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                   Quantity                        Symbol     Coherent SI unit
Absorptivity (radiation) α
Amount of substance N mol
Molar flow rate mol/s
Molar mass velocity(= N/Ac) mol/m2s
Plane α, β, γ, θ, Ø rad
Solid ω sr
Cross-sectional S m2
Surface A m2
Coefficient of cubic expansion
(1/v)(∂v/∂T)p β K−1
Mass (= M/V) c kg/m3
Molar (= N/V) mol/m3
Contact angle θ rad.
Mass (= M/V) ρ kg/m3
Molar (= N/V) mol/m3
Diameter D m
Diffusion coefficient D m2/s
Diffusivity δ m2/s
Thermal (= λ/ρcp) κ m2/s
Dryness fraction (quality) x
Emissivity (radiation) ε
Energy E J = kg m2/s2
Enthalpy (= U + pV) H J = kg m2/s2
Specific, molar h, J/kg, J/mol
Of phase change at constant p:
Melting hsl, J/kg, J/mol
Sublimation hsg, J/kg, J/mol
Evaporation hlg, J/kg, J/mol
Entropy S J/K = kg m2/s2K
Specific, molar s, J/kg K, J/mol K
Force F N = kg m/s2
Mass, of species i xiyi
Mole, of species i
Phase ε
Frequency f Hz, s−1
Frequency circular ω rad/s
Gas constant J/mol K
Molar (universal) = kg m2/s2· mol K
Specific, of species i Ri J/kgK = m2/s2K
Gravitational acceleration g m/s2
Gibbs function (H – TS) G J = kg m2/s2
Specific (h – Ts) g J/kg = m2/s2
Molar ( ) J/mol
= Kgm2/s2mol
Quantity of Q J = kg m2/s2
Rate W = kg m2/s3
Flux ( /A) W/m2 = kg/s3
Heat capacity c J/K = kg m2/s2 K
Specific, at constant volume or cv, cp J/kg K = m2/s2 K
Molar at constant volume or , J/mol K
pressure = kg m2/s2mol K
Ratio cp/cv γ
Heat transfer coefficient α W/m2K = kg/s3K
Overall U W/m2K = kg/s3K
Internal energy U J = kg m2/s2
Specific u J/kg = m2/s2
Molar J/mol = kg m2/s2mol
Joule Thompson coefficient
[(δT/δP)h] μJT m2K/N = rns2K/kg
Length 1(L) m
Diameter d(D) m
Radius r(R) rn
Breadth b(B) m
Height z(Z) m
Thickness δ m
Liquid film thickness δ m
Liquid holdup εL
Mass M kg
Flow rate kg/s
Mass velocity or mass flux kg/m2s
( /A)
Flux of species i kg/m2s
Mass transfer coefficient β m/s
Molar mass kg/kmol
Pressure p Pa = N/m2
= Kg/ms2
Drop δp Pa = N/m2
= kg/ms2
Reflectivity p
Shear stress τ Pa = N/m2
= kg/ms2
Surface tension σ N/m = kgs2
Absolute T K
Difference or interval ΔT K
Logarithmic mean difference ΔTLM K
Mean temperature difference ΔTM K
Thermal conductivity λ W/m K = kg m/s3K
Time t s
Velocity u m/s
Component in Cartesian
coordinates x,y,z u,v,w m/s
View factor (geometric or Ø12 sr
configuration factor)
Dynamic (absolute) η Pas = N s/m2
= kg/ms
Kinematic (= η/ρ) ν m2/s
Void fraction εG
Volume V m3
Volumetric flow rate m3/s
Specific, molar volume v, m3/kg
Work W J = kg m2/s2
Rate (power) W = kg m2/s3
Wavelength λ m
Subscripts and Superscripts
Solid or saturated solid S (s)
Liquid or saturated liquid L(l)
Gas or saturated vapor G(g)
Change of phase at constant p:
Melting SL (sl)
Sublimation SG (sg)
Evaporation LG (lg)
Critical state c
Initial o
Inlet in,
Outlet out,
At constant value of property p,v,t. etc
Molar (per unit of amount of ~
Stagnation o
Wall W(w)
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