My company is setting up an test to measure the airflow rate in a duct. Airflow measurement techniques are necessary especially in many market sectors. A number of the common airflow measuring applications include air flow evaluation, air balancing, ductwork, air planes etc. Many research and studies have been placed into enhancing and inventing new accessories to measure air flow. This is so to enable end user to get the most accurate result and at the same time using the least cost.
This report outlines the different airflow dimension techniques and devices that exist today. There are various types and ways to assess air flow but I will concentrate on the ones that are more popular and popular. They will be the Pitot-tube, Orifice dish, Venturi meter, Glass anemometer, Sphere anemometer and Hot-Wire anemometer.
Techniques and devices for airflow rate measurements
2. 1 Pitot tube
A pitot-static pipe is used in blowing wind tunnel tests and on airplanes to measure the ventilation rate. It is also found in many professional applications. It was developed by the French engineer Henri Pitot in the first 1700s and was later revised to its modern form in the mid 1800s by People from france scientist Henry Darcy. It really is a slender pipe that has two slots onto it (Physique 1). Leading hole is put in the airstream to assess what's called the stagnation pressure. The medial side hole steps the static pressure. By measuring the difference between these pressures, we're able to get the energetic pressure you can use to calculate mid-air velocity.
By Bernoulli's rule,
Stagnation pressure = static pressure + active pressure
Solving that for speed we get:
Where V: smooth speed; pt: stagnation or total pressure; ps: static pressure; : liquid density
Figure 1: Pitot-Static Tube
The incorporation of detectors to measure the air temp, barometric pressure, and comparative moisture can further improve the exactness of the speed and flow measurements. The Pitot tube can also gauge the velocity by using a pressure transducer that creates an electrical signal which is proportional to the difference between your pressures generated by the full total pressure and the static pressure. The volumetric stream is then computed by measuring the average velocity of the air stream passing through a passage of a known diameter. When measuring volumetric flow, the 'passage of the known diameter' must be designed to reduce air turbulence as air mass flows over the Pitot tube.
To obtain an estimate of the volumetric movement in the duct from some pitot-static pipe velocity
measurements, one must integrate the velocity above the duct area.
There are a lot of different methods for approximating the aforementioned integral. One of the methods is to divide the duct cross-section into lots of equivalent area sectors, and then evaluate the
"average" speed at the guts of each sectors.
For example, we can split the cross-section of the duct in the body below:
The speed will be by calculating
the total:
2. 1. 1 Advantages and Disadvantages
The benefits of using pitot-static tube is that it can be placed in small airstream and it reveals little level of resistance to flow. It is simple, inexpensive and suited for a variety of environmental conditions including extremely high heat and a variety of pressures.
The cons would be that if the flow rate is low, the difference in stresses will be too small to effectively assess with the transducer. If the ventilation is high (supersonic), assumptions of Bernoulli's formula will be violated and thus leading us to wrong way of measuring. Furthermore, if the pipes are clogged, the reading by the transducer will be inaccurate resulting in dire effect in the context on airplane. Icing of the pitot pipe had caused aircraft to crash.
2. 2 Orifice Plate
Orifice plate is used for circulation rate measuring in tube systems. An orifice plate is put in a tube containing a smooth movement, which constricts the simple flow of the substance inside the pipe. By restricting the move, the orifice meter causes a pressure drop over the plate. By measuring the difference between the two pressures over the plate, the orifice meter determines the movement rate through the pipe.
Figure 2: Orifice Dish in a duct
Applying Bernoulli's formula to a streamline moving down the axis of the pipe gives,
Where,
P: pressure
: density of the fluid
V: Velocity of the fluid
As shown in the above diagram(Figure 2), point 1 is upstream of the orifice, and point 2 is behind the orifice. It is recommended that point 1 be positioned one tube diameter upstream of the orifice, and point 2 be positioned one-half pipe diameter downstream of the orifice. Because the pressure at 1 will be higher than the pressure at point 2, the pressure difference will be a positive amount.
From continuity equation, the velocities can be substituted by cross-sectional regions of the stream and the volumetric movement rate Q,
Where,
A: cross sectional area
Solving for the volumetric stream rate Q gives,
The above formula remains true with correctly laminar, inviscid moves. For real flows like normal water or air, we have to take into account of the viscosity and turbulence that are present. To take into account this impact, a discharge coefficient Disc is introduced into the above formula to marginally decrease the flow rate Q,
Since the genuine flow account at point 2 downstream of the orifice is quite complicated, the following substitution launching a move coefficient Cf is made,
Where,
Ao: section of the orifice
As a result, the volumetric move rate Q for real moves is given by the equation,
The stream coefficient Cf is available from experiments and is tabulated in research books. It runs from 0. 6 to 0. 9 for some orifices. Because it depends upon the orifice and pipe diameters (as well as the Reynolds Amount), one will often find Cf tabulated versus the proportion of orifice diameter to inlet diameter, sometimes defined as b,
The mass stream rate are available by multiplying Q with the fluid density,
There are mainly 3 different kinds of orifice plates. They are simply Concentric, Segmental and Eccentric. This is to accommodate for different applications so that the meter has the optimum composition. The denseness and viscosity of the smooth, and the condition and width of the tube do influence the choice of plate condition to be used.
The concentric orifice is the most frequent of the 3 types. In this design, the orifice is equidistant. It is generally used for clean water and gas flow in pipes under six inches, where Reynolds numbers range from 20, 000 to 107. We will therefore use concentric orifice for our experiment purposes. ( which handles air).
Segmental orifice is comparable to concentric orifice with regard to its working. The round section is concentric with the pipe as the segmental part is installed in a horizontal tube. This installation helps to eliminate of overseas materials on the upstream side of the orifice.
Eccentric orifice plates were created in such a way that the border of the orifice is reallocated towards the inside of the tube wall. It can be used in similar manner as the segmental orifice dish.
Figure 3 below shows the various types of orifice plates:
2. 2. 1 Advantages and Disadvantages
With no moving parts and a simple design, the orifice is easily machined. It really is low lost and can be easily inserted into a duct or a preexisting pipeline with a minimum alteration to the design. Therefore orifice dish is a popular device for flow measurement.
The disadvantage is the fact it creates a fairly large non-recoverable pressure due to the turbulence about the plate, resulting in high energy intake (Foust, 1981).
2. 3 Venturi meter
Most of the unrecoverable loss of pressure with an orifice is due to the sudden change in the combination sectional area. The immediate increase of area following the air moves the portion of least area: the immediate convergence of the stream on the upstream aspect contributes considerably to the total loss. We're able to recover the majority of the pressure by leading the stream by using a conical amount of pipe, using its smaller end of the same cross section as the jet, and gradually extending in size along the direction of flow until the full tube diameter is reached. An arrangement of the kind, with a conical accessibility is known as a venturi tube.
The Venturi impact is named after Giovanni Battista Venturi (1746-1822), an Italian physicist.
A venturi meter consists of a cylindrical span, a converging span with an included perspective of 20o or even more, and short parallel neck, and a diverging section with an covered angle around 6o. The inner coatings and proportions were created so to allow us to attain the most appropriate readings while guaranteeing minimum head losses.
Assuming that the fluid is inviscid with no losses scheduled to viscosity, the velocity at section 1 and 2 are V 1 and V 2 respectively. The velocities are continuous and uniform over areas A 1 and A 2
Applying Bernoulli's equation to a streamline transferring across the axis between your two portions ( 1 & 2 ).
Where,
V:Velocity of the fluid
P: Pressure
: thickness of the fluid
Z: Height
Using continuity equation,
Q = A1 V 1 = A 2 V 2
When real life results such as fluid friction and turbulence are included a correction factor, called the coefficient of release, Cd is presented in to the venturi equation giving
For low viscosity fluids C d = 0, 98.
2. 3. 1 Advantages and Disadvantages
The venturi pipe introduces considerably lower non-recoverable pressure drops (Foust, 1981). Therefore project tube can be utilized on more viscous fluid.
However it has limited range capability. It must be used only on installations where the flow rate is well known and varies significantly less than 3 to at least one 1. It is extremely expensive and should be circulation calibrated to provide correctness into the range of +/- 1. 00%, Systems are big and weigh more than equivalent head devices and thus which makes it difficult to install and check.
2. 4 Anemometer
An anemometer, also called wind flow vane is a tool for measuring the ventilation rate in a covered stream such as duct or unconfined flow. The term is derived from the Greek expression anemos, meaning wind. In around 1450, the Italian skill architect Leon Battista Alberti developed the first mechanical anemometer which consisted of a disk put perpendicular to the wind. To look for the velocity, an anemometer detects change in a few physical property of the liquid or the effect of the liquid on a mechanical device inserted in to the flow. They are most likely best used installed on light, preferably streamlined, aids and inserted into the airstream from one side.
2. 4. 1 Glass anemometer
This device involves three or four hemispherical cups attached at the ends of horizontal spokes which rotates about a low-friction vertical shaft. A power device is used to track record the revolutions of the cups and measures the ventilation rate. (Figure below)
As the anemometer is located inside the stream stream, the concave areas of the mugs have higher wind level of resistance than their convex counterparts and therefore producing an unbalanced point in time with respect to the centre axis. This forces the mugs to rotate (see schematic). Under regular stream condition, the rotational quickness of the anemometer is straight related to the wind flow speed, that is: V=rw.
There are quantity of fundamental physical variables and characteristics associated with an anemometer that influences the cup anemometer performance. They are really:
rotor arm length
cup area
rotor inertia
drag coefficient on convex face of cup
drag coefficient on concave face of cup
static, dynamic and parabolic mechanical friction coefficients for temperature range
sensitivity quality to out-of-plane attack
linearised calibration curve.
A "smartly designed" glass anemometer should have the next characteristics as shown in the Figure 4 below:
Let us analyze a glass anemometer rotating at quickness w in a free wind rate U:
The instantaneous aerodynamic torque on the rotor, MA, is distributed by:
where A: frontal area of the anemometer
r: the air density
Cdv : drag coefficients for the concave encounters of cup
Cdx pull coefficients for the convex faces of cup
In the constant state, there is perfect torque balance (MA=0), and the equation reduces to:
defining l and as the rate and move ratios respectively:
allows further re-expression in a quadratic form:
Typical values of Cdv and Cdx are 1. 4 and 0. 4 respectively, providing a value of of 3. 5. The above
equation predicts that the consequential swiftness proportion l will be 0. 303, meaning the rotor will turn at about 1 / 3 of the wind flow speed. Remember that this solution also shows the theoretically linear awareness of the glass anemometer to wind flow speed. In addition, it implies that the speed proportion is dependent on the move characteristics of the glass and not the size. Furthermore, the rotational velocity is inversely proportional to rotor radius.
2. 4. 2 Advantages and disadvantages
The advantages of the mugs are their stability and ruggedness. The disadvantages will be the relatively high threshold speed (the minimum breeze velocity needed to start the cups to carefully turn). It really is mainly used to only gauge the horizontal component of the wind flow.
Another problem with cup anemometry is the several response time for increasing and lowering wind velocities due to its second of inertia. This results in an overestimation of wind flow speed under turbulent breeze conditions as present in nature, the so-called over-speeding. Additionally, the rotation of the anemometer triggers a wear of bearing and leads to a recalibrations with time.
2. 5 Sphere anemometer
Many research and studies have eliminated into the enhancing of such a device (Cup anemometer). For example, the sphere anemometer. It had been developed at the School of Oldenburg.
This sphere anemometer, as shown in figure 4 below, can measure the air flow rate as well as simultaneous recognition of the ventilation direction. It minimizes the challenge of wear of bearing as experienced in glass anemometer.
Figure 5:
The sphere anemometer uses the relationship between the point pressure F functioning on the tip of a
rod and its resulting deflection s.
(1)
Where
l: the distance of the rod
E: the elasticity modulus
Ja: the next second of area.
In circumstance of the sphere anemometer, with a sphere radius r much bigger than the radius of the fishing rod rR, the push can be assumed to do something only on the tip. The second minute of area is then given by
(2)
Together with the drive acting on the sphere
(3)
where cd: the pull coefficient of the sphere
A: the cross portion of the sphere
: the thickness of air
V: the wind flow velocity
Equation 1 becomes
(4)
Therefore the deflection of the pole is proportional to the move coefficient compact disk and the wind velocity squared. To get a calibration it is necessary to know the way the drag coefficient disc changes with wind flow velocities. Desk 1 below shows the move coefficient of your sphere plotted contrary to the Reynolds quantity (Re) (cf [1]). It can be seen that for Reynolds numbers in the number from about 800 to 200000 the change in pull coefficient cd is negligible.
For a sphere with a radius r = 40mm this range in Re corresponds to a range in blowing wind velocities
from 0. 17m/s to 38m/s using
where v = 1. 51 x 10-5 m2/s is the kinematic viscosity of air. Within this velocity range the
deflection s of the fishing rod is directly proportional to the breeze velocity squared. With this direct
relation it is not hard to calibrate the sphere anemometer over a wide range of wind velocities.
Table 1:
2. 6 Hot line anemometer
Thermal anemometry is the most frequent method used to measure instantaneous fluid velocity. The technique depends upon the convective high temperature loss to the encompassing fluid from an electrically heated up sensing element or probe. If only the fluid speed varies, then your heat reduction can be interpreted as a way of measuring that changing.
Working Principle
It's principle program is the way of measuring of quick fluctuations, specially the study of turbulent flow; in this field it is the only instrument with sufficiently swift response, and the associated electronic digital equipment lends itself conveniently to signal handling needed to track record immediately such properties of a turbulence as r. m. s prices, relationship functions, and spectral distributions.
Governing equation
Consider a slender heated wire mounted to supports and exposed to a speed U
Where,
W: power produced by Joule heating system (W=I2Rw)
Q: heat transferred to surrounding
Qi: CwTw=thermal energy stored in wire
Cw: heat capacity of wire
Tw: line temperature
The cable is heated up electrically and placed in the stream stream. The energy balance of the heated wire at equilibrium is (equation 1):
Where,
I: a power current
Rw: the wire resistance
h: heat transfer coefficient
A: heat transfer area
Tw: the cable temperature
Tf: the substance temperature
D: line diameter
Kf: heat conductivity of fluid
Nu: dimensionless warmth transfer
In the forced convection regime (0. 02<Re<140):
Reynolds amount: Re= (where r is air density and U is the velocity and is the environment strong viscosity). (equation 2)
Where
Substituted Eq(2) into Eq(1),
There are two types of hot-wire anemometer used in practice but I am going to touch on Regular Temperature Anemometer which is additionally used.
For an instance of Constant Temps Anemometer
Where
And
The voltage is a assessed of speed U.
2. 6. 1 Advantages and disadvantages
It has good rate of recurrence response as it could measure up to many hundred kHz possible. With the ability to measure an array of velocity. It is small in proportions and has quick response. -
Thermal anemometry likes its acceptance because the technique involves the use of very small probes that provide high spatial resolution. The basic principles of the approach are relatively uncomplicated and the probes are difficult to ruin if reasonable good care is taken.
However, deposition of impurities in circulation on sensor can transform the calibration characteristics and reduce rate of recurrence response. Probe may or burnt out easily if not carefully taken care of. It is unable to fully map speed fields that rely upon space coordinates and all together promptly. Furthermore, it cannot work very well in hostile environment like combustion. The wire diameter needs to be really small - of the order of 0. 02mm or less.
Conclusion
In this record, I have touched on different techniques and various devices for the measurement of airflow. There are many different devices in the market but many use similar techniques with abit of new innovations or add-ons occasionally. Different airflow measuring devices utilize different technologies and thus, one must grasp the characteristics, techniques and its pros and cons before selecting the optimal one for use.
In summary, a perfect device to measure air flow rate must have the following characteristics
good signal level of sensitivity. It should be able to discover result for small changes in speed.
High Frequency Response: to accurately follow transients without any time lag
Wide velocity range
Create minimal movement disturbance
Good Spatial Resolution
Inexpensive
High Accuracy
User friendly
