Chapter2 - 02-R3868 7/20/06 10:09 AM Page 23 CHAPTER...

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PROPERTIES OF FLUIDS I n this chapter, we discuss properties that are encountered in the analysis of fluid flow. First we discuss intensive and extensive properties and define density and specific gravity . This is followed by a discussion of the properties vapor pressure , energy and its various forms, and the specific heats of ideal gases and incompressible substances. Then we discuss the property viscosity , which plays a dominant role in most aspects of fluid flow. Finally, we present the property surface tension and determine the capillary rise from static equilibrium conditions. The property pressure is discussed in Chap. 3 together with fluid statics. 23 CHAPTER 2 OBJECTIVES When you finish reading this chapter, you should be able to n Have a working knowledge of the basic properties of fluids and understand the continuum approximation n Have a working knowledge of viscosity and the consequences of the frictional effects it causes in fluid flow n Calculate the capillary rises and drops due to the surface tension effect A drop forms when liquid is forced out of a small tube. The shape of the drop is determined by a balance of pressure, gravity, and surface tension forces.
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2–1 n INTRODUCTION Any characteristic of a system is called a property . Some familiar proper- ties are pressure P , temperature T , volume V , and mass m . The list can be extended to include less familiar ones such as viscosity, thermal conductiv- ity, modulus of elasticity, thermal expansion coefficient, electric resistivity, and even velocity and elevation. Properties are considered to be either intensive or extensive . Intensive properties are those that are independent of the mass of the system, such as temperature, pressure, and density. Extensive properties are those whose values depend on the size—or extent—of the system. Total mass, total vol- ume V , and total momentum are some examples of extensive properties. An easy way to determine whether a property is intensive or extensive is to divide the system into two equal parts with an imaginary partition, as shown in Fig. 2–1. Each part will have the same value of intensive properties as the original system, but half the value of the extensive properties. Generally, uppercase letters are used to denote extensive properties (with mass m being a major exception), and lowercase letters are used for intensive properties (with pressure P and temperature T being the obvious exceptions). Extensive properties per unit mass are called specific properties . Some examples of specific properties are specific volume ( v 5 V / m ) and specific total energy ( e 5 E / m ). The state of a system is described by its properties. But we know from experience that we do not need to specify all the properties in order to fix a state. Once the values of a sufficient number of properties are specified, the rest of the properties assume certain values. That is, specifying a certain number of properties is sufficient to fix a state. The number of properties required to fix the state of a system is given by the
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Chapter2 - 02-R3868 7/20/06 10:09 AM Page 23 CHAPTER...

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