Môn Hóa Lý: Chapter 6 thermodynamic cycles

Brayton cycle Ideal Diesel cycle Ideal Otto cycle... • Thermodynamic cycle for heat engines• Describes the thermodynamic energy conversion process for the most efficient heat engine.. •

Dr Ngo Thanh An

Classification of thermodynamic cycle:

• Power / refrigeration cycle

• Gas / vapor cycle

• Closed / open cycle

Introduction

Power cycle

External combustion engine

Introduction

Brayton cycle Ideal Diesel cycle Ideal Otto cycle

Ideal Rankine cycle

Introduction

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

Carnot cycle

• Thermodynamic cycle for heat engines

• Describes the thermodynamic energy conversion process for the most efficient heat engine.

• The cycle has 4 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

Carnot cycle

W = Q1 - Q2

Since then

Efficiency:

• 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 1.

• Observation #3: Stirling engines operation approximates a Carnot Cycle.

Carnot cycle

Q dS

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

• 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.

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 (374oC)

• Process 2-3: quality of steam is low  corrosion for turbine blade

• Process 4-1: compress a two phase fluid is impossible

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

Rankine cycle

1 2

,

1 2

,

v v

P f

in pump

in pump

v h

h

P P

v w

h h

4 3

, h h

1

4 h h

The thermal efficiency of the Rankine cycle is determined from

in

out in

net th

q

q q

turb out

in

w