CFD analysis to determine the local Nusselt number over a heated micro wire in multicomponent flow
Author
Werner, Gerrit Nils
Term
4. term
Education
Publication year
2016
Submitted on
2016-06-01
Abstract
Formålet er at forstå, hvordan et varmetrådsanemometer (en tynd opvarmet tråd, der måler hastighed via varmeafledning) påvirker og påvirkes af strømningen omkring det, samt at sammenholde resultater med laboratorieforsøg. Vi udviklede en CFD-model (computersimulering af strømning og varmeoverførsel) af et rør med en tråd placeret i midten. Modellen kan køres med forskellige gassammensætninger og fugtniveauer, hvor de fysiske egenskaber behandles som temperaturafhængige. For at få hurtigere netgenerering (meshing) og beregning er hovedmodellen todimensionel. Simulationerne viser, at ved lave strømningshastigheder ledes varme fra tråden opstrøms imod strømretningen, og at gasens udløbstemperatur afhænger af hastigheden: højere hastighed giver lavere temperatur, fordi mere varme transporteres væk. I de undersøgte tilfælde kan naturlig konvektion (buoyansedrevet strømning) negligeres. De bestemte koefficienter i en Nusselt-tal-korrelation (en standardformel for konvektiv varmeoverførsel) var overordnet set i overensstemmelse med laboratorieværdierne, men med nogle afvigelser, der kan skyldes evalueringsmetoden. En supplerende tredimensionel model blev opbygget for at vurdere forskellen mellem 2D- og 3D-antagelserne; den viser, at 2D-modellen overfører mere varme pr. trådlængde til væsken end 3D-modellen.
The goal is to understand how a hot-wire anemometer (a thin heated wire that senses flow by how fast it loses heat) affects and is affected by the surrounding flow, and to compare against laboratory measurements. We built a CFD model (a computer simulation of fluid flow and heat transfer) of a pipe with a centrally located wire. The model can be run with different gas mixtures and humidity levels, with thermophysical properties treated as functions of temperature. To speed up mesh generation and computation, the main model is two-dimensional. The simulations show that at low flow speeds, heat from the wire conducts upstream against the flow direction, and that the gas temperature at the outlet depends on flow speed: higher velocities lead to lower temperatures because more heat is removed. Under the cases studied, natural convection (buoyancy-driven flow) can be neglected. The fitted coefficients in a Nusselt-number correlation (a standard convective heat-transfer relation) broadly match laboratory values but show some deviations that may stem from the evaluation method. A supplementary three-dimensional model was built to quantify the difference between 2D and 3D assumptions; it indicates that the 2D model transfers more heat per unit wire length to the fluid than the 3D model.
[This abstract was generated with the help of AI]
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