I want to do some cowl work to reduce my cooling exit flow area and at the same time reduce overall frontal area.
But before I can do that, I have to do two other things. One is make a seal around the crankshaft/spinner gap. As you close off the cooling exit, raising the pressure in the lower cowl plenum, you want the higher pressure to result in higher exit velocities out of the cooling exit, not just spill out around the spinner. So I have to do that.
The other thing I need to do is increase the flow through the oil cooler. The angle-valve engines put a lot more of the heat load into the oil, and efficient oil cooling is crucial. I've been very happy with my firewall-mounted cooler arrangement with the large 20006A cooler, I have good oil temperatures even during a long climb at Vy on a hot day. But as I close off the cowl exit and increase the pressure in the lower plenum, the oil cooler flow will decrease because of the reduced pressure differential across the oil cooler. So..how to get that flow back? put a diffuser on the cooler exit. The trick is to do this smoothly enough get good pressure recovery without a lot of length that wouldn't fit in the cowl area. Short, wide-angle diffusers can work pretty well. The pressure in the oil cooler flow at the exit of the diffuser will match the local static pressure in the area around it. Ideally, if the diffuser is flowing well, this results in lower pressure at the exit face of the cooler itself. Its like immersing the cooler in the throat of a venturi.
I made a mold out of wood, painted and waxed it, and laid up a fiberglass diffuser. The diffuser geometry has an area ratio of 1.45. This will also be a good trial of the high-temperature hardener system (3136R) I got for the Jeffco 3107 (Rhino) epoxy we use. The diffuser was room temperature cured, then removed from the mold and post-cured, 1/2 hr at 150F, 1/2 hr at 175F, then 2 hr at 200F. Hopefully this will give a high enough glass-transition temperature (T_g) for the epoxy to survive the hot environment in the lower cowl. Here are a couple of pictures of the installation:
You can see, especially in the second picture, that I made the diffuser a little bit too small. I had measured the mounting flange length and assumed that the cooling channels were the same length, when they actually extend about a 1/4" beyond. So I will add a 1/2" plank on the mold and make another one. In the mean time, I can test this one.
Another issue is that I have an engine mount tube crossing the exit face of the oil cooler. Without the diffuser, this likely reduced the flow through the cooler a little because of the "blockage". With the diffuser, there is some suction pulling on the exit of the cooler that will help pull flow around that tube. Hard to say if the unsteady flow around the tube will help or harm the flow in the diffuser itself. It may cause an oscillating flow separation on the diffuser walls, but hopefully the average pressure recovery will be OK. When/if I put a slightly wider diffuser on, I will also put a spiral wrap of wire on the tube to break up the coherence of the oscillating separated flow downstream of the tube. I'll report back when I get a chance to fly with this.