Chapter 4

Energy Analysis of Closed Systems

(Analisas Tenaga untuk sistem tertutup)

Oleh Lt Kol Ir Khalid bin Abd Jalil TUDM Jabatan Kejuruteraan Mekanikal Universiti Pertahanan Nasional Malsysia. 012-2094234

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

Moving Boundary Work (Pergerakan Kerja Sempadan)

-Closed system. -Involved with expansion and compression work of gas in a piston cylinder device. -In automobile engine, during expansion, the combustion gases force the piston to move, which in turn forces the crankshaft to rotate. - when the piston moved (expansion & compression) at low velocities in the boundary system, the process called quasi-equilibrium process or quasi-static process (proses mirip static)

A gas (P) does a differential amount of work Wb as it forces the piston to move by a differential amount ds (displacement of piston). 3-2

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

The area under the process curve on a P-V diagram represents the boundary work.

ds=dV=Adl

Where A- area

dl - displacement

Compression (work –ve) 3-3 Expansion (work +ve)

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

Boundary Work – for closed system.

There were 4 types of process involved in the closed system. i. Constant-Volume Process (Proses seisipadu) V1 = V2 ii. Constant-Pressure Process (Proses Setekanan) P1 = P2 iii. Isothermal process (Proses sesuhu) pV =constant iv. Polytropic process (Proses politropik) pVⁿ =constant

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

P

v1=v2

v

Note: Under the Constant Volume Process, the boundary work done has a constant volume and the dV=0, the process is always zero. - work under rigid tank/container.

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

P

P1=P2

Note:

v v v2 v1

Volume changed (expansion & compression) under constant pressure.

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

Constant Temperature Process

P2

2

pV=C

P1

1

v2

v1

Note:

pV = mRT

To ensure that the temperature is always constant, heat is supplied continuously during the expansion process and rejected during the compression process.

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

FIGURE 2-49 Percentage of error involved in assuming steam to be an ideal gas, and the region where steam can be treated as an ideal gas with less than 1 percent error. 2-12

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

Polytrophic Process

P2

2

pVⁿ =C

Liquid/vapor

P1

1

Ideal gas

v2 v1

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

Politropik Process …cont

Liquid/vapor

Ideal gas

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

Boundary Work/

Polytrophic Work/

Constant Pressure Work/

Constant Temperature Work/

Constant Volume Work/

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

Energy Balance for Closed system

Change in internal, kinetic, potential, etc… energy

Net Energy transfer by heat, work & mass

Thermodynamics I (EMM2503) Chapter 4 – Energy Analysis of Closed Systems Duty, Honor and Integrity

1st Law Thermodynamic & Conservation Principle of energy for Closed System

Total Energy Transfer by haet, work & mass

Rate of net energy change in the system

For stationary system -

&

=0...