Pulsating Flow Conditions Research Articles

Theoretical study on temperature oscillation of a parallel-plate in pulsating flow condition

Abstract Temperature distribution in a parallel-plate heater with the cooling of pulsating flow is theoretically studied. The results indicate the temperature of the heated plate fluctuates periodically as the flow pulsates. Effects of the wall thickness and its thermal properties, pulsating period and amplitude are analyzed based on the theoretical result. The fluctuation amplitude and the phase-lag of the outer surface temperature are magnified by the increase of plate thickness and the decrease of thermal diffusivity and pulsating period, and vice versa. It should be noted that the temperature difference between the outer wall and the inner wall, and the heat difference between the heat source and transported by the fluid caused by the unsteady condition should not be ignored. The results of the present study can be used to obtain a precise temperature fluctuation and the variation of the heat flux in unsteady flow condition.

Heat transfer enhancement with laminar pulsating nanofluid flow in a wavy channel

In this study, the heat transfer characteristics of Al2O3–water based nanofluids in a wavy mini-channel under pulsating inlet flow conditions are investigated numerically. The simulations are performed for nanofluid volume fractions, pulsating frequency and amplitude while the other parameters are kept constant by using control volume based cfd solver. The flow is both thermally and hydrodynamically developing while the channel walls are kept at a constant temperature. Results indicate that there is a good potential in promoting the thermal performance enhancement by using the nanoparticles under pulsating flow. Pulsation in nanofluids is a new idea for enhancement of heat transfer. Furthermore, the pulsating flow has an advantage to prevent sedimentation of nanoparticles in the base fluid. Results show that the heat transfer performance increases significantly with increase in nanoparticle volume fraction and with the amplitude of pulsation while the pulsation frequencies have a slight effect. In the pulsating flow conditions the combined effect of pulsation and nanoparticles is favorable for the increasing Nusselt number when compared to the steady flow case. The obtained results are given as dimensionless parameters.

A Three-Dimensional Computational Study of Pulsating Flow Inside a Double Entry Turbine

The double entry turbine contains two different gas entries, each feeding 180 deg of a single rotor wheel. This geometry can be beneficial for use in turbocharging and is uniquely found in this application. The nature of the turbocharging process means that the double entry turbine will be fed by a highly pulsating flow from the exhaust of an internal combustion engine, most often with out-of-phase pulsations in each of the two entries. Until now research on the double entry turbine under pulsating flow conditions has been limited to experimental work. Although this is of great value in showing how pulsating flow will affect the performance of the double entry turbine, the level of detail with which this can be studied is limited. This paper is the first to use a three-dimensional computational analysis to study the flow structures within a double entry turbine under conditions of pulsating flow. The analysis looks at one condition of pulsating flow with out-of-phase pulsations. The computational results are validated against experimental data taken from the turbocharger test facility at Imperial College and a good agreement is found. The analysis first looks at the degree of mass flow storage within different components of the turbine and discusses the effect on the performance of the turbine. Each of the volute limbs is found to be subject to a large degree of mass storage throughout a pulse cycle demonstrating a definite impact of the unsteady flow. The rotor wheel shows a much smaller degree of mass flow storage overall due to the pulsating flow; however, each rotor passage is subject to a much larger degree of mass flow storage due to the instantaneous flow inequality between the two volute inlets. This is a direct consequence of the double entry geometry. The following part of the analysis studies the loss profile within the turbine under pulsating flow using the concept of entropy generation rate. A significant change in the loss profile of the turbine is found throughout the period of a pulse cycle showing a highly changing flow regime. The major areas of loss are found to be due to tip leakage flow and mixing within the blade passage.

Pulsating flow driven alteration in moving contact-line dynamics on surfaces with patterned wettability gradients

The contact line dynamics over surfaces patterned with wettability gradients under pulsating flow condition are of essential importance in application areas ranging from the design of smart and effective microfluidic devices to the understanding of blood flow dynamics in narrow conduits. In the present study, we probe the capillary filling dynamics in a pulsatile flow environment, in an effort to explore the underlying flow physics. Presenting the results of frequency assisted contact line motion of two immiscible fluids over surfaces patterned with wettability gradients, we show how the interfacial dynamics are affected by the interplay of both the surface characteristics and flow pulsation. Our results reveal that the competition between two control parameters, the frequency and the amplitude of the imposed flow pulsation, may effectively be tuned to control the capillary filling dynamics significantly. The study, we present here, also suggests that by suitably tuning the control parameters, it is possible to control the capillary residence time over engineered locations which may, in turn, facilitate improved mixing and/or design of chemically active reaction stations.

Integration of meanline and one-dimensional methods for prediction of pulsating performance of a turbocharger turbine

Stringent emission regulations are driving engine manufacturers to increase investment into enabling technologies to achieve better specific fuel consumption, thermal efficiency and most importantly carbon reduction. Engine downsizing is seen as a key enabler to successfully achieve all of these requirements. Boosting through turbocharging is widely regarded as one of the most promising technologies for engine downsizing. However, the wide range of engine speeds and loads requires enhanced quality of engine-turbocharger matching, compared to the conventional approach which considers only the full load condition. Thus, development of computational models capable of predicting the unsteady behaviour of a turbocharger turbine is crucial to the overall matching process. A purely one-dimensional (1D) turbine model is capable of good unsteady swallowing capacity predictions, however it has not been fully exploited to predict instantaneous turbine power. On the contrary, meanline models (zero-dimensional) are regarded as a good tool to determine turbine efficiency in steady state but they do not include any information about the pressure wave action occurring within the turbine.This paper explores an alternative methodology to predict instantaneous turbine power and swallowing capacity by integrating one-dimensional and meanline models. A single entry mixed-flow turbine is modelled using a 1D gas dynamic code to solve the unsteady flow state in the volute, consequently used as the input for a meanline model to evaluate the instantaneous turbine power. The key in the effectiveness of this methodology relies on the synchronisation of the flow information of different time scales. The model is validated against experimental data generated at Imperial College London under steady and pulsating flow conditions. Three rotational speeds (27.0, 43.0, and 53.7rps/K) and four pulse flow frequencies (20 to 80Hz) are considered for performance validation. In addition to the turbine performance, the common level of unsteadiness is also compared based on Strouhal number evaluations. Furthermore, comparisons are made with the quasi-steady assumption in order to understand the strengths and weaknesses of the current method for effective unsteady turbine performance prediction.

Influence of speed and frequency towards the automotive turbocharger turbine performance under pulsating flow conditions

The ever-increasing demand for low carbon applications in automotive industry has intensified the development of highly efficient engines and energy recovery devices. Even though there are significant developments in the alternative powertrains such as full electric, their full deployment is hindered by high costing and unattractive life-cycle energy and emission balance. Thus powertrain based on highly efficient internal combustion engines are still considered to be the mainstream for years to come. Traditionally, turbocharger has been an essential tool to boost the engine power, however in recent years it is seen as an enabling technology for engine downsizing. It is a well-known fact that a turbocharger turbine in an internal combustion engine operates in a highly pulsating exhaust flow. There are numerous studies looking into the complex interaction of the pulsating exhaust gas within the turbocharger turbine, however the phenomena is still not fully integrated into the design stage. Industry practice is still to design and match the turbine to an engine based on steady performance maps. The current work is undertaken with the mind to move one step closer towards fully integrating the pulsating flow performance into the turbocharger turbine design. This paper presents the development efforts and results from a full 3-D CFD model of a turbocharger turbine stage. The simulations were conducted at 30,000rpm and 48,000rpm (50% and 80% design speed respectively) for both 20Hz and 80Hz pulsating flow inlet conditions. Complete validation procedure using cold-flow experimental data is also described. The temporal and spatial resolutions of the incidence angle at the rotor leading edge suggest that the circumference variation is little (7%) as compared to its variation in time as the pulse progresses. The primary aim of this paper is to investigate the relationship of the turbine speed, as well as the pulsating flow frequency to its performance. It was found that there are no direct instantaneous relationship between the pulsating pressure at the turbine inlet and the turbine efficiency, except when one considers an additional parameter, namely the incidence angle. This paper also intends to investigate the potential loss of information if the performance parameters are simply averaged without considering the instantaneous effects.

Radial Turbine Rotor Response to Pulsating Inlet Flows

The performance of automotive turbocharger turbines has long been realized to be quite different under pulsating flow conditions compared to that under the equivalent steady and quasi-steady conditions on which the conventional design concept is based. However, the mechanisms of this phenomenon are still intensively investigated nowadays. This paper presents an investigation of the response of a stand-alone rotor to inlet pulsating flow conditions by using a validated unsteady Reynolds-averaged Navier–Stokes solver (URANS). The effects of the frequency, the amplitude, and the temporal gradient of pulse waves on the instantaneous and cycle integrated performance of a radial turbine rotor in isolation were studied, decoupled from the upstream turbine volute. A numerical method was used to help gain the physical understanding of these effects. A validation of the numerical method against the experiments on a full configuration of the turbine was performed prior to the numerical tool being used in the investigation. The rotor was then taken out to be studied in isolation. The results show that the turbine rotor alone can be treated as a quasi-steady device only in terms of cycle integrated performance; however, instantaneously, the rotor behaves unsteadily, which increasingly deviates from the quasi-steady performance as the local reduced frequency of the pulsating wave is increased. This deviation is dominated by the effect of quasi-steady time lag; at higher local reduced frequency, the transient effects also become significant. Based on this study, an interpretation and a model of estimating the quasi-steady time lag have been proposed; a criterion for unsteadiness based on the temporal local reduced frequency concept is developed, which reduces to the Λ criterion proposed in the published literature when cycle averaged. This in turn emphasizes the importance of the pressure wave gradient in time.

Pulsating flow performance of a turbocharger compressor for automotive application

Downsizing with turbocharging is the most promising way, especially in terms of cost, to get reduced fuel consumption and CO2 emissions particularly in the case of Spark Ignition engines. In automotive applications the turbocharger turbine usually operates under heavy unsteady flow conditions due to the opening and closing of engine valves. However, in the case of extremely downsized engines with a reduced number of cylinders and a small intake circuit volume also the compressor performance can be affected by the unsteady flow generated by the engine intake valves.To make simulation models able to accurately predict engine performance, a better understanding of compressor and turbine pulsating flow performance can be accomplished through measurements performed on specialized test facilities, using suitable measuring equipment. As regards the turbocharger compressor, the surge line position under pulsating flow conditions is another important aspect to be considered.In the paper the results of a broad experimental investigation performed on a small turbocharger compressor matched to a downsized gasoline engine are presented. Measurements were developed on the test facility operating at the University of Genoa, which allows investigations on automotive turbochargers both under steady and unsteady flow conditions. Tested turbocharger compressor was coupled to the automotive engine intake circuit and the pulsating flow was generated by a motor-driven cylinder head fitted with a variable valve actuation system.Different levels of turbocharger rotational speed and different intake valve opening strategies were considered. For each operating condition compressor unsteady performance was evaluated starting from measurement of several instantaneous parameters (inlet and outlet static pressure, mass flow rate and turbocharger rotational speed). A significant deviation of compressor instantaneous performance from steady state was observed, resulting in a hysteresis loop surrounding the steady state curve.

Instantaneous pressure measurement in pulsating high temperature internal flow in ducts

Research tasks in internal combustion engines often require instantaneous pressure measurement of the gases inside the exhaust manifold including absolute value information. In such environment, flow operation includes high temperature and extremely pulsating conditions. The work described in the paper identifies the problems that arise when acquiring absolute pressure with a piezoresistive transducer. A methodology based on using an additional cooler for a piezoresistive sensor placed between the duct and the original sensor cooler adaptor is proposed for dynamic pressure measurement. Specific tests have been performed in cold and hot pulsating flow in order to check the influence of the additional cooler geometry. It is possible to remove the effect of the additional cooler but the procedure depends on the pressure in the duct. If relevant frequencies in the signal are similar to the natural frequency of the cooler, a simple filtering technique is not recommended. In this case, direct application of the acoustic theory inside ducts allows the reconstruction of the instantaneous pressure in the duct. Comparisons between the reconstructed pressure and measurements carried out with piezoelectric transducers, which do not capture the absolute value of the signal but can be placed in the duct without using an additional cooler, in hot and pulsating flow are presented. Results confirm that the method for pressure reconstruction from additional cooled piezoresistive transducers provides accurate information for instantaneous pressure measurement in hot and pulsating flow conditions inside ducts.

Importance of Mechanical Losses Modeling in the Performance Prediction of Radial Turbochargers under Pulsating Flow Conditions

Importance of Mechanical Losses Modeling in the Performance Prediction of Radial Turbochargers under Pulsating Flow Conditions

Investigation of the mixed flow turbine performance under inlet pulsating flow conditions

Turbochargers are widely used in Diesel engines as a means of increasing the output power. Most of them are fitted with radial or mixed flow turbines. In applications where high boost pressure is required, radial turbines are replaced by mixed flow turbines which can achieve a maximum efficiency at a lower value of blade speed to isentropic expansion velocity ratio than the usual 0.7 (for radial turbines). This study, performed with the ANSYS-CFX software, presents a numerical performance prediction of a mixed flow turbine under inlet pulsating flow conditions. In addition, the influence of the pulse frequency is studied and the numerical results are compared with those of a one-dimensional model and experimental data.

Engine turbocharger performance prediction: One-dimensional modeling of a twin entry turbine

As the automotive industry develops technology strategies to meet increasingly stringent vehicle emission regulations, turbocharging has become the primary enabler for engine downsizing, a building block for improving fuel consumption and reduced CO2 emissions. Engine manufacturers routinely use one-dimensional engine cycle simulation for performance and emissions prediction, and accurate engine-turbocharger matching is a key aspect. Turbocharger turbines are subject to the highly unsteady, pulsating flow inherent to reciprocating engines, however standard 1D turbine models rely on steady state test measurements. Simplification of turbine geometry is unavoidable, especially in 1D performance studies, yet this must not be taken so far that it prohibits acceptable prediction accuracy. This paper presents the geometrical effects of 1D numerical models of a twin entry turbocharger turbine under full admission pulsating flow conditions. Several turbine volute models of increasing complexity were developed and the corresponding performance predicted using a 1D compressible flow solver. The predicted mass flow rate is strongly dependent on local total state flow parameters, and higher secondary mass flow rate fluctuation was noticed as model complexity increased. Finally, a two-inlet constant cross-section model with junction tongue gave the best compromise of flow prediction accuracy and geometrical complexity.

Effects of pulse flow and leading edge sweep on mixed flow turbines for engine exhaust heat recovery

Recovery of heat energy from internal combustion engine exhaust will achieve significant road transportation CO2 reduction. Turbocharging and turbogenerating are most commonly used technologies to recover engine exhaust heat energy. Engine exhaust pulse flow can significantly affect the turbine performance of turbocharging and turbogenerating systems, and it is necessary to consider the pulse flow effects in turbine design and performance analysis. An investigation was carried out by numerical simulation on the mixed flow turbine pulse flow performance and flow fields. Results showed that the variations of the turbine efficiency and flowfiled under pulsating flow conditions demonstrate significant unsteady effects. The effect of blade leading edge sweep on turbine pulse flow performance was studied. It is shown that increasing of the leading edge sweep angle can improve the turbine average instantaneous efficiency by about 2 percent under pulsating flow conditions.

Steady and pulsating flow efficiency of a waste-gated turbocharger radial flow turbine for automotive application

In the paper the results of an experimental investigation developed on a small turbocharger turbine for automotive application are presented, including the effect of the waste-gate valve opening. The study was focused on the evaluation of turbine efficiency, especially under unsteady flow conditions typically occurring in automotive turbocharged engines. Turbine efficiency values measured under steady and pulsating flow conditions are compared, also considering a quasi-steady flow approach.

Unsteady Effect in a Nozzled Turbocharger Turbine

The unsteady behavior of a nozzled turbocharger turbine under pulsating flow conditions has been studied experimentally in a cold flow test facility that replicates engine pulses. The results presented are obtained at a turbine speed of 48,000 rpm for pulsating frequencies of 40 Hz and 60 Hz (which correspond to 1600 rpm and 2400 rpm in a twin turbocharger six cylinder internal combustion engine). The turbine unsteady behavior is compared for nozzle vane angles ranging between 40 deg and 70 deg. A nozzled turbocharger turbine is found to behave differently from a nozzleless turbine under pulsating flow. The existence of a nozzle ring “damps” the unsteady flow leading to a reduced level of flow dynamics affecting the turbine wheel for all vane angles. The bigger volume in the nozzled turbine is also another contributing factor to the observed characteristics. The results are more pronounced in the higher frequency and maximum vane opening condition. Given this “damping” behavior, the concept of unsteady efficiency is questioned. The level of unsteadiness in the flow is characterized by the relevant nondimensional parameters, and the onset of the unsteadiness in the flow and its effect on a nozzled turbocharger turbine is discussed. The onset of the unsteady effect is suggested to be at 40 Hz flow condition. However, the nozzled turbine exhibits more of filling and emptying characteristics for both the frequency conditions, especially at close nozzle position cases. The effect of unsteadiness on the instantaneous efficiency calculation is more pronounced in the nozzled turbine compared with a nozzleless turbine.

Viscoelastic changes in the blood and vascular wall in a pulsating circular flow

Viscoelastic changes in the blood and vascular wall in a pulsating circular flow Altered flow conditions, such as separation and recirculationg zones, low and oscillatory shear stress, play an important role in the development of arterial disease. Endothelial denudation by the blood flow is the first step in atherosclerosis. The description of blood flow in vivo is complicated due to the viscoelasticity of vessel walls. However, conventional researches of the effect of the blood vessel viscoelasticity on the blood pressure wave propagation using non-linear one-dimensional models do not take into account the viscoelasticity, despite it being importance in the analysis of pulse wave propagation in arteries. The purpose of this paper is to study the impact of the arterial pulse wave on the viscoelastic blood flow and initial factors of atherosclerosis. In 12 healthy men (25-39 years of age) peak velocity, mean velocity, mean flow and net flow in the aorta have been investigated by MR angiography. Initial velocity was registered after 43msec of the ECG-R wave, and it differed from zero at all sites of the aorta, although net flow was equal to zero. Womersley's number from the ascending to the thoracic aorta decreased from 12.5 ± 1.5 to 7.3 ± 1.2; flow modified from inertio-elastic to viscous. Pulse pressure wave move on artery walls fifteen or more times more rapidly than the blood flow. In the aortic arch in protodiastole blood flow separated into the opposite directed streams resulting in wave superposition with the high net flow. At the isthmus area separated waves interferences and reflects to anterograde direction. Pulse oscillation increases strain rate to the contiguous vessel wall flow layers. At the sites with the flow wave negative interference vessel pulse oscillation attenuates and at the boundary reflection flow wave can shift the vessel wall.

Investigation of Flow Structure in a Turbocharger Turbine under Pulsating Flow Conditions

Investigation of Flow Structure in a Turbocharger Turbine under Pulsating Flow Conditions

Effect of Superimposed Pulsating Flow on Liquid-Solid Mass Transfer in Fixed Beds

The liquid-solid mass transfer behaviour of a fixed bed of cylinders has been studied using the dissolution of copper in acidified dichromate solution under pulsating flow conditions. The bed diameter was 100 mm. The depth of the working section was 95 mm and the length and diameter of the cylinders were 19.1 mm. Variables studied were steady flow superficial liquid velocity, and oscillation amplitude and frequency. Data have been correlated as the Sherwood number in terms of Schmidt number and the cylinder Reynolds numbers for steady and oscillatory flow. The form of the correlation permits comparison with literature data for the limiting cases where either of these Reynolds numbers is zero.

High-Precision Digital Microflow Controllers Using Fluidic Digital-to-Analog Converters Composed of Binary-Weighted Flow Resistors

This paper presents digital microflow controllers (DMFC), where a fluidic digital-to-analog converter (DAC) is used to achieve high-linearity and fine-levels in flow-rate control for applications to precise biomedical dosing systems. The fluidic DAC, composed of binary-weighted flow resistance, is designed to control the flow-rate based on the ratio of the flow resistance to achieve finer flow-rate levels. The binary-weighted flow resistance has been specified by a serial or a parallel connection of an identical flow resistor to improve the linearity of the flow-rate control by making the flow-resistance ratio insensitive to the size errors of flow resistors in micromachining process. We have designed and fabricated three types of 4-digit DMFC: Prototype S and P with serial and the parallel combinations of an identical flow resistor and Prototype V with width-varied flow resistors. In the experimental study, we perform static DMFC tests at forward and backward flow conditions as well as dynamic DMFC tests at pulsating flow conditions. The present DMFC shows the nonlinearity of 5.0% and finer flow-rate levels of 16(2/sup N/) for 4(N) digital valves. From the measured flow-rate deviation due to micromachining errors, Prototype S and P show 27.2% and 27.6% of the deviation measured from Prototype V, respectively; thus, verifying that Prototype S and P are less sensitive to the micromachining errors than Prototype V.

Steady and Unsteady Velocity Measurements in a Small Turbocharger Turbine with Computational Validation

The detailed flow characteristics of three high-pressure-ratio mixed-flow turbines were investigated under both steady and pulsating flow conditions. Two rotors featured a constant inlet blade angle, one with 12 blades and the second with 10. The third rotor was shorter and had a nominally constant incidence angle. The rotors find application on an automotive high-speed large commercial diesel turbocharger.The steady flow entering and exiting the blades has been quantified by a laser Doppler velocimetry system. The measurements were performed at a plane 3.0-mm ahead of the rotor leading edge and 9.5-mm downstream the rotor trailing edge. The turbine test conditions corresponded to the peak efficiency point at two rotational speeds, 29,400 and 41,300-rpm. The results were resolved in a blade-to-blade sense to examine fully the nature of the flow at turbocharger representative conditions. A correlation between the combined effects of incidence and exit flow angle with the isentropic efficiency has been verified. Regarding pulsating flow, the velocity data and their corresponding instantaneous velocity triangles were resolved in a blade-to-blade sense to understand better the complex phenomenon. The results highlighted the potential of a nominally constant incidence design to absorb better the inadequacy of the volute to discharge the exhaust gas uniformly along the blade leading edge. A double vortex rotating in a clockwise sense propagated on the plane normal to the meridional direction. This should be attributed to the effect of the passing blade that was acting as a blockage to the flow. The phenomenon was more pronounced near the suction and pressure surfaces of the blade, but diminished at the mid-passage region where the flow exhibited its best level of guidance.The full mixed flow turbine stage under transient conditions was modelled firstly with a ‘steady’ inlet and secondly with a ‘pulsating’ inlet boundary condition. In both cases comparison was made to experiment performance and LDV measurements. With the steady inlet boundary condition, a high level of accuracy was achieved when compared to the experimental performance and velocity field. The velocity along the leading edge showed the same discrepancy as the single passage analysis that is with the radial and axial component from mid span to the blade tip. At the trailing edge features identified in the experimental data are identified in the numerical results; the velocity field appears more ‘diffused’ across the plane as per the experimental data than from the single passage analysis. With the pulsating inlet boundary, the predicted velocity traces in the volute and close to the turbine lead and trailing edge show excellent agreement in both form (against time) and magnitude.