Consider the Brayton cycle described in Problem 1. The heat is to be supplied by the combustion of octane (C8H18) with air, with 10% excess air being used in the combustion and the combustion proceeding to completion. The air enters the process at the exit temperature from the compressor, the fuel enters at 77°F, and the products exit at 2200°F. For both parts (a) and (b) of Problem 2, determine the mass flow rate of octane needed for the combustion process.
Problem 1
A Rankine cycle power plant has a thermal efficiency of 40% and produces 750 MW of power. The heat is to be supplied to the water in the steam generator through the combustion of methane (CH4) with 10% excess air. The methane and air both enter the steam generator at 298 K and burn to completion, and the products exit the steam generator at 650 K. The combustion process takes place at 101 kPa. Determine the required mass flow rate of the methane, and the corresponding volumetric flow rate of the fuel and the volumetric flow rate of the air entering the steam generator.
Problem 2
A Diesel cycle with a compression ratio of 20 and a cut-off ratio of 2.5 produces 200 kW of power. Model the Diesel cycle as using an ideal gas with constant specific heats and the heat provided in a constant-pressure closed system process. The air enters the compression stroke at 300 K, and the combustion process begins at the end of the compression stroke. The fuel is decane, and it enters the process at 500 K. (Note: This is not a perfect model for the complicated process in an actual engine.) Consider the overall equivalence ratio of the combustion to be 0.60. The products exit the combustion process at 1300 K. Determine the mass flow rate of decane required to produce the necessary heat for this combustion process.
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