Pulsating Flow Conditions Research Articles

An Experimental and Modeling Strategy for Obtaining Complete Characteristic Maps of Dual-Volute Radial Inflow Turbines

Abstract Despite the importance of turbocharged engines with dual-volute turbines, their characteristic maps and fully predictive modeling using 1D gas dynamic codes are not well established yet. The complexity of unsteady flow and the unequal admission of these turbines, when operating with pulses of engine exhaust gas, makes them a challenging system. This is mainly due to the unequal flow admission, which generates an additional degree-of-freedom with respect to well-known single entry vanned or vaneless turbines. This paper has as the main novelty a simple procedure for characterizing experimentally and elaborating characteristic maps of these turbines with unequal flow conditions. This method of analysis allows for easy interpolation within the proposed characteristic maps or conceiving simple models for calculating and extrapolating full performance parameters of dual-volute turbines. Two innovative 0D mean-line models are described that require a minimum quantity of experimental data for calibrating both: the mass flow parameter model and the isentropic efficiency model. Both models are predictive either in partial or unequal flow conditions using as inputs: the mass flow ratio and the total temperature ratio between branches; the blade speed ratio and the pressure ratio in each branch. These six inputs are generally instantaneously provided by 1D gas-dynamics codes. Therefore, the novelty of the model is its ability to be used in a quasi-steady way for dual volute turbines performance prediction. This can be done instantaneously when turbines are calculated operating at turbocharged engines under pulsating and unequal flow conditions.

Effects of turbocharger rotational inertia on engine and turbine performance in a turbocharged gasoline direct injection engine under transient and steady conditions

The effects of turbocharger (T/C) rotational inertia on engine and turbine performance under transient and steady engine conditions were analyzed in a 2.0 L 4-cylinder turbocharged-gasoline direct injection (T-GDI) engine. The test T/Cs consisted of heavy and light compressor wheels (C/W) and turbine wheels (T/W). The study was conducted in two research stages. First, transient engine load tests were conducted to evaluate the effect of T/C rotational inertia on transient response of the T/C, combustion performance, and fuel consumption. Seconds, steady engine load tests were conducted to find out if a light inertia T/C can run at higher efficiency under the same exhaust pulsating flow conditions within one engine cycle. In order to evaluate the engine on-board turbine instantaneous performances in the units of crank angle degree (CAD), T/C rotation speed and pressure data were measured in the experiment. The instantaneous exhaust gas mass flow rate and the temperature of upstream and downstream of the turbine were extracted by 1-D simulation. Turbine efficiency and mass flow rate parameters were calculated by combining these data. In the results, there existed positive effects of light inertia T/C on response and specific fuel consumption under transient conditions. It was also found that the light inertia T/C could show higher T/C speed fluctuation under the same exhaust pulsating flow conditions. Consequently, blade speed ratio (BSR) and turbine efficiency of light inertia T/C were partially higher than that of conventional one. However, it was not led to higher engine efficiency.

Stall behaviour of centrifugal compressor impeller for automotive turbocharger under pulsating flow conditions

Stall behaviour of centrifugal compressor impeller for automotive turbocharger under pulsating flow conditions

Influence of flow pulsations and yield stress on heat transfer from a sphere

In this work, the momentum and heat transfer characteristics of a time-dependent flow of Bingham plastic fluid over a heated isothermal sphere have been investigated numerically over wide ranges of conditions: Reynolds number, 5 ≤ Re ≤ 120, Bingham number, 0.1 ≤ Bn ≤ 100, Prandtl number, 0.7 ≤ Pr ≤ 100, frequency, π/4 ≤ ω* ≤ π and amplitude, 0 ≤ A ≤ 0.8. The influence of the fluid yield stress and inertia due to the imposed flow pulsations on the flow and thermal fields have been examined in detail. Detailed structure of the flow and temperature fields are analysed in terms of the instantaneous streamlines, isothermal contours, yielded/unyielded zones, surface pressure profiles, time-average drag coefficient and surface- and time-average Nusselt number. The influence of the frequency and amplitude of pulsations on the size of yielded (fluid-like) and unyielded (solid-like) zones is considered in order to understand convective heat transport. The size of yielded zones is seen to be in phase with the imposed pulsating velocity. However, the yield stress effects suppress the influence of flow pulsations. The temporal evolution of the drag coefficient and Nusselt number lag the imposed pulsating flow by different degrees thereby indicating the inherently different evolution of the momentum and thermal boundary layers. Broadly, the pulsating flow conditions may lead to up to 20% augmentation in heat transfer provided there is a moderate degree of advection and/or when the fluid yield stress effects are weak, i.e., small Bingham numbers. Thus, the maximum benefits of pulsating flow accrue in Newtonian fluids only. Finally, the present values of the time-average Nusselt number have been consolidated in the form of a predictive expression.

Impact of Lorentz force on the pulsatile flow of a non-Newtonian Casson fluid in a constricted channel using Darcy\u2019s law: a numerical study

The present paper examines the flow behavior and separation region of a non-Newtonian electrically conducting Casson fluid through a two-dimensional porous channel by using Darcy’s law for the steady and pulsatile flows. The vorticity-stream function approach is employed for the numerical solution of the flow equations. The effects of various emerging parameters on wall shear stress and stream-wise velocity are displayed through graphs and discussed in detail. It is noticed the increasing values of the magnetic field parameter (Hartman number) cause vanishing of the flow separation region and flattening of the stream-wise velocity component. The study also reveals that the non-Newtonian character of Casson fluid bears the potential of controlling the flow separation region in both steady and pulsating flow conditions.

Active Control of Passive and Active Particle Distribution at the Outlet of Double Y-Microchannel Using Pulsatile Flow

Abstract While a variety of active and passive techniques have been proposed for steady flows, pulsatile flow has received much less attention. Pulsation makes more control parameters available for passive methods and enables them to separate particles. The purpose of this work is to determine the effects of the phase shift between two entering flows (only one includes the particles) on particle separation inside a double Y-microchannel. Numerical simulations were carried out for both steady and pulsating flow conditions. The results showed that when the velocity amplitude ratio (β) is less than 2, the separation index increases with the phase shift (φ) and the highest efficiency occurs at φ = 180 deg. A similar trend can be observed for higher values of β only if the pulsation period is short enough. A series of experiments qualitatively validated the numerical results.

Optimization of a Radial Turbine for Pulsating Flows

Abstract A turbocharger turbine is exposed to pulsating flow conditions when it is connected to an engine exhaust system due to the opening and closing of the exhaust valves. However, many radial turbines are designed and tested under steady-state conditions without taking into account these unsteady exhaust flows. In order to seek the optimal aerodynamic design of a radial flow turbine (RFT) under pulsating flow conditions, the present research utilizes a numerical simulation approach to optimize the blade shape of a small-scale mixed flow turbine (MFT) under 50 Hz pulses. This corresponds to a four-stroke, three-cylinder engine rotating at 2000 rpm. In order to understand how a less computationally intensive, steady-state optimization compares, the blade shape was also optimized using the peak power point of the pulse. Three turbine features were modified during the optimization process, including blade cone angle, blade axial location, and blade camber angles. The optimization was carried out using a computational fluid dynamics (CFD)–genetic algorithm (GA) coupled approach, targeting at maximizing both energy-weighted efficiency and energy output during a predefined pulse period. To ensure that the new design maintains a similar matching to the engine, the maximum deviation of turbine swallowing capacity is controlled to within ±5% of the baseline for all new blade designs. The design that achieves the maximum pulse cycle-averaged efficiency was produced from unsteady optimization, with a performance benefit of 0.66%. The unsteady optimization also produced a blade shape that delivers the maximum energy output, with an improvement of 5.42%.

Pulsating Flow of CNT–Water Nanofluid Mixed Convection in a Vented Trapezoidal Cavity with an Inner Conductive T-Shaped Object and Magnetic Field Effects

Mixed convection of carbon-nanotube/water nanofluid in a vented cavity with an inner conductive T-shaped object was examined under pulsating flow conditions under magnetic field effects with finite element method. Effects of different parameters such as Richardson number (between 0.05 and 50), Hartmann number (between 0 and 30), cavity wall inclination (between 0 ∘ and 10 ∘ ), size (between 0.1 H and 0.4 H) and orientation (between −90 ∘ and 90 ∘ ) of the T-shaped object, and amplitude (between 0.5 and 0.9) and frequency (Strouhal number between 0.25 and 5) of pulsating flow on the convective flow features were studied. It was observed that the average Nusselt number enhanced with the rise of strength of magnetic field, solid nanoparticle volume fraction, and amplitude of the pulsation, while the effect was opposite for higher values of Ri number and cavity wall inclination angle. The presence of the T-shaped object and adjusting its size and orientation had significant impact on the main flow stream from inlet to outlet and recirculations around the T-shaped object and in the vicinity of hot wall of the cavity along with the magnetic field strength. Pulsating flow resulted in heat transfer enhancement as compared to steady flow case for all configurations. However, the amount of increment was different depending on the variation of the parameters of interest. Heat transfer enhancements were 41.85% and 20.81% when the size of the T-shaped object was increased from 0.1 H to 0.4 H. The T-shaped object can be utilized in the vented cavity as an excellent tool for convective heat transfer control. As highly conductive CNT particles were used in water, significant enhancements in the average Nusselt number between 97% and 108% were obtained both in steady flow and in pulsating flow cases when magnetic field was absent or present.

Analysis of the wall shear stress in a generic aneurysm under pulsating and transitional flow conditions

In this study, the flow through a generic abdominal aneurysm under realistic pulsating flow conditions is examined with magnetic resonance velocimetry (MRV), laser Doppler velocimetry (LDV) and computational fluid dynamics (CFD). The influence of flow phenomena on the wall shear stress (WSS) is examined. It is seen that a strong vortex ring develops during systole at the proximal end of the aneurysm and subsequently travels downstream and decays. The vortex formation plays a major role in the temporal and spatial distribution of the WSS, which is analyzed in detail. A peak of the WSS is observed for a very limited time and in a very localized region where the vortex ring initially develops. The intrinsic temporal averaging during the acquisition of the MRV data is found to significantly decrease this peak. CFD and LDV results, which are averaged in the same manner, show a similar behavior. This indicates that besides the spatial resolution, the temporal resolution is a crucial factor, which needs to be considered especially in flows where vortex rings are observed. Results from LDV and CFD show excellent agreement for the velocity field obtained by MRV. While the flow is found to be laminar in the undilated diameter, results show laminar–turbulent transitional behavior for specific phases of the cycle within the aneurysm bulk. Although MRV is not capable of measuring instantaneous velocity fluctuations, we show that the periodic increase in turbulence intensity can be observed from image artifacts in the MRV data. These artifacts increase the velocity uncertainty, which correlates well with the velocity fluctuations measured with LDV. Although the flow encounters laminar and transitional conditions as well as multiple vortices and stagnation and reattachment points, the improved instability-sensitive Reynolds stress model, which is used for the numerical simulations of this work, shows very good agreement with the measurements. Significant effort has been expended by numerous research groups in recent years in improving the estimation of WSS from MRV data. However, an assessment of these various post-processing methods is only possible if the true values of the WSS are known. The present study is therefore aimed at providing such ground truth WSS values as well as the corresponding MRV data, allowing also other research groups to validate their WSS estimation methods using the experimental data set presented in this work.Graphic

Application of particle image velocimetry measurement technique to study pulsating flow in a rod bundle channel

The flow rate of the primary coolant in a nuclear reactor will fluctuate during accident conditions or in floating reactors that are affected by inertial forces in the ocean. These fluctuations may have a substantial impact on heat transfer by the primary coolant in the reactor. In the current research, the flow field distribution and turbulence structure at different planes of a 5 × 5 rod bundle under pulsating flow conditions were measured using phase-locked particle image velocimetry and matching index refractive. The local ensemble-averaged velocity and root-mean-square (RMS) component distributions of different pulsating flow phases were obtained from the measurement results. This experimental study covered a pulsation period T range of 3 – 7 s and an imposed pulsation amplitude Au range of 0.31 – 0.65. For quasi-steady, intermediate-frequency, and high-frequency pulsating flows, the ensemble-averaged velocity of different phases and the distribution of velocity amplitude modulation in different subchannels were studied. Moreover, the phase shifts of the velocity modulation amplitude and RMS components of the rod bundle channel relative to the imposed pulsating flow with different pulsation parameters (Au, T) were also analyzed. Results for the rod bundle channel under the pulsating flow condition and for a conventional channel were compared; the discrepancies and agreements are discussed in detail. These research results can help to understand pulsating flow in a rod bundle channel better, and the experimental results can provide reference and validation data for numerical simulations.

Unsteady Internal Flow and Loss Mechanism of a Turbocharger Turbine Under Pulsating Flow Condition

Unsteady Internal Flow and Loss Mechanism of a Turbocharger Turbine Under Pulsating Flow Condition

Experimental investigation of pulsating flow structures and heat transfer characteristics in sinusoidal channels

In the present work, hydrodynamics and heat transfer characteristics in the sinusoidal channel are investigated experimentally for both steady and pulsating flow conditions. The experiments for heat transfer investigations were performed under a constant heat flux in the range of Strouhal number, St 0.11≤ St ≤2.07for the Reynolds number, in the range of 4 × 103≤Re≤7 × 103. After seeing the improvement of heat transfer with employing pulsation to the working fluid the hydrodynamics of pulsating flow was analyzed by considering the pulsating flow characteristics such as the time-averaged streamlines topology, 〈Ψ〉, streamwise velocity distribution, 〈u〉, cross-streamwise velocity distribution, 〈v〉, and turbulent Reynolds stress, u′v′¯/U2 using instantaneous flow data measured by the Particle image velocimetry (PIV) system. The results revealed that pulsating flow is highly effective for the lower turbulent flow case in the sinusoidal channel. As the Reynolds number increases, the effect of Strouhal number, St becomes less effective. There is an optimum Strouhal number,St value for different Reynolds numbers, Re to reach the maximum enhancement compared to steady flow cases.. The entrainment between the core flow and recirculating flow enhances the heat transfer rates in a steady flow. But the pulsating flow forces the recirculating flow zones in the diverging-converging section of the channel wave to exchange the flud from the core flow region further and that is an additional mechanism to upgrade the rate of heat transfer comparing to the steady flow cases.

Unsteady Flow Loss Mechanism and Aerodynamic Improvement of Two-Stage Turbine under Pulsating Conditions

The developments of two-stage turbocharging and turbocompounding promote the application of the two-stage turbine system in internal combustion engines. Since the turbine suffers from the pulsating exhaust, the performance deteriorates significantly from steady conditions. In the paper, the pulsating flow losses in the two-stage turbine are analyzed and a control method is proposed to improve the turbine performance. ANSYS CFX, which is a commercial software for computational fluid dynamic, is applied to resolve the three-dimensional unsteady flow problem. The accuracy of the simulation method is verified by the experimental data from each turbine. Firstly, the impacts of pulse amplitudes on transient loss of each component of the two-stage turbine are studied. Then flow field analysis is carried out to understand details of the unsteady flows. It is found that the variation of incidence angle at the low-pressure turbine (LPT) rotor tip is significantly larger than that at rotor hub, which causes severe flow loss near leading edge. As a result, the LPT performance drops down significantly. To improve the LPT performance, the blade shape at tip is modified. The aerodynamic performances of turbines with three different shapes under high- and low-load pulsating flow conditions are evaluated. It is found that increased inlet blade angle and medium thickness achieves good aerodynamic performance. The rotor averaged efficiency is improved by 2.27% under high-load pulsating condition.

Impact of the wastegate opening on radial turbine performance under steady and pulsating flow conditions

The present article attempts to describe the behavior of wastegated turbines under various steady and pulsating flow conditions. For this, meanline and one-dimensional numerical codes including appropriate modeling approaches for wastegated turbines have been developed with the FORTRAN language. These codes were validated against experiments with an established test rig at the National School of Engineers of Sfax. The discharge coefficient map of the wastegate was determined with a developed correlation built from experiments, and it was served as an input to the developed codes for interpolations during computation. This correlation is based on a two-dimensional non-linear dose-response fitting relationship instead of classical polynomial function which is one novelty of the article in addition to the one-dimensional modeling methodology. The normalized root mean square error (NRMSE) of both cycle-averaged efficiency and mass flow parameter (MFP) remains below 2% which confirms the validity of the proposed calculation approach. The results indicated a large deviation in the turbine performance under pulsating flow conditions compared to the steady state ones. The shape of the hysteresis loop of the turbine efficiency remains unchanged toward the variation of the wastegate valve angle at the same pulse frequency. The mass flow hystereses loop area is decreased by around 50% as the pulse frequency increases from 33 up to 133.33 Hz. An increase of less than 1% of the cycle-averaged efficiency has been reported when the bypass flow through the wastegate increases. The fluctuation of the efficiency is decreased by 1.5% when the wastegate valve becomes fully opened under the whole range of the pulse frequency.

Heat transfer in a triangular wavy channel with CuO-water nanofluids under pulsating flow

In this paper, heat transfer and pressure drop characteristics of CuO-water nanofluid flow in a isothermally heated triangular-wavy channel under pulsating inlet conditions are numerically investigated. A numerical simulation is conducted by solving the governing continuity, momentum, and energy equations for laminar flow using the finite volume approach. In the studies, the main parameters including the Reynolds number, pulsating amplitude and frequency, are changed while the nanoparticle volume fraction and the other parameters are kept constant for all cases. Numerical results are compared with the steady flow conditions, which showed that heat transfer performance significantly increases due to improve thermal conductivity and the use of nanoparticles in the pulsating flow conditions. The results indicate that there is a high potential for promoting the thermal performance enhancement by using nanoparticles under pulsating flow in wavy channels. It is found that the heat transfer enhancement increases with increasing pulsating amplitude and Reynolds number, and there is a slight increase in pressure drop. The obtained results are given as a function of dimensionless parameters.

A numerical study of heat transfer in the in-line tube bundle under pulsating fluid flow conditions

Shell-and-tube heat exchangers are widely used in different industries. Even a small increase in the efficien-cy of shell-and-tube heat exchangers can lead to significant energy savings. One of the ways to improve the efficiency of shell-and-tube heat exchangers is the use of pulsating flows for the enhancement of heat ex-change. Despite the fact that heat transfer in the tube bundle cross flow in steady-state conditions has been studied quite well, there is limited data on heat transfer in pulsating flow, which means that the problem of finding regularities of heat transfer with pulsating flows in tube bundles is still important. The work employs the incompressible Reynolds averaged Naviere-Stokes (URANS) equations and the continuity equation. Heat transfer is described by the convective heat transfer (Fourier-Kirchhoff) equation. The calculations are performed using Ansys Fluent. A numerical study has been conducted of the effects of forced asymmet-rical pulsating flow on heat exchange in in-line tube bundle cross-flow conditions. In the numerical experi-ment the Reynolds number Re ranged from 1000 to 2000, the relative pulsating amplitude A/D – from 1 to 2, the Strouhal number Sh – from 0,77 to 1,51, the Prandtl number and the duty cycle had fixed values: Pr = 7,2,  = 0,25. The relative transverse and longitudinal pitch was s1,2/D = 1,3. It has been found that pulsating flows lead to the enhancement of heat transfer in the whole range of the studied operating parameters. An increase in A/D and Sh leads to bigger Nusselt number Nu. An increase in the Re number leads to a de-crease in the Nu ratio in pulsating and steady flow conditions. The general correlation obtained based on the numerical study results can be used to predict heat transfer in a pulsating flow in the range of the studied geometric and operating parameters. More research is needed to predict heat transfer in a wider range of operating parameters and with other tube bundle configurations.

Experimental validation of a quasi-two-dimensional radial turbine model

This article presents the experimental validation of a quasi-two-dimensional radial turbine model able to be used in turbocharged reciprocating internal combustion engine simulations. A passenger car variable-geometry turbine has been tested under steady and pulsating flow conditions, instrumented with multiple pressure probes, temperature sensors and mass flow sensors. Using the data obtained, a pressure decomposition has been performed. The pressure at the turbine inlet and outlet has been split into forward and backward travelling waves, employing the reflected and transmitted waves to verify the goodness of the model. The experimental results have been used to compare the quasi-two-dimensional radial turbine model as well as a classic one-dimensional model. The quasi-two-dimensional code presents a good degree of correlation with the experimental results, providing better results than the one-dimensional approach, especially when studying the high-frequency spectrum.

Study on Heat Transfer Performance of Pulsating Flow in Convergent-Divergent Tube

The heat transfer performance of the pulsating flow in the convergent-divergent tube is analyzed. The influence of pulsation amplitude, pulsation frequency and divergent segment-convergent segment ratio on heat transfer and resistance of the tube was simulated by numerical simulation. The results show that the heat transfer performance of the pulsating flow in the convergent-divergent tube is better than the fluid steady flow in the tube. The heat transfer is enhanced by about 11.4% compared with the fluid flow in the tube was steady. The pulsating flow in the tube strengthens the heat transfer, but also increases the resistance. Through the analysis of enhanced heat transfer comprehensive index, the heat transfer performance of the tube under pulsating flow conditions is significantly enhanced.

Characterization of two-stage turbine system under steady and pulsating flow conditions

As the development of turbocompounding and two-stage turbocharging technology, there are increasingly more two-stage turbine systems applied on vehicles. The two-stage turbine recovers waste heat from the exhaust and thus affects the engine performance significantly. However, the interaction pattern and mechanism between the two turbines have not been fully understood. The paper focuses on the characteristic of the two-stage turbine under steady and pulsating flows.Firstly, an analytical model for two-stage turbine is developed to investigate the relationships between the turbines load split and equivalent area ratio under steady condition. The pattern of the load split between the high pressure turbine (HPT) and low pressure turbine (LPT) is disclosed by the model and verified by experimental data.Second, the impact of pulse frequency and amplitude on the two-stage turbine unsteady characteristic is studied by 3D computational fluid dynamics. As frequency increases, the peak values of the HPT expansion ratio and rotor torque increase drastically while those of LPT change little. As the pulse amplitude increases, the cycle-averaged expansion ratio of HPT decreases while that of LPT increases. The cycle-averaged rotor efficiencies of HPT and LPT reduce 3.7% and 8.1% respectively under 1.6 A amplitude condition, when compared with steady condition. In order to understand the phenomenon, the reasons are also discussed in detail in the paper.

Numerical investigation for convective heat transfer and friction factor under pulsating flow conditions

For pulsating flow, the behaviours of the convective heat transfer and friction factor for a periodic corrugated channel are investigated numerically. The finite volume method is used in the numerical study. Three different Reynolds Averaged Numerical Simulation based turbulence models, namely the k-ω model, the Shear Stress Transport (SST) model and the transition SST model are used and compared. The results are also compared with the previous experiments for non-pulsating flow. Analyses are conducted for air flow through a corrugated channel which has sharp corrugation peaks with an inclination angle of 30° and a 5mm minimum channel height. Reynolds number is changed in the range 6294 to 7380, while keeping the Prandtl number constant at 0.70. A sinusoidal pulsatile flow condition which is F=400 and uA*=0.5 is used. Variations of the Nusselt number and the friction factor with the Reynolds number are studied. Non-pulsating flow results and pulsating flow results are compared with each other.