Lucky V. Tran

Effect of Rib Aspect Ratio on Heat Transfer and Friction in Rectangular Channels

This paper is an investigation of the heat transfer augmentation in the fully-developed portion of a 2:1 aspect ratio channel with orthogonal ribs at Reynolds numbers based on the open channel hydraulic diameter of 20,000, 30,000, and 40,000. Ribs are applied to the two opposite wide walls. The rib aspect ratio is varied systematically from 1, 3, and 5, with a constant rib height and constant rib pitch (rib pitch-to-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. Data reported includes the local Nusselt number augmentation as well as the friction factor augmentation. With increasing rib width, the overall heat transfer augmentation decreased accompanied by a reduction in pressure drop.

Comparison 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 fullydeveloped 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.

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 num...

Heat Transfer in a Rectangular Channel with Dimples Applied to One Wall

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.

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

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.

An Experimental Study of Detailed Flow and Heat Transfer Analysis in a Single Row Narrow Impingement Channel

An experimental investigation of detailed flow and heat transfer in a narrow impingement channel was studied; the channel included 15 inline jets in a single row with a jet-to-target wall distance of 3 jet diameters. The spanwise length of the channel was 4 jet diameters, and a streamwise jet spacing of 5 jet diameters was considered for the current study. Both the flow physics and heat transfer tests were run at an average jet Reynolds number of 30,000. Temperature sensitive paint was used to study heat transfer at the target wall. Along with other parameters, jet-to-jet interaction in a narrow row impingement channel plays a significant role on heat transfer distribution at the side and target walls as the self-induced jet cross flow tends to bend the downstream jets. The present work shows detailed information of flow physics using Particle Image Velocimetry (PIV). PIV measurements were taken at planes normal to the target wall along the jet centerline for several jets. The flow field and heat transfer data was compared between the experiment and CFD in order to understand the relationship between flow characteristics and heat transfer. The experimental data gathered from PIV can be used as benchmark data for validating the current state of the art RANS turbulence models as well as for Large Eddy Simulation (LES).Copyright

Microturbine Recuperation: Turbulators and Their Effect on Power Density and Thermal Efficiency

Microturbines have proven to be a vital part of the distributed power generation field due to their low emissions, compact size, high reliability and low maintenance. However, microturbines operate at low pressure ratios and relatively low turbine inlet temperatures that limit cycle efficiency. In order to overcome these limitations, microturbines often utilize a recuperator or regenerator to achieve the optimal balance between improved heat rates and reduced pressure ratios across the turbine. Recuperator design aims to achieve maximum effectiveness while staying reasonably compact, which creates the need to study novel heat transfer surfaces for compact heat exchanger application. In this study, experimental data of heat transfer augmentation and friction factor augmentation values for various turbulator geometries is used to determine the required heat exchanger volume to achieve 85%, 90%, and 95% effectiveness. A parametric analysis of various recuperator channel surface areas and turbulator geometry data will be utilized to determine the feasibility of increasing thermal efficiency while remaining compact to avoid large, negative effects on power density for a hypothetical gas turbine modeled after the Turbine Technologies, Ltd. SR-30 Turbo-Jet Engine. The turbulators considered in this study consist of 4 wedges, 4 ribs, and a dimpled geometry. The results will highlight the applicability of surface features in recuperator designs that can improve overall efficiency for microturbines. Results present the power density, thermal efficiency, and specific fuel consumption as functions of heat exchanger channel Reynolds number for heat exchangers implementing different turbulators. It is shown that dimples at low Reynolds numbers yield 85% effectiveness with only a 8% reduction in power density and 90% effectiveness with only a 12% reduction in power density. Ribs and wedges also perform well but suffer from high pressure losses due to their obtrusive design.Copyright

PIV Study on the Dimple Mid-Plane of a Narrow Rectangular Channel With Dimples Applied to One Wall

This paper presents an investigation of the fluid flow in the fully developed portion of a rectangular channel (Aspect Ratio of 2) with dimples applied to one wall at channel Reynolds numbers of 20,000, 30,000, and 40,000. The dimples are applied in a staggered-row, racetrack configuration. Results for three different dimple geometries are presented: a large dimple, small dimple, and double dimple. Heat transfer and aerodynamic results from preceding works are presented in Nusselt number and friction factor augmentation plots as determined experimentally. Using particle image velocimetry, the region near the dimple feature is studied in detail in the location of the entrainment and ejection of vortical packets into and out of the dimple; the downstream wake region behind each dimple is also studied to examine the effects of the local flow phenomenon that result in improved heat transfer in the areas of the channel wall not occupied by a feature. The focus of the paper is to examine the secondary flows in these dimpled channels in order to support the previously presented heat transfer trends. The flow visualization is also intended to improve the understanding of the flow disturbances in a dimpled channel; a better understanding of these effects would lead the development of more effective channel cooling designs. Copyright © 2011 by ASME.

A Study of Heat Transfer Augmentation in a Rectangular Channel with Dimples Applied to One Wall

This study is an investigation of the heat transfer augmentation through the fullydeveloped portion of a narrow rectangular duct (AR=2) characterized by the application of dimples to the bottom wall of the channel. The geometries are studied at channel Reynolds numbers of 20000, 30000, and 40000. 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 includes the local and Nusselt number augmentation of the channel walls and the overall friction augmentation throughout the length of the duct. The large dimple feature was found to promote significant intensification of convective heat transfer, as high as eighty percent, at a Reynolds number of 30000. Furthermore, the double dimple surface feature was found to promote heat transfer augmentation comparable to 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.