Q7. A gaseous fuel consists of a mixture of methane (CH4), 80% by volume, and pentane (C5H12), 20% by volume. It is burned in 10% excess air. The dry combustion gases contain 1% carbon monoxide (CO) by volume.
(a) Formulate the full combustion equation per kmol of fuel.
(b) Calculate the percentage by volume of O2 in the dry combustion products.
Note: Atomic mass relationships: H = 1; C = 12; O = 16; N = 14
Air contains 21% oxygen by volume.
Q8. The layout of a gas turbine plant is illustrated in Fig. Q3. The plant operates between pressures of 1.00 bar and 23.00 bar. The HP turbine drives the compressor, and the LP turbine drives the load. Air enters the compressor at temperature of 300 K. Combustion gases enter the HP turbine at 1530 K. The isentropic efficiency of the compressor is 0.80, and that of each turbine is 0.90. For the compression process, γ = 1.4 and Cp = 1.005 kJ/kg K. For the remaining processes, γ = 1.33 and Cp = 1.150 kJ/kg K. The mass flow rate of exhaust gas is 110 kg/s, and the calorific value of the Fuel is 40 MJ/kg.
(a) Sketch the cycle on a T-s diagram.
(b) Calculate EACH of the following:
(i) The temperature and pressure at HP turbine exhaust
(ii) The power output
(iii) The specific fuel consumption in kg/kWh.
Q9. Ammonia (R717) is used as the refrigerant in a simple vapour compression cycle to maintain the contents of a container at a temperature of 8°C. The temperature of the surroundings is 34°C. To achieve the required heat transfer, the temperature difference between the cold container and the evaporating refrigerant should be 6 K and the temperature difference between the condensing refrigerant and the surroundings should be 10 K. The refrigerant enters the compressor dry and saturated, and there is no undercooling in the condenser. The isentropic efficiency of the compressor is 0.85.
(a) Sketch the cycle on p-h and T-s diagrams.
(b) Determine EACH of the following:
(i) The evaporating and condensing pressures
(ii) The temperature at compressor outlet
(iii) The coefficient of performance of the cycle.
Q9. River water is to be used to cool engine cooling water in a single pass shell and tube heat exchanger. The cooling water is to enter the tubes at a temperature of 85°C and to be cooled to 28°C. The flow rate of cooling water will be 2.5 kg/s. The river water will enter at a temperature of 14°C and its flow rate will be 15 kg/s. The specific heat capacities of both cooling water and river water may be taken as 4.2 kJ/kg K. The overall heat transfer coefficient is expected to be 3200 W/m2 K, based on the outside surface area of the tubes. The tube outside diameter is to be 60 mm.
Calculate EACH of the following:
(a) The outlet temperature of the river water;
(b) The logarithmic mean temperature difference for EACH of the following cases:
(i) counter flow;
(ii) Parallel flow;
(c) The total length of tubing required for EACH of the following cases:
(ii) Parallel flow.
Q8. At the beginning of compression in a single stage, single acting reciprocating air compressor, the air is at a pressure of 1.03 bar and a temperature of 35°C. The delivery valve opens at a pressure of 8.3 bar. The delivery temperature is 195°C. The bore diameter and stroke length are 0.48 m and 0.53 m respectively. The clearance volume is 5.2% of the swept volume and the compressor runs at 600 rev/min.
(a) Sketch the p-V diagram.
(i) The index of compression
(ii) The volumetric efficiency
(iii) The indicated work per kg of air
(iv) The free air capacity in m3/min, given that free air conditions are 1.013 bar and 25°C.
Note: For air, R = 0.287 kJ/kg K.
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