How is the efficiency of a gas turbine system calculated?

The efficiency of a gas turbine system is calculated by assessing how effectively it converts the energy input into useful work output. This ratio is expressed as the comparison of useful work produced by the turbine to the total heat energy input, which comes from burning fuel. When evaluating a gas turbine's performance, this efficiency metric is crucial, as it directly indicates how well the system utilizes the fuel's energy content to perform work—whether that be mechanical energy for propulsion, electricity generation, or other forms of useful energy.

In context, although other choices discuss aspects of the gas turbine operation, they do not provide a direct measure of efficiency as defined by the relationship between work and energy input. For instance, fuel output, airflow, and speed ratios may inform certain operational characteristics of the system but do not encapsulate the fundamental assessment of efficiency as the correct option does. Thus, the ratio of useful work output to heat input most accurately reflects the efficiency in a gas turbine system.