CFD Simulation of a Fin-and-Tube Heat Exchanger: Heat transfer, fluid flow, and turbulence model analysis using 3D open source CFD code

Hansen, Anna Margrete

4. term

Chemical Engineering, Master

2008

89

This thesis uses three-dimensional computational fluid dynamics (CFD) to study how heat and air move through a two-row, plane fin-and-tube heat exchanger. The work is carried out in OpenFOAM, a free open-source CFD program, and every step—from geometry and meshing to steady-state solving and post-processing—uses open-source tools. We simulate flow conditions with Reynolds numbers from 330 to 7000 (a measure that indicates whether flow is laminar, transitional, or turbulent). We compare a laminar model with two turbulence models (k-epsilon and Menter SST k-omega) and compute pressure drop as well as velocity and temperature fields. To validate the model, we compare the simulated friction factor f (a measure of flow resistance/pressure loss) and the Colburn factor j (a measure of heat transfer performance) with published experimental data. For friction factor, the models give similar results in laminar flow; in transitional flow the laminar model is most accurate, and in turbulent flow the SST k-omega model is more accurate. For heat transfer, the laminar model is most accurate in laminar flow, the SST k-omega model performs best in transitional flow, and the k-epsilon model is more accurate in turbulent flow. Overall, the simulations agree reasonably well with experiments, and open-source software is sufficient for simulating flow and heat transfer in tube-fin heat exchangers.

Denne afhandling bruger tredimensionel computational fluid dynamics (CFD) til at undersøge, hvordan varme og luft bevæger sig gennem en to-rækket, plan ribbe-rør varmeveksler. Arbejdet udføres i OpenFOAM, et gratis open source CFD-program, og alle trin fra opbygning af geometri og mesh til steady-state beregninger og efterbehandling er udført med open source-værktøjer. Vi simulerer strømningstilstande med Reynolds-tal fra 330 til 7000 (en skala, der angiver om strømningen er laminær, overgangs- eller turbulent). Vi sammenligner en laminær model med to turbulensmodeller (k-epsilon og Menter SST k-omega) og beregner trykfald samt hastigheds- og temperaturfelter. For at validere modellen sammenlignes friktionsfaktoren f (et mål for modstand/tryk-tab) og Colburn-faktoren j (et mål for varmeoverførselsydelse) med publicerede forsøgsdata. For friktionsfaktoren er der kun små forskelle mellem modellerne i laminær strømning; i overgangsstrømning er den laminære model mest nøjagtig, og i turbulent strømning er SST k-omega mere nøjagtig. For varmeoverførsel er den laminære model bedst i laminær strømning, SST k-omega er bedst i overgangsstrømning, og k-epsilon er mere nøjagtig i turbulent strømning. Overordnet er der rimelig overensstemmelse mellem simuleringerne og forsøgene, og open source-software er tilstrækkeligt til at simulere strømning og varmeoverførsel i ribbe-rør varmevekslere.

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