Heat Transfer 2

Front 1

____ is a measure of the amount of energy possessed by the molecules of a substance. It is a relative measure of how hot or cold a substance is and can be used to predict the direction of heat transfer

Back 1

Temperature

Front 2

is energy in transit

Back 2

Heat

Front 3

The transfer of energy as_____, occurs at the molecular level as a result of a temperature difference. ____ is capable of being transmitted through solids and fluids by conduction, through fluids by convection, and through empty space by radiation. Heat is measured in BTU’s

Back 3

Heat

Front 4

The symbol for temperature is___. The common scales for measuring temperature are ___

Back 4

T

Fahrenheit, Rankine, Celsius, and Kelvin temperature scales.

Front 5

DESCRIBE the difference between heat and work (2).

Back 5

Both represent energy in transition. Work is the transfer of energy resulting from a force acting through a distance. Heat is energy transferred as the result of a temperature difference. Neither heat nor work, are thermodynamic properties of a system. Heat can be transferred into or out of a system and work can be done on or by a system. Heat into a system and work out of a system are considered positive quantities.

Front 6

DESCRIBE the Second Law of Thermodynamics and how it relates to heat transfer

Back 6

). Heat will be lost during work, etc. Nothing can be 100% efficient

Front 7

Involves the transfer of heat by the interactions of atoms or molecules of a material through which the heat is being transferred. (contact)

Back 7

Conduction -

Front 8

Involves the transfer of heat by the mixing and motion of macroscopic portions of a fluid. (cooling by a fan)

Back 8

Convection -

Front 9

Involves the transfer of heat by electromagnetic radiation that arises due to the temperature of a body. (boiling water on stove consists of all three modes of heat transfer at one time)(snow melting from heat of the sun).

Back 9

Radiation (or radiant heat transfer) -

Front 10

Heat transfer rate per unit area, measured in BTU’s (BTU/hr-ft2)(heat surface)

Back 10

Heat flux –

Front 11

It is a measure of a substance's ability to transfer heat through a solid by conduction. The thermal conductivity of most liquids and solids varies with temperature. For vapors, it depends upon pressure.

Back 11

Thermal conductivity -

Front 12

In heat exchanger applications, the inlet and outlet temperatures are commonly specified based on the fluid in the tubes. The temperature change that takes place across the heat exchanger from the entrance to the exit is not linear.

Back 12

Log mean temperature difference -

Front 13

The convective heat transfer coefficient (h) is dependent upon the physical properties of the fluid and the physical situation. Typically, the convective heat transfer coefficient for laminar flow is relatively low compared to the convective heat transfer coefficient for turbulent flow. This is due to turbulent flow having a thinner stagnant fluid film layer on the heat transfer surface.

Back 13

Convective heat transfer coefficient -

Front 14

The overall heat transfer coefficient combines the heat transfer coefficient of the two heat exchanger fluids and the thermal conductivity of the heat exchanger tubes.

Back 14

Overall heat transfer coefficient -

Front 15

Looks at the overall majority of the container. Hotter at the bottom and colder at the top, but the overall or Bulk temperature is the most important. Point at the the greatest number of water molecules. Basically this is the average temperature.

Back 15

Bulk temperature –

Front 16

A body that emits the maximum amount of heat for its absolute temperature is called a black body

Back 16

Black body radiation -

Front 17

is simply a factor by which we multiply the black body heat transfer to take into account that the black body is the ideal case.

Back 17

Emissivity

Front 18

The most common arrangements for flow paths within a heat exchanger are ____

Back 18

counter flow and parallel flow heat exchangers

Front 19

A ____ is one in which the direction of flow of one of the working fluids is opposite to the direction of flow of the other fluid.

Back 19

counter flow heat exchanger

Front 20

In a _____ both fluids in the heat exchanger flow in the same direction.

Back 20

parallel flow heat exchanger,

Front 21

Occurs at the heating surface. Steam bubbles collapse when carried into the bulk coolant stream. Bulk coolant temperature is below the saturation point for system pressure. Region II of the Pool Boiling Curve represents a rise in heat input until the heat transfer surface temperature is slightly above the saturation temperature of the water.

Back 21

Nucleate boiling -

Front 22

A ____ is one in which the same fluid is used as the cooling fluid and the cooled fluid.

Back 22

regenerative heat exchanger

Front 23

Critical Heat Flux: If the heat applied rises further, more bubbles form on the heat transfer surface. Eventually the bubbles start to collect on the surface. As bubbles collect and blanket the surface the ΔT between the fuel rod and the coolant rises significantly. Critical Heat Flux:They begin to coalesce into a vapor film (steam blanket). This causes a transition to takes place resulting in a Departure from Nucleate Boiling (DNB).

Back 23

Film boiling -

Front 24

is the end of nucleate boiling and the beginning of reduced heat transfer. This is the Transition Boiling mechanism, Region III of the Pool Boiling Curve. Raising heat input in Region III causes heat transfer by conduction and radiation through the vapor film. DNB is also called The Onset of Transition Boiling.

Back 24

Departure from nucleate boiling (DNB)

Front 25

occurs when the bubbles start to collect on the heat transfer surface. They begin to coalesce into a vapor film (steam blanket). This causes a transition to takes place resulting in a Departure from Nucleate Boiling (DNB). Critical Heat Flux generally results in a rapid transition to stable film boiling.

Back 25

Critical heat flux -

Front 26

The heat flux associated with DNB is called the _____

Back 26

Critical Heat Flux (CHF). CHF

Front 27

the ratio of the total kW heat generation along the fuel rod with the highest total kW to the total kW of the average fuel rod.

Back 27

Nuclear enthalpy rise hot channel factor-

Front 28

In nuclear reactors, the fuel is usually distributed in individual components which sometimes resemble rods, tubes, or plates. It is possible to determine the average power produced per unit length of fuel component by dividing the total thermal output of the core by the total length of all the fuel components in the core. This quantity is called the average linear power density. Common units for measuring average linear power density are kW/ft.

Back 28

Average linear power density -

Front 29

The heat generation rate divided by the volume of fuel will give the average

Back 29

Volumetric thermal source strength -

Front 30

______ is a particular problem associated with nuclear reactors. Even though the reactor is shut down, heat is produced from the decay of fission fragments. Limits for each particular reactor are established to prevent damage to fuel assemblies due to decay heat. Some radioactive atoms will decay while the reactor is operating and the energy released by their decay will be removed from the core along with the heat produced by the fission process. All radioactive materials that remain in the reactor at the time it is shut down and the fission process halted will continue to decay and release energy. This release of energy by the decay of fission products is called decay heat.

Back 30

Decay heat production

Front 31

All radioactive materials that remain in the reactor at the time it is shut down and the fission process halted will continue to decay and release energy. This release of energy by the decay of fission products is called ____

Back 31

decay heat.

Front 32

Neither heat nor work are ______

Back 32

Thermodynamic properties of a system

Front 33

Heat into a system and work out of a system are both considered ___

Back 33

Positive quantities

Front 34

The 2nd Law of THERMO indicates the natural flow of energy is from ____ to ___

Back 34

hotter body to colder body

Front 35

____ denies the possibility of converting all the heat into work in a cyclic system

Back 35

2nd law of thermo

Front 36

What are the three types of heat transfer?

Back 36

conduction convection and radiation

Front 37

The rate at which heat is transferred is represented by the symbol __

Back 37

Q ^ dot

Front 38

Common units for heat transfer rate is ___

Back 38

Btu/hr

Front 39

heat transfer rate per unit area

Back 39

Heat flux

Front 40

units for heat flux are ___

Back 40

btu/hr-ft^2

Front 41

Emissivity is a dimensionless number and has a maximum value of __

Back 41

1

Front 42

What are the 4 stages of the Pool boiling curve?

Back 42

1. Natural Convection
2. Nucleate Boiling
3. Partial Film boiling
4. Film Boiling

Front 43

______ is the ratio of the maximum heat flux expected at any area to the average heat flux for the core.

Back 43

Nuclear heat flux hot channel factor (HFHCF)

Front 44

Areas with the_____ will produce the most heat and have the highest temperatures

Back 44

highest heat generation rate

Front 45

What are the two methods for removing decay heat from a reactor core?

Back 45

circulating fluid througha the reactor core in a closed loop, using some sort of heat exchanger to transfer heat out of the system.

open system, drawing in cool fluid from some source and discharging warmer fluid to some storage area or the environment

Front 46

The dominant heat transfer mechanism that occurs when film boiling is present is ___

Back 46

Radiation

Front 47

Order of reactor coolant heat transfer mechanisms, from most efficient to least efficient

Back 47

Nucleate boiling, transition boiling, single phase convection, and stable film boiling

Front 48

_____ allow the exiting cooled fluid temperature to be below the exiting cooling fluid temperature.

Back 48

Counter flow heat exchangers

Front 49

Nucleate boiling affects heat transfer from a fuel rod primarily by increasing the _____ from the fuel rod to the coolant

Back 49

convective heat transfer

Front 50

The driving head for natural circulation flow through the core is developed by differences in the _____

Back 50

water density