The Prandtl number (Pr) is a dimensionless quantity that characterizes the relative importance of momentum diffusion (viscosity) to thermal diffusion (thermal conductivity) in a fluid. It is defined as: Pr = μ · c_p / λ, where μ is dynamic viscosity [mPa·s], c_p is isobaric specific heat capacity [kJ/(kg·K)], and λ is thermal conductivity [W/(m·K)].

Fluids with Pr ≪ 1 conduct heat efficiently relative to momentum (liquid metals, Pr ≈ 0.003–0.03). Fluids with Pr ≫ 1 are viscosity-dominated (oils, Pr ≈ 10–10,000). Water has Pr ≈ 6–7 near room temperature. MIL-PRF-23699 has Pr ≈ 550 near room temperature, dropping to about 57 at 107 °C (225 °F), reflecting the steep viscosity decrease with temperature.

Dynamic viscosity values are from NIST IR 8263, Table B4 (oscillating-piston viscometer measurements at near-ambient pressures, 0.03–0.2 MPa). Specific heat capacity is from Table 14 (DSC, 1-K intervals). Thermal conductivity is from Table A4 (transient hot-wire, near-ambient pressure).