Doctoral dissertation reveals how boiling starts
22.12.2011
In his dissertation, Teemu Laurila managed for the first time in the world to model the initial moments of a steam bubble in a boiling liquid.
The creation of boiling bubbles has been studied with experiments at the Mir space station and in laboratory conditions by observing as liquid hydrogen boils in a temperature of -240 degrees. Instead of carrying out experiments, Laurila and his colleagues in the Multiscale Statistical Physics Group decided to utilise the computational capacity of the latest computers.
“The most important result of the work is a computational model that predicts the behaviour of the boiling bubbles when they are too small for experimental observations,” explains Laurila.
Water boiling in a kettle on a hot stove or a trickle of bubbles appearing on the surface of a glass of beer is a common phenomenon that, however, fascinates physicists.
“The creation phase of the bubbles determines how much heat can be transferred through boiling,” says Laurila, describing the phenomenon.
Understanding the fundamental physics of boiling is crucial as nearly 90 per cent of the world’s energy is generated using boilers. Power generation is based on boiling water into steam irrespective of the fuel used (wood, natural gas, uranium, etc.)
A great deal is already known about boiling. When the temperature of a kettle filled with water approaches the boiling point, microscopic gas bubbles start appearing at the hot bottom of the kettle. Gravitation causes the bubbles to rise towards the surface of the liquid. On their way up they grow into clearly visible boiling bubbles.
If the temperature of the bottom of the kettle is raised still further, something unexpected happens. At about 130 degrees, bubble boiling gives way to layer boiling: a gas layer insulating the liquid from the source of heat starts forming at the bottom of the kettle. Even though heating power is increased, less heat is transferred into the boiling liquid.
“You can observe the same phenomenon when a water drop falls on a hot stove. It rises on top of the gas layer and it will take a while before it boils away,” explains Laurila.
Researchers at Aalto University wanted to know how the changeover from bubble boiling to layer boiling occurs as the temperature of the boiling surface increases. They decided to employ a model for calculating diffuse interfaces developed as early as 1901. Researchers could not use the model before computer capacity had reached its present level.
What is happening at the bottom of the kettle? Laurila has produced computer graphics to describe the birth of a gas bubble of less than 100 nanometres.
“The steam generated at the interface of the bubble and the vessel bottom streams inside the bubble, causing the bubble to swell. At the same time, the recoil of the steam spray causes the bubble to spread at the bottom of the kettle,” explains Laurila, describing the chain of events.
In a computing method, jointly developed with the researchers of the Kungliga Tekniska Högskolan in Sweden, the properties of the bottom of the kettle (water repellence and surface roughness) can be freely changed. This is of great use when the method is applied to designing boilers with optimal heat transfer. In information technology, processor cooling is a potential application. Computers have power processors that require an effective liquid cooling system.
Researchers believe that cooling could be made more effective through nanocoating that would optimise the boiling. The manner in which steam bubbles are created also interests such sectors as metal processing companies. Sooner the cast metal plate cools down, finer the microstructure of the metal becomes. Thus, understanding the fundamental physics of boiling helps to cast metal plates of higher quality.
Teemu Laurila’s dissertation in the field of theoretical and computational physics, “Interface Dynamics in Two-Phase Flows with Diffuse Interface Methods” was examined on 9 December 2011 at the Aalto University School of Science. In his dissertation, Laurila studied two different two-phase flows (boiling and kinetic roughening) using computational methods.
Further information:
Teemu Laurila
Tel. +358407181795
E-mail: teemu.laurila [at] aalto [dot] fi
Multiscale Statistical Physics Group of Aalto University
Tekst: Petja Partanen