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

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

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%

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

A Study Of Heat Transfer Augmentation For Recuperative Heat Exchangers: Comparison Between Three Dimple Geometries

This study presents an investigation of the heat transfer augmentation for the purpose of obtaining high effectiveness recuperative heat exchangers for regeneration. The focus of the present work is in the fully developed portion of a 2:1 aspect ratio rectangular channel characterized by dimples applied to one wall at channel Reynolds numbers of 10, 000, 18, 000, 27, 000, and 36, 000. The dimples are applied in a staggered-row, racetrack configuration. In this study, a segmented copper test section was embedded with insulated dimples in order to minimize (to a negligible level) the heat transfer within the dimpled feature. The insulated material used to create a dimpled geometry isolates the heat transfer within the dimple cavity from the heat transfer augmentation on the surrounding smooth walls promoted by the flow disturbances induced by the dimple. Results for three different geometries are presented, a small dimple feature, a large dimple, and a double dimple. The results of this study indicate that there is significant heat transfer augmentation even on the non-featured portion of the channel wall. Overall heat transfer augmentations for the small dimples are between 13-27%, large dimples between 33-54%, and double dimples between 22-39%, with highest heat transfer augmentation at the lowest Reynolds number for all three dimple geometries tested. Copyright © 2011 by ASME

Heat Transfer Measurements Using The Hybrid Heat Transfer Technique With Thermally Adiabatic And Participating Ribs

This work is focused on the application of a number of improvements to the traditional transient thermochromic liquid crystals technique, in particular the hybrid heat transfer experiment, in order to provide more detailed and accurate measurements of the surface heat transfer coefficient in internal cooling passages. More accurate measurements of heat transfer coefficient are necessary to provide a clearer understanding of the performance of the cooling channels and to not misrepresent the channel performance so that more optimal designs and progress can be achieved. Detailed Nusselt number measurements were performed for a square channel with ribs on one wall in the Reynolds number range of 50 000 to 150 000, based on channel hydraulic diameter, using the transient thermochromic liquid crystals technique. The rib aspect ratio is 1:1, the rib height-to-hydraulic diameter ratio is 0.10, the rib-pitch-to-rib-height ratio is 10, and the ribs are oriented orthogonal to the streamwise direction. Heat transfer measurements were taken on all four walls so that the bulk temperature variation throughout the channel during the experiment can also be taken into account. Adiabatic and aluminum ribs were used simultaneously. The recently developed Coupled 0D-1D model is used to resolve the average heat transfer of the metallic rib features. A comparison of the data obtained using adiabatic and metallic rib features is made to quantify experimentally the influence of the rib-induced contamination. Friction augmentation, overall heat transfer augmentation, and overall thermal performance are also reported. Copyright © 2013 by ASME

Comparision Of Heat Transfer And Friction Augmentation For Symmetric And Non-Symmetric Wedge Turbulators On Two Opposite Walls

This paper is an investigation of the heat transfer and friction augmentation in the fully-developed portion of a narrow rectangular duct (AR=2) with wedge turbulators applied to the top and bottom walls. Tests are conducted at 10,000, 20,000, 30,000, and 40,000 Reynolds numbers based on the channel hydraulic diameter. The purpose of the paper is to find the overall thermal performance of four different wedge-shaped transport promoters, two symmetric and two non-symmetric wedges varying in height and footprint. Experimental setup consists of 40 segmented and individually heated copper blocks (10 for each wall) with temperature data measured with thermocouples embedded in each block on all four walls. Data reported includes the Nusselt number augmentation of the side walls of the channel in addition to the top and bottom featured walls to quantify the influence of flow disturbances caused by these wedge geometries to the surrounding smooth walls. Overall thermal performance is presented for each case to determine which wedge shape contributes to high heat transfer with a lower pressure loss. The non-symmetric (half) wedge shapes resulted in heat transfer augmentations 30% to 50% lower than the symmetric (full) wedge cases, and 20% to 50% lower friction factor augmentations. A better understanding of the effects produced by these geometries will help in the design and development of more effective cooling-channel design. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

A Study Of Heat Transfer Augmentation For Recuperative Heat Exchangers: Comparison Between Three Dimple Geometries

This study presents an investigation of the heat transfer augmentation for the purpose of obtaining high effectiveness recuperative heat exchangers for waste heat recovery. The focus of the present work is in the fully developed portion of a 2:1 aspect ratio rectangular channel characterized by dimples applied to one wall at channel Reynolds numbers of 10,000, 18,000, 28,000, and 36,000. The dimples are applied in a staggered-row, racetrack configuration. In this study, a segmented copper test section was embedded with insulated dimples in order to isolate the heat transfer within the dimpled feature. The insulated material used to create a dimpled geometry isolates the heat transfer within the dimple cavity from the heat transfer augmentation on the surrounding smooth walls promoted by the flow disturbances induced by the dimple. Results for three different geometries are presented, a small dimple feature, a large dimple, and a double dimple. The results of this study indicate that there is significant heat transfer augmentation even on the nonfeatured portion of the channel wall resulting from the secondary flows created by the features. Overall heat transfer augmentations for the small dimples are between 13-27%, large dimples between 33-54%, and double dimples between 22-39%, with the highest heat transfer augmentation at the lowest Reynolds number for all three dimple geometries tested. Heat transfer within the dimple was shown to be less than that of the surrounding flat regions at low Reynolds numbers. Results for each dimple geometry show that dimples are capable of promoting heat transfer over the entire bottom wall surface as well as the side walls; thus the effects are not confined to within the dimple cavity. © 2012 American Society of Mechanical Engineers