Large-Eddy VS. RANS Simulations In Predicting Flow And Heat Transfer In A U-Duct With A Trapezoidal Cross Section

2018 ◽  
Author(s):  
Kenny Hu ◽  
Tom Shih
Keyword(s):  
Heat Transfer ◽  
Cross Section ◽  
Trapezoidal Cross Section ◽  
Flow And Heat Transfer ◽  
Large Eddy ◽  
Rans Simulations

Large-Eddy and RANS Simulations of Heat Transfer in a U-Duct With a High-Aspect Ratio Trapezoidal Cross Section

2018 ◽  
Author(s):  
Kenny S.-Y. Hu ◽  
Tom I-P. Shih
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Unsteady Flow ◽  
Cross Section ◽  
High Aspect Ratio ◽  
Scale Model ◽  
Trapezoidal Cross Section ◽  
Large Eddy ◽  
Rans Simulations ◽  
Straight Duct

Large-eddy and RANS simulations were performed to examine the details of the heat-transfer mechanisms in a U-duct with a high-aspect ratio trapezoidal cross section at a Reynolds number of 20,000. ANSYS-Fluent was used to perform the simulations. For the large-eddy simulations (LES), the WALE subgrid-scale model was employed, and its inflow boundary condition was provided by a concurrent LES of incompressible fully-developed flow in a straight duct with the same cross section and flow conditions as the U-duct. The grid resolution required to obtain meaningful LES solutions were obtained via a grid sensitivity study of incompressible fully-developed turbulent flow in a straight duct of square cross section, where data from direct numerical simulation (DNS) and experiments are available to validate and guide the simulation. In addition, the grid used satisfies Celik’s criterion, and resolves the Kolmogorov’s −5/3 law. Results were also obtained for the U-duct by using RANS, and three widely used turbulence models were examined — realizable k-ε with the two-layer model in the near-wall region, shear-stress transport (SST) model, and stress-omega full Reynolds stress model (RSM). Results obtained from LES showed unsteady flow separation to occur immediately after the turn region, which none of the RANS models could predict. By being able to capture this unsteady flow mechanism, LES was able to predict the measured heat-transfer downstream of the U-duct. The maximum relative error in the predicted local heat-transfer coefficient was less than 10% in the LES results, but up to 80% in the RANS results.

Steady RANS of Flow and Heat Transfer in a Smooth and Pin-Finned U-Duct With a Trapezoidal Cross Section

2018 ◽  
Author(s):  
Kenny S.-Y. Hu ◽  
Xingkai Chi ◽  
Tom I-P. Shih ◽  
Minking Chyu ◽  
Michael Crawford
Keyword(s):  
Heat Transfer ◽  
Cross Section ◽  
Relative Error ◽  
Turbulence Models ◽  
Separated Flow ◽  
Maximum Relative Error ◽  
Trapezoidal Cross Section ◽  
Flow And Heat Transfer ◽  
Pin Fins ◽  
Turn Region

Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε), shear-stress transport (SST), Reynolds stress model with linear pressure strain (RSM-LPS), and stress-omega RSAM (RSM-τω) — to predict the turbulent flow and heat transfer in a U-duct with a trapezoidal cross section and with and without a staggered array of pin fins. Results generated for the heat-transfer coefficient (HTC) were compared with experimentally measured values. For the smooth U-duct, the maximum relative error in the averaged HTC in the up-leg is 2.5% for k-ε, SST, and RSM-τω and 9% for RSM-LPS. In the turn region, that maximum is 14.5% for RSM-τω, 29% for SST, and 50% for k-ε and RSM-LPS. In the down-leg, SST gave the best predictions and RSM-τω being a close second with maximum relative error less than 10%. The ability to predict the secondary flow in the turn region and the separated flow downstream of the turn dominated in how well the models predict the HTC. For the U-duct with pin fins, k-ε predicted the lowest and the least accurate HTCs, and SST and RSM-τω predicted the best. For k-ε, the maximum relative error in the averaged HTC is about 25%, whereas it is 15% for the SST and RSM-τω, and they occur in the turn. In the turn region, the staggered array of pin fins was found to behave like guide vanes in turning the flow. The pin fins also reduced the size of the separated region just after the turn.

scholarly journals Large-Eddy Simulation Study of Flow and Heat Transfer in Swirling and Non-Swirling Impinging Jets on a Semi-Cylinder Concave Target

2021 ◽  
Vol 11 (15) ◽  
pp. 7167
Author(s):  
Liang Xu ◽  
Xu Zhao ◽  
Lei Xi ◽  
Yonghao Ma ◽  
Jianmin Gao ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Large Eddy Simulation ◽  
Wall Temperature ◽  
Target Surface ◽  
Impinging Jets ◽  
Small Scale ◽  
Complex Flow ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Large Eddy

Swirling impinging jet (SIJ) is considered as an effective means to achieve uniform cooling at high heat transfer rates, and the complex flow structure and its mechanism of enhancing heat transfer have attracted much attention in recent years. The large eddy simulation (LES) technique is employed to analyze the flow fields of swirling and non-swirling impinging jet emanating from a hole with four spiral and straight grooves, respectively, at a relatively high Reynolds number (Re) of 16,000 and a small jet spacing of H/D = 2 on a concave surface with uniform heat flux. Firstly, this work analyzes two different sub-grid stress models, and LES with the wall-adapting local eddy-viscosity model (WALEM) is established for accurately predicting flow and heat transfer performance of SIJ on a flat surface. The complex flow field structures, spectral characteristics, time-averaged flow characteristics and heat transfer on the target surface for the swirling and non-swirling impinging jets are compared in detail using the established method. The results show that small-scale recirculation vortices near the wall change the nearby flow into an unstable microwave state, resulting in small-scale fluctuation of the local Nusselt number (Nu) of the wall. There is a stable recirculation vortex at the stagnation point of the target surface, and the axial and radial fluctuating speeds are consistent with the fluctuating wall temperature. With the increase in the radial radius away from the stagnation point, the main frequency of the fluctuation of wall temperature coincides with the main frequency of the fluctuation of radial fluctuating velocity at x/D = 0.5. Compared with 0° straight hole, 45° spiral hole has a larger fluctuating speed because of speed deflection, resulting in a larger turbulence intensity and a stronger air transport capacity. The heat transfer intensity of the 45° spiral hole on the target surface is slightly improved within 5–10%.

LES Simulations of Rotating Two-Pass Ribbed Duct With Coriolis and Centrifugal Buoyancy Forces at Re=100,000

2016 ◽  
Author(s):  
Cody Dowd ◽  
Danesh Tafti
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Large Eddy Simulations ◽  
The Other ◽  
Coriolis Forces ◽  
Flow And Heat Transfer ◽  
Large Eddy ◽  
Buoyancy Forces ◽  
First Pass ◽  
Duct Geometry

The focus of this research is to predict the flow and heat transfer in a rotating two-pass duct geometry with staggered ribs using Large-Eddy Simulations (LES). The geometry consists of a U-Bend with 17 ribs in each pass. The ribs are staggered with an e/Dh = 0.1 and P/e = 10. LES is performed at a Reynolds number of 100,000, a rotation number of 0.2 and buoyancy parameters (Bo) of 0.5 and 1.0. The effects of Coriolis forces and centrifugal buoyancy are isolated and studied individually. In all cases it is found that increasing Bo from 0.5 to 1.0 at Ro = 0.2 has little impact on heat transfer. It is found that in the first pass, the heat transfer is quite receptive to Coriolis forces which augment and attenuate heat transfer at the trailing and leading walls, respectively. Centrifugal buoyancy, on the other hand has a bigger effect in augmenting heat transfer at the trailing wall than in attenuating heat transfer at the leading wall. On contrary, it aids heat transfer in the second half of the first pass at the leading wall by energizing the flow near the wall. The heat transfer in the second pass is dominated by the highly turbulent flow exiting the bend. Coriolis forces have no impact on the augmentation of heat transfer on the leading wall till the second half of the passage whereas it attenuates heat transfer at the trailing wall as soon as the flow exits the bend. Contrary to phenomenological arguments, inclusion of centrifugal buoyancy augments heat transfer over Coriolis forces alone on both the leading and trailing walls of the second pass.

Large Eddy Simulation of Flow and Heat Transfer Mechanism in Matrix Cooling Channel

2017 ◽  
Author(s):  
Yigang Luan ◽  
Lianfeng Yang ◽  
Bo Wan ◽  
Tao Sun
Keyword(s):  
Heat Transfer ◽  
Flow Field ◽  
Large Eddy Simulation ◽  
Gas Turbine ◽  
Transfer Mechanism ◽  
Heat Transfer Mechanism ◽  
Longitudinal Vortices ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Large Eddy

Gas turbine engines have been widely used in modern industry especially in the aviation, marine and energy fields. The efficiency of gas turbines directly affects the economy and emissions. It’s acknowledged that the higher turbine inlet temperatures contribute to the overall gas turbine engine efficiency. Since the components are subject to the heat load, the internal cooling technology of turbine blades is of vital importance to ensure the safe and normal operation. This paper is focused on exploring the flow and heat transfer mechanism in matrix cooling channels. In order to analyze the internal flow field characteristics of this cooling configuration at a Reynolds number of 30000 accurately, large eddy simulation method is carried out. Methods of vortex identification and field synergy are employed to study its flow field. Cross-sectional views of velocity in three subchannels at different positions have been presented. The results show that the airflow is strongly disturbed by the bending part. It’s concluded that due to the bending structure, the airflow becomes complex and disordered. When the airflow goes from the inlet to the turning, some small-sized and discontinuous vortices are formed. Behind the bending structure, the size of the vortices becomes big and the vortices fill the subchannels. Because of the structure of latticework, the airflow is affected by each other. Airflow in one subchannel can exert a shear force on another airflow in the opposite subchannel. It’s the force whose direction is the same as the vortex that enhances the longitudinal vortices. And the longitudinal vortices contribute to the energy exchange of the internal airflow and the heat transfer between airflow and walls. Besides, a comparison of the CFD results and the experimental data is made to prove that the numerical simulation methods are reasonable and acceptable.

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