A PROCEDURE
TO ANALYZE AIRFLOW IN AUTOMOTIVE AIR-CONDITIONING SYSTEMS IN AN EARLY STAGE
OF DESIGN
Jörn Hofhaus, BMW AG, Climate Control
Concepts, Simulation, 80788 Munich, Germany
Hooshang Didandeh, Technalysis Inc., 5856 W. 74th Street,
Indianapolis, Indiana 46278, USA
Abstract
A procedure for a
computer-aided analysis of the airflow in the entire automotive
air-conditioning system is introduced. The system is modeled as a
one-dimensional network of flow resistances. We analyzed four typical
operating modes of the air-conditioning system at different flow rates. Only
rough CAD data were necessary to build the models. Results are obtained in
terms of resistance curves, operating points, flow distributions at the
outlets, and local pressure losses. Once the models are set up, sensitivity
analysis is used to examine critical components or design modifications very
rapidly.
Introduction
In recent years
the requirements to reduce the costs in the design process in automotive
engineering have resulted in two major tasks. On the one hand, the overall
time to develop a new car has been reduced by approximately 30 percent. On
the other hand, fewer prototypes and less testing hardware are available in
the comparatively late stage of the design. Yet, in order to cover all
functions of the car at early stage, numerous computer models and programs
are applied to simulate nearly all characteristics of a new car, including
the air-conditioning system.
A common approach to simulate
flow characteristics inside the air-conditioning system is the use of
three-dimensional analysis tools by solving the full Navier-Stokes equations
to obtain a detailed description of the flow field (1-3). The results are
fairly accurate, however generating them is time consuming as:
- A complete and closed description of the
geometry’s surface in CAD is necessary. This is not always available. In
general it is necessary to modify the CAD data for the special
requirements of the 3-D analysis.
- A computational mesh has to be generated,
which is still an extensive amount of work, especially for complex
geometries.
- The iterative solution of the 3-D
conservation equations for turbulent flows requires hours and sometimes
weeks of computing time on high performance computers.
This expense grows very rapidly
with the size of the problem. For analyzing a 3-D flow in a single duct, the
time to obtain the results is relatively acceptable. However, in predicting
the overall performance of an air-conditioning system or the flow
distribution at the several outlets in the passenger compartment, the time
to build and compute the model is critical. Furthermore, the flexibility for
a quick comparison between different design variants is, in general, not an
easy task to achieve.
In the early stage of a design,
frequent changes to the package are encountered and very little CAD data is
available. Often, only 2-D geometry information is available. At this point,
the accuracy of a 3-D analysis is generally not necessary. Only a rough
approximation of the new design or qualitative comparisons between different
design alternatives are needed. In this study, a procedure is introduced to
analyze the flow in the air-conditioning system with the help of a
one-dimensional network of flow resistances. The resulting pressure drop for
a given flow rate and the flow distribution in the various branches of the
network is the primary output of the program. Together with the performance
curve of the blower, the operating point for the system can be determined.
Although methods for flow
network analysis are certainly not new and the results are naturally
approximate in comparison with a detailed 3-D analysis, the procedure has
some decisive advantages which made it attractive to shorten the design
process:
- The time for building up the model is
substantially smaller.
- Detailed description of the surface
geometry is not necessary. A few geometric dimensions are sufficient.
- The computational time is in the range of
minutes on Workstations or PC’s.
- The effect of different component
parameters such as area ratios or expansion angles are quickly
evaluated. Without any CAD data, many different parameters can be
investigated.
- The critical components of the entire
system can be determined with the help of sensitivity analysis before a
detailed 3-D simulation or testing is carried out.