Water Boils at 20°C Too — The Physics of Cavitation

Water boils at 20°C. You only need to drop the pressure enough. The schoolbook line "water boils at 100°C" hides a missing footnote: at 1 atm. What actually controls boiling is the balance between the saturation vapor pressure and the local static pressure. When water accelerates past a pump impeller or the tip of a ship's propeller, the static pressure drops, and room-temperature water suddenly boils. We call that cavitation. This post explains why it starts, how to measure the danger with a single dimensionless number, and how a vanishing bubble launches a 10,000-bar shock that eats steel. The last two sections give you a small Python tool and a venturi simulator you can drag around.

Boiling is not about temperature#

Water's saturation vapor pressure is a function of temperature.

There is nothing magical about 100°C. It is just the temperature where $p_v$ matches 1 atm. A pressure cooker (about 2 atm) shifts the boiling point up to 120°C. On top of Mount Everest (0.3 atm) water boils at 70°C.

Run the same logic backward. To boil 20°C water, you only have to drop the pressure to about 2.5 kPa — roughly 1/40 of an atmosphere. A vacuum pump on the kitchen counter is enough.

The catch is that flowing water does not need a vacuum pump.

How a flow makes its own low pressure#

Along a streamline of an inviscid steady flow, Bernoulli holds.

$p$ is static pressure, $\rho$ density, $u$ speed, $z$ elevation. At a fixed height, wherever the speed rises, the static pressure falls. The throat of a venturi, the suction side of an airfoil, the inlet of a pump impeller — every spot where the fluid accelerates is also a spot where the pressure dives.

Push it far enough and $p$ drops below the vapor pressure $p_v$ at the operating temperature. The liquid can no longer hold itself together. Tiny gas nuclei seed an explosive growth of vapor bubbles. That is the birth of cavitation.

In one line: cavitation is boiling triggered by hydrodynamic depressurization, not by heating.

The cavitation number σ — one number for the whole risk#

Designers turn "will this pump cavitate" into a single dimensionless number, the cavitation number.

$p_\infty$ is the reference static pressure, $U_\infty$ the reference speed, $p_v$ the vapor pressure at the operating temperature. The numerator is "how much pressure margin sits above $p_v$"; the denominator is "the largest dynamic pressure the flow can convert".

A bigger $\sigma$ is safer: even at full acceleration, the dynamic pressure cannot drag the static pressure under $p_v$. Once $\sigma$ falls below a critical value $\sigma_i$, cavitation begins. Each foil or impeller has its own $\sigma_i$, usually measured in a wind or water tunnel.

In rotating-machinery design the equivalent quantity is the NPSH (net positive suction head). Low NPSH means low $\sigma$, which means the pump is boiling.

The death of a bubble — a 10,000-bar shock#

If cavitation only made bubbles, we would not care. The destruction comes from how those bubbles die.

A bubble drifts past the low-pressure zone into a region where the pressure recovers. Suddenly the ambient liquid crushes the vapor inside. The Rayleigh–Plesset equation describes the radius $R(t)$ of a spherical bubble.

$p_B$ is the bubble interior pressure, $\gamma$ surface tension, $\nu$ kinematic viscosity. When $p_\infty \gg p_B$, $\ddot R$ goes deeply negative and the bubble implodes in microseconds. Just before the wall meets itself, the surrounding liquid moves at near-acoustic speed; the moment after, a shock of order 10,000 bar fires off where the bubble used to be.

Repeat this often enough and metal pits like a moonscape. Pump impellers, propeller tips, hydroturbine runners — that is the calling card of cavitation erosion.

Code: predicting cavitation in a venturi#

Once you give me an area profile $A(x)$, continuity and Bernoulli hand back the pressure field for free.

import numpy as np
 
def venturi_pressure_field(x, area_ratio, U_in, rho=1000.0, p_in=101300.0):
    """Cosine-tapered venturi: contracts to area_ratio, then recovers."""
    a0, a1, a2, a3 = 0.30, 0.42, 0.58, 0.70
    A = np.ones_like(x)
    m1 = (x >= a0) & (x < a1)
    s1 = 0.5 - 0.5 * np.cos(np.pi * (x[m1] - a0) / (a1 - a0))
    A[m1] = 1.0 + (area_ratio - 1.0) * s1
    A[(x >= a1) & (x <= a2)] = area_ratio
    m2 = (x > a2) & (x < a3)
    s2 = 0.5 - 0.5 * np.cos(np.pi * (x[m2] - a2) / (a3 - a2))
    A[m2] = area_ratio + (1.0 - area_ratio) * s2
 
    u = U_in / A                              # continuity
    p = p_in + 0.5 * rho * (U_in**2 - u**2)   # Bernoulli
    return A, u, p
 
def cavitation_zone(p, p_vapor):
    """Indices where the static pressure drops below p_vapor."""
    return np.where(p < p_vapor)[0]
 
x = np.linspace(0, 1, 400)
A, u, p = venturi_pressure_field(x, area_ratio=0.4, U_in=5.0)
p_vapor = 2339.0  # vapor pressure of water at 20°C, in Pa
 
zone = cavitation_zone(p, p_vapor)
sigma_min = (p.min() - p_vapor) / (0.5 * 1000.0 * 5.0**2)
 
print(f"min static pressure: {p.min()/1000:.1f} kPa")
print(f"sigma_min = {sigma_min:.3f}   (negative means cavitation)")
print(f"cavitating length: {zone.size/x.size*100:.0f}% of the tube")

Drop area_ratio from 0.5 to 0.3. The throat speed jumps by ~1.7x, the dynamic pressure roughly triples, and the static pressure tunnels through $p_v$ without breaking a sweat. The instant $\sigma_{\min}$ goes negative is the start of cavitation.

Cavitate it yourself#

Drag the inlet velocity, throat ratio, and water temperature in the simulator below and watch the pressure curve dip under the dashed vapor line.

Crank the temperature to 60°C and cavitation starts at much lower velocities — vapor pressure jumps from about 2 kPa to 20 kPa. Reverse it: open the throat to 0.6 and even fast flow stays clean. Watch the bubbles die downstream as red rings — that is the implosion that scars metal.

Where you actually meet it — pumps, propellers, medicine#

• Pumps and turbines: insufficient NPSH at the impeller inlet is the dominant cause. Suction-line losses, fluid temperature, and rotation rate all eat into $\sigma$.
• Ship propellers: the tip-vortex core cavitates first, sapping thrust and broadcasting audible noise. Submarines deliberately throttle back to stay quiet.
• Valves and orifices: a narrow passage with poor pressure recovery erodes the downstream piping.
• Medical ultrasound (HIFU): clinicians intentionally cavitate inside tissue to shatter kidney stones or to heat tumors. The implosion that ruins propellers becomes the cure.
• Water hammer: when a turbine stops, the back-flowing column slamming a closed valve briefly carves out negative-pressure pockets and lays down a cavitation pulse.

Next time you see a bubble#

Three tensile limits are enough.

1. Liquids can boil without heat — drop the absolute pressure below $p_v$ and they oblige.
2. Smaller $\sigma = (p_\infty - p_v)/(\tfrac{1}{2}\rho U_\infty^2)$ means more risk. That is why designers obsess over a single number.
3. The damage is not the birth of bubbles, it is their collapse. Protect the region where pressure recovers.

Pull up the suction line drawing again. Trace the NPSH margin in fat highlighter. That single line decides how long the impeller lives.