Wikipedia:Reference desk/Archives/Science/2019 April 7

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April 7

Electric fan vs propeller

I've noticed that common electric fans lack an airfoil-shaped blade. That is, unlike a prop, they are constant thickness, and work by just ramming the air at an angle. I understand that this doesn't work for aircraft, because if creates too much drag.

• So, why don't fans use airfoil shapes ? Is it just that airfoils are more expensive to make and they don't mind the inefficiency caused by the drag ?

• Do high-end fans, as in wind tunnels, use an airfoil-shaped prop ?

SinisterLefty (talk) 15:40, 7 April 2019 (UTC)

First, most common electric fans do have an airfoil shape. The old stamped metel blades were constant thickness to keep the costs down, but molded plastic is now cheaper than stamped metal.

The goals for engineering each are different. An electric fan designer wants it to be quiet and move a lot of air with a small diameter. Efficiency is desirable, but not the most important thing. For an airplane, efficiency is everything, and keeping the noise and diameter down are way down on the priority list. Airplane props also need to work well in thin air at altitude. Also an electric fan that puts out a lot of air at a fairly low velocity is acceptable, but an airplane cannot go faster than the velocity of the air coming off the prop. --Guy Macon (talk) 18:48, 7 April 2019 (UTC)

(after edit conflict)

Everything is an airfoil - you just have to apply the correct equations of fluid-dynamics to model it! In particular, the air speed and pressure that you'll commonly find in a small consumer-grade electric fan fall into a range where the exact shape of the airfoil - that is, its camber - is probably less important than the blade's angle of attack. In practice, this means you can blow air using a totally flat piece of material as the fan-blade, as long as you spin it at the right angle. And, it's a lot easier to build a flat blade than a shaped blade.

A modern consumer-grade fan may have flat- or shaped- blades, and the dominant factor is almost surely the cost to manufacture it at high volumes. Even if a shaped blade is better at blowing air, it might be harder to build cheaply. This might change a little bit as you look farther and wider at different types of fans - standard fans made of injection-molded plastic can be quite sophisticated!

So, if you look hard, you can find specially-shaped blades! You just have to know where to look:

• Here's a news article on airflow in recent consumer-electronics fans: Retina MacBook Pro's quiet asymmetric fans... (2012).

Regarding wind tunnels: these vary far and wide as well, depending on size, budget, and purpose. If the tunnel is really big, it probably uses a turbine engine (almost identical to a jet engine from an airplane) and ducts the air through specially-shaped ducts to keep it laminar; but it depends on what the operators want to do with the airflow. Sometimes, wind tunnel people want turbulence!

• Here's the main page for the wind tunnel facility complex at Ames, and we would be remiss to omit the adjacent National Full-Scale Aerodynamics Complex operated by our United States Air Force. "Power is derived from six 40-foot diameter fan blades, each motor rated at 23,500 hp." Here is a picture of those blades, featured during the facility's 25th anniversary celebration, (2012). It seems that '12 was a good year for airfoils.

Here's the main page for the Wind Tunnel at Cal Poly. They have great videos, showing off the air supply and the research they do with it. They just overhauled their main tunnel, so it's pretty state-of-the-art!

Hopefully you can find even more information about the powerplant that drives your local wind tunnel!

Nimur (talk) 18:52, 7 April 2019 (UTC)

I suspect electrical efficiency of fans isn't that bad. They don't face the same tradeoffs as props and wind tunnels since they don't particularly try to impart the maximal possible speed to the air. But they get a known volume (and therefore mass) per minute of air moving at a known speed, so that gives a known kinetic energy, which is probably not that far off the electrical energy going into the fan. The ratio between the two is the electrical efficiency. 173.228.123.166 (talk) 08:42, 8 April 2019 (UTC)

I would propose that the design-goal of most airplane propellers is to maximally impart momentum to the vehicle, not to impart speed to the airflow. In a moving air stream, in which different quantities of air can go in all sorts of different directions, speed and momentum are very different objectives.

Have a look at the FAQs at Hartzell Propeller, one of the major manufacturers of props for small and mid-size airplanes. They have wise words on the topic of efficiency:

"If 2-blade propellers are more efficient, then why don’t all propellers have 2 blades?

The short answer is because efficiency doesn’t propel the airplane, thrust does. The most efficient propeller blade count for a particular aircraft is a function of the aircraft mission and a number of other factors. These include the amount of engine power, operating RPM for the propeller, diameter limitations, aircraft performance requirements (high speed cruise, takeoff, loiter, etc), noise requirements, and others. Depending on the combination of these parameters a 2-blade propeller may be most efficient, but as power increases additional blades are generally required to efficiently utilize the increased power."

We might construct a similar argument about a cooling fan: the objective isn't to maximize airspeed, it's trying to maximize cooling. If the fan is designed to a tight specification, the engineers determine what airflow will cool the overall system best, accounting for rates of thermal transfer, weird complexities about heat flow into moving air, and parasitic heat added by the fan motor and electronics; and the balance of aesthetics, acoustics, and lifetime reliability. It's really hard to estimate the energy-transfer efficiency in an airflow problem; rather, the key figure of merit is probably the final heat-removal-rate per unit of time, measured as a heat flux. Put simply: more air, or faster-moving air, does not linearly relate to more cooling, because this simple heat flow scenario is a classic example of a nonlinear system. We have complexity from airflow and fluid dynamics; and complexities from heat transfer and material properties; and complexities from the geometry; and so on. In plain english: more air that moves slower, versus less air moving faster, cool differently. Engineers have to balance heat capacity and heat conductivity, which sometimes work against each other!

Many times, the design-space is very constrained: the only controllable factor the engineers might have is the variable speed range of a standard fan part. (Few engineers are privileged enough to actually design and manufacture for a specific aerodynamic performance of their fan blade). Instead, they estimate a linearized relation between fan-speed and heat removal rate; and a relation between input electrical power and fan speed... So, one can normalize heat-flux by input power, and obtain a sort of "efficiency" figure that makes the pure physicist in me cringe - it's not exactly in the right units, but it's probably more useful in this specific context!

Anyway, if our esteemed readers are really interested in this topic, I can dig out a textbook on heat transfer. Moving imperfect airflow over a heat source, with simultaneous heat transfer by conduction, convection, and radiation... This is the sort of calculation that mechanical engineers have to study in gory detail, all while solving just some of the most horrible fin heat transfer parameterizations I've ever set eyes on. In the real world, this is what I call "overparameterization," and I would caution against it! Whenever possible, use the simplest possible model, and experimentally validate it!

Nimur (talk) 11:45, 8 April 2019 (UTC)

Thanks everyone. That thought about maximizing cooling not being the same as maximizing air flow made me think of two different uses for portable fans:

• Window fans, where the goal is to move as much air as possible, probably could do with airfoil-shaped wings, to provide laminar flow. However, if the fan is to blow outwards, then any inefficiency in terms of extra heat will be dumped outside, so that lessens the penalty.

• Fans that blow directly on people, on the other hand, could benefit from a bit of turbulence and, as such, would do just fine with constant-thickness blades.

Is this correct ? SinisterLefty (talk) 23:34, 8 April 2019 (UTC)