06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
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Item Open Access Experimental investigation of unsteady convective heat transfer under airflow velocity and temperature variations(2021) Brack, Stefan; Poser, Rico; Wolfersdorf, Jens vonAn experimental approach to study unsteady local heat transfer characteristics due to airflow velocity and/or airflow temperature variations is presented. It uses controlled electrical heaters and rotating vanes to independently vary the flow and thermal boundary conditions. Time-resolved surface temperatures are measured using an in situ calibrated infrared thermography camera. Those surface temperatures are analyzed by modeling the transient conjugate heat transfer process in the wall to obtain locally resolved surface heat flux distributions. The applicability is illustrated for a flow and heat transfer behind a tetrahedral vortex generator on a flat plate.Item Open Access A comparison between transient heat transfer measurements using TLC and IR thermography(2017) Brack, Stefan; Poser, Rico; Wolfersdorf, Jens vonNarrowband thermochromic liquid crystals (TLCs) and infrared thermography (IRT) are compared in the context of spatially resolved and transient heat transfer measurements. For accurate measurements the TLC coating was calibrated with a stationary method before the experiment. The IRT camera was in-situ calibrated with a surface thermocouple. A good agreement on temperature was achieved for both methods. The TLC data as a single-point-in-time measurement was evaluated for a time-independent heat transfer coefficient hTLC. The surface temperature history measured with the IRT camera enables an additional evaluation for a time-dependent hIR(t). In the case of one-dimensional heat conduction situations and late TLC indications, hTLC and hIR(t) agree well after the first 10 s of the experiment. An investigation of the heat transfer in the wake region of a vortex generator illustrated the influence of lateral conduction. This effect is taken into account by an analytical-empirical correction method for the TLC data. For the IRT data an evaluation method based on an analytical solution of the three-dimensional heat conduction equation is presented. Equally to the one-dimensional case the evaluation methods considering the lateral conduction effects agree well after 10 s while the other methods deliver erroneous results.Item Open Access A comparison between transient heat transfer measurements using TLC and IR thermography(2016) Brack, Stefan; Poser, Rico; Wolfersdorf, Jens vonNarrowband thermochromic liquid crystals (TLCs) and infrared thermography (IRT) are compared in the context of spatially resolved and transient heat transfer measurements. For accurate measurements the TLC coating was calibrated with a stationary method before the experiment. The IRT-camera was in-situ calibrated with a surface thermocouple. A good agreement on temperature was achieved for both methods. The TLC data as a single point measurement was evaluated for a time-independent heat transfer coefficient ℎTLC. The surface temperature history measured with the IRT-camera enables an additional evaluation for a time-dependent ℎCF(t). In the case of one-dimensional heat conduction situations and late TLC indications ℎTLC and ℎCF(t) agree well after the first 10 s of the experiment. An investigation of the heat transfer in the wake region of a vortex generator illustrated the influence of lateral conduction. This effect is not taken into account by any method and leads to greater differ-ences between ℎTLC and ℎCF(t).Item Open 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 vonTransient 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.Item Open 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 vonExperimental 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.Item Open Access Experimental detection of organised motion in complex flows with modified spectral proper orthogonal decomposition(2023) Schneider, Nick; Köhler, Simon; Wolfersdorf, Jens vonSpectral proper orthogonal decomposition (SPOD) has seen renewed interest in recent years due to its unique ability to decouple organised motion at different timescales from large datasets with limited available information. This paper investigated the unsteady components of the flow field within a simplified turbine centre frame (TCF) model by applying SPOD to experimental, time-resolved flow speed data captured by particle image velocimetry (PIV). It was observed that conventional methods failed to capture the two significant active bands in the power spectrum predicted by preliminary hot wire anemometry measurements. Therefore, a modification to the SPOD procedure, which employs subsampling of the time sequence recorded in the experiment to artificially lower the PIV data acquisition frequency, was developed and successfully deployed to analyse the TCF flow field. The two dynamically active bands were identified in the power spectra, resulting in a closer match to the preceding analyses. Within these bands, SPOD’s ability to capture spatial coherence was leveraged to detect several plausible coherent, fluctuating structures in two perpendicular planes. A partial three-dimensional reconstruction of the flow phenomena suggested that both bands were associated with a distinct mode of organised motion, each contributing a significant percentage of the system’s total fluctuating energy.