Thermodynamic Cycles: Introduction and Analysis PDF - Engineering

Thermodynamic Cycles: Introduction and Analysis Joseph Aldrin Chua Thermodynamic Cycle A thermodynamic cycle is a sequence of processes that begins and ends at the same state. At the conclusion of a cycle all properties have the same values they had at the beginning. Consequently, ove...

Thermodynamic Cycles: Introduction and Analysis Joseph Aldrin Chua Thermodynamic Cycle A thermodynamic cycle is a sequence of processes that begins and ends at the same state. At the conclusion of a cycle all properties have the same values they had at the beginning. Consequently, over the cycle the system experiences no net change of state. Cycles that are repeated periodically play prominent roles in many areas of application. Cycle Energy Balance ∆𝐸𝐶𝑦𝑐𝑙𝑒 = 𝑄𝐶𝑦𝑐𝑙𝑒 − 𝑊𝐶𝑦𝑐𝑙𝑒 “no net change of state.” ∴ ∆𝐸𝐶𝑦𝑐𝑙𝑒 = 0 𝑾𝑪𝒚𝒄𝒍𝒆 = 𝑸𝑪𝒚𝒄𝒍𝒆 Refrigeration and Heat Power Cycles Pump Cycles Power Cycles The devices or systems used to produce a net power output are often called engines, and the thermodynamic cycles they operate on are called power cycles. Heat engines are designed for the purpose of converting thermal energy to work, and their performance is expressed in terms of the thermal efficiency ηth Examples of Power Cycles: Otto Cycle Examples of Power Cycles: Diesel Cycle  Refrigeration Cycles (Reversed cycles) The devices or systems used to produce refrigeration are called refrigerators, air conditioners, or heat pumps, and the cycles they operate on are called refrigeration cycles (Reversed cycles). The objective of a refrigeration cycle is to cool a refrigerated space or to maintain the temperature below that of the surroundings. The objective of a heat pump is to maintain the temperature above that of the surroundings. Coefficient of Performance (COP) Vapor Compression Refrigeration Cycle Power vs Refrigeration Cycles Examples 1. The net work of a power cycle operating is 10,000 kJ, and the thermal efficiency is 0.4. Determine the heat transfers Qin and Qout, each in kJ. Ans: Qin = 25000 kJ; Qout = 15000 kJ 2. A refrigeration cycle operating has Qout = 1000 Btu and Wcycle = 300 Btu. Determine the coefficient of performance for the cycle. Ans: COP = 2.33 Example 3 A gas within a piston–cylinder assembly undergoes a thermodynamic cycle consisting of three processes in series: Process 1–2: Compression with U2 = U1. Process 2–3: Constant-volume cooling to p3 = 140 kPa, V3 = 0.028 m3. Process 3–1: Constant-pressure expansion with W31 = 10.5 kJ. For the cycle, Wcycle = -8.3 kJ. There are no changes in kinetic or potential energy. Determine (a) the volume at state 1, in m3, (b) the work and heat transfer for process 1–2, each in kJ. (c) Can this be a power cycle? A refrigeration cycle? Explain. Ans: V1 = 0.103 m3; W12 = -18.8; Q12 = -18.8; Refrigeration Cycle Exercises Q12 = 74 kJ; Q31 = 22kJ; U3 = 540 kJ; Reversed Cycle Exercises Wnet = -280.52 kJ ; Q23 = 80kJ