).2) Primary zone may operate near 15:1 to 25:1 locally, but overall includes secondary and dilution air.3) Typical overall AFR used in introductory design problems and many texts ≈ 60:1.4) Therefore, among the choices, 60:1 best represents real engine practice. Verification / Alternative check: Back-of-envelope: If only ~25–30% of compressor air is used for primary combustion and the rest is for dilution/cooling, then overall AFR ≈ (air total) / fuel ≈ 3 * (stoich AFR) to 4 * (stoich AFR) ≈ 45:1 to 60:1, consistent with 60:1 being a reasonable pick. Why Other Options Are Wrong: 30:1 or 40:1: Too low for typical cruise/full-load overall AFR; would overheat turbine components. 50:1: Plausible but slightly on the lower side of typical representative values. 80:1: Possible at some conditions, but not commonly quoted as the representative value. Common Pitfalls: Confusing stoichiometric AFR with overall combustor AFR, or assuming all air takes part in the initial chemical reaction. Also, mixing local zone AFR with overall system AFR leads to underestimation. Final Answer: 60:1

Question 3

For a compressor, the input work decreases as the suction temperature decreases. From which source should air be taken to minimize the work required for compression?

Accepted Answer

Introduction / Context: The power required by compressors depends strongly on the suction (inlet) state. Lower inlet temperature reduces the specific volume and the enthalpy rise, thereby lowering the compression work. This question checks understanding of how suction conditions influence compressor work. Given Data / Assumptions: Same delivery pressure and mass flow requirement. Ideal gas behavior for air. Comparison across different air sources with different inlet temperatures. Concept / Approach: For isentropic or polytropic compression of an ideal gas, the shaft work per unit mass increases with inlet temperature. For a given pressure ratio, temperature rise is proportional to inlet absolute temperature. Therefore, the colder the suction air, the lower the work to reach the same discharge pressure. Step-by-Step Solution: 1) For ideal gas, isentropic work per kg is proportional to cp * (T2 - T1).2) For a fixed pressure ratio, T2 / T1 is fixed by isentropic relations, so (T2 - T1) scales with T1.3) Lower T1 directly reduces (T2 - T1) and thus reduces work input.4) Hence, air should be drawn from the coldest available source to minimize compressor work. Verification / Alternative check: Industrial practice uses inlet chillers or precoolers to reduce compressor power in gas turbines and large air systems, confirming the principle that colder inlet air reduces work and increases density (higher mass flow per unit volume). Why Other Options Are Wrong: The atmosphere: Only correct if it is the coldest option, which is not guaranteed. A source at 0°C: Colder than typical ambient, but option (C) is more general and always correct. A source of high temperature air: Increases work due to higher T1. A source of saturated steam–air mixture: Adds moisture issues and usually higher enthalpy; not work-minimizing. Common Pitfalls: Confusing volumetric flow with mass flow; colder air has higher density, affecting volumetric throughput and sometimes leading to misinterpretation of power changes. Final Answer: A source of low temperature air

Question 4

In a two-stage air compressor with an intercooler, perfect intercooling means the temperature after intercooling returns to the initial suction temperature. Under perfect intercooling, what is the optimal intermediate pressure p2 in terms of suction…

Accepted Answer

Introduction / Context: Staging a compressor with intercooling reduces work input. The question tests knowledge of the condition for minimum work in a two-stage compressor with ideal (perfect) intercooling and asks for the correct expression for the intermediate pressure p2. Given Data / Assumptions: Two stages with an intercooler that cools the air back to the initial suction temperature (perfect intercooling). Same mass flow through both stages. Identical polytropic/isentropic behavior in both stages for simplicity. Concept / Approach: For minimum total work in two-stage compression with perfect intercooling, the pressure ratio should be split equally between stages. That means the pressure ratio in the first stage equals the pressure ratio in the second stage. This leads directly to the geometric mean relation for p2. Step-by-Step Solution: 1) Let r1 = p2 / p1 and r2 = p3 / p2.2) For minimum work with perfect intercooling, set r1 = r2.3) Therefore, p2 / p1 = p3 / p2.4) Rearranging gives p2^2 = p1 * p3, hence p2 = sqrt(p1 * p3). Verification / Alternative check: If p2 is the geometric mean of p1 and p3, both stages perform equal pressure ratio and roughly equal work, minimizing the sum. This well-known result appears in standard compressor design references and is consistent with energy minimization analyses. Why Other Options Are Wrong: Arithmetic mean (p1 + p3)/2: Does not equalize stage ratios; leads to higher total work. p2 = p1 or p2 = p3: Collapses one stage and overloads the other; not minimum work. p2 = p1 * p3: Dimensional nonsense for pressure without normalization. Common Pitfalls: Confusing geometric mean with arithmetic mean; forgetting the equal-ratio requirement; ignoring that perfect intercooling restores temperature to the initial suction temperature. Final Answer: p2 = sqrt(p1 * p3)

Question 5

In a vane (rotary) blower, some work is wasted due to backflow or slip. If W1 is the useful work due to compression and W2 is the extra work required due to backflow, what is the blower efficiency?

Accepted Answer

Introduction / Context: Vane blowers experience internal leakage/backflow that adds to the work input. Defining efficiency properly helps quantify the fraction of input that is usefully expended on compression versus losses. Given Data / Assumptions: W1 is the useful work for compressing the fluid to the delivery condition. W2 is the extra work consumed due to backflow/slip (loss). Total input work to the blower is W1 + W2. Concept / Approach: Device efficiency in this context is defined as useful output divided by input. For a blower, the useful effect is the theoretical/ideal compression work W1, while the actual input includes the loss W2. Therefore, efficiency must be W1 divided by the total W1 + W2. Step-by-Step Solution: 1) Identify useful work = W1.2) Identify total input work = W1 + W2.3) Define efficiency η = useful / input.4) Hence, η = W1 / (W1 + W2). Verification / Alternative check: If W2 = 0 (no losses), η = W1 / W1 = 1, which makes physical sense. As W2 increases, η decreases, also consistent with intuition. Why Other Options Are Wrong: (W1 + W2)/W1: >1 for any loss; efficiencies cannot exceed 1 in this definition. W2/(W1 + W2): This is a loss fraction, not efficiency. W1 - W2: Not nondimensional and can be negative; not an efficiency. W1/W2: A ratio of useful work to loss, not efficiency. Common Pitfalls: Confusing loss fraction with efficiency and forgetting to include all loss mechanisms in the energy balance when estimating actual power draw. Final Answer: η = W1 / (W1 + W2)

Question 6

In a two-stage reciprocating air compressor with an intercooler, how does the work saved with incomplete intercooling compare to that with complete (perfect) intercooling?

Accepted Answer

Introduction / Context: Intercooling between compression stages reduces the temperature before the next stage, thereby lowering the work required. The question distinguishes between complete (perfect) and incomplete intercooling and asks which saves more work. Given Data / Assumptions: Two stages, same mass flow. Intercooler effectiveness may be less than 100% for incomplete intercooling. Same final delivery pressure and overall pressure ratio. Concept / Approach: Compression work per kg is proportional to the enthalpy rise. Perfect intercooling restores the air to the initial suction temperature before the second stage, minimizing the second stage work. Any shortfall (incomplete intercooling) means higher inlet temperature into the second stage and therefore more work than the perfect case. Step-by-Step Solution: 1) For perfect intercooling: T2,in(second stage) ≈ T1,suction.2) For incomplete intercooling: T2,in(second stage) > T1,suction.3) Higher inlet temperature into a given pressure ratio stage raises the enthalpy rise and thus the work.4) Therefore, the work saved is maximized under perfect intercooling; any deviation saves less. Verification / Alternative check: Plotting compressor power vs. intercooler effectiveness shows monotonic decrease in power as effectiveness approaches 1. This corroborates that incomplete intercooling yields less work saving than perfect intercooling. Why Other Options Are Wrong: More than complete: Impossible since perfect intercooling is the limiting best case. Exactly the same: Only if incomplete effectiveness were 100%, which contradicts the definition. Independent of performance/Zero: Intercooler effectiveness directly affects power; not independent or zero. Common Pitfalls: Assuming diminishing returns eliminate benefit; while returns diminish, maximum saving still occurs at perfect intercooling. Final Answer: Less than with complete intercooling

Question 7

Centrifugal compressor impeller blade shape strongly affects stability and efficiency. Which statement best reflects the industry-standard behavior of backward-curved blades?

Accepted Answer

Introduction / Context: Impeller blade geometry (backward-curved, radial/straight, or forward-curved) determines the characteristic curves of centrifugal compressors and fans, affecting efficiency, surge margin, and power characteristics. This question checks recognition of the typical stability advantage of backward-curved blades. Given Data / Assumptions: Comparison among common blade shapes at similar operating conditions. Focus is on qualitative behavior regarding stability and efficiency trends. Concept / Approach: Backward-curved blades generally provide higher efficiency and a non-overloading power characteristic with a relatively flat or drooping pressure-flow curve that is more stable. Forward-curved blades tend to offer higher pressure rise at low speeds in fans but commonly at lower efficiency and with overloading characteristics, which can reduce stability in compressor applications. Step-by-Step Solution: 1) Identify target trait: stable performance (good surge margin, non-overloading power).2) Backward-curved blades are known for stable operation and high efficiency.3) Therefore, the statement that backward-curved blades lead to stable performance is the best choice. Verification / Alternative check: Typical compressor maps and design texts show backward-curved impellers used widely for their efficiency and stability advantages, especially in industrial compressors where surge avoidance is important. Why Other Options Are Wrong: Poor efficiency (backward-curved): Opposite of typical behavior. Forward-curved higher efficiency: Usually not; forward-curved are less efficient and more prone to overloading. Forward-curved lower pressure ratio: They often show higher head at low speeds; statement is misleading. Straight radial blades always highest efficiency: Not universally true; they can be robust but not necessarily most efficient. Common Pitfalls: Generalizing fan behavior to compressor maps without considering Mach effects and diffusion limits; ignoring surge margin and power characteristics when judging “performance.” Final Answer: Backward curved blades lead to stable performance

Question 8

Compared with single-stage compression, multi-stage air compression with intercooling offers several advantages. Which combination below correctly states the benefits of multi-stage compression?

Accepted Answer

Introduction / Context: Staging a compressor distributes the overall pressure ratio across multiple steps, often with intercooling between stages. This arrangement yields a set of practical and thermodynamic advantages that are widely exploited in industrial compressors. Given Data / Assumptions: Same overall delivery pressure and mass flow rate. Effective intercooling between stages. Mechanical design allows appropriate speed/torque matching. Concept / Approach: Intercooling reduces the inlet temperature to subsequent stages, lowering specific work. Splitting the ratio also improves volumetric efficiency because each stage sees lower discharge temperatures and reduced leakage/slip effects. Multiple stages smooth torque fluctuations in reciprocating machines and improve operating range for rotary machines. Step-by-Step Solution: 1) Work reduction: Intercooling lowers T before the next stage, decreasing enthalpy rise and shaft power.2) Volumetric efficiency: Cooler suction after intercooling raises density, improving volumetric efficiency.3) Torque uniformity: Distributing load across stages reduces torque pulsations in reciprocating compressors.4) Therefore, all listed benefits apply. Verification / Alternative check: Power vs. intercooler effectiveness curves show decreased power as effectiveness improves. Practical experience confirms higher volumetric efficiency from denser intercooled suction and smoother torque from multi-throw arrangements. Why Other Options Are Wrong: Options (a), (b), (c) alone: Each captures one benefit but ignores the others. None of these: Contradicts well-established compressor design practice. Common Pitfalls: Assuming only thermodynamic (work) benefit and overlooking mechanical/operational advantages; forgetting volumetric efficiency depends on suction temperature and clearance effects. Final Answer: All of these

Compressors, Gas Dynamics and Gas Turbines Questions