A new CFD modeling method for flow blockage accident investigations

Culvert blockage at the inlet can dramatically alter the local hydraulic characteristics that often increase backwater rise, change the flow patterns, and thereby increase the risk of upstream flooding. The impact of culvert blockage is not usually included in the initial hydraulic design, although it can cause significant losses once the culvert becomes blocked. Culvert hydraulics including backwater rise with respect to blockage slope and ratio has been neglected in the studies conducted so far. In this study, we conducted a series of narrow flume tests to investigate the effects of four bottom blockage slope angles and five blockage ratios on the hydraulic characteristics of box culverts, including backwater rise, and upstream mean velocity at various Froude numbers. The results showed that Froude number, blockage ratio, and slope angle were regarded as the controlling variables for culvert backwater rise. Backwater rise increased dramatically with increasing blockage ratio and Froude number. Compared to blockage ratio B = 0.20, ΔH/hu increased by 2, 5, 9, and 13 times for B = 0.40, 0.60, 0.80, and 1, respectively. However, when the blockage slope angle decreased, the backwater rise reduced and upstream mean velocity increased in compared with vertical blockage. The relative backwater rise reduced from 0.32 to 0.29, when blockage slope angle decreased from 90° to 18.44° at B = 1 and Fo = 0.10. In addition, the length and maximum depth of water surface depression inside the culvert itself were larger with increasing blockage ratio and Froude number. For Fo = 0.10, d/ho = 0.13 and L/ho = 0.94 for B = 0.20 compared to d/ho = 0.32 and L/ho = 4.34 for B = 1. The results were also combined to develop an equation to predict backwater rise in the range 0.10 ≤ Fo ≤ 0.25, 0 ≤ B ≤ 1, and 18.44° ≤ α ≤ 90°. To enhance the accuracy, it is recommended to apply the equation for ΔH/hu ˃ 0.10. It is useful and practical in applications to add the probability of backwater rise or losses owing to culvert blockage within accounting of losses during the hydraulic design of culverts, especially in channels exposed to debris.

A numerical study of magnetohydrodynamic flows and heat transfer past a circular cylinder in a duct under a strong magnetic field parallel to the cylinder axis is presented. In this configuration, the flow is quasi-two-dimensional and the modified Navier–Stokes equations are solved in a two–dimensional domain. The numerical simulations have been performed over a range of parameters including the Reynolds number 50 ≤ Re ≤ 3000, modified Hartmann number 50 ≤ Ha⋆ . 500, blockage ratio 0.1 ≤ β ≤ 0.5, offset ratio 0.25 ≤ γ ≤ 1, velocity amplitude 0 ≤ A ≤ 3 and forcing frequency 0 ≤ Ste ≤ 10. The primary aim of this study is to understand the fundamental mechanism that governs transition to unsteady flow in this system and exploit this for further improvements in heat transfer for MHD cooling duct flows. With this aim in mind, a spectral-element method is employed to compute the MHD flow and heat transfer past a confined circular cylinder in a rectangular duct. Meshes have been constructed to deal with the significant number of geometric flow parameters combinations. Thorough validation and grid resolution studies have been performed to ensure adequate domain sizes, and spatial and temporal resolutions to accurately resolve all flow and thermal features for the reported flow variable ranges. Studies show that the reported flow parameters are converged to better than 0.3% in terms of spatial and temporal accuracy and 1% with respect to the domain size. For the optimal growth studies, the dependence of energy growth on upstream domain length is also considered through the calculation of the energy growth over a fixed time span. This verifies that the effect of truncating the upstream length from 32d to 8d causes an error of less than 3% in the growth rate prediction. The critical Reynolds number Rec for the transition from steady to periodic flow is determined as a function of Ha⋆ and β, and this is found to increase with increasing Ha⋆ and β. In addition, the variation of the wake recirculation length in the steady flow regime is determined as a function of Reynolds number, Hartman number and blockage ratio, and a universal expression is proposed. The characteristics of heat transfer depend strongly on the proximity of the cylinder to the heated wall. For small blockage ratios, it increases significantly as the gap ratio decreases from 1 to 0.25. However, there is a substantial drop in heat transfer for high blockage ratio. Downstream cross-stream mixing induced by the cylinder wake is found to increase the heat transfer augmentation by more than a factor of two in some cases. The maximum gain in heat transfer generated by placing the cylinder in the channel near to the wall with that at the centerline was obtained for the lowest blockage ratio β = 0.1, as the cylinder is further approached the heated wall. For all β, a very significant transient energy growth is found in the subcritical regime below the onset of vortex shedding. This suggests a potential for the design of an actuation mechanism to invoke vortex shedding and then enhance heat transfer in these ducts. The energy amplification of the disturbances is found to decrease significantly with increasing Hartmann number and the peak growth shifts towards smaller times while it increases significantly with increasing blockage ratio. The structure of the optimal initial disturbance is found to be consistent across the all blockage ratios being tested. In line with similar problems, it convects along the separating region being amplified to the peak growth state downstream of the recirculation bubble. The maximum time for maximum energy growth τmax is found to increase significantly as recirculation length increases which demonstrates the amplifying nature of the separated shear layers in the wake. The critical Reynolds number for the onset of positive growth at different Hartmann numbers and blockage ratios is determined. It is found that it increases rapidly with increasing Hartmann number and blockage ratio. For all β, the peak energy amplification grows exponentially with Re from low Hartmann numbers. Direct numerical simulation studies in which the inflow is perturbed by random white noise confirms the predictions arising from the transient growth analysis: that is, the perturbation excites and feeds energy into the global mode. A considerable increase in heat transfer from the heated channel wall occurs from rotational oscillation of the cylinder, with a maximum enhancement of approximately 22% observed at higher amplitude over that for steady flow. The oscillation frequency range for effective enhancement is widened in both directions, while the frequency at which the peak of the Nusselt number occurs is shifted slightly to a lower frequency as A is increased. It is found that as the amplitude was reduced, the forcing frequency approaches the global frequency mode.

Effects of trapezoidal bluff bodies on blow out limit of methane/air combustion in a micro-channel

Adsorption chillers can be a promising part of sustainable development concept of the global economy due to the utilization of low grade thermal energy sources for cooling production. Therefore, research aiming at improving their performance i.e. Coefficient of Performance (COP) by optimizing the heat and mass transfer condition in the adsorption beds are crucial. Innovative modification of the sorbent layer structure are proposed in the paper in order to improve the heat transfer characteristics in the heat exchanger boundary layer. The analysis of desorption conditions in the parametric model of a coated adsorption bed construction is presented in the paper. The computational fluid dynamics with conjugate heat transfer analysis are used to determine the crucial input parameters for further analytical calculations. The heat transfer condition in novel coated design and a conventional fixed bed are compared in the paper. The developed computational model consisted of three subdomains representing heating water, heat exchanger material (copper) and sorbent (silica gel).

Effect of blockage on critical ventilation velocity in longitudinally ventilated tunnel fires

With increasing inlet temperature of gas turbines, turbine blades need to be effectively protected by using cooling technologies. However, the deposition from the fuel impurities and dust particles in the air is often found inside film holes, which results in partial hole blockage. In this paper, the deposition geometry is simplified as a rectangular channel, and the effect of three blockage ratios is investigated by using the computational fluid dynamics. In addition, water droplets are also released from the coolant inlet to provide a comparison of the results with and without mist injection. It is found that the lateral film cooling effectiveness is reduced with increasing blockage ratio. For all the cases with the blowing ratio 0.6, 1% mist injection provides an improvement of the cooling performance by approximately 10%.

In this research, a Computational Fluid Dynamics (CFD) technique was used to investigate the effect of choking on the flow and heat transfer characteristics of a typical micro-channel heat sink. Numerical simulations have been carried out using Spalart–Allmaras model. Comparison of the numerical results for the heat transfer rate, mass flow rate and Stanton number with the experimental data were conducted. Relatively good agreement was achieved with maximum relative error 16%, and 8% for heat transfer and mass flow rate, respectively. Also, average relative error 9.2% was obtained for the Stanton number in comparison with the experimental values. Although, the results show that the majority of heat was transferred in the entrance region of the channel, but the heat transfer in micro-channels can also be affected by choking at channel exit. Moreover, the results clearly show that, the location where the flow is choked (at the vicinity of the channel exit) is especially important in determining the heat transfer phenomena. It was found that Spalart–Allmaras model is capable to capture the main features of the choked flow. Also, the effects of choking on the main characteristics of the flow was presented and discussed.

This paper researches the parametric optimization of a two-stage transonic compressor having a large air bypass at partial rotating speed according to flow analysis for a turbine-based combined cycle engine (TBCC). To obtain adequate thrust, the inlet transonic compressor of the turbofan part of the TBCC is required to have a wider frequently used corrected rotating speed range and a larger mass-flow rate at low rotating speed, which is different from a typical transonic compressor. The one-dimensional blade design parameters and flow path of the baseline two-stage transonic compressor are introduced. With the widely used CFD software Numeca, the three-dimensional flow fields of the baseline transonic compressor and effects of the flow path between Stage 1 and Stage 2 on the inlet mass flow rate are analyzed for indicating the further improvement direction. For design speed (NC = 1.0), to improve the efficiency at the design point, parametric research is carried out on Rotor 2 to optimize the shock structure and strength, resulting in enhanced efficiency at the design point due to reduced shock loss of Rotor 2. For partial speed (NC = 0.8 and 0.7), since the flow field analysis indicates that the flow blockage in S1 limits the entire mass flow rate, the parametric redesign of stator S1 aims at obtaining an increased blade throat width to enhance the flow capacity of S1. Simulation confirms the increase in the mass-flow rate and efficiency at partial speed due to the reduction in flow blockage and related viscous losses. Aerodynamic analysis at representative operation points indicates that the modifications of R2 and S1 lead to obvious aerodynamic improvement at all rotating speeds (NC = 1.0 to 0.7), while maintaining sufficient stall margin.

Hydrodynamic and heat transfer analyses were carried out for laminar fluid flow past a heated sphere placed centrally in a pipe using Computational Fluid Dynamics (CFD) simulations. Fully developed parabolic velocity profile characteristic of laminar flow was specified at the pipe inlet. The effects of blockage ratio, defined as the ratio of the sphere diameter to the pipe diameter, on the drag coefficient and Nusselt number are reported. The particle Reynolds numbers were varied up to a maximum of 500 while the Prandtl number for the water system studied was fixed at 5.12. At lower blockage ratios (<0.3), transient and three-dimensional simulations were required for particle Reynolds numbers exceeding 270. The effect of blockage was more significant at lower particle Reynolds numbers and influenced the drag coefficient more than the Nusselt number for this system. At high particle Reynolds numbers, reasonable predictions of the drag coefficient could be obtained even at high blockage ratios, by using the standard correlations available for a sphere immersed in an unbounded domain and the fluid approaching the sphere with a uniform velocity profile. Conversely, at low blockage ratios and low Reynolds numbers, considerable deviation from the standard drag correlation predictions was observed. Higher blockage ratios delayed boundary layer separation. The local Nusselt number distribution trends along the sphere are also reported. Correlations were developed for the drag coefficient, boundary layer separation angle and the average Nusselt number as a function of the blockage ratio and particle Reynolds number.

CFD based study of fluid flow and heat transfer effect for novel turning tool configured with internal cooling channel

Flashing-induced deposition poses a significant challenge in industrial heat exchange systems, where rapid pressure drops or local boiling lead to vapor generation and solute precipitation in confined regions. This mechanism is particularly critical in nuclear steam generators, where flow blockage near tube support plate (TSP) holes can compromise long-term operational safety and efficiency. To investigate this phenomenon, a three-dimensional four-channel computational fluid dynamics (CFD) model incorporating a two-fluid and phase-change approach was developed. Simulations reveal that flashing initiates at the TSP inlet, accompanied by a sharp increase in void fraction and the formation of low-pressure vortices. Variations in mass flow rate and inlet vapor quality significantly influence pressure drop and evaporation intensity, affecting bubble redistribution and pressure recovery. An empirical model was formulated to quantify flashing-enhanced deposition based on pressure-drop characteristics. The pressure-drop coefficient K follows a blockage-dependent power-law form (K = K0·Re−n), and a two-phase multiplier C = 4.84 accounts for vapor-phase effects. Model predictions show good agreement with CFD results across multiple blockage scenarios. This study not only reveals the formation mechanism of flashing-induced local deposition, but also provides theoretical support for fouling risk assessment and cleaning strategies in full-scale steam generator systems.

Effect of blade thickness on the hydraulic performance of a Francis hydro turbine model

Results are presented from an experimental study designed to obtain detailed radial heat transfer coefficient distributions applicable to the cooling of disk-cavity regions of gas turbines. An experimental apparatus has been designed to obtain local heat transfer data on both the rotating and stationary surfaces of a parallel geometry disk-cavity system. The method employed utilizes thin thermochromic liquid crystal coatings together with video system data acquisition and computer-assisted image analysis to extract heat transfer information. The color display of the liquid crystal coatings is detected through the analysis of standard video chromanance signals. The experimental technique used is an aerodynamically steady but thermally transient one, which provides consistent disk-cavity thermal boundary conditions yet is inexpensive and highly versatile. A single circular jet is used to introduce fluid from the stator into the disk cavity by impingement normal to the rotor surface. The present study investigates hub injection of coolant over a wide range of parameters including disk rotational Reynolds numbers of 2 to 5 × 105, rotor/stator spacing-to-disk radius ratios of 0.025 to 0.15, and jet mass flow rates between 0.10 and 0.40 times the turbulent pumped flow rate of a free disk. The results are presented as radial distributions of local Nusselt numbers. Rotor heat transfer exhibits regions of impingement and rotational domination with a transition region between, while stator heat transfer shows flow reattachment and convection regions with evidence of an inner recirculation zone. The local effects of rotation, spacing, and mass flow rate are all displayed. The significant magnitude of stator heat transfer in many cases indicates the importance of proper stator modeling to rotor and disk-cavity heat transfer results.

Results are presented from an experimental study designed to obtain detailed radial heat transfer coefficient distributions applicable to the cooling of disk-cavity regions of gas turbines. An experimental apparatus has been designed to obtain local heat transfer data on both the rotating and stationary surfaces of a parallel geometry disk-cavity system. The method employed utilizes thin thermochromic liquid crystal coatings together with video system data acquisition and computer-assisted image analysis to extract heat transfer information. The color display of the liquid crystal coatings is detected through the analysis of standard video chromanance signals. The experimental technique used is an aerodynamically steady but thermally transient one which provides consistent disk-cavity thermal boundary conditions while yet being inexpensive and highly versatile. A single circular jet is used to introduce fluid from the stator into the disk-cavity by impingement normal to the rotor surface. The present study investigates hub injection of coolant over a wide range of parameters including disk rotational Reynolds numbers of 2 to 5 · 105, rotor/stator spacing-to-disk radius ratios of .025 to .15, and jet mass flow rates between .10 and .40 times the turbulent pumped flow rate of a free disk. The results are presented as radial distributions of local Nusselt numbers. Rotor heat transfer exhibits regions of impingement and rotational domination with a transition region between, while stator heat transfer shows flow reattachment and convection regions with evidence of an inner recirculation zone. The local effects of rotation, spacing, and mass flow rate are all displayed. The significant magnitude of stator heat transfer in many cases indicates the importance of proper stator modeling to rotor and disk-cavity heat transfer results.

Blockage effect on wakes of various bluff bodies: A review of confined flow