Isobaric combustion

Heat engines that use gases as the working fluid in an open system model are treated in this chapter. The modern gas turbine engine operates on the Brayton cycle. The basic Brayton cycle consists of an isentropic compression process, an isobaric combustion process, an isentropic... 

Figure 2, Enthalpy, entropy diagram with exergy for reactants e and reaction products after an adiabatic, isobaric combustion (4).

Figure 4. Enthalpy, entropy diagram with exergy for reactants e- and reaction products 2 after an isothermal, isobaric combustion Q4) The change of state from 1 to la is a release of heat at constant temperature, therefore the entropy decreases.

Only the ideal cases of an isothermal-isobaric combustion process will be assumed. This combustion is superior to the usual isobaric-adiabatic process. Such an assumption can be verified more easily than in a normal combustion process, since in the cases studied here the chemical reactions take place at the surface of the oxygen carriers. 

In an isobaric combustion, the combustion occurs without any loss of heat to the surroundings (adiabatic) and at a constant pressure (isobar), while the combustion products are found in a chemical equilibrium (e.g. rocket propellants). 

Habisreuther P, Philipp M, EickhofF H, Leuckel W Mathematical modeling of mrbrJent swirling flames. In High intensity combustors—steady isobaric combustion, Weinheim, Germany, 2005, Wiley-VCH Verlag, pp 156-175. 

The combustion process in internal combustion engines as an isobaric or isometric heat-addition process is oversimplistic and not realistic. A real cycle p-v diagram of the Otto or Diesel cycle looks like a curve (combination of isobaric and isometric) rather than a linear line. Are the combustion processes in the dual cycle more realistic ... 

Assume a process for each of the four devices (1) compressor as adiabatic with efficiency of 85%, (2) combustion chamber as isobaric, (3) turbine as adiabatic with efficiency of 89%, and (4) heat exchanger as isobaric on both hot and cold sides. Input the given information (1) working fluid is air, (2) inlet pressure and temperature of the compression device are 14.7 psia and 60°F, (3) inlet pressure and temperature of the turbine are 120 psia and 2000°F, (4) mass flow rate of air is 1 Ibm/sec, (5) exit pressure of the turbine is 14.7 psia, (6) display the exit temperature of the compressor (it is 562.5°F), and (7) input the exit temperature of the exhaust turbine gas... 

Assume compressor as adiabatic and 80% efficient, combustion chamber and mixing chamber as isobaric, and turbines as adiabatic and 80% efficient. 

A Braysson cycle (Fig. 4.32) uses air as the working fluid with 1 kg/ sec of mass flow rate through the cycle. In the Brayton cycle, air enters from the atmospheric source to a compressor at 20°C and 1 bar (state 1) and leaves at 8 bars (state 2) air enters an isobaric heater (combustion chamber) and leaves at 1100°C (state 3) and air enters a high-pressure turbine and... 

A Braysson cycle (Fig. 4.32) uses air as the working fluid with 1 kg/sec mass flow rate through the cycle. In the Brayton cycle, air enters from the atmospheric source to a compressor at 20° C and 1 bar (state 1) and leaves at 8 bars (state 2) air enters an isobaric heater (combustion chamber) and leaves at 1100°C (state 3) air enters a high-pressure isentropic turbine and leaves at 1 bar (state 4). In the Ericsson cycle, air enters a low-pressure isentropic turbine and leaves at 0.04 bar (state 5) air enters a first-stage compressor and leaves at 0.2 bar (state 6) air enters an isobaric intercooler and leaves at 20°C (state 7) air enters a second-stage compressor and leaves at 1 bar (state 8) and air is discharged to the atmospheric sink. Assume all compressors have 85% efficiency. 

Combustion occurs under isobaric, steady-state conditions. 

As previously stated for internal energy, the change in enthalpy is solely a reflection of the initial and final states of the system. It does not matter whether the change is the result of one reaction or many intermediate reactions. For instance, the test tube combustion of glucose to carbon dioxide and water has the same enthalpy of reaction in an isothermic, isobaric, closed system as the algebraic sum of the enthalpies for all the reactions in the respiratory degradation of the sugar by cells in a similar system. This is an example of Hess s Law. 

A third way of combustion would be isothermal, isobaric as indicated in Figure 4. This reaction requires a heat reservoir... 

Figure 13 shows the exergetic efficiency vs. the maximum temperature for different combustion processes with and without intermediate reactions. As a comparison, the adiabatic, isobaric... 

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