Peter Newton

The Effect of Unequal Admission on the Performance and Loss Generation in a Double-Entry Turbocharger Turbine

The current work investigates a circumferentially divided turbine volute designed such that each gas inlet feeds a separate section of the turbine wheel. Although there is a small connecting interspace formed between the nozzle and the mixed-flow rotor inlet, this design does well to preserve the exhaust gas energy in a pulsed-charged application by largely isolating the two streams entering the turbine. However, this type of volute design produces some interesting flow features as a result of unequal flows driving the turbine wheel. To investigate the influence of unequal flows, experimental data from the turbocharger facility at Imperial College have been gathered over a wide range of steady-state, unequal admission conditions. These test results show a significant drop in turbine performance with increasing pressure difference between inlets. In addition, the swallowing capacities of each gas inlet are interdependent, thus indicating some flow interaction between entries. To understand the flow physics driving the observed performance, a full 3D computational fluid dynamics (CFD) model of the turbine was created. Results show a highly disturbed flow field as a consequence of the nonuniform admission. From these results, it is possible to identify the regions of aerodynamic loss responsible for the measured performance decrease. Given the unequal flows present in a double-entry design, each rotor passage sees an abrupt change in flow conditions as it rotates spanning the two feeding sectors. This operation introduces a high degree of unsteady flow into the rotor passage even when it operates in steady conditions. The amplitude and frequency of this unsteadiness will depend both on the level of unequal admission and the speed of rotor rotation. The reduced frequency associated with this disturbance supports the evidence that the flow in the rotor passage is unsteady. Furthermore, the CFD model indicates that the blade passage flow is unable to fully develop in the time available to travel between the two different sectors (entries).

A comparison of timescales within a pulsed flow turbocharger turbine

Most modern turbocharger turbines are driven by a highly pulsating flow generated at the exhaust valve of an internal combustion engine. The amplitude and frequency of the exhaust pulses can influence the performance of the turbine when compared to steady state operation. It is useful to seek to simplify the problem of unsteadiness such that greater understanding may result. This paper uses a combination of experimental and computational results to study the various timescales associated with pulsed turbine operation. The effect of pulse amplitude and frequency on the unsteady flow in the volute and rotor is discussed.

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.

Effect of Exit Pressure Pulsation on the Performance and Stability Limit of a Turbocharger Centrifugal Compressor

It is well known that compressor surge imposes a significant limit on the flow range of a turbocharged internal combustion engine. The centrifugal compressor is commonly placed upstream of the inlet manifold and hence, it is exposed to the intermittent flow regime of the inlet valves. Surge phenomena has been well studied over the past decades, there still remains limited information with regards to the unsteady impact caused by the inlet valves. This study presents an experimental evaluation of such a situation. Engine representative pulses are created by a downstream system comprising a large volume, two rotating valves, a throttle valve and the corresponding pipe network. Different pulsation levels are characterized by means of their frequency and the corresponding amplitude at the compressor inlet. The stability limit of the system under study is evaluated with reference to the parameter B proposed by Greitzer [7-9]. B describes the dynamics of the compression system in terms of volume, area, equivalent length and compressor tip speed as well as the Helmholtz frequency of the system. For a given compressor, as B goes beyond a critical value, the system will exhibit surge as the result of the flow instability progression. The reduced frequency analysis shows that the scroll-diffuser operates in an unsteady regime, while the impeller is nearly quasi-steady. In the vicinity of the surge point, under a pulsating flow, the instantaneous operation of the compressor showed significant excursions into the unstable side of the surge line. Furthermore, it has been found that the presence of a volume in the system has the greatest effect on the surge margin of the compressor under the unsteady conditions.

A 3-Dimensional Computational Study of Pulsating Flow Inside a Double Entry Turbine

The double entry turbine contains two different gas entries, each feeding 180° 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 3 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.Copyright