Most of the studies on heat transfer augmentation using extended surfaces (i.e., fins) primarily analyze the underlying thermo-fluid system in the framework of first law of thermodynamics, which limits the scope of the studies to investigate heat transfer improvements and the associated increase of pressure drops. Second law, on the other hand, casts both of these quantities into single measure called entropy production.  This entropy production gives a measure of exergy (useful energy) destruction, therefore, it can be used to study the effectiveness of the system.

The entropy production in a thermo-fluid system stems from the presence of thermal gradients (heat transfer) and viscous dissipation (friction) effects. Helical fins are proposed for heat transfer enhancement in solar receiving tubes where large radial and circumferential thermal gradients exist due to circumferentially non-uniform heating. These helical fins aid fluid mixing by introducing a degree of swirl in the flow, thereby decreasing these thermal gradients (and the associated entropy production).

The objective of this paper is to study the relation between total entropy, pressure drop and heat transfer in the context of design optimization. For this purpose, the thermo-hydraulic performance of the tube with internal helical fins is analyzed from an entropy production perspective. The underlying conjugate heat transfer problem is simulated via computational fluid dynamics (CFD) using the k-e turbulence model. The entropy production, heat transfer coefficient and friction factor computed using this CFD model are validated with existing literature for the case of smooth tube. 

The entropy production (mean and fluctuating) contributions due to heat conduction and viscous dissipation in helical finned tubes are computed for Reynolds number (Re) ranging from 500 to 50,000 using the validated numerical model and analyzed for operational Reynolds number corresponding to low entropy production rate. The effects of helical fin geometry (i.e., helical pitch, height, thickness) on entropy production, heat transfer and pressure drop are parametrically studied.

A pareto-front corresponding to best feasible tradeoffs between heat transfer enhancement and pressure drop is constructed via multi-objective optimization using NSGA-II algorithm. Due to computationally expensive nature of these CFD simulations, it is not feasible to directly use this CFD model during the optimization process. Therefore, Kriging surrogate models are constructed to approximate entropy production, pressure drop and heat transfer coefficient, thus making the optimization tractable. The pareto-front from multi-objective optimization is interpreted from the entropy perspective and compared with the design corresponding to minimum total entropy production.


Entropy-Based Optimization of Helical Fins for Heat Transfer Enhancement Inside Tubes

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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