HEAT TRANSFER EXAM 1 QUESTION
WITH AND ANSWERS
Gas - spacing between molecules, conduction, how the heat transfer occurs -
ANSWER-Large spacing between particles/molecules; low conductivity; collisions and
diffusion
Fourier's Law - ANSWER-Heat flux[W/m^2] = -(thermal conductivity)*(delta T)
The negative sign is because heat flux is in the direction of decreasing temperature
k - ANSWER-Thermal Conductivity -the ability for a material to conduct heat
-material property
- function of material, temp, and sometimes direction
- higher for a solid
- low k means that material is an insulator
- k is normally highest in the solid phase and lowest in the gas phase
**We generally assume that k is isotropic and constant with respect to temperature but it
can change with direction
Thermal diffusivity - ANSWER-(heat conduction or thermal conductivity)/(heat storage
or heat capacity) = (k)/(𝜌*c_p)
How well a material conducts heat
Convection - ANSWER--advection of the thermal energy away with the flow (carried
along with flow) + conduction into the fluid
- has the combined effects of conduction and fluid motion
- occurs when thermal energy is transferred to a fluid
- is the mode of energy transfer between a solid surface and the adjacent liquid or gas
that is in motion
Turbulent flow ___ more effectively that laminar b/c faster flow = faster ___
Newton's Law of Cooling - ANSWER-Heat flux = convection HT coefficient * (surface
temp - free stream temp of flow)
Note: At the surface, the fluid temperature equals the surface temperature of the solid
Convection heat transfer coefficient - ANSWER-- h units: [W/Km^2]
- h is not a material property, but related to material properties
- h is a function of geometry, flow characteristics, fluid properties
- Experimentally determined parameter
- H is the proportionality factor between heat flux and temperature difference
Forced Convection - ANSWER--fluid is forced to flow over the surface be external
means (ex. fan, pump, wind)
WITH AND ANSWERS
Gas - spacing between molecules, conduction, how the heat transfer occurs -
ANSWER-Large spacing between particles/molecules; low conductivity; collisions and
diffusion
Fourier's Law - ANSWER-Heat flux[W/m^2] = -(thermal conductivity)*(delta T)
The negative sign is because heat flux is in the direction of decreasing temperature
k - ANSWER-Thermal Conductivity -the ability for a material to conduct heat
-material property
- function of material, temp, and sometimes direction
- higher for a solid
- low k means that material is an insulator
- k is normally highest in the solid phase and lowest in the gas phase
**We generally assume that k is isotropic and constant with respect to temperature but it
can change with direction
Thermal diffusivity - ANSWER-(heat conduction or thermal conductivity)/(heat storage
or heat capacity) = (k)/(𝜌*c_p)
How well a material conducts heat
Convection - ANSWER--advection of the thermal energy away with the flow (carried
along with flow) + conduction into the fluid
- has the combined effects of conduction and fluid motion
- occurs when thermal energy is transferred to a fluid
- is the mode of energy transfer between a solid surface and the adjacent liquid or gas
that is in motion
Turbulent flow ___ more effectively that laminar b/c faster flow = faster ___
Newton's Law of Cooling - ANSWER-Heat flux = convection HT coefficient * (surface
temp - free stream temp of flow)
Note: At the surface, the fluid temperature equals the surface temperature of the solid
Convection heat transfer coefficient - ANSWER-- h units: [W/Km^2]
- h is not a material property, but related to material properties
- h is a function of geometry, flow characteristics, fluid properties
- Experimentally determined parameter
- H is the proportionality factor between heat flux and temperature difference
Forced Convection - ANSWER--fluid is forced to flow over the surface be external
means (ex. fan, pump, wind)
