This page contains news about my research such as new publications, new project grants, happenings, collaborations, or other topics of significance which I think could be of interest to others.
New article in International Journal of Hydrogen Energy
Our paper entitled: Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes has been published in the International Journal of Hydrogen Energy.
A present key barrier for implementing large-scale hydrogen liquefaction plants is their high power consumption. The cryogenic heat exchangers are responsible for a significant part of the exergy destruction in these plants and we evaluate in this work strategies to increase their efficiency.
In the work, we present a detailed mathematical model of a plate-fin heat exchanger that incorporates the geometry of the heat exchanger, nonequilibrium ortho-para conversion and correlations to account for the pressure drop and heat transfer coefficients due to possible boiling/condensation of the refrigerant at the lowest temperatures. Plate fin heat exchangers have fins that increase the heat transfer area. Catalyst is placed in the layers where there is conversion from ortho-hydrogen to para-hydrogen as illustrated in the figure below.
Based on available experimental data, a correlation for the ortho-para conversion kinetics is developed, which reproduces available experimental data with an average deviation of 2.2%. The correlation (solid lines) is compared to some of the available experimental data in the figure below.
In a plate-fin heat exchanger that is used to cool the hydrogen from 47.8 K to 29.3 K with hydrogen as refrigerant, we find that the two main sources of exergy destruction are thermal gradients and ortho-para hydrogen conversion, being responsible for 69% and 29% of the exergy destruction respectively. A route to reduce the exergy destruction from the ortho-para hydrogen conversion is to use a more efficient catalyst, where we find that a doubling of the catalytic activity in comparison to ferric-oxide, as demonstrated by nickel oxide-silica catalyst, reduces the exergy destruction by 9%. A possible route to reduce the exergy destruction from thermal gradients is to employ an evaporating mixture of helium and neon at the cold-side of the heat exchanger, which reduces the exergy destruction by 7%. We find that a combination of hydrogen and helium-neon as refrigerants at high and low temperatures respectively, enables a reduction of the exergy destruction by 35%. A combination of both improved catalyst and the use of hydrogen and helium-neon as refrigerants gives the possibility to reduce the exergy destruction in the cryogenic heat exchangers by 43%. The limited efficiency of the ortho-para catalyst represents a barrier for further improvement of the efficiency.
The work describes new routes to follow in order to improve the efficiency and reduce the power requirement to liquefy hydrogen and was carried out in in collaboration with D. Berstad, Ailo Aasen, Petter Nekså and Geir Skaugen from SINTEF Energy Research and the Norwegian university of science and technology.
New article in Phys. Rev. E on nonlocal entropic contributions to interfacial properties
Our paper entitled: Temperature anisotropy at equilibrium reveals nonlocal entropic contributions to interfacial properties has been accepted for publication in Physical Review E.
In the work, we show that the configurational part of the temperature has different contributions from the parallel and perpendicular directions at the vapor-liquid interface, even at equilibrium. This has been illustrated in the figure below. Let us assume that north/south are the directions perpendicular, and east/west are the directions parallel to the vapor-liquid interface. Particles located about 1 nanometer towards the vapor-side of the equimolar surface would feel contributions to the configurational temperature from the north/south direction which, if they would have been in a single-phase fluid, would correspond to a hot temperature. These directions would perhaps feel like the Sahara desert. From the east/west directions on the other hand, the contributions would be equivalent to those of a cold temperature in a single-phase fluid. These directions would perhaps feel like the arctic winter. The hot and cold contributions compensate each other, such that the particle at the interface experiences the equilibrium temperature overall.
The article starts by explaining why anisotropy in the contributions to the configurational temperature is expected across the vapor-liquid interface from a theroretical point of view. We next show that the anisotropy can also be found in molecular dynamics simulations and obtain a qualitative agreement between theory and simulations. The theory shows that the temperature anisotropy originates in nonlocal entropic contributions, which are missing from the classical description of interfacial phenomena.
The nonlocal entropic contributions discussed in this work are likely to play a role in the description of both equilibrium and nonequilibrium properties of interfaces. At equilibrium, they influence the temperature- and curvature-dependence of the surface tension. Across the vapor-liquid interface of the Lennard Jones fluid, we find that the maximum in the temperature anisotropy coincides precisely with the maximum in the thermal resistivity relative to the equimolar surface, where the integral of the thermal resistivity gives the Kapitza resistance. This links the temperature anisotropy at equilibrium to the Kapitza resistance of the vapor-liquid interface at nonequilibrium.
I believe the work to be of importance to future research on interfacial phenomena, in particular for the description of nonequilibrium interfacial processes. The work was performed in in collaboration with Thuat T. Trinh and Anders Lervik from the Department of Chemistry at the Norwegian University of Science and technology.