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Extra heat transfer > Mec203 + > Flashcards

Mec203 + Flashcards

1
Q

What is heat?

A

The form of energy that can be transferred from one system to another as a result of temperature difference

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2
Q

What is specific heat capacity?

A

The energy required to raise the temperature of a unit mass of a substance by one degree

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3
Q

What is an incompressible substance?

A

A substance whose specific volume does not change with temperature or pressure.

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4
Q

What is heat transfer rate?

A

The amount of heat transferred per unit time

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5
Q

What is heat flux?

A

The rate of heat transfer per unit area normal to the direction of heat transfer

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6
Q

What is the first law of thermodynamics?

A

The first law of thermodynamics states that energy can neither be created or destroyed during a process, it can only change forms.

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7
Q

A surface contains……

A

no body or mass and thus no energy

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8
Q

What is conduction?

A

the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles

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9
Q

Fourier’s law of heat conduction?

A

Q = -kA dT/dx

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10
Q

What is thermal conductivity, k?

A

a measure of the ability of a material to conduct heat

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11
Q

What is thermal diffusivity?

A

α = k/ρcp how fast heat diffuses through a material

Heat conduction/Heat stored

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12
Q

What is convection?

A

The mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion.

Faster fluid motion the greater the convection

Absence of any fluid motion heat transfer between surface and gas is pure conduction

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13
Q

Forced convection?

A

if the fluid is forced to flow over the surface by external means such as a fan, wind or pump.

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14
Q

Natural convection?

A

If the fluid motion is caused by the buoyancy forces that are induced by density difference due to the variation of temperature in a fluid.

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15
Q

Newtons law of cooling?

A

Q = hA(Ts - Tinf)

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16
Q

What does the convection heat transfer coefficient depend on?

A
  • surface geometry
  • the nature of the fluid motion
  • the properties of the fluid
  • the bulk fluid velocity
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17
Q

What is radiation?

A

The energy emitted by matter in the form of electromagnetic waves as a result of the change in the electronic configurations of the atoms or molecules

All bodies above 0K emit radiation
It is a volumetric phenomenon but usually considered a surface phenomenon for solids

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18
Q

Stephen-Boltzmann Law

A

Q = σAT^4

Radiation emitted by real surfaces: Q = σεAT^4

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19
Q

Define a lumped system

A

Variation with time but not with position, the temperature of the medium changes uniformly with time

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20
Q

Heat generation ……….

A

is a volumetric phenomenon

E = eV (w)

21
Q

What is a boundary condition?

A

a mathematical expression of the thermal conditions at the boundaries

22
Q

Thermal resistance concept …….

A

rate of heat transfer —-> electric current
thermal resistance —–> electrical resistance
temperature difference ——> voltage differnce

thermal resistance depends on the geometry and the thermal properties of the medium

23
Q

Thermal resistance network for two parallel layers

A

1/Rtotal = 1/R1 +1/R2

24
Q

When Ts and Tinf are fixed the two ways of increasing heat transfer is………………….

A
  • increase convection heat transfer coefficient by installing a fan
  • increase the surface areas by attaching fins made of highly conduction material (Al)
25
Q

Fin effectiveness

A

heat transfer rate from the fin of base area Ab / heat transfer rate from the surface of area Ab

26
Q

Adiabatic

A

An adiabatic process occurs without the transfer of heat or mass of substances between a thermodynamic system and its surroundings.

27
Q

Semi-infinite solid

A

an idealized body that has a single plane surface and extends to infinity in all directions

28
Q

Limitations of analytical solution methods

A

limited to highly simplified problems in simple geometries

29
Q

Convection heat transfer coefficient

A

the rate of heat transfer between a solid surface and a fluid per unit surface area per unit temperature difference

30
Q

Viscous flow

A

flows in which the frictional effects are significant

31
Q

inviscid flow regions

A

A region where viscous forces are negligibly small compared to inertial or pressure forces

32
Q

Laminar flow

A

the highly ordered fluid motion characterised by smooth layers of fluid.

33
Q

Turbulent flow

A

the highly disordered fluid motion that typically occurs at high velocity and is characterised by velocity fluctuations

34
Q

Velocity boundary layer

A

the region of flow over a plate in which the effects of the vicious and shearing forces caused by the fluid’s viscosity are felt

35
Q

irrotational flow

A

the frictional effects are negligible and the velocity remains essentially constant

36
Q

free stream velocity

A

the velocity of the fluid relative to an immersed solid body

37
Q

Drag

A

the force a flowing fluid exerts on a body in the flow direction

38
Q

separated region

A

the low-pressure region behind the body, here recirculating and backflow occurs

39
Q

What does the transition from laminar to turbulent depends on

A
  • surface geometry
  • surface roughness
  • upstream velocity
  • surface temperature
  • type of fluid
40
Q

In the fully developed region of the tube what remains constant?

A

friction and convection coefficients

41
Q

what is the effect of the entrance region

A

to increase the average friction factor and heat transfer coefficients for the entire tube

42
Q

Buoyancy force

A

the upward force exerted by a fluid on a body completely or partially immersed in it in a gravitational field.magnitude of buoyancy force is equal to the weight of the fluid displaced by the body

43
Q

Archimedes principle

A

a body immersed in a fluid will experience a ‘weight loss’ in an amount equal to the weight of the fluid it displaces

44
Q

Volume expansion coefficient

A

1/T

45
Q

compact heat exchanger

A

it has a large heat transfer surface area per unit volume.

46
Q

cross flow heat exchanger

A

in compact heat exchangers, the two fluids usually move perpendicular to each other. the cross-flow is further classified as unmixed and mixed flow

47
Q

Fouling factor increases with

A

operating temperature, length of service and decreases with the velocity of fluids

48
Q

irradiation

A

radiation flux incident on a surface, G

49
Q

what is the second law of thermodynamics

A

The second law of thermodynamics states that the entropy of any isolated system always increases.

Extra heat transfer (1 decks)

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