The SR-30 Turbojet Engine Performance and Work Analysis - Research Paper Example

Turbojet engine analysis Turbojet engine analysis Objective: This turbojet analysis project calculates the actual engine performance on basis of the simulation results as obtained in enginesimU application. Summary: The analysis is for SR-30 Turbojet engine and was performed after data was gathered from simulation. Pressures and temperatures alongside fuel flow rate, thrust force as well as engine RPM were obtained. Theory and Analysis: The SR-30 Turbojet Engine is theoretically modeled using Brayton cycle and is constituted of 4 processes: Isentropic compression in compressor Constant-pressure heat addition in combustor Isentropic expansion in turbine Constant-pressure heat rejection across nozzle 452136 Fresh air is drawn into compressor via nozzle. The compressor increases pressure of air before it is sent into combustion chamber. In combustor, air is mixed with injected fuel and burned at constant pressure. The high pressure and high temperature air is forced across turbine blades which spins the turbine. The air exiting turbine is accelerated across nozzle to result into a thrust. This analysis has a number of overriding assumptions. These include: 1. One dimensional analysis 2. Working fluid is air treated as ideal gas 3. Processes are internally reversible 4. Combustion is modeled as heat addition from external source 5. Constant specific heats 6. Steady state 7. Steady flow The stagnation temperature for isentropic compression is calculated using Equation 1 below, Equation 1: Stagnation Temperature for Isentropic Compression Equation 1 is used in each location using subscript 02 as exit and subscript 01 as inlet. The density anywhere in system can be calculated from ideal gas law using gas constant for air (Rair = 287J/kg K). Equation 2: Density from Ideal Gas Law The entropy in system is calculated using Equation 3. This is assuming flow is classified as Rayleigh flow. Rayleigh flow is steady, one dimensional flow of ideal gas having constant specific heats passing through a constant area duct with heat transfer and negligible friction. Equation 3: Entropy in the System Compressor: The basic operation of compressor consists of stationary casing and a rotating impeller producing high velocity air. The compressor rotates at a very high rate imparting a high speed to the outer edge of the vanes. The high speeds present require high strength materials to be used in the construction. The compressor has a diffuser section which decelerates the air thus creating high pressure. The compressor contains relatively low temperatures and can be considered to have a constant heat capacity cp=1.005 kJ/kg K and ratio of specific heat capacities k=1.4. The compressor is assumed to be isentropic, meaning constant entropy from the compressor inlet to compressor outlet / turbine inlet. Compressor work can be calculated from equation 1. Equation 4: Compressor work The efficiency of the compressor is calculated from the ideal isentropic temperature and the actual temperature at the stagnation state. Equation 5: Compressor efficiency Combustor: The SR-30 Turbojet engine uses a reverse flow annular combustion chamber. The annular type combustor is used because of the low frontal area and weight for the given volume. The reverse type allows the turbine to be radially inboard of the combustor permitting a shorter engine. The air entering into the combustor is at a high pressure from the compressor. The high pressure air mixes with the fuel as it is sprayed in. The fuel is assumed to burn at constant pressure and the resulting high temperature, high pressure air is exited to the turbine. The combustion process has a high air to fuel ratio because the excess air is used to cool the turbine. The air to fuel ratio in the combustor can be calculated as: Equation 6: Air to Fuel Ratio The total energy produced from the combustion process can be calculated using the lower heating value of the fuel and the mass flow rate of the fuel. Equation 7: Combustion Energy From Fuel Turbine: Like the compressor, the turbine has a very high rotational rate. The added effect of the high temperature from the combustion gasses make the strength of the blades crucial. The strength of the turbine blades is the limiting factor to how fast the engine can spin. The axial flow type turbine is used in the SR-30 to convert the high pressure gasses exiting the combustor to rotational energy. The axial flow turbine is used in aircraft because of the lower frontal area required for a given mass flow and pressure ratio. The turbine has high temperatures and the heat capacity is assumed to be cp=1.148 kJ/kg K and the ratio of specific heat capacities is k = 1.333. Turbine efficiency can be calculated from the ideal isentropic temperature and the actual temperature at the stagnation state. Equation 8: Turbine Efficiency Turbine work is a function of the total mass flow rate, specific heat and the change in temperature across the turbine. Equation 9: Turbine Work Nozzle: The purpose of the nozzle in an aircraft turbine is to accelerate the exhaust gasses to produce thrust. In subsonic flow, i.e. Ma < 1, a converging nozzle is employed to accelerate the air. For supersonic flow, i.e. Ma > 1, a diverging nozzle is used to accelerate the air. The exit velocity in the SR-30 Turbojet is less than the speed of sound so a converging nozzle is used. Like the turbine, the nozzle has high temperatures also. The specific heat capacity and the ratio is the same at the turbine. Nozzle efficiency is calculated like the rest of the components with the ideal isentropic temperature and the actual temperature at the stagnation state. Equation 10: Nozzle Efficiency RESULTS Engine Air Flow / Compressor Sizing Calculations     Air Properties Molecular weight 29.00 Z 1.00 k-1/k 0.288 Engine Data engine rpm 5000 revs/min displacement 231.0 cu inch volumetric efficiency 75% number of turbos 1 compressor efficiency 65% Ambient Conditions local baro pressure 29.92 in Hg 14.70 psia ambient temp 65 deg F Conditions at Compressor Inlet Vacuum drawn at inlet 2.0 in Hg Inlet Pressure 13.71 psia Inlet density 0.071 lb/ft3 Conditions at Compressor Outlet outlet pres 22.0 psig outlet temp 329.5 deg F P2/P1 2.68 outlet density 0.126 lb/ft3 Conditions at Intercooler Outlet manifold pres 19.0 psig manifold temp 110.0 deg F manifold density 0.160 lb/ft3 IC pressure drop 3.0 psi Results, mass and volume flows compressor air flow 53.4 lb/min, ideal compressor air flow 40.1 lb/min, actual compressor air flow 303.0 gm/sec, actual total engine air flow 303.0 gm/sec, actual compressor air flow 567.2 ACFM, actual inlet compressor air flow 318.9 ACFM, actual outlet Data for use with Turbonetics curves pressure correction 0.983 temperature correction 0.981 corrected suction flow 40.0 lb/min Pressure ratio 2.68 Fuel Injector Sizing Calculations         Fuel Injectors No. of injectors 6 Desired duty cycle 80% A/F ratio 11.5 :1 fuel specific gravity 0.735 fuel required 3.48 lb/min fuel required 34.1 gal/hr Injector size reqd 43.6 lb/hr engine hp potential 298.7 hp @ BSFC=0.7 engine hp potential 348.5 hp @ BSFC=0.6 engine hp potential 418.2 hp @ BSFC=0.5 Turbine Performance / Exhaust Pressure Calculations     Compressor Head and Horsepower reqd polytropic factor 0.4431 (n-1)/n head reqd 34,484 ft horsepower reqd 64.4 hp required to drive compressor Exhaust Properties EGT 1600 deg F exhaust MW 28.36 (k-1)/k 0.222 Exhaust flow 43.56 lb/min % bypassed to WG 30.0% of total to wastegate Flow thru turbine 30.49 lb/min Turbine Power Recovery isentropic efficiency 80% mechanical efficiency 99% hp delivered 64.4 hp Pres. ratio reqd 2.37 inlet/outlet pressure post turbine EGT 1313 deg F Exhaust Pressures TOP 5.0 psig, turbine outlet pressure TIP 31.9 TIP/boost 1.45 Pipe Velocity Calculations         Velocities Compressor Inlet Pipe 3.0 inch inside diameter Inlet Pipe Velocity 192.6 ft/sec Compressor Outlet Pipe 3.0 inch inside diameter Outlet Pipe Velocity 108.3 ft/sec IC Outlet Pipe 2.5 inch inside diameter Up Pipe Velocity 122.5 ft/sec Read More