Typical fan shroud installations

To be able to predict cooling performance and underhood airflow with good precision within a virtual design process, it is of utmost importance to model and simulate the cooling fan efficiently and accurately, and this has turned out to be challenging. Simulating the cooling fan in a vehicle installation involves capturing complex fluid dynamic interaction between rotating blades and stationary objects in the vicinity of the fan.

This interaction is a function of fan rotation rate, fan blade profile, upstream and downstream installation components. The flow is usually highly turbulent and small geometry details, like the distance between the blade tip and the fan shroud, have strong impact on the fan performance characteristics. Fan installations therefore have a large influence on cooling performance which the fan data from the supplier cannot capture.

Improved simulation capabilities in this area are critical for optimizing the design of energy efficient vehicles since the performance of these fans, which provide airflow to the heat exchangers used for engine cooling and HVAC system operation, have a big impact on the vehicles' overall energy efficiency.

This paper presents a comparison of two methodologies for simulating fan air flows. Simulation results are compared to experimental data obtained in a fan test rig with representation of a typical truck fan installation, including fan shroud, ring, seal and an engine silhouette downstream.

Results of MRF simulations are known to be sensitive to the MRF domain, which is highly constrained in tight fan installations. For typical truck installations, SM provides a more robust alternative, and better accuracy than MRF in the transitional and radial regime of the fan curve.

Subscribers can view annotate, and download all of SAE's content. When obstructions cause a rattle or whistle, the reasons may be dampers, sharp elbows, a sudden expansion or contraction in ductwork, leaks in ductwork, turning vanes, or fins on coils. Rattle or rumbles are caused by vibrating ductwork, vibrating cabinet parts, or vibrating parts not isolated from the building. Fan is too quiet. Fan performance is wrong.

Causes of inadequate airflow to the fan include a backwards installed or running impeller, improper blade angle setting, missing cut-off, an impeller not centred with inlet collars, slow fan speed, dirty or clogged inlet, improper running clearance or improper inlet cone-to-wheel fit.

Not enough airflow to the duct system indicates the actual system is more resistant to airflow than expected. Other probable causes are closed dampers or registers, loose insulating duct liner or leaks in the supply ducts. If airflow to the filters or coils is inadequate, look for dirty or clogged filters, replacement filters with greater than specified pressure drop, or install a replacement coil with tighter fin spacing.

Airflow issues arising from poor inlet conditions may be elbows, cabinet walls or other obstructions. Poor airflow due to poor outlet conditions results from a sudden expansion or contraction at the fan outlet, or a duct bend. Too much airflow is caused by oversized ductwork, damper set to bypass, filters not in place, too fast a fan speed, low system resistance or an open access. Other probable causes are improper blade angle setting, missing cut-off, too slow fan speed, dirty or clogged impeller or inlet, improper running clearance, or improperly set inlet vane or damper.

Fan is vibrating. Ask yourself whether the fan impeller is properly balanced, motor and sheaves are balanced, key lengths are correct, fan shaft seals are rubbing, and does the motor have the right bearings.

If the fan foundation is inadequate, are isolators properly sized and levelled; shims properly installed; anchor bolts properly sized, intact and installed on all anchor points; and is ducting twisting or rubbing?

Check bearings: are they upgraded for a higher load capacity? Do you use the correct type and amount of grease? Is one bearing free to float? Finally, check for changes in the system such as higher or lower pressure. Have expansion joints or dampers failed? When the fan fails. This is the worst-case scenario. Higher trending vibration and rapidly increasing bearing temperatures indicate bearing ring failure. Other reasons include failing belt drive or coupling, shaft failure and impeller failure due to abrasive or corrosive wear.