Precision Meters: Laminar Flow – Air


Definition / Descriptions:
a) Non-turbulent flow of a viscous fluid in layers near a boundary, as that of lubricating oil in bearings.

b) Smooth, orderly movement of a fluid, in which there is no turbulence, and any given sub-current moves more or less in parallel with any other nearby sub-current. Laminar flow is common in viscous fluids, especially those moving at low velocities.

c) Non-turbulent streamline flow in parallel layers (laminae)

A long time ago, 1962 to be precise, working in the diamond mines run by De Beers Consolidated Mines Ltd., in Kimberley was when I first came face to face with the effects of poor laminar flow.

Management was planning for the next development stage which entailed going deeper to exploit the Kimberlite that held the diamonds resulting from volcanic action millions of years previously.

The main and ventilation shafts in the bedrock on either side of the extinct volcano were to be extended by another 600ft and a series of haulage tunnels connecting to a pattern of tunnels crossing between the shafts would then follow.

The expansion as well as the existing working areas would need to be adequately ventilated by the massive exhaust fan, sucking some 500,000 cubic feet of air per minute, that sat atop the ventilation shaft. As may be imagined, the walls of the shafts and the tunnels along with all the impediments in the main shaft presented very rough surfaces that caused resistance to air flow.

The particular project to which I was assigned was to carefully measure the air flow at various points throughout the mine, correlate these with the length of certain tunnels and their dimensions and using this information to determine the probable improvement that would be brought about if smoother surfaces were constructed by lining these tunnels with concrete.

Using hand held anemometers, moved in a controlled pattern over the height and width of a tunnel, a precise measurement of the air flow was possible especially if the measurement was repeated over given time periods so as to average out any unusually high or low readings.

An anemometer is a finely balanced fan, shaped like a propeller, that is geared to a dial that records the revolutions turned by the fan. The dial is reset before each reading.

These are available as a hand held unit which is held at arms length, so as to reduce air disturbance caused by the body of the person doing the measuring and then moved at a timed rate, in a pattern, across the extent of the area being measured.

Each revolution recorded is equal to a known volume of air that has passed over the fan blades. Using the total volume of air multiplied by the cross section of the aperture measured results in the total volume of air that is moving through the aperture – water meters operate in much the same manner, being driven instead by water flowing over the blades, with each revolution being recorded and directly being converted into the cubic measure of water that has passed through the meter since the last reading of the dial.

The project was able to demonstrate the degree of air flow improvement that could be expected and the large cost saving in not having to shut the entire mine down so as to exchange the main ventilation fan for a larger unit.

Nikola Tesla (Serbian: 10 July 1856 – 7 January 1943) was an inventor, mechanical engineer, and electrical engineer. He was an important contributor to the birth of commercial electricity, and is best known for his many revolutionary developments in the field of electromagnetism in the late 19th and early 20th centuries. Tesla’s patents and theoretical work formed the basis of modern alternating current (AC) electric power systems, including the polyphase system of electrical distribution and the AC motor. This work helped usher in the Second Industrial Revolution.

He well understood the effects of laminar flow when he designed the Tesla Turbine which differs from the standard turbine in that it has no vanes that are prone to damage by mechanical fatigue or particle intake with resultant high repair or replacement costs.

His turbine used two disks, with a narrow space between them, that were mounted on a shaft with bearings.

The disks were easy to manufacture and had a series of exhaust slots or holes drilled through them in a circle spaced a certain distance from the shaft.

By directing a tangential flow of air, under pressure, through a nozzle to the narrow space between the disks, laminar flow causes the disks to begin to spin with the air flow spiralling down between the disks and exiting via the holes or slots.

Initially the air forms a spiral around the shaft as it moves through to exit through the slots or holes until, at the rotational speed reached relevant to the pressure / air flow, the air flows in a more direct path exhausting through these holes or slots.

Particles small enough to pass through the space between the disks pose no impediment or damage as these would merely flow through the gap and exit via the slots or holes.

The following link will take you to a video that explains Laminar Flow.

Precision Meters: Laminar Flow – Air

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