Brayton cycle

The Joule- cycle or Brayton cycle is a fairly continuous thermodynamic cycle, which is named after James Prescott Joule or Brayton George. He is a comparative process for the processes occurring in gas turbines and jet engines process and consists of two isentropic and two isobaric state changes.

Description

• Schematic diagram and state diagrams

Joule process in the p- v chart

Joule- process in the T- s diagram

The four process steps are:

• Heat exchanger ( combustion chamber )
• Supply of the specific heat Pressure remains constant
• Temperature rises from to
• Specific volume is increasing on
• Specific entropy increases from to

• By adiabatic turbine
• Withdrawal of the turbine work Pressure drops by on
• Temperature decreases from to
• Specific volume is increasing on
• Specific entropy remains constant

• A heat exchanger (cooler)
• Withdrawal of the specific heat Pressure remains constant
• Temperature decreases from to
• Specific volume decreases from to
• Specific entropy decreases from to

The trace of (1 - 2 - 3 - 4) enclosed area corresponds to the specific process work w.

In contrast to the closed Joule process is omitted in the open cooling, as cold gas is continuously sucked and compressed.

The supply of heat, which is shown only schematically, is actually realized by the combustion of fossil fuel usually. In jet engines, kerosene is used for this purpose generally illustrating an intermediate fraction of gasoline and diesel in petroleum distillation.

The following pictures show scale diagrams and a table with the state variables and process data from a computationally active file.

• State diagrams and data table

T- s-diagram

Data table

Efficiency

Efficiency in the single-stage Joule- cycle

In general, the thermal efficiency is defined as the ratio of benefits to costs.

When Joule process is the use of the votes in the technical working wNutz, the effort is qzu in the required heat, so can be formulated:

The reported warming is replaced by the enthalpy differences.

Also applies to an ideal gas, the specific enthalpy h is only a function of the temperature and is independent of pressure.

Therefore

The final relationship is derived from the use of the equation for the temperature change in isentropic compression.

Under atmospheric conditions for noble gases such as helium and argon about 1.66; 2 diatomic gases like hydrogen, oxygen, air and about 1.4 to 3 atomic gases having rigid molecules such as water vapor approximately 1.33 (see the derivation of the heat capacity of ideal gases ). Therefore, a Joule- cycle process are most effectively used with inert gases.

However, if the heat capacity, thermal conductivity and viscosity for a real Joule cycle is observed with, then hydrogen is also a very beneficial working medium.

For a given material by the maximum temperature T3 is an optimum temperature T2 can be determined after compression, in which the cycle yields the maximum useful work:

Possibility of increasing the efficiency of regenerative heat transfer and by a multi-stage Joule- cycle

Since the output of the expander usually a temperature prevails, which is above the temperature at the outlet of the compressor, a heat exchanger can here a recuperative heat transfer to take place. This amount of heat then need not be supplied from the outside.

The efficiency is then calculated as follows:

Through a multi-stage compression with respective heat dissipation and a multistage expansion with respective heat supply and regeneration, the efficiency

• Increased by the regeneration temperature range to be covered, and
• By the heat dissipation can be reduced and the compression work
• By the heat supply in the expansion step, the work of expansion can be increased.

With an infinite number of stages of compression - Heat dissipation of the process in an isotherm compression goes. The process is then described by the Ackeret - basement or Ericsson cycle, whose efficiency is calculated analogous to the Carnot process:

Low-maintenance heat engines according to the Joule - cycle

Like a Stirling heat engine, a heat engine in accordance with the Joule cycle is operated with an external supply of heat, and thus has many of the same advantages of a Stirling engine.

For nuclear technology ( nuclear power plants ) Turbo compressors are designed for helium, which can be equipped with magnetic bearings and permamentmagnetischen emergency bearings, which in the working gas liquids, such as lubricating oils must be introduced that may contaminate the gas circuit.

So that a gas turbine is conceivable as the comparison process operates according to the Joule cycle with helium or hydrogen as a working gas, which may have low maintenance, efficient, and a high energy density.

The real gas turbine process

The real gas turbine process is characterized by the irreversibility of the technical state changes (1-2, 3-4) from the theoretical Joule process. In addition, pressure losses occur in the combustion chamber (2-3 ) (or the heat exchanger 4-1, in the closed gas turbine cycle ) on. The pressure change due to the heat losses in the combustion chamber can now be minimized by suitable measures ( high-temperature resistant ceramic), while the pressure drop in the heat exchanger ( 4-1) is only partially reduced. These differences are clearly in the Ts diagram displayed ( T- temperature - specific entropy s ).

The technical work for the compressor and the turbine are in the h, s- diagram illustrates (h specific enthalpy, specific entropy s ).

Other comparison processes

• Carnot cycle
• Rankine process
• Diesel process
• Otto cycle
• Seiliger process