Turbulator

Grob G 102 Astir prototype with turbulator Tape under the wing

A turbulator is a device that turns a laminar flow into a turbulent flow. Turbulent flow can be desired on parts of the surface of an aircraft wing (airfoil) or in industrial applications such as heat exchangers and the mixing of fluids. The term “turbulator” is applied to a variety of applications and is used as a derivative of the word turbulent. However, the word has no commonly accepted technical or scientific meaning. As such, it has been approved as a trademark in the U.S. and other countries in conjunction with machine parts used within rotating drums, sterilizers, heat transfer ovens, mixing and pelletizing machines, and air destratification fans for horticultural and agricultural uses, among others.

Airfoil turbulators

When air flows over the wing (an airfoil) of an aircraft, there is a layer of air called the boundary layer between the wing's surface and where the air is undisturbed. Depending on the profile of the wing, the air will often flow smoothly in a thin boundary layer across much of the wing's surface. The boundary layer will be laminar near the leading edge and will become turbulent a certain distance from the leading edge depending on surface roughness and Reynolds Number (speed). However there comes a point, the separation point, in which the boundary layer breaks away from the surface of the wing due to the magnitude of the positive pressure gradient. Beneath the separated layer, bubbles of stagnant air form, creating additional drag because of the lower pressure in the wake behind the separation point.

These bubbles can be reduced or even eliminated by shaping the airfoil to move the separation point downstream or by adding a device, a turbulator that trips the boundary layer into turbulence. The turbulent boundary layer contains more energy, so will delay separation until a greater magnitude of negative pressure gradient is reached, effectively moving the separation point further aft on the airfoil and possibly eliminating separation completely. A consequence of the turbulent boundary layer is increased skin friction relative to a laminar boundary layer, but this is very small compared to the increase in drag associated with separation.

In gliders the turbulator is often a thin zig-zag strip that is placed on the lower side of the wing and sometimes on the vertical stabilizer.^[1] The DG 300 glider used small holes in the wing surface to blow air into the boundary layer, but there is a risk that these holes will become blocked by polish, dirt and moisture.

For the aircraft with low Reynolds numbers (i.e. where minimizing turbulence and drag is a major concern) such as gliders, the small increase in drag from the turbulator at higher speeds is minor compared with the larger improvements at best glide speed, at which the glider can fly the farthest for a given height.

In pipe flow

The heat transfer coefficient for liquids and gases flowing through pipes in heat exchangers tends to be limited due to a fluid boundary layer close to the pipe wall that is stagnant or moves at slow speed, thus acting as an insulating layer. Such heat exchangers are, for example, in domestic central heating systems. This boundary layer can be broken or reduced in thickness if turbulators are placed in the pipe, which create a turbulent flow that reduces the boundary-layer thickness and thereby increase the heat-transfer coefficient. This increase in heat transfer coefficient comes at the price of increased pressure drop through the pipe as well as increased manufacturing cost and complexity.

Examples of turbulators for pipe flow are:

Twisted-tape turbulators, a twisted ribbon that forces the fluid to move in a helicoidal path rather than in a straight line;
Brock turbulators, a zig-zag folded ribbon;
Wire turbulators, typically an open structure of looped and entangled wires that extends over the entire pipe length.

Turbulators can also be put to use in certain internal combustion engines - particularly, a ramjet engine. A simple porous wire mesh placed in the diffuser of the ramjet can increase turbulence in the flow entering the combustion chamber, which aids in fuel mixing.

In rotating drums

Axial bars are placed on the inside of steam-heated drums to create turbulence in the rimming condensate layer. If the spacing of the bars is chosen correctly, resonant waves are created in the condensate film, between each pair of bars. This significantly increases the level of turbulence, even at high drum speeds. This increase in turbulence reduces the resistance to heat transfer from the steam, through the condensate film, to the drum shell. The increase in heat transfer can be as large as 50%, even when compared to the performance of drums with modern, close clearance rotating syphons. The maximum increase, however, requires the bars to operate with the optimum condensate layer thickness. ^[2]

In addition, these axial bars can reduce the speed at which the condensate layer will move from the cascading stage to the rimming stage. This can provide a significant reduction in the power and torque required to drive the rotating cylinder. ^[3] When operating with the right configuration of bars and the corresponding optimum condensate depth, these axial bars achieve a high rate of heat transfer and a high degree of heat transfer uniformity.

Axial bars are commonly used inside rotating drums in the paper and fabric manufacturing industries, for example, in paper machine drying cylinders and similar steam-heated cylinders that are rotating at high speeds where the condensate would normally be in a rimming condition. These bars increase the drying rate and uniformity of the cross-machine moisture profile of the paper. A number of different configurations of axial bars have been used in these applications, with differing mechanical constructions. The term “Turbulator” is a registered trademark of Kadant Johnson and defines a specific configuration of axial bars used in these rotating drum applications.

References

↑ Proper Turbulator Placement. Maughmer, Swan, Willits (2003).
↑ Wedel, G. L., & Timm, G. L. (May, 2009). Heat Transfer Performance with Dryer Bars. Three Rivers, Mich.
↑ Wedel, G. L., & Timm, G. L. (April, 2009). Drive Power and Torque in Paper Machine Dryers. Three Rivers, Mich.

External links

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