Right idea, wrong orientation.

You need to place the fan 90 degrees to the fan to allow for maximum thermal loss.
You're current design allows for maximum cooling of one U1, but then causes turbulence for the rest. You want the air flow to run across the heat sinks then be removed from the environment.

I calculate the thermal loss of the U1's (with no active cooling) as:

Heatsink Area (25x40mm): 0.001m2
U-Value: 5.56W/m2K
Ambient Temp: 24.2oC
Object Temp: 55.7oC

This gives a thermal loss (0.001*5.56*31.5) calculation as 175.14W per U1 (an average person sitting down, doing nothing, pumps out 140W of heat by way of comparison).

This obviously is for a single U1 on it's own as any U1 in close proximity will impact the loss rate (ie slow it down).
The airflow will increase the thermal loss by lowering the ambient temp and causing a higher thermal difference but the most effective way is to have the airflow across the heatsinks as it allows for a higher surface to volume ratio.