Compression Expansion Gases

Reference data and engineering information about compression expansion gases for combustion applications.

compressionexpansiongasesCalculatorData Table

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.

3 rows

Fundamental relationships for gas compression/expansion processes.
Process	Condition	Pressure-Volume Relationship	Exponent
Isothermal	Constant temperature (slow process)	$p_1 V_1 = p_2 V_2$	$n=1$
Isentropic (Adiabatic)	No heat transfer (fast process)	$p_1 V_1^k = p_2 V_2^k$	$n=k$
Polytropic	Intermediate process (real-world)	$p_1 V_1^n = p_2 V_2^n$	$1 < n < k$

Source: engineeringtoolbox.com

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), $k = \frac{c_p}{c_v}$ : $\frac{p}{\rho^k} = \text{constant}$

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 $1.4$ .