Termofluida W10 - PART 1a Pembangkit Listrik Tenaga Gas [Brayton Cycle]

Summary

In this video, the speaker explains various power cycles, including the Brayton cycle, Rankine cycle, Otto cycle, and Diesel cycle. The focus is on understanding thermal efficiency, which is the ratio of net power to the heat input in a system. The video walks through the details of the Brayton cycle used in gas turbines, distinguishing between open and closed cycle systems. It covers key concepts like pressure, temperature, enthalpy, and entropy changes within components such as compressors, heat exchangers, turbines, and condensers. Additionally, it highlights the assumptions commonly used in power cycle analysis.

Takeaways

• 😀 The video introduces four types of power cycles: Gas Cycle (Brighton Cycle), Rankine Cycle, Otto Cycle, and Diesel Cycle.
• 😀 Thermal efficiency (η) is defined as the ratio between net work output (W_net) and heat input (Q_in) to the boiler in a power cycle.
• 😀 The basic steps in the power cycle process include: compression of fluid, heat addition, expansion, and heat rejection.
• 😀 The ideal Carnot cycle is a theoretical model used to understand the basic principles of heat engines, but it is not practical for real-world applications.
• 😀 The Brighton cycle, used in gas turbines, consists of an open-cycle system where the air is compressed, mixed with fuel, combusted, expanded in a turbine, and the exhaust gas is expelled.
• 😀 In the closed Brighton cycle, the exhaust gas is recycled in a heat exchanger to recover energy and reduce waste heat.
• 😀 The diagram of the Brighton cycle involves stages of compression, heat addition, expansion, and heat rejection, with isentropic processes (constant entropy) occurring in the compression and expansion stages.
• 😀 Pressure ratio in the power cycle refers to the ratio of the pressure at the output of the compressor to the input of the compressor (P2/P1) and similarly for the turbine (P3/P4).
• 😀 Assumptions for analysis of power cycles include neglecting friction (pressure drop), treating expansion and compression as quasi-equilibrium (steady-state), and assuming perfect insulation in pipes.
• 😀 The heat rejection and heat addition processes occur at constant pressure, with temperature increasing during heat addition and decreasing during heat rejection, ensuring the cycle remains efficient.

Q & A

What is thermal efficiency in the context of power cycles?

Thermal efficiency refers to the ratio of net power output (Wnet) to the heat input (Qin) into the system, and it measures how effectively a power cycle converts heat energy into work.

What is the primary difference between an open cycle and a closed cycle in the Brayton cycle?

In an open cycle, like the one used in gas turbines, the exhaust gases are expelled into the atmosphere after passing through the turbine. In a closed cycle, exhaust gases are recirculated through a heat exchanger to recover heat before being cooled and reintroduced into the cycle.

What role does the condenser play in a Rankine cycle?

The condenser in a Rankine cycle removes heat from the exhaust steam after it passes through the turbine, cooling it down to condense it back into water for reuse in the cycle.

How does the concept of pressure ratio affect the performance of a compressor in a Brayton cycle?

The pressure ratio is the ratio of the pressure at the compressor exit to the pressure at the compressor inlet. A higher pressure ratio generally results in a more efficient compressor and higher potential energy recovery in the cycle.

What is the significance of the isentropic process in the Brayton cycle?

An isentropic process is one in which entropy remains constant. In the Brayton cycle, this idealized process ensures that the compression and expansion in the compressor and turbine, respectively, are as efficient as possible without losses due to friction or heat generation.

Why are kinetic and potential energy often neglected in power cycle analysis?

Kinetic and potential energy are typically negligible in power cycle analysis because the main focus is on the conversion of heat energy to work, and these forms of energy have minimal impact on the overall efficiency or operation of the cycle.

What is the key difference between the Rankine cycle and the Brayton cycle?

The Rankine cycle uses steam as the working fluid, where water is heated in a boiler to form steam that drives a turbine. The Brayton cycle, on the other hand, uses gas as the working fluid and involves compressing and expanding air in a turbine to generate work.

How does the heat exchanger function in the closed Brayton cycle?

In the closed Brayton cycle, the heat exchanger adds heat to the compressed air (or gas) to raise its temperature before it enters the turbine. It also removes heat from the exhaust gases to cool them down before they are recirculated.

What does the term 'quasi-equilibrium expansion and compression' mean in the context of power cycles?

Quasi-equilibrium expansion and compression refer to processes that occur slowly enough for the system to remain nearly in thermodynamic equilibrium, meaning that temperature, pressure, and volume changes happen gradually, minimizing losses due to non-equilibrium effects.

What happens during the heat addition and heat rejection phases of a Brayton cycle?

During heat addition, heat is supplied to the compressed gas to increase its temperature and energy before it enters the turbine. During heat rejection, heat is removed from the exhaust gases to cool them down, ensuring they are ready for compression again.