ME3451 Thermal Engineering Important Questions

ME3451 Thermal Engineering Important Questions

Unit 1

1. The boiler produces dry and saturated steam at 30 bar. The steam expands in the turbine to a condenser pressure of 20 kPa. Compare the cyclic work done and thermal efficiency of Carnot and Rankine cycles for these conditions.
2. A Diesel engine has an inlet temperature and pressure of 17°C and 1 bar, respectively. The compression ratio is 15 and the maximum cycle temperature is 1400 K. Calculate the air-standard efficiency of the Diesel cycle. Take y = 1.4.
3. In an ideal Brayton cycle, air is compressed from 1 bar to a pressure ratio of 6. Calculate the cyclic efficiency. If the ratio of lower to upper temperature is 0.3 then calculate the work ratio.
4. The pressure ratio and maximum temperature of a Brayton cycle are 5:1 and 923 K, respectively. Air enters the compressor at 1 bar and 298 K. Calculate for 1 kg of air flow, the compressor work, turbine work and the efficiency of the cycle.
5. The maximum pressure and temperature in an Otto cycle are 10 kPa and 27 deg * C The amount of heat added to the air per cycle is 1500 kJ/kg.
   (i) Determine the pressure and temperatures and pressures at all points of the air standard Otto cycle.
   (ii) Calculate the specific work and thermal efficiency of the cycle for a compression ratio of 8:1.
   Take for air: C_{v} = 0.72kJ / k  gK and gamma = 1.4
6. In an engine working on Dual cycle, the temperature and pressure at the beginning of the cycle are 90 deg * C and 1 bar respectively. The compression ratio is 9.2. The maximum pressure is limited to 68 bar and total heat supplied per kg of air is 1750 kJ. Calculate:
   (i) Pressure and temperature at all salient points
   (ii) Air standard efficiency
   (iii) Mean effective pressure.
7. For the same compression ratio, prove that the efficiency of the Otto cycle is greater than that of the diesel cycle.
8. In an air standard diesel cycle with a compression ratio of 14, the condition of air at the start of the compression stroke are 1 bar and 300 K. After addition of heat at constant pressure, the temperature rises to 2775 K. Determine the thermal efficiency of the cycle, network done per kg of air.

Unit 2

1. Explain the significance of critical pressure ratios in steam nozzles.
2. Illustrate how variations in mass flow rate with pressure ratios impact the overall efficiency of the system.
3.  A nozzle is to be designed to expand steam at the rate of 0.10 kg/s from 500 kPa, 210°C to 100 kPa. Neglect inlet velocity of steam. For a nozzle efficiency of 0.9, determine the exit area of the nozzle.
4. Calculate the critical pressure and throat area per unit mass-flow rate of steam, expanding through a convergent-divergent nozzle from 10 bar, dry saturated, down to atmospheric pressure of 1 bar. Assume that the inlet velocity is negligible, and that the expansion is isentropic.
5. Steam at a pressure of 10.5 bar and 0.95 dry is expanded through a convergent divergent nozzle. The pressure of steam leaving the nozzle is 0.85 bar. Find the velocity of steam at the throat for maximum discharge. Take Also find the area at the exit and steam discharge if the n = 1.135 throat area is 1.2c * m ^ 2 Assume flow is isentropic and there are no friction losses.
6. Brief the following in case of steam nozzles:
   (i) Critical pressure ratio
   (ii) Effect of friction
   (iii) Metastable flow and its effect.
7.  Calculate the critical pressure ratio and throat area per unit mass flow rate of steam, expanding through a convergent-divergent steam nozzle from 10 bar, dry saturated down to atmospheric pressure of 1 bar. Assume that the inlet velocity is negligible and that the expansion is isentropic.
8. A nozzle is to be designed to expand steam at the rate of 0.1 kg/sec from 500 kPa, 210°C to 100 kPa. Neglect the inlet velocity of steam. For a nozzle efficiency of 0.9, determine the exit area of the nozzle.

Unit 3

1. Discuss the principles of compounding and governing to optimize the performance of a gas turbine with its significance.
2. Compare regenerative, intercooled, and reheat cycles in terms of how each enhance gas turbine performance individually.
3. The gas turbine has an overall pressure ratio of 5:1 and the maximum cycle temperature is 550°C. The turbine drives the compressor and an electric generator, the mechanical efficiency of the drive being 97%. The ambient temperature is 20 he turbine drives the compressor and an electric 20°C and the isentropic efficiencies for the compressor and the turbine are 0.8 and 0.83 respectively. Calculate the power output in megawatts for an air flow of 15 kg/s. Also calculate the thermal efficiency and work ratio.
   Neglect the changes in kinetic energy and loss of pressure in combustion chamber.
4. A steam power plant operates on an ideal reheat Rankine cycle between the pressure limits of 15 MPa and 10 kPa. The mass flow rate of steam through the cycle is 12kg/s. Steam enters both stages of the turbine at 500°C. If the moisture content of the steam at the exit of the low-pressure turbine is not to exceed 10%, determine the following.
   (i) Reheat pressure
   (ii) Heat input to the Boiler
   (iii) Thermal efficiency of the cycle. Represent the cycle on T-s diagram.
5. In a gas turbine power plant, air enters the compressor at 15°C and it is compressed through a pressure ratio of 4 with isentropic efficiency of 85%. The air-fuel ratio is 80 and the calorific value of the fuel is 42,000 kJ/kg. The turbine inlet temperature is 1000 K and the isentropic efficiency of the turbine is 82%. Find the overall plant efficiency.
6. Explain the concept of advanced techniques adapted in gas turbine power plant with neat line schematic diagram. Also represent the cycle in all P-v, T-s and h-s diagrams. Give merits of the advance techniques.

Unit 4

1. Define the detonation. Give its effects on Spark Ignition Engines.
2. Explain the working principle of simple carburetor with neat sketch. Give its limitations.
3. What do you mean by knocking? Describe the phenomenon of knocking in SI engine. What are the factors affect the knocking? How can it be controlled?
4. Explain the different types of combustion chambers used in CI engines.
5.  Compare valve and port timing diagrams for internal combustion engines, highlighting differences in their operational characteristics and efficiency.
6.  Differentiate the operating characteristics of SI and CI engines by emphasizing the combustion processes, fuel delivery, and ignition mechanisms.
7. Explain influence of each factor in engine performance.

Unit 5

1. Compare operating principles or supercharging and turbocharging by emphasizing the impact on air intake, combustion efficiency, and overall power output in internal combustion engines.
2. An engine has a displacement of three liters and operates on a four-stroke cycle at 3600 RPM. The engine features a compression ratio of 9.5, square geometry (bore diameter is equal to the stroke length), and connecting rods with a length of 16.6 cm. Combustion concludes at 20° after TDC. The engine is connected to a dynamometer, registering a brake torque of 205 N-m at 3600 RPM. With air entering the cylinders at 85 kPa and 60°C, and a mechanical efficiency of 85%, calculate:
   (i) Brake power
   (ii) Indicated power
   (iii) Brake mean effective pressure
   (iv) Indicated mean effective pressure
3. A six cylinder, gasoline engine operates on the four stroke cycle. The bore of each cylinder is 80 mm and the stroke is 100 mm. The clearance volume per cylinder is 70 cc. At the speed of 4100 rpm, the fuel consumption is 5.5 gm/sec and the torque developed is 160 Nm. Calculate:
   (i) Brake power
   (ii) Brake mean effective pressure
   (iii) Brake thermal efficiency if the calorific value of the fuel is 44000 kJ/kg and
   (iv) Relative efficiency on a brake power basis assuming the engine works on the constant volume cycle y 1.4 for air.
4. During the trial of a four stoke, single cylinder, oil engine the following observations were recorded: bore 300 mm. stroke 400 mm, speed = 200 rpm, duration of trial = 60 minutes. fuel consumption = 7.050 kg, calorific value = 14000 kJ/kg, area of indicator diagram = 322 mm², length of indicator diagram = 62 mm, spring index = 1.1 bar/mm, dead load on the brake drum = 140 kg, spring balance reading = 5 kg, brake drum diameter 1600 mm, total weight of cooling water 495 kg, temperature rise of cooling water = 38°C, temperature of exhaust gases = 300°C, air consumption = 311 kg; specific heat of exhaust gases = 1.004 kJ/kg K; specific heat of water 4.186 kJ/kg K; room temperature = 20°C. Determine
   (1) Brake power
   (ii) Indicated power
   (iii) Mechanical efficiency
   (iv) Indicated thermal efficiency.
5. A full load test was conducted on a two stroke engine and the following results were obtained:
   Speed of engine 500 rpm; Brake load 500 N; Air/fuel ratio 30; oil consumption = 5kg/hr; Room temperature 25°C; Atmospheric pressure = 1 bar; diameter of cylinder = 22cm; stroke length 28cm; Brake diameter 1.6m. Calculate the volumetric efficiency and brake specific fuel consumption.
6. The following results refer to at test on a four stroke petrol engine: The diameter of the cylinder is 30 cm and stroke length of the piston is 45 cm. The Engine runs at the speed of 1000 rpm. The brake specific fuel consumption is 0.35 kg/kWh. The calorific value of the fuel is 43,900 kJ/kg. The indicated mean effective pressure is 540 kPa. Calculate the following:
   (i) Indicated thermal efficiency
   (ii) Brake thermal efficiency
   (iii) Mechanical efficiency