Flow Resistance Coefficient Research Articles

Experimental and theoretical investigations on reverse flow characteristics in vertically inverted U-tube steam generator under transient condition

Under natural circulation conditions, the primary circuit of a steam generator (SG) has a tendency to occur flow instabilities. These instabilities due to reverse flow in some vertically inverted U-tubes will increase the flow resistance coefficient and reduce the natural circulation ability in the primary system. To address the lack of existing research on transient reversed flow, in the present work, the reverse flow characteristics in the U-tubes of vertically inverted U-tube steam generator (UTSG) are investigated experimentally and theoretically. By gradually rising the inlet plenum mass flow of steam generator simulator at a constant fluid temperature, the reverse flow characteristics are observed in a thermal hydraulic test facility. Based on the experiment, the flow and heat transfer model in U-tubes are established to study the effects of change rate of mass flow and flow distribution on the reverse flow inside the UTSG. The experimental and theoretical results demonstrate that during the transient process, some U-tubes get reverse flow with the continuous decrease of mass flow and the reverse flow U-tubes also have a significant impact on other U-tubes, which results in a maximum temperature drop of about 35%. With the decrease of mass flow in U-tubes, the critical mass flow of the UTSG will increase and the reverse flow will occur more easily during the transient process. The results can provide a basis for avoiding the occurrence of reverse flow in actual SGs and improving the ability of natural circulation.

Flow resistance coefficient analysis of left anterior descending artery stenosis: A preliminary study.

As a non-invasive diagnosis method, computed tomographic angiography-based computational fluid dynamics is used to estimate fractional flow reserve of coronary arteries (FFRct). However, FFRct uses hypothetical hemodynamic flow conditions, and thus FFRct might cause mismatches (fractional flow reserve > 0.8) with invasive fractional flow reserve (≤0.8). Additional computational fluid dynamics-based criteria are still needed to improve the accuracy of non-invasive diagnosis. In this article, a new concept of computed tomographic angiography-based flow resistance coefficient (FRCct) is proposed, and it tests pressure drops at coronary arteries under different blood flow rates and returns two constant flow resistance coefficients (A and B) for each artery. Specifically, 30 patients who were suspected to meet the treatment indication of their left anterior descending stenosis were tested with invasive fractional flow reserve and FRCct. The invasive fractional flow reserve divided the patients into a safe group (invasive fractional flow reserve > 0.8, 15 patients) and a sick group (invasive fractional flow reserve ≤ 0.8, 15 patients). A following FRCct indicated that the flow resistance coefficient always displayed a low value (A = 0.0039 ± 2.7e-5; B = 0.079 ± 0.0025) for the safe group, while the flow resistance coefficient always exhibited a high value (A = 0.0235 ± 0.001; B = 0.270 ± 0.108) for the sick group. The results of the statistical test indicated that the p-value was less than 0.05 for both A and B of the two groups. In conclusion, in addition to the FFRct, FRCct is a supplementary non-invasive method to evaluate the treatment indication of left anterior descending stenosis.

One Dimensional Hydraulic Flow Routing Incorporating a Variable Grain Roughness Coefficient

The reach-average impacts of frictional forces, which retard flows in man-made channels and natural streams, are basically taken into account by flow resistance coefficients. These coefficients have been commonly treated as either a constant or a variable parameter, while the latter is only feasible through a tedious calibration process considering different flow and channel-boundary conditions. When neither historical records are available nor flow measurement is possible, applying a fixed-value roughness coefficient is practically inevitable. Although variation of Manning’s coefficient (n) with flow characteristics has been established in the literature, it has not been systematically implemented into hydraulic flow routing models, particularly because of the absence of a flow-dependent bed roughness predictor (BRP) suitable for numerical applications. In this study, a new grain roughness predictor, which provides derivations of n in respect with discharge and stage, is proposed. This grain roughness estimator, which enables to consider flow-dependent variation of n, is implemented in casting of governing equations of one-dimensional hydraulic flow routing method. In the numerical experiments designed to assess this implementation, three scenarios for n were considered: (1) constant n, (2) variable n computed using the new roughness predictor, and (3) variable n calculated based on the observed data. The third scenario, which requires a significant amount of field measurements, was considered as the benchmark solution. The obtained results showed that applying the proposed BRP to the hydraulic flow routing improved estimated outflows more than 40% based on the mean absolute relative error. The achieved improvement obviously demonstrates that considering variable resistance coefficient, like the one suggested in this study, may considerably improve the results of the flow-related numerical modeling.

Investigation on reverse flow characteristics in UTSGs with coupled heat transfer between primary and secondary sides

Under natural circulation conditions, the reverse flow phenomenon may occur within some U-tubes. This will result in the flow resistance coefficient of the steam generator (SG) obviously larger than that under forced circulation conditions, and the decrease of nature circulation capability in the primary circuit (PC). The heat transfer characteristics between the secondary circuit (SC) and PC in the SG are very complicated. Expressly, there is an obvious preheating section due to a certain fluid under-heat degree at the inlet of the ascending passage in the SC. In this paper, a type of marine SG is studied and a theoretical model of the SC single channel is established for the preheating sections (PS) corresponding to the ascending section (AS) and the descending section (DS) of U-tubes. Based on this model, the reverse flow characteristics of U-tubes are researched, and the influence of SC operating pressure, U-tubes pitch and circulation ratio on the U-tube reverse flow characteristics are calculated and analyzed. The results show that compared with the existing reverse flow calculation methods, the temperature distribution in the PS of SC increases exponentially and remains unchanged in the boiling section, and the critical flow rate is relatively higher. The increase of SC operating pressure and U-tubes pitch will decrease the mass flow rate of reverse flow, and the influence of circulation ratio has the opposite conclusion.

Multi-phase flow assessment for the fermentation process in mono-substrate reactor with skeleton bed

Abstract The selected techniques were reviewed and their technological aspects were characterized in the context of multi-phase flow for biogas production. The conditions of anaerobic fermentation for pig slurry in a mono-substrate reactor with skeleton bed were analysed. The required technical and technological criteria for producing raw biogas were indicated. Design and construction of the mono-substrate model, biogas flow reactor, developed for cooperation with livestock buildings of various sizes and power from 2.5 kW to 40 kW. The installation has the form of a sealed fermentation tank filled with a skeletal deposit constituting a peculiar spatial system with regular shapes and a rough surface. Incorporating a plant in such a production cycle that enables the entire slurry stream to be directed from the cowshed or pig house underrun channels to the reactor operating in the flow mode, where anaerobic digestion will take place, allows to obtain a biogas. The paper presents preliminary results of experimental investigations in the field of hydrodynamic substrate mixing system for biogas flow assessment by the adhesive bed in the context of biogas production. The aim of the study was to assessment and shows the influence of the Reynolds number on the biogas resistance factor for the fermentation process in mono-substrate reactor with adhesive deposit. The measurement results indicate a clear effect of the Reynolds number in relation to the descending flow resistance coefficient for the adhesive bed.

Evaluation of the parameters affecting the roughness coefficient of sewer pipes with rigid and loose boundary conditions via kernel based approaches

One of the important issues in water transport and sewer systems is determining the flow resistance and roughness coefficient. An accurate estimation of the roughness coefficient is a substantial issue in the design and operation of hydraulic structures such as sewer pipes, the calculation of water depth and flow velocity, and the accurate characterization of energy losses. The current study, applies two kernel based approaches [Support Vector Machine (SVM) and Gaussian Process Regression (GPR)] to develop roughness coefficient models for sewer pipes. In the modeling process, two types of sewer bed conditions were considered: loose bed and rigid bed. In order to develop the models, different input combinations were considered under three scenarios (Scenario 1: based on hydraulic characteristics, Scenarios 2 and 3: based on hydraulic and sediment characteristics with and without considering sediment concentration as input). The results proved the capability of the kernel based approaches in prediction of the roughness coefficient and it was found that for prediction of this parameter in sewer pipes Scenario 3 performed better than Scenarios 1 and 2. Also, the sensitivity analysis results showed that Dgr (Dimensionless particle number) for a rigid bed and wb/y (ratio of deposited bed width, wb, to flow depth, y) for a loose bed had the most significant impact on the modeling process.

The effect of roughness and rainfall on hydrodynamic properties of overland flow

AbstractOverland flow is the initial driver of slope surface erosion. To discover resistance characteristics of overland flow influenced by rainfall intensity and roughness, indoor simulated rainfall experiments with six kinds of roughness, five flow discharges, and five rainfall intensities were investigated. Results showed that overland flow over rough surfaces could be considered as laminar and turbulent flow when using flow Reynolds number. According to roll waves observed, flow regimes belonged to the laminar transitional zone based on the viscosity-to-depth ratio. A critical water depth formula for overland flow was re-derived, and it showed that this test water flow consisted of supercritical flow in most cases, and subcritical flow in only a few cases. The flow resistance coefficient increased with increasing roughness, whereas it decreased as rainfall intensity increased. Considering the ‘increasing resistance’ phenomenon, this study focused on frictional resistance, thickness of the viscous sublayer, pressure drag and roll waves. Finally, a formula for sheet flow resistance was proposed based on resistance segmentation and multi-element linear regression. These findings are of significance both for understanding the characteristics and development of overland flow and overland flow dynamics.

Flow resistance of ducting with triple-threaded auger inserts in laminar flow regimes

This paper describes the mathematical model of a laminar Newtonian flow in ducting with triple-threaded auger inserts and delivers the results of the solution to the problem. The values of the flow resistance coefficients have been calculated to demonstrate the purpose of the approach.

Influence of Geometrical Parameters on Heat Transfer of Transversely Corrugated Helically Coiled Tube for Deicing Fluid

In order to ensure flight safety in cold winter, aircraft ground deicing is crucial and necessary. In Chinese deicing fluid heating system, the helically coiled tube is paramount exchanger to heat deicing fluid. The deicing fluid is ethylene-glycol-based mixture with high viscosity. Aiming at heat transfer enhancement of deicing fluid, ring rib is formed by an embossed tube wall toward the internal of the tube; thus, transversely corrugated helically coiled tube (TCHC) is achieved. Depth and width are two key geometrical parameters of ring rib. Based on field synergy principle, the influence of depth–diameter ratio (H/D) and width-diameter ratio (w/D) is investigated through numerical simulation. The results show that outlet temperature, mean convection heat transfer coefficient, and Nusselt number have similar trends, which first increase and then decrease nonlinearly. The variation of flow resistance coefficient is inversely proportional to Reynolds number. Especially, the effect of H/D is more significant than that of w/D. Field synergy angle and velocity field are also analyzed to reveal the mechanism of heat transfer. TCHC performs better than the original tube. Orthogonal experiment calculates the outlet temperature of TCHC when H/D and w/D change. The combination of H/D=0.075 and w/D=0.5 is best solution. TCHC effectively enhances heat transfer of deicing fluid. Therefore, TCHC is beneficial to improve the deicing efficiency and ensure the flight punctuality.

Performance analysis and design optimization of heat pipe sink with a variable height fin array under natural convection

To address the cooling problems in electronic equipment with high heat flux but small area under natural convection, a heat pipe sink with a novel fin array of unequal and continuously changing heights is proposed to improve heat transfer by reducing flow resistance. By means of three-dimensional numerical calculations, the comparison of heat transfer performance of heat pipe sinks with different variable-height fin arrays is performed. The results show that the fin arrays with larger height difference have lower flow resistance and higher local heat transfer coefficient. Parameter studies show that increasing the fin spacing (s) and the maximum fin height difference (p) can greatly reduce the material cost per unit power, Mtot, but their effects on the total thermal resistance, Rtot, are more complex. A multi-objective optimization analysis is further conducted based on the response surface method with consideration of minimum Rtot and minimum Mtot as objectives. Additionally, the optimization technique was integrated with the Non-dominated Sorting Genetic Algorithm II (NSGAII) and the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) in order to improve the search speed and accuracy of Pareto solutions. By analyzing the Pareto solutions obtained, it is found that the influence of p and s on the system performance is related rather than independent.

Flow pattern of double-cavity flow at high Reynolds number

The flow structure in a turbulent double-cavity flow has been studied experimentally and numerically. The dynamics of the two-component instantaneous velocity vector fields measured by an optical smoke image velocimetry method and calculated using the ANSYS Fluent 19.2 software has been derived. For a wide range of dynamic similarity numbers of shape factor and ReL, the flow resistance coefficients for the cavity and relative flow mass transfer with the cavities have been estimated; three characteristic flow regimes of double-cavity flow have been distinguished and described; the flow pattern map via the ReL number and shape factor has been obtained.

Numerical Simulation of Small Defects Flow Resistance Characteristics of Thin-walled Components of Titanium Alloy with Adding Material Manufacturing Based on CT Data

In order to study the influence of different types of small defects on the flow resistance characteristics of thin-walled components, the method of micro-focus industrial CT images and numerical simulation was used. According to CT imaging technology, there were mainly three types of small defects on the inner surface of the runner: surface pores, surface inclusions, and surface cracks. Extracting small defects, refining the local grid, and establishing the finite element analysis model. Through finite element calculation software Fluent, a low Reynolds number k-e model was adopted to carry out numerical simulation and analysis on the flow channels with three small defects on the inner surface. The results showed that under the same Reynolds number condition, the flow resistance coefficient of surface cracks was the largest, which was much larger than that of surface inclusions and surface pores. The resistance of surface pores and surface inclusions to the flow passage was similar and relatively small. The flow resistance coefficient of three kinds of minor defects decreased with the increase of Reynolds number. In the case of low Reynolds number, the separation of flow boundary layer occurred at the surface cracks.

Multi-objective optimization of a new special-shaped tube for heating deicing fluid

The ground deicing heating system is an important equipment to ensure flight safety of civil aircraft in winter. As the main element, the heating tube directly affects the working efficiency of the system. The deicing fluid is high viscosity mixture. To improve heating efficiency of deicing fluid, a new special-shaped tube is proposed, which is named double-spiral trapezoidal grooved tube. The upper base width (m), groove depth (h) and base angle (θ) are the important structural parameters of the new tube. The optimal Latin hypercube design is applied to carry out the experiment design of m, h and θ. Nusselt number (Nu), flow resistance coefficient (f) and performance evaluation criteria (PEC) are selected as the optimization objectives. Then, the response surface approximation model is established to conduct sensitivity analysis. Based on the model, NCGA and NSGA-II algorithms are, respectively, introduced to perform the multi-objective optimization. The results show that NSGA-II is better than NCGA. When \(m \in \left[ {0.17, \, 0.33} \right]\), \(h \in [1.06, \, 1.08]\) and \(\theta \in \left[ {84.06, \, 84.75} \right]\), the best range of optimization objectives is \(Nu \in \left[ {86.04, \, 87.81} \right]\), \(f \in \left[ {0.70, \, 0.73} \right]\) and \({\text{PEC}} \in \left[ {1.60, \, 1.62} \right]\). With the growth of turbulence intensity, PEC > 1, and Nu increases by 4–27%. As a result, the heating efficiency of the new tube is superior to the original tube.

Computational Fluid Dynamic Modeling to Determine the Resistance Coefficient of a Saturated Steam Flow in 90 Degree Elbows for High Reynolds Number

The pressure drop in 90 deg elbows under the operating conditions of geothermal power plants in Mexico is studied using the computational fluid dynamics model. The elbow resistance coefficient was calculated for a steam flow with high Reynolds numbers (1.66–5.81 × 106) and different curvature ratios (1, 1.5, and 2). The simulations were carried out with the commercial software ANSYScfx, which considered the Reynolds-averaged Navier–Stokes (RANS) compressible equations and the renormalization group (RNG) k–ε turbulence model. First, the methodology was validated by comparing the numerical results (velocity and pressure) with published data of airflow (25 °C, 0.1 MPa) with high Reynolds numbers. Then, scenarios with different diameters (0.3–1.0 m) and conditions of the working fluid (0.8–1.2 MPa) were simulated to obtain velocity, pressure, density, and temperature profiles along the pipeline. The temperature and density gradients combined with the compressible effects achieved in the 90 deg elbows modified the flow separation, pressure drop, and resistance coefficient. Based on the resistance coefficient, factors were generated for a new equation, which was integrated into Geosteam.Net to calculate the pressure drop in a pipeline at the Los Azufres geothermal power plant. The difference with the data measured by a pressure transducer was 7.59%, while the equations developed for water or air showed differences between 11.23% and 45.22%.

Aerodynamic CFD simulations of experimental and industrial thermal flow reversal reactors

The paper presents a simplified CFD model suitable for simulating a Thermal Flow Reversal Reactor (TFRR) aerodynamics. This model may be used for an analysis of flow phenomena in a pilot reactor which is planned to be implemented for Ventilation Air Methane (VAM) combustion in a hard coal mine. Moreover, the model was to be used to compare the designed versions of geometries from the point of view of expected pressure drop of future plant, and also uniformity of flow velocity in the cross-section of the ceramic monolith packing.The ANSYS FLUENT simulations presented in the paper were divided into the two following stages:(a)Development of a simulation method allowing to couple simplified CFD simulations with results of separately performed calculations applying a dynamic model based on spatially one-dimensional partial differential equations of the dynamic mass and energy balances with own thermal combustion kinetics. The aerodynamic model was verified by comparing CFD simulation results with the experimental records taken from the research and demonstration (R&D) plant (cf. Section 2.2). The simulations revealed acceptable accordance with experimental results what shows that the model may be useful in practice. This stage was aimed to get reliability of further, forecasted results for an industrial plant in a larger size scale.(b)CFD simulations for currently designed larger pilot industrial flow reversal reactor with use the flow resistance coefficients obtained and verified during simulations of the much smaller experimental reactor aerodynamic model, referred to in the Section 3.1.

COMPUTATION CHEZY’S COEFFICIENT IN A SEMI-ELLIPTICAL PIPE

In the hydraulic field, the Chezy’s flow resistance coefficient in canals and pipes is often chosen arbitrarily. This value is tabulated independently of the depth of the flow or hydraulic radius and even less of the Reynolds number. This coefficient is usually influenced by a number of parameters that must be defined and considered. The objective of this study is to examine, on the one hand, the variation of the Chezy’s flow resistance coefficient for a semi-elliptical pipe under the hypothesis of an uniform flow with free surface, and to determine on the other hand, the expressions which govern them. Further, it must search expression for Chezy’s with consideration of the geometric characteristics of the pipe and hydraulic flow. This study is based on the rough model method (RMM).

Numerical investigation on turbulent oscillatory flow through a jet pump.

A jet pump with an asymmetrical channel can induce a time-averaged pressure drop in oscillatory flow, which can effectively suppress Gedeon streaming in looped thermoacoustic engines. In this work, the flow characteristics and time-averaged pressure drop caused by a jet pump in turbulent oscillatory flow are investigated through numerical simulation. Through the analysis of the dimensionless governing equations, the emphasis is put on the effects of Womersley number and maximum acoustic Reynolds number on the performance of the jet pump. Meanwhile, the steady flow resistance coefficients are also measured numerically. The results indicate that the oscillatory flow resistance coefficients are relatively insensitive to Womersley number when it is less than 46. Moreover, the oscillatory flow resistance coefficients agree well with the steady state flow results, which validate the quasi-static assumption in turbulent oscillatory flow. However, further increasing Womersley number will lead to a reduction in the time-averaged pressure drop. The simulation method and results, as well as the hydrodynamic mechanism beneath the results, are presented and discussed in detail.

Flow Pattern and Hydraulic Parameter Characteristics of the Different Topographic Position in the Small Catchment

Flow pattern and hydraulic parameter characteristics of the different topographic position in the “slope-gully-basin” system under the rainfall intensities of 60, 90 and 120 mm/h using generalized small watershed model with the simulated rainfall experiment. The results show that the increase of the rainfall intensity will result in the increase of the Reynolds number. During the whole experiment, only when the rainfall intensity is 60 mm/h, the flow pattern of the hilly-slope is laminar flow. The flow patterns of the other geomorphic position are all turbulent flow. Moreover, the Reynolds number of slope flow is much less than that of channel flow. With the increase of rainfall intensity, flow patterns of the all different geomorphic position changed from the stratum flow into torrent flow. Furthermore, the Froude number increases first and then decreases with the increase of rainfall intensity. For the resistance coefficient of the overland flow, with the increase of rainfall intensity, the resistance coefficient of overland flow and channel flow decreases obviously. For the spatial distribution of resistance coefficient, the maximum occurs at the hilly-slope and the minimum at the channel.

The research on coherent structure in 7-rod bundle lattice

The generation and development of coherent structure in 7-rod bundle lattice are researched by Unsteady Reynolds-Averaged Navier-Stokes (URANS) method with the Reynolds Stress Model (RSM). The influence of the coherent structure on the flow resistance and heat transfer coefficient is also analyzed. It is revealed that the coherent structure is helpful to improve the performance of lattice. Decreasing the pitch-to-diameter (P/D) or properly changing the shape of rod can contribute to the generation of coherent structure. The flow redistribution caused by corner channel has great influence to the coherent structure.

Performance and design of eccentrically-loaded concrete-filled round-ended elliptical hollow section stub columns

Concrete-filled round-ended elliptical hollow section (CFREHS) columns are gradually coming into use as piers and arches in engineering practice, owing to their unique properties concerning aesthetic perception and low flow resistance coefficients. However, there has been a paucity of studies focusing on the structural behaviour and design methods for CFREHS columns. The present study has involved investigating the behaviour of eccentrically-loaded CFREHS stub columns. A nonlinear numerical model that adopts an equivalent stress-strain model for the novel type of confined core concrete was established, and verified via experimental data. Subsequently, the effects of various parameters on the eccentric compressive response of CFREHS stub columns were analysed, including the diameter-to-thickness ratio, load eccentricity ratio, and the cross-section slenderness. Eccentric compressive capacities, failure patterns, strength indexes, stress–strain responses, contact stress, typical force-displacement curves, and M–N curves of eccentrically pressured CFREHS stub columns were also evaluated. The findings of the numerical analysis indicated that the eccentric load bearing capacity of CFREHS stub columns evidently increased for increases in the cross-section area, steel strength, and concrete strength, while the opposite was observed for increases in the load eccentricity ratio and diameter-to-thickness ratio. Finally, simplified empirical formulae were presented to predict the eccentric load bearing capacities of CFREHS stub columns. The results of the study are expected to provide a reliable reference for application to the proposed CFREHS columns in concrete-filled steel tube (CFST) structures.