Compression Expansion Gases

Overview

Engineering reference data for Compression Expansion Gases in combustion engineering.

Key Formulas

Heat Release

$Q = \dot{m} \cdot HV$

Fuel energy release rate.

Air-Fuel Ratio

$AF = \frac{m_{air}}{m_{fuel}}$

Mass of air per mass of fuel.

Excess Air

$EA = \frac{O_2}{21 - O_2} \times 100\%$

From flue gas oxygen measurement.

Variables

Symbol	Description	Unit
$Q$	Heat release rate	W
$\dot{m}$	Mass flow rate	kg/s
$HV$	Heating value	J/kg
$AF$	Air-fuel ratio	—

Comparison of Compression/Expansion Processes

The table below summarizes the key characteristics of the three primary gas compression and expansion processes.

Process Details & Key Relationships

Isothermal Process

For an ideal gas undergoing an isothermal process, the product of pressure and volume remains constant. The relationship with density is: $\frac{p}{\rho} = \text{constant}$

Isentropic (Adiabatic) Process

For an adiabatic process with no heat loss, the relationship involves the isentropic exponent (ratio of specific heats), $k = \frac{c_p}{c_v}$: $\frac{p}{\rho^k} = \text{constant}$

Polytropic Process

Most real processes are polytropic. The polytropic exponent $n$ characterizes the path, typically ranging between $1$ (isothermal) and $k$ (isentropic). For air, $k \approx 1.4$, so $n$ often lies between $1.0$ and $1.4$.

Interactive Charts

References

• Original Source