A_{Bk }  m²^{ }  BC cross sectional area (at BC node boundary k) 
A_{Nn}, A_{Mn }  m²^{ }  SC cross sectional area (at SC node boundary n, mean value) 
A_{TWMn }  m²  Surface area of a (single) tube wall along a node n 
C C_{0},     Dimensionless constant Phase distribution parameter and its slope at α = 0 
d_{HY }  m  Hydraulic diameter 
f(z, t), f_{Nn}, f_{Mn}_{ }    General and nodal (boundary and mean) solution functions 
f_{LIMCA }    Upper or lower limit of the approx. function f(z,t) 

 Mass flow Mass flux 
h, h^{P}, c_{P} = h^{T} 
 Specific enthalpy and its partial derivatives with respect to pressure and temperature (=specific heat) 
h_{SW} = h^{//} − h^{/}, h^{//}, h^{/ } 
 Latent heat, saturation steam and water enthalpy 
K_{EYBC }    Characteristic key number for each channel (BC) within an overall design 
L_{FTYPE} = 0, 1, 2 or 3    SC with saturated water/steam mixture, subcooled water, superheated or supercritical steam 
L_{FTBE} (=L_{FTYPE} of 1st N_{SC}) L_{HEATB} = 0 or = 1     L_{FTYPE} of 1st SC within BC Nonheated or heated wall 
L_{NOCCF} = 0 or = 1    Eventual CCF situation allowed or not allowed 
N_{BT} = N_{BCA} − N_{BCE} + 1_{ }    Total number of BC nodes 
N_{BCA} = N_{CT} + N_{BCE} − 1, N_{BCE}_{ }    BC node numbers containing BC outlet or entrance 
N_{CT} = N_{CA} − N_{CE} + 1 N_{CA}, N_{CE}, N_{CT} = N_{CA} − N_{CE} + 1    Total number of SC nodes BC node number containing SC outlet and entrance and total number of SC nodes 
N_{SC} = N_{SCE}, N_{SCA}    Characteristic number of each SC, setting N_{SCE} = 1, 2, 3 or 4 if L_{FTYPE} = 0, 1, 2 or 3. Then N_{SCA} = N_{SCE} + N_{SCT} − 1 
P, ΔP_{T} = P_{A} − P_{E}  ^{ }  Pressure and pressure difference (in flow direction) 
Q_{BT}, Q_{BMk}  W  Total and nodal power into BC node k 

 Mean nodal BC power density into the fluid (=volumetric heat transfer rate) 

 Heat flux from (heated) wall to fluid at BC node k 
_{ } 
 Linear power along BC node k 

 Mean nodal SC power density into fluid (=volumetric0 heat transfer rate) 
T, t  C, s  Temperature, time 
U_{TW }  m  Perimeter of a heated (single) tube wall 
_{ }  m^{3}  Mean nodal SC volume 

 Water velocity 

 Steam velocity 
   Steam quality (extended to singlephase flow too) 