Illustrated Physics
physics, drawn
Concept

Boiling — where it appears

The change of a liquid into vapour by bubbles forming within it, which happens when its vapour pressure reaches the pressure on it. Because the boiling point rises with pressure, it varies with altitude and with depth, and the heat a surface can pass to a boiling liquid depends on how the bubbles form and leave.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

The boiling curve, which turns back. Heat flux from a horizontal surface into water boiling at one atmosphere, on a logarithmic axis, against how far the surface is above the boiling point, also logarithmic, from standard correlations. At a few degrees heat leaves by convection; from about 5 K bubbles nucleate and the flux climbs as the cube of the superheat, to the critical heat flux of 1.11 MW/m² at 20 K. Beyond it the flux FALLS as the surface gets hotter — the transition regime, drawn dashed as the interpolation it is — down to 19 kW/m² at 67 K, the Leidenfrost point, where a continuous film of vapour insulates the surface. Past that, film boiling and radiation let the flux rise again, slowly: 310 kW/m² at 1000 K above boiling. Between the crisis and the Leidenfrost point, a surface 3 times hotter passes 58 times less heat.

The boiling curve that turns back

Heat a surface under water and more heat flows from it the hotter it gets — up to a point. Past about twenty degrees above boiling the bubbles crowd together, merge, and begin to blanket the surface with vapour, and from then on a hotter surface passes less heat, not more, until by a few hundred degrees above boiling it is wrapped in an insulating film and passes fifty times less than it did at its best. Shigeo Nukiyama found the turn in 1934 by melting wires. It is why a water drop skates for a minute on a very hot pan but vanishes in two seconds on a merely hot one, and why every boiler, steam generator and water-cooled reactor is designed around a number called the critical heat flux.

thermodynamics · Phase change The wait after an eruption, against the eruption. For 400 simulated eruptions of the model geyser: the interval to the next eruption against the duration of the eruption just finished (dots), and the straight line through them, interval = 33.8 min + 10.5 × duration, with a correlation of 0.94. The eruptions fall in two clusters, short ones of about 2.2 minutes followed by waits near an hour and long ones of about 5.0 minutes followed by waits near an hour and a half, and the line runs through both. Its slope is the ratio of the discharge rate to the refill rate, 10.7: each minute of eruption throws out water that takes 10.7 minutes to replace. Plotted against the eruption that follows instead, the same intervals show a correlation of 0.05 — nothing. The interval is set by the past and tells nothing about the future.

The geyser that remembers one eruption

Old Faithful is famous for being predictable, and its predictability has an odd shape. Its eruptions come in two kinds, short and long, and the wait after each is short or long to match — about an hour after a short eruption, about an hour and a half after a long one. So the next eruption can be forecast from the last, to within about ten minutes, and from nothing else. The reason is a bookkeeping of water. An eruption throws out a volume of water that the geyser must take back from below before it can erupt again, and a long eruption throws out more. The geyser carries a memory of exactly one eruption: how much it has to refill.

thermodynamics · Phase change

Named alongside it

The objects these essays reach for when they reach for this one.

Bimodal distributionCritical heat fluxEruption forecastingGeyserHeat-transferHydrothermal systemHysteresisInstabilityLatent heatLeidenfrost effectNucleationRecharge

All concepts