Monday, 5 December 2016

WORKING PRINCIPLE OF CLOSED CYCLE GAS TURBINE


We were discussing Otto cycle, an ideal cycle for internal combustion spark ignition reciprocating engines or simply petrol engines and also Diesel cycle, the ideal cycle for the operation of internal combustion compression ignition reciprocating engines in our previous posts. We have also discussed the working principle of open cycle gas turbine.

Today we will see here the another very important topic i.e. Closed cycle gas turbine power plant or closed cycle gas turbine engine with the help of this post.

Closed cycle gas turbine engine

Closed cycle gas turbine engines are usually used in nuclear power stations and also used as standby power unit for the hydro electric power stations. Compressor, Turbine, heat exchanger for heating the working fluid termed as heating chamber and heat exchanger for cooling the working fluid termed as cooling chamber are the main components of closed cycle gas turbine engine.

Open cycle gas turbine engine could be modelled as closed cycle gas turbine engine. Combustion process will be replaced here by constant pressure heat addition from an external source in heating chamber and discharge process will be replaced by constant pressure heat rejection in cooling chamber.

Let us see the arrangements of various components of closed cycle gas turbine engine

Air will enter in to the compressor, where pressure and temperature of air will be increased. Now air at high pressure and high temperature will enter to the heating chamber as shown in above figure. 

Working fluid i.e. high pressure and high temperature air will be heated from an external source in heating chamber. High temperature nuclear rods are used here for heating the working fluid i.e. air. Hence working fluid i.e. air will have high pressure and high temperature at the discharge of the heating chamber.

High pressure and high temperature air will enter in to the turbine, where high pressure and high temperature air will be expanded through the turbine. Pressure and temperature of the air, both will be dropped here.

There will be drop in temperature of air but still temperature of air will be high, while pressure of air will be reduced up to the pressure at which air will enter in to the cooling chamber.

Air will be cooled in to the cooling chamber at constant pressure up to its original temperature with the help of continuous circulating cold water and hence heat will be rejected here at constant pressure. Again cold air coming from cooling chamber will enter to compressor for repeating the cycle.

As we can observe here that exhaust air is not rejected to atmosphere but also exhaust air re-circulated to the cooling chamber and therefore this cycle will be termed as closed cycle gas turbine engine.

Work energy will be generated from the turbine during the expansion of high pressure and high temperature air and some part of this generated work will be used to drive the compressor and hence compressor and turbine are assembled with common shaft as shown in above figure.

Let us see the processes involved in closed cycle gas turbine engine

Process 1-2: Isentropic compression process, air entering in to the compressor will be compressed here at high pressure and high temperature. Pressure will be increased from P1 to P2 and volume will be decreased here from V1 to V2. Temperature will be increased from T1 to T2 and entropy will remain constant as this process will be isentropic process.

Process 2-3: Constant pressure heat addition in to the heating chamber. Air will be heated from an external source in heating chamber. Temperature of working fluid i.e. air will be increased here from T2 to T3 and entropy will also increased from S2 to S3.
Process 3-4: Isentropic expansion process, high pressure and high temperature air will be expanded through the turbine. Pressure of working fluid i.e. air will be reduced here from P3 to P4 and volume will be increased here from V3 to V4. Temperature will also be reduced from T3 to T4 and entropy will remain constant as this process will be isentropic process.

Process 4-1: This process indicates the constant pressure heat rejection process, where Air will be cooled in to the cooling chamber at constant pressure up to its original temperature with the help of continuous circulating cold water. Working fluid i.e. air will be cooled here from T4 to T1 and entropy will also reduced from S4 to S1.

Do you have any suggestions? Please write in comment box.
We will see another topic i.e. “Brayton cycle: the ideal cycle for gas turbine engines” in our next post in the category of thermal engineering.

Reference:

Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof S. K. Som
Image courtesy: Google

Also read

Continue Reading

Sunday, 4 December 2016

WORKING PRINCIPLE OF OPEN CYCLE GAS TURBINE

We were discussing Otto cycle, an ideal cycle for internal combustion spark ignition reciprocating engines or simply petrol engines and also Diesel cycle, the ideal cycle for the operation of internal combustion compression ignition reciprocating engines in our previous posts. We have also discussed the derivation of efficiency of Otto cycle and efficiency of Diesel cycle.

We will see here the very important topic i.e. Open cycle gas turbine power plant or open cycle gas turbine engine with the help of this post.

An open cycle gas turbine engine

Open cycle gas turbine engines are usually used in aircraft engines, marine engines and also in automobile engines. Compressor, Turbine and combustion chamber are main components of an open cycle gas turbine engine.
Let us see the arrangements of various components of an open cycle gas turbine engine.
Fresh air will enter in to the compressor at atmospheric pressure and temperature, where pressure and temperature of air will be increased. Now air at high pressure and high temperature will enter to the combustion chamber as shown in above figure. 

Fuel will be injected from outside in to the combustion chamber and therefore burning of fuel will take place within the combustion chamber in presence of high pressure and high temperature air. Hence combustion product i.e. gas will have high pressure and high temperature at the discharge of the combustion chamber.

High pressure and high temperature gas will enter in to the turbine, where high pressure and high temperature gas will be expanded through the turbine. Pressure and temperature of the gas, both will be dropped here.

There will be drop in temperature of gas but still temperature of gas will be high, while pressure of gas will be reduced to atmospheric pressure and hence we can say that high temperature gas will be rejected to the atmosphere at atmospheric pressure.

As we can observe here that fresh air is entering in to the compressor at atmospheric pressure and temperature and high temperature gas is rejected here to the atmosphere at atmospheric pressure and exhaust gases leaving the turbine are not re-circulated but also going to the atmosphere therefore this cycle will be termed as open cycle gas turbine engine.

Work energy will be generated from the turbine during the expansion of high pressure and high temperature gas and some part of this generated work will be used to drive the compressor and hence compressor and turbine are assembled with common shaft as shown in above figure.

Let us see the processes involved in an open cycle gas turbine engine

Process 1-2: Isentropic compression process, fresh air entering in to the compressor at atmospheric pressure and temperature will be compressed here at high pressure and high temperature.
Process 2-3: Constant pressure heat addition in to the combustion chamber
Process 3-4: Isentropic expansion process, high pressure and high temperature gas will be expanded through the turbine.

Let us see here the thermal efficiency of the open cycle gas turbine engine

We will see here the various energy calculations for unit mass
Input work energy to the compressor, WC = CP (T2-T1)
Output work energy from the turbine, WT = CP (T3-T4)
Heat added in the combustion chamber at constant pressure, QA= CP (T3-T2)
Net work output, W= WT - WC = CP (T3-T4) - CP (T2-T1)
η thermal = Net work output / Heat added

η thermal = [CP (T3-T4) - CP (T2-T1)]/ CP (T3-T2)

Do you have any suggestions? Please write in comment box.
We will see another topic in our next post in the category of thermal engineering.

Reference:

Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof S. K. Som
Image courtesy: Google

Also read

Continue Reading

Tuesday, 15 November 2016

DIESEL CYCLE: VOLUMETRIC COMPRESSION RATIO, EXPANSION RATIO, CUT-OFF RATIO AND EFFICIENCY

We were discussing Otto cycle, an ideal cycle for internal combustion spark ignition reciprocating engines or simply petrol engines and also Diesel cycle, the ideal cycle for the operation of internal combustion compression ignition reciprocating engines in our previous posts. We have also discussed the derivation of efficiency of Otto cycle and efficiency of Diesel cycle.

Today we will see here few more very important terms i.e. volumetric compression ratio, Expansion Ratio, Cut-off ratio and simultaneously we will also see here the efficiency of Diesel cycle in terms of these ratios with the help of this post.

First we will have to see brief introduction and background to remind the concept of Diesel cycle. Diesel cycle is one type of air standard cycle which is designated as the ideal cycle for the operation of internal compression ignition reciprocating engines

Let us see an overview of Diesel cycle with the help of PV diagram and TS diagram as displayed here in following figure. As we can see in below figure, there will be two isentropic or adiabatic processes, one constant pressure and one constant volume process.

Let us understand now the term volumetric compression ratio, Expansion Ratio and Cut-off ratio

Volumetric compression ratio will be defined as the maximum volume to minimum volume and it will be indicated by rv.
rv = V1/V2

Now we must have to understand here that why we are focusing here to understand the volumetric compression ratio and why we are giving stress to determine the result or formula to calculate the efficiency of the Diesel cycle in term of volumetric compression ratio.

As we know very well that Diesel cycle is one theoretical cycle or ideal cycle for internal compression ignition reciprocating engines or simply Diesel engines. Space is one very important design criteria for any automobile and automobile designer. Designer will have to surely consider this design criteria i.e. space restriction. 

We will have to design our engine with considering the space permitted for respective automobile as we may not design one engine which accommodate large area as it will not be in favour of the user of automobile and therefore there will be one maximum volume and one minimum volume and we will have to design our engine with considering the maximum and minimum volume.

Now we will write here the expression of efficiency of the Diesel cycle in terms of above mentioned ratios. As we have discussed in our recent post, efficiency of diesel cycle will be determined with the help of following formula.
Do you have any suggestions? Please write in comment box.
We will see another topic in our next post in the category of thermal engineering.

Reference:

Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof S. K. Som
Image courtesy: Google

Also read

Continue Reading

Monday, 14 November 2016

DIESEL CYCLE EFFICIENCY DERIVATION

We were discussing Diesel cycle, the ideal cycle for the operation of internal combustion compression ignition reciprocating engines, in our recent post. We have also discussed basic operations and arrangements of various components of ideal cycle for the operation of internal combustion compression ignition reciprocating engines. We have also seen there the PV diagram for a Diesel cycle.

  Today we will see here the calculation of efficiency of Diesel cycle with the help of this post and we will also see here the TS diagram for a Diesel cycle.

Diesel cycle: Efficiency

Diesel cycle is one type of air standard cycle which is designated as the ideal cycle for the operation of internal combustion compression ignition reciprocating engines. Before understanding the method for determination of the efficiency of the Diesel cycle, we will have to remind here various processes involved.

Therefore first let us see an overview of a Diesel cycle with the help of PV diagram and TS diagram as displayed here in following figure. As we can see in below figure, there will be two isentropic or adiabatic processes, one constant volume process and one constant pressure process. We will determine the various properties for unit mass of working fluid.

Process 1-2: Compression stroke

Let us use the concept of first law of thermodynamics 
Q= ΔU + W
As we have already seen that process 1-2 will follow constant entropy process and therefore Q= 0
Hence we will have from first law of thermodynamics ΔU = - W
ΔU = CV (T2-T1)
W = CV (T1-T2)

Process 2-3: Combustion stroke

As we have already seen that process 2-3 will follow constant pressure process and therefore
W = P2 (V3-V2)
ΔU = CV (T3-T2)
Hence, heat energy addition to the system Q = Q1 = CP (T3-T2)

Process 3-4: Expansion or power stroke

As we have already seen that process 3-4 will follow constant entropy process and therefore Q= 0
Hence we will have from first law of thermodynamics ΔU = - W
ΔU = CV (T4-T3)
W = CV (T3-T4)

Process 4-1: Blow down

  As we have already seen that process 4-1 will follow constant volume process and therefore W= 0
Q= ΔU = CV (T1-T4)
Hence, heat energy rejection from the system Q = Q2 = CV (T4-T1)

Efficiency of the Otto cycle

Efficiency of the Otto cycle will be determined with the help of following formula
Do you have any suggestions? Please write in comment box.
We will see another topic in our next post in the category of thermal engineering.

Reference:

Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof S. K. Som
Image courtesy: Google

Also read


Continue Reading

Archivo del blog

Copyright © ENGINEERING MADE EASY | Powered by Blogger | Designed by Dapinder