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HÓA LÝ Chapter – Thermodynamic cycles Introduction Classification of thermodynamic cycle: • Power / refrigeration cycle • Gas / vapor cycle • Closed / open cycle Chapter – Thermodynamic cycles Introduction Otto cycle Internal combustion engine Diesel cycle Gas turbine Brayton cycle Jet engine Power cycle Rankine cycle External combustion engine Stirling cycle Ericsson cycle Steam turbine Chapter – Thermodynamic cycles Introduction Carnot cycle Gas cycle Stirling cycle Vapor cycle Rankine cycle Chapter – Thermodynamic cycles Introduction Chapter – Thermodynamic cycles Brayton cycle Ideal Diesel cycle Ideal Otto cycle Chapter – Thermodynamic cycles Introduction Ideal Rankine cycle Chapter – Thermodynamic cycles Carnot cycle Diagrams for a Carnot Cycle 1–2 isothermal expansion (in contact with TH) 2–3 isentropic expansion to TC 3–4 isothermal compression (in contact with TC) 4–1 isentropic compression to TH Chapter – Thermodynamic cycles Carnot cycle • Thermodynamic cycle for heat engines • Describes the thermodynamic energy conversion process for the most efficient heat engine • The cycle has states • Q1 is the heat (i.e., energy) provided to the Carnot engine • Q2 is the heat that the engine returns to the environment (heat rejection) • W is the work (i.e., energy) produced in one cycle Chapter – Thermodynamic cycles Carnot cycle W = Q1 - Q Q Since dS T TdS then Q � S2 S4 S1 S3 W Q1 Q2 �T1dS �T2 dS W Q1 Q2 (T1 T2 )( S2 S1 ) Efficiency: W Q1 (T1 T2 )( S2 S1 ) T 1 T1 ( S2 S1 ) T1 • Observation #1: The efficiency increases as T1 increases (higher quality heat) and T2 (typically the ambient temperature) decreases • Observation #2: Since T2 can never be zero, the efficiency can never be • Observation #3: Stirling engines operation approximates a Carnot Cycle Chapter – Thermodynamic cycles Carnot cycle The Carnot engine is useful as an idealized model All of the heat input originates from a source at a single temperature, and all the rejected heat goes into a cold reservoir at a single temperature Since the efficiency can only depend on the reservoir temperatures, the ratio of heats can only depend on those temperatures eCarnot QC TC 1 1 QH TH Chapter – Thermodynamic cycles • Vapor power cycle: the one in which the working fluid is alternatively vaporized and condensed • Steam is the most common working fluid used in vapor power cycles because of its many desirable characteristics, such as low cost, availability, and high enthalpy of vaporization Chapter – Thermodynamic cycles Carnot cycle is the most efficient cycle operating between two specified temperature limits However, the Carnot cycle is not a suitable model for power cycles Several impracticalities are associated with this cycle: Temperature can not be higher than the critical temp (374 oC) Process 2-3: quality of steam is low corrosion for turbine blade Process 4-1: compress a two phase fluid is impossible Chapter – Thermodynamic cycles Rankine cycle Ideal cycle for vapor power cycles, consists of the following four processes: • 1-2 Isentropic compression in a pump • 2-3 Constant pressure heat addition in a boiler • 3-4 Isentropic expansion in a turbine • 4-1 Constant pressure heat rejection in a condenser Chapter – Thermodynamic cycles Chapter – Thermodynamic cycles Energy analysis Pump (q = 0) w pump ,in h2 h1 w pump ,in v P2 P1 h1 h f @ P1 and v v1 v f @ P1 Boiler (w = 0): qin h3 h2 Turbine (q = 0) w turb,out h3 h4 Condenser (w = 0): qout h4 h1 Chapter – Thermodynamic cycles The thermal efficiency of the Rankine cycle is determined from w net qout th 1 qin qin where : w net qin qout w turb,out w pump ,in Chapter – Thermodynamic cycles Chapter – Thermodynamic cycles Chapter – Thermodynamic cycles Chapter – Thermodynamic cycles Chapter – Thermodynamic cycles Chapter – Thermodynamic cycles Chapter – Thermodynamic cycles Chapter – Thermodynamic cycles