Experimental validation of a periodic heat transfer CFD model of a vertical shell and tube heat exchanger
Abstract: Flow obstructions are used as a passive design element in heat exchangers to enhance heattransfer. Further, a change in flow structure can also have a positive effect on the heat transfer. Avertical shell and tube heat exchanger, used to recover heat in the greywater stream, isinvestigated in this study. The heat exchanger consists of flow obstructions such as annulargrooves and a helical string. The flow structure can be modified to a swirling film flow byadding a passive design element, called a Cyclone generator. This study aims to experimentallyvalidate a periodic heat transfer CFD model of a shell and tube heat exchanger, with uniformflow at steady-state laminar conditions. The study further analyses the heat transfercharacteristics of the annular grooves and the helical string, and the modified flow due to aswirling film. A calibrated test rig is constructed to consist of a heat source and a heat sink, as well as a meansfor measuring the flow and temperature of a vertical heat exchanger at elevated temperatures.The experimental results were evaluated using the Ɛ -NTU method and uncertainty analysis ofone standard deviation. The heat exchanger geometry had periodically repeating sectionsbetween the inlet and the outlet. Hence the large geometry was simplified to a smaller periodicmodule. The module was subjected to periodic boundary conditions and was simulated using apressure-based coupled algorithm on ANSYS Fluent. Further, the distribution of pressure andvelocity flow fields are examined for uniform flow in CFD. The experiment investigated the heattransfer of a swirling flow at a wide range of flow rates. The CFD model could not be validated by the experiment due to a difference between the overallheat transfer coefficients, calculated in the model and the experiment. The error in validationcould be pointed to an ambiguous energy result in one of the streams. However, the model couldsimulate real-life pressure drop conditions. It was found that the helical string contributed to asubstantial increase in the local turbulence, which translates to an increase in heat transfer. Theheat transfer was also increased in the presence of the annular grooves. From the experiment, a higher heat transfer is noticed at the entrance region of the heatexchanger compared to the middle section. The heat transfer characteristics of the swirling filmwere found to be significantly higher than that of the uniform flow. Finally, for uniform andswirling flows, the heat exchanger effectiveness, Ɛ, can be described as a single logarithmicfunction of the NTU.
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