Question

If regenerator is added to a Brayton cycle, the thermal efficiency is improved due to

• A. Increase in temperature of gases at turbine inlet
• B. Decrease in compressor input
• C. Increase in turbine output
• D. Decrease in quantity of heat in combustion chamber

Hint:

Regeneration = Heat Recovery from exhaust. It doesn't alter the compressor or turbine work profiles ($W_{\text{net}} = \text{constant}$). Its sole benefit is preheating the air, which directly cuts down on the fuel required in the combustion chamber ($Q_{\text{in}} \downarrow$).

Answer 1

The Correct Option is D

Concept: A regenerator (or recuperator) is a counter-flow heat exchanger incorporated into a gas turbine plant to recover thermal energy from the hot exhaust gases leaving the turbine. This recovered energy is transferred directly to the cooler compressed air exiting the compressor before it enters the combustion chamber. The thermal efficiency ($\eta$) of any power cycle is given by the ratio of net work output to heat input: \[ \eta = \frac{W_{\text{net}}}{Q_{\text{in}}} = \frac{W_t - |W_c|}{Q_{\text{in}}} \] Adding an ideal regenerator has the following effects on the cycle parameters:
• The compressor work input ($W_c$) remains unchanged because the compressor operates between the same pressure limits.
• The turbine work output ($W_t$) remains unchanged because the turbine expansion profile is unaltered.
• Consequently, the net work output ($W_{\text{net}} = W_t - |W_c|$) stays constant.

Step 1: Analyze how regeneration affects heat input (\(Q_{\text{in}}\)).
Because the compressed air is preheated by the turbine exhaust gases before entering the combustor, less fuel needs to be burned to raise the air temperature up to the maximum required turbine inlet temperature ($T_3$). Therefore, the external heat input required in the combustion chamber ($Q_{\text{in}}$) decreases significantly.

Step 2: Evaluate the impact on thermal efficiency.
Looking back at our efficiency equation: \[ \eta \uparrow = \frac{W_{\text{net}} \quad (\text{constant})}{Q_{\text{in}} \quad (\text{decreases}\downarrow)} \] Since a smaller value divides the constant net work output, the overall thermal efficiency increases. This improvement stems directly from the reduction in fuel energy required within the combustion chamber, which perfectly matches Option (D).