Turbocharger Design and Performance Analysis

Performance testing identifies many aspects of turbocharger performance. Although, when performance is less than satisfactory, test cell mapping only identifies secondary or tertiary troubles demonstrating inconsistencies with expected performance. Such tasks as seeking out and eliminating efficiency losses or increasing operational surge margin are unrealistic expectations when basing your conclusions solely on inlet and discharge operating conditions. Identifying the root cause such as a mismatched impeller / diffuser or a poorly matched rotor / stator requires a complete aerodynamic analysis employed through a systematic investigation. Turbomachinery design and analysis software predicts the interactions of a working fluid with its geometrical surroundings and operational environment. Accurately predicting these interactions is highly dependent on understanding the energy loss models embedded within the design code. These loss models dictate how severely performance diminishes due to inherent or sometimes improper geometrical and operational constraints. Such energy losses include skin friction, excessive pressure recovery, airfoil incidence, flow recirculation, and blade tip leakage to name a few. Working with aerodynamicists, Globe Turbocharger has fully integrated multiple centrifugal compressor and axial turbine performance codes into its design procedure. This procedure outlines a system of embedded relationships between component geometry, efficiency, and performance margin. Combining detailed aerodynamic analysis with a systematic design methodology provides the turbomachinery designer and turbocharger end user a system wide perspective of how and why the turbocharger will perform under all operating conditions. INTRODUCTION Within the last decade, it has become customary for emission reduction companies to reduce the level of pollutants such as NO, NO2, and CO primarily thru improving combustion efficiency and reducing power cylinder temperatures. These operational attributes are typically accomplished through custom designed medium –to– high pressure fuel injection systems complimented by a higher air/fuel ratio (leaner charge). For some situations, simply operating the engine at a greater air/fuel ratio attains the desired emissions level. The increased airflow rate and air density used to operate the engine at a greater air/fuel ratio is achieved by turbocharging the either naturally aspirated or pump scavenged reciprocating combustion engine. In some cases, as with a turbocharged engine, retrofitting the existing turbocharger with new aerodynamic components provides the desired air mass flow rate and air manifold pressure increase. Over the last few years, turbocharger testing prior to installation on an emissions reduction engine has become more frequent. For some pipeline companies pre-installation testing is standard practice. This testing does not typically include any instrumentation of the turbocharger itself other than to monitor stage inlet and discharge conditions. The primary driver for turbocharger performance testing is the sensitivity of the air specification provided to the turbomachinery designer combined with the high cost of installation, removal, and engine or project “down time”. This “down time” occurs when turbocharger performance, mechanical or aerodynamic, is not as expected. Unfortunately, performance testing a turbocharger in this manner only provides the interested party with “flange conditions”. This terminology implies that the test cell instrumentation is only collecting data at the compressor inlet, compressor discharge(s), turbine inlet(s), and the turbine discharge. It does not distinguish between the performance of each turbocharger component. The test cell data is global, assessing the overall compressor and the overall turbine as a whole. When turbocharger performance is not as expected, identifying a solution requires the ability to assess the individual performance of each component through a detailed aerodynamic analysis. A turbocharger design and performance analysis provides insight as to which individual component or set of components is causing the problem.