Universität Stuttgart

Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1

Browse

Subject: search.filters.subject.620Start date: 2020Institute: search.filters.UBSInstitut.Institut für Thermodynamik der Luft- und RaumfahrtAuthor: search.filters.author.Hartmann, Christopher

Search Results

Now showing 1 - 3 of 3
  • Thumbnail Image
    ItemOpen Access
    Determination of local heat transfer coefficients and friction factors at variable temperature and velocity boundary conditions for complex flows
    (2024) Hartmann, Christopher; Wolfersdorf, Jens von
    Transient conjugate heat transfer measurements under varying temperature and velocity inlet boundary conditions at incompressible flow conditions were performed for flat plate and ribbed channel geometries. Therefrom, local adiabatic wall temperatures and heat transfer coefficients were determined. The data were analyzed using typical heat transfer correlations, e.g., Nu=CRemPrn, determining the local distributions of C and m . It is shown that they are closely linked. A relationship lnC=A-mBis observed, with A and B as modeling parameters. They could be related to parameters in log-law or power-law representations for turbulent boundary layer flows. The parameter m is shown to have a close link to local pressure gradients and, therewith, near wall streamlines as well as friction factor distributions. A normalization of the C parameter allows one to derive a Reynolds analogy factor and, therefrom, local wall shear stresses.
  • Thumbnail Image
    ItemOpen Access
    Experimental validation of a numerical coupling environment applying FEM and CFD
    (2023) Hartmann, Christopher; Schweikert, Julia; Cottier, François; Israel, Ute; Gier, Jochen; Wolfersdorf, Jens von
    Experimental results for the transient heat transfer characteristics over a flat plate and over a plate with V-shaped ribs were compared to numerical results from a coupling environment applying FEM and CFD. In order to simulate transient effects in the cooling process of engine components during typical flight missions, the temperature and the velocity at the inlet of the channel were varied over time. The transient temperature distribution at the plate was measured using infrared thermography. Five different plate materials (perspex, PEEK, quartz, aluminum, and steel) were considered to investigate the influence of thermal conduction on the heat transfer between solid and fluid depending on the Biot number. The experimental results represent a reference database for a Python-based coupling environment applying CalculiX (FEM) and ANSYS CFX (CFD). The results were additionally compared to numerical results simulating the complete transient conjugated heat transfer with CFD. A good agreement between the numerical and the experimental results was achieved using different coupling sizes at different Biot numbers for the flat plate and the plate with V-shaped ribs.
  • Thumbnail Image
    ItemOpen Access
    Experimental and numerical investigations of transient conjugate heat transfer processes
    (2025) Hartmann, Christopher; Weigand, Bernhard (Prof. Dr.-Ing. habil.)
    Effective cooling of components exposed to high thermal loads is a key challenge in aircraft engine development. Analyzing thermal loads during flight missions is critical, as they fluctuate with varying operating conditions. Accurate assessment requires considering coupled heat transfer processes and transient effects. The calculation of slow, transient phenomena was optimized by enhancing a coupling environment between a finite element and a finite volume solver. A wide range of boundary conditions and geometries were experimentally investigated. An existing ITLR test rig was adapted, and four geometries were examined. The rig enables independent, reproducible control of inlet velocity and temperature, allowing the study of various test cycles. Wall temperatures were measured with high resolution using infrared thermography, and wall heat fluxes were calculated. Numerical simulations complemented the experiments. The data support validation of the coupling environment and showed good agreement with simulations. A variable, adaptive, experiment-specific coupling step size reduced computation time while preserving accuracy. A method was developed to enhance prediction accuracy and account for local dissipation, targeting heat transfer coefficients and friction factors in complex flows. Experimental data were analyzed using heat transfer correlations. A close relationship between two local parameters was observed, which enabled the development of a simplified correlation. The resulting model included coefficients that could be linked to established laws for turbulent boundary layer flows. One parameter correlates with local pressure gradients, near wall streamlines and friction factor distributions, while the other yields a Reynolds analogy factor that was used to estimate wall shear stresses. The model agreed well with simulations and proved universally applicable.