Development of Full Surface Transient Thermochromic Liquid Crystal Technique for Internal Cooling Channels

Proper design of high performance industrial heat transfer equipment relies on accurate knowledge and prediction of the thermal boundary conditions. In order to enhance the overall gas turbine efficiency, advancements in cooling technology for gas turbines and related applications are continuously investigated to increase the turbine inlet temperature without compromising the durability of the materials used. For detailed design, local distributions are needed in addition to bulk quantities. Detailed local distributions require advanced experimental techniques whereas they are readily available using numerical tools. Numerical predictions using a computational fluid dynamics approach with popular turbulence models are benchmarked against a semi-empirical correlation for the friction in a circular channel with repeated-rib roughness to demonstrate some shortcomings of the models used. Numerical predictions varied widely depending on the turbulence modelling approach used. The need for a compatible experimental dataset to accompany numerical simulations was discussed. An exact, closed-form analytical solution to the enhanced lumped capacitance model is derived. The temperature evolution in a representative 2D turbulated surface is simulated using Fluent to validate the model and its exact solution. A case including an interface contact resistance was included as well as various rib sizes to test the validity of the model over a range of conditions. The analysis was extended to the inter-rib region to investigate the extent and magnitude of the influence of the metallic rib features on the apparent heat transfer coefficients in the inter-rib region. It was found that the thermal contamination is limited only to the regions closest to the base of the rib feature. An experimental setup was developed, capable of measuring the local heat transfer distributions on all four channel walls of a rectangular channel (with aspect ratios between 1 and 5) at Reynolds numbers up to 150,000. The setup utilizes a transient thermochromic liquid crystals technique using narrow band crystals and a four camera setup. The setup is used to test a square channel with ribs applied to one wall. Using the transient thermochromic liquid crystals technique and applying it underneath high conductivity, metallic surface features, it is possible to calculate the heat transfer coefficient using a lumped heat capacitance approach. The enhanced lumped capacitance model is used to account for heat conduction into the substrate material. Rohacell and aluminum ribs adhered to the surface were used to tandem to validate the hybrid technique against the standard technique. Local data was also used to investigate the effect of thermal contamination. Thermal contamination observed empirically was more optimistic than numerical predictions. Traditional transient thermochromic liquid crystals technique utilizes the time-to-arrival of the peak intensity of the green color signal. The technique has been extended to utilize both the red and green color signals, increasing the throughput by recovering unused data while also allowing for a reduction in the experimental uncertainty of the calculated heat transfer coefficient. The over-determined system was solved using an un-weighted least squares approach. Uncertainty analysis of the multi-color technique demonstrated its superior performance over the single-color technique. The multi-color technique has the advantage of improved experimental uncertainty while being easy to implement

Effect of rib aspect ratio on heat transfer and friction in rectangular channels

The heat transfer and friction augmentation in the fully developed portion of a 2:1 aspect ratio rectangular channel with orthogonal ribs at channel Reynolds numbers of 20,000, 30,000, and 40,000 is studied both experimentally and computationally. Ribs are applied to the two opposite wide walls. The rib aspect ratio is varied systematically at 1, 3, and 5, with a constant rib height and constant rib pitch (rib-pitch-to-rib-height ratio of 10). The purpose of the study is to extend the knowledge of the performance of rectangular channels with ribs to include high aspect ratio ribs. The experimental investigation is performed using transient Thermochromic Liquid Crystals technique to measure the distribution of the local Nusselt numbers on the ribbed walls. Overall channel pressure drop and friction factor augmentation is also obtained with the experimental setup. A numerical simulation is also performed by solving the 3-D Reynolds-averaged Navier-Stokes equations using the realizable-k-Greek lowercase letter episilon] turbulence model for closure. Flow visualization is obtained from the computational results as well as numerical predictions of local distributions of Nusselt numbers and overal channel pressure drop. Results indicate that with increasing rib width, the heat transfer augmentation of the ribbed walls decreases with a corresponding reduction in channel pressure drop

Dynamics of a Perfectly Premixed Jet Flame Exhibiting Self-Excited High-Frequency, Transverse Thermoacoustic Instabilities

This work is an investigation of the behavior of a premixed turbulent jet flame in a cylindrical dump combustor. The degeneracy of the simple configuration in this study lends itself for a detailed study of inherent mechanisms of a self-excited thermoacoustic instability in isolation from system coupling effects, enabling detailed numerical simulations to be carried out to supplement experimental findings. Tests were done at a nominal pressure of 8 bar and inlet temperature around 450 ˚C. Self-excited large eddy simulations were also carried out in OpenFOAM, using a b-Ξ flame-wrinkling model to model the combustion process. Eigenfrequency analysis in COMSOL was also done to support and explain the findings from both the numerical simulations and trends observed in the experiments. Measurements from high frequency pressure transducers were analyzed to determine the frequencies of the excited modes in the rig test and compared to the spectra from the LES simulation. The time-resolved fields from the LES simulation were phase-averaged to deduce the acoustic-flame interactions. Despite the (axis)symmetry in this configuration, the non-axisymmetric 1T and 1T1L modes were (simultaneously) excited. Two distinct behaviors are noted for the dynamic flame behavior. In the downstream region, the flame motion is well described by a bulk kinematic displacement as a result of the interaction of the flame front with the local acoustic perturbation. In the upstream region, near the combustor dump plane, large-scale wrinkles are observed in the flame front that have characteristics of a convective wave. The current findings provide additional evidence supporting and further establishing the theory of inherent acoustic-flame interactions as an excitation mechanism (distinct from acoustically-induced hydrodynamic oscillations) for high-frequency, transverse thermoacoustic instabilities

A Coupled 0D-1D Model For Hybrid Heat Transfer Measurements

This paper covers the discovery, development, and preliminary application of an improved model to address errors associated with transient heat transfer experiments which also include the application of lumped capacitance. Previous studies using Transient TLC methods typically omit featured areas from the heat transfer calculations; by applying TLC underneath lump-able features it is possible to calculate the heat transfer with a lumped capacitance model. In utilizing lumped capacitance, the heat loss into the surface underneath the lumped features was previously erroneously unaccounted for. In this paper, the original hybrid heat transfer technique is discussed in detail. The erroneous assumptions are highlighted and an improved model is then derived. Results from previous experiments are then recalculated using the improved model to demonstrate the difference in the calculated heat transfer coefficients in a practical experiment and to demonstrate the importance of accounting for the heat conducted through the substrate in the hybrid heat transfer technique. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

Coupled Zero-Dimensional/One-Dimensional Model For Hybrid Heat Transfer Measurements

This paper covers the application of an improved model to address errors associated with transient heat transfer experiments, which also include the application of lumped capacitance. Using transient thermochromic liquid crystals techniques, and applying thermochromic liquid crystals underneath lumpable features, it is possible to calculate the heat transfer using a lumped heat capacitance approach. In previous studies using the classical lumped capacitance model, the heat loss into the surface underneath the lumped features was not accounted for. In this paper, an exact, closed-form analytical solution to the enhanced lumped capacitance model is derived for discrete bodies for the case of perfect thermal contact. To validate the model and its exact solution, the transient heat conduction in a representative two-dimensional ribbed surface is simulated numerically using the finite volume method. The modeled behavior of the coupled zero-dimensional/one-dimensional model has reasonable agreement with the numerical simulation. The solution for perfect contact can also be extended for imperfect contact. © 2013 by the American Institute of Aeronautics and Astronautics, Inc

Numerical Benchmark Of Friction Factor In Pipes With Repeated-Rib Roughness

Numerical predictions using a computational fluid dynamics approach with popular turbulence models are benchmarked against a semi-empirical correlation for the friction in a circular channel with repeated-rib roughness to demonstrate some shortcomings of the models used. Numerical predictions varied widely depending on the turbulence modelling approach used. The need for a compatible experimental dataset to accompany numerical simulation is discussed. Data reported includes numerical predictions of the friction factor and friction similarity parameter for each case. The Reynolds Stress Model most accurately reproduced the friction factors from the correlation, to within 10%

Coupled Zero-Dimensional/One-Dimensional Model for Hybrid Heat Transfer Measurements

This paper covers the application of an improved model to address errors associated with transient heat transfer experiments, which also include the application of lumped capacitance. Using transient thermochromic liquid crystals techniques, and applying thermochromic liquid crystals underneath lumpable features, it is possible to calculate the heat transfer using a lumped heat capacitance approach. In previous studies using the classical lumped capacitance model, the heat loss into the surface underneath the lumped features was not accounted for. In this paper, an exact, closed-form analytical solution to the enhanced lumped capacitance model is derived for discrete bodies for the case of perfect thermal contact. To validate the model and its exact solution, the transient heat conduction in a representative two-dimensional ribbed surface is simulated numerically using the finite volume method. The modeled behavior of the coupled zero-dimensional/one-dimensional model has reasonable agreement with the numerical simulation. The solution for perfect contact can also be extended for imperfect contact. © 2013 by the American Institute of Aeronautics and Astronautics, Inc

The Role of Flexible Loops in Folding, Trafficking and Activity of Equilibrative Nucleoside Transporters.

Equilibrative nucleoside transporters (ENTs) are integral membrane proteins, which reside in plasma membranes of all eukaryotic cells and mediate thermodynamically downhill transport of nucleosides. This process is essential for nucleoside recycling, and also plays a key role in terminating adenosine-mediated cellular signaling. Furthermore, ENTs mediate the uptake of many drugs, including anticancer and antiviral nucleoside analogues. The structure and mechanism, by which ENTs catalyze trans-membrane transport of their substrates, remain unknown. To identify the core of the transporter needed for stability, activity, and for its correct trafficking to the plasma membrane, we have expressed human ENT deletion mutants in Xenopus laevis oocytes and determined their localization, transport properties and susceptibility to inhibition. We found that the carboxyl terminal trans-membrane segments are essential for correct protein folding and trafficking. In contrast, the soluble extracellular and intracellular loops appear to be dispensable, and must be involved in the fine-tuning of transport regulation

Heat Transfer In A Rectangular Channel With Dimples Applied To One Wall

This study is an investigation of the heat transfer augmentation through the fully developed portion of a narrow rectangular duct (aspect ratio = 2) characterized by the application of dimples to the bottom wall of the channel. The geometries are studied at channel Reynolds numbers of 20,000,30,000, and 40,000. The purpose is to understand the contribution of dimple geometries in the formation of flow structures that improve the advection of heat away from the channel walls. Experimental data reported include local heat transfer coefficient distributions, channel-averaged Nusselt number augmentation, and overall friction augmentation throughout the length of the duct. The largedimple feature was found to promote significant intensification of convective heat transfer (as high as 80%) at a Reynolds number of 30,000. Furthermore, the double-dimple surface feature was found to promote heat transfer augmentation comparable with the large-dimple feature, accompanied by the pressure loss penalty of the modest small dimple. By contributing to a clearer understanding of the effects produced by these geometries, the development of more effective channel-cooling designs can be achieved. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

Large Eddy Simulations Of The Hydrodynamic And Thermal Fields From A Cylindrical Film Cooling Hole

Large Eddy Simulations are used to explore the unsteady jet-in-crossflow interactions arising from discrete hole film cooling from a cylindrical hole. The numerical grids are created using GridPro and exported into OpenFOAM for solution with specified initial and boundary conditions. A recycling-rescaling technique is used to generate a realistic turbulent incoming boundary layer upstream of injection. The geometry and flow conditions are specified to match conditions of an experiment in open literature for robust validation of the numerical solution and turbulence modeling. While LES has been demonstrated to be very successful in free shear flows, wall-bounded flows remain a hot topic of research. The current study tests the ability of LES in predicting film cooling flows using detailed experimental measurements. The LES results compared favorably with the experimental data except in areas close to the injection site and close to the wall. Grid resolution is discussed in terms of the percent turbulent kinetic energy resolved and related to the success of the LES predictions in different regions of the jet. With a dynamic Smagorinsky model used for the subgrid turbulent stresses, the benefit of a dynamic procedure for the subgrid turbulent heat fluxes instead of a supplied constant Prandtl number is minimal at the current grid resolution. The trajectories, spreading rates and large turbulent structures of the jet are discussed in terms of the hydrodynamic parameters such as velocity ratio and momentum ratio. © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved