In my previous blog post I explained how my new improved indoor cycling setup now includes two Cleva Vacmaster fans. The fans are excellent, providing powerful jets of air to help keep me cool during hard workouts.
The fans have three speed settings, with the fastest #3 setting delivering a airspeed of 31 kph at the centre of the jet, at a distance of 1 metre from the nozzle. Directly in front the nozzle, the airspeed is obviously faster, 55 kph, but the measurements at 1 metre are more appropriate to how I use the fan. On its lowest #1 setting, the speeds at 1 metre are approximately 60-70% of the airspeeds at the highest setting.
Why a Diffuser?
The problem I've found is that during the colder winter months, when it's around 10 degrees Celsius in my garage, the airspeed at even the lowest fan setting is a bit too fast. This is especially true when doing easier Zone 2 endurance rides. I find that need some airflow to avoid getting sweaty, but I only need a very light breeze. With the more powerful Cleva fans I found myself often getting too cold, even with the fans on their lowest setting. I would get too sweaty when the fans were off, though, so found myself cycling them on and off.
I decided that I would try to make a diffuser. A diffuser should slow the flow down by causing it to 'spread out'. This would, if it works, also cause the jet to become broader, having the additional benefit that the flow would cover more of my body, which should be helpful at times when I wanted the fan to be on it's full setting, to keep me cool. To do that though, the diffuser would have to work properly, meaning no flow separation within the diffuser, in order to maintain full aerodynamic efficiency. If flow separations occur inside the diffuser, then aerodynamic losses occur (total pressure losses), the result of which is that the air would slow down somewhat, but the jet would not widen correctly. In that case, the same result could be achieved simply just by restricting the flow with an object that partially blocks the nozzle (e.g. a grid, or a gauze).
Diffusers are also used in a couple of other applications that people may be familiar with:
- On the rear underside of Formula 1 cars, and other race cars (see here). Race car diffusers achieve the same result, slowing down the air flow by causing it to spread out, to expand. The objective is slightly different though - when the flow slows down, it increases in pressure, returning to the ambient pressure as it leaves the back of the car. This in turn allows the flow under that car to be faster, operating like a venturi, which reduces the pressure below the floor of the car to a pressure below the ambient air pressure, 'sucking' the car downwards, thereby creating aerodynamic downforce.
- Diffusers are also used in wind tunnels, behind the working section, to slow down the flow, allowing slower moving airflow in the return loop of the wind tunnel, reducing losses in the wind tunnel.
Fan Diffuser Mk.1
The photo below shows how the two parts fit together. The fixed part has a forward facing slot around the edge of the nozzle. The diffuser has a tapered flange which then fits into that slot.
The next step was to see whether the diffuser worked correctly. I taped down a number of small ~30mm lengths of wool onto the inside of the diffuser to help determine the flow quality.
Wool tufts are a common type flow visualisation technique used by aerodynamicists to determine where the flow is attached to the surface or separated. Attached flow means the air is moving in the intended direction, flowing smoothly across the surface. Separated flow means the flow has broken away from the surface, causing regions of recirculation where the flow can be moving in the opposite direction, creating eddies that restrict the flow and reduce the efficiency of the diffuser.
The video below shows the that some of the wool tufts are quite stable. However, other tufts are quite active, showing that the flow is separated, or nearly separated. One tuft on the lower surface is being blown backwards, indicating it's in a region of re-circulating air caused by flow separation further upstream inside the diffuser.
Generally, the result was a bit disappointing. I measured the characteristics of the airflow downstream, just to check it (see plot on the left).
This plot confirms that the airflow speed was being slowed down, but the jet wasn't getting much wider. This is another indication that the diffuser was lacking efficiency because the flow separation in the diffuser causes losses that slow the flow down but do not cause it to spread out, meaning the mass flow rate was not being conserved because the flow through the fan was being constricted. My feeling is that the angle of divergence I chose for the nozzle was too severe, causing the adverse pressure gradients in the diffuser to be too high, prompting the flow separation.
Fan Diffuser Mk.2
In my mind, these multiple guide vanes would operate in a similar way to how corner vane cascades work in the corners of closed return wind tunnels to efficiently turn the flow through 90 degrees (see diagram on the right).
Airspeed profile measurements
