Tallinn University of Technology

Recently, Monika Kollo defended her PhD thesis "Uni- and Bidirectional Stratified Flows in Submerged Openings of Built Environment" at TTÜ Department of Civil Engineering and Architecture.

Stratified flows, i.e. layered water or air flows due to density variations, have a significant impact on the usage conditions of built environment. In order to solve a number of related engineering problems, the PhD candidate Monika Kollo developed the model of an integrated system of stormwater collection and domestic water heating, which can be used to determine the stormwater harvesting and storage units parameters required for domestic hot water production.

Heat extraction from stormwater results in temperature difference between inflow and outflow in a storage tank, i.e. stratified flow with small density difference. The author of the PhD thesis proposed simple mathematical models for determining uni- and bidirectional stratified flows, which allow calculation of flow quantities at submerged openings in buildings or ships. A submerged opening means a vent of a building or an oil-spill hole in a ship’s hull. Thus, it is an opening in a man-built structure, where fluids of different densities – outdoor and indoor air in case of a building or oil and water in case of a leaking tanker – are connected.

Monika Kollo says, "In the PhD thesis, I modified to some extent the mathematical model for stratified flows used so far by adapting it to submerged openings and stormwater tanks. Further development of mathematical models used only for stratified flows in the natural environment, for instance in a river estuary or sea strait, allows analysis of stratified flows of different liquids or gases at openings in the built environment. In the PhD thesis, I explained the possibilities for applying the developed mathematical models for specific clarification of the energy efficiency of buildings, improvement of the risk assessment of tankers and making use of thermal energy of water available in the urban environment.”

There are already first examples of utilization of thermal energy of low-temperature water in Estonia – seawater is used as an energy carrier for the heating system of Tallinn Seaplane Harbour and river water is used for cooling in the cooling station of the shopping centre Lõunakeskus in Tartu. The PhD thesis proposed a solution for using stormwater thermal energy.

"The comparison of three types of buildings – a shopping centre, an office building and an apartment building – indicated that an underground stormwater storage tank is an optimal solution for a shopping centre, since it has the best conditions for the rainwater harvesting and hot-water consumption. Thus, the PhD thesis provides a novel solution for utilization of stormwater as a low-temperature water thermal energy source for buildings, as well as for cooling of buildings, if necessary," Monika Kollo explains.

The supervisor of the TTÜ doctoral thesis was Associate Professor Janek Laanearu (TTÜ), the opponents were Professor Paul Linden (University of Cambridge) and Associate Professor Claudia Adduce (University Roma Tre).

The PhD thesis has been published in the digital collection of TTÜ library: https://digi.lib.ttu.ee/i/?9257

Kersti Vähi, TTÜ Research Administration Office