in Architectural Engineering2. The first law

1

Thermal and Fluids

in Architectural Engineering 2. The first law

Jun-Seok Park, Dr. Eng., Associate Prof.

Dept. of Architectural Engineering Hanyang Univ.

Where do we learn in this chaper

1. Introduction 2.The first law

3.Thermal resistances

4. Fundamentals of fluid mechanics

5. Thermodynamics 6. Application

7.Second law 8. Refrigeration,

heat pump, and power cycle

9. Internal flow 10. External flow

11. Conduction 12. Convection 14. Radiation

13. Heat Exchangers 15. Ideal Gas Mixtures

and Combustion

2.1 The First law of thermodynamic 2.2 Heat Transfer

2.3 Internal Energy

2.4 Specific Heat of Ideal Liquids and Solids 2.5 Fundamental Properties of a System

2.6 Ideal gas 2-8 Work

2-11 Specific Heat of Ideal gas

2-14 The “Pizza” procedure for problem solving

2. The First Law

2.1 The First Law of Thermodynamic

□ The central idea of thermodynamic is the principle of conservation of energy

□ Total energy is always conserved in all processes regardless of the forms of energy

system boundary

system boundary

system or Leaving or

Energy Entering

Energy Energy

in Change



 



 



 



 



 





W -

 Q

E

2.1 The First Law of Thermodynamic

□ The Energy, ΔE, of a system consists

- Kinetic energy dew to the velocity of the system

- Potential energy dew to the a gravitational fields

- Internal energy is stored at molecular or atomic in the system (There is no simple expression)

 _{2 1}

mgz ΔPE mgz mgz

PE   





ΔU U U

U  



 



  

 ² ₂² m ₁²

2 m 1

2 1

2m

1  ΔKE  

KE

W -

Q 

ΔE

2.1 The First Law of Thermodynamic

□ Finally, the energy of a system, ΔE, will be expressed as below,



















n n

W ΔU Q

ΔPE ΔKE

ΔU ΔPE

ΔKE

ΔE

) (

W - Q )

(

W - Q

W -

Q 

ΔE

2.2 Heat Transfer

□ Heat is defined as transferred energy dew to temperature differences through boundary

- Conduction by molecular vibration in contact

- Radiation by electrons fall and emit electromagnetic - Convection by fluids

W -

Q 

ΔE

2.3 Internal Energy

□ There is no simple expression in all cases

□ The properties of a system will be changed whenever energy is added to or removed from the system

- The vibration of atoms or molecular

> Temperature change - The change of phase > Temperature un change - The chemical bonds

W -

Q



ΔU

2.4 Specific Heat of Ideal Liquids and Soilds

□ Internal energy will be changed, whenever

- Heat is transferred

- Work is done on or out a system

- Heat and Work is done on or out a system

□ In a case of property, temperature, changes internal energy will be expressed as below,

W -

Q  ΔU

dT T

c m dU

ΔT ΔU m













) (

at const volume

2.4 Specific Heat of Ideal Liquids and Soilds

□ From the integrations of internal energy

W -

Q  ΔU

 

K]

[J/kg, 1

const.) c

( ) ( )

(

1 2

1 2

1 2

1 2

2 1 2

1

2 1 2

1

T T

U U

c m

T T

c m U

U

dT c

m du

dT T

c m du

dT T

c m dU



 



















































2.5 Fundamental Properties of a System

□ Density, pressure, and temperature are the most basic quantities of a system

- Density - Pressure

W -

Q  ΔU

] [kg/m

³

V

 m



N ] 1 Pa

[1

₂

m A

P  F 

2.5 Fundamental Properties of a System

□ Temperature

- Newton : Oil filled thermometer

> 0=freezing point, 12=body temp.

- Gabriel Fahrenheit : Mercury in glass device

> 8 times at Newton scale

> 0=freezing point of salt water, 96=body temp.

32=freezing point of pure water - Anders Celsisus scale

> 0=freezing point, 100=boiling point of pure water - Kelvin scale

> 0=Pressure 0, 273.15=freezing of pure water

W -

Q



ΔU

2.5 Fundamental Properties of a System

□ Temperature and Internal energy

- Example 1: Compression of gas

> Work > increase internal energy > Temp. rise - Example 2 : heating of water with ice

> heating > increase internal energy > Temp. steady - Example 3: Current Work

> Current dose work > internal energy rise

W -

Q



ΔU

2.6 Ideal gases

□ Relationship from kinetic energy of molecular in gases

- Between temp. and pressure in gases - Between volume and pressure in gases

W -

Q  ΔU

) (

M T R

M) n

m (

M T R T R n

V P m

PV m PV

















 





2.8 Work

□ Compression or expansion work in gases

□ Work depends on the path taken between start and end points

-Example > Figure 2-17/2-18

W -

Q  ΔU

) (

) P

(

dV dx

A PdV

W

A dx F

PA W

dx F

W































2.8 Work

□ Electrical Work

-Example 2-7

W -

Q  ΔU

























Idt V

W

dt I dq Idt

V W

dt dt V dq W

dq V

W

) (

 





2.8 Work

□ Shaft Work

W -

Q  ΔU

) (

) Rd dx

FR;

(

dt dt d

W

R Rd W

Fdx W

 







 



























2.8 Work

□ Radian

W -

Q  ΔU

• From Figure, we see that the differential plane angle dθ is defined by a region between the rays of a circle and is measured as the ratio of the element of arc length dl on the circle to the radius r of the circle.

□ In a case of compressible substances (gases), there are two different types of specific heat

(1) Volume of a system is const.

W -

Q  ΔU

T c

m U

T m

U Q U

v























 ( W 0)

2.11 Specific Heat of Ideal gases

at const. specific heat

(2) Pressure of a system is const.

ΔU  Q - W

T c m

T c m Q

H H H

PV U

PV U

V V

P U

U

V P U Q

V P V

V P dV

P W

V P Q U

Q U

p p























































































H

PV) U

H (

) (

) (

) (

) (

) )

( (

0) W

( W -

1 2

1 1

2 2

1 2

1 2

1

 2



2.11 Specific Heat of Ideal gases

-Approach to solve engineering problems involves below procedures

(1) Analysis

(2) Application of governing concepts (3) Evaluation Properties

-Cut a the problem into manageable pieces ! 2.14 The “Pizza” procedure for problem solving