Validation of a Heat Transfer Prediction Approach Inside Turbochargers and its Application on Turbocharged Engine Performance Analysis
Authors: Tomm Uwe , Weiske Sascha , Coksen Ahmet , Rafaa Youness , Münz Stefan 
Published: August 2017

In the present study, the entire energy balance of a turbocharger is investigated applying an experimentally validated numerical approach with the intention of examining the heat transfer mechanism inside the turbo. The heat transfer results thus obtained are used amongst others to determine the diabatic effects on the turbine and compressor flow resulting in heat-transfer corrected performance maps. These maps are applied as matching data to 1D engine performance calculation and are utilized in the engine process simulation procedure with GT Power™. In detail, the numerical approach of the entire energy balance is based on a thermal network model (RC-resistance / capacity) where the 3D geometry of the turbocharger is subdivided into segments. These segments are defined as lumped mass elements of the thermal network. The entire energy balance of the modeled turbo is fulfilled by coupling the thermal network of the structure to the enthalpy flows of the turbine, compressor, oil circuit, and water coolant as well as the heat losses to ambient. The heat transfer between the structure and the enthalpy flows, respectively, is achieved by using heat transfer coefficients (HTC) performed in accordance with Nusselt-No. laws. Heat loss to ambient is expressed by natural convection and radiation. In general it would be possible to perform the energy balance of the turbo model in the steady state or transient regime. A time-governed finite volume calculation scheme is used for the solution algorithms. The code of the turbo heat transfer approach (THT) is written in Matlab™, something which facilitates flexible adjustments on the algorithm and good post-processing capabilities. Two routes are resorted to for validating the THT approach. Gas stand tests with instrumented turbochargers using thermocouples and pressure sensors are conducted in the first assignment for generating the essential experimental data. Segmentation of the 3D turbocharger geometry into discrete elements is accomplished in the second assignment by means of CAD technology and used for both, the setup of the THT thermal network model and in parallel for the generation of an AnsysCFX™ 3D CAE model. The same HTC thermal boundary conditions are applied to both models which is favorable in as far as it provides a one-to-one comparison of the heat flux and mean temperature in each segment of the two models, Matlab™ THT and CFX™. Ansys™ model heat flux and mean segment temperature results are validated by the measured experimental temperature data. The THT network model properties such as segment volume, areas, volume, and element distances are calibrated applying the results of the 3D CAD and CAE Ansys™ model. The results of the two numerical models are compared with each other, thus demonstrating the qualitative and quantitative level of agreement. The THT approach that has been developed is successfully applied to GT Power™ gasoline engine model. A thermal network model of that applied turbocharger was setup and validated by gas-stand and engine data obtained on an experimental basis with an instrumented turbo. Finally it was possible to demonstrate that the heat-transfer corrected turbocharger performance map data which was provided utilizing the THT model approach brings about a significant benefit to the determination process aimed at achieving a tailored turbocharger thermodynamic layout.