Design and Performance of a Gas-Turbine Engine from an Automobile Turbocharger



  Design and Performance of a Gas-Turbine.pdf (Size: 2.03 MB / Downloads: 1)

Introduction

The Massachusetts Institute of Technology Department of Mechanical Engineering teaches
thermodynamics and fluid mechanics through a pair of classes, Thermal Fluids Engineering
I & II. The courses cover the basic principles of thermodynamics, fluid mechanics, and heat
transfer. This includes the rate processes involved with heat and work transfer, steady flow
components of thermodynamic plants, and energy conversion cycles.
The purpose of this project was to design and fabricate a gas-turbine engine that is
suitable for use as a classroom demonstration. The gas turbine designed and built for this
project illustrates the operation of the Brayton cycle. The Brayton cycle is a convenient cycle
to demonstrate because it involves a combination of standard components, which are used
in many other energy conversion applications. The Brayton cycle consists of a compressor, a
heat exchanger, a turbine, and another heat exchanger. The gas turbine engine operates on
an open version of the Brayton cycle and allows students to measure the temperature and
pressure changes associated with each system component. The students can then use these
values to calculate the performance of the engine components as well as the overall cycle
efficiency.


Gas Turbine Power Plants

Gas turbines are thermodynamic systems that use fuel and air to produce a positive work
transfer. They convert the chemical potential energy of the fuel to mechanical energy. The
gas turbine operates on an open cycle consisting of a compressor, a combustor, and a turbine
combined in series (Figure 2). Air from the atmosphere enters the compressor where it is
compressed by a negative shaft work transfer. The compressed air is then combined and
burned with fuel in the combustion chamber. The combustor increases both the temperature
and the specific volume of the air. The hot air is then fed into the turbine where it is
expanded. The expansion of the air creates a positive shaft work transfer. The expanded air
is then exhausted to the atmosphere. A net positive shaft work transfer is produced because
the negative shaft work transfer required to power the compressor is less than the positive
work transfer produced by the turbine.



The Brayton Cycle

The Brayton cycle consists of two adiabatic work transfers and two constant pressure heat
transfer heat processes (Figure 3). From State 1 to State 2 the gas undergoes an isentropic,
adiabatic compression. This process increases the temperature, pressure, and density of the
gas. From State 2 to State 3, heat is added at constant pressure. For a gas-turbine, heat
is added through a combustion process. From State 3 to State 4 the gas passes through an
adiabatic isentropic turbine which decreases the temperature and pressure of the gas. For
the closed Brayton cycle, heat is removed from the gas between State 4 and State 1 via a
heat exchanger.



Combustion Chambers
The combustion chamber is the component of the gas turbine in which the fuel is combined
with the air from the compressor and burned. The combustion chamber functions like a heat
exchanger and can be modeled as a constant pressure device. The combustion process raises
the temperature of the air in the system by converting the chemical potential energy of the
reactants to thermal energy. There is no work transfer involved in the reaction.