Open Cycle Regenerator Gas Turbine Power Plant Engineering Essay

A performance analysis and optimization of an open cycle regenerator gas turbine power-plant is to be performed. The analysis is performed by considering the eight pressure-drop losses within the open cycle during the calculation of the power output which is not considered in classical thermodynamic analysis. The power output can be optimized by adjust the mass flow rate and the distribution of pressure losses along the flow path. The power output also can be optimize by having the optimum fuel flow rate or any of the overall pressure drops as well as the overall pressure ratio. A computer program is to be produced to allow user to obtain the performance analysis based on the parameter entered by the user.

Keywords

Open cycle regenerator gas turbine power plant, pressure drop, pressure ratio.

1. Introduction

The gas turbine is unquestionably one of the most important inventions of the 20th century, and it has changed our lives in many ways. Early gas turbines for power generation applications were of low power and their thermal efficiency was too low to be competitive. By the end of the 20th century, however, gas turbines were capable of output up to 300MW with thermal efficiencies of 40 per cent and the gas turbine became widely used in power generation.

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The power plant usually consists of an air compressor, a heat exchanger, a combustion chamber and a gas turbine. First, the air is being compressed by the air compressor and then being raised its temperature by the heat exchanger before being combusted in the combustion chamber. The air then undergoes expansion in the gas turbine and finally being channel back to the heat exchanger before being released to ambient environment.

The gas turbine is used in a wide range of applications. Common uses include power generation plants and military and commercial aircraft. In Jet Engine applications, the power output of the turbine is used to provide thrust for the aircraft.

Gas turbines operate on the principal of the Brayton Cycle, which is defined as a constant pressure cycle, with four basic operations which it accomplishes simultaneously and continuously for an uninterrupted flow of power.

The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine that can be used in both internal combustion engines (such as jet engines) and for external combustion engines. It usually consists of a compressor, a combustion chamber and a turbine.

The four steps of the cycle are:

(1-2) Isentropic Compression-Ambient air compressed in the compressor

(2-3) Isobaric Heat Addition-Pressurized air heated in the combustion chamber

(3-4) Isentropic Expansion-Expansion of heated pressurized air in the turbine

(4-1) Isobaric Heat Rejection-Heat rejection to the atmosphere

Pressure ratio

Thermal efficiency of a Brayton cycle

Isentropic relation

2. Literature review

Every gas turbine has three fundamental elements in common, an axial compressor, a combustor and a turbine. These elements work together to produce usable energy. First it converts fuel energy into heat energy and then it harness as much of that heat as possible and converts it into mechanical energy. The more heat it produces, the more energy it can extract. However, basic cycle gas turbine can only achieve maximum efficiency of less than 50%. Thus element such as regenerator, intercooler or reheater can be added to increase its thermal efficiency and the power output.

2.1 Open Cycle Gas Turbine

Gas turbines usually operate on an open cycle, as shown in Figure 1. Fresh air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high-pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. The resulting high-temperature gases then enter the turbine, where they expand to the atmospheric pressure through a row of nozzle vanes. This expansion causes the turbine blade to spin, which then turns a shaft inside a magnetic coil. When the shaft is rotating inside the magnetic coil, electrical current is produced. The exhaust gases leaving the turbine in the open cycle are not re-circulated.

2.2 Closed Cycle Gas Turbine

The open gas-turbne cycle can be modeled as a closed cycle as shown in Figure 2 by utilizing the air-standard assumptions Here the compression and expansion processes remain the same, but the combustion process is replaced by a constant-pressure heat-addition process from an external source, and the exhaust process is replaced by a constant pressure heat-rejection process to the ambient air.

2.3Principal irreversibilities and Losses

In real gas turbine, the T-S diagram deviates from an actual gas turbine as a result of irreversibility. There are pressure losses due to fluid friction during compression and expansion. There are also pressure losses during heat addition and heat rejection due to fluid flow.

Efficiency of compressor

Efficiency of turbine

2.4 Open Cycle Regenerator Gas Turbine

Regeneration involves the installation of a regenerative heat exchanger through which the turbine exhaust gases pass.

In gas-turbine engines, the temperature of the exhaust gas leaving the turbine is often considerably higher than the temperature of the air leaving the compressor. Therefore, the high-pressure air leaving the compressor can be heated by transferring heat to it from the hot exhaust gases in a counter-flow heat exchanger, which is known as a regenerator.

The highest temperature occurring within the regenerator is T4, the temperature of the exhaust gases leaving the turbine and entering the regenerator. Under no conditions can the air be preheated in the regenerator to a temperature above this value. Air normally leaves the regenerator at a lower temperature, T5. In the limiting (ideal) case, the air exits the regenerator at the inlet temperature of the exhaust gases T4.

The thermal efficiency of the Brayton cycle increases as a result of regeneration since the portion of energy of the exhaust gases that is normally rejected to the surroundings is now used to preheat the air entering the combustion chamber.

2.5 Brayton cycle with regeneration

Thermal efficiency of a Brayton cycle with regeneration:

Degree of regeneration

3. Methodologies

3.1 Open regenerated Brayton-cycle for a gas-turbine power-plant

Performance analysis will be based on the open cycle regeneration gas turbine power plant model shown above. The cycle consists of a compressor, a regenerator, a combustion chamber, and a gas turbine.

3.2 The temperature-entropy diagram and the flow resistances of the power-plant model

The performance analysis will include the with considerations of the eight pressure-drop losses in the intake, compression, regeneration, combustion, expansion and discharge processes and flow process in the piping, the heat-transfer loss to the ambient environment, the irreversible compression and expansion losses in the compressor and the turbine, and the irreversible combustion-loss in the combustion chamber.

3.3 Procedure

Perform theoretical analysis on the performance of an open cycle regenerator gas turbine power plant by considering the pressure losses.

Write a computer program to analyze the performance of an open cycle regenerator gas turbine power plant with and without pressure losses.

Compare the performance of the open cycle regenerator gas turbine power plant with and without pressure losses.

3.4 Expected results

4. Progress report based on Gantt chart

Table 1 — Progress Report

Task done

July

August

September

October

Information

research

Preparing proposal

Study on thermodynamic

Study on related journal paper

Study on Matlab

Study on related mathematical formulae

Attempt to plot desired graph

Preparation of progress report

4.1 Current progress

The above are some of the sample graph plotted that are similar to the expected results. However, the similarities are limited as the degree of regeneration of the regenerator in the expected results remains unknown.

The remaining graphs are still in the progress as there are so difficulties encountered with the mathematical formulae.

5. Conclusion

The project is going according to the timeline given. Further analysis on the turbine’s temperature ratio and the regenerator’s temperature ratio will help to create the program desired.

6. Recommendation

Further study into thermodynamic will ease the progress of this project. There are one particular journal related to the project that is yet to be purchased. Purchasing this journal will solve most of the problem encountered.