Exam 4

Question 1

What are the two types of Heat Transfer

Answer 1

Macroscopic in nature
Microscopic in nature

Question 2

What type/types of heat transfer are macroscopic in nature

Answer 2

Convection

Why?
The driving mechanisms such as density changes may have microscopic causes, but the actual heat transfer happens through large-scale fluid flow, so convection is considered macroscopic.

Question 3

What type/types of heat transfer are microscopic in nature

Answer 3

Conduction

This is the behavior of individual particles that collectively leads to the flow of heat

Radiation

Radiation starts at the atomic or subatomic level, involving the movement of charged particles and quantum energy transitions

Question 4

Describe Radiation

Answer 4

Dark surfaces absorbing electromagnetic waves and transforming it into heat. Also described as the transfer of energy by electromagnetic waves from a hot body and absorbed by a cool body.

Question 5

What are the three types of conduction

Answer 5

Solids conductors
Solids and Liquids insulators
Gases

Question 6

What are the two types of convection

Answer 6

Forced
Natural

Question 7

Define conduction through gases

Answer 7

Brownian (random) motion of molecules

This is the heat transfer of thermal energy via collisions between gas molecules, from a region of higher temperature to lower temperature. In gases, molecules are farther apart so fewer collisions happen compared to solids, making gases poor conductors of heat.

Question 8

Define conduction through solids and liquids insulators

Answer 8

Transport of momentum of electrons

Question 9

Define conduction through solids conductors

Answer 9

Motion of unbound electrons

Question 10

Define forced convection

Answer 10

Flow made to move by external influence (such as a pump forcing flow)

Question 11

Define Natural Convection

Answer 11

Natural convection is when density differences causes mixing .

Question 12

What type of heat transfer is Heating a room?

Answer 12

Natural Convection

When a heater warms the air in the room, the warm air becomes less dense and rises, while the cool air moves in to take its place. The cycle continues, causing the air to circulate and warm the entire room.

Question 13

What type of heat transfer is Cooling Fans in Electronics?

Answer 13

Forced convection

Many electronic devices, such as computers, use fans to force air over components like CPUs and GPUs. This helps remove heat more effectively than natural convection alone.

Question 14

What is an insulator?

Answer 14

Materials that resist the flow of heat, electricity, or sound

Question 15

How does rubber transfer heat? How well does it do that?

Answer 15

Solids insulators as conductors

Just like solids, rubber transfers heat internally through molecular vibration, and interaction, which is thermal conduction . It has a low thermal conductivity, meaning it does not transfer heat efficiently.

Question 16

What type of heat transfer is heat from a fire?

Answer 16

Radiation

Common example of infrared radiation. As the fire burns, it emits infrared radiation, which we feel as heat even if we aren’t in direct contact with the flames.

Question 17

What is the formula for heat transfer by conduction?

Answer 17

qx = -kA ( dT/dx ) (Watts)
Steady State : q = -kA(delta T), k = constant

Where
q = rate of heat transfer in the x direction
A= Area normal to the direction of heat flow
dT/dx = the temperature gradient in the x direction
k = the thermal conductivity

Question 18

What type of convection is drying your hair with a hairdryer?

Answer 18

Forced convection

Question 19

What type of convection is hot air rising above a fire?

Answer 19

Natural convection

Question 20

What type of convection is turning on the air conditioner to cool yourself on a July day in Tucson?

Answer 20

Forced convection

Question 21

What type of convection is formation of thunderstorms/tornadoes?

Answer 21

Natural convection

Movement of fluid caused by differences in temperature and density without any external force

Fluid is heated, warm part becomes less dense and rises, cooler denser fluid sinks to take its place. This creates a circulating flow.

Question 22

True or false?
Heat flow occurs from colder bodies to warmer bodies.

Answer 22

False

According to the second law of thermodynamics, The direction of heat flow is always from regions of higher temperature to regions of a lower temperature.

Question 23

You are walking, barefoot on concrete during a hot sunny day and you could feel your feet burning. The dominant mechanism of heat transfer between the concrete and your feet is:

Answer 23

Conduction

Question 24

You get into a car on a hot and sunny day and turn on the air conditioner to cool you down. The dominant mechanism of heat transfer cooling off is:

Answer 24

Convection

Question 25

We are generally concerned with three phases of matter in this course: what are they? And rank these phases from lowest to highest thermal conductivity.

Answer 25

The three phases of matter are gas, liquid, solid. From lowest to highest thermal conductivities: gas, liquid, solid Why? Solids typically have the highest thermal conductivity because their particles are closely packed, allowing heat to transfer efficiently through them. Liquids have moderate thermal conductivity because their particles are not as tightly packed as solids. Gases have the lowest thermal conductivity since their particles are far apart, resulting in less efficient heat transfer.

Question 26

There are two slabs of concrete, slab one and slab two, which are equidistant from a furnace. The side of the slabs facing the furnace are hotter than the sides facing away from the furnace. If slab two is twice as thick a slab one and both slabs having a identical rate of heat transfer. How do the temperatures on the walls facing away from the furnace compare?

Answer 26

The temperature of slab two is lower than slab one. Why? The heat will need to travel a longer distance through slab 2, meaning that the temperature difference on the cold side of slab 2 will be generally lower than that of slab one because the temperature at the furnace side is the same for both slaps, and the gradient in slab 2 has to cover more distance.

Question 27

What is the equation that describes radiation?

Answer 27

Q = 𝜎AT^4

Question 28

What equation describes convection?

Answer 28

Q = -hA∆T Where h is the heat transfer coefficient A is the outside surface area of the pipe And delta T is ( T of the outside of the pipe - T of the surroundings )

Question 29

Other books often say we have a semi infinite slab when we use this equation: Q=-kA( dT/dx ) Why is this for a semi infinite slab?

Answer 29

If we assume semi infinite, we don’t have to worry about heat, leaving the edges. This makes the mathematics easier and cleaner.

Question 30

In the summer, I like to turn the fan on. Why?

Answer 30

Convection

Question 31

In the summer, I like to stand in the shade when I’m outside. Why?

Answer 31

Radiation

Question 32

In the summer, I like to put an ice cube in my mouth to cool off. Why?

Answer 32

Conduction

Question 33

When comparing plates versus pipes for heat transfer problems, what do we need to account for in pipes that we do not for plates?

Answer 33

The area for heat transfer is no longer constant when looking at pipes instead of plates.

Question 34

Under which conditions will there be no heat transfer between an object and its surroundings?

Answer 34

Adiabatic, well insulated, or when T object = T surroundings

Question 35

What factors go into choosing insulation material?

Answer 35

Cost; thermal, conductivity; K; material of construction; compatibility; safety

Question 36

If you want to reduce the heat losses from a pipe to be half that of the bear pipe, while making the pipe safe to touch do you want a low, medium, or high k material?

Answer 36

We want a low k material to prevent heat from reaching the surface of insulation to make it safe to touch, but also be thin so it costs less

Question 37

What type of heat transfer involves k, the thermal conductivity?

Answer 37

Conduction

Question 38

What type of heat transfer involves h, the heat transfer coefficient?

Answer 38

Convection

Question 39

The prandtl number is the ratio of:

Answer 39

Momentum diffusivity/ thermal diffusivity

Question 40

The Nusselt number is the ratio of:

Answer 40

Convective heat transfer / conductive heat transfer

Question 41

The Grashof number is a ratio of :

Answer 41

Buoyancy / viscous forces

Question 42

What equation gives the effective diameter of a non-cylindrical shape?

Answer 42

de = 4(cross sectional area/ perimeter)

Question 43

True or False? The heat transfer coefficient, h, is inversely related to the boundary layer thickness.

Answer 43

True Why? As boundary layer thickness increases the heat transfer coefficient decreases . A thicker boundary layer acts like insulation, it reduces the rate of which heat can be transferred A thinner boundary layer allows for more efficient heat transfer, meaning a higher h value. The heat transfer coefficient h is strongly influenced by the thermal boundary layer that forms at the surface between a fluid (like air or water) and a solid (like a pipe wall).

Question 44

In conductive heat transfer through a cylindrical or spherical geometry, we use dT/dr because:

Answer 44

The area the heat is transferring through is constantly changing and must be accounted for

Question 45

True or False? The overall heat transfer coefficient can be calculated by treating the heat transfer processes as a circuit with processes occurring in series and/or parallel .

Answer 45

True

Question 46

What are the steps to solve for h for natural convection?

Answer 46

1. Solve for Gr and Pr 2. Solve for x = (Gr*Pr) 3. Solve for Y based on what flow regime X is in 4. Set Y = (h*L)/k to solve for h

Question 47

In the Gr equation for natural convection from a pipe to the surrounding air, what L, density, viscosity, beta and delta T are needed?

Answer 47

The characteristic length for a long pipe = r^2/ 2 Density and viscosity of the air at the T wall temperature Beta = 1/T wall Delta T = T wall - T surrounding air

Question 48

In the Prindtl equation for natural convection between a pipe and its surrounding air, what are the Cp, viscosity, and k value needed?

Answer 48

Cp of air at the temperature of the wall (T wall) Viscosity of air at T wall k of air at Twall

Question 49

What are greybodies? What variables are constant for all greybodies?

Answer 49

An object whose absorptivity is the same for all temperature radiation. α (absorptivity) = ε (emissivity) = const, at all temperatures

Question 50

What is absorptivity (α)?

Answer 50

How well a surface absorbs radiation

Question 51

What is emissivity (ε)?

Answer 51

How well a surface emits thermal radiation

Question 52

What is k? For conduction from a fluid to a steel pipe, what k is needed?

Answer 52

k is the thermal conductivity which describes how easily heat can pass through a material by conduction. The k of steel.

Question 53

What is a blackbody and what is the absorptivity of a blackbody?

Answer 53

The sun is a blackbody a.k.a the perfect emitter and absorber α = 1 E = 1

Question 54

What does a smaller U mean? How does this impact the SA needed?

Answer 54

A smaller U means a higher thermal resistance . For a higher thermal resistance, more surface area is needed to transfer the same amount of heat.

Question 55

What is scaling in a heat exchanger?

Answer 55

Scaling is a very different phenomenon from fouling. Scaling occurs when a mineral film coats the entire surface of a heat exchanger. The most common forms of scale are usually from calcium based salts such as calcium carbonate

Question 56

What does the prandtl number tell us? What does a higher prandtl number mean in convection?

Answer 56

Pr is the ratio of momentum diffusivity to thermal diffusivity. A higher prandtl number means it has a greater resistance to flow, the fluid has a higher viscosity (or lower thermal conductivity),and the momentum (how the fluid resists motion) diffuses faster than heat

Question 57

What is momentum diffusivity? And what does a higher number mean for the flow?

Answer 57

Momentum diffusivity is kinematic viscosity and a higher number means it is more resistant to flow/deformation.

Question 58

What is thermal diffusivity ? What does a higher number mean for the heat transfer ?

Answer 58

Thermal diffusivity is the thermal conductivity divided by the heat transfer and specific heat capacity at constant pressure, and is related to a materials ability to conduct thermal energy versus store thermal energy ; higher means heat transfers rapidly

Question 59

In the overall heat transfer coefficient equation, can we include all forms of heat transfer? Why?

Answer 59

Radiation cannot be included in the overall heat transfer coefficient equation. This is because radiation is not linear, it depends on T^4. Using radiation in this equation would make U nonlinear and temperature dependent.

Question 60

Is a higher or lower k material needed for insulation in order to make a pipe safe enough to touch while still remaining thin so it costs less to add the insulation ?

Answer 60

A low k material to prevent heat from reaching the surface of the insulation to make it safe to touch. k measures how easily heat flows through a material. A higher k means the material conducts heat easily, a low k means the material resists heat flow which is ideal for insulation purposes.

Question 61

For conduction through and insulated pipe, how does the thickness of the insulation change the temperature on the outer surface of the insulation?

Answer 61

The temperature on the outer surface of the insulation will get lower if the thickness increases, and higher if the thickness decreases.

Question 62

In the smokestack problem from class, why was there no radiation inside the smokestack fluids?

Answer 62

In the smokestack, the gas is hotter than the surface of the smokestack, so the smokestack does not transfer energy via radiation since it is in a lower energy state (it’s cooler)

Question 63

What is the characteristic length? How can you calculate it for a long cylinder?

Answer 63

The characteristic length is a representative length that describes the size of an object in the direction of heat flow or fluid flow. It is volume / area. For a long cylinder the end areas are negligible so… Characteristic length = (pi r^2L) / (2pi*r*L)

Question 64

When it is 70 degrees Fahrenheit I am comfortable in shorts and a T-shirt. However, when I am swimming in 70 degrees Fahrenheit water, I need a 5 mm thick wetsuit. Why?

Answer 64

the convection coefficient h, which in this example h is how effectively the fluid pulls heat away from your skin. Water has an h of about 500 W/m^2*K while air has an h of about 5 W/m^2*K . Water has a much higher thermal conductivity than air. Heat will leave your body much faster in water than in air. This rapid heat loss makes you feel cold in water, even at 70 degrees Fahrenheit.