Hammered and very hot.
To the Editor: A physicist, I am visiting my "mech-eng" daughter and come to read R.A. Leishear's thought-provoking article, "From Water Hammer to Ignition," in your December 2014 issue. It raises a probable but serious safety concern on installations containing pipelines, such as nuclear power plants.
Not being an engineer, I previously assumed that water hammer causes only mechanical damage. Leishear suggests that it can ignite explosive gas, too, a conjecture that I find convincing, having known that a gas bubble within a liquid medium, when it collapses during the passage of shock waves, will reach an extreme temperature due to adiabatic compression. Physicists have measured, e.g., its light emission.
The temperature-pressure relationship depends on parameters such as bubble size and the equation of state of the gas, but the magnitudes mentioned in the article certainly look reasonable. Indeed, the initiation of explosion by temperature rise at shock wave front is also responsible for the supersonic propagation of detonation in an explosive medium.
Tong B. Tang, Hong Kong S.A.R.