Mechanical Engineering 2022 Paper II 50 marks Compulsory Explain

Q5

(a) Can alcohols be used as fuel in IC engine? Explain with advantages and disadvantages. (10 marks) (b) A water-filled reactor with a volume of 1 m³ is at 20 MPa and 360 °C, and is placed inside a containment room as shown in Fig. 5(b). The room is well-insulated and initially evacuated. Due to a failure, the reactor ruptures and the water fills the containment room. Find the minimum room volume so that the final pressure does not exceed 200 kPa. [Use steam table data given at the end of the Paper] (10 marks) (c) Using a schematic and T-s diagram, explain how with perfect regeneration for a simple steam power plant (Rankine) cycle, thermal efficiency can approach Carnot efficiency. (10 marks) (d) Discuss the effect of the following parameters on the performance of a vapor compression refrigeration system with the help of p-h diagram: (i) Suction pressure (ii) Delivery pressure (iii) Subcooling of liquid (iv) Superheating of vapors (10 marks) (e) The room air is recirculated at the rate of 40 m³ per minute and the outdoor air enters a cooling coil of an air conditioner at 32 °C DBT and 18 °C WBT. The effective surface temperature of the coil is 4·5 °C. The surface area of the coil is such as would give 12 kW of refrigeration with the given entering conditions of air. Determine the DBT and WBT of the air leaving the coil and the coil bypass factor. [Psychrometric chart is given at the end of this Paper] (10 marks)

हिंदी में प्रश्न पढ़ें

(a) क्या अल्कोहल आइ० सी० इंजन में ईंधन के रूप में प्रयुक्त हो सकता है? लाभ तथा हानियों के साथ समझाइये। (10 अंक) (b) 1 m³ आयतन वाले जल से भरे एक रिएक्टर, जो कि 20 MPa तथा 360 °C पर है, को चित्र 5(b) के अनुसार एक नियंत्रण कक्ष में रखा गया है। कक्ष अच्छी तरह से रोधित है तथा इसे आरंभ में निर्वात किया गया है। असफलता के कारण रिएक्टर फट जाता है तथा नियंत्रण (कंटेनमेंट) कक्ष में जल भर जाता है। कक्ष का न्यूनतम आयतन ज्ञात कीजिये जिससे कि अंतिम दाब 200 kPa से अधिक न हो। [इस पत्र के अंत में दी हुई भाप (स्टीम) सारणी में दत्त सामग्री का उपयोग कीजिये] (10 अंक) (c) एक योजनाबद्ध तथा T-s आरेख का उपयोग करते हुए समझाइये कि कैसे आदर्श पुनर्जनन वाले एक साधारण भाप शक्ति संयंत्र (रैंकिन) चक्र की तापीय दक्षता, कार्नो दक्षता के सदृश हो सकती है। (10 अंक) (d) एक वाष्प संपीडन प्रशीतन तंत्र के निष्पादन पर निम्न प्राचलों से पड़ने वाले प्रभाव की विवेचना p-h आरेख की सहायता से कीजिये: (i) चूषण दाब (ii) प्रदान दाब (iii) द्रव का उपशीतन (iv) वाष्प का अतितापन (10 अंक) (e) एक कमरे की वायु 40 m³ प्रति मिनट की दर पर पुनःसंचारित की जाती है तथा 32 °C DBT और 18 °C WBT अवस्था वाली बाहरी वायु एक वातानुकूलक की शीतलन कुण्डली में प्रविष्ट होती है। कुण्डली का प्रभावी सतह तापमान 4·5 °C है। कुण्डली की सतह का क्षेत्रफल इस प्रकार है कि यह वायु के प्रवेश की दी हुई अवस्था के लिये 12 kW का शीतलन प्रदान करेगा। कुण्डली से बाहर जाने वाली वायु का DBT तथा WBT और कुण्डली के बाइपास गुणक का निर्धारण कीजिये। [इस पत्र के अंत में साइक्रोमीट्रिक चार्ट दिया गया है] (10 अंक)

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Directive word: Explain

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How this answer will be evaluated

Approach

Explain the suitability of alcohols as IC engine fuels with balanced advantages and disadvantages for part (a). For (b), apply first law for unsteady flow with steam tables to find containment volume. Part (c) requires schematic and T-s diagram with clear regeneration explanation. Part (d) needs p-h diagram analysis of four parameters. Part (e) involves psychrometric calculations with bypass factor. Allocate ~15% time each to (a), (c), (d); ~25% to (b) and (e) due to numerical complexity.

Key points expected

  • Part (a): Alcohols (methanol, ethanol) as fuels—higher octane rating, lower emissions vs lower energy density, corrosion, cold-start issues; mention India's ethanol blending programme (E20, E85)
  • Part (b): Unsteady flow energy equation for ruptured reactor; initial state: compressed liquid/subcooled water at 20 MPa, 360°C; final state: saturated mixture at 200 kPa; mass and energy balance to find quality and containment volume
  • Part (c): Rankine cycle with perfect regeneration—open feedwater heater schematic; T-s diagram showing heat addition at variable temperature approaching Carnot's isothermal; mean temperature of heat addition increases to T_max
  • Part (d): p-h diagram effects—suction pressure ↑ increases refrigeration effect and COP; delivery pressure ↑ decreases COP; subcooling ↑ increases refrigeration effect; superheating ↑ may increase or decrease COP depending on useful cooling
  • Part (e): Psychrometric process—mixing of recirculated and outdoor air, cooling and dehumidification to 4.5°C EST; bypass factor = (T_out - T_est)/(T_in - T_est); energy balance for 12 kW to find air exit conditions
  • Part (b) specific: Use steam tables—initial v ≈ 0.0015 m³/kg (compressed liquid), u ≈ 1700 kJ/kg; final v_f and v_g at 200 kPa; solve for mass then V_room = m·v_final
  • Part (e) specific: Locate 32°C DBT, 18°C WBT on chart; find humidity ratio; process follows constant sensible heat factor to 4.5°C EST; read exit DBT and WBT

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10For (a), correctly identifies alcohol fuel properties and India's E20 policy; for (b), applies correct unsteady flow formulation with proper steam table state identification; for (c), explains why regeneration raises mean temperature of heat addition; for (d), correctly identifies each parameter's thermodynamic effect; for (e), understands bypass factor and coil bypass mixing process.Basic concepts correct but some confusion—treats (b) as steady flow, or misidentifies regeneration as reheat; partial understanding of psychrometric processes.Fundamental errors—confuses regeneration with reheating, treats rupture as isothermal, or completely misinterprets bypass factor as efficiency.
Numerical accuracy20%10Part (b): correct initial specific volume (~0.0015 m³/kg), mass ~667 kg, final quality calculation, room volume ~100-120 m³; Part (e): correct bypass factor (~0.15-0.25), exit DBT ~8-12°C, WBT ~6-9°C; all calculations with proper significant figures and units.Correct approach but steam table interpolation errors or arithmetic slips; final answers within 10-15% of correct value; units mostly consistent.Order of magnitude errors in (b) (e.g., room volume 1 m³ or 1000 m³); wrong steam table values used (saturated instead of compressed liquid); no units or wrong units throughout.
Diagram quality20%10Part (c): Clear T-s diagram with saturated dome, pump, boiler, turbine, condenser, and FWH showing feedwater heating process approaching T_max; Part (d): Labelled p-h diagram with saturation dome and four process variations shown; Part (e): Sketch of psychrometric chart with process path or coil schematic with bypass air shown.Diagrams present but incomplete—T-s diagram missing FWH details, p-h diagram shows only one parameter change, or psychrometric process not clearly marked.No diagrams where required (parts c, d, e); or completely wrong diagrams—T-s diagram for Brayton cycle, p-h diagram without saturation dome.
Step-by-step derivation20%10Part (b): Shows complete unsteady flow energy equation, mass balance, identifies initial state from compressed liquid tables or approximation, final state quality calculation, explicit solve for V_room; Part (e): Shows energy balance on coil, mass flow rate calculation, bypass factor definition and application; all steps logically sequenced with justification.Key equations stated but some steps skipped—jumps from initial to final state without showing energy balance, or assumes quality without calculation; final answer correct but working unclear.No derivation shown—final answers only; or completely wrong approach (e.g., ideal gas law for steam in part b); equations written without variables defined.
Practical interpretation20%10Part (a): Links to India's National Biofuel Policy, ethanol procurement from sugarcane molasses, vehicle compatibility issues; Part (b): Discusses containment design for nuclear/pressurized systems; Part (c): Explains why real plants use multiple FWHs, not perfect regeneration; Part (d): Relates to actual refrigeration system optimization; Part (e): Discusses coil selection and energy efficiency implications.Brief mention of practical relevance without depth—states 'important for India' but no specifics; or generic statements about energy saving.No practical interpretation; treats all parts as purely academic exercises with no connection to engineering applications or policy context.

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