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50 Cards in this Set
- Front
- Back
____ 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
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Temperature
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is energy in transit
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Heat
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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
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Heat
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The symbol for temperature is___. The common scales for measuring temperature are ___
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T
Fahrenheit, Rankine, Celsius, and Kelvin temperature scales. |
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DESCRIBE the difference between heat and work (2).
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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.
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DESCRIBE the Second Law of Thermodynamics and how it relates to heat transfer
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). Heat will be lost during work, etc. Nothing can be 100% efficient
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Involves the transfer of heat by the interactions of atoms or molecules of a material through which the heat is being transferred. (contact)
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Conduction -
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Involves the transfer of heat by the mixing and motion of macroscopic portions of a fluid. (cooling by a fan)
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Convection -
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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).
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Radiation (or radiant heat transfer) -
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Heat transfer rate per unit area, measured in BTU’s (BTU/hr-ft2)(heat surface)
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Heat flux –
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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.
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Thermal conductivity -
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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.
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Log mean temperature difference -
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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.
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Convective heat transfer coefficient -
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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.
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Overall heat transfer coefficient -
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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.
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Bulk temperature –
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A body that emits the maximum amount of heat for its absolute temperature is called a black body
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Black body radiation -
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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.
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Emissivity
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The most common arrangements for flow paths within a heat exchanger are ____
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counter flow and parallel flow heat exchangers
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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.
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counter flow heat exchanger
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In a _____ both fluids in the heat exchanger flow in the same direction.
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parallel flow heat exchanger,
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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.
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Nucleate boiling -
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A ____ is one in which the same fluid is used as the cooling fluid and the cooled fluid.
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regenerative heat exchanger
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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).
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Film boiling -
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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.
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Departure from nucleate boiling (DNB)
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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.
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Critical heat flux -
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The heat flux associated with DNB is called the _____
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Critical Heat Flux (CHF). CHF
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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.
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Nuclear enthalpy rise hot channel factor-
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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.
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Average linear power density -
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The heat generation rate divided by the volume of fuel will give the average
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Volumetric thermal source strength -
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______ 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.
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Decay heat production
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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 ____
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decay heat.
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Neither heat nor work are ______
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Thermodynamic properties of a system
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Heat into a system and work out of a system are both considered ___
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Positive quantities
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The 2nd Law of THERMO indicates the natural flow of energy is from ____ to ___
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hotter body to colder body
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____ denies the possibility of converting all the heat into work in a cyclic system
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2nd law of thermo
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What are the three types of heat transfer?
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conduction convection and radiation
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The rate at which heat is transferred is represented by the symbol __
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Q ^ dot
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Common units for heat transfer rate is ___
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Btu/hr
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heat transfer rate per unit area
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Heat flux
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units for heat flux are ___
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btu/hr-ft^2
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Emissivity is a dimensionless number and has a maximum value of __
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1
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What are the 4 stages of the Pool boiling curve?
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1. Natural Convection
2. Nucleate Boiling 3. Partial Film boiling 4. Film Boiling |
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______ is the ratio of the maximum heat flux expected at any area to the average heat flux for the core.
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Nuclear heat flux hot channel factor (HFHCF)
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Areas with the_____ will produce the most heat and have the highest temperatures
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highest heat generation rate
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What are the two methods for removing decay heat from a reactor core?
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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 |
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The dominant heat transfer mechanism that occurs when film boiling is present is ___
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Radiation
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Order of reactor coolant heat transfer mechanisms, from most efficient to least efficient
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Nucleate boiling, transition boiling, single phase convection, and stable film boiling
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_____ allow the exiting cooled fluid temperature to be below the exiting cooling fluid temperature.
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Counter flow heat exchangers
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Nucleate boiling affects heat transfer from a fuel rod primarily by increasing the _____ from the fuel rod to the coolant
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convective heat transfer
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The driving head for natural circulation flow through the core is developed by differences in the _____
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water density
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