Sassan Etemad

Numerical Investigation of Turbulent Heat Transfer in a Rectangular-Sectioned 90° Bend

ABSTRACT The flow and thermal fields in a rectangular-sectioned 90° bend with a cross-section aspect ratio of 6 were investigated using four turbulence models. All models managed to reproduce the general flow and thermal patterns. Chens high-Re k–ϵ model and Sugas cubic low-Re k–ϵ model performed well. The V2F k–ϵ model was found to be the least diffusive model and delivered good results. The RSM-GGDH model showed convergence difficulties and gave poor results. It was found that the boundary-layer thickness and the flow upstream of the bend are crucial for the character of the secondary flow, velocity profile, turbulence level, and heat transfer in the bend.

CFD-analysis of fully developed turbulent flow and heat transfer in a unitary cell of a cross corrugated plate pattern heat exchanger

The turbulent flow and the heat transfer in a unitary cell of a cross corrugated plate pattern heat exchanger has been studied using Chen’s high-Re k-e model, Suga’s low-Re k-e model, the RSM and the V2F model at a Reynolds number of 4930. The ability of these models in predicting the mean Nusselt number and Fanning friction factor has been investigated. The V2F model predicted higher heat transfer and friction factors than the other models. It was observed that the upper and lower flow in the unitary cell interact throw a shear process. This in turn initiates a complex secondary flow pattern which promotes the heat transfer. The V2F model predicted the strongest shear process. This may explain the fact that it also predicted the highest values of heat transfer and friction factor compared to the other models. The shear flow also caused high levels of turbulent kinetic energy in the centre of the unitary cell. The observed secondary motion is believed to be an efficient means of increasing the heat transfer coefficient with limited pressure drop penalty. It is also demonstrated that despite the geometrical complexity, high quality computational grids can be created and thereby details of the flow and heat transfer phenomena can be studied. The RSM appeared to be instable and gave results similar to Chen’s k-e model. Therefore, its use is not motivated for such applications.Copyright

Hydraulic and Thermal Simulations of a Cross-Corrugated Plate Heat Exchanger Unitary Cell

Simulations have been carried out by means of computational fluid dynamics for turbulent convective heat transfer in a cross-corrugated plate pattern heat exchanger unitary cell at a Reynolds number of 4930. Chens high-Reynolds-number k-ϵ model, the Reynolds stress model (RSM), Sugas low-Reynolds-number k-ϵ model, and the V2F model were used. The predicted hydraulic and thermal behaviors using these models have been compared and explained. The simulations showed that the interaction between the upper flow and lower flow in the cell creates a shear zone at their interface with a rotating motion and as a consequence the heat transfer is enhanced. This phenomenon was captured best by the V2F model, which predicted the highest Nusselt number and friction factor. Chens k-ϵ model provided results similar to the RSM. The fact that the RSM was instable and required more computational power makes it less interesting for further use in similar studies. It was also found rewarding to generate multiblock all-hexahedral grids to resolve the existing thermal and hydraulic phenomena in the domain.

On the Numerical Modeling of Impinging Jet Heat Transfer

Steady state numerical simulation of impinging jet heat transfer at H/D = 2 is methodically investigated for the following parameters: Turbulence models (k-ε low Re, k-ω low Re and V2F), grid density (0.25E06, 0.5E06, 1E06 and 2E06 cells), grid topology (2 types), inlet velocity corresponding to Re (10000, 20000 and 30000) and inlet velocity profile (uniform flow and fully developed flow). Effect of numerical discretization scheme is also investigated. The study resulted in 95 simulations. The simulation results are compared with different published experimental data. It was found that the V2F turbulence model performs best for these types of simulations. Furthermore, the choice of turbulence model and inlet velocity profile significantly affects the results. Grid topology is also found to be important. Careful CFD simulations are capable of predicting the heat transfer of impinging jets with good accuracy. (This work can be seen as a best practice guide for this type of simulations).

Turbulent flow and heat transfer in a square-sectioned U-bend

Turbulent flow and heat transfer in a square-sectioned U-bend are investigated. Turbulence models with linear and non-linear expressions for the Reynolds stresses are used. The near wall turbulence is treated by the damping functions approach and a two-layer model with Wolfshteins sub-layer treatment. The inlet conditions have a significant effect on the predictions. The results from the non-linear low-Reynolds number k-e models including Sugas cubic low-Re model were closest to experimental data. These models predicted the stress-induced secondary motion in the straight inlet duct well. This secondary motion had a small impact on the flow in the bend.

Prediction of developing turbulent flow in a rectangular-sectioned curved duct

The developing turbulent flow in a rectangular-sectioned curved duct with an aspect-ratio of 6 was explored using linear and non-linear high- and low-Re k-e turbulence models. The impact of the inlet flow conditions and grid density was also studied. It was found that the boundary layer thickness upstream the bend has fundamental influence on the secondary flow, velocity profile and turbulence level. The flow in the straight inlet duct was nearly 2-dimensional. The predicted data using Chen’s model and the quadratic high-Re model agreed well with the experimental data from Kim and Patel [1]. Also Suga’s cubic low-Re model performed well. The quadratic low-Re model, however, predicted thicker boundary layer which gave magnified secondary flow with high levels of shear and turbulence. It is not certain that the used high-Re models would perform well in a fully three-dimensional flow. Suga’s model, however, might perform well in other more complex flows.Copyright

Validation of URANS Simulation of Truck Cooling Fan Performance

The performance of an axial heavy duty truck cooling fan was investigated by measurements in a test rig and by CFD simulations. In order to account for the unsteadiness of the flow, URANS simulations were employed. Good agreement was achieved between the simulation and test data, in particular in the axial regime, despite the constant density assumption. To improve the simulation accuracy in the radial and transitional regime it is most likely insufficient to assume constant density. New simulations with ideal gas assumptions for these regimes are believed to give better agreement with the test data. The simulations show that URANS CFD can produce results very close to the ones obtained in the test facilities and thereby can be used for the industrial applications when flow unsteadiness has to be taken into account. The fact that it requires long computational time and is CPU-demanding can no longer be regarded as a major preventing factor for its application in the industry. In addition, it provides valuable information about the details of the flow which can contribute to the optimization of the geometry for improved efficiency and higher performance.Copyright

CFD Simulation of Thermocouple Measurement of Hot Gas Jets

Turbulent convective heat transfer and radiation is simulated for a hot gas jet, impinging perpendicular on a flat surface at 2 jet diameters away from the jet nozzle. A small solid spherical bead, located in the jet centre half way from the wall, represents a thermocouple sensitive point. The bead becomes so hot that it radiates some heat to the colder surrounding surfaces. This phenomenon is responsible for a gap between the jet temperature and the bead temperature. The jet Reynolds number ranged from 7.67*103 to 4.52*104 . Bead sizes 1.0 and 2.0 mm are used in jets at 500°C and 900°C. The simulations show that the mentioned temperature differences are significant and grow rapidly with high temperatures but decrease with Reynolds number. The temperature gap, which can be regarded as the thermocouple measurement error, increases also with the bead size. Simulations can be conducted for specific thermocouples with other shapes and materials to assist the measurement process. The modelling methodology is found to be promising for such demanding simulations that require a fine grid for resolving the field near the bead without using excessive cells in the rest of the domain. Hence, further work in this field is envisaged using the same methodology for solving convection, conduction and radiation in one single model and at a reasonable computational cost together with validating measurements. Hopefully this study contributes to a better understanding of the measurement of hot gas jet temperature and its improvement with the aid of simulations.Copyright

Analysis of Developing and Fully Developed Turbulent Flow and Heat Transfer in a Square-Sectioned U-Bend Duct

Turbulent flow and thermal field were predicted in a square-sectioned 180° bend at a Reynolds number of 56000. Sugas low-Re cubic k-e model [5-6] and the RSM [7-8] were used. The results were compared to experimental data [1]. Identical inlet boundary conditions were used in both cases. The inlet length impact on the flow-heat transfer in the bend was investigated. The velocities are higher near the inner wall and lower near the outer wall when a short inlet section is used. As the inlet length increases, the boundary layer grows thicker and the pressure-driven secondary vortex near the side wall becomes stronger. This vortex contributes significantly to the mixing process and heat transfer. It also alters the velocity distribution to a higher velocity near the outer wall and a lower velocity near the inner wall. When using a very long inlet length the vortex grows so strong that it generates a second counter-rotating vortex which isolates the fluid near the inner wall and prevents from further mixing. Consequently the local Nusselt number decreases. Both models reproduced the experimental data fairly well. Sugas model performed better and converged without problems. It is believed that Sugas model would be more suitable for industrial applications. (Less)