At what psi does water boil
Simulations for more efficient power plants
On Yohei Sato's screen, a water vapor bubble is much more detailed than our eyes are used to. You can see the computer simulation of a bubble that sticks to a heated wall and is surrounded by water. If you slowly bring the water to a boil, the bubble grows and one after the other takes on shapes that are reminiscent of a chocolate kiss, a stamp handle and a balloon. At some point it finally separates from the wall and stumbles upwards through the imaginary fluid.
PSI researchers Sato and the head of his research group, Bojan Niceno, put many hours of thought into this seemingly simple sequence of images of the growth of a bubble. says Sato. And one might be amazed: As common as the formation of bubbles when boiling water is, no one has yet been able to reproduce the physics behind it with such accuracy on a computer.
Dealing with the bubbles is no gimmick for the PSI scientists. To generate around 85 percent of the world's electricity, water is heated and converted into steam. Fossil fuels such as coal, gas or oil are burned for heating; in nuclear power plants, the heat is generated in the fuel rods. The steam then drives a generator, which in turn produces electricity. In power plants, steam bubbles play a key role in ensuring that the water boils efficiently.
If the bubbles cover the hot wall closely, the heat is better transferred to the water. The vapor bubbles grow by absorbing the water that evaporates around them. At some point they tear off the wall and carry the heat with them into the water layers away from the wall. These conditions - experts speak of - only occur when the temperature on the wall is moderately above the boiling point of water. If the temperature continues to rise, the rate of bubble formation becomes so high that a continuous, thin layer of steam forms on the hot wall. Because steam is not a good conductor of heat, the heat no longer reaches the water as quickly. The result of this so-called film boiling: the wall dries out or, in the worst case, even burns through.
Using computer power to create a picture of the bubble
Finding the right temperature is therefore a tightrope walk: you want to have as many bubbles as possible on the wall, but you definitely don't want to get into the area where the film is boiling. The researchers therefore want to be able to calculate the growth of the bubbles and their movements in the liquid as precisely as possible on the computer. Then you could derive the temperature on the wall at any point in time. says Niceno.
Sato and Niceno went to work meticulously when they began to develop the PSI-BOIL computer program for this purpose: the program divides a bubble and its surroundings into tiny cubes, the edges of which are only a few thousandths of a millimeter long. For each cube, the temperature, the pressure, the physical state (liquid or gaseous) and the speed of the steam and water currents in the vicinity of the bubble are determined. So even simulating a single bubble is tough. With the calculations, Sato's PC with its four processor cores would be at full capacity for at least two years. The researchers therefore fall back on Merlin: a PSI-specific computer that is actually a network of several hundred processor cores. But even with the concentrated computing power of 128 Merlin‘ processor cores, the simulation still takes a whole three weeks. The process simulated in this way, on the other hand, takes no more than five seconds.
Reality depicted correctly
With PSI-BOIL, the researchers have now succeeded in making nucleate boiling more predictable, first for one, then for several bubbles. In the simulations, the bubbles went through the same phases that had previously been recorded in real experiments with video and infrared cameras. More importantly, the simulations provided the correct value for the temperature at each point on the wall under the bubble. says Sato happily. adds Niceno.
This success is hard-earned: eight man-years have gone into research projects and collaborations with industry. The scientists are still a long way from reaching their goal: says Niceno.
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