Stay cool by phase transition
Researcher Haiyu Wang was able to better understand the mechanisms of phase transitional flow as a cooling mechanism. He will defend his PhD on May 13th.
Mankind exploited phase transition for cooling long before we fully understood it. Human developed the function of sweating when our ancestors were chasing prey on the grasslands of Africa. The sweat on the skin evaporates, while absorbing heat, so that the body temperature does not exceed its normal level. In the same way, phase transitional technologies nowadays transport heat away from chips in laptops and cellphones. Researcher Haiyu Wang developed a new numerical model to better understand the mechanisms of phase transitional flow and to quantify the effect of cooling. He will defend his PhD on May 13th.
The demand for ever smaller chips by the semiconductor industry has raised a new challenge for cooling technologies at small length scales. It is reported that so-called 3D chips have the potential to be more power efficient and computationally faster than conventional chips. The challenge, however, is to transport the heat out of 3D chips without using too much space in the chips nor interfering the computation.
GLASS OF BEER
Phase transitional cooling is a suitable method since it has high efficiency, but there is still a lack of understanding. Therefore, the aim of Wang’s research was to investigate phase transitional flow at small length scales. In order to accomplish this, the vapor-liquid two-phase flow and the surface tension need to be handled carefully before the phase transitional flow.
To illustrate what this means, imagine you are enjoying a glass of your favorite beer. Bubbles rise to create a thick layer of foam. The boundary that divides the gas and the liquid phases is called the interface. Wang proposed and implemented a model to accurately calculate the position of the interface. With this model, rising bubbles with different shapes as well as stationary bubbles can be simulated.
At small length scales, the surface tension dominates in vapor-liquid two-phase flow. For example, the reading on a medical mercury-in-glass thermometer is fixed and resistant to tilting when it is taken out of the warm human body. This indicates that the surface tension force in the thermometer is stronger than gravity. Therefore, the accurate inclusion of surface tension is important to the accurate simulation of phase transitional flow. Wang succeeded in calculating surface tension accurately by proposing a modified Continuum Surface Force model and evaluating the accuracy of different curvature methods.
ENERGY JUMP CONDITION
Previous researchers have developed models for phase transition that employed empirical coefficients. Different from for example the gravitational constant in Newton’s law of universal gravitation, the empirical coefficients are not constant. This approach ruins the universality treasured by many scientists. Therefore, more and more researchers are devoting their time to investigate models for phase transition based on the “energy jump condition”. In these models, no empirical coefficient is necessary.
Wang also developed such a model. Different from previous work, he suggested to solve the conservative form of the energy equation, so energy conservative solutions are obtained. Simulation results agree well with analytical solutions on different test cases with variable density ratios. It turns out that because of this, the cooling effect can be quantified accurately.
This research is an important step towards the accurate and versatile numerical simulation of phase transitional flow at small length scale.
Title of PhD-thesis: Towards numerical simulation of phase transitional flow for cooling purposes. Supervisors: Hans Kuerten, TU/e; Bart van Esch, TU/e.