HEAT TRANSFER Flashcards
measure of the amount of energy possessed by the molecules of a substance
TEMPERATURE (T)
transfer of energy as heat occurs at the molecular level as a result of a temperature difference
HEAT (Q)
transfer of energy resulting from a force acting thru a distance
WORK (W)
heat transfer by the interactions of atoms or molecules of a material
CONDUCTION
heat transfer by the mixing and motion of macroscopic portions of a fluid
CONVECTION
heat transfer by electromagnetic radiation that arises due to the temp. of a body
RADIATION (Radiant Heat Transfer)
heat transfer rate per unit area (Q/A)
HEAT FLUX
measure of a substance’s ability to transfer heat thru a solid by conduction
THERMAL CONDUCTIVITY
The thermal conductivity of most liquids and solids varies with ______.
TEMPERATURE
Thermal conductivity of vapors depends upon ______.
PRESSURE
average temp. difference
LOG MEAN TEMPERATURE DIFFERENCE
also called as a film coefficient
CONVECTIVE HEAT TRANSFER COEFFICIENT
the thermal resistance of a relatively stagnant layer of fluid between a heat transfer surface and the fluid medium
CONVECTIVE HEAT TRANSFER COEFFICIENT
combines the heat transfer coefficient of the two heat exchanger fluids and the thermal conductivity of the heat exchanger tubes
OVERALL HEAT TRANSFER COEFFICIENT
For flow adjacent to a hot or cold surface, it is the temp. of a fluid that is far from the surface
BULK TEMPERATURE
For boiling or condensation, Tb is equal to the _______.
SATURATION TEMPERATURE
Heat transfer by conduction depends upon the driving force of ________ and the _______ to heat transfer.
TEMPERATURE DIFFERENCE, RESISTANCE
It depends upon the nature and dimensions of the heat transfer medium.
RESISTANCE TO HEAT TRANSFER
All heat transfer problems involve the temp. difference (ΔT), the ______, and the physical properties of the objects being studied.
GEOMETRY (AREA)
Fourier’s Law of Conduction for Rectangular
Q = kA (ΔT/Δx)
Fourier’s Law of Conduction for Cylindrical
Q = kA (ΔT/Δr)
current flow is analogous to _______
HEAT TRANSFER RATE (Q)
resistance is analogous to combination of _____, _____, and ______
THERMAL CONDUCTIVITY (k), THICKNESS (x), AREA (A)
potential (emf) is analogous to ______
TEMPERATURE DIFFERENCE (ΔT)
Fourier equation (Equivalent Resistance Method)
Q = ΔT/(Δx/kA)
analogous to I = V/R
Across a cylindrical wall, the heat transfer ______ is continually increasing or decreasing.
SURFACE AREA
The evaluation of heat transfer thru a cylindrical wall can be extended to include a ________ composed of several concentric, cylindrical layers.
COMPOSITE BODY
the motion and mixing is caused by density variations resulting from temp. differences within the fluid.
NATURAL CONVECTION
the motion and mixing is caused by an outside force (such as pump)
FORCED CONVECTION
T or F. Heat transfer by conduction is more difficult to analyze than heat transfer by convection.
FALSE (convection > conduction)
T or F. Heat transfer by convection varies from situation to situation (upon fluid flow conditions), and is frequently with the mode of fluid flow.
TRUE
In practice, analysis of heat transfer by convection is treated _______ or by direct observation.
EMPIRICALLY
Factors that affect the stagnant film thickness
- Fluid velocity
- Fluid viscosity
- Heat flux
- Surface roughness
- Type of flow (single/two-phase)
T or F. Density of hot gas is higher than cold gas.
FALSE (cold > hot)
For flow in pipe, Tb is the _______ measured at a particular cross-section of the pipe.
AVERAGE TEMPERATURE
Convection heat transfer equation
Q = hAΔT
______ is dependent upon the physical properties of the fluid and the physical situation.
CONVECTIVE HEAT TRANSFER COEFFICIENT (h)
Typically, convective heat transfer coefficient for laminar flow is _______ than turbulent flow.
relatively low
T or F. Laminar flow has a thinner stagnant fluid film layer on the heat transfer surface than turbulent flow.
FALSE (turbulent < laminar)
Where applications involving convection heat transfer take place
within pipes, tubes, or some similar cylindrical device
________ is used for heat transfer processes encountered in thermal facilities involving both conduction and convection.
OVERALL HEAT TRANSFER COEFFICIENT (Uo)
The convective coefficient (h), the overall coefficient (Uo), and the other fluid properties may vary substantially for the fluid if it experiences a large ________ change during its path thru the convective heat transfer device.
TEMPERATURE
Under such circumstances, the temperature at which the properties are looked up must be some type of _____ value rather than using either the inlet or outlet temperature value.
AVERAGE
