Dimples in corrugated channels to improve efficiency in plate heat exchangers

The development of new heat exchanger patterns is ever changing. Companies are constantly pushing innovation to satisfy costumer needs in terms of thermal efficiency, reliability and serviceability. In this thesis, the implementation of dimples in the corrugated pattern of a plate heat exchanger is investigated to increase the thermal efficiency. The thesis comprises three studies, moving from research of a dimpled surface to an implementation of dimples on a real-life heat exchanger model used by Alfa Laval. Computational fluid dynamics was applied to model heat transfer, pressure drop and thermal efficiency through three Reynolds Averaged Navier Stokes (RANS) models and one Large Eddy Simulation (LES). The RANS-models were shown to provide insufficient resolution which heavily affects mixing, vortex formation and flow distribution in the heat exchanger channel. The RANS-model suggested that the thermal efficiency decreases when dimples are added, but failed to accurately capture key phenomena such as vortex formation and mixing. In the more advanced LES model, dimples were shown to increase the thermal efficiency by 18 % compared to a flat channel. This study not only provides insight into the flow behavior in dimpled heat exchanger channels, but also shows the importance of using the correct turbulence model for a given problem. The flow dynamics in a dimpled plate heat exchanger is complex, and a RANS-model is not sufficient for an accurate description of the velocity field. In this regard, LES performs better and the result of the LES simulation is therefore more trustworthy than results from the RANS. As no experimental data exists, it is difficult to compare models to reality, laboratory trials are nessessaryPlate heat exchangers find their main use as a heat recovery unit in refrigeration, petrochemical plants, pulp and paper, pharmaceutical production, food industry and waste water treatment. Energy and utilities are of great importance in today’s world, as companies shift towards reduced carbon emissions and minimizing water and energy consumption to protect the environment. The energy saving capability of a plate heat exchanger is dictated by the corrugated pattern, as this is where most of the heat transfer occurs. New plate heat exchanger products are constantly developed to satisfy costumer needs in terms of thermal efficiency, reliability and serviceability. A heat transfer enhancement technique used to improve thermal efficiency through mixing, is the addition of dimples. Dimples are commonly used today, on aeroplane wings to reduce fuel consumption and on the surface of golf balls to increase travel distance. Dimples in corrugated plate heat exchanger channels is an area that has not yet been explored sufficiently, but recent advancements in the field has sparked the interest of an investigation. The implementation of dimples on a corrugated heat exchanger pattern through a computational fluid dynamic model was performed in Ansys CFX, which revealed formation of vortex structures emanating from the dimple cavities. The dimples were shown to enhance mixing, vortex formation and provide beneficial flow distribution in the model of the plate heat exchanger channel. The results also included state of the art fluid dynamics simulations with conclusions about the insufficiency of certain models in regards to mixing and vortex formation. Through this research, an increased understanding of dimples in plate heat exchangers has been achieved which can contribute to better products with higher thermal efficiency. In the future, the next generation of heat exchangers will continue to contribute to save reduced energy consumption, possibly with the addition of dimples