
Area (m , ft )  
Acceleration (m/s , ft/s )  
Local speed of sound (m/s, ft/s)  
Concentration (mass/volume, moles/volume)  
Drag coefficient, defined different ways (dimensionless)  
,  Heat capacity at constant pressure, volume (J/kgK, Btu/lb  F) 
Diameter; , particle diameter (m, ft)  
Hydraulic diameter (m, ft)  
Mass diffusion coefficient (m /s, ft /s)  
Total energy, activation energy (J, kJ, cal, Btu)  
Mixture fraction (dimensionless)  
Force vector (N, lb )  
Drag force (N, lb )  
Gravitational acceleration (m/s , ft/s ); standard values = 9.80665 m/s , 32.1740 ft/s  
Gr  Grashof number ratio of buoyancy forces to viscous forces (dimensionless) 
Total enthalpy (energy/mass, energy/mole)  
Heat transfer coefficient (W/m K, Btu/ft h F)  
Species enthalpy; , standard state enthalpy of formation (energy/mass, energy/mole)  
Radiation intensity (energy per area of emitting surface per unit solid angle)  
Mass flux; diffusion flux (kg/m s, lb /ft s)  
Equilibrium constant = forward rate constant/backward rate constant (units vary)  
Kinetic energy per unit mass (J/kg, Btu/lb )  
Reaction rate constant, e.g., , , , (units vary)  
Thermal conductivity (W/mK, Btu/fth F)  
Boltzmann constant ( J/moleculeK)  
,  Mass transfer coefficient (units vary); also , 
Length scale (m, cm, ft, in)  
Le  Lewis number ratio of thermal diffusivity to mass diffusivity (dimensionless) 
Mass (g, kg, lb )  
Mass flow rate (kg/s, lb /s)  
Molecular weight (kg/kgmol, lb /lb mol)  
M  Mach number ratio of fluid velocity magnitude to local speed of sound (dimensionless) 
Nu  Nusselt number dimensionless heat transfer or mass transfer coefficient (dimensionless); usually a function of other dimensionless groups 
Pressure (Pa, atm, mm Hg, lb /ft )  
Pe  Peclet number for heat transfer, and for mass transfer (dimensionless) 
Pr  Prandtl number ratio of momentum diffusivity to thermal diffusivity (dimensionless) 
Flow rate of enthalpy (W, Btu/h)  
Heat flux (W/m , Btu/ft h)  
Gaslaw constant (8.31447 J/kgmolK, 1.98588 Btu/lb mol F)  
Radius (m, ft)  
Reaction rate (units vary)  
Ra  Rayleigh number ; measure of the strength of buoyancyinduced flow in natural (free) convection (dimensionless) 
Re  Reynolds number ratio of inertial forces to viscous forces (dimensionless) 
Total entropy (J/K, J/kgmolK, Btu/lb mol F)  
Species entropy; , standard state entropy (J/kgmolK, Btu/lb mol F)  
Sc  Schmidt number ratio of momentum diffusivity to mass diffusivity (dimensionless) 
Mean rateofstrain tensor (s )  
Temperature (K, C, R, F)  
Time (s)  
Freestream velocity (m/s, ft/s)  
Velocity magnitude (m/s, ft/s); also written with directional subscripts (e.g., , , , )  
Volume (m , ft )  
Overall velocity vector (m/s, ft/s)  
We  Weber number ratio of aerodynamic forces to surface tension forces (dimensionless) 
Mole fraction (dimensionless)  
Mass fraction (dimensionless)  
Permeability, or flux per unit pressure difference (L/m hatm, ft /ft h(lb /ft ))  
Thermal diffusivity (m /s, ft /s)  
Volume fraction (dimensionless)  
Coefficient of thermal expansion (K )  
Porosity (dimensionless)  
Ratio of specific heats, c /c (dimensionless)  
Change in variable, final initial (e.g., , , , , )  
Delta function (units vary)  
Emissivity (dimensionless)  
LennardJones energy parameter (J/molecule)  
Turbulent dissipation rate (m /s , ft /s )  
Void fraction (dimensionless)  
Effectiveness factor (dimensionless)  
,  Rate exponents for reactants, products (dimensionless) 
Radiation temperature (K)  
Molecular mean free path (m, nm, ft)  
Wavelength (m, nm, Å, ft)  
Dynamic viscosity (cP, Pas, lb /fts)  
Kinematic viscosity (m /s, ft /s)  
,  Stoichiometric coefficients for reactants, products (dimensionless) 
Density (kg/m , lb /ft )  
StefanBoltzmann constant (5.67 W/m K )  
Surface tension (kg/m, dyn/cm, lb /ft)  
Scattering coefficient (m )  
Stress tensor (Pa, lb /ft )  
Shear stress (Pa, lb /ft )  
Time scale, e.g., , , (s)  
Tortuosity, characteristic of pore structure (dimensionless)  
Equivalence ratio (dimensionless)  
Thiele modulus (dimensionless)  
Angular velocity; , Mean rate of rotation tensor (s )  
Specific dissipation rate (s )  
Solid angle (degrees, radians, gradians)  
Diffusion collision integral (dimensionless) 