Page 20 - Modelling in Transport Phenomena A Conceptual Approach

Chapter 1

Introduction

1.1 BASIC CONCEPTS

A concept is a unit of thought. Any part of experience that we can organize into
an idea is a concept. For example, man's concept of cancer is changing all the time
as new medical information is gained as a result of experiments.
Concepts or ideas that axe the basis of science and engineering are chemical
species, mass, momentum, and energy. These axe all conserved quantities. A
conserved quantity is one which can be transformed. However, transformation does
not alter the total amount of the quantity. For example, money can be transferred
from a checking account to a savings account but the transfer does not affect the
total assets.
For any quantity that is conserved, an inventory rate equation can be written to
describe the transformation of the conserved quantity. Inventory of the conserved
quantity is based on a specified unit of time, which is reflected in the term, rate.
In words, this rate equation for any conserved quantity cp takes the form
Rate of ) - ( Rate of ) + ( Rate of

input of cp output of cp generation of cp
Rate of
= ( accumulation of cp ) (1.1-1)
Basic concepts, upon which the technique for solving engineering problems is
based, are the rate equations for the
Conservation of chemical species,
Conservation of mass,

Conservation of momentum,
Conservation of energy.

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