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One Dimensional models for front end air flows through the cooling system package are very useful for evaluating the effects of component and front end geometry changes. To solve such models for the air flow requires a robust iterative process that involves a number of non-linear sub-models. The cooling fan (s) constitute a major part of the difficulty, especially when they employ a viscous or “thermal” fan drive. This drive varies the torque coupling between the input and output shafts based on the radiator outlet air temperature. The coupling is achieved by viscous shear between two grooved disks and is regulated by a bimetal strip valve that varies the amount of fluid between the disks. This paper presents a mathematical model by which the input/output speed ratio may be determined as a function of the air temperature and input speed. Coefficients in the model are estimated from standard supplier performance information.

Computer simulation of an aircraft's environmental control system enable proper understanding and characterization of the system, help lower design time and cut down on costly iron bird and in-flight testing. Components such as the control valves, ram air heat exchangers, recirculation fans, and boot strap compressors can be sized and optimized by the design engineers through simulation. System analysis is not possible with 3d CFD, but can be done with 1d tools that allows engineers to model a variety of test cases very early in the design process - at the concept stage and can be continued to be utilized all the way to the released stage to identify potential issues and investigate solutions before the aircraft is sent into service.

Advanced design of modern engine cooling and vehicle HVAC components involves sophisticated simulation. In particular, front end air flow models must be able to cover the complete range of conditions from idle to high road speeds involving multiple fans of varying types both powered and unpowered. This paper presents a model for electric radiator cooling fans which covers the complete range of powered and unpowered (freewheel) operation. The model applies equally well to mechanical drive fans.