thermofluids definition

FV1201 – Energy Transfer and Thermodynamics Definition of terms

©HO Yuet Ming/08-09 1

Extensive property

An extensive property is one that depends upon the mass of materials involved. Examples are

volume, internal energy, enthalpy, entropy and total energy.

Two-property rule

If two independent properties of a pure substance are defined, then all other properties on the state

of the substance are also defined.

Control volume (CV)

A CV is a volume of space, chosen for the purpose of analysis, surrounded and defined by control

surfaces around it. A control volume may involve heat and work transfer, just as a closed system,

in addition to mass interaction.

Temperature

Temperature is the `thermodynamic potential’ of a body which controls the direction of heat

transfer; in absolute terms it is measured in kelvins, K.

Reversible process

Reversible process is a process after which the system and its surrounding can be returned to their

initial states without any trace left on them by this action.

Total energy of a flowing fluid

It is the sum of the enthalpy, kinetic, and potential energies of the flowing fluid.

Specific heat capacity

Specific thermal capacity is the ability of a material to contain thermal energy in relation to its

temperature, measured in J/kg.K.

Perfect gas

It is an ideal gas obeying the perfect gas equation PV = mRT, i.e. having a constant specific gas

constant and constant specific heat capacity.

Constant volume process undergone by a perfect gas

Constant volume gas process normally implies zero work done, and is therefore a simple heating

process with Q12 = m cv (T2 – T1) and P1/T1 = P2/T2.

Heat supply at constant pressure for a perfect gas

The energy required to heat a given mass of gas at constant pressure: Q = m cp (T2 – T1)



Adiabatic process undergone by a perfect gas

Adiabatic means having no heat transfer, Q = 0. For a perfect gas, PVγ = constant.

Thermodynamic cycle

It is the sequence of processes a control mass undergoes as it passes through a number of

thermodynamic state and ultimately returns to its original state.

First Law of Thermodynamics

If a closed system is taken through a cycle, the net work done is directly proportional to the net

heat transfer, i.e., ΣW + ΣQ = 0.

Second Law of Thermodynamics

It is impossible to construct a system which will operate in a cycle, extract heat from a reservoir,

and do an equivalent amount of work on the surrounding, or equivalent statement.

Heat engine

A device operates continuously, or cyclically, and produces a net work output while energy

transfer by heating takes place across the boundary of the device.

Heat pump

A heat pump is a device which moves thermal energy from a lower-temperature source to a

higher-temperature sink, using a work input to do so.

Entropy

It is a thermodynamic property designated by S and is defined by dS = (δQ/T)rev. Entropy

remains constant during a reversible and adiabatic(isentropic) process.

Heating value of a fuel

It is the amount of heat released when a specified amount of fuel at room temperature is

completely burned and the combustion products are cooled to the room temperature.

Adiabatic flame temperature

It is the maximum attainable temperature of the product gases under adiabatic operation of the

reactor and complete combustion conditions.

Absolute pressure

It is the actual pressure at a given position and it is measured relative to absolute vacuum (i.e.,

absolute zero pressure).



Perfect(ideal) fluid

A perfect(ideal) fluid has zero viscosity.

Steady flow

The mass flow rate is steady and the properties across any section at right angles to the flow are

invariable with time.

Uniform flow

The velocity vector at all points in the flow is same in magnitude and direction at any instant of

time, ( t( v / s) 0∂ ∂ = .

Laminar and turbulent flow

In a laminar flow, fluid particles travel in smooth lines parallel to the mean direction of flow. In

turbulent flow, eddies are present and particles travel in random path causing a rapid exchange of

fluid momentum.

Continuity equation

It is the conservation of mass equation referred to in fluid mechanics. Net mass transfer to or

from a system during a process equals to the net change in the total mass of the system.

Bemoulli’s equation

Bernoulli’s equation states that the total energy of each particle of a body of fluid is the same

provided that no energy enters or leaves the system at any point. The total head(energy) is the

sum of pressure, kinetic and potential head, each of which may vary from point to point.

Static pressure

Static pressure is the normal component of stress exerted across a surface moving with the fluid,

especially across a surface which lies in the direction of the flow.

Dynamic pressure

The pressure produced when the flowing fluid is brought to rest by a frictionless process and is

given by Pdy = ½ρu2.

Stagnation pressure or total head pressure

Stagnation pressure is static pressure plus dynamic pressure. It is measured by a Pitot tube.

Dimensionless number (group)

Dimensionless number (group) is a product of several variables which combine to be

dimensionless, e.g. Reynolds number and Froude number.



Reynolds number Reynolds number Re = ρVD/μ and its numerical value indicates whether a pipe flow will be

laminar or turbulent.

Pump(fan) and system matching

It is a process of selecting a pump(fan) to operate in conjunction with a given system so that it

delivers the required flow rate, operating at its best efficiency, which corresponds to the

pump’s(fan’s) design point.

Boundary layer

A boundary layer is the thin layer of fluid immediately adjacent to a moving body. Within the

boundary layer, the speed is reduced by viscosity and severe velocity gradients occur.

Flow visualization

It is an experimental means of examining the flow pattern around a body or over its surface. The

flow is "visualized" by introducing dye, smoke or pigment to the flow area under investigation.

Conduction

Heat energy is transferred from one part of a substance to another part of the substance, or from

one substance to another in physical contact with it, without appreciable displacement of the

molecules forming the substance.

Convection

Energy is transferred between a solid surface and the adjacent fluid that is in motion, and it

involves the combined effects of conduction and fluid motion. The movement of the fluid may be

caused by differences in density or the motion may be produced by mechanical means

Radiation

Radiant energy is transferred through electromagnetic waves emitted due to the agitation of the

molecules of a substance and without the need for any physical contact between bodies. It

requires no medium for its propagation, and will pass through a vacuum.

Black body

A black body is an ideal body, which absorbs all the radiation, which fall upon it. That is, for a

black body, absorptivity equal to unity, i.e., absortivity 1=α and relectivity 0=ρ .

Grey body

A grey body is an ideal body with its surface having constant emissivity ε over all wavelengths

and for all temperatures.

thermofluids definition

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