Anh Văn CN Nhiệt chapter 3

The first law of thermodynamics1 2 The thermodynamic cycles 3 The second law of thermodynamics Contents Chapter 3 The laws of thermodynamics... The change in internal energy of a system

LOGO Anh văn Chuyên ngành Nhiệt English for thermal engineering

The first law of thermodynamics

1

2

The thermodynamic cycles

3

The second law of thermodynamics

Contents

Chapter 3 The laws of thermodynamics

Tài liệu tham khảo

1 Fundamentals of thermal-fluid science, Y A Çengel.

2 Fundamentals of thermodynamics (sixth edition),

Sonntag, Borgnakke and van Wylen.

3 Fundamentals of engineering thermodynamics (Fifth

edition), Michael J Moran, Howard N Shapiro.

Chapter 3 The laws of thermodynamics

The first law

Statements

1 The first law of thermodynamics states that energy can be

neither created nor destroyed; it can only change forms

2 The change in internal energy of a system is equal to the

heat added to the system minus the work done by the

system

The first law of thermodynamics is essentially an expression

of the conservation of energy principle, also called the energy balance.

Chapter 3 The laws of thermodynamics

The first law

For closed system

or The energy balance in differential form:

The instantaneous time rate form of the energy balance:

Chapter 3 The laws of thermodynamics

The first law

For closed system

Stationary systems:

Per unit mass:

The energy change of a system during

a process is equal to the net work and heat transfer between the system and

its surroundings.

Figure 1

Chapter 3 The laws of thermodynamics

The first law

Example 1: Heating of a Gas by a Resistance Heater

A piston-cylinder device initially contains 0.5 m3 of nitrogen gas at 400 kPa and 27°C An electric heater within the device is turned on and is allowed to pass a current of 2 A for 5 min from a 120-V source Nitrogen expands at

constant pressure, and a heat loss of 2800 J occurs during the process

Determine the final temperature of nitrogen

Assumptions:

1 Nitrogen is an ideal gas

2 KE = PE = 0 and E = U

3 The pressure is constant

4 Specific heat is constant

Figure 2

Chapter 3 The laws of thermodynamics

The second law

Kelvin-Planck Statement:

“It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir In other words, a perpetual motion machine of the second kind is impossible”

Heat is transferred to a heat engine from a furnace

at a rate of 80 MW And the rate of waste heat

rejection to a nearby river is 50 MW

Figure 3

Chapter 3 The laws of thermodynamics

The second law

The Clausius statement

“It is impossible to construct a device that operates in a cycle and produces

no effect other than the transfer of heat from a lower-temperature body to a higher- temperature body.”

A refrigerator that violates the Clausius statement of

the second law

Figure 4

Chapter 3 The laws of thermodynamics

Thermodynamic cycles

Power cycle

Figure 5: Schematic diagrams of power cycles

Systems undergoing cycles of the type shown in Fig 5 deliver a net work transfer of energy to their surroundings during each cycle

The thermal efficiency:

Chapter 3 The laws of thermodynamics

Thermodynamic cycles

Refrigeration and Heat Pump Cycles

Figure 6: Schematic diagrams of Refrigeration and heat pump cycles

The refrigeration and heat pump cycles shown in

Fig 6

Chapter 3 The laws of thermodynamics

Thermodynamic cycles

Refrigeration Cycle

Figure 6: Schematic diagrams

of Refrigeration and heat pump

cycles

The performance of refrigeration cycles can be

described by the coefficient of performance, COP, is:

Heat pump Cycle

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