Heat Flux
The rate at which heat is transferred is symbolized by the symbol Q. General units for heat transfer rate is Btu/hr. At times it is significant to determine the heat transfer rate per unit area, or heat flux that has the sign Q". the units for heat flux are Btu/hr-ft2. The heat flux can be established by dividing the heat transfer rate by the area via which the heat is being transferred.
Here:
Q" = heat flux (Btu/hr-ft2)
Q = heat transfer rate (Btu/hr)
A = area (ft2)