Position #223 – Experimental characterisation of LN2-2 phase flow during chill down

Host institution
BE - von Karman Institute/Université Libre de Bruxelles
Mechanical Engineering
Sub-domain or keywords
Liquid nitrogen, void fraction, chill down, optical measurements, heat transfer, flow visualization
Type of mobility
Duration (months)
From 6 months

Full description at

In the framework of space propulsion systems, operating at cryogenic temperatures, the chill down is one of the major phenomena. The word “chill down” defines a transient time happening at the initial start of a cryogenic system prior to operation. Indeed chill down appears when a cryogenic liquid is introduced into a system at ambient. Heat exchanges between the cryogenic fluid and the system lead to vaporization and boiling phenomena as well as a cool down of the whole system. Indeed the cryogenic liquid flowing inside a pipe with wall at ambient temperature turns instantaneously into vapour at the wall, thus creating an outer ring of vapour. The liquid inner ring slowly warm up evaporating, consequently the vapour becomes superheated. This process cools down the walls allowing the liquid inner ring to reach a new downstream portion of the component eventually filling it completely with liquid.
The phenomenon of chill down is very important since it has an impact on the design of the propellant systems. As an example the type and thickness of transfer pipeline insulation are determined by the heat transfer rate as well as the magnitude of the pressure oscillation determines the thickness of the pipeline material.
Even if the chill down is a well-known process its prediction remains uncertain due both to the complexity of the physical phenomena involved and to the fact that it applies to a whole cryogenic system including engines, pumps, valves, and pipelines. Indeed the difficulty of the chill down process is due to the presence of both two- phase flow and the boiling heat transfer, moreover the specific physical properties of cryogenic fluids do not allow the use of empirical and semi-empirical correlations obtained in non-cryogenic conditions.
The von Karman Institute is developing a dedicated facility to study the chill-down phenomena in cryogenic flows. This facility will allow measurement of temperature, pressure and flow rate.
Moreover, since an important quantity to be measured during chill down will be the quantity of gas (as bubbles for example) present in the flow, non-intrusive measurement techniques are developed to measure the void fraction that allows quantification of the gas percentage present in the two-phase flow. The cross sectional void fraction is measured by means of either optical techniques or attenuation (Capacitance, Radio Frequency) devices. The bubble velocity as well as the liquid phase velocity is also measured.
The drawings of the experimental facility are reported in figure 1 (in the PDF file).
The successful candidate will help in the design of the experimental techniques, contribute to the experimental campaign and analyze the data. During the experimental campaign the candidate will team up with other researchers involved in the cryogenic research program. The candidate will be also responsible of the experimental data analysis and will develop the necessary image processing tools using a number of commercial and in-house software.

From January 2016
Maximum available positions
Dr Maria Rosaria Vetrano <vetrano@vki.ac.be>