Question 1

Ideal Gas Relation Between Specific Heats and Gas Constant For a calorically perfect gas, the gas constant R equals the difference between the specific heats at constant pressure and constant volume: R = Cp − Cv.

Accepted Answer

Introduction / Context: Linking specific heats to the gas constant is foundational for ideal-gas property relations and for deriving many isentropic and energy equations used in thermodynamics and compressible flow. Given Data / Assumptions: Ideal-gas behavior with constant specific heats (calorically perfect). Definitions: Cp is at constant pressure, Cv at constant volume. R is the specific gas constant (per unit mass). Concept / Approach: The Mayer relation gives Cp − Cv = R. Physically, Cp exceeds Cv because at constant pressure the gas must do boundary work during heating, requiring extra heat input per unit temperature rise compared to constant-volume heating. Step-by-Step Solution: Start from h = u + pv for ideal gases (with h = CpT, u = CvT).Differentiate: dh/dT − du/dT = vdp/dT + pdv/dT.For ideal gas, pv = R*T and manipulations lead to Cp − Cv = R.Conclude R equals the difference of the two specific heats. Verification / Alternative check: For air near room temperature: Cp ≈ 1.005 kJ/kg-K, Cv ≈ 0.718 kJ/kg-K, difference ≈ 0.287 kJ/kg-K, matching R for air. Why Other Options Are Wrong: Sum, product, or ratio do not appear in the Mayer relation and have no general meaning for R. Geometric mean is irrelevant. Common Pitfalls: Using molar quantities without noting units; mixing universal gas constant Ru with specific R (R = Ru/M). Final Answer: difference

Question 2

Second Law – Choose the Correct Formal Statements Select the correct formulation(s) of the second law of thermodynamics. The Kelvin–Planck engine statement and the Clausius heat-pump statement are both valid and equivalent expressions.

Accepted Answer

Introduction / Context: The second law can be stated in multiple, equivalent ways. Two classical forms are the Kelvin–Planck statement (about the impossibility of 100% heat-to-work conversion in a cyclic engine) and the Clausius statement (about the impossibility of heat flowing spontaneously from cold to hot without external aid). Given Data / Assumptions: Cyclic devices interacting with thermal reservoirs. Macroscopic classical thermodynamics; no statistical exceptions considered. Standard sign conventions for heat and work. Concept / Approach: Statement (a) is the Kelvin–Planck form: no engine can completely convert heat from a single reservoir to work. Statement (b) is the Clausius form: heat cannot, of itself, flow from colder to hotter bodies. These two are logically equivalent; violating one enables violation of the other. Statement (c) is not a correct second-law statement; it resembles an inaccurate interpretation of the mechanical equivalent of heat and ignores the second-law efficiency limits. Step-by-Step Solution: Evaluate (a): matches Kelvin–Planck → valid.Evaluate (b): matches Clausius → valid.Evaluate (c): incorrect; the second law does not guarantee a definite full conversion of heat to work.Therefore, choose the option asserting both (a) and (b) are correct. Verification / Alternative check: Textbook proofs show that a hypothetical device violating Kelvin–Planck can be used to construct a device violating Clausius, and vice versa, establishing equivalence. Why Other Options Are Wrong: Option (e) includes (c), which is false. Picking only (a) or only (b) ignores the presence of the other correct statement. Common Pitfalls: Confusing first-law energy conversion statements with second-law limitations; assuming (c) is a conservation statement when it actually misstates second-law restrictions. Final Answer: Both (a) and (b) are correct second-law statements.

Question 3

Carnot Cycle – Process During Which Heat Is Rejected In the ideal Carnot refrigeration/heat-pump representation reversed, heat rejection in the forward (heat-engine) Carnot cycle occurs during the isothermal compression at the higher reservoir; conv…

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Introduction / Context: The Carnot cycle consists of two isothermal and two isentropic processes and sets the theoretical upper bound on heat-engine efficiency. Identifying where heat is added and rejected is essential for understanding temperature-limit performance. Given Data / Assumptions: Reversible processes with ideal gas as working fluid for visualization. Heat addition at the higher temperature T_H; heat rejection at the lower temperature T_L. No internal irreversibilities; quasi-static processes. Concept / Approach: In the forward heat-engine Carnot cycle: 1–2 isothermal expansion at T_H (heat addition), 2–3 isentropic expansion, 3–4 isothermal compression at T_L (heat rejection), 4–1 isentropic compression. Thus, heat rejection occurs during the isothermal compression at the low-temperature reservoir. Many texts simply say: heat is rejected isothermally during compression. Step-by-Step Solution: List processes and their heat interactions.Identify the low-temperature isotherm (compression) as the segment with Q_out.Select the option that states isothermal compression. Verification / Alternative check: p–v and T–s diagrams of the Carnot cycle clearly show the Q_out segment aligned with the lower isotherm during compression. Why Other Options Are Wrong: Isothermal expansion is the Q_in leg. The isentropic legs are adiabatic (no heat transfer). Polytropic compression is not part of the ideal Carnot sequence. Common Pitfalls: Mixing the forward (engine) and reversed (refrigerator) directions; assuming both isotherms have heat addition, whereas only the high-temperature isotherm adds heat in the engine mode. Final Answer: Isothermal compression

Question 4

Basic Property Identification – Hydrogen Compared to Other Substances Among chemical elements and common substances, hydrogen is the lightest in terms of density at standard conditions.

Accepted Answer

Introduction / Context: Recognizing basic physical properties such as relative density helps in safety, storage, and application decisions. Hydrogen's low density influences buoyancy, diffusion rates, and its use as a lifting gas and potential energy carrier. Given Data / Assumptions: Standard ambient conditions (near 1 atm, room temperature). Comparison across common elements and gases. Density basis by mass per unit volume. Concept / Approach: Hydrogen (H2) has a very low molecular mass (approximately 2 g/mol), leading to extremely low density compared with air (≈ 29 g/mol average) or other gases. Consequently, a hydrogen-filled balloon experiences strong buoyant force in air and rises rapidly. This property also impacts storage design (e.g., high-pressure tanks) and safety (rapid dispersion). Step-by-Step Solution: Compare molecular masses: H2 ≈ 2 g/mol, helium ≈ 4 g/mol, nitrogen ≈ 28 g/mol, oxygen ≈ 32 g/mol.Relate molar mass to gas density via ideal gas relation: rho = p * M / (R_u * T).Lower M implies lower rho at the same p and T; hydrogen therefore has the smallest density.Conclude that hydrogen is the lightest common substance/gas under comparable conditions. Verification / Alternative check: Empirical density at 1 atm, 0°C: hydrogen ≈ 0.0899 kg/m^3; air ≈ 1.29 kg/m^3; helium ≈ 0.1785 kg/m^3, confirming hydrogen's status as lightest among these. Why Other Options Are Wrong: Heaviest/most dense contradicts data. Viscosity and volatility are different properties and do not define “lightest”. Common Pitfalls: Confusing helium with hydrogen in lifting applications; overlooking safety concerns like flammability despite the low density benefit. Final Answer: lightest

Question 5

Fuels and Energy Content — Comparing Calorific Value by Fuel Type In practical thermal engineering, is the mass-based calorific value (MJ/kg) of typical liquid fuels (e.g., petrol/gasoline, diesel, kerosene) generally higher than that of common soli…

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Introduction: The calorific value of a fuel is the heat released by complete combustion of a unit mass (or volume). Comparing typical liquid and solid fuels helps in selecting fuel for boilers, engines, furnaces, and process heaters. The question probes basic fuel science: do common liquid fuels usually exhibit higher calorific value per kilogram than widely used solid fuels? Given Data / Assumptions: Representative liquid fuels: petrol/gasoline, diesel, kerosene, furnace oil. Representative solid fuels: air-dried wood, lignite/sub-bituminous coal, bituminous coal, coke, agricultural residues. Comparison on a mass basis (MJ/kg), neglecting minor variability due to additives and moisture beyond typical handling conditions. Concept / Approach: Calorific value depends on elemental composition, especially carbon and hydrogen content, and on moisture/ash fractions. Liquids refined from petroleum tend to have high hydrogen-to-carbon ratios and low ash, which raise the higher heating value (HHV) per kilogram. Many solid fuels (wood, low-grade coal, biomass) carry higher inherent moisture and ash, lowering effective MJ/kg. Step-by-Step Solution: Typical HHV ranges: gasoline ≈ 44–46 MJ/kg; diesel ≈ 44–46 MJ/kg; kerosene ≈ 43–46 MJ/kg.Typical solids: wood (air-dried) ≈ 15–18 MJ/kg; lignite ≈ 10–20 MJ/kg; sub-bituminous/bituminous coal ≈ 20–32 MJ/kg; metallurgical coke ≈ 28–31 MJ/kg.Observation: For the mainstream solid fuels used widely, the MJ/kg is commonly lower than for petroleum liquids. Hence the general statement that liquid fuels have higher calorific value than solid fuels is correct in most practical contexts. Verification / Alternative check: Fuel specification sheets and handbooks show petroleum liquids with low ash/moisture and high hydrogen content, which elevate HHV. Even when some high-grade coals approach 32 MJ/kg, they still seldom exceed typical petroleum liquids on a mass basis. Why Other Options Are Wrong: Choosing “No” would contradict the majority of field data for commonly used fuels. While niche solids (e.g., certain synthetic char) can be energy-dense, the general engineering rule remains that liquids are higher on a mass basis. Common Pitfalls: Mixing mass-based MJ/kg with volume-based MJ/L (where densities differ); ignoring moisture corrections for biomass; assuming all coals have identical quality. Final Answer: Yes

Question 6

Thermodynamic Processes — Are Isothermal and Isentropic Necessarily Reversible Non-Flow? Consider the statements: “Isothermal processes” and “isentropic processes” are reversible non-flow processes. Evaluate the correctness of this claim for general…

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Introduction: Engineers frequently idealize processes as isothermal (constant temperature) or isentropic (constant entropy). The question tests whether these labels inherently imply “reversible non-flow,” which is often assumed in classroom examples but is not universally true in practice. Given Data / Assumptions: No restriction to closed (non-flow) systems is given. No guarantee of reversibility is stated; real devices may be irreversible. Processes may occur in closed systems (piston–cylinder) or open systems (turbines, compressors, nozzles). Concept / Approach: Isothermal simply means constant temperature. A process can be isothermal yet irreversible (e.g., finite temperature difference heat transfer, throttling of an ideal gas that keeps T roughly constant). Isentropic means constant entropy; in engineering, “isentropic” typically implies an ideal reversible adiabatic model, but real devices only approximate it and can be open-flow (e.g., turbines and compressors). Therefore, neither term inherently enforces “reversible non-flow” across all contexts. Step-by-Step Solution: Identify the logical claim: both process types “are reversible non-flow.”Counterexample 1 (isothermal): Gas expansion against a finite pressure drop with heat exchange at finite temperature difference is isothermal but irreversible.Counterexample 2 (isentropic): Turbine expansion is modeled as isentropic in an open steady-flow system, not a non-flow system; real expansions are not perfectly reversible. Verification / Alternative check: Textbook cycle analyses use isentropic efficiencies precisely because real devices deviate from reversibility. Similarly, isothermal compression in practice requires intercooling with irreversibilities, not perfect reversibility. Why Other Options Are Wrong: “Agree” would mistakenly equate the property labels with a specific mode (non-flow) and idealization (reversibility) that is not inherently required. Common Pitfalls: Assuming “isentropic” automatically equals “reversible adiabatic in a closed system”; forgetting that isothermal/isentropic can describe both closed and open systems. Final Answer: Disagree

Question 7

Basic Properties of Gases — Identifying Minimum Molecular Mass Among the following common gases, which one has the lowest molecular mass (kg/kmol or g/mol): Oxygen (O2), Nitrogen (N2), Hydrogen (H2), Methane (CH4)?

Accepted Answer

Introduction: Molecular mass (molar mass) is a foundational concept that influences gas properties such as the gas constant R_specific, speed of sound, and diffusion rates. This question checks quick recall of molar masses for common gases used in thermodynamics and fluid mechanics problems. Given Data / Assumptions: Diatomic molecules for oxygen and nitrogen under standard conditions (O2, N2). Diatomic hydrogen (H2) and methane as CH4. Standard atomic masses are used for estimation. Concept / Approach: Calculate or recall typical molar masses: H ≈ 1 g/mol, C ≈ 12 g/mol, N ≈ 14 g/mol, O ≈ 16 g/mol. Combine these according to molecular composition to obtain the molecular masses of the listed gases and identify the minimum value. Step-by-Step Solution: M(O2) = 2 * 16 = 32 g/mol.M(N2) = 2 * 14 = 28 g/mol.M(H2) = 2 * 1 = 2 g/mol.M(CH4) = 12 + 4 * 1 = 16 g/mol.Compare: the smallest is clearly hydrogen at 2 g/mol. Verification / Alternative check: Because R_specific = R_universal / M, hydrogen's very small molar mass yields the largest specific gas constant and very high molecule speeds, consistent with the identification. Why Other Options Are Wrong: Oxygen (32 g/mol), nitrogen (28 g/mol), and methane (16 g/mol) are all much heavier than hydrogen (2 g/mol). Common Pitfalls: Confusing atomic and molecular masses (e.g., taking H as 1 g/mol and forgetting the diatomic nature H2); mixing up methane's composition. Final Answer: Hydrogen

Question 8

Power Cycles — Composition of the Ericsson Cycle Does the ideal Ericsson cycle consist of two constant-pressure (isobaric) processes and two isothermal processes with regeneration?

Accepted Answer

Introduction: The Ericsson cycle is an external-combustion or gas-cycle model emphasizing heat regeneration. Understanding its process composition helps distinguish it from Brayton (Joule) and Stirling cycles and clarifies where ideal efficiency originates. Given Data / Assumptions: Idealized, reversible processes. Perfect regenerator with no losses. Working fluid treated as an ideal gas. Concept / Approach: In the Ericsson cycle, heat addition and rejection occur isothermally, while compression and expansion occur at constant pressure with regeneration. The regeneration transfers heat from the exhaust isobar to the intake isobar, theoretically enabling Carnot efficiency between the same temperature limits when regeneration is perfect. Step-by-Step Solution: Identify the four processes: (1) isothermal heat addition at high temperature, (2) isobaric expansion with heat exchange in regenerator, (3) isothermal heat rejection at low temperature, (4) isobaric compression with regenerator heat recovery.Confirm that two of these are isothermal and two are isobaric.Hence the statement is correct. Verification / Alternative check: Comparing with the Brayton (Joule) cycle (two isobars + two isentropes) and the Stirling cycle (two isothermals + two isochoric processes) further confirms the Ericsson composition. Why Other Options Are Wrong: “Disagree” would conflate Ericsson with Brayton or Stirling and ignore the defining isothermal segments with regeneration. Common Pitfalls: Omitting the regenerator in the ideal description; mixing constant-volume with constant-pressure segments. Final Answer: Agree

Question 9

Basic Chemistry for Engineers — Definition of an Element What do we call substances that have not yet been resolved into any simpler substances by chemical means and are regarded as fundamental in composition?

Accepted Answer

Introduction: Engineers routinely encounter material classifications. Knowing the difference between elements, compounds, atoms, and molecules is essential for material selection, combustion calculations, and corrosion analysis. This question focuses on the foundational definition of an element. Given Data / Assumptions: Chemical decomposition methods (e.g., electrolysis, thermal decomposition) are considered. Modern periodic table context is implied. Macroscopic samples comprised of a single chemical species are under consideration. Concept / Approach: An element is a pure substance consisting of only one type of atom; it cannot be chemically broken down into simpler substances. Compounds are chemical combinations of two or more elements in fixed proportions. Atoms are the smallest units of elements that retain chemical identity. Molecules are assemblies of atoms bonded together; they can be elemental (e.g., O2) or compound (e.g., H2O). Step-by-Step Solution: Identify the phrase “cannot be resolved into simpler substances by chemical means.”Match to the definition of “element,” not to “compound,” “atom,” or “molecule.”Therefore, the correct term is “elements.” Verification / Alternative check: Examples: Copper, sulfur, and oxygen are elements; water (H2O) is a compound of hydrogen and oxygen; O2 is a molecular form of the element oxygen. Why Other Options Are Wrong: Compounds can be chemically decomposed into elements; atoms are submicroscopic units, not macroscopic substances; molecules denote bonded groups, not necessarily irreducible substances. Common Pitfalls: Equating “molecule” with “element”; forgetting that some elemental species are monatomic (e.g., noble gases) while others are diatomic (e.g., N2, O2). Final Answer: elements

Question 10

Carnot Cycle — Identifying the Incorrect Statement Which of the following statements about the Carnot cycle is incorrect? Choose the single most accurate answer.

Accepted Answer

Introduction: The Carnot cycle is a theoretical construct comprising two reversible isothermal and two reversible adiabatic processes. It sets the absolute upper bound on efficiency for any heat engine operating between two fixed thermal reservoirs. However, real cycles (Otto, Diesel, Brayton, Rankine) differ in process structure and practicality. Given Data / Assumptions: Idealized, reversible operation for Carnot. Heat engines compared on the same temperature limits. Real engines experience irreversibilities and have different process constraints. Concept / Approach: Efficiency_Carnot = 1 - T_cold / T_hot places a strict ceiling on what is achievable. While Carnot provides a benchmark and conceptual guidance, it is not a practical template from which “all heat engines are based.” Most real engines instead follow cycles optimized for combustion, heat transfer, and mechanical feasibility, not reversible isothermal heat addition. Step-by-Step Solution: Evaluate option A: Using Carnot as a standard of comparison is correct.Evaluate option C: The Carnot model introduces the idea of maximizing work by widening temperature difference and minimizing irreversibility—also correct.Option B claims all engines are based on Carnot—this is incorrect, as real cycles differ fundamentally.Option D “all of the above” cannot be correct because A and C are true. Verification / Alternative check: Textbook cycles (Otto/Diesel for reciprocating engines, Brayton for gas turbines, Rankine for steam plants) contain isentropic and isobaric/isochoric segments, not isothermal heat addition at reversible conditions; they are not “based on” Carnot. Why Other Options Are Wrong: A and C are true; D is false because it incorrectly groups true and false statements together. Common Pitfalls: Confusing a benchmark (Carnot) with a blueprint; assuming maximum efficiency is reachable in practice. Final Answer: All the heat engines are based on Carnot cycle.

Question 11

Thermodynamic Paths — Law Governing a Hyperbolic Process A “hyperbolic process” on a p–V diagram is governed by which gas law or relation?

Accepted Answer

Introduction: Different thermodynamic processes trace distinctive curves on the pressure–volume plane. Recognizing these shapes allows quick identification of the underlying relation. The term “hyperbolic” typically refers to the rectangular hyperbola characterized by a constant p * V product for ideal gases. Given Data / Assumptions: Ideal-gas behavior for the working fluid. Quasi-static process representation on a p–V diagram. Temperature variations allowed unless constrained by the law. Concept / Approach: Boyle’s law states p * V = constant at constant temperature (isothermal) for a given mass of ideal gas. The locus p = C / V is a rectangular hyperbola in the p–V plane. Hence, “hyperbolic process” commonly denotes the isothermal ideal-gas path governed by Boyle's law. Step-by-Step Solution: Start from p * V = constant.Express p = C / V, which is the mathematical form of a rectangular hyperbola.Therefore, the hyperbolic process is identified with Boyle’s law. Verification / Alternative check: On semilog plots, isothermal compression/expansion of an ideal gas aligns with hyperbolic behavior; experimental deviations occur at high pressures where real-gas effects appear. Why Other Options Are Wrong: Charles’ law relates volume to temperature at constant pressure; Gay-Lussac relates pressure to temperature at constant volume; Avogadro’s law connects volume to moles at fixed T and p—none generate a hyperbola in p–V like Boyle’s law. Common Pitfalls: Assuming any curved p–V line is “hyperbolic”; forgetting that isothermal ideal-gas paths are the canonical hyperbolas. Final Answer: Boyle's law

Question 12

Fuel Utilization — Do Liquid Fuels Have Lower Efficiency Than Solid Fuels? Evaluate the statement: “Liquid fuels have lower efficiency than solid fuels.” Select True or False based on general engineering practice and thermodynamic performance.

Accepted Answer

Introduction: Fuel “efficiency” in engineering typically refers to how effectively a fuel can be converted into useful work or heat in a specific device. This depends on calorific value, controllability of combustion, mixing, ash and moisture content, and equipment design. The statement claims liquid fuels are less efficient than solid fuels, which generally contradicts field experience. Given Data / Assumptions: Comparison across commonly used fuels: solid (coal, biomass) vs. liquid (diesel, furnace oil, gasoline, kerosene). Similar-scale, well-designed burners and engines. Efficiency considered on practical device performance, not just theoretical maxima. Concept / Approach: Liquid fuels typically have higher mass-based calorific value, negligible ash, better atomization, precise metering, and cleaner combustion. These traits often yield higher combustion efficiency, easier control of excess air, and higher overall thermal efficiency in engines and industrial burners compared to many solid-fuel systems, which suffer from heterogeneity, ash, clinker formation, and slower mixing kinetics. Step-by-Step Solution: Assess energy content: liquids ~43–46 MJ/kg; many solids (wood, low-grade coal) are lower.Assess combustion control: liquid fuel injectors/atomizers enable efficient mixing and stable flames.Assess losses: lower unburnt residues and reduced particulate carryover raise overall efficiency.Therefore, the generalized claim that liquid fuels have lower efficiency is false. Verification / Alternative check: Power plants and engines often achieve higher efficiencies with liquid or gaseous fuels due to better combustion control and lower auxiliary losses, supporting the conclusion. Why Other Options Are Wrong: Choosing “True” ignores common industrial and automotive data showing equal or superior efficiencies with liquids compared to many solids. Common Pitfalls: Equating specific applications where coal is economical with superior “efficiency”; conflating device efficiency with fuel price or availability. Final Answer: False

Question 13

Irreversibility in Throttling — Effect of Pressure Drop Magnitude Consider a throttling (adiabatic, no-shaft-work) process of a real fluid through a restriction. Is the following statement correct? “The greater the pressure difference across the thr…

Accepted Answer

Introduction: Throttling is a widely used expansion in valves and capillary tubes. It is inherently irreversible because pressure drops at essentially constant enthalpy with significant entropy generation. The claim suggests irreversibility decreases as pressure drop increases, which contradicts thermodynamic reasoning. Given Data / Assumptions: Steady-flow throttling: adiabatic, no shaft work, negligible changes in kinetic and potential energies (usually). Real fluid behavior; Joule–Thomson effects may change temperature but not enthalpy. Irreversibility measured by entropy generation s_gen > 0. Concept / Approach: For throttling, the second law indicates s_out - s_in = s_gen (adiabatic, no heat transfer). Larger pressure drops at similar inlet states commonly increase viscous dissipation and mixing, raising s_gen. Hence, greater pressure difference generally means more irreversibility, not less. Step-by-Step Solution: Model as an adiabatic, steady-flow control volume with one inlet and one outlet.Apply first law: h_out ≈ h_in (enthalpy approximately constant).Apply second law: s_out - s_in = s_gen ≥ 0; stronger throttling raises s_gen due to larger dissipative losses.Therefore, the statement is false. Verification / Alternative check: Refrigeration expansion valves show increased entropy and lost work potential as the pressure ratio increases, matching the qualitative conclusion. Why Other Options Are Wrong: Answering “Yes” would invert the entropy-generation trend for throttling and misrepresent second-law behavior. Common Pitfalls: Confusing throttling with isentropic expansion through a turbine (which would produce work and lower entropy ideally); assuming larger pressure drop always yields more cooling without acknowledging irreversibility. Final Answer: No

Question 14

Joule (Brayton) Cycle — Efficiency Compared to Carnot Between Same Temperature Limits How does the ideal Joule (Brayton) cycle efficiency compare to the Carnot efficiency when both operate between the same maximum and minimum temperatures?

Accepted Answer

Introduction: The Carnot efficiency is the theoretical maximum for any heat engine operating between two thermal reservoirs at temperatures T_hot and T_cold. Realistic cycles like the Joule (Brayton) cycle have different process structures and therefore lower efficiencies than the Carnot limit for the same temperature bounds. Given Data / Assumptions: Ideal Brayton cycle with two isentropic and two isobaric processes. Same maximum and minimum cycle temperatures as the reference Carnot engine. Working fluid treated as an ideal gas with constant specific heats. Concept / Approach: Carnot efficiency depends only on temperatures: eta_Carnot = 1 - T_cold/T_hot. The Brayton efficiency depends on pressure ratio and the ratio of specific heats, and even in the ideal reversible limit it is strictly less than Carnot for the same temperature limits because heat addition/rejection in Brayton occurs over a temperature range, not isothermally at the extremes. Step-by-Step Solution: State the benchmark: eta_Carnot is the upper bound between the given T_hot and T_cold.Identify Brayton processes: isentropic compression/expansion + isobaric heat addition/rejection.Because heat is exchanged over finite temperature ranges, entropy generation equivalents prevent matching Carnot performance.Therefore, Brayton efficiency is less than Carnot between the same temperature limits. Verification / Alternative check: Even at the optimal pressure ratio for a given T_max/T_min, Brayton's efficiency remains below the Carnot bound, confirming the strict inequality. Why Other Options Are Wrong: “Greater than” or “equal to” would violate the second law or misinterpret the differing process paths; “none of these” is unnecessary since “less than” is correct. Common Pitfalls: Comparing at equal pressure ratios rather than equal temperature limits; forgetting that Carnot requires isothermal heat transfer at the extremes, which Brayton does not provide. Final Answer: less than Carnot cycle

Question 15

Fuel selection for steam boilers (general practice in thermal engineering): Which fuel is most commonly used in steam boilers for power and process applications?

Accepted Answer

Introduction / Context: Industrial and utility steam boilers require a fuel that ignites reliably, burns steadily, and delivers high calorific value with manageable ash and handling properties. Among solid fuels, different ranks of coal and their coking tendency influence suitability for boiler firing versus metallurgical applications. Given Data / Assumptions: Conventional grate, stoker, or pulverized-fuel fired steam boilers. Need for steady flame, good grindability (for PF systems), and non-sluggish ash behavior. Typical industry practice rather than niche or waste-to-energy cases. Concept / Approach: Coking (agglomerating) coals soften and become plastic, then resolidify to form coke when heated without air. This property is advantageous for coke ovens, not for boiler furnaces where agglomeration can cause clinker formation and unstable combustion. Non-coking bituminous coal, with good volatile matter and high heating value, provides rapid ignition and stable flames without problematic caking, making it the workhorse fuel for many boilers worldwide. Step-by-Step Solution: Identify boiler fuel requirements: stable combustion, adequate volatile matter, and consistent handling.Compare coal types: peat and brown coal have high moisture/low calorific value; coking coal agglomerates (undesirable for boilers).Select non-coking bituminous coal due to favorable combustion and handling characteristics. Verification / Alternative check: Standard boiler-firing handbooks and utility practice list non-coking bituminous (and sometimes sub-bituminous) as the predominant solid fuel classes for steam generation, with pulverized-coal systems specifically optimized for such ranks. Why Other Options Are Wrong: Brown coal: High moisture and low calorific value; special handling and drying needed. Peat: Very low grade and rarely used for industrial boilers. Coking bituminous coal: Preferred for metallurgical coke, not boilers, due to caking. Common Pitfalls: Assuming “higher rank is always better” without considering coking behavior; ignoring furnace design constraints and slagging/ash fusion temperature issues. Final Answer: non-coking bituminous coal

Question 16

Dual (mixed) combustion cycle efficiency: Does the thermal efficiency of a dual combustion (limited pressure) cycle depend upon the cut-off ratio?

Accepted Answer

Introduction / Context: The dual (mixed) cycle models modern compression-ignition engines more realistically than either the pure Otto or Diesel cycles by splitting heat addition into two parts: constant-volume followed by constant-pressure. Its efficiency depends on several non-dimensional ratios that define the state points of the ideal cycle. Given Data / Assumptions: Air-standard assumptions with constant specific heats. Heat addition divided between constant-volume and constant-pressure processes. Compression ratio r, pressure ratio during constant-volume addition, and cut-off ratio ρ define the cycle. Concept / Approach: In the dual cycle, the cut-off ratio ρ = V3/V2 (volume after constant-pressure addition to volume at the end of constant-volume addition) directly affects the temperature at the end of heat addition and thus the work output and efficiency. For fixed compression ratio and the proportion of heat added at constant volume, increasing ρ generally lowers efficiency, similar to the Diesel trend where larger cut-off extends heat addition to lower average temperatures during expansion. Step-by-Step Solution: Define ρ = V3/V2 for the constant-pressure heat addition segment.Recognize that higher ρ increases exhaust (state 3) temperature for the same compression, shifting average heat addition to lower thermodynamic quality.Conclude: η_dual is a function of r, pressure ratio during CV addition, and ρ; therefore it depends on cut-off ratio. Verification / Alternative check: Textbook efficiency expressions for the dual cycle explicitly include ρ along with r and the pressure rise ratio, confirming dependence. Why Other Options Are Wrong: Does not depend / independent: Contradicts the parametric form of η_dual. Depends only on compression ratio: Incomplete; other ratios influence efficiency. Common Pitfalls: Confusing dual with Otto (no cut-off) or Diesel (single constant-pressure addition); ignoring how splitting heat addition changes mean effective temperature. Final Answer: depends

Question 17

Metallurgical by-products: Is blast furnace gas produced as a by-product during the manufacture of pig iron in a blast furnace?

Accepted Answer

Introduction / Context: Blast furnaces reduce iron ore to pig iron using coke and fluxes. In the process, large volumes of off-gas are generated. Understanding the nature and utilization of this by-product is important for integrated steel plant energy balances and environmental control systems. Given Data / Assumptions: Standard blast furnace operation with iron ore, coke, and limestone/dolomite. Counter-current flow of gases and solids. Gas cleaning (dust catchers, scrubbers) applied before reuse. Concept / Approach: Blast furnace gas (BFG) is a low-calorific-value fuel containing mainly nitrogen, carbon monoxide, carbon dioxide, and small amounts of hydrogen and hydrocarbons. It is indeed a by-product of the ironmaking reaction sequence, recovered and commonly used to fire stoves, boilers, and sometimes mixed with coke-oven or basic-oxygen-furnace gases to stabilize heating value. Step-by-Step Solution: Recognize that reduction of iron oxides releases CO/CO2-rich gas.BFG is captured at the top of the furnace and cleaned.The plant reuses BFG as a secondary fuel, confirming its by-product status. Verification / Alternative check: Plant energy diagrams list BFG production rates and its internal consumption, underscoring its role as a by-product energy stream. Why Other Options Are Wrong: No: Factually incorrect—BFG is characteristic of blast furnaces. Other qualifiers: Confuse with different furnaces or raw materials; BFG arises regardless of these variations. Common Pitfalls: Assuming BFG is waste only; in practice it is a valuable, though low-grade, fuel when properly cleaned. Final Answer: Yes

Question 18

Comparing ideal-cycle efficiencies: Why is Otto cycle efficiency higher than Diesel cycle efficiency for the same compression ratio and heat input?

Accepted Answer

Introduction / Context: The Otto and Diesel cycles are idealized models for spark-ignition and compression-ignition engines. For the same compression ratio, the Otto cycle typically attains higher thermal efficiency because of the way heat is added. Understanding this distinction is central to thermodynamic comparison of engine cycles. Given Data / Assumptions: Air-standard analysis with constant specific heats. Equal compression ratio and equal total heat input for both cycles. Reversible compression/expansion (isentropic) in both cycles. Concept / Approach: In the Otto cycle, heat addition occurs at constant volume, raising pressure and temperature more steeply at top dead center for a given heat input. The Diesel cycle adds heat at constant pressure over a finite cut-off, leading to a lower average temperature of heat addition and thus lower efficiency at the same compression ratio. The key differentiator is the constant-volume heat addition of the Otto cycle, not the isentropic legs (which both cycles share). Step-by-Step Solution: Assume equal compression ratio r for both cycles.Otto: heat at constant volume → high peak temperature for same Q_in.Diesel: heat at constant pressure → larger specific volume during heat addition → lower average temperature of heat addition.Result: η_Otto > η_Diesel at same r and Q_in. Verification / Alternative check: Standard efficiency expressions show η_Otto = 1 − 1/r^(γ−1), while Diesel efficiency contains the cut-off ratio; for identical r, Diesel's η decreases as cut-off increases. Why Other Options Are Wrong: Isentropic legs: Present in both cycles; not the differentiator. Maximum temperature is higher: A consequence of constant-volume addition, but the fundamental reason is the heat-addition mode. Lower heat rejection: Not inherently guaranteed; it is the higher average temperature of heat addition that drives higher η. Common Pitfalls: Attributing differences to compression alone; forgetting the role of cut-off in Diesel efficiency. Final Answer: combustion is at constant volume

Question 19

Avogadro’s law (ideal gas mixtures and properties): Which statement correctly expresses Avogadro’s law?

Accepted Answer

Introduction / Context: Gas laws summarize idealized relationships among pressure, volume, temperature, and amount. Avogadro’s law isolates the effect of the amount of substance by holding temperature and pressure constant and comparing volumes across different gases. Given Data / Assumptions: Ideal gas behavior. Equal temperature and equal pressure conditions for comparisons. Use of amount in moles as the measure of quantity. Concept / Approach: Avogadro’s law states that equal volumes of different ideal gases at the same temperature and pressure contain the same number of molecules (moles). This leads directly to molar volume at standard conditions and underpins mixture relations like y_i = n_i/n_total. Dalton’s law (not Avogadro’s) treats additive partial pressures. The statement about gas constant and molecular mass relates to R_specific = R_universal / M, which, while true, is not Avogadro’s law. Step-by-Step Solution: Hold T and p constant; vary gas identity.Ideal relation: pV = nR_uT ⇒ V ∝ n for fixed p and T.Thus, equal volumes imply equal moles (equal molecule count). Verification / Alternative check: At STP, one mole occupies about 22.4 L (older) or 22.71 L (depending on convention), regardless of gas identity in the ideal limit. Why Other Options Are Wrong: Option A: Property of specific gas constant, not Avogadro’s law. Option B: That is Dalton’s law of partial pressures. Option D: Includes incorrect or unrelated statements. Common Pitfalls: Confusing Avogadro’s and Dalton’s laws; mixing universal and specific gas constants. Final Answer: equal volumes of all gases, at the same temperature and pressure, contain equal number of molecules

Question 20

Stoichiometry of complete combustion: For complete oxidation, 1 kg of carbon forms how many kilograms of carbon dioxide (CO2)?

Accepted Answer

Introduction / Context: Combustion stoichiometry uses atomic/molecular mass ratios to relate fuel mass to product mass. For elemental carbon burned to carbon dioxide, the mass ratio follows directly from molecular weights and is frequently used in flue-gas calculations and emissions estimates. Given Data / Assumptions: C + O2 → CO2 (complete combustion). Atomic mass: C = 12; Molecular masses: O2 = 32, CO2 = 44. No side products (no CO or unburned C) in the ideal case. Concept / Approach: Balance the reaction stoichiometrically and use molecular masses. One mole of carbon (12 kg per 1000 moles basis) combines with one mole of oxygen gas (32) to produce one mole of carbon dioxide (44). Hence, per unit mass of carbon, product mass is scaled by 44/12. Step-by-Step Solution: Write balanced reaction: C + O2 → CO2.Compute mass ratio: m_CO2 / m_C = 44 / 12 = 11 / 3.Therefore, 1 kg C → 11/3 kg CO2 ≈ 3.666… kg. Verification / Alternative check: Carbon content and ultimate analysis in fuels often use this factor to estimate theoretical CO2 emissions from complete combustion. Why Other Options Are Wrong: 3/7, 7/3, 3/11: Do not match the stoichiometric mass ratio derived from molecular weights. Common Pitfalls: Confusing mole ratio (1:1) with mass ratio (44:12); forgetting that oxygen mass comes from O2 in air, not from the carbon itself. Final Answer: 11/3

Thermodynamics Questions