Quasi-steady Method Research Articles

Data-based estimation and simulation of compressible pulsating flow with reverse-flow through an orifice

Highly compressible pulsating flows are often encountered in devices where knowledge of the flow rate is required but elimination of pulsations is not an option. The current work is a continuation of a previous investigation that characterized the orifice discharge coefficient Cd as a function of dimensionless groups based on pulsation characteristics. The experimental apparatus has been rebuilt in the current work to mitigate temperature and vibration problems, allowing pressure and ΔP measurements to be made very close to the test section with 159-mm of nylon tubing. Data was acquired for 77 operating conditions spanning a range of pulsation frequencies, mass flow rates and system pressures. They confirm previously reported low Cd's in 0.20 range (calculated from time-averaged pressures) at some high-pressure low-flow operating conditions. Computational Fluid Dynamics (CFD) simulations of 12 of these data points suggest that the low Cd's result from reverse flow. Flow direction changed several times during each pulsation cycle closely tracking the orifice ΔP. A ‘core-and-sheath’ phenomena was observed for reverse-flow operating conditions: a positive core flow surrounded by a sheath of negative flow transitioned to a negative core and positive sheath several times during each pulsation cycle. The simulations also suggested that velocity profiles at the orifice stay stable and similar to steady-state profiles except for periods of rapid transitions. Based on these results a data-based quasi-steady method of estimating pulsating flow has been proposed. A pair of forward and reverse flow Cd's chosen by the data are used to predict instantaneous forward and reverse flows using the steady-state orifice discharge equation for compressible flow. The instantaneous values are then summed up over the pulsation cycle to estimate average mass flow rate. Average prediction errors were within 6%. A previously proposed method where regression was used to model Cd as a function of dimensionless groupings was shown to produce similar results. Both methods are designed to extract information from experimental data in order to overcome theoretical limitations and experimental error. The data is available upon request for further understanding of the flow physics.

Experimental study on gas permeability of granite close to natural joint

The metal corrosion, organic degradation and waste radiolysis in the geological disposal repository of high-level radioactive waste (HLW) will lead to gas generation and form additional pressure, which will drive the nuclide migration and thereby influence the long-term safety of geological disposal. It is crucial to study the gas permeability of the host rock for the geological disposal of HLW. Therefore, the gas permeability of some granite samples close to a natural joint in the Beishan pre-selected area for China’s geological disposal project was studied in this paper. Firstly, the effective gas permeability under different confining pressures, different gas pressures and different directions was measured by the quasi-steady method. Secondly, the absolute gas permeability and slip factor were obtained according to Klinkenberg’s law. Finally, the effect of confining pressure, direction and distance to the joint surface on gas permeability were analyzed. The gas seepage in granite is found to conform to Klinkenberg’s law. The absolute gas permeability is gradually decreased with the increase of the confining pressure. The gas permeability of granite is isotropic and homogeneous because the intrinsic permeability measured in different directions and at different distances from the joint is found to be similar. The above results can provide references for the performance assessment of the surrounding rock of the HLW disposal repository.

An Integrated Turbocharger Matching Program for Internal ‎Combustion Engines

Following the global environmental concerns, many automobile manufacturers intend to produce smaller ‎engines, aiming to lower emissions and fuel consumption. As compensation for performance reduction, these ‎engines are equipped with turbochargers. One of the challenges is to select the right turbocharger for a specific ‎engine. In this regard, an integrated zero-dimensional turbocharger and engine simulation program is ‎developed, employing a quasi-steady compressible flow method. The program gives the designer the ‎possibility of observing the performance of different compressor-turbine combinations on an internal ‎combustion engine. Engine details such as fuel, cylinder geometry, heat transfer conditions, valve timing, and ‎spark timing are considered within the engine modeling, and the designer can investigate their effects on the ‎overall performance of the system. Compressor and turbine performances are predicted by their previously ‎provided performance maps (steady-state characteristic curves) and special inter- and extrapolation methods. ‎A new algorithm for turbocharger matching is suggested, and its logics, basic convergence loops, and ‎thermodynamic equations have been described in detail. A database containing digitized compressor and ‎turbine maps and details provided by different manufacturers is created and integrated into the program. An ‎existing turbocharged engine has been tested and also simulated by the program. The accuracy of the program ‎results is evaluated by comparing them with experimental results. The maximum error of modeling the engine ‎and the whole simulation is 1.5% and 16%, respectively. Two other compressor-turbine combinations have ‎been evaluated using the program, one of which is suggested as an alternative‎‎.

Optimization of Nonlinear Pressure-Flow Characteristics of a Spring-Loaded Pressure Relief Valve Based on Computational Fluid Dynamics Simulation

Abstract In this study, the nonlinear pressure-flow characteristics of a spring-loaded pressure relief valve (PRV), which is used in the automotive fuel supply system for pressure control is analyzed, and its characteristics are improved by means of geometrical modifications of the valve structure. Given the complexity of the coupling mechanism between the valve internal flow characteristics and spring system, a quasi-steady computational fluid dynamics (CFD) method is introduced to predict the nonlinear pressure-flow characteristic curve of the valve and the accuracy is validated by experimental data. The total hydraulic force on the valve spool and diaphragm are divided into three parts according to the loading position and the correlation between the internal flow characteristics, hydraulic force, and pressure-flow characteristics of the valve are explained by CFD analysis and visualization. The results show that the quasi-steady CFD method can accurately predict the trends of the valve nonlinear pressure-flow characteristic curve, which is mainly determined by the hydraulic force produced in the middle chamber of the valve. When the valve opening reaches a certain value, a main vortex would be formed in the middle chamber and lead to the sudden increase of hydraulic force which causes the fluctuation of the pressure-flow characteristic curve of the valve. It is also found that the toggle point for the flow regimes seen in the valve is affected by the geometric structure of the middle chamber and the pressure-flow characteristics can be improved by the round corner size modification.

Aerodynamic Effects of an Incoming Vortex on Turbines With Different Tip Geometries

Abstract Experimental and numerical methods were used to investigate the aerodynamic effects of a near-casing streamwise incoming vortex flow on the tip leakage flow of different tip geometries in an unshrouded high-pressure turbine. A flat tip, a cavity tip, and a suction side winglet tip were investigated with the quasi-steady method first. A swirl generator was used to produce the incoming vortex in a linear cascade. In the flat tip case, the incoming vortex interacts with the tip leakage flow and the two vortices gradually mix together. The tip leakage loss is reduced due to the streamwise momentum supplement within the tip leakage vortex core. For the cavity tip, the tip leakage vortex appears at a location relatively downstream in the blade passage compared with the flat tip and no evident vortex interaction is observed. The incoming vortex causes extra viscous dissipation within the blade passage and increases the aerodynamic loss for the cavity tip. For the winglet tip, the extension of the suction side winglet tends to push the incoming vortex and the tip leakage vortex move and mix together, thus reducing the loss. Then, the effects of periodic unsteady vortex transportations were investigated by conducting unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations. The incoming vortex is stretched as it transports downstream. The unsteady incoming vortex is easier to interact with the tip leakage vortex for the winglet tip. As a result, the winglet tip is the most efficient tip design with unsteady incoming flow among the three tips and achieves a 3.7% reduction of mixed-out loss coefficient compared with the flat tip, larger than 2.8% reduction in the uniform inlet condition. The detailed loss mechanism is discussed in this paper.

A tribo-dynamic model of coupled journal-thrust water-lubricated bearings under propeller disturbance

The main purpose of this study is to develop a tribo-dynamic numerical model for coupled journal-thrust water-lubricated bearings (also referred as coupled bearings) used in rim driven thruster under propeller disturbance, in which the four degree of freedom (4-DOF) dynamic model of the propeller rotor is bridged with the transient mixed friction model of the coupled bearing. A validating experiment for the coupled water-lubricated bearing is performed to support the correctness of the developed tribo-dynamic model. In the model, the disturbing forces and moment caused by the propeller are calculated by the quasi-steady method. Based on the numerical study, the transient coupling effects between the dynamic characteristic of the propeller rotor and the mixed friction of the coupled bearing are revealed. Furthermore, parametric studies are carried out to identify the effects of the structural and working condition parameters on the tribo-dynamic responses of the coupled bearing. Numerical results indicate that disturbing moment leads to the increase in the transient contact force of the coupled bearing, and the coupling hydrodynamic pressure at the common boundary significantly affects the tribo-dynamic behaviors of the coupled bearing. Parametric studies demonstrate that there exists the optimal radial clearance and tilting angle to minimize the transient contact force of the coupled bearing during operation. In addition, numerical studies reveal that the propeller rotor mass is an important parameter affecting the tribo-dynamic responses of the thrust bearing.

Trajectory and attitude deviations for internal store separation due to unsteady and quasi-steady test method

The deviations of trajectory and attitude angle for internal store separation are evaluated by two wind tunnel test methods. One is the Freedrop Test (FDT), which is known as unsteady and time-dependent method of scaled model. The other is the Captive Trajectory System (CTS) test, which is usually regarded as a quasi-steady and time-averaged test technology. The result shows that there is a streamwise adverse pressure gradient on the cavity resulting in a nose-up pitching moment coefficient (>0) acting on the store model. When the initial pitch angular velocity is 0, the store exits the shear-layer with a nose-up pitch attitude causing the store to climb back towards and collide with the aircraft. However, the store passes through the shear-layer into the freestream with a nose-down pitch, which causes a successful separation event when the initial pitch angular velocity < 0. The pitch angle obtained by unsteady test method is different from that by quasi-steady test method. The time-dependent test includes the aerodynamic force induced by pitch angular velocity whereas the time-averaged method (CTS) cannot reflect the effect of unsteady aerodynamic force. The deviation of vertical displacement is not obvious for FDT and CTS test since the store has an initial vertical velocity, which is dominant for the vertical displacement. This means that the highly unsteady flow can create unpredictability in aerodynamic pitching moment of the store, which can lead to the deviation of pitch angle for internal store separation.

Pressure-based lift estimation and its application to feedforward load control employing trailing-edge flaps

Abstract. This experimental load control study presents results of an active trailing-edge flap feedforward controller for wind turbine applications. The controller input is derived from pressure-based lift estimation methods that rely either on a quasi-steady method, based on a three-hole probe, or on an unsteady method that is based on three selected surface pressure ports. Furthermore, a standard feedback controller, based on force balance measurements, is compared to the feedforward control. A Clark-Y airfoil is employed for the wing that is equipped with a trailing-edge flap of x/c=30% chordwise extension. Inflow disturbances are created by a two-dimensional active grid. The Reynolds number is Re=290 000, and reduced frequencies of k=0.07 up to k=0.32 are analyzed. Within the first part of the paper, the lift estimation methods are compared. The surface-pressure-based method shows generally more accurate results, whereas the three-hole probe estimate overpredicts the lift amplitudes with increasing frequencies. Nonetheless, employing the latter as input to the feedforward controller is more promising as a beneficial phase lead is introduced by this method. A successful load alleviation was achieved up to reduced frequencies of k=0.192.

Simulation of heat transfer processes in a diesel engine equipped with pistons coated by using Galvanic-plasma Method

The methodology for analyzing the thermodynamics and heat transfer of the combustion chambers of a diesel engine is presented. The work is devoted to the modified quasi-steady method (MQM) for the analysis of a diesel engine piston coated with an aluminum alloy. The oxide-coated piston using a galvanic-plasma modification (GPM) depending on the thickness of the coating, including comparison with the results for uncoated piston temperature, to achieve higher characteristics of the Cummins KTA-50 diesel engine. In thermodynamic modeling of a diesel engine, instantaneous gas temperatures and convective heat exchanges are first predicted. The time-dependent boundary conditions are then applied to the gas-blown surfaces of two-dimensional, transitional finite element models of the components of the combustion chamber. Further, the predictions on the finite elements of the instantaneous heat flux passing through each surface of the component are used to determine when the engine goes into quasi-stationary operation. The results show that our path in methodology can identify the complex transition paths of the heat process in the engine combustion chamber and significantly improve heat conduction and convection heat models when modeling a diesel engine.

CFD simulation of snow redistribution on a bridge deck: Effect of barriers with different porosities

Snow redistribution caused by snowdrift may worsen the traffic condition of bridge decks in snowy regions, hence it needs to be considered in bridge designs. In this study, a quasi-steady numerical method is adopted to simulate snowdrift on a typical bridge deck with barriers. The whole process of snowdrift on bridge decks is divided into several phases to simulate the variation in profile of snowpack, and a two-dimensional (2D) steady Computational Fluid Dynamics (CFD) simulation method is employed in each phase. To investigate the effect of barriers on the snow redistribution on bridge decks, the simulation results of wind velocity field, wall friction velocity, snow concentration field and snow redistributions are analyzed in detail. Generally, as the barrier porosity decreases, the total amount of snow erosion decreases due to the blocking effect of the barrier, but the decrease is limited when the barrier porosity is below 37.5%. Heavy snow accumulation mainly occurs behind the windward barrier when the barrier porosity is below 62.5%. To avoid snow accumulation, barrier porosity above 75% is suggested as the optimal choice for improving the traffic condition of bridge decks.

Does the Darcy–Buckingham Law Apply to Flow through Unsaturated Porous Rock?

The Darcy–Buckingham (DB) law, critical to the prediction of unsaturated flow, is widely used but has rarely been experimentally tested, and therefore may not be adequate in certain conditions. Failure of this law would imply that the unsaturated hydraulic conductivity is not constant for a given water content, as assumed in nearly all subsurface flow models. This study aims to test the DB law on unsaturated porous rock, complementing the few previous tests, all done on soils. Two lithotypes of calcareous porous rocks were tested. The quasi-steady centrifuge method was used to measure the flux density for different centrifugal driving forces while maintaining essentially constant water content, as required. Any deviations from the direct proportionality of the measured flux and the applied force would indicate a violation of the DB law. Our results show that, for the tested rocks and conditions, no physical phenomena occurred to cause a failure of the DB law.

Study on tie rod force characteristics in electro-hydraulic power steering system for heavy vehicle

The tie rod is one of the key components affecting the steering stability and reliability in the steering system of heavy vehicles. In order to meet the heavy load steering requirements of heavy vehicles, the steering system is universally the electro-hydraulic power steering system (EHPSS). The hydraulic actuators used in the EHPSS are double hydraulic cylinders, which are connected in series and symmetrically installed in this study. The hydraulic drive system in the steering system will affect the steering mechanism of stress state, leading to the tension of the tie rod in most working conditions. The tie rod be compressed only in extremely unequal loading of dual-tires. The tie rod force model is established based on Lagrange equation and verified by the tie rod test platform. Three key parameters describing the force rule of tie rod are obtained by adopting quasi-steady state analysis method, including pump pressure, difference value and sum value of left and right tires steering resistance torque. Pump pressure is the key factor affecting the tie rod force. The higher the pump pressure is, the greater the pull force of tie rod is. The steering resistance torque will further affect the variation law of the tie rod force. In the analysis when the tire at middle position, with the increase of difference value of steering resistance torque, the tie rod force offset between loading conditions and no-load conditions increases. When the tire turns from the middle to both sides, the increase of sum value of steering resistance torque will result in the rising of tie rod force change rate. This study reveals the variation law and key parameters of the tie rod force of heavy vehicles. It can provide basis for stability and reliability design of steering system.

Flow modeling and experimental verification of flow resistors used in microfluidic chips driven by capillary force

A microfluidic chip driven by capillary force has the advantage of low cost and ease of manufacturing in batches, and its flow behavior is dominated by the geometry and surface characteristics of the microchannel. The design of mirochannel structures is very important for the microfluidic chips. This study presents a semi analytical method for the design of capillary microchannel. In this method, the quasi-steady state approximate solution method of the Young–Laplace equation is used to derive the capillary surface tension, and the parallel flow assumption based on the Reynolds equation is used to derive the resistance of the channel wall. A computational fluid dynamics simulation is used to provide the inlet effect coefficient and channel shape effect coefficient of this model. The availability of the semi analytical model is verified by the experiment. This model realizes the flow analysis of two-dimensional capillary flow channel with a continuous shape of the flow channel wall, providing a fast and accurate method for the structural design of the microfluidic chip driven by capillary force.

Numerical study on coupling flight characteristics of a plate-type windborne debris

In this paper, the flight characteristics of plate-type windborne debris are examined by the CFD numerical simulation method taking account of two-way fluid-structure interaction (FSI) between the plate debris and the wind field. Two kinds of plates with different sizes and masses are selected as the research objects, and the SST k-ω turbulent model based on the RANS method is used to solve the flow field around the plates. The unsteady three-degree-of-freedom responses of the plates are obtained by solving the motion equations. The real-time position updating of the flying plates in the wind field is realized by dynamic mesh technique. The effects of different grid numbers and time steps on the parameters of flight time, flight velocities and horizontal displacement are analyzed in detail. At the same time, the flight trajectories, time histories of the flight displacement and velocity, and change laws of the flight attitude of the plate are obtained. The results of the comparative analysis show that the numerical results of the present unsteady method are close to the measurement of wind tunnel tests. Compared with the quasi-steady method and the analytical method, the coupling effect will significantly increase the horizontal velocity and the resultant velocity of the plate, which will cause the plate debris to fly a farther distance in the horizontal direction.

Colloidal hydrodynamics using a quasi-steady algorithm in lattice Boltzmann method

Hydrodynamics can play an important role in determining the behaviour of colloidal particles in many soft matter systems. Analytical solutions for fluid dynamics are limited and incorporating the particle dynamics in numerical methods is challenging, since grid points belonging to fluid and solid phases are exchanged during the simulations. As a solution, here, we introduce a quasi-steady method to simulate dynamics of particles within the frame work of lattice Boltzmann method. This method not only carries the advantages of lattice Boltzmann, namely the simple and straight forward algorithm of programming and the simplicity in imposing the boundary conditions, but it also avoids the complications associated with exchange of particle and fluid nodes. Exploiting the smallness of Reynold’s number associated with colloidal hydrodynamics, the proposed algorithm works in an instantaneous frame of reference and particle velocities are then calculated by imposing additional constraints of force and torque acting on the particle. We illustrate the method using the classic examples of settling particles and a system of recent interest-dynamics of active particles, both in the presence of a wall. Therefore, we expect the proposed method to be suitable and useful in variety of soft and active matter systems.

Building energy modeling at neighborhood scale

The urban climate and outdoor air quality of cities that have a positive thermal balance depending on the thermal consumptions of buildings cause an increase of the urban heat island and global warming effects. The aim of this work has been to develop an energy balance using the energy consumption data of the district heating network. The here presented engineering energy model is at a neighborhood scale, and the energy-use results have been obtained from a heat balance of residential buildings, by means of a quasi-steady state method, on a monthly basis. The modeling approach also considers the characteristics of the urban context that may have a significant effect on its energy performance. The model includes a number of urban variables, such as solar exposition and thermal radiation lost to the sky of the built environment. This methodology was applied to thirty-three 1 km × 1 km meshes in the city of Turin, using the monthly energy consumption data of three consecutive heating seasons. The results showed that the model is accurate for old built areas; the average error is 10% for buildings constructed before 1970, while the error reaches 20% for newer buildings. The importance and originality of this study are related to the fact that the energy balance is applied at neighborhood scale and urban parameters are introduced with the support of a GIS tool. The resulting engineering models can be applied as a decision support tool for citizens, public administrations, and policy makers to evaluate the distribution of energy consumptions and the relative GHG emissions to promote a more sustainable urban environment. Future researches will be carried out with the aim of introducing other urban variables into the model, such as the canyon effect and the presence of vegetation.

Quasi-steady state method for unsteady flow calibration of triangular weir

Calibration of a triangular weir discharge under unsteady flow condition using a quasi-steady state analysis is presented in this study. The weir was initially subjected to high flow supply corresponding high water head above the weir crest. The stop of this flow induced continuous drop in the water height with time. Measuring the initial and final water heights and the time elapsed for this height drop by using a stop watch, the weir coefficient of discharge can be obtained using a formula based on a quasi-steady state analysis for the unsteady flow over the triangular weir.The uncertainty in the measuring instruments and the obtained Cd was computed, allowing accurate representation of the experimental data. The uncertainty in the experimental measurements and calculated Cd was found to be lower than 5% for 87.5% of the test results. Uncertainty analysis was carried out to provide confidence in the proposed method and calculated data.

A Robust Adiabatic Model for a Quasi-Steady Prediction of Far-Off Non-Measured Performance in Vaneless Twin-Entry or Dual-Volute Radial Turbines

The current investigation describes in detail a mass flow oriented model for extrapolation of reduced mass flow and adiabatic efficiency of double entry radial inflow turbines under any unequal and partial flow admission conditions. The model is based on a novel approach, which proposes assimilating double entry turbines to two variable geometry turbines (VGTs) using the mass flow ratio ( MFR ) between the two entries as the discriminating parameter. With such an innovative approach, the model can extrapolate performance parameters to non-measured MFR s, blade-to-jet speed ratios, and reduced speeds. Therefore, the model can be used in a quasi-steady method for predicting double entry turbines performance instantaneously. The model was validated against a dataset from two different double entry turbine types: a twin-entry symmetrical turbine and a dual-volute asymmetrical turbine. Both were tested under steady flow conditions. The proposed model showed accurate results and a coherent set of fitting parameters with physical meaning, as discussed in this paper. The obtained parameters showed very similar figures for the aforementioned turbine types, which allows concluding that they are an adequate set of values for initializing the fitting procedure of any type of double entry radial turbine.

A Supercritical CO2 Waste Heat Recovery System Design for a Diesel Generator for Nuclear Power Plant Application

After the Fukushima accident, the importance of an emergency power supply for a nuclear power plant has been emphasized more. In order to maximize the performance of the existing emergency power source in operating nuclear power plants, adding a waste heat recovery system for the emergency power source is suggested for the first time in this study. In order to explore the possibility of the idea, a comparison of six supercritical carbon dioxide (S-CO2) power cycle layouts recovering waste heat from a 7.2 MW alternate alternating current diesel generator (AAC DG) is first presented. The diesel engine can supply two heat sources to the waste heat recovery system: one from exhaust gas and the other from scavenged air. Moreover, a sensitivity study of the cycles for different design parameters is performed, and the thermodynamic performances of the various cycles were evaluated. The main components, including turbomachinery and heat exchangers, are designed with in-house codes which have been validated with experiment data. Based on the designed cycle and components, the bottoming S-CO2 cycle performance under part load operating condition of AAC DG is analyzed by using a quasi-steady state cycle analysis method. It was found that a partial heating cycle has relatively higher net produced work while enjoying the benefit of a simple layout and smaller number of components. This study also revealed that further waste heat can be recovered by adjusting the flow split merging point of the partial heating cycle.

Towards nearly Zero Energy Buildings: Shape optimization of typical housing typologies in Ibero-American temperate climate cities from a holistic perspective

This research analyses the reduction of primary energy consumption (PE)11PE: primary energy. of slab and high-rise housing typologies through the optimization of their building shapes. The process considered applies passive strategies at the concept stage of the architectural design.A quasi-steady state method provides an estimation of building PE consumption in heating and cooling. Then, a multi-objective Genetic Algorithm (GA)22GA: Genetic algorithm. optimizes the building shapes with three competitive objective functions: minimization of PE consumption per m2 (PEI), and maximization of passive volume ratio (PVR)33PVR: Passive volume ratio is the ratio between the building passive and the useful volumes., and the sum of roofs and best oriented surfaces (RBOS)44RBOS: Best oriented surfaces are those looking South for the Northern Hemisphere and North for the Southern Hemisphere, and horizontal roofs.. The performance-driven design workflow shows the Pareto front solutions that reflect the influence of passive strategies on the building shapes. A performance simulation of the single tower with Energy Plus permits to evaluate the quasi-state method sensibility. Finally, a multivariate linear regression evaluates the interdependence among the optimized parameters.The methodology is tested in four cities with temperate climate: Resistencia and Buenos Aires in Argentina, and Seville and Madrid in Spain. It is suitable for the early stages of design since it allows designers to relate architectural morphologies with environmental issues to reduce energy consumption and Green-House Gasseś emissions within the current Climate Change scenario.