Dimensionless Numbers
The table shows the definitions of a lot of dimensionless quantities used in chemistry, fluid flow and physics engineering. Below the table, the symbols used in the formulas are explained and given with SI units.
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| Name | Symbol | Formula | Areas of application |
| Alfvén number | Al | Al = ν(ρ μ) ½ /B | Study of magnetic fields |
| Cowling number | Co | Co = B2/(μ ρ ν2) | Study of magnetic fields |
| Euler number | Eu | Eu = Δp /(ρ ν2) | Characterization of energy losses in fluid flows |
| Fourier number | Fo | Fo = a t / l2 | The ratio of diffusive or conductive heat transport rate to the heat storage rate |
| Fourier number for mass transfer | Fo* | Fo* = D t / l2 | The ratio of diffusive mass transport rate to the mass storage rate |
| Froude number | Fr | Fr = ν /(l g) ½ | Determine the resistance of a partially submerged object moving through water |
| Grashof number | Gr | Gr = l3 g α ΔT ρ2/ η2 | Study situations involving natural heat convection |
| Grashof number for mass transfer | Gr* | Gr* = l3 g (∂p/∂x) T,p (Δx p / η) | Predictions of mass flow patterns |
| Hartmann number | Ha | Ha = B l (κ/η) 1/2 | Describes the ratio of electromagnetic force to the viscous force |
| Knudsen number | Kn | Kn = λ / l | Determine whether statistical mechanics or the continuum mechanics formulation of fluid dynamics should be used to model a situation |
| Lewis number | Le | Le = a / D | Characterize fluid flows where there is simultaneous heat and mass transfer |
| Mach number | Ma | Ma = ν / c | Determine the approximation with which a flow can be treated as an incompressible flow |
| Nusselt number | Nu | Nu = h l / k | The ratio of convective to conductive heat transfer across (normal to) a boundary surface, predicts flow patterns. |
| Nusselt number for mass transfer | Nu* | Nu* = k d l / D |
Predicts mass flow patterns |
| Peclet number | Pe | Pe = ν l / a | For transport phenomena in a continuum, the ratio of advective to diffusive heat transport rates, to decide the simplicity/complexity of computational models |
| Peclet number for mass transfer | Pe* | Pe* = ν l / D | The ratio of advective to diffusive mass transport rates |
| Prandtl number | Pr | Pr = η / (ρ a) | Determine the thermal conductivity of gases at high temperatures |
| Rayleigh number | Ra | Ra = l3 g α ΔT ρ /(η a) | Predict if heat transfer appear as conduction or convection |
| Reynolds number | Re | Re = p ν l / η | Predictions of fluid flow patterns |
| Magnetic Reynolds number | Rem | Rem = ν μ κ l | Estimates of the relative effects of advection or induction of a magnetic field |
| Schmidt number | Sc | Sc = η /(ρ D) | Characterization of fluid flows in which there are simultaneous momentum and mass diffusion convection processes |
| Stanton number | St | St = h /(ρ ν cp ) | Characterization of heat transfer in forced convection flows, the ratio of heat transferred into a fluid to the thermal capacity of fluid |
| Stanton number for mass transfer | St* | St* = k d / ν | To characterize mass transfer in forced convection flows |
| Strouhal number | Sr | Sr = l f / ν | Describing oscillating flow mechanisms |
| Weber number | We | We = ρ ν2l / γ | Analysing fluid flows where there is an interface between two different fluids |
where
ν = speed [m/s]
η = viscosity [kg/(m s)]
ρ = density, mass density, [kg/m3 ]
m = mass [kg]
V = volume [m3 ]
l = length [m]
a = thermal diffusivity [m2/s]
t = time [s]
μ = permeability [kg m/(s2A2)]
B = magnetic flux density [kg/(s2A)]
Δp = pressure difference [kg/(m s2)]
g = acceleration of free fall [m/s2]
α = cubic expansion coefficient [1/K]
ΔT = temperature difference
κ = electric conductivity [s3 A2/(kg m3 )]
λ = mean free path [m]
D = diffusion coefficient [m2/s]
c = speed of sound [m/s]
h = coefficient of heat transfer [kg/(s3 K)]
k = thermal conductivity [kg m/(s3 K)]
cp = specific heat apacity at constant pressure [kg m2/(s2K)]
f = frequency [1/s]
γ = surface tension [kg/s2]
x = mole fraction [1]
k d = mass transfer coefficient [m/s]
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