Heat Transfer Flashcards
Describe three mechanisms of heat transfer.
- Conduction - transfer of heat by the interactions between adjacent molecules of the material through which the heat is being transferred.
- Convection - transfer of heat by a process of bulk motion and mixing of
macroscopic portions of a fluid. - Radiation - transfer of heat by electromagnetic radiation that arises due to the
temperature of a body.
Describe thermal conductivity.
A measure of how well a material conducts heat.
Explain the overall heat transfer coefficient (Uo)
For the simplest case of steady-state heat transfer by conduction through
a single slab of material, the overall heat transfer coefficient (Uo) equals the thermal conductivity (k) divided by the thickness of the slab (x).
Explain heat flux.
Heat flux (Q”, Q double prime) is defined as heat
transfer rate (Q) divided by area (A). It can be
viewed as the heat transfer rate per unit area. It
can also be defined as the product of the heat
transfer coefficient (U) and the difference in
temperature (T).
What is the formula for the overall heat transfer rate?
What is the formula for heat flux?
What is the formula for heat transfer using thermal conductivity of the material?
Discuss factors impacting convection that are important to nuclear power.
In a heat exchanger tube, the heat is transferred through the tube wall by
conduction, but the fluid flowing in the tube carries the heat away by convection. Solids transfer heat by conduction, and with a few very important exceptions, fluids (that is, liquids and gases) transfer heat by convection. The two most important exceptions in a nuclear power plant are the helium gap between the fuel pellet and the cladding, and the boundary, or laminar liquid layer, which is the water film in actual contact with the heat transfer surface. In both cases, conduction across these two layers impedes the entire heat transfer process, and these results in higher fuel pellet and cladding temperatures.
Distinguish between regenerative and non-regenerative heat exchangers.
In a regenerative heat exchanger, the same fluid is
used as the cooling fluid and the cooled fluid.
Discuss parallel flow and counter flow heat exchangers.
In a parallel flow heat exchanger, both fluids in the heat exchanger flow in the same direction. The heat transfer rate decreases, as the fluids travel through the
parallel heat exchanger.
In a counter flow heat exchanger, the direction of flow of one working fluid is primarily opposite to the direction of flow of the other fluid. A counter flow
heat exchanger can transfer a relatively large amount of heat energy for its size, compared with the parallel flow and cross flow types, because of the large T present through out the heat exchanger. For this reason, most shell and tube heat exchangers are of the counter flow type.
What is the formula for the log meant temperature difference?
Describe the relationship between heat transfer rate in a heat exchanger and the factors which affect it.
There are several factors affecting the heat transfer coefficient for a fluid film. Most of these affect the film thickness. Film thickness, in conjunction with the fluid thermal conductivity, determines the heat transfer coefficient.
- Fluid velocity - The greater the velocity of the fluid stream, the thinner the fluid film will be which causes the heat transfer coefficient to increase.
- Fluid thermal conductivity - An increase in thermal conductivity results in an increase of the heat transfer coefficient.
- Fluid viscosity - The smaller the viscosity, the thinner the film, and the larger the heat transfer coefficient.
- Heat flux/nucleate boiling - If the heat flux is sufficient to cause nucleate boiling, the film thickness will effectively decrease (due to the turbulence caused by the bubbles). This will increase the heat transfer coefficient.
List the functions of the main condenser in a power plant.
The exhaust steam from the turbine gives up heat to the circulating water in the tubes of the condenser, and condensation of the steam takes place. The condensate collects in the bottom of the condenser and is pumped back to the boiler.
Define condensate depression and discuss its opeational implications.
The temperature difference between the saturation temperature for the existing condenser vacuum and the temperature of the condensate is condensate depression. It is expressed as the number of degrees condensate depression or the number of degrees subcooled.
Excessive condensate depression decreases the operating efficiency of the plant since the subcooled condensate must be reheated. Excessive condensate depression also allows for an increased absorption of air by the condensate that causes accelerated oxygen corrosion of plant materials.
Explain core thermal power.
Core thermal power (CTP) is defined as the
average power density of the core times the core volume.
It can be more simply defined as the measure
of heat input per unit time from the core to the
reactor coolant system.
Explain methods of determining core thermal power using reactor coolant system parameters.
Explain methods of determining core thermal power using steam generator parameters.
Explain percent reactor power.
Nuclear instrumentation indicates percent reactor
power. This is simply the percent of rated CTP
that the reactor is operating at (actual core
thermal power divided by rated core thermal
power).
