Effect Of Inlet Turbulence Intensity Research Articles

Effects of inlet turbulence intensity on wall heat transfer in a turbine guide vane

Heat transfer is an important phenomenon that exists in many industrial applications, especially for gas turbines, aeronautical engines. In this work, two different turbulence models ([Formula: see text] and SAS model) are used to investigate the effects of inlet turbulence on wall heat transfer and the characteristics of flow field in a well-known turbine guide vane (LS89). In order to handle the transition, Menter’s [Formula: see text] transition model is used. The simulations show that the inlet turbulence has an apparent effect on the wall heat transfer of the vane. Not only the maximum wall heat transfer coefficient is increased, the distribution of wall heat flux at the suction side is also modified. The isentropic Mach number along the vane surface is insensitive to the variance of inlet turbulence intensity. Besides, a shock appears in the throat and a laminar-to-turbulence transition position moves forward after the main flow turbulence is enhanced. Moreover, the results indicate that SAS model is capable of capturing more flow structures such as reflecting pressure waves and shedding vortexes while the [Formula: see text] model misses them due to the dissipation.

Numerical analysis of micro-pin-fin heat sink cooling in the mainboard chip of a CPU

The cooling capability of heat sinks is important for a central processing unit (CPU). In this work, simulation has been done to investigate heat transfer (HT) in a heat sink (HS) mounted on the circular cylinder chip of a CPU that is studied and to explore the thermofluid behavior of the designed micro-pin-fin heat sink (MPFHS). Air cooling methods are used for heat extraction. This numerical work considers the effects of inlet turbulence intensity (TI) and fin diameter (D) of the micro-pin-fin on the performance of the HS. Turbulent SST model is used to explore turbulence regime in the system. The HT and pressure coefficient were obtained at different Reynolds number (Re) (i.e., different inlet velocities). As shown in this study, the Nusselt number (Nu) rises with the increase in air flow velocity which enhances the heat extraction from CPU.

CPU heat sink cooling by triangular shape micro-pin-fin: Numerical study

The cooling capability of heat sinks is important for a central processing unit (CPU). In this work, simulation has been done to study heat transfer (HT) in a heat sink (HS) mounted on the triangular cylinder chip of a CPU is studied, to explore the thermofluid behaviour of the designed micro-pin-fin heat sink. Air cooling methods are used for heat extraction. This numerical work considers the effects of inlet turbulence intensity (TI) and fin diameter (D) of the micro-pin-fin on the performance of the heat sink. Turbulent SST model is applied to capture turbulence regime in the system. The heat transfer and pressure coefficient were obtained at different Reynolds number (Re) (i.e. different inlet velocities). As shown in this study, the Nusselt number (Nu) increases with increase in air flow velocity which enhance the heat extraction from CPU.

The Effects of Inlet Turbulence Intensity and Computational Domain on a Nonpremixed Bluff-Body Flame

A bluff body burner was investigated using computational fluid dynamics (CFD) to assess the effects of inlet turbulence intensity and compare the combustion characteristics with and without the bluff-body modeled in the computational domain. The effects of the CFD modeling techniques were assessed for inlet turbulence intensity, using a two-dimensional (2D) versus three-dimensional (3D) computational domain, and whether to include the bluff body in the domain. The simulations were compared with experimental data from the Turbulent Nonpremixed Flames workshop. The results showed that the turbulence intensity specified as a boundary condition at the fuel-jet inlet had a substantial impact on the axial decay of mixture fraction and temperature, which was overlooked by previous researchers when the bluff body was not modeled. The numerical results of the 2D axisymmetric and 3D domains without the bluff body showed that the 3D domain provided the best predictions when the turbulence intensity was defined using a published correlation versus experimental estimates since the k–ε turbulence model underestimated dissipation. It was shown that a 2D axisymmetric domain can be used to obtain predictions with acceptable numerical errors without the inclusion of the bluff body, and that a uniform inlet velocity can be specified, provided that the inlet turbulence intensity is defined using the correlation by Durst et al. (“Methods to Set Up and Investigate Low Reynolds Number, Fully Developed Turbulent Plane Channel Flows,” ASME J. Fluids Eng., 120(3), pp. 496–503.). Finally, further analysis of flow and flame characteristics demonstrated that when the bluff-body was included for the 2D axisymmetric domain, predictions improved and the flow was insensitive to inlet turbulence intensities because the bluff-body provided an entrance region for the flow to develop before mixing, thus reducing inlet effects. Thus, if experimental inlet data are not available, the addition of the bluff-body in the computational domain provides a more accurate jet velocity profile entering the reacting domain and eliminates errors caused by the inlet boundary condition.

Comparison of Turbulence Modeling Strategies for Indoor Flows

Turbulence modeling techniques are compared for the simulation of low speed indoor air flow in a simple room. The effect of inlet turbulence intensity on the flow field is investigated using the constant coefficient large eddy simulation (LES) model with uniform mean inlet conditions at several levels of inlet turbulence intensities. The results show significant differences between the simulations with laminar inflow conditions and those in which turbulence was introduced at the inlet. For simulations with turbulent inlet conditions, it is noticed that the jet transitions to a state of fully developed turbulence wherein the dynamics of the flow become nearly insensitive to any further increase in the level of inlet turbulence. For laminar flow conditions, it is seen that the jet slowly spreads and mixes with the quiescent room air. As a result, the jet reaches a fully developed turbulent state further away from the inlet relative to the simulations with inlet turbulence. The effect of using experimental inlet profiles is also investigated. It is seen that, close to the inlet, the flow is sensitive to the inflow details, whereas further away from the inlet, these effects become less pronounced. The results from the constant coefficient and the dynamic LES models are compared. The most noticeable differences in the flow occur at the locations where the subgrid-scale’s contribution to the turbulent kinetic energy is highest. Finally, the results from the dynamic LES and the k-ϵ models are compared. It is found that there are significant differences between the two models for the zero inlet turbulence limit where the flow is most probably transitional in nature and turbulence has not yet reached a fully developed state. It is seen that in the laminar inflow case the k-ϵ model predicts a fully turbulent jet very close to the inlet and thus fails to capture the slow development of the jet found in LES. Accordingly, the k-ϵ model results are nearly insensitive to the level of inlet turbulence especially far from the origin of the flow. It is also seen that for cases with nonzero inlet turbulence level, the k-ϵ model predicts the general features of the mean flow reasonably well; however, the k-ϵ model overpredicts the jet spreading rate and the turbulent kinetic energy close to the inlet. Furthermore, the k-ϵ model under predicts the turbulence level near the corner of the ceiling as it fails to capture the complicated mean velocity and turbulent kinetic energy, most likely because of the highly intermittent flow pattern found there in LES.

Nusselt Numbers and Flow Structure on and Above a Shallow Dimpled Surface Within a Channel Including Effects of Inlet Turbulence Intensity Level

Abstract Experimental results from a channel with shallow dimples placed on one wall are given for Reynolds numbers based on channel height from 3,700 to 20,000, levels of longitudinal turbulence intensity from 3% to 11% (at the entrance of the channel test section), and a ratio of air inlet stagnation temperature to surface temperature of approximately 0.94. The ratio of dimple depth to dimple print diameter δ∕D is 0.1, and the ratio of channel height to dimple print diameter H∕D is 1.00. The data presented include friction factors, local Nusselt numbers, spatially averaged Nusselt numbers, a number of time-averaged flow structural characteristics, flow visualization results, and spectra of longitudinal velocity fluctuations which, at a Reynolds number of 20,000, show a primary vortex shedding frequency of 8.0Hz and a dimple edge vortex pair oscillation frequency of approximately 6.5Hz. The local flow structure shows some qualitative similarity to characteristics measured with deeper dimples (δ∕D of 0.2 and 0.3), with smaller quantitative changes from the dimples as δ∕D decreases. A similar conclusion is reached regarding qualitative and quantitative variations of local Nusselt number ratio data, which show that the highest local values are present within the downstream portions of dimples, as well as near dimple spanwise and downstream edges. Local and spatially averaged Nusselt number ratios sometimes change by small amounts as the channel inlet turbulence intensity level is altered, whereas friction factor ratios increase somewhat at the channel inlet turbulence intensity level increases. These changes to local Nusselt number data (with changing turbulence intensity level) are present at the same locations where the vortex pairs appear to originate, where they have the greatest influences on local flow and heat transfer behavior.

Computational Modeling of Three-Dimensional Endwall Flow Through a Turbine Rotor Cascade With Strong Secondary Flows

A steady, three-dimensional Navier–Stokes solver that utilizes a pressure-based technique for incompressible flows is used to simulate the three-dimensional flow field in a turbine cascade. A new feature of the numerical scheme is the implementation of a second-order plus fourth-order artificial dissipation formulation, which provides a precise control of the numerical dissipation. A low-Reynolds-number form of a two-equation turbulence model is used to account for the turbulence effects. Comparison between the numerical predictions and the experimental data indicates that the numerical model is able to capture most of the complex flow phenomena in the endwall region of a turbine cascade, except the high gradient region in the secondary vortex core. The effects of inlet turbulence intensity and turbulence length scale on secondary vortices, total pressure loss, and turbulence kinetic energy inside the passage are presented and interpreted. It is found that higher turbulence intensity energizes the vortical motions and tends to move the passage vortex away from the endwall. With a larger turbulence length scale, the secondary flow inside the passage is reduced. However, the total pressure loss increases due to higher turbulence kinetic energy production.

The Effect of Inlet Turbulence Intensity on the Reattachment Process Over a Backward-Facing Step

Behavior of a separated shear layer over a backward-facing step and its reattachment is presented when a two-dimensional cavity or rod is installed upstream of the step in order to change local turbulence intensity in addition to grid turbulence in the free-stream. The reattachment length has a strong negative correlation with maximum turbulence intensity near the wall at the separation point. Turbulence in the entrainment region immediately downstream of the step plays an important role in determining the reattachment length.