Abrasive Machining

1. Abrasives

       Abrasives can be natural or manmade.

Abrasive Requirements

•High hardness at room & elevated temperure

•Controlled toughness or rather ease of fracture, allows fracture to occur under imposed mechanical and thermal stresses

•Low adhesion to the workpiece of material

•Chemical stability

•The grains must have a shape that presents several sharp cutting edges

Hardness dan Tempratur

2. Abrasive Grain Size

        Grains are separated by mechanical sieving machines.  The number of openings per linear inch in the sieve(or screen) through which the particles can pass determines the grain size.

*Typical classifications:

–Course, medium, and fine

–Silicon Carbides range from 2-240 in size

–Aluminum Oxides range from 4-240 in size

   •600 range are generally used in honing or lapping operations

3. Forms

Abrasive particles can be:

    •A Free slurry

    •Adhered to resin on a belt   

    •Close packed into wheels or stones held together by a bonding agent.

4. Cylindrical Grinding

     •Cylindrical grinding is used to produce external cylindrical surfaces

    •In cylindrical grinding the workpiece is mounted and rotated on a longitudinal axis, the grinding wheel rotate in the same axis, but in opposite directions.

     •With long workpieces, the workpiece typically is moved relative to the wheel.

     •With smaller high production parts, a chuck-type external grinder is used, and the wheel moves relative to the workpiece.

5. Centerless Grinding

     •In centerless grinding the workpiece can be ground internally or externally without requiring the material to be mounted in a center or chuck.

     •The workpiece rests between two wheels, one providing the grinding and the other providing regulation of the grinding speed.

Steady Two-Phase Flow to Fluid Mechanics Solutions

         A phase is simply one of the states of matter and can be either a gas, a liquid, or a solid. The general subject of two-phase flow includes gas-liquid, gas-solid, and solid-liquid flow. The term multicomponent is used to describe flows in which the phases do not consist of the same chemical substance. In the petrochemical industries many processes involve the evaporation (and condensation) of binary (n 2) and multicomponent mixtures. Pure single-component, two-phase flows are those during evaporation and condensation of the same chemical substance. For example, steamwater flow is a single-component, two-phase flow, while air-water is a two-phase, two-component flow. The main emphasis of the following presentation is on the two-phase flow of water.

Regimes of Gas-Liquid Flow

         Description. Cocurrent, simultaneous flows of gases and liquids occur in numerous components of plant equipment such as steam generators, drain lines, and oil and natural gas pipelines. Ever since the earliest visual observations of two-phase flow, it has been recognized that there are natural varieties of flow patterns. In addition to the random character of each flow configuration, two-phase flows are never fully developed. In fact, the gas phase expands due to the pressure drop along a pipe leading to a modification of the flow structure. The flow pattern depends also upon the geometry changes of a flow channel (bends, valves, etc.). Flow patterns will be classified according to pipe geometry and flow direction (upward, downward, cocurrent, countercurrent), and several shown are discussed in the following subsections.

Upward Cocurrent Flow in Vertical Pipes. 

          The main flow patterns encountered in a vertical pipe are shown in Bubbly flow is certainly the most widely known configuration, although at high velocity its milky appearance prevents it from being easily recognized. Bubbles are spherical only if their diameters do not exceed 1 mm; whereas beyond 1 mm their shape is variable. Roumy distinguishes two bubbly flow patterns. In the independent bubble configuration, bubbles are spaced and do not interact with each other. On the other hand, in the packed configuration, bubbles are crowded together and strongly interact with each other. Slug flow is composed of a series of gas plugs. The head of a gas plug is generally blunt, whereas its end is flat with a bubbly wake. A simple visual observation reveals that the liquid film which surrounds a gas plug moves downward with respect to the pipe wall.

Characteristics of Incompressible Flow to Fluid Mechanics Solutions

Characteristics of Incompressible Flow

         Although there is no such thing in reality as an incompressible fluid, this term is applied to liquids. Yet sound waves, which are really pressure waves, travel through liquids. This is an evidence of the elasticity of liquids. In problems involving water hammer, it is necessary to consider the compressibility of the liquid. The compressibility of a liquid is expressed by its bulk modulus of elasticity which influences the wave speed in the liquid. 

         It should be explained here that when density changes of compressible fluids (gases or steam) are gradual and do not change by more than about 10 percent, the flow may be treated as incompressible with the use of an average density. Bernoulli’s equation is a special case of  applied to nonviscous, incompressible fluids (ideal fluid) which do not exchange shaft work with surroundings:

            In real flow systems, however, the Bernoulli equation must be suplemented by a frictional head loss Hf (expressed in feet of a column of the fluid) and by a pumphead term Hp (total dynamic head, TDH, expressed in feet of a column of fluid). Then, for real systems the following is found:

        

      Expressions for calculating the loss of pressure in turbulent flow are based upon experimental data. An empirical transition function for commercial pipes for the region between smooth pipes and the absolute roughness , expressed in feet (millimeters), is used as a measure of pipe wall irregularities of commercial pipes. Formula is the basis for the Moody diagram in the ‘‘transition zone’’ flow. The Moody diagram is widely accepted for hand calculations. For computerized calculations of the pressure drop, however, the Colebrook equation is built into the software.

            Pressure losses which occur in piping systems due to bends, elbows, joints, valves, and so forth are called form losses. Recommended values of local flow resistance coefficients (K-factors) may be found losses may also be expressed in terms of the equivalent length Le of pipe that has the same pressurehead loss for the same flow rate:

*Note : After solving for Le.

Characteristics of Incompressible Flow to Fluid Mechanics Solutions

Characteristics of Incompressible Flow

         Although there is no such thing in reality as an incompressible fluid, this term is applied to liquids. Yet sound waves, which are really pressure waves, travel through liquids. This is an evidence of the elasticity of liquids. In problems involving water hammer, it is necessary to consider the compressibility of the liquid. The compressibility of a liquid is expressed by its bulk modulus of elasticity which influences the wave speed in the liquid. 

         It should be explained here that when density changes of compressible fluids (gases or steam) are gradual and do not change by more than about 10 percent, the flow may be treated as incompressible with the use of an average density. Bernoulli’s equation is a special case of  applied to nonviscous, incompressible fluids (ideal fluid) which do not exchange shaft work with surroundings:

            In real flow systems, however, the Bernoulli equation must be suplemented by a frictional head loss Hf (expressed in feet of a column of the fluid) and by a pumphead term Hp (total dynamic head, TDH, expressed in feet of a column of fluid). Then, for real systems the following is found:

        Expressions for calculating the loss of pressure in turbulent flow are based upon experimental data. An empirical transition function for commercial pipes for the region between smooth pipes and the absolute roughness , expressed in feet (millimeters), is used as a measure of pipe wall irregularities of commercial pipes. Formula is the basis for the Moody diagram in the ‘‘transition zone’’ flow. The Moody diagram is widely accepted for hand calculations. For computerized calculations of the pressure drop, however, the Colebrook equation is built into the software.

            Pressure losses which occur in piping systems due to bends, elbows, joints, valves, and so forth are called form losses. Recommended values of local flow resistance coefficients (K-factors) may be found losses may also be expressed in terms of the equivalent length Le of pipe that has the same pressurehead loss for the same flow rate:

*Note : After solving for Le.

Conservation of Energy to Fluid Mechanics

Conservation of Energy

          The first law of thermodynamics can be formulated as the principle of conservation of energy2:

          where Ein represents all types of energy added to the system within a defined control boundary during a specified time interval, Es is the change in the total energy of the system, and Eout represents all kinds of energy substracted from the system in the same time interval. In the steady-flow system Es 0, and

            With fluid flow through the system, the energy entering (or leaving) this system by a duct (or a pipeline) will consist of potential (external), kinetic, and internal energy of the fluid, and also the external pumping work, the energy transmitted by a pump to force the fluid to flow continuously across the boundary of the system. Then the total rate of steady-flow energy transported in a duct (or a pipe) is:

where J 778.169 (ft lbf)/Btu (dimensional conversion factor).

Introduction to Fluid Mechanics

Introduction
          The primary objective of this chapter is to show the user the most important logical milestones and the general background of equations and formulas recommended for specific practical applications of fluid flow in pipes, nozzles, and orifices. For details, Refs. 1 through 4 or other equivalent textbooks should be consulted.


Basic Fluid Properties
          A fluid is a substance which can flow and which deforms continuously under the action of shearing forces. Fluids offer no resistance to distortion of form; they yield continuously to tangential forces, no matter how small. Ordinarily, fluids are classified as being liquids or gases. Some classifications also include the vapor form within the group of fluids. Liquids change volume and density very slightly with considerable variation in pressure, and when the pressure  is removed, they do not dilate significantly. They are practically incompressible.
           A gas is a fluid which tends to expand to fill completely any vessel in which it is contained. It is easily compressed, and a change in pressure is accompanied by a considerable change in its volume and density.


Theoretical Background Viscosity and Pressure
Some basic fluid properties used in this chapter are discussed in the following paragraphs; for more detailed information see Refs. 


Viscosity
        It is an experimental fact that a fluid in immediate contact with a solid boundary has the same velocity as the boundary itself. For the case of Fig. B8.2, in which a fluid separates closely spaced parallel plates, the force F applied to the upper, moving plate, is directly proportional to the surface area A of the upper plate and its velocity w, and is inversely proportional to the distance y between the plates. The last statement is expressed in the form of Newton’s law of viscosity: 

Pressure Variation in a Static Fluid
         The static pressure existing at a point within a fluid body is known also as the hydrostatic pressure. In the case of gaseous fluids, the density of the fluid column is relatively small unless great vertical heights are involved. In such cases the average density of fluid should be used for the static pressure calculation. With denser fluids such as liquids, the increase in pressure due to depth within the liquid can be of great significance. When applied to the two-fluid mixture in a container filled with a gas at the pressure p1.

Heat Transfer about Introduction to Convection

Introduction to Convection
 

• Convection denotes energy transfer between a surface and a fluid moving over the surface.  
• The dominant contribution due to the bulk (or gross) motion of fluid particles.

• In this chapter we will 

        –Introduce the convection transfer equations 

        –Discuss the physical mechanisms underlying convection 

        –Discuss physical origins and introduce relevant dimensionless parameters that can help   

          us to perform convection transfer calculations in subsequent chapters.

 

*Note similarities between heat, mass and momentum transfer 

 

 

Heat Transfer Coefficient

Recall Newton’s law of cooling for heat transfer between a surface of arbitrary shape, area As and temperature Ts and a fluid:

 

Generally flow conditions will vary along the surface, so q” is a local heat flux and h a local convection coefficient. 

The total heat transfer rate is :

where,

 

 is the average heat transfer coefficient.

 

 

Mass Transfer Coefficient

 

Recall mass transfer between a surface of arbitrary shape, area As and molar concentration species A of CAs and a fluid flow with molar concentration species A of CA∞: 

 

Generally flow conditions will vary along the surface, so NA” is a local mass flux and hm is a local convection coefficient. 

The total mass transfer rate is

where,

is the average mass transfer coefficient.