English for Thermal engineering De

English for Thermal engineering

Designed by: Huy Quoc Nguyen

Week 1: Introduction to Thermodynamic
The aim of this chapter is to understand the definition of basic thermodynamic parameters.

1 Pressure
Absolute pressure: The absolute pressure (P) is the force acting on unit area. In the SI system,
the unit for pressure is Pa, Pascal. In the English system, it is psi.
Atmospheric pressure: The air surrounding the earth can be treated as a homogeneous gas,
called atmosphere. Atmospheric pressure (Pa) is the pressure due to the force by the atmosphere
mass. Standard atmospheric pressure is 101325 Pa.
Gage pressure (Pg) is the difference between the absolute pressure and the atmospheric
pressure if the difference is positive. If the difference is negative, it is called vacuum pressure
(Pv).
Barometer is a device used to measure the atmospheric pressure.
Pa = ρ g h
where:
ρ = The density of the working liquid, kg/m3

g = The acceleration of gravity, 9.8 m/s2

h = The height of the working liquid in the tube, m

2 Temperature and the Zeroth Law
The measurement of the degree of hotness or coolness is temperature.
If two bodies at different temperatures are brought together, the hot body will warm up the cold
one. At the same time, the cold body will cool down the hot one. This process will end when
the two bodies have the same temperatures. At that point, the two bodies are said to have
reached thermal equilibrium. English for Thermal engineering

Designed by: Huy Quoc Nguyen

The Zeroth Law of thermodynamics states: Two bodies each in thermal equilibrium with a
third body will be in thermal equilibrium with each other
Notice: The Zeroth Law of thermodynamics is a basis for the validity of temperature
measurement.
Thermometers measure temperature, by using materials that change in some way when they
are heated or cooled. In a mercury or alcohol thermometer the liquid expands as it is heated
and contracts when it is cooled, so the length of the liquid column becomes longer or shorter
depending on the temperature. Modern thermometers are calibrated in standard temperature
units such as Fahrenheit or Celsius. Three practical points for using thermometer are:
 The thermometer should be isolated to everything except the body which temperature
is measured. The general method is to immerse the thermometer in a hole in a solid
body, or directly in a fluid body.
 When thermal equilibrium is reached, the thermometer can indicate its own temperature
as well as the body measured. The thermometer should be small relative to the body so
that it only has a small effect upon the body.
 The thermometer must not be subject to effects such as pressure changes, which might
change the volume independently of temperature.
Digital thermometers almost replace the mercury ones in nowadays because they are more
accurate and easier to use.
3 Heat
Heat is energy transferred from one system to another solely by reason of a temperature
difference between the systems. Heat exists only as it crosses the boundary of a system and the
direction of heat transfer is from higher temperature to lower temperature.
For thermodynamics sign convention, heat transferred to a system is positive; Heat transferred
from a system is negative.
The heat needed to raise an object's temperature from T1 to T2 is:
Q = cp m (T2 - T1)
where: cp = specific heat of the object; m = mass of the object English for Thermal engineering

Designed by: Huy Quoc Nguyen

Modes of heat transfers:
Conduction: Heat transferred between two bodies in direct contact.
Convection: Heat transfer between a solid surface and an adjacent gas or liquid. It is
the combination of conduction and flow motion. Heat transferred from a solid surface to a
liquid adjacent is conduction. And then heat is brought away by the flow motion.
Radiation: The energy emitted by matter in the form of electromagnetic waves as a
result of the changes in the electronic configurations of the atoms or molecules.
4 Work
Work, like heat, is an energy interaction between a system and its surroundings. In other word,
work is the energy transfer associated with a force acting through a distance. A rising
piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated
with work interactions.
In thermodynamics sign convection, work transferred out of a system is positive with respect
to that system. Work transferred in is negative.
5 Energy
Energy is the capacity for doing work. It may exist in a variety of forms such as thermal,
mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear. It may be transferred
from one type of energy to another. For example,
Heating water by gas: Chemical energy ---> thermal energy
Heating water by electricity: electric energy ---> thermal energy
Running nuclear power plant: Nuclear energy ---> e