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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-83282 (the 1980 NADC reference lot, lot and revision not stated), 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 = 186.7, 51.8, 25.8 and 19.1. MIL-PRF-83282 is a fire-resistant, synthetic hydrocarbon aircraft hydraulic fluid. A high Pr means the fluid is poor at carrying heat away compared with how easily it flows, as with most oils.
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 = 854.002 − 0.65704 · T°C kg/m³, specific heat is cp = 1946.01 + 3.6523 · T°C J/(kg·K), and conductivity is λ = 0.17956 − 0.000327925 · T°C W/(m·K). The curve reproduces the tabulated Pr to within 1.0 % 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. The 1980 lot is more viscous at 38 °C than current MIL-PRF-83282D products, so Pr at the cool end is probably higher than a current fluid would show. See MIL-PRF-83282 viscosity.