Numerical Optimization of Finned Heat Exchanger Using Ansys CFX


Numerical Optimization of Finned Heat Exchanger Using Ansys CFX

Ayob, A. A.; Unger, S.

his report investigates the effect of tube geometry and different fin parameters towards heat transfer and flow properties in a heat exchanger using Ansys CFX for a single phase system. The purpose of this study was focused on improving heat transfer efficiency around a single tube within a heat exchanger. Stainless steel was used as the construction material for the tube and the flowing medium is air at ambient conditions. Tube length was kept constant at 212 mm. The temperature of the tube wall was kept constant at 330.15 K and inlet air temperature at 300.15 K. Inlet air velocity was varied from 0.5 m/s to 5 m/s to observe how the structure performed at different Reynolds number. The main results that were observed were “goodness” factor (ratio of Colburn number to friction factor), pressure drop (∆P), increase in air temperature (∆T), efficiency, and average heat transfer coefficient (HTC).
An initial simulation was modelled and compared to literature. This was done to ensure that it was performing as intended. The accuracy of Ansys CFX was studied to ensure that changing the software properties such as mesh, residual target and turbulence model would not have much impact on the results. It was decided that manipulating software properties changed the results by less than 5% would suffice. The effect of tube geometry was studied by changing the shape from annular to oval shaped and by changing the outer diameter of oval tubes from 27mm to 16 mm. From the results, it was concluded that an oval tube with 16 mm outer diameter was the best design as it gave the lowest pressure drop (0.05 – 1.9 Pa) and highest “goodness” factor (0.4), even though it had the lowest value for heat transfer coefficient. The 16 mm oval tube was used for the simulation of different fin parameters The fin parameters varied were fin thickness, fin height, and fin pitch. Only one parameter was manipulated while other parameters were maintained at a constant value. The fin height was manipulated from 6mm to 46mm. It was observed that a smaller fin height would give efficiencies up to 96%, lower pressure drop but the lowest “goodness” factor. At low air velocities, a 6 mm fin height had a lower value for ∆T compared to 46 mm tall fin by 58%. The difference decreases to 0.016% when air velocity reaches 5 m/s. Next, f in thickness was changed from 0.25 mm to 4mm. The results showed that increasing the fin thickness increases the average heat transfer coefficient and ∆T but resulted in higher ∆P. When air velocity is low, the difference in ∆T was 53%. This decreased with increasing velocity. Lastly, fin pitch was manipulated from 0.25 mm to 6 mm. It was found that a fin pitch value of 0.25-1 mm reduced the amount of air able to flow between the fins. At low velocities of around 0.5 m/s, increasing the fin pitch beyond 4 mm had a negative effect on HTC and ∆T. This value changes when air velocity exceeds 3 m/s, where a fin pitch larger than 2 mm had lower values for average heat transfer coefficient and ∆T.

  • Master thesis
    The University of Edinburgh, 2017
    Mentor: Sebastian Unger

Permalink: https://www.hzdr.de/publications/Publ-25877
Publ.-Id: 25877