## Pressure Ratio – Brayton Cycle – Gas Turbine

The** thermal efficiency** in terms of the compressor **pressure ratio** (PR = p_{2}/p_{1}), which is the parameter commonly used:

In general, **increasing the pressure ratio** is the most direct way to increase the overall thermal efficiency of a Brayton cycle, because the cycle approaches the Carnot cycle.

According to Carnot’s principle higher efficiencies can be attained by increasing the temperature of the gas.

But there are also **limits on the pressure ratios** that can be used in the cycle. The highest temperature in the cycle occurs at the end of the combustion process, and it is limited by the **maximum temperature** that the **turbine blades** can withstand. As usual, metallurgical considerations (about 1700 K) place an upper limits on thermal efficiency.

Consider the effect of compressor pressure ratio on thermal efficiency when the turbine inlet temperature is restricted to the maximum allowable temperature. There are two Ts diagrams of Brayton cycles having the same turbine inlet temperature but different compressor pressure ratios on the picture. As can be seen for a fixed-turbine inlet temperature, the net work output per cycle (W_{net} = W_{T} – W_{C}) decreases with the pressure ratio (**Cycle A**). But the Cycle A has the greater efficiency.

On the other hand, the **Cycle B** has a larger net work output per cycle (enclosed area in the diagram) and thus the greater net work developed per unit of mass flow. The work produced by the cycle times a mass flow rate through the cycle is equal to the power output produced by the gas turbine.

Therefore with less work output per cycle (Cycle A), a larger mass flow rate (thus a **larger system**) is needed to maintain the same power output, which may not be economical. This is the key consideration in design of gas turbine, since here engineers must balance the thermal efficiency and the compactness. In most common designs, the pressure ratio of a gas turbine ranges from about 11 to 16.

We hope, this article, **Pressure Ratio – Brayton Cycle – Gas Turbine**, helps you. If so, **give us a like** in the sidebar. Main purpose of this website is to help the public to learn some interesting and important information about thermal engineering.