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Rajaram Lakkaraju (promotion date: 11 January 2013)

Boiling turbulent Raleigh-Bénard convection

Promotion date: December ?

Promotor: Prof.dr. Detlef Lohse

Assistant promotor: Prof.dr. Andrea Prosperetti

This work highlights the importance of nucleated vapor bubbles on the local and the global properties of the flow (e.g., energy dissipation rates, velocity, temperature, heat flux etc.) in a boiling convective environment. Studied are boiling phenomena of water at 100 °C under normal pressures in a Rayleigh-Bénard (RB) cell with the help of numerical simulations.

A fundamental understanding of liquid-vapor phase transitions, mainly boiling phenomena, is essential due to its omnipresence in science and technology. In industries, many empirical correlations exist on the heat transport to get an optimized and efficient thermal design of the boiling equipment.

Vapor bubbles absorb heat from the hot plate (and also from surrounding liquid), grow in size and rise in the flow due to enhanced buoyancy. While in motion, they condense by releasing heat when they encounter a cooler liquid or by reaching the cold plate. Their complex interactions with the liquid significantly change the mentioned local and global properties of the flow. It is useful to illuminate the relative importance of enhanced convection due to the increased buoyancy of vapor bubbles and their motion relative to the liquid in both the laminar and the turbulent states. This work is a preliminary step to better understand boiling thermal convection in the perspective of fluid dynamics and heat transfer. 

How do you scale your thesis project? Is it application oriented or mainly fundamental of nature?

Studying this kind of phenomena is helpful in understanding many processes we encounter on an everyday basis, such as: making better coffee, understanding cloud formation in the environment, extracting organic products from crude oils, or better refrigeration and air-conditioning; also for controlled inkjet printing, or to generate large amounts of power in thermal plants.

Say in the steel industry, it is still unknown how to cool steel billets or molten metals in the most efficient and optimized way. Sprinkling jets of water on hot objects surely help in removing unwanted heat to make a desired object, with controlled surface and physical properties. However, questions like how much to sprinkle, where to sprinkle, and for how long to sprinkle, are still difficult to answer. Perhaps another fluid will do nicely also.

The topic of research has even applications in medical and food product industries, e.g. in sterilizing medical equipment, to kill unwanted micro-organisms, making hygienic, tasty food and so many more. Though cooking has wide applications in nature and in industries, it is still at a basic level in terms of understanding the processes.

So, a fundamental way of looking at phase change phenomena is essential for a better scientific and technological design. I looked specifically at the problem on boiling phenomena where significant amount of nucleated vapour bubbles strongly influence the heat and the flow properties of a typical system.

Were your findings published in scientific magazines?

Two articles were published in Physical Review E. Another is submitted to Proceedings of the National Academy of Sciences, The USA (PNAS). Two more manuscripts are in final stages and will be submitted soon. I was a speaker on a few conferences in Europe and USA, for example at the Division Fluid Dynamics (DFD) session in American Physical Society, USA.

In what way did you develop as a scientist and researcher during your PhD project?

In the past four years, I was trained as a computational researcher and learned the necessary skills in understanding continuum mechanics, especially writing computational codes to solve general fluid flow problems at a fundamental level.

I started appreciating the power of numerical techniques, in order to understand the physics at a basic level, where theoretical approaches and skilful experiments are difficult to perform. These techniques allow modern scientists to understand which experiments will really matter, suggesting theoretical scenarios or discovering more about the way the actual phenomena are taking place.

Performing simulations is challenging and really informative, giving a complete insight in the problem. For example, take a cup of hot coffee and pour a few sugar crystals into it, wait for a minute without any stir and taste the coffee. It tastes bitter. When you do the same with a little stir it tastes sweet. What is the reason for the quick spreading of sweetness (or sugar) when we stir it and how does it happened? This problem might look very simple for a general observer. But, it is not simple at all in terms of understanding or at least getting information through experiments on the entire 3D-field of sweetness in time.

Here computational techniques come at help. They are useful not only for fluid mechanics but also for general physics and engineering problems. Scientists and engineers can learn a lot from these `computational experiments’. These can contribute to a better design of equipments.

What are your future plans?

I like to stay in academics and I would like to perform research for a while now and develop my skills even further. At the same time I would like to pursue a teaching career. Working with students sharpens my brain and makes me a stronger person, in thinking in terms of logical connections to solve a situation or problem. Though it is a bit out of context, the logical deductions made by Sherlock Holmes always motivated and influenced me. The easily reachable (perhaps good) place for most of us to do that kind of thinking work is academic research. So, I would like to be in that field.

Though I didn’t make use of the experimental facilities, I enjoyed the expertise present at Mesa+ by listening to talks and courses. Here, in the Netherlands, every person gets a fair chance of developing his talent, no matter if one is rooted at a higher, middle or lower educational background.