Question 1

Pressure–Velocity Compounding in Impulse Turbines – Impact on Stage Count In a pressure–velocity compounded impulse turbine, a larger allowable pressure drop per stage is achieved. How does this influence the number of stages needed for a given over…

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Introduction / Context: Compounding strategies—velocity compounding (Curtis), pressure compounding (Rateau), and pressure–velocity compounding—are used to manage blade speeds, stage efficiency, and practical rotor diameters in high-pressure steam turbines. Understanding their effect on the number of stages is central to preliminary layout and cost estimation. Given Data / Assumptions: Impulse principles: pressure drops mainly in stationary elements. Pressure–velocity compounding combines partial pressure drop in multiple nozzle sets with multiple rotor rows per pressure stage. Goal: achieve target overall expansion without excessive stage count or impractical blade speeds. Concept / Approach: Allowing a larger pressure drop in each compounded stage means more of the total expansion is handled per stage. Consequently, fewer stages are required to accommodate a given boiler-to-condenser pressure ratio. Velocity compounding within a stage also allows lower individual rotor-tip speeds for a given jet speed, aiding mechanical integrity while keeping stage count down compared with pure pressure compounding at the same limits. Step-by-Step Solution: Define objective: cover total pressure ratio with staged drops.Increase per-stage pressure drop via compounding.Result: fewer stages to span the same overall expansion.Select ‘‘less’’ stages as the correct qualitative outcome. Verification / Alternative check: Typical large turbines use combinations of compounding to balance efficiency and mechanical limits; design examples show reduced stage counts when greater per-stage drops are feasible without unacceptable losses. Why Other Options Are Wrong: ‘‘More’’ contradicts the logic of larger per-stage drop. ‘‘Unchanged’’ ignores the definition of compounding. Conditions such as partial admission and moisture do not reverse the fundamental trend. Common Pitfalls: Assuming compounding only affects velocity triangles; it directly influences allowable pressure distribution and hence stage count. Final Answer: less

Question 2

Velocity Compounding (Curtis) – Where Does the Expansion Occur? State whether, in a velocity-compounded impulse turbine stage, the steam expansion (pressure drop) takes place entirely in a nozzle or set of nozzles while the moving blade rows operate…

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Introduction / Context: Velocity compounding (Curtis staging) is a classic method to extract work from a very high-velocity jet without requiring impractically high rotor-tip speeds. Clarifying where the pressure drop occurs distinguishes Curtis stages from pressure-compounded (Rateau) arrangements and from reaction stages. Given Data / Assumptions: Impulse principle: pressure drop localized in stationary nozzles. Multiple rotor rows with intermediate redirecting fixed guide blades at near-constant pressure. Overall pressure drop for the Curtis stage produced by the nozzle set feeding the first rotor. Concept / Approach: In a velocity-compounded stage, the steam expands (experiences its pressure drop) in a single nozzle set (or grouped sets), emerging at high velocity. The kinetic energy is then extracted over multiple rotor rows separated by fixed guide blades that merely redirect the flow at approximately constant static pressure. No further intentional expansion occurs in the rotor passages, distinguishing Curtis from reaction designs. Thus, within the Curtis stage envelope, expansion is confined to the nozzles. Step-by-Step Solution: Recognize ‘‘impulse’’ hallmark: pressure drop in stator, not rotor.Add ‘‘velocity compounding’’: multiple rotor rows share one nozzle jet’s kinetic energy.Conclude: expansion is in the nozzle(s) only; rotor rows operate near constant pressure.Therefore, the statement is true. Verification / Alternative check: Textbook pressure/velocity diagrams across a Curtis stage show a single pressure drop across the nozzle, with flat pressure profiles across rotor and guide rows, while velocity steps down over successive rotors. Why Other Options Are Wrong: Reaction stages deliberately create pressure drops in rotors; Rateau (pressure compounding) divides pressure drops among multiple nozzle groups between single rotors, not within one velocity-compounded stage. Common Pitfalls: Confusing multi-stage turbines (many Curtis stages in series) with the internal distribution within a single Curtis stage; the truth here concerns the single-stage compounding principle. Final Answer: True

Question 3

Critical Pressure Ratio for Steam – Identify the Initial State from the Value 0.546 If the measured critical pressure ratio (static back pressure to upstream total/static as defined for choking) for steam is approximately 0.546, the steam at inlet i…

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Introduction / Context: The critical pressure ratio p*/p0 (or p2/p1 depending on convention) marks the onset of choking in nozzle flow. For steam, its value depends slightly on the thermodynamic state (wet, saturated, or superheated) because the effective specific heat ratio and real-gas effects vary with quality and temperature. Recognizing typical benchmark values helps infer inlet condition when only the ratio is known. Given Data / Assumptions: Nozzle or orifice with steady flow of steam to a lower-pressure region. Critical pressure ratio reported as about 0.546. Classical steam-nozzle practice where saturated vs. superheated states have slightly different characteristic ratios. Concept / Approach: Empirical and semi-theoretical treatments show that for steam initially dry saturated at the nozzle inlet, the critical pressure ratio is close to 0.546. For superheated steam, an often-quoted value is around 0.577 (reflecting an effective specific heat ratio nearer 1.3–1.33 rather than that typical at saturation). Wet steam values can differ further due to mixture properties. Therefore, observing 0.546 suggests the inlet is dry saturated steam. Step-by-Step Solution: Recall indicative values: saturated ~0.546; superheated ~0.577.Compare the given ratio (0.546) with these benchmarks.Conclude the inlet condition most consistent is dry saturated steam.Select ‘‘dry saturated steam’’ accordingly. Verification / Alternative check: Design handbooks and nozzle test data reproduce these ratios for standard conditions; deviations arise with significant moisture or high superheat, but 0.546 is widely associated with saturated entry. Why Other Options Are Wrong: ‘‘Superheated’’ corresponds to a higher critical ratio around 0.577. ‘‘Wet’’ is not tied to 0.546 as a typical value and, depending on quality, can show different behavior. ‘‘None of these’’ and ‘‘compressed liquid’’ do not match compressible vapor flow at choking. Common Pitfalls: Mixing up the definition (p*/p0 versus its inverse) and confusing air’s value (~0.528) with steam’s saturated value (~0.546). Always be clear about state and ratio orientation. Final Answer: dry saturated steam

Question 4

Parsons (50% reaction) turbine — minimum leaving kinetic energy A Parsons stage has blade peripheral speed U = 320 m/s at the mean radius, and the rotor-blade exit (relative) angle β2 = 30°. For minimum kinetic energy of steam leaving the stage (i.e…

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Introduction / Context: This problem tests steam-turbine velocity triangles for a Parsons (50% reaction) stage and the design condition for minimum leaving kinetic energy (LKE). Minimizing the absolute exit velocity reduces exhaust kinetic-energy losses and improves stage efficiency. The key geometric link is between blade speed, relative flow angle at rotor exit, and the absolute exit velocity. Given Data / Assumptions: Blade (peripheral) speed at mean radius U = 320 m/s. Rotor exit relative flow angle β2 = 30° (measured from the direction of wheel motion). Design for minimum LKE implies zero whirl component at exit, Vw2 = 0. Axial velocity is the absolute exit velocity when Vw2 = 0. Concept / Approach: For the rotor exit triangle: the relative velocity Vr2 leaves at angle β2 to wheel direction. Its tangential component is Vr2 * cosβ2 and axial component is Vr2 * sinβ2. The absolute tangential (whirl) component at exit is Vw2 = Vr2 * cosβ2 − U. Setting Vw2 = 0 gives Vr2 = U / cosβ2. Then the absolute exit speed equals its axial component Va2 = Vr2 * sinβ2 = U * tanβ2. Therefore, V2 = U * tanβ2 at the minimum LKE condition. Step-by-Step Solution: Write the exit-whirl condition: Vw2 = Vr2 * cosβ2 − U = 0.Solve for Vr2: Vr2 = U / cosβ2.Compute absolute exit velocity: V2 = Va2 = Vr2 * sinβ2 = (U / cosβ2) * sinβ2 = U * tanβ2.Substitute U = 320 m/s and β2 = 30°: tan30° ≈ 0.577 ⇒ V2 ≈ 320 * 0.577 ≈ 184.6 m/s ≈ 185 m/s. Verification / Alternative check: With Vw2 = 0, the leaving velocity is purely axial and minimized for the given U and β2. Any nonzero Vw2 would increase the magnitude of V2, raising exhaust losses (V2^2/2), confirming 185 m/s as the minimum-LKE result. Why Other Options Are Wrong: 213 m/s and 107 m/s correspond to other arbitrary combinations (e.g., 640/3 or 320/3) not supported by the zero-whirl condition. 640 m/s implies an unrealistically high exit speed. 355 m/s would require tanβ2 ≈ 1.11, not 0.577. Common Pitfalls: Mixing up β2 with an inlet angle, or using φ = U/V1 relationships mistakenly at exit. Also, confusing the “minimum LKE” condition with “maximum work” generally—here both align with Vw2 = 0 for the single stage. Final Answer: 185 m/s

Question 5

Impulse turbine expansion location In an impulse steam turbine stage, where does the expansion (pressure and enthalpy drop) of the steam occur?

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Introduction / Context: Steam turbines are broadly categorized as impulse or reaction based on where the pressure drop occurs. Identifying the expansion location is essential for drawing velocity triangles, estimating losses, and choosing appropriate blading for a given duty. Given Data / Assumptions: Single impulse stage: one row of fixed nozzles followed by a row of moving blades. Idealized impulse behavior: negligible pressure drop across moving blades. Steady flow with well-formed jets issued by the nozzles. Concept / Approach: In an impulse stage, the entire pressure (enthalpy) drop of the stage occurs in the nozzles, converting thermal energy into kinetic energy. The moving blades then redirect the high-speed jet, changing its momentum at nearly constant static pressure to extract shaft work. This is contrasted with reaction stages, where pressure drops occur in both fixed and moving blades. Step-by-Step Solution: Nozzles: pressure ↓, velocity ↑ (enthalpy converted to kinetic energy).Moving blades: pressure ≈ constant; jet is turned, momentum change produces rotor work.Therefore, in an impulse turbine, expansion is wholly in the nozzles. Verification / Alternative check: Static-pressure measurements across a classical De Laval or Curtis stage show major pressure drop across stators, negligible across rotors—consistent with impulse theory. Why Other Options Are Wrong: “Wholly in moving blades” describes neither impulse nor reaction correctly. “Partly in both” is reaction behavior. “None of these” ignores established definitions. Common Pitfalls: Assuming pressure must drop in the rotor to get work; impulse stages rely on momentum change at near-constant pressure in the rotor. Final Answer: Wholly in the nozzles (fixed blades)

Question 6

Definition check — convergent–divergent nozzle A nozzle is termed a convergent–divergent (C–D) nozzle when its cross-section decreases from inlet to throat and then increases from throat to exit. Is this definition correct?

Accepted Answer

Introduction / Context: Gas dynamics and steam-nozzle design use specific geometries to accelerate flow. The convergent–divergent (de Laval) nozzle is fundamental for achieving supersonic speeds after sonic choking at the throat. Recognizing the geometric definition is a prerequisite for understanding performance maps and shock behavior. Given Data / Assumptions: Single-phase compressible flow. Well-defined minimum-area section (throat). Isentropic analysis as a first approximation. Concept / Approach: In a C–D nozzle, flow is accelerated subsonically in the convergent section to Mach 1 at the throat under choked conditions, then further accelerated supersonically in the divergent section when back pressure is suitably low. Hence, the geometry necessarily converges to a throat and diverges thereafter. The statement given correctly captures this shape definition. Step-by-Step Solution: Identify the convergent entrance: area decreases, velocity rises for subsonic flow.At the throat: Mach = 1 when choked; mass flow is maximized for given upstream state.In the divergent exit: area increases; if properly expanded, velocity becomes supersonic. Verification / Alternative check: Rocket nozzles and turbine-stage nozzle blocks use C–D profiles to achieve design Mach numbers and pressure ratios, validating the definition in practice. Why Other Options Are Wrong: “False” would deny the standard textbook geometric meaning of a C–D nozzle. Common Pitfalls: Confusing “diverging duct” with diffuser; in compressible flow, function depends on Mach regime, not just geometry. Final Answer: True

Question 7

Ideal impulse turbine — velocity relationships In an ideal impulse turbine stage (neglecting friction), which equality holds true?

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Introduction / Context: Impulse-stage kinematics are captured by velocity triangles. With friction neglected, certain components remain unchanged across the rotor, while others must change to allow work extraction. Understanding which components are conserved clarifies stage design and performance estimation. Given Data / Assumptions: Ideal impulse rotor (no frictional losses in blades). Constant axial velocity assumption through the stage. Steady flow with well-defined velocity triangles. Concept / Approach: In an ideal impulse rotor, the magnitude of the relative velocity is unchanged across the rotor, i.e., Vr1 = Vr2, since no energy is added or removed in the rotor aside from direction change. Axial velocity is typically assumed constant (Va1 = Va2) for uniform flow area, while whirl components must change to produce torque: Vw1 ≠ Vw2 (indeed, Vw2 is often reduced toward zero for high efficiency). Absolute velocities generally differ in magnitude and direction between inlet and outlet due to the wheel speed and momentum change. Step-by-Step Solution: Recognize that with no friction: |Vr| is conserved through the rotor.Torque requirement demands ΔVw ≠ 0, so whirl cannot be equal.Absolute speeds differ because the vector sum with wheel speed changes between inlet and outlet.Therefore, the correct equality is Vr1 = Vr2. Verification / Alternative check: Many introductory derivations of diagram efficiency start from Vr1 = Vr2, Va1 = Va2, and analyze changes in Vw to compute work and power. Why Other Options Are Wrong: Absolute speeds equal (A) is not generally true; whirl equal (D) would imply zero work; (C) contradicts the common constant-axial-velocity design assumption. Common Pitfalls: Confusing absolute and relative components; forgetting that work in turbines comes from change in whirl velocity. Final Answer: Relative velocity at rotor inlet equals relative velocity at rotor exit

Question 8

Effect of steam bleeding (regenerative feed heating) What is the combined effect of steam bleeding (extraction for feedwater heating) in a Rankine-cycle turbine?

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Introduction / Context: Bleeding (extraction) routes a fraction of expanding steam from intermediate stages to feedwater heaters. This regenerative process improves the average temperature of heat addition in the boiler, raising thermal efficiency, but it also reduces the mass flow continuing to the low-pressure stages, lowering net power. Given Data / Assumptions: Rankine cycle with one or more regenerative feedwater heaters. Fixed main steam conditions at the boiler and fixed condenser pressure. Negligible pump work relative to turbine work for conceptual discussion. Concept / Approach: Raising feedwater temperature reduces the irreversibility of heat addition because heat is supplied at a higher average temperature. This increases cycle efficiency (less fuel per unit power). However, steam diverted to heaters no longer expands through downstream stages, reducing turbine power for the same boiler flow. Practical designs trade efficiency gains against hardware cost and reduced specific output. Step-by-Step Solution: Extract steam from an intermediate pressure to a heater; condense it to warm feedwater.Feedwater returns hotter to the boiler, increasing average heat-addition temperature.Because mass flow to later stages is lower, the turbine’s net work decreases.Overall cycle efficiency (net work / heat input) increases despite lower absolute power for a fixed boiler flow. Verification / Alternative check: TS-diagrams show reduced area of condenser heat rejection and elevated feedwater line, demonstrating improved thermal efficiency with regeneration. Why Other Options Are Wrong: Choosing any single effect ignores the coupled thermodynamic and mass-flow consequences; all listed effects occur together. Common Pitfalls: Expecting both higher efficiency and higher specific power; in basic regeneration, specific power usually drops when bleed fraction increases. Final Answer: All of the above

Question 9

Single-stage turbines and speed reduction A single-stage steam turbine is rarely used for large pressure drops because the required wheel speed becomes extremely high, necessitating large reduction gearing. Do you agree with this statement?

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Introduction / Context: Large pressure drops create very high nozzle exit velocities. To extract work efficiently in a single stage, blade peripheral speed must be a significant fraction of jet speed, pushing rotor r.p.m. to impractical values and demanding heavy speed-reduction gear trains. Multi-stage compounding resolves this issue. Given Data / Assumptions: High overall pressure ratio across the turbine. Single impulse stage with one nozzle row and one moving row. Generator speed and mechanical stresses impose limits on allowable rotor speed. Concept / Approach: For high-efficiency impulse operation, blade speed ratio φ = U/V1 is typically around 0.4–0.5. If V1 is several hundred m/s to over 1000 m/s for large drops, U must also be very large, yielding r.p.m. values incompatible with direct coupling to standard generators. Staging distributes the enthalpy drop, lowering per-stage velocities and enabling practical shaft speeds with better efficiency and reduced exit losses. Step-by-Step Solution: Relate nozzle speed to required blade speed via φ ≈ 0.4–0.5.Note that single-stage U then implies excessive r.p.m. for typical diameters.Conclude that reduction gearing would need to be large and inefficient; multi-stage designs avoid this. Verification / Alternative check: Historical Curtis wheels were used to reduce speed but still required multiple velocity steps; modern turbines universally use multistage arrangements for utility service. Why Other Options Are Wrong: “Disagree” ignores well-established design practice and speed limitations of rotors and generators. Common Pitfalls: Assuming only electrical frequency drives the choice; mechanical stress and tip-speed limits are equally important. Final Answer: Agree

Question 10

Choked flow facts — identify the wrong statement Which of the following statements about compressible flow in a steam/gas nozzle is wrong?

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Introduction / Context: Understanding choking and the role of nozzle geometry is central to turbine, rocket, and ejector design. Several statements are listed; only one contradicts fundamental gas-dynamics principles. Given Data / Assumptions: Steady, adiabatic, single-phase compressible flow. Converging–diverging nozzle with a well-defined throat. Isentropic flow used for ideal reasoning. Concept / Approach: Choking occurs when Mach = 1 at the throat; the corresponding downstream (back) pressure equals the critical pressure. Upstream of the throat in a convergent passage, the flow is subsonic. To achieve supersonic speeds, the flow must pass through a diverging section after reaching Mach 1 at the throat; a mere convergent nozzle cannot produce supersonic flow from subsonic inlet conditions. Step-by-Step Solution: Evaluate A: correct — critical condition sets throat velocity to sonic.Evaluate B: correct — subsonic accelerates in convergence up to Mach 1 at the throat.Evaluate C: correct — beyond the throat, a diverging section accelerates flow to supersonic if back pressure is low enough.Evaluate D: wrong — a divergent section is required to go beyond sonic; without it, a purely convergent nozzle cannot deliver supersonic outlet flow from subsonic inlet. Verification / Alternative check: Standard area–Mach relations mandate that for M > 1, area must increase; experimental data confirm that supersonic velocities occur only in diverging ducts past a sonic throat, not in a purely converging nozzle. Why Other Options Are Wrong: They are consistent with textbook compressible-flow theory and observed nozzle behavior. Common Pitfalls: Confusing throttling valves (constant area) with nozzles; mixing up “critical pressure” with “critical point” in thermodynamics. Final Answer: To exceed sonic velocity by expanding below the critical pressure, a divergent portion is not necessary

Question 11

Energy exchange in nozzles — heat transfer assumption During steady flow through a steam or gas nozzle, it is commonly assumed that no heat is supplied to or rejected by the fluid (adiabatic flow). Do you agree with this modelling assumption?

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Introduction / Context: Nozzles are designed to convert enthalpy into directed kinetic energy. For tractable analysis and because residence times are short with insulated casings, engineers often assume adiabatic flow (Q ≈ 0). This question checks understanding of that standard assumption. Given Data / Assumptions: Steady one-dimensional flow through a nozzle. Insulated walls and short flow residence time. No shaft work; negligible potential-energy change. Concept / Approach: The steady-flow energy equation reduces to h1 + V1^2/2 = h2 + V2^2/2 for adiabatic, no-work nozzles. Thus, the isentropic enthalpy drop approximates the rise in kinetic energy. While small heat losses may occur in practice, the first-order model treats the nozzle as adiabatic to predict velocities and areas accurately enough for design. Step-by-Step Solution: Assume Q ≈ 0 and W_s = 0; write SFEE neglecting PE change.Solve for velocity change using Δh ≈ −Δ(KE): V2^2/2 − V1^2/2 ≈ h1 − h2.Use isentropic relations to compute area–Mach numbers for given pressures. Verification / Alternative check: Measured nozzle efficiencies close to unity in well-designed stages justify the adiabatic approach; deviations are handled via a nozzle efficiency factor based on exit kinetic energy vs. ideal enthalpy drop. Why Other Options Are Wrong: “Disagree” would claim that heat transfer is essential to the model; while possible, it is secondary and usually neglected for primary sizing and performance predictions. Common Pitfalls: Confusing adiabatic with isentropic; adiabatic flow can still have entropy generation due to friction, so nozzle efficiency may be less than 1. Final Answer: Agree

Question 12

Steam turbines — select the correct statement Which of the following statements about steam turbines and their operation is correct?

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Introduction / Context: Steam turbines replaced reciprocating steam engines in power generation due to superior efficiency, compactness, and smoother rotation. This question probes basic qualitative understanding of turbine behavior versus engines and the role of blades in momentum exchange. Given Data / Assumptions: Comparison at similar rating and modern design practice. Standard impulse or reaction blading assumptions. Well-designed governor and control systems. Concept / Approach: Turbines achieve higher isentropic and overall efficiencies than piston engines because they admit continuous flow with lower mechanical friction, better expansion matching, and multistaging. Turbines also deliver nearly steady torque (so large flywheels are unnecessary), and their blades are explicitly shaped to change the steam’s direction, producing a momentum change and hence work. In reaction turbines, pressure drops in both stator and rotor; it does not increase. Step-by-Step Solution: Evaluate A: correct — turbines are typically more efficient than reciprocating engines for power generation.Evaluate B: incorrect — turbines have smooth torque; large flywheels are generally not required.Evaluate C: incorrect — blades turn and redirect flow to extract work.Evaluate D: incorrect — in reaction stages, pressure decreases in both fixed and moving blades. Verification / Alternative check: Utility-scale steam plants universally use turbines; piston engines are obsolete for large-scale continuous power due to lower efficiency and maintenance demands. Why Other Options Are Wrong: They conflict with fundamental turbine mechanics and standard stage descriptions. Common Pitfalls: Assuming all rotating machines need flywheels; misinterpreting “reaction” as pressure rise rather than pressure drop in the rotor. Final Answer: Steam turbines generally achieve higher efficiencies than reciprocating steam engines

Question 13

Steam turbine classifications Steam turbines can be classified on multiple bases. Which grouping below correctly lists common classification criteria?

Accepted Answer

Introduction / Context: Correctly classifying turbines helps in selecting the right machine for a duty and in understanding performance maps. Steam turbines are grouped by how steam moves, how many stages share the total enthalpy drop, and whether expansion occurs only in stators (impulse) or in both stators and rotors (reaction). Given Data / Assumptions: Industrial and utility steam-turbine practice. Standard terminology in turbomachinery. Single- or multi-cylinder configurations may exist under each category. Concept / Approach: Common classification axes include: (1) direction of flow (axial, radial-inflow, mixed), (2) number of stages (affecting speed and efficiency), and (3) mode of action (impulse versus reaction). Many real machines are axial multi-stage with mixed impulse–reaction blading, but the categories remain foundational for study and specification. Step-by-Step Solution: Review flow direction: axial is most common; radial-inflow is used in small stages; mixed combines components.Review staging: single-stage for small drops and controls; multi-stage for large pressure ratios and high efficiency.Review mode of action: impulse stages expand in nozzles only; reaction stages expand in both rows.Hence, all listed criteria are valid classification bases. Verification / Alternative check: Manufacturers’ catalogs and standards specify turbines by these axes (e.g., axial multi-stage reaction machine for condensing service). Why Other Options Are Wrong: Picking a single axis omits other equally standard bases used in engineering documentation. Common Pitfalls: Assuming “impulse” or “reaction” refers to flow direction; they are independent concepts. Final Answer: All of these

Question 14

Steam nozzles and metastable expansion: In a convergent–divergent nozzle, if the expansion is supersaturated (condensation is delayed), the effective dryness fraction of the steam within the nozzle will tend to be ________ compared with the equilibr…

Accepted Answer

Introduction / Context: When wet steam expands rapidly in a nozzle, condensation may not occur instantly even when the equilibrium saturation line is crossed. This metastable phenomenon is called supersaturation. Understanding how supersaturation affects the dryness fraction (mass fraction of vapour) is important for predicting exit velocity, heat drop, and erosion risks in steam turbines and ejectors. Given Data / Assumptions: Flow is through a nozzle with rapid pressure drop. Condensation kinetics are finite; nucleation and droplet growth take time. Comparison is at the same static pressure between supersaturated and equilibrium states. Concept / Approach: In equilibrium expansion, part of the vapour condenses as pressure and temperature fall, producing a wet mixture with lower dryness fraction. In supersaturated expansion, condensation is delayed; the steam remains vapour for longer, so at a given pressure it is effectively drier (higher dryness fraction) than the equilibrium wet mixture. Because fewer liquid droplets exist, the specific volume is slightly larger and the isentropic-like heat drop available for kinetic energy tends to increase. Step-by-Step Solution: Recognize that equilibrium wet expansion decreases dryness fraction as pressure falls.Account for delayed condensation: metastable vapour persists → fewer droplets.Conclude that the dryness fraction in the nozzle under supersaturation is higher than the equilibrium value at the same pressure. Verification / Alternative check: Classical nozzle experiments (Wilson line) show onset of condensation delayed below the saturation line, confirming higher effective dryness fraction until nucleation occurs downstream. Why Other Options Are Wrong: No: Opposite of observed metastable behaviour. Depends only on inlet superheat: Kinetics and rate of pressure drop matter; it is not solely a function of initial superheat. Cannot be predicted: While exact onset is complex, the qualitative trend (higher dryness fraction before nucleation) is well established. Common Pitfalls: Confusing supersaturated (metastable) expansion with superheated steam; supersaturation refers to delayed condensation below saturation temperature, not simply high temperature above saturation. Final Answer: Yes

Question 15

Effect of supersaturation inside a steam nozzle: During metastable expansion (supersaturation), which of the following qualitative effects is observed within the nozzle?

Accepted Answer

Introduction / Context: Supersaturation occurs in fast nozzle expansions when condensation lags behind the thermodynamic equilibrium. Designers care about how this affects available energy conversion, velocities, and blade erosion downstream. One key qualitative outcome is the change in effective heat drop between inlet and exit conditions in the flow passage. Given Data / Assumptions: Rapid expansion of steam through a nozzle. Condensation kinetics delayed (metastable vapour persists). Comparison at nominally similar inlet and outlet pressures to equilibrium cases. Concept / Approach: If condensation is delayed, less latent heat is released inside the nozzle during the same pressure decrease. The flow retains higher vapour content (higher dryness fraction). This effectively increases the portion of enthalpy drop available for conversion into kinetic energy across the nozzle, often described as an increase in the heat drop and hence a higher ideal exit velocity compared with the fully equilibrated wet expansion at the same pressure ratio. Step-by-Step Solution: Supersaturation → delayed droplet formation → mixture stays drier.Less latent heat release inside the nozzle for the same pressure fall.Higher effective enthalpy drop converts to kinetic energy → higher exit velocity. Verification / Alternative check: Wilson line observations and measured pressure-velocity profiles in steam nozzles show increased velocity (and thus greater effective heat drop) before spontaneous condensation sets in and forms a visible fog region downstream. Why Other Options Are Wrong: Decrease dryness fraction: Supersaturation raises, not lowers, dryness fraction prior to nucleation. Decrease specific volume: Drier mixture has, if anything, higher specific volume at the same pressure. Increase entropy: The metastable path is not an isentropic improvement; entropy generation is primarily associated with non-equilibrium and shock/viscous effects, but the simple qualitative hallmark is increased effective heat drop, not a guaranteed entropy rise along the nozzle passage considered ideally. Common Pitfalls: Assuming “more heat drop” always means more efficiency; once nucleation occurs, droplet drag and shocks may introduce losses. Final Answer: increase the heat drop

Question 16

Parsons (50% reaction) stage geometry: For a pure Parsons reaction turbine stage, if α1 and α2 are the fixed (stator) blade inlet and exit angles, and β1 and β2 are the moving (rotor) blade inlet and exit angles, identify the correct angular relatio…

Accepted Answer

Introduction / Context: Parsons turbines are designed for 50% degree of reaction with symmetrical velocity triangles. Recognizing the symmetry between stator and rotor flow angles is vital for quick sketches of velocity diagrams and for understanding how reaction is shared between fixed and moving rows. Given Data / Assumptions: Idealized 50% reaction stage (Parsons stage). Negligible profile and leakage losses for geometric relations. Axial velocity approximately constant across the stage. Concept / Approach: In a Parsons stage, the velocity triangles are geometrically similar and mirror each other about the axial direction. This leads to equality of corresponding angles: the stator exit angle equals the rotor inlet angle, and the stator inlet angle equals the rotor exit angle. Hence α1 (stator inlet) = β2 (rotor exit) and α2 (stator exit) = β1 (rotor inlet). This symmetry underpins the 50% reaction distribution between stator and rotor. Step-by-Step Solution: Construct the ideal stator exit and rotor inlet velocity triangles with equal axial components.Apply 50% reaction symmetry → corresponding angles equal in magnitude.Read off relations: α2 = β1 and α1 = β2. Verification / Alternative check: Textbook Parsons stage diagrams confirm mirrored triangles about the axial line, giving the same equal-angle relations stated above. Why Other Options Are Wrong: α1 = α2 and β1 = β2: Implies no flow turning in rows, which is incorrect. α1 = β1 and α2 = β2: Confuses inlet/exit pairings; these are generally not equal. α1 < β1 and α2 > β2: No general rule of this type applies for a pure Parsons stage. Common Pitfalls: Mixing absolute (α) and relative (β) angle definitions; ensure α refers to absolute flow angles at stator, β to relative flow angles at rotor. Final Answer: α1 = β2 and β1 = α2

Question 17

Nozzle classification by area change: A steam nozzle is termed a convergent nozzle when its cross-sectional area from inlet to outlet ________ continuously.

Accepted Answer

Introduction / Context: Nozzles are classified by how their flow area changes along the axis. This classification dictates acceleration or deceleration for subsonic and supersonic regimes and is foundational for turbine and rocket nozzle design. Given Data / Assumptions: Steam/gas as a compressible working fluid. Geometry alone defines the term (independent of actual Mach number attained). Convergent means monotonically decreasing area. Concept / Approach: A convergent nozzle has a decreasing cross-sectional area from entrance to exit. In subsonic flow, such a duct accelerates the fluid; the maximum Mach number attainable with a purely convergent nozzle is M = 1 at the exit if the flow is choked. To reach supersonic speeds, a divergent section must follow a throat (convergent–divergent, or De Laval nozzle). Step-by-Step Solution: Identify definition: convergent → area decreases along flow direction.Recall area–Mach relations: subsonic acceleration in convergent passages.Distinguish from C–D nozzle required for supersonic acceleration. Verification / Alternative check: Classic compressible-flow equations show that for subsonic flow, dV > 0 when dA < 0, consistent with the definition. Why Other Options Are Wrong: Increases: That is a divergent nozzle. First increases then decreases: Reverse of a C–D nozzle. Remains constant: That is a constant-area duct, not a nozzle. Common Pitfalls: Assuming convergence alone guarantees M > 1; a divergent section after a throat is required for supersonic acceleration. Final Answer: decreases

Question 18

Simple impulse turbine optimum speed ratio: For a simple impulse stage with nozzle exit speed V at angle α to the wheel tangent, the rotor (diagram) efficiency is maximum when the blade speed Vb equals ________.

Accepted Answer

Introduction / Context: In a simple impulse stage, the pressure drop occurs in the nozzle and the rotor primarily turns the flow to extract work. The rotor (diagram) efficiency depends on the blade speed ratio and the nozzle exit angle. There is an optimal blade speed that maximizes this efficiency for given V and α. Given Data / Assumptions: Impulse stage, negligible losses in ideal analysis. Nozzle exit absolute velocity magnitude V at angle α to the tangent. Constant axial velocity through the rotor, symmetric turning for the ideal case. Concept / Approach: Maximizing rotor efficiency amounts to maximizing the work extracted per unit kinetic energy at rotor inlet. The classical result for the ideal impulse stage gives the optimum blade speed ratio φ_opt = Vb/V = (cos α)/2. Therefore Vb = 0.5 * V * cos α at the condition for maximum diagram efficiency. Step-by-Step Solution: Write work ∝ change in whirl component: ΔVw depends on V, α, Vb.Express efficiency in terms of φ = Vb/V and cos α.Differentiate with respect to φ and set derivative to zero → φ_opt = cos α / 2.Thus Vb,opt = 0.5 V cos α. Verification / Alternative check: Substituting φ_opt back into the expression yields the well-known maximum ideal rotor efficiency η_r,max = cos^2 α, consistent with impulse stage theory when inlet and outlet relative flow angles are equal and friction is neglected. Why Other Options Are Wrong: Vb = V cos α: Too high; would not maximize rotor efficiency. Terms with V^2: Dimensionally incorrect for a speed; these combine velocity and velocity squared improperly. Common Pitfalls: Confusing the optimum for rotor efficiency with overall stage efficiency, which also depends on nozzle and mechanical losses. Final Answer: Vb = 0.5 V cos α

Question 19

Thermodynamic path inside a steam nozzle: The expansion of steam through a well-insulated nozzle is best idealized as which type of thermodynamic behaviour?

Accepted Answer

Introduction / Context: Although a nozzle is not a thermodynamic cycle, its internal expansion is typically modeled as adiabatic and with negligible shaft work. In steam power plants, the overall process belongs to the Rankine cycle, and the nozzle segment is treated as an isentropic (or near-isentropic) expansion for engineering calculations. Given Data / Assumptions: Well-insulated nozzle (adiabatic), negligible heat exchange with surroundings. No shaft work; changes in kinetic energy dominate. Steam plant context where the broader process is the Rankine cycle. Concept / Approach: Within the Rankine cycle, both nozzle and turbine expansions are commonly approximated as isentropic for ideal analysis. While the Carnot, Joule (Brayton), and Stirling cycles involve different idealized processes (isothermal or constant-pressure heat addition, etc.), the steam plant context aligns the nozzle expansion with the Rankine framework and its isentropic expansion assumption. Step-by-Step Solution: Recognize the plant-level cycle is Rankine.Identify the nozzle segment as an (approximately) isentropic expansion element within that cycle.Therefore, in cycle classification terms, it is associated with the Rankine cycle. Verification / Alternative check: Textbook T–s diagrams of Rankine show expansion lines (turbine/nozzle) modeled as vertical (isentropic) for the ideal case, reflecting the same assumption applied to nozzle flows. Why Other Options Are Wrong: Carnot: Requires isothermal heat addition and rejection; not applicable to practical steam plants. Joule (Brayton): Constant-pressure heat addition for gas turbines. Stirling: Isothermal processes with regeneration; not representative of water–steam cycles. Common Pitfalls: Interpreting the question as a pure process classification; strictly speaking a nozzle process is not a cycle, but in steam plant context its idealization belongs under Rankine. Final Answer: Rankine cycle

Question 20

Definition check — stage efficiency in steam turbines: Stage efficiency is defined as the ratio of the energy transferred to (and thus available for work in) the rotor blades per kilogram of steam to the total ideal energy supplied across the entire…

Accepted Answer

Introduction / Context: Several efficiency definitions exist in turbomachinery: nozzle efficiency, blade (or rotor/diagram) efficiency, stage efficiency, and overall (isentropic) efficiency. Being precise about stage efficiency helps engineers compare real stages against their ideal energy conversion potential. Given Data / Assumptions: One complete stage comprises a fixed (stator/nozzle) row and a moving (rotor) row. Ideal reference is the isentropic energy drop across the whole stage. Losses occur in stator and rotor; measured output is the work actually transferred to the rotor. Concept / Approach: Stage efficiency (sometimes called isentropic efficiency of the stage) is defined as: η_stage = (actual work output per kg across the stage) / (ideal isentropic enthalpy drop per kg across the stage). Since the actual work output equals the energy effectively supplied to and extracted by the rotor blades, the verbal statement aligns with the formal definition. This measure includes losses in both fixed and moving rows within that stage. Step-by-Step Solution: Identify numerator: actual rotor work per kg (energy transferred to blades).Identify denominator: total isentropic energy drop for that stage.Form the ratio → definition of stage efficiency. Verification / Alternative check: On T–s diagrams, the ideal vertical (isentropic) drop represents the denominator, while the actual expansion curve enclosing a smaller area corresponds to the numerator (after accounting for exit kinetic energy and losses). The ratio gives η_stage. Why Other Options Are Wrong: False: Conflicts with standard turbomachinery definitions. Only true for impulse/reaction stages: The definition applies to both; the isentropic reference and measured work are stage-level quantities regardless of degree of reaction. Common Pitfalls: Mixing stage efficiency with diagram (rotor) efficiency, which uses kinetic energy at rotor inlet as the reference rather than the stage isentropic drop. Final Answer: True

Steam Nozzles and Turbines Questions