Flow visualization of downward condensing ammonia in a gasketed plate heat exchanger 

Recently, Maaike Leichsenring graduated from the TU Delft in Engineering Thermodynamics on flow visualization of the ammonia condensing process in a plate type heat exchanger. Generally, two-phase flow in plate heat exchangers is yet not fully understood which leads to inaccurate performance calculations. The uniqueness of the experiment is described by the fact that condensing flow of pure ammonia in a corrugated plate heat exchanger was successfully captured for the first time. These results can be used to improve the performance calculations of such condensers. In August Maaike presented her work on the International Congress of Refrigeration 2019 in Montreal and competed as one of the three finalists for the Best Student Paper Competition.

Maaike Leichsenring presenting her research at the International Congress of Refrigeration 2019 in Montreal

Maaike Leichsenring presenting her research at the International Congress of Refrigeration 2019 in Montreal

Plate heat exchangers (PHEs) are known to have a wide range of applications due to their superior performance in relation to favourable heat transfer coefficients (HTCs), compactness, design flexibility and thermal effectiveness. However, the two-phase behaviour inside the PHEs is yet not fully understood, resulting in over- or underestimating by several published heat transfer and pressure drop correlations, which are limited by the range of conditions they cover. Tao et al. (2018) conclude that better predictions of flow patterns and their behaviour in PHEs will improve the calculation of heat transfer and pressure drop.

In cooperation with the Delft University of Technology, Bluerise B.V., recently aqcuired by Allseas Engineering B.V., constructed a 100 W OTEC demo based on an organic Rankine cycle using pure ammonia as the working fluid that is downward condensed in a gasketed plate heat exchanger (GPHE). A visualization section is designed in the OTEC demo that allows for flow visualization which involves a Gasketed PHE, a transparent visualization plate, illumination by a LED-strip and a 3000 fps high speed camera. The visualization section contained three looking windows distributed over the length of the plate.

Flow visualization experiments are performed in a single-channel configuration of the GPHE. During the experiments, the mass flux and average vapor quality were varied at a constant condenser pressure and cold-water temperature. These test conditions were chosen for OTEC applications but are also applicable for refrigeration systems.

A low mass flux shows partial film flow with a smooth film, and a high mass flux corresponds to film flow with rough film characteristics. Partial film flow is defined by areas of dry-out on the plate, which can be explained by the phenomenon that the thin liquid film does not fully cover the entire plate and that the vapor core of the channel is in direct contact with the plates. It is observed that these areas of dry-out increase with vapor quality and volumetric void fraction. For increasing mass flux, it is observed that the areas of dry-out vanish, indicating that mass flux determines a transition from partial film flow to film flow.

Partial film flow shows higher absolute values for the HTCs with respect to film flow. It is expected that this is due to the decrease of the liquid film thickness within partial film flow, which corresponds to a reduction of the thermal resistance of the liquid. Partial film flow shows a lower pressure drop than film flow, which is expected as pressure drop is proportionally related to the square of the mass flux.

Maaike Leichsenring (3rd from the left) together with her supervisors Xuan Tao (left) and Carlos Infante Ferreira (4th from the left). Her work was selected for the finale of the 'Best student competition'.

Maaike Leichsenring (3rd from the left) together with her supervisors Xuan Tao (left) and Carlos Infante Ferreira (4th from the left). Her work was selected for the finale of the ‘Best student competition’.

Proposed flow patterns maps by previous studies are not in accordance with the observed flow patterns inside the GPHE for the current experimental conditions. For this reason, a flow pattern map for downward condensing ammonia is proposed for this experimental configuration and conditions. This flow pattern map can be used to increase the accuracy of general flow pattern maps for two-phase flows in plate heat exchangers.

Source: Maaike Leichsenring

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