Evaluation of steady and pulsating flow performance of a double-entry turbocharger turbine

The turbocharger remains one of the best means available to the engine developer to satisfy the power density demands on a modern internal combustion engine. This simple device uses the otherwise waste exhaust gas energy to provide significant improvements in the volumetric efficiency or ‘breathing capacity’ of an engine. In order to maximize the energy of the exhaust driving the turbine, most applications utilize pulse turbocharging where a compact exhaust manifold feeds the highly pulsating exhaust flow directly into the turbine wheel. This thesis considers the influence that this pulse-charging has on a double-entry turbocharger turbine. The design of this turbine plays an important role in much of the research presented in this thesis. The turbine is equipped with a mixed-flow rotor with 12 blades that are fed by a 24 blade nozzle ring. The circumferentially divided volute is designed with two gas inlet passages that each feed a separate 180° section of the nozzle ring. Thus, there is no communication between the entries from the volute inlet to the exit of the nozzles. At the exit to the nozzle, the fluid from both inlets expands into an interspace that spans the circumference of the rotor inlet. This small volume that is formed between the nozzle and the mixed flow rotor is the first area where interaction between the flows can occur. The core of this report contains three main divisions: Steady flow experimental results, CFD modelling, and unsteady flow experimental results. These sections are preceded by an introduction explaining the background of the research study, and an essential outline of the equipment and the method of experimentation. The aim of this work is to use a combination of experiments and computational modelling to build up a picture of the performance of the turbine under a wide variety of flow conditions that will eventually lead to further insight into its unsteady performance. First, a comprehensive steady-state experimental data set was obtained to establish the base-line turbine performance. Steady, equal admission tests yielded excellent performance, peaking at 80% efficiency. Owing to the double-entry arrangement, steady flow could also be introduced in the two inlets unequally. During unequal, steady-state operation a notable decrease in performance was observed. The correlation between the ratios of entry pressures and the efficiency of operation was apparent but essentially independent of which flow was varied. In the extreme, when the turbine was only partially supplied with air, the consequence was a 28 point decrease in performance at the optimal velocity ratio. Despite the division between the two entries, the experiments showed that the flows through each inlet were interdependent. Compared to full flow,of the performance of the turbine under a wide variety of flow conditions that will eventually lead to further insight into its unsteady performance. First, a comprehensive steady-state experimental data set was obtained to establish the base-line turbine performance. Steady, equal admission tests yielded excellent performance, peaking at 80% efficiency. Owing to the double-entry arrangement, steady flow could also be introduced in the two inlets unequally. During unequal, steady-state operation a notable decrease in performance was observed. The correlation between the ratios of entry pressures and the efficiency of operation was apparent but essentially independent of which flow was varied. In the extreme, when the turbine was only partially supplied with air, the consequence was a 28 point decrease in performance at the optimal velocity ratio. Despite the division between the two entries, the experiments showed that the flows through each inlet were interdependent. Compared to full flow, 4 when the pressure in one entry was low, the second entry could swallow more mass, and when it was high, the second entry swallowed less. A three-dimensional CFD model was constructed in order to permit a detailed study of the flow in the double-entry design and answer specific questions regarding the observed steady-state performance. For both equal and unequal admission simulations, the model showed close agreement with the experimental mass flow behaviour and reproduced the measured efficiency trends quite well. The interdependence of the swallowing capacity of the two inlets was also predicted by the model, thereby allowing the analysis of the physical flow effects that drive this trend. It was found that the interspace region near the tongues was the site of much of the interaction between inlets. A major emphasis of this modelling work was also to discover areas of loss generation that could lead to the decrease in performance. By focussing on partial admission, this study found that the windage loss in the interspace region of the non-flowing entry proved to be one of the more significant areas of loss generation. Pulsating air flow was then introduced using the range of frequencies typically produced by an internal combustion engine. The operating point of the turbine, traced an orbit within a 3-D space defined by three non-dimensional parameters: velocity ratio, pressure ratio across inlet one, and pressure ratio across inlet two. Direct comparison between steady and unsteady values at the same pressure ratios and velocity ratio was possible due to the large amount of steady data measured. Thus, a quasi-steady versus unsteady comparison was made on the basis of efficiency, mass flow and output power. In general, under pulsating flow conditions, the turbine behaved quite differently than that predicted by the quasi-steady assumption. Lower frequency, higher amplitude pulsations produced the lowest unsteady cycle-averaged efficiency and also produced the most significant departure from quasi-steady behaviour

NASA three-laser airborne differential absorption lidar system electronics

The system control and signal conditioning electronics of the NASA three laser airborne differential absorption lidar (DIAL) system are described. The multipurpose DIAL system was developed for the remote measurement of gas and aerosol profiles in the troposphere and lower stratosphere. A brief description and photographs of the majority of electronics units developed under this contract are presented. The precision control system; which includes a master control unit, three combined NASA laser control interface/quantel control units, and three noise pulse discriminator/pockels cell pulser units; is described in detail. The need and design considerations for precision timing and control are discussed. Calibration procedures are included

Expanded mixed multiscale finite element methods and their applications for flows in porous media

We develop a family of expanded mixed Multiscale Finite Element Methods (MsFEMs) and their hybridizations for second-order elliptic equations. This formulation expands the standard mixed Multiscale Finite Element formulation in the sense that four unknowns (hybrid formulation) are solved simultaneously: pressure, gradient of pressure, velocity and Lagrange multipliers. We use multiscale basis functions for the both velocity and gradient of pressure. In the expanded mixed MsFEM framework, we consider both cases of separable-scale and non-separable spatial scales. We specifically analyze the methods in three categories: periodic separable scales, $G$- convergence separable scales, and continuum scales. When there is no scale separation, using some global information can improve accuracy for the expanded mixed MsFEMs. We present rigorous convergence analysis for expanded mixed MsFEMs. The analysis includes both conforming and nonconforming expanded mixed MsFEM. Numerical results are presented for various multiscale models and flows in porous media with shales to illustrate the efficiency of the expanded mixed MsFEMs.Comment: 33 page

An Isocurvature CDM Cosmogony. I. A Worked Example of Evolution Through Inflation

I present a specific worked example of evolution through inflation to the initial conditions for an isocurvature CDM model for structure formation. The model invokes three scalar fields, one that drives power law inflation, one that survives to become the present-day CDM, and one that gives the CDM field a mass that slowly decreases during inflation and so ``tilts'' the primeval mass fluctuation spectrum of the CDM. The functional forms for the potentials and the parameter values that lead to an observationally acceptable model for structure formation do not seem to be out of line with current ideas about the physics of the very early universe. I argue in an accompanying paper that the model offers an acceptable fit to main observational constraints.Comment: 11 pages, 3 postscript figures, uses aas2pp4.st

Classical theory of radiating strings

The divergent part of the self force of a radiating string coupled to gravity, an antisymmetric tensor and a dilaton in four dimensions are calculated to first order in classical perturbation theory. While this divergence can be absorbed into a renormalization of the string tension, demanding that both it and the divergence in the energy momentum tensor vanish forces the string to have the couplings of compactified N = 1 D = 10 supergravity. In effect, supersymmetry cures the classical infinities

Constraints on string networks with junctions

We consider the constraints on string networks with junctions in which the strings may all be different, as may be found for example in a network of $(p,q)$ cosmic superstrings. We concentrate on three aspects of junction dynamics. First we consider the propagation of small amplitude waves across a static three-string junction. Then, generalizing our earlier work, we determine the kinematic constraints on two colliding strings with different tensions. As before, the important conclusion is that strings do not always reconnect with a third string; they can pass straight through one another (or in the case of non-abelian strings become stuck in an X configuration), the constraint depending on the angle at which the strings meet, on their relative velocity, and on the ratios of the string tensions. For example, if the two colliding strings have equal tensions, then for ultra-relativistic initial velocities they pass through one another. However, if their tensions are sufficiently different they can reconnect. Finally, we consider the global properties of junctions and strings in a network. Assuming that, in a network, the incoming waves at a junction are independently randomly distributed, we determine the r.m.s. velocities of strings and calculate the average speed at which a junction moves along each of the three strings from which it is formed. Our findings suggest that junction dynamics may be such as to preferentially remove the heavy strings from the network leaving a network of predominantly light strings. Furthermore the r.m.s. velocity of strings in a network with junctions is smaller than 1/\sqrt{2}, the result for conventional Nambu-Goto strings without junctions in Minkowski spacetime.Comment: 12 pages, 6 figures. Version to appear in PRD. (2 new references and slightly extended discussion in section VII

Reheating and gravitino production in braneworld inflation

We consider the constraints that can be imposed on a wide class of Inflation models in modified gravity scenarios in which the Friedmann equation is modified by the inclusion of $\rho^2$ terms, where $\rho$ is the total energy density. In particular we obtain the reheating temperature and gravitino abundance associated with the end of inflation. Whereas models of chaotic inflation and natural inflation can easily avoid the conventional gravitino overproduction problem, we show that supersymmetric hybrid inflation models (driven by both F and D-terms) do not work in the $\rho^2$ dominated era. We also study inflation driven by exponetial potentials in this modified background, and show that the gravitino production is suppressed enough to avoid there being a problem, although other conditions severely constrain these models.Comment: 24page