Passive Control and Enhancement of Low Reynolds Number Slot Jets Through the Use of Tabs and Chevrons

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Andrew Sexton ◽  
Jeff Punch ◽  
Jason Stafford ◽  
Nicholas Jeffers
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Integrated Circuits ◽  
Passive Control ◽  
Low Reynolds Number ◽  
Hydraulic Diameter ◽  
Stagnation Region ◽  
Number Range ◽  
Low Reynolds

Liquid microjets are emerging as candidate primary or secondary heat exchangers for the thermal management of next generation photonic integrated circuits (PICs). However, the thermal and hydrodynamic behavior of confined, low Reynolds number liquid slot jets is not yet comprehensively understood. This investigation experimentally examined jet outlet modifications—in the form of tabs and chevrons—as techniques for passive control and enhancement of single-phase convective heat transfer. The investigation was carried out for slot jets in the laminar flow regime, with a Reynolds number range, based on the slot jet hydraulic diameter, of 100–500. A slot jet with an aspect ratio of 4 and a fixed confinement height to hydraulic diameter ratio (H/Dh) of 1 was considered. The local surface heat transfer and velocity field characteristics were measured using infrared (IR) thermography and particle image velocimetry (PIV) techniques. It was found that increases in area-averaged Nusselt number of up to 29% compared to the baseline case could be achieved without incurring additional hydrodynamic losses. It was also determined that the location and magnitude of Nusselt number and velocity peaks within the slot jet stagnation region could be passively controlled and enhanced through the application of outlet tabs of varying geometries and locations.

Passive Control and Enhancement of Low Reynolds Number Slot Jets Through the Use of Tabs and Chevrons

2016 ◽  
Author(s):  
Andrew Sexton ◽  
Jeff Punch ◽  
Nicholas Jeffers ◽  
Jason Stafford
Keyword(s):  
Reynolds Number ◽  
Nusselt Number ◽  
Optical Networks ◽  
Passive Control ◽  
Low Reynolds Number ◽  
Hydraulic Diameter ◽  
Nozzle Geometry ◽  
Critical Element ◽  
Number Range ◽  
Low Reynolds

Optical networks are a critical element of contemporary communications infrastructure, due to their efficacy in transmitting high-speed data over large distances. Photonic integrated circuits (PICs) offer compelling advantages in terms of performance and miniaturization, but the increase in power density of these components, coupled with shrinking packaging restrictions, presents a significant thermal management challenge. This has driven the need for the integration of liquid-based microfluidic cooling artefacts into next generation PIC packages. Liquid micro-jets are emerging as candidate primary or secondary heat exchangers for such packages, however the thermal behavior of confined, low Reynolds number liquid slot jets is not comprehensively understood. This investigation utilized a hot foil technique to experimentally determine the influence of implementing jet outlet modifications — in the form of tabs and chevrons — as techniques for passive control and enhancement of single-phase convective heat transfer. The investigation was carried out for slot jets in the laminar flow regime, with a Reynolds number range, based on the conventional slot jet hydraulic diameter, of 100 to 500. The investigation was carried out with a slot jet aspect ratio of 4, and a fixed confinement height to hydraulic diameter ratio (H/Dh) of 1. It was found that all outlet modifications increased local and area-averaged Nusselt number compared to a conventional slot jet. Modifications to the major axis (or long edge) of the slot jet were most effective, achieving increases in area-averaged Nusselt number of up to 61%. It was also determined that the location and magnitude of Nusselt number peaks within the slot jet stagnation region, could be passively controlled and enhanced through the application of outlet tabs at varying locations, allowing for more flexible targeted hotspot cooling. Therefore, it was concluded that enhancements in an integrated microjet cooling artefact can be achieved through passive geometry devices, without compromising the stringent packaging restrictions of such systems, such as confinement height and nozzle geometry.

A Comparative Study of Performance of Low Reynolds Number Turbulence Models for Various Heat Transfer Enhancement Simulations

2019 ◽  
Vol 141 (7) ◽  
Author(s):  
Ankit Tiwari ◽  
Savas Yavuzkurt
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Heat Transfer Enhancement ◽  
Friction Factor ◽  
Fluid Dynamic ◽  
Low Reynolds Number ◽  
Turbulence Models ◽  
Transfer Enhancement ◽  
Low Reynolds

The goal of this study is to evaluate the computational fluid dynamic (CFD) predictions of friction factor and Nusselt number from six different low Reynolds number k–ε (LRKE) models namely Chang–Hsieh–Chen (CHC), Launder–Sharma (LS), Abid, Lam–Bremhorst (LB), Yang–Shih (YS), and Abe–Kondoh–Nagano (AKN) for various heat transfer enhancement applications. Standard and realizable k–ε (RKE) models with enhanced wall treatment (EWT) were also studied. CFD predictions of Nusselt number, Stanton number, and friction factor were compared with experimental data from literature. Various parameters such as effect of type of mesh element and grid resolution were also studied. It is recommended that a model, which predicts reasonably accurate values for both friction factor and Nusselt number, should be chosen over disparate models, which may predict either of these quantities more accurately. This is based on the performance evaluation criterion developed by Webb and Kim (2006, Principles of Enhanced Heat Transfer, 2nd ed., Taylor and Francis Group, pp. 1–72) for heat transfer enhancement. It was found that all LRKE models failed to predict friction factor and Nusselt number accurately (within 30%) for transverse rectangular ribs, whereas standard and RKE with EWT predicted friction factor and Nusselt number within 25%. Conversely, for transverse grooves, AKN, AKN/CHC, and LS (with modified constants) models accurately predicted (within 30%) both friction factor and Nusselt number for rectangular, circular, and trapezoidal grooves, respectively. In these cases, standard and RKE predictions were inaccurate and inconsistent. For longitudinal fins, Standard/RKE model, AKN, LS and Abid LRKE models gave the friction factor and Nusselt number predictions within 25%, with the AKN model being the most accurate.

scholarly journals Effect of Pin Diameter Degressive Gradient on Heat Transfer in a Microreactor with Non-Uniform Pin-Fin Array under Low Reynolds Number Conditions

Energies ◽  
2019 ◽  
Vol 12 (14) ◽  
pp. 2702
Author(s):  
Miao Qian ◽  
Jie Li ◽  
Zhong Xiang ◽  
Chao Yan ◽  
Xudong Hu
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Pressure Drop ◽  
Friction Factor ◽  
Low Reynolds Number ◽  
Hydrodynamic Characteristics ◽  
Pin Fin ◽  
Low Reynolds ◽  
Pin Fin Array

To improve the efficiency of hydrogen-producing microreactors with non-uniform pin-fin array, the influence of the pin diameter degressive gradient of the non-uniform pin-fin array (NPFA) on heat transfer and pressure drop characteristics is analyzed in this study via numerical simulation under low Reynolds number conditions. Because correlations in prior studies cannot be used to predict the Nusselt number and pressure drop in the NPFA, new heat transfer and friction factor correlations are developed in this paper to account for the effect of the pin diameter degressive gradient, providing a method for the optimized design of the pin diameter degressive gradient for a microreactor with NPFA. The results show that the Nusselt number and friction factor under a low Reynolds number are quite sensitive to the pin diameter degressive gradient. Based on the new correlations, the exponents of the pin diameter degressive gradient for the friction factor and Nusselt number were 6.9 and 2.1, respectively, indicating the significant influence of the pin diameter degressive gradient on the thermal and hydrodynamic characteristics in the NPFA structure.

Numerical Investigation of Micro-Channeled Louver Fin Aluminum Heat Exchangers at Low Reynolds Number

2016 ◽  
Author(s):  
Pradeep Shinde ◽  
Mirko Schäfer ◽  
Cheng-Xian Lin
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Exchanger ◽  
Heat Exchangers ◽  
Low Reynolds Number ◽  
Poor Performance ◽  
Geometrical Parameters ◽  
Number Range ◽  
Reynolds Number Range ◽  
Low Reynolds

Extensive studies are being carried out by several researchers on the performance prediction of aluminum heat exchangers with different fin and tube geometrical configurations mostly for Reynolds number higher than 100. In the present study, the air-side heat transfer and pressure drop characteristics of the louvered fin micro-channeled, Aluminum heat exchangers are systematically analyzed by a 3D numerical simulation for very low Reynolds number from 25 to 200. Three different heat exchanger geometries obtained for the experimental investigation purposes with constant fin pitch (14 fins per inch) but varied fin geometrical parameters (fin height, fin thickness, louver pitch, louver angle, louver length and flow depth) are numerically investigated. The performance of the heat exchangers is predicted by calculating Colburn j factor and Fanning friction f factor. The effect of fin geometrical parameters on the heat exchanger performance at the Reynolds number range specified is evaluated. The air-side performance of the studied heat exchangers for the specified Reynolds number range is compared with experimental heat exchanger performance data available in the open literature and a good agreement is observed. The present results show that at the studied range of Reynolds number the flow through the heat exchanger is fin directed rather than the louver directed and therefore the heat exchanger shows poor performance. The effect of geometrical parameters on the average heat transfer coefficient is computed and design curves are obtained which can be used to predict the heat transfer performance for a given geometry.

scholarly journals Rayeilgh number effect on the turbulent heat transfer within a parallelepiped cavity

2011 ◽  
Vol 15 (suppl. 2) ◽  
pp. 341-356 ◽  
Author(s):  
Mohamed Aksouh ◽  
Amina Mataoui ◽  
Nassim Seghouani ◽  
Zoubida Haddad
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Natural Convection ◽  
Low Reynolds Number ◽  
Turbulence Models ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Practical Applications ◽  
Low Reynolds

This purpose is about a three dimensional study of natural convection within cavities. This problem is receiving more and more research interest due to its practical applications in the engineering and the astrophysical research The turbulent natural convection of air in an enclosed tall cavity with high aspect ratio (AR=H/W=28.6) is examined numerically. Two cases of differential temperature have been considered between the lateral cavity plates corresponding, respectively, to the low and high Rayleigh numbers: Ra=8.6?105 and Ra=1.43?106 [1]. For these two cases, the flow is characterized by a turbulent low Reynolds number. This led us to improve the flow characteristics using two one point closure low-Reynolds number turbulence models: RNG k-e model and SST k-w model, derived from standard k-e model and standard k-w model, respectively. Both turbulence models have provided an excellent agreement with the experimental data. In order to choose the best model, the average Nusselt number is compared to the experiment and other numerical results. The vorticity components surfaces confirm that the flow can be considered two-dimensional with stretched vortex in the cavity core. Finally, a correlation between Nusselt number and Rayleigh number is obtained to predict the heat transfer characteristics.

Internal Low Reynolds-Number Turbulent and Transitional Gas Flow With Heat Transfer

1966 ◽  
Vol 88 (2) ◽  
pp. 239-245 ◽  
Author(s):  
D. M. McEligot ◽  
L. W. Ormand ◽  
H. C. Perkins
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Gas Flow ◽  
Low Reynolds Number ◽  
Bulk Temperature ◽  
Temperature Ratio ◽  
Heating Rates ◽  
Reynolds Analogy ◽  
Number Range ◽  
Low Reynolds

The results of a semitheoretical and experimental investigation of the heat-transfer and frictional effects in air, nitrogen, and helium in steady flow in the downstream region of round tubes are presented. The constant-properties analysis for low Reynolds-number turbulent flow is evolved from an improved description of the adiabatic velocity profile, without modifying the Reynolds analogy assumption of equal eddy diffusivities. Data cover peak wall-to-bulk temperature ratios from near unity to 4.8 and entering Reynolds numbers from 1450 to 45,000. Low and moderate temperature-ratio data are used to confirm and to extend the analysis, while high temperature-ratio results are utilized for classification of flow regimes at high heating rates in the low Reynolds-number range.

Numerical and experimental investigation of tip leakage flow and heat transfer using idealised rotor-tip models at transonic conditions

2009 ◽  
Vol 113 (1141) ◽  
pp. 165-175 ◽  
Author(s):  
S. K. Krishnababu ◽  
H. P. Hodson ◽  
W. N. Dawes ◽  
P. J. Newton ◽  
G. D. Lock
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Discharge Coefficient ◽  
Fluid Dynamic ◽  
Low Reynolds Number ◽  
Analytical Models ◽  
Tip Leakage Flow ◽  
Mach Numbers ◽  
Low Reynolds

Abstract The effect of tip geometry on discharge coefficient and heat transfer is investigated both experimentally and numerically using idealised models of an unshrouded rotor blade. A flat tip was compared with two squealer-type geometries (a cavity and suction-side squealer) under the transonic conditions expected in the gas turbine engine. Heat transfer measurements were performed using a transient liquid crystal technique while a duplicate test section was used for measuring the pressure field. Computations were carried out using an unstructured, fully compressible, three-dimensional RANS (Reynolds averaged Navier Stokes) solver. Initial computations performed using a low Reynolds number k-ε model demonstrated the inability of the model to predict the Nusselt number with reasonable accuracy. Further computations performed using a low Reynolds number k-ω model improved the predictions dramatically. The computed discharge coefficient and the average Nusselt number over the blade tip agreed well with the experiments. Three upstream-total to exit-static pressure ratios were used to create a range of engine-representative Mach numbers. Both experimental and numerical studies at the lower pressure ratio of 1·3 (exit Mach number ~ 0·65) established the cavity geometry as the best performer from an aerodynamic perspective by reducing the discharge through the tip. However, from the heat transfer perspective, both the peak Nusselt number and the average heat transfer to the tip were higher than the flat tip. At the higher pressure ratios of 1·85 and 2·27 (corresponding to exit Mach numbers ~ 0·98 and 1·12) the discharge coefficient and heat transfer to the tip increases. This paper explores the fluid dynamics associated with these flows and shows that the highest heat transfer is caused by reattachment and flow impingement. The fluid dynamic computations provide insight into the experimental measurements and were successfully compared with simple analytical models.

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