Introduction | Fluid Mechanics | Fluid properties

Fluid Mechanics – Introduction

Fluid Mechanics is that section of applied mechanics, concerned with the statics and dynamics of liquids and gases.

A knowledge of fluid mechanics is essential for the Mechanical engineer, because the majority of Mechanical processing operations are conducted either partially or totally in the fluid phase.

The handling of liquids is much simpler, much cheaper, and much less troublesome than handling solids.

Even in many operations a solid is handled in a finely divided state so that it stays in suspension in a fluid.

Fluid Statics: Which treats fluids in the equilibrium state of no shear stress

 

Fluid Mechanics: Which treats when portions of fluid are in motion relative to other parts.

Fluids and their Properties

Fluids

In everyday life, we recognize three states of matter:

• solid,
• liquid and
• gas.

Although different in many respects, liquids and gases have a common characteristic in which they differ from solids: they are fluids, lacking the ability of solids to offer a permanent resistance to a deforming force.

A fluid is a substance which deforms continuously under the action of shearing forces, however small they may be.Conversely, it follows that:
If a fluid is at rest, there can be no shearing forces acting and, therefore, all forces in the fluid must be perpendicular to the planes upon which they act.

Shear stress in a moving fluid

Although there can be no shear stress in a fluid at rest, shear stresses are developed when the fluid is in motion, if the particles of the fluid move relative to each other so that they have different velocities, causing the original shape of the fluid to become distorted. If, on the other hand, the velocity of the fluid is same at every point, no shear stresses will be produced, since the fluid particles are at rest relative to each other.

Vertical screw conveyors

A vertical screw conveyor conveys material upward in a vertical path. It requires less space than some other types of elevating conveyors. Vertical screw conveyor can handle most of the bulk materials provided there is no large lump. The maximum height is usually limited to 30m.

A vertical screw conveyor consists of a screw rotating in a vertical casing. The top bearing for the screw shaft must be designed to stand against radial and thrust loads. A suitable inlet port at the lower end and a discharge port at the upper end of the casing are provided. Feeding a vertical screw conveyor deserves careful consideration. Most materials are fed to the vertical conveyor by a straight or offset horizontal feeder conveyor. The ideal operation of a vertical screw conveyor is to have a controlled and uniform volume of material feeding.

Uneven feeding and start stop operation may adversely affect the performance of the vertical screw conveyor in terms of speed, capacity and horse power.

Average capacities and speeds of vertical conveyor

Nominal diameter of screw in mm	Capacities in m3/hr	Speed of screw
150	10	Up to 400 RPM
250	35	300 RPM
300	75	250 RPM
400	170	200 RPM

Vertical screw conveyors or some special design of vertical screw conveyor finds wide application in ship unloading.

Practical experience with these conveyors has shown that the resistance factor for vertical conveyors is higher than those of the horizontal conveyors. Resistance factor λ may be taken as 5.5 to 7.5 for grains. 6.5 to 8.3 for salt.

The driving power of the loaded screw conveyor is given by:

P = P_H + P_N + P_st

Where,

P_H = Power necessary for the progress of the material

P_N = Driving power of the screw conveyor at no load

P_st = Power requirement for the inclination of the conveyor

Power necessary for the progress of the material P_H:

For a length L of the screw conveyor (feeder), the power PH in kilo watts is the product of the mass flow rate of the material by the length L and an artificial friction coefficient λ, also called the progress resistance coefficient.

P_H = I_m.L. λ.g / 3600 (kilowatt)

= I_m.L. λ / 367 (kilowatt)

Where,

I_m = Mass flow rate in t/hr

λ = Progress resistance coefficient

Each material has its own coefficient λ. It is generally of the order of 2 to 4. For materials like rock salt etc, the mean value of λ is 2.5. For gypsum, lumpy or dry fine clay, foundry sand, cement, ash, lime, large grain ordinary sand, the mean value of λ is 4.0.

In this connection it should be noted that the sliding of the material particles against each other gives rise to internal friction. Other resistance due to grading or shape of the output discharge pattern contributes to the resistance factor. That is why the parameter λ is always higher than that due to pure friction.

Drive power of the screw conveyor at no load, P_N:

This power requirement is very low and is proportional to the nominal diameter and length of the screw.

P_N = D.L / 20 (Kilowatt)

Where,

D = Nominal diameter of screw in meter

L = Length of screw conveyor in meter

Power due to inclination: P_st

This power requirement will be the product of the mass flow rate by the height H and the acceleration due to gravity g.

P_st = I_m.H.g / 3600

= I_m.H / 367

H should be taken positive for ascending screws and will be negative for descending screws.

Total power requirement:

The total power requirement is the sum total of the above items

P = (I_m (λ.L + H) / 367) + (D.L /20) (Kilowatt)

FINISHING OPERATIONS

Sizing:

Repressing the sintered component in a die to meet required tolerances.

Coining:

Repressing the sintered component in a die to increase the density and to give additional strength.

Infiltration:

Filling the pores of sintered product with molten metal to improve the physical properties.

Impregnation:

Filling of Oil, Grease or other Lubricants in a Sintered components such as Porous Heating

Machining:

Removing excess material by using cutting tool to imparts specific features such as Threads, Grooves, Undercuts etc, which are not practicable in powder metallurgy process.

Heat Treatment:

Process of Heating & Cooling at a desired rate to improve Grain Structure, Strength & Hardness.

Plating:

Used for obtaining Resistance to Corrosion or better appearance.

Powder metallurgy is used in the following industries:

• Automotive (Brake pads, Gear parts, Connecting rods, Planetary carriers, Sintered Engine Bearings);

• Aerospace (Light weight Aluminum base structural materials, High temperature Composite materials);

• Cutting tools (Hard metals, Diamond containing materials);

• Medicine (Dental implants, Surgical instruments);

• Abrasives (Grinding and Polishing wheels and Discs);

• Electrical, Electronic and Computer parts (Permanent magnets, Electrical contacts).

 

FINISHING OPERATIONS

Sizing:

Repressing the sintered component in a die to meet required tolerances.

Coining:

Repressing the sintered component in a die to increase the density and to give additional strength.

Infiltration:

Filling the pores of sintered product with molten metal to improve the physical properties.

Impregnation:

Filling of Oil, Grease or other Lubricants in a Sintered components such as Porous Heating

Machining:

Removing excess material by using cutting tool to imparts specific features such as Threads, Grooves, Undercuts etc, which are not practicable in powder metallurgy process.

Heat Treatment:

Process of Heating & Cooling at a desired rate to improve Grain Structure, Strength & Hardness.

Plating:

Used for obtaining Resistance to Corrosion or better appearance.

Powder metallurgy is used in the following industries:

• Automotive (Brake pads, Gear parts, Connecting rods, Planetary carriers, Sintered Engine Bearings);

• Aerospace (Light weight Aluminum base structural materials, High temperature Composite materials);

• Cutting tools (Hard metals, Diamond containing materials);

• Medicine (Dental implants, Surgical instruments);

• Abrasives (Grinding and Polishing wheels and Discs);

• Electrical, Electronic and Computer parts (Permanent magnets, Electrical contacts).