Water Thermal Diffusivity Liquid and Gaseous States vs Temperature and Pressure

Thermal diffusivity (α) is the rate at which heat spreads through a material. It is defined as:

α = k / (ρ C_P)

where k is thermal conductivity (W/(m·K)), ρ is density (kg/m³), and C_P is specific heat capacity at constant pressure (J/(kg·K)). The SI unit is m²/s. Higher thermal diffusivity means heat propagates faster.

Liquid water has thermal diffusivity in the range of 0.13–0.17 × 10⁻⁶ m²/s, increasing slightly with temperature. Steam (gaseous water) has dramatically higher values — at atmospheric pressure it rises from about 20 × 10⁻⁶ m²/s just above the boiling point to over 280 × 10⁻⁶ m²/s at 1700 °F. This is why separate charts with different scales are used for each phase.

For liquid water, pressure has only a modest effect on thermal diffusivity — the three pressure curves (1, 10, 100 bara) are close together. For steam, the effect is very large: at 100 bara (1450 psia), steam thermal diffusivity near the critical point is roughly two orders of magnitude lower than at atmospheric pressure, reflecting the much higher density at elevated pressure.