The high boost pressures and fuel–air ratios required for the next generation of turbocharged diesel engines imply an increased turbine expansion ratio without an increase in the speed of rotation. This leads to a requirement for high peak efficiency at lower values of blade speed/isentropic expansion velocity U/C than are normal today. The objective of this project was to achieve this with a mixed flow rotor with a positive inlet blade angle. Two rotors were manufactured and tested: one a ‘constant blade angle’ design and the other a ‘constant incidence’ design. In practice both achieved a peak efficiency at a low U/C value, but the constant blade angle design, at 0.84 total to static efficiency, was significantly more efficient than the constant incidence design at 0.77. These efficiencies are highly competitive, compared to current radial turbine design. It is suggested that the reasons for this difference are a lack of understanding of the incidence and its effects on a mixed flow rotor, and a region of diffusion in the shroud-trailing edge corner of the suction surface, apparently worse for the constant incidence design.
A broad operating range between surge and choke is so important for turbocharger compressors and many other applications that a vaneless diffuser, with its reduced efficiency, is usually adopted. With the demand for increased pressure ratio the operating range naturally reduces and techniques to extend the range are necessary. The inducer bleed slot is a technique which has been adopted in turbocharger compressors. This approach was first reported by Fisher (1988) and was described as a Map Width Enhancement slot (MWE). The flow conditions in the MWE slot and impeller inlet duct were investigated with a view to developing an improved understanding of the flow mechanisms involved as the flow rate was reduced from choice to surge. Mean temperature and pressure measurements were recorded in the MWE passage, the main inducer duct to the impeller and the inlet duct upstream of the compressor. In addition the development of flow pulsations were monitored with pressure transducers in the MWE passage, the main inducer duct and the inlet duct, together with the application of flow visualisation techniques. The transient pressure measurements showed that low frequency flow pulsations developed in the MWE passage at high flow rates. As the flow rate was reduced the low frequency pulsations disappeared and flow reversal through the MWE passage developed. It was shown that flow reversal through the MWE passage commenced at flow rates close to the peak efficiency point for the compressor.
Understanding of the effects of blade loading and blade number in radial and mixed flow turbines is often based on analogies with axial turbine cascade tests or centrifugal compressors rather than direct measurement. In this paper test results from a series of similar mixed flow turbines are described. The turbine rotors differ only in blade number or inlet incidence variation. The results comprise performance data and hub and shroud pressure measurements, from which it is possible to deduce parameters such as incidence angle with good accuracy. In addition, predictions of blade loading using three-dimensional computations are shown. The results are correlated against a loading coefficient and a slip factor, both derived for the general case of a mixed flow turbine. The influence of these parameters on the performance of the tested turbines, and for comparison of other radial turbines, is shown.
The performance of two turbocharger impeller designs was evaluated experimentally. The compressor design requirement was for a pressure ratio of 3.6, with a peak pressure ratio of 4.3 at a maximum non-dimensional impeller speed of 1.66. Due to the stress-limited speed the impeller discharge blade backsweep had to be restricted and the application of prewhirl was considered from the outset as a means of extending the operating range. An impeller, designated A, was designed with 25° of prewhirl applied. A second impeller, designated B, was designed with zero prewhirl for comparison purposes, but was not manufactured. A third impeller, C, was manufactured, in place of impleller B, through the modification of an existing design. This experimental study includes the assessment of this third impeller together with impeller A.
The compressor design requirement was for a pressure ratio of 3.6, with a peak pressure ratio of 4.3 at the maximum non-dimensional speed of the impeller of 1.66. Due to the stress-limited speed, an aluminium alloy impeller was specijied, the impeller discharge blade backsweep had to be restricted and the application of prewhirl was considered from the outset as a means of extending the operating range. A non-dimensional conceptual design procedure, including the effect of inlet prewhirl, was applied to the design of three turbocharger impellers. An impeller, designated A , was designed with the inclusion of 25" of prewhirl. A second impeller, designated B, was designed with zero prewhirl for comparison purposes, but was not manufactured. A third impeller, C, was manufactured through the modijication of an existing design and the design study was applied to the assessment of this third design. a A b c Cslip d A00593/1 Q IMechE 1993
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