Stephen Spence

A Comparison of the Flow Structures and Losses Within Vaned and Vaneless Stators for Radial Turbines

This paper details the numerical analysis of different vaned and vaneless radial inflow turbine stators. Selected results are presented from a test program carried out to determine performance differences between the radial turbines with vaned stators and vaneless volutes under the same operating conditions. A commercial computational fluid dynamics code was used to develop numerical models of each of the turbine configurations, which were validated using the experimental results. From the numerical models, areas of loss generation in the different stators were identified and compared, and the stator losses were quantifaed. Predictions showed the vaneless turbine stators to incur lower losses than the corresponding vaned stator at matching operating conditions, in line with the trends in measured performance. Flow conditions at rotor inlet were studied and validated with internal static pressure measurements so as to judge the levels of circumferential nonuniformity for each stator design. In each case, the vaneless volutes were found to deliver a higher level of uniformity in the rotor inlet pressure field.

An experimental investigation into the pressure drop for turbulent flow in 90° elbow bends:

Abstract The prediction of the pressure drop for turbulent single-phase fluid flow around sharp 90° bends is difficult owing to the complexity of the flow arising from frictional and separation effects. Several empirical equations exist, which accurately predict the pressure loss due to frictional effects. More recently, Crawford et al. [1] proposed an equation for the prediction of pressure loss due to separation of the flow. This work proposes a new composite equation for the prediction of pressure drop due to separation of the flow, which incorporates bends with ratio R/r < 2. A new composite equation is proposed to predict pressure losses over the Reynolds number range 4 × 103-3 × 105. The predictions from the new equation are within a range of −4 to +6 per cent of existing experimental data.

A Direct Performance Comparison of Vaned and Vaneless Stators for Radial Turbines

An extensive performance investigation has been conducted on a radial turbine with three different vaneless volutes and three corresponding vaned stators. Previously published comparisons have been based on turbines with unmatched flow rates, meaning that the impact of stator losses was not isolated from rotor and exit losses. Each vaned stator configuration tested in this investigation matched the flow rate of the corresponding vaneless volute to within 1%. The volutes and the vaned stators were all machined in order to achieve high quality and comparable surface finishes. At all operating conditions, the vaneless volutes were shown to deliver a significant efficiency advantage over the vaned stators. However, the vaneless volute turbines did not demonstrate any greater tolerance for off-design operating conditions than the vaned stator configurations. Full performance data are presented for the six different turbine configurations tested and a one-dimensional turbine performance model is evaluated as a means of predicting and extrapolating turbine performance.

Experimental performance evaluation of a 99.0 mm radial inflow nozzled turbine with different stator throat areas

Abstract The performance of a single-stage, radial inflow turbine was measured with seven vaned stators of different throat areas. The turbine test facility is described and details of the turbine rotor and stator geometry are provided. Efficiency, mass flowrates and rotor static pressure ratios are presented for each turbine configuration at a range of speeds and pressure ratios. The experimental data clearly shows the turbine mass flowrate increasing approximately in proportion to the stator throat area, while the maximum attainable efficiency decreased as the stator throat area decreased.

An experimental assessment of incidence losses in a radial inflow turbine rotor.

Abstract The non-optimum angle of incidence at the rotor inlet is the primary cause of reduced efficiency at off-design operating conditions in a radial turbine. Prediction of the complete performance characteristic, rather than just the design point, is desirable when considering the radial turbine as part of a dynamic system, such as a gas turbine engine, air cycle refrigeration plant or a turbocharger. The existing, rather unsatisfactory, incidence loss models are summarized and any published experimental information about the effect of incidence angle is reviewed. Through the analysis of data obtained from experiments conducted by the authors and reported elsewhere, correlations are drawn between the magnitude of the rotor loss and the gas incidence angle at the rotor inlet. As a consequence of the analysis, a negative value for the optimum incidence angle is confirmed and consistent trends are presented for the effects of varying incidence angle at different rotor speeds.

Using continuous assessment to promote student engagement in a large class

The authors have developed a first-year fluids course for a class of around 230 aerospace, civil and mechanical engineering students. This paper aims to show how the teaching and assessment methodology was applied to the challenge of a large class. The lectures featured formal teaching interspersed with active learning elements. Smaller group (about 25–30 students) tutorial classes involved student practice. A 10-minute test occurred in each tutorial during weeks 3–11. Each test was based on the previous weeks lecture material and the marks contributed towards 20% of the course mark. A condition for passing the course was that a student must pass at least six of the nine tests. The assessment promoted student involvement with the course – tutorial attendance was greater and more uniform than previously and exam performance improved significantly. Students recognised that the assessment system was useful in encouraging continuous learning over the semester and building confidence.

An investigation of the flowfield through a variable geometry turbine stator with vane endwall clearance

Abstract Variable geometry turbines provide an extra degree of flexibility in air management in turbocharged engines. The pivoting stator vanes used to achieve the variable turbine geometry necessitate the inclusion of stator vane endwall clearances. The consequent leakage flow through the endwall clearances impacts the flow in the stator vane passages and an understanding of the impact of the leakage flow on stator loss is required. A numerical model of a typical variable geometry turbine was developed using the commercial CFX-10 computational fluid dynamics software, and validated using laser doppler velocimetry and static pressure measurements from a variable geometry turbine with stator vane endwall clearance. Two different stator vane positions were investigated, each at three different operating conditions representing different vane loadings. The vane endwall leakage was found to have a significant impact on the stator loss and on the uniformity of flow entering the turbine rotor. The leakage flow changed considerably at different vane positions and flow incidence at vane inlet was found to have a significant impact.

The technical merits of turbogenerating shown through the design, validation and implementation of a one-dimensional engine model

Turbocompounding is the process of recovering a proportion of an engine’s fuel energy that would otherwise be lost in the exhaust process and adding it to the output power. This was first seen in the 1930s and is carried out by coupling an exhaust gas turbine to the crankshaft of a reciprocating engine. It has since been recognised that coupling the power turbine to an electrical generator instead of the crankshaft has the potential to reduce the fuel consumption further with the added flexibility of being able to decide how this recovered energy is used. The electricity generated can be used in automotive applications to assist the crankshaft using a flywheel motor generator or to power ancillaries that would otherwise have run off the crankshaft. In the case of stationary power plants, it can assist the electrical power output. Decoupling the power turbine from the crankshaft and coupling it to a generator allows the power electronics to control the turbine speed independently in order to optimise the specific fuel consumption for different engine operating conditions. This method of energy recapture is termed ‘turbogenerating’. This paper gives a brief history of turbocompounding and its thermodynamic merits. It then moves on to give an account of the validation of a turbogenerated engine model. The model is then used to investigate what needs to be done to an engine when a turbogenerator is installed. The engine being modelled is used for stationary power generation and is fuelled by an induced biogas with a small portion of palm oil being injected into the cylinder to initiate combustion by compression ignition. From these investigations, optimum settings were found that result in a 10.90% improvement in overall efficiency. These savings relate to the same engine without a turbogenerator installed operating with fixed fuelling.

Experimental performance evaluation of a 99.0 mm radial inflow nozzled turbine at larger stator/rotor throat area ratios.

Abstract Following on from experimental data already presented by Spence and Artt in 1997, further comprehensive performance tests were carried out on a 99.0 mm radial turbine with three larger vaned stators. The results have helped to confirm some of the trends noted in the earlier tests. Measurements of the efficiency, mass flowrate and rotor pressure ratio are presented together with the results of a simple loss analysis. For smaller stator sizes the turbine mass flowrate is shown to be proportional to the stator throat area. The significance of the stator-rotor throat area ratio is examined and its dependence on the stage pressure ratio is noted.

Improved Performance of a Radial Turbine Through the Implementation of Back Swept Blading

This report details the numerical investigation of the performance characteristics and internal flow fields of an 86 mm radial turbine for a turbocharger application. A new blade was subsequently designed for the 86 mm rotor which departed from the conventional radial inlet blade angle to incorporate a 25° inlet blade angle. A comparative analysis between the two geometries is presented. Results show that the 25° back swept blade offers significant increases in efficiency while operating at lower than optimum velocity ratios (U/C). This enhanced efficiency at off-design conditions would significantly improve turbocharger performance where the turbine typically experiences lower than optimum velocity ratios while accelerating during engine transients. A commercial CFD code was used to construct single passage steady state numerical models. The numerical predictions show off-design performance gains of 2% can be achieved, while maintaining design point efficiency. Primary and secondary flow patterns are examined at various planes within the turbine blade passage and reasons for the increase in performance are discussed. A finite element analysis has been conducted to assess the stress implications of introducing a non-radial angle at turbine rotor inlet. A modal analysis was also carried out in order to identify the natural frequencies of the turbine geometry, thus calculating the critical speeds corresponding to the induction of the excitational frequencies from the stator vanes. Although the new blade design has resulted in stress increases in some regions, the numerical study has shown that it is feasible from both an aerodynamic and structural point of view to increase the performance characteristic of a radial turbine through the implementation of back swept blading.Copyright