Industrial Turbocharger

Turbocharger System Sample Model

A simple model of an industrial turbocharger has been developed to illustrate key concepts for modeling systems involving more than one turbomachinery component. These concepts include the calculation of net torque, the calculation of the shaft speed that balances torque, and shaft speed transients based on transient equations of motion (namely, T = I*dw/dt).

In the case of a turbocharger, a turbine provides the torque to drive a compressor. There is no gear box in this system, though representations of gearing, gear losses, bearing losses, etc. do not represent significant modeling challenges if the data (gear ratios, torque coefficients, etc.) is readily available. Similarly, starter motors and loads (e.g., generators) can be modeled as well.

The concepts and modeling methods developed are applicable to other systems involving multiple, linked turbomachines including:

  • Brayton cycles, including jet engines
  • Rankine cycles
  • liquid rocket turbopumps

System Description
The figure below represents the system schematic.

Air at ambient pressure and 20°C enters the compressor at point 1, and is discharged at point 2 (nominally 3.5:1 pressure ratio), the engine inlet. The nominal (design point) flow rate into the compressor is 10.47 kg/s, and the nominal shaft speed is 16000 rpm. The engine is modeled as a source of hot air (with combustion products neglected for simplicity), with a constant flow rate of 0.52 kg/s.

The engine representation is very simple: it adds 5.93MW of energy to the air. The nominal flow rate through the turbine, from point 3 to 4, is the sum of the flows through the compressor and engine: 10.99 kg/s.

The nominal exhaust pressure of the turbine is 1.9MPa. The exhaust system resistance (from turbine outlet to ambient) is estimated to be equivalent to a K-factor loss of about 16.8 at the dynamic head corresponding to the turbine exhaust. (This exhaust system resistance value will be varied parametrically later to test sensitivity).

The compressor is a centrifugal compressor, with an inlet meanline diameter of 230mm, a rotor outer diameter of 474mm, and a stator outer diameter of 676mm. The turbine is a radial design, with a stator inlet diameter of 709mm, rotor inlet diameter of 541mm, and a meanline outlet diameter of 252mm.

Basic Model Description
The model was developed using Thermal Desktop® and FloCAD®. The compressor was modeled using the performance map information (flow and efficiency versus pressure ratio). EZXY® plots of this information are provided below.

Performance Map Input for Compressor

Similarly, the performance of the turbine is plotted below. The basis for the turbine is total-static, which was defined as part of the TURBINE device information.

Solving for RPM at Zero Net Torque

In the above example, shaft speed is constant and the net torque is predicted. Often, the balance point is required: what shaft speed will result in equal but opposite compressor and turbine torques?

In SINDA/FLUINT, the Solver module can be used to find a traditional input (speed) given a traditional output (net torque), in a manner similar to the Excel goal seeking capability. The balance point was found to be about 16,050 RPM.

Shaft Speed Transient Example

To illustrate the solution of a combined mechanical and thermohydraulic set of equations, an artificial transient is run by perturbing the shaft speed from its equilibrium value (just above 16000 rpm) to 14000 rpm … the lowest value for which turbine and compressor data are available. Initially, this lower speed will cause a net positive value of torque. The shaft will then be allowed to speed back up to its design point.

A co-solved first-order ordinary differential equation (ODE) is set up to for the current shaft RPM, following the formula T = I*dw/dt (where T is the net torque, I is the rotational inertia, w is the rotor/shaft speed, and t is time).

An event duration of 360 seconds (6 minutes) proves enough for the shaft speed to return to its equilibrium value, as shown in the responses below:

Click here to fetch the Turbocharger Example from our User Forum

FloCAD online training

Class times: May 2 & 4 from 10am to 2pm MT

Cost: $425

This online class will provide an introduction to fluid modeling components within FloCAD. The class will be held over a 2-day period, with daily sessions running approximately 4 hours each. The class uses a mixture of lecture, demonstrations, and self-paced tutorials to allow attendees to practice building fluid system models and interpreting results. The presentations will comprise 2 - 3 hours of each session, and the instructor will be available during the remainder of the time for questions during tutorials. Attendees must have basic working knowledge of SINDA and Thermal Desktop as these topics will not be covered but their usage is required for FloCAD.

Register here

Thermal Desktop, RadCAD, and TD Direct in-class training

Date: April 25-28, 2017, 8:00 a.m. to 5:00 p.m., daily
Location: Lakewood, CO

CRTech will be hosting introductory training for Thermal Desktop, RadCAD and TD Direct. Lecture and hands-on tutorials introduce attendees to basic Thermal Desktop and RadCAD usage and allow practice building models and interpreting results. The class will also introduce students to SpaceClaim direct modeling CAD interface and advanced meshing tools in TD Direct.

Daily Schedule

Day 1 and 2: Introduction to SINDA and Thermal Desktop
Day 3: Introduction to RadCAD
Day 4: Introduction to TD Direct

To learn more about this class and to register, visit our Training Page.

Anode and cathode of a flow battery

Using Sinaps? It is not too soon to get started with TD/FloCAD!

This webinar describes the process for converting from Sinaps to Thermal Desktop (TD) and FloCAD. This process includes using an exporter which works with Version 6.0 of the CRTech tool suite (expected to be released in May of 2017).

Come learn about the basics of TD/FloCAD, including many compelling features not available in Sinaps. The webinar will also cover how to manage the transition period, during which you may be using both programs simultaneously. This is also a chance to ask questions. 

If you missed this webinar, please contact us for the presentation material and recording.