Heat Transfer

Question 1

The second law tells us that energy has quality as well as…?

Answer 1

…quantity

Question 2

The direction of heat flow is governed by the…?

Answer 2

…2nd Law of Thermodynamics

Question 3

Heat can be transferred by…?

Answer 3

…conduction, convection, and radiation

Question 4

The rate at which heat transfers is measured in…?

Answer 4

Joules per second (J/s), or Watts (W)

Question 5

The rate of heat transfer is given the symbol…

Answer 5

Question 6

In fluids, conduction is due to…?

Answer 6

…collision and diffusion of the molecules

Question 7

In solids, conduction is due to the…

Answer 7

…vibration of the molecules in a lattice

Question 8

The rate at which conduction occurs in a medium depends on…(3 things)

Answer 8

1. Geometry of the medium
2. Thickness of the medium
3. Temperature difference across the medium

Question 9

Give a property of an insulator?

Answer 9

Low rates of heat conduction

Question 10

Steady-state heat conduction is governed by Fourier’s Law

What is his law? (Equation)

Answer 10

Question 11

“Rate of heat conduction is proportional to…”

What is k?

Answer 11

• Rate of heat conduction is proportional to the area of the body, the temperature difference across it, and its thickness
• k is the thermal conductivity

Question 12

What is Natural convection (or free convection)

Answer 12

Natural convection is when the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) but only by density differences in the fluid occurring due to temperature gradients

Question 13

What is Forced convection?

Answer 13

Fluid is forced to flow over the solid surface by external forces (i.e. a fan, the wind)

Question 14

Convection processes are expressed through Newton’s Law of Cooling

What is the equation for this?

Answer 14

Question 15

What is this called?

And what does each symbol stand for?

Answer 15

Newton’s Law of Cooling

• h is the convection heat transfer coefficient
• As is the surface area of the body
• Ts is the surface temperature
• T∞ is the temperature of the fluid far from the surface

Question 16

Radiation is a volumetric phenomenon

Define this

Answer 16

all solids, liquids and gases absorb and emit radiation to some degree

Question 17

An idealised body which emits radiation at this maximum rate is called a…?

Answer 17

…blackbody

Question 18

Radiation heat transfer is expressed by the Stefan-Boltzmann Law

What is the equation for this?

Answer 18

Question 19

What is σ (sigma) in this equation?

Answer 19

σ is the Stefan-Boltzmann constant 5.67x10^-8Wm-²K^-4

Question 20

What form of heat transfer does

Opaque, Gas and Vacuum do?

Answer 20

Question 21

The rate of heat conduction is…

Answer 21

…inversely proportional to the thickness of the layer

Question 22

A high value of k implies that the material is a ____ ____

A low value of k implies ____ ____ ____ - i.e. an ____

Answer 22

A high value of k implies that the material is a good conductor

A low value** of k implies **poor heat conduction - i.e. an insulator

Question 23

When you intergrate and rearrange what do you get?

Answer 23

Question 24

R_wall = ?

And what is R_wall ?

Answer 24

R_wall​ = L/kA

R_wall​ is called the thermal resistance of the wall

Question 25

* Rate of heat transfer corresponds to the ____ \_\_\_\_ * Thermal resistance corresponds to the ____ \_\_\_\_ * Temperature difference corresponds to the ____ \_\_\_\_

Answer 25

* Rate of heat transfer corresponds to the **_electric current_** * Thermal resistance corresponds to the **_electrical resistance_** * Temperature difference corresponds to the **_potential difference_**

Question 26

What does this correspond to in terms of an electric current? (Equation)

Answer 26

Question 27

Q_conv = ?

Answer 27

Question 28

R_conv = ?

Answer 28

Question 29

Under steady conditions, the rate of heat convection into the wall... = ? = ?

Answer 29

Under steady conditions, **the rate of heat convection into the wal**l = **rate of heat conduction through the wall** = **rate of heat convection from the wall**

Question 30

R_cond = ? R_conv = ?

Answer 30

R_cond = L / kA R_conv = 1 / hA_s

Question 31

Area of a sphere A = ?

Answer 31

A = 4πr²

Question 32

For a hollow **cylinder** (or sphere) undergoing **heat transfe**r by **conduction** with **convection**, the total thermal resistance can be denoted as R_total = ?

Answer 32

R_total = R_conv,1 + R_cyl + R_conv,2

Question 33

Plot the variation of heat transfer with insulating material radius, r₂

Answer 33

* The variation is not linear * There is a critical radius at which heat transfer is a maximum

Question 34

For a cylinder, the critical radius is... For a sphere, the critical radius is...

Answer 34

_rcr,cylinder = k/h _rcr,sphere = 2k/h

Question 35

For common insulating materials, the critical radius is of the order of...? Insulation on electrical wires is of the order of...?

Answer 35

...1 cm ...1 mm thick

Question 36

The convective heat transfer coefficient depends on several flow properties and also depends on whether the flow is...

Answer 36

...**laminar** or **turbulent** in nature

Question 37

What is the **no-temperature-jump condition**

Answer 37

That if a surface has some temperature T, it follows from the no-slip condition that the temperature of the fluid one wall should also be T

Question 38

Place a fluid layer between two plates of differing temperature; if the fluid is motionless, the heat transfer will be by ____ only, if the fluid is in motion, heat transfer will be by \_\_\_\_

Answer 38

Place a fluid layer between two plates of differing temperature; if the fluid is motionless, the heat transfer will be by **_conduction_** only, if the fluid is in motion, heat transfer will be by **_convection_**

Question 39

Define **heat flux**

Answer 39

Rate of heat transfer per unit time per unit surface area

Question 40

**Heat flux equation** for **conduction**

Answer 40

Question 41

**Heat flux equation** for **convection**

Answer 41

Question 42

What is the equation for the **Nusselt number**?

Answer 42

Question 43

Define **Prandtl's number** and the equation

Answer 43

The Prandtl number is defined as the ratio of momentum diffusivity to heat diffusivity (i.e. the relative thickness of the velocity and thermal boundary layers)

Question 44

A **low Prandtl number** means that...

Answer 44

...the **thermal boundary layer** is **thicker** than the **velocity boundary layer** (**high heat diffusivity rate**)

Question 45

For a flat plate, the critical Reynolds number (i.e. the Reynolds number at the x-location where transition occurs)... = ?

Answer 45

Question 46

For Re number, Laminar \< ? \< Turbulent

Answer 46

Laminar \< 5x10⁵​ \< Turbulent

Question 47

Fluid properties are evaluated at the...? (Name and equation)

Answer 47

***Film temperature***

Question 48

* µs is the flow dynamic viscosity at the ____ of the \_\_\_\_ * µ∞ is the dynamic viscosity in the ____ \_\_\_\_

Answer 48

* µs is the flow dynamic viscosity at the **_surface_** of the **_sphere_** * µ∞ is the dynamic viscosity in the f**_reestream flow_**

Question 49

As with flat plate flows, the flow in a pipe depends on the...(Equation as well)

Answer 49

...Reynolds number

Question 50

* For Re \< 2300, the flow is \_\_\_\_ * For Re \> 4000, the flow is \_\_\_\_

Answer 50

* For Re \< 2300, the flow is **_laminar_** * For Re \> 4000, the flow is **_turbulent_**

Question 51

For the thermal boundary layer, the entrance length for laminar flow can be obtained from is...(equation)

Answer 51

Question 52

For turbulent flow, the entrance length for the thermal boundary layer is approximately...

Answer 52

...10 pipe diameters

Question 53

For a rough pipe, the ____ Chart can be used to determine the ____ \_\_\_\_

Answer 53

For a rough pipe, the **_Moody_** Chart can be used to determine the **_Nusselt number_**

Question 54

High temperature fluids have a ____ \_\_\_\_ than colder fluids

Answer 54

High temperature fluids have a **_lower density_** than colder fluids

Question 55

What is the equation of state for an ideal gas?

Answer 55

Question 56

The Grashof number represents the ratio of...?

Answer 56

...the **buoyant force** to the **viscous** **force**

Question 57

What is the equation for the **Grashof Number**?

Answer 57

(We are expected to remember this)

Question 58

What does every symbol in this equation stand for?

Answer 58

Question 59

What is the equation for the **Rayleigh number** of the flow?

Answer 59

Question 60

But can alos be obtained by...?

Answer 60

...

Question 61

Natural convection from a vertical plate - flow pattern is different when the flow is...?

Answer 61

...laminar or turbulent

Question 62

When is the **Nusselt number _vertically_ and _horizontally_** **laminar?**

Answer 62

**Vertically** when 10⁴ \< Ra \< 10⁹ **Horizontally** when 10⁴ \< Ra \< 10⁷

Question 63

When is the **Nusselt number _vertically_ and _horizontally_ turbulent**?

Answer 63

**Vertically** when Ra \> 10⁹ **Horizontally** when Ra \> 10⁷

Question 64

Nusselt number when **Laminar** **flow**, **horizontal** **plate** **face** **down**

Answer 64

10⁵ \< Ra \< 10¹¹

Question 65

c (speed of light) = ? And define all symbols

Answer 65

c is the speed of light funny thing is the wavelength v is the frequency

Question 66

e (energy) = ? = ? And what is h?

Answer 66

h is **Planck’s constant** (6.6256x10^-34 Js)

Question 67

The shorter the wavelength of the photon, the higher its...?

Answer 67

...energy

Question 68

Define a **blackbody** and give an example

Answer 68

A **blackbody** is defined as a **perfect** **emittier** and **absorber** of **radiation** A good **blackbody** is the **sun** (or every star) because sthe surface termperature barely changes

Question 69

What is the **Stefan-Boltzmann Law** (equation)

Answer 69

Question 70

A radiator works by...?

Answer 70

...convection current

Question 71

What is ε?

Answer 71

ε is the emissivitiy of the surface

Question 72

The **Stefan-Boltzmann Law** tells us the...

Answer 72

...**heat** **transfer** **rate** over all **wavelengths**

Question 73

The colour of room temperature object is due to selective reflection of radiation by the surface (try to remember picture/diagram)

Answer 73

Question 74

Kirchoff's law for absorbtions states...

Answer 74

...that the **emissivity** and **absorptivity** for a body are the **same**

Question 75

If a smaller body is enclosed in a larger body, the net rate of heat transfer is given by...(equation)

Answer 75

(Not sure how important this is)

Question 76

What is the **solar** **flux** at the sun, **S₀**, using the **Stegan** **Boltzmann** **law**? (Equation)

Answer 76

Question 77

Solar flux decreases with the square of distance from the object so S = ?

Answer 77

Question 78

The Earth is 1 Astronomical Unit (AU) away from the Sun - 1AU = ?

Answer 78

1.49x10¹¹m

Question 79

T_ln = ?

Answer 79

Question 81

Q = \__{_} - \__{_} / \__{____}

Answer 81

Q = T₁ - T_∞ / R_total