Properties of Fluids

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CONTINUITY EQUATION Fluid Flow Even  though  a  detailed  analysis  of  fluid  flow  can  be  extremely  difficult,  the  basic  concepts involved in fluid flow problems are fairly straightforward.   These basic concepts can be applied in solving fluid flow problems through the use of simplifying assumptions and average values, where appropriate.   Even though this type of analysis would not be sufficient in the engineering design of systems, it is very useful in understanding the operation of systems and predicting the approximate response of fluid systems to changes in operating parameters. The basic principles of fluid flow include three concepts or principles; the first two of which the student  has  been  exposed  to  in  previous  manuals.    The  first  is  the  principle  of  momentum (leading to equations of fluid forces) which was covered in the manual on Classical Physics.  The second is the conservation of energy (leading to the First Law of Thermodynamics) which was studied  in  thermodynamics.    The  third  is  the  conservation  of  mass  (leading  to  the  continuity equation) which will be explained in this module. Properties of Fluids fluid is any substance which flows because its particles are not rigidly attached to one another. This includes liquids, gases and even some materials which are normally considered solids, such as glass.   Essentially, fluids are materials which have no repeating crystalline structure. Several properties of fluids were discussed in the Thermodynamics section of this text.   These included temperature, pressure, mass, specific volume and density.  Temperature was defined as the relative measure of how hot or cold a material is.  It can be used to predict the direction that heat  will  be  transferred.   Pressure  was  defined  as  the  force  per  unit  area.   Common  units  for pressure  are  pounds  force  per  square  inch  (psi).   Mass  was  defined  as  the  quantity  of  matter contained in a body and is to be distinguished from weight, which is measured by the pull of gravity  on  a  body.    The  specific  volume  of  a  substance  is  the  volume  per  unit  mass  of  the substance.   Typical units are ft3/lbm.   Density, on the other hand, is the mass of a substance per unit  volume.    Typical  units  are  lbm/ft3.    Density  and  specific  volume  are  the  inverse  of  one another.   Both density and specific volume are dependant on the temperature and somewhat on the pressure of the fluid.  As the temperature of the fluid increases, the density decreases and the specific volume increases.   Since liquids are considered incompressible, an increase in pressure will result in no change in density or specific volume of the liquid.   In actuality, liquids can be slightly  compressed  at  high  pressures,  resulting  in  a  slight  increase  in  density  and  a  slight decrease in specific volume of the liquid. Buoyancy Buoyancy is defined as the tendency of a body to float or rise when submerged in a fluid.   We all have had numerous opportunities of observing the buoyant effects of a liquid.   When we go swimming, our bodies are held up almost entirely by the water.   Wood, ice, and cork float on water.  When we lift a rock from a stream bed, it suddenly seems heavier on emerging from the water.   Boats rely on this buoyant force to stay afloat.   The amount of this buoyant effect was first computed and stated by the Greek philosopher Archimedes.   When a body is placed in a fluid, it is buoyed up by a force equal to the weight of the water that it displaces. HT-03 Page 2 Rev. 0


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