Fluid Flow Operations

Fluid Flow Operations is the name adopted in chemical engineering from the Fluid Mechanics branch. It deals with studying the behaviour of fluid in static or dynamic conditions.

Fluid mechanics is classified as Fluid Statics and Fluid Dynamics.

The study of fluids at static (rest) condition is called Fluid Statics.

The study of fluids in motion relative to stationary solid walls is called Fluid Dynamics.

Importance of Fluid Flow Operations in Chemical Engineering

Fluid Flow Operations is important in chemical engineering because it mostly deals with fluids. Understanding fluids and their flow are important to deal with the problems based on the movement of fluids through the pipes, pumps, fans, blowers, and all kind of process equipment.

Fluid flow operations are also important to study heat flow and separation operations that depend on diffusion and mass transfer operations.

Fluid Definition

A fluid is a substance capable of flowing if allowed to do so.
A fluid is a substance that undergoes continuous deformation under the action of shear force.

All liquids, gases, and vapors are called fluid.

Characteristics of Fluid

It has no definite shape, but it takes the shape of its container.

Even a small amount of shear force exerted on fluid will cause it to undergo a deformation which continues as long as the force continues to be applied.

A solid suffers strain when subjected to shear forces, where a fluid suffers a rate of strain.

Properties of Fluids

Density

The density \(\rho\) or mass density of a fluid is the mass of the fluid per unit volume. Unit of density in SI system is \(kg/m^3\). The density of pure water at \(277 K (4^oC)\) is taken as 1000 \(kg/m^3\).

Weight Density

The weight density of a fluid is the weight of the fluid per unit volume. In the SI system, it is expressed in \(N/m^3\). Specific weight or weight density of pure water at \(277 K (4^oC)\) is taken as 9810 \(N/m^3\).

Specific Volume

The fluid volume per unit mass is called Specific Volume. In the SI system, it is expressed in \(m^3/kg\).

Specific Gravity

The specific gravity of a fluid is the ratio of the density of the fluid to the density of a standard fluid. For liquids, water at \(277 K (4^oC)\) is considered as standard fluid, and for gases, air at NTP \((0^oC\) and \(760\; torr)\) is considered as a standard fluid.

Vapour Pressure

It is defined as the absolute pressure at which the liquid and its vapor are in equilibrium at a given temperature, or the pressure exerted by the vapour (on the surface of a liquid) at equilibrium conditions is called the vapour pressure.

Surface Tension

The property of liquid surface film to exert tension is called surface tension. It is the force required to maintain a unit length of film in equilibrium. It is denoted by the symbol \(\sigma\), and its SI unit is \(N/m\).

Viscosity

A fluid undergoes continuous deformation when subjected to shear stress. The resistance offered by a fluid to its continuous deformation (when subjected to a shear stress or shear force) is called viscosity.

It measures its fluid's resistance to flow at a given temperature.

The viscosity of a gas or a liquid is practically independent of the pressure for the range that is normally encountered in practice. However, it varies with temperature.

For gases, viscosity increases with increased temperature, and for liquids, viscosity decreases with increased temperature.

Ideal fluid and real fluid

Ideal Fluid

An ideal fluid has no viscosity, no surface tension, and is incompressible. However, an ideal fluid does not exist in nature, and therefore, it is only an imaginary fluid.

Real Fluid

A real fluid has viscosity, surface tension. It offers resistance when it is set in motion. All naturally occurring fluids are real fluids.

Classification of Fluids

Fluids can be classified in two ways.

Based upon the behaviour of fluids under the action of externally applied pressure and temperature

Compressible Fluids
Incompressible Fluids

Based upon the behaviour of fluids under the action of shear stress

Newtonian Fluids
Non-Newtonian Fluids

Fluid has a definite density at a given temperature and pressure, and density changes with temperature and pressure variation. The change may be small or large.

Compressible Fluid

Suppose the fluid density is not appreciably affected by moderate changes in temperature and pressure. In that case, the fluid is said to be incompressible.

Suppose the fluid density is almost insensitive to moderate changes in temperature and pressure. In that case, the fluid is said to be incompressible.

All gases are considered to be compressible fluids.

Newtonian Fluids

Newtonian fluid is defined as the fluid which follow Newton's law of viscosity. All gases and most pure liquids are Newtonian fluids examples.

Non Newtonian Fluids

Fluids that do not follow Newton's law of viscosity or the ratio of the shear stress to the shear rate are not constant or are called Non Newtonian fluids. Toothpaste, paints, gels, jellies, slurries, and polymer solutions are Non Newtonian fluids examples.

Types of Non-Newtonian Fluids

Bingham Plastic Fluids
Pseudoplastic Fluids
Dilatent Fluids

Bingham Plastic Fluids

These fluids resist small shear stress indefinitely but flow linearly under the action of larger shear stress; these fluids do not deform, flow unless a threshold shear stress value is not exceeded. Examples: Toothpaste, jellies, paints, sewage sludge, and some slurries.

Pseudoplastic Fluids

The viscosity of these fluids decreases with an increase in shear rate (velocity gradient). Examples: Blood, solution of high molecular weight polymers, paper pulp, muds, most slurries, and rubber latex.

Dilatent Fluids

The viscosity of these fluids increases with an increase in velocity gradient. Examples: Suspensions of starch in water, pulp in water, and sand-filled emulsions.

Some important definitions of fluid flow operations

Steady and Unsteady Flow

If the flow does not change with time is called steady-state flow, i.e., the mass flow rate is constant, and the quantities, such as temperature, pressure, etc., are independent of time, do not vary with time.

If the mass flow rate and other quantities such as temperature, pressure, etc., vary with time, the flow is unsteady.

Stream Line and Stream Tube

A stream tube is a small or large cross-section that is entirely bounded by streamlines. It may be of any convenient cross-sectional shape, and no net flow occurs through the walls of the stream tube.

Potential Flow

The flow of incompressible fluids without shear is referred to as potential flow. In potential flow, eddies and cross-currents cannot form within the stream, and friction cannot develop.

Fully Developed Flow

The flow with unchanging velocity distribution is called fully developed flow.

Types of flow

There are mainly two types of flow

Laminar Flow
Turbulent Flow

Laminar Flow

The flow in which the fluid flows in parallel, straight lines is called laminar flow. It occurs at low fluid velocities.

The flow in which the streamlines remain separated over their entire flow length is termed laminar flow. This flow is also called streamline flow or viscous flow. It is characterized by the absence of lateral mixing, cross currents, and eddies.

Turbulent Flow

The flow in which the fluid, instead of flowing in an orderly manner, moves erratically in cross currents and eddies is called turbulent flow.

It occurs at high fluid velocities, and there is a lateral mixing in this type of flow.

References

"Unit Operations of Chemical Engineering", McCabe W L, Smith J C, Harriott P, Mc Graw Hill Publication, 7th edition 2005.
“Chemical Engineering” Vol. I – Fluid flow, Heat Transfer and Mass Transfer; Coulson & Richardson’s, Butterworth – Heinemann Publication, 6 th Edition.
"Unit Operations-I" K A Gavhane, Nirali Prakshan.

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