Drag Reduction by Leidenfrost Vapor Layers

We demonstrate and quantify a highly effective drag reduction technique that exploits the Leidenfrost effect to create a continuous and robust lubricating vapor layer on the surface of a heated solid sphere moving in a liquid. Using high-speed video, we show that such vapor layers can reduce the hydrodynamic drag by over 85%. These results appear to approach the ultimate limit of drag reduction possible by different methods based on gas-layer lubrication and can stimulate the development of related energy saving technologies.

Figure 1

(a) Digital camera snapshot of a heated 15 mm steel sphere held stationary in fluorinated liquid with sphere temperature $T_S$ above the Leidenfrost temperature $T_L$. A thin vapor layer streaming around the sphere can be observed by the ripples moving along the sphere surface. (b) Snapshot at the instant when the sphere has cooled to the Leidenfrost temperature that is marked by an explosive release of bubbles. (See supplemental video 1 [21]).

Figure 2

Variation of the terminal velocity with the sphere temperature measured for a 20 mm steel sphere falling through the liquid (FC-72). Open square data points (blue) are for temperature below the Leidenfrost temperature ($T_L=130°\mathrm{C}$) and solid square (red) for temperature above $T_L$ (see supplemental video 2 [21]).

Figure 3

(a) Dependence of the drag coefficient on the Reynolds number for different sphere diameters and densities [21]. Open (blue) data points are taken at room temperature ($25°\mathrm{C}$) for steel (square), tungsten carbide (triangles) and agate (circles) spheres of varying sizes. Solid (red) data points (same symbols) are for the same spheres heated to $200°\mathrm{C}$. Data for the heated, ascending steel sphere (red crosses) are also included for comparison. (b) Dependence of the observed separation angle, $\theta$ of the boundary layer on the Reynolds number (the bottom pole of the falling sphere is $\theta =0$). Open squares (blue) are for steel sphere at temperature just below the $T_L$ ($T_S=110°\mathrm{C}$) and the solid squares (red) for the same spheres heated to $200°\mathrm{C}$ (see also Fig. 4).

Figure 4

Snapshots of 20 mm steel sphere falling at different temperatures and velocities. (a) Sphere temperature, $T_S=110°\mathrm{C}$ slightly below the Leidenfrost temperature ($T_L=130°\mathrm{C}$) with no continuous vapor layer on the sphere surface and terminal velocity $U=1.7\mathrm{m}/\mathrm{s}$ ($\mathrm{Re}=7.6\times {10}^4$). (b) Sphere temperature, $T_S=200°\mathrm{C}$, above Leidenfrost temperature with a continuous vapor layer on the sphere surface but prior to reaching terminal velocity at $U=0.9\mathrm{m}/\mathrm{s}$ ($\mathrm{Re}=5.0\times {10}^4$). (c) Sphere temperature, $T_S=200°\mathrm{C}$, above Leidenfrost temperature and having reached terminal velocity of $U=3.6\mathrm{m}/\mathrm{s}$ ($\mathrm{Re}=2.0\times {10}^5$). Arrows indicate the approximate position at which flow separation occurs (See supplemental video 4 [21]).