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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 · cp / λ, where μ is dynamic viscosity [mPa·s], cp is isobaric specific heat capacity [kJ/(kg·K)], and λ is thermal conductivity [W/(m·K)].
The Prandtl number is derived, not tabulated. NADC-79248-60 (Conte & Hammond, Naval Air Development Center, 1980, DTIC ADA081597) reports kinematic viscosity (Table 1), density (Table 2), specific heat (Table 7) and thermal conductivity (Table 8) for MIL-H-5606, but no Prandtl number. Pr is calculated from those values with μ = ν · ρ, so Pr = ν · ρ · cp / λ. The four tabulated points at 38, 93, 149 and 204 °C (100, 200, 300 and 400 °F) are computed this way from the table values: Pr = 172.4, 72.6, 42.6 and 29.9.
The smooth curve is computed from fits to the same four tables. Kinematic viscosity uses piecewise ASTM D341 interpolation between the measured points (linear in log10(log10(ν + 0.7)) against log10(T + 273.15)), so it passes through each measured viscosity. Density is ρ = 869.517 − 0.71295 · T°C kg/m³, specific heat is cp = 1786.96 + 4.3331 · T°C J/(kg·K), and conductivity is λ = 0.13822 − 0.000077493 · T°C W/(m·K). The curve reproduces the tabulated Pr to within 0.4 % at all four temperatures.
The curve is limited to 38–204 °C because the density fit is valid only over that range, and μ needs density. The viscosity data reach down to −54 °C, but without density no Pr can be computed below 38 °C, so nothing is extended there. The NADC report gives no measurement uncertainty and does not state the lot or specification revision tested.