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Influence of condensation effects on screw expander operation

Titel: Influence of condensation effects on screw expander operation
Thema: Simulation and experimental investigation of condensation phenomena in screw expanders


Processes without superheating of a vapourous working fluid might offer energetic advances, for instance in heat recovery with an organic-Rankine-cycle. In contrast to turbo machinery or reciprocating expanders, twin screw expanders are well adapted for operation with wet or saturated vapours. Liquid in the working chambers of screw expanders effects a partial seal of the clearances and lubricates the profile mesh.

When an supersaturated state of the vapourous working fluid is reached in the working chambers, condensate might emerge. In the scope of this project a simulation-based investigation of various condensation phenomena is carried out and the interactions between condensate and screw expander are investigated. Corresponding experiments are carried out on the steam test rig at the Chair of Fluidics.

Simulation results for an idealised screw expander cycle are shown in figures 1 and 2. The spontaneous condensation process during the expansion of saturated steam is simulated by means of classical nucleation theory. The consideration of a non-equilibrium condensation process in the simulations shows a lower chamber pressure during the early expansion phase compared to the simulations performed in thermodynamic equilibrium. The corresponding history of droplet growth is depicted in figure 2. Classical nucleation theory provides information on the expected droplet size during the expansion of vapourous fluids.

condensation_cnt2 condensation_cnt1

Figure 1:

Pressure-volume diagram for idealised expander cycle with non-equilibrium condensation model (classical nucleation theory - CNT) and comparison to process in thermodynamic equilibrium and purely metastable expansion.

Figure 2:

Growth of emerging condensate droplets and size of critical clusters for an idealised expander cycle simulated with classical nucleation theory.

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Univ. Prof. Dr.-Ing. Andreas Brümmer
Tel.: 0231 755-5720
Dipl.-Ing. Manuel Grieb
Tel.: 0231 755-5727