Mounting of Fans on a Radiator
There are two ways to mount fans on a radiator: mount the fan directly to the radiator, or build a shroud to hold the fan a short distance away from the surface of the radiator.
If you mount the fan directly to the radiator, it is normally done with plastic ties that are pushed through the fins and coils of the radiator. This seems to be an undesirable technique in itself since it will deform the fins, and with the vibrations of the fan and vehicle, it might eventually wear through the coils and cause a leak or flatten the fins. The second problem is that the fan blades are literally next to the radiator fins which will cause a large flow degradation compared to running the fan in free air. Further, the air will only pass through the area below the fan blades and the air distribution is not uniform across the area of the fan since there will be no flow at the center of the fan. Thus, only a small area of the radiator has air flow through it.
If you mount the fan on a shroud to create a plenum behind the fans, then the fan blades are not next to the radiator fins and hence act more like it was in free air. The fan will pull a vacuum over the entire area of the shroud and will hence have a more uniform flow over the entire area, and that area will be larger than the area below the fan blades. Based on this reasoning, it appears that the shroud mounting would be the clear solution; however, I have tested the various combinations of mounting techniques and radiator flow on a flow bench to verify this assumption.
We have a large flow bench that can provide substantial flow to a test section. The first test was to measure the pressure-flow characteristics of the fan mounted on a flat plate with a circular opening as shown by the first photo. This gave us the free flow characteristics of an individual fan. Since there will be two fans, we can double the flow measurement with no increase in pressure to get what the ideal flow from two fans would be in free air.
A pressure-flow curve for the fan can be generated by turning the fan on and providing a variable restriction upstream of the fan to get a progression from the blocked pressure (with the valve closed) and a maximum flow (with the valve open).
The next test was with the radiator mounted to the test section as shown by the second photo. We can generate a pressure-flow curve by varying the speed of the flow bench pump.
The fans were then connected directly to the radiator as shown by the next picture.
Finally, a shroud was built that encompassed the entire radiator surface (in two parts) with fans mounted to the shroud.
First of all, consider the free flow pressure drop across the radiator with no fans. As shown in the graph, it has a pressure drop of 0.30 inches of water at 1000 cfm. When the fans are mounted directly to the radiator (but turned off), the pressure drop increases to 0.38 inches of water. You might think that the shroud would cause a large restriction to free flow; however, the shroud with fans attached (but turned off) only increases the pressure drop to almost 0.40 inches of water.
Next consider the air flow with the fans turned on. From the test of a single fan and then doubling the flow, we can see that the ideal maximum free air flow of the fan is the top red line ranging from 0.60 inches of water to well over 1400 cfm. When the fans are mounted directly on the radiator and turned on the pressure and flow ranges from 0.20 inches of water to 600 cfm. When the fans are mounted on the shroud and turned on the pressure and flow ranges from 0.50 inches of water to 900 cfm.
The intersection of the fan and radiator curves illustrate the actual operating point. For the fans mounted directly on the radiator, the actual flow will be about 350 cfm. When the fans are mounted on the shroud the flow will be about 530 cfm. Even though this is a 50% increase in flow, it is actually a much larger increase in cooling effect since the fans mounted directly on the radiator only have a small area of air flow through the fan and hence do not cool as much water as a uniform flow over the entire area of the radiator. Thus, it is clear that the shroud will provide much better cooling.
I will build an angled shroud that has the stand-off to the bottom of the radiator larger than the top. This will give more volume in the shroud’s plenum and will direct the flow more upward and into the nostrils better. I will further mount the fans with some sort of vibration isolation so they won’t make so much noise. I will post a photo of this when I get it built.
-Bob Woods