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SCIENCE

SERIES

27

Performance of Louvered Devices

as Air IVIixers

4

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UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS , • Lewis M. Branscomb, Director

Performance of Louvered Devices as Air Mixers

T. K. Faison, Jr., J. C. Davis, and

P. R. Achenbach

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Nat. Bur. Stand. (U.S.), Bid?. Sci. Ser. 27, 22 pages (Mar. 1970) CODEN: BSSNB

Issued March 1970

For sale by the Sunerintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402. (Order by SD Catalog No. C 13.29/2:27). Price, 30 cents
Contents

Page

1. Introduction 1

2. Apparatus and test procedure 1

2.1. Instrumentation 3

2.2. Conditioning and control 3

3. Description of mixing devices 3

3.1. Louvered strip 3

3.2. Louver-baffle 4

3.3. Concentric louver 4

4. Performance 5

4.1. Louvered strip 5

4.2. Louver-baffle 8

4.3. Concentric louver 13

4.8,1 Applications of the concentric louver mixer 14

4.4. Comparison of other air mixer designs 17

5. Summary 18

6. References 18

7. Appendix. Construction techniques for fabrication of mixing

devices 19

Library of Congress Catalog Card No. 73-603859
Performance of Louvered Devices as Air Mixers

T. K. Faison, Jr., J. C. Davis, and P. R. Achenbach

As part of a study of evaluating methods for reducing thermal gradients within the

cross section of an air stream, three louvered mixing devices were investigated. Each of these devices was found to be capable of reducing the cross-sectional nonuniformity of

air temperature to a few percent of the entering value. The three devices covered in this report contain combinations of louvers (directing vanes) and baffles as mixing elements. Two of the devices were designed at the National Bureau of Standards ; the third was a modification of a previous design. The three mixers (the louvered strip, the concentric

louvers, and the louvered-baffle) required 4.75, 3.8, and 3.0 duct diameters, respectively,

to reach a mixing effectiveness level of 97 percent. The mixing effectiveness of the louvered strip and concentric louver models was independent of the approach velocity,

whereas the effectiveness of the louver-baffle model was somewhat dependent on the approach velocity. The pressure drops accompanying air flow through the mixers,

expressed as multiples of the velocity head of the entering air, were approximately 7, 5, and 38 for the louvered strip, concentric louver, and louver-baffle mixers, respectively.

Key words: Effectiveness; forced mixing; mixing device; pressure drop; temperature;

uniformity.

1. Inti

The lack of uniformity of temperature

within the cross section of air streams has long

posed a problem to investigators trying to

' make accurate measurements of air-stream

temperature. Several mixing devices have been

developed and many rather complicated designs

have been devised to promote uniformity of

temperature, although the literature contains

little information of a quantitative nature on

their effectiveness. Each of the methods seems

to have its ov^^n particular drawback—too complicated to use, the pressure drop too high, the

.1 length for mixing too long, etc.

( A recent study [1] ^ at the National Bureau

of Standards showed that thermal gradients

within the cross section of an air stream can be

effectively diminished through forced mixing.

, With the removal of the undesirable condition

, of temperature nonuniformity, the average

temperature of the air stream can be more accurately obtained and air stream temperaturedependent properties, such as enthalpy, can be

determined with greater precision.

Several types of air mixing devices were

evaluated in the laboratory but evalutations of

only three types are presented here; louvered

strip, louvered-baffle combination, and concentric louvers. The performance of each is given

for various initial conditions of nonuniformity.

The names given these mixers tend to describe

the geometric configuration of each. The au- thors have previously described the apparatus

in which the devices were evaluated and discussed the performance of the orifice as a mixer for removing thermal gradients in an

air stream [1, 2]. The three types presented

here and the orifices previously reported [1] were studied in detail Several other designs,

which were not as effective, were evaluated

only qualitatively.

2. Apparatus and Test Procedure

The test appartus [2] was designed to produce in a 24-in-diam duct a condition (nonuniformity of temperature in the air stream)

that might be commonly encountered in heating and air conditioning systems and laboratory applications. A schematic of the apparatus is shown in figure 1. Figure 2 is a photograph of the apparatus. A detailed description

of the apparatus, the instrumentation, and procedure is given in reference [2]. Briefly, the

apparatus functioned as follows: Air preconditioned and maintained at a constant temperature was drawn into the inlet blower at a controlled rate. Downstream from the blower, just

^ Figures in brackets indicate the literature references at the end

of this paper.

1
STATIC PRESSURE TAPS CONTROL THERMOCOUPLE WATER

il VAPOR INLETS ' 7 CONSTANT VOLTAGE i \\ fn, TRANSFORNCR

EXHAUST TO

RECONDITIONMG ZONE

PRESSURE TAPS

latliol

Figure 1. A sketch of the mixing apparatus used to produce the test condition and

hou^e the mixing devices to he evaluated. iPIMfP'

Figure 2. Overall view of assembled test apparatus.

after an enlarging transition, the duct cross-sectional area was divided into four equal 1-ft squares (sect. A-A, fig. 1). The dividing partitions were made to extend from beginning of

the 24-in square conditioning portion of the apparatus on through a portion of the round duct

and terminated at a position a short distance

upstream from the section shown as B-B in

figure 1. The partitions in the circular duct di- vided the duct space or air stream into four

equal pie-shaped quadrants. The portions of

the apparatus immediately upstream and

downstream of section A-A in figure 1 con- tained electrical resistance heaters in each of

the spaces to produce and maintain selected

conditions of temperature.

Immediately following the conditioning section

teal

of the apparatus, a measuring station was posi

tioned for determining the temperature pat

tern of the air prior to its entering the mixing,

device to be tested. From the first measuring

station, the air passed through the mixing device and along the duct to a second measuring

station. The second station could be repositioned along the length of the duct to determine the uniformity of the air stream at different locations downstream from the mixing(i^t6r(

device. The air was then passed from the apparatus through the exhaust blower. The two

blowers, the supply and exhaust, were dampered to control both the flow rate and the

static pressure within the apparatus.

Temperature patterns shown in figure 3 were used to evaluate the performance of the

mixing devices. These patterns indicate distinct differences in temperature within the

cross section of air streams prior to entering a

device under test. The cases shown in the center and left part of the figure might represent

situations where two streams of fluid merge to- gether and form patterns of nonuniformity.

The case shown at the right in the figure illus- trates a situation where air flows for a distance in a duct located in an ambient temperature that is higher or lower than the initially

uniform temperature in the duct, thus causing

the temperature near the wall of the duct to be different from that at the center of the

stream. These patterns are fairly common in

practice and should give meaningful information on the ability of a mixing device to promote mixing and thus produce a homogeneous

stream.

2
r AND t REPRESENT AREAS OF AN AIR STREAM AT DIFFERENT TEMPERATURES

Figure 3. Patterns of temperature distribution within the cross section of the

test stream.

2.1. Instrumentation

Temperature measurements were made at each of two stations. Measuring station No. 1 (see fig. 1) was located just downstream from

the termination of the quadrant partitions,

where the nonuniform temperature pattern of

the air was measured as it existed prior to passing through the mixing device. The second

measuring station was used to make measure- ments of the temperature distribution at se- lected points along the duct length as the mixing progressed. Both stations were constructed

having the same physical pattern as shown in

figure 1, with the only difference being that the

upstream station was stationary and the downstream station was movable. At each station an

array consisting of 24 copper-constantan thermocouples constructed of 30 gage (A.W.G.)

wire was used. Thermocouples were calibrated

at the National Bureau of Standards and were

manually read on a precision-type potentiometer capable of direct reading to 1.0 /xV (0.05

deg F for the wire used) with interpolation to

0.1 ju,V. A zone box [3] was used in the thermocouple circuit to permit the use of a common ice

reference junction and all switching was accomplished in the copper portion of the thermocouple circuit.

From time to time comparison tests to determine the magnitude of variations amongst the

24 thermocouples at each station were made by

immersing all of the thermocouples in a Dewar

containing room-temperature water. The difference between any two of the 24 thermocouples was always less than 0.7 /aV (0.03 deg F).

2.2. Conditioning and Control

Air flow rate in the test apparatus was controlled by varying adjustable dampers at the

inlet of the first blower and at the exhaust

downstream from the second blower. By proper adjustment of these two dampers, the pres- sure inside the apparatus was maintained at a

positive level to prevent leakage of unconditioned air into the system from the room. Flow

rates through the four quadrants were approximately equal. The volume rate of air flow was

determined by using pitot-static tubes to measure the velocity head at 18 positions in the

cross-sectional area of the rectangular inlet

duct. The duct had a straight length of 8 ft

ahead of the station of measurement. The

pressures from the pitot-static tubes were

measured with a Hook gage, which could be

read to the nearest 0.001 in of vertical water

column. Using potentiometer-type thermostat

controllers and manually operated base heaters,

air temperature was maintained constant at preselected temperature levels in the apparatus.

The major increase of the air temperature was

obtained from base heaters. Final control was

obtained by thermostatic operation of trimmer

heaters of smaller capacity. Each of the four

quadrants was individually controlled to provide flexibility in selection of temperature

patterns. Electrical resistance heaters positioned in a circular pattern around the

periphery of the duct near the duct wall were

used to produce concentric temperature patterns.

3. Description of Mixing Devices

In this study, three types of mixers were investigated: louvered strip, a louver-baffle combination, and concentric louvers. These mixers were designed with the objective of obtaining a

satisfactory combination of mixing effective- ness, pressure drop, and space requirements

for their use. The design and construction of

the three types of mixers are described in the

following sections.

3.1. Louvered Strip

In designing the louvered strip mixer, the area enclosed by a circular band 24 in in diameter was divided into 6 horizontal strips, each

4 in high, as shown in figure 4, left view. The

areas bounded by the strips were fitted with

vertical louvers for deflecting the air from its normal path. Louvers were designed so that

3
Figure 5. A view of the louver-baffle mixing device showing the two elements in series.

Figure 4. A view of the louvered strip mixing device showing the two elements in series.

the air passing through any two adjacent louvered areas was deflected through equal and

opposing angles from its normal path. For ex- ample, the air passing through one area was

deflected left by the louvers and in the adjacent area the deflection of the air was to the

right. The air streams issuing through adjacent louvered strips moved in opposing directions, thus creating a shearing action between

the streams at their interface. Both the forces

generated bv the shearing action and the resist- ance set up by opposing flow caused the formation of a highly turbulent field downstream

from the mixing device.

Two louvered strip mixers of this construction were placed in series for this study. The

first mixer provided shearing action in the horizontal plane. The second, rotated 90° in res- pect to the first mixer and located downstream,

provided shearing action in the vertical plane

(see fig. 4)

.

3.2. Louver-Baffle

The louver-baflle mixing device, as shown in

figure 5, is a modification of one developed by

Wile [4]. It was modified to fit into a round

duct rather than a square or rectangular duct.

For this mixer, the circular area was divided

into six vertical strips each 4 in wide (left, fig.

5). One-half of the area of each strip was completely covered by a fixed metal baffle, thus

making up a pattern of staggered baffles and

openings over the large circular area. Behind

the opening in each strip a set of louvers was

attached to direct the flow of air from one side

of the duct to the other. By blocking half of

the total cross section of the duct, the set of

baffles approximately doubled the average air

velocity in the plane of each mixing element.

The baffles created in their wake, low-pressure

regions into which the deflected air could flow.

A pattern of turbulence was set up by the ac- tion of the adjacent layers of air moving in opposing directions. Two of the louver-baffle ele- ments were used in series. Through the first element the air stream was deflected vertically and in the second element a horizontal deflection was achieved by rotating the element

90° from the first.

3.3. Concentric Louver

Shown in figure 6 is a set of two concentric

louver elements which were placed in series.

Figure 6. A view of the concentric louver mixing

device showing the two elements in series.
The large circular area was divided by three

concentric bands of 4 in in radial vi^idth. The

two outer annular areas and the central circular area were fitted with louvers, which deflect

these streams of air in a clockwise or counterclockwise direction. Because of counterrotation

of the adjacent annular air streams, turbulence was generated at the interface of the annuli

downstream from the mixing element.

Through the shearing movement of the rotat- ing air streams, a general condition of mixing

or energy transfer was developed. As shown,

a second set of concentric louvers caused the

air to be directed in a direction opposite to that

of the corresponding areas of the first set of

concentric louvers.

4. Performance

For each of the three mixers, tests were con- ducted to determine how a variation of each of

the following parameters would affect mixer

performance.

1. Temperature pattern

2. Initial temperature nonuniformity

3. Distance between mixing elements

4. Overall distance required for mixing

5. Air flow rate

6. Deflection angle.

In each test, temperature values were re- corded for each of the 24 thermocouple positions at the upstream and downstream measuring stations. A statistical analysis was made of

the temperature values of the two stations.

Five sets of temperature measurements were made for each test, and standard deviations of

the upstream and downstream distribution were made for each set. An average upstream stan- dard deviation and an average dovnistream

standard deviation were obtained from the five

sets. A value of effectiveness, which would give an indication of mixing capability, was calculated by subtracting from unity the ratio of

downstream to upstream standard deviations.

The mathematical relationships used for determining both the standard deviation and the

effectiveness are defined by eqs (1) and (2),

S.D.

Effectiveness SD = ( 1.0 --MiBg-

)

\ S.D.us /

X 100% (2)

where

;

n

Xi

X

= number of measurements, = individual measurements, = arithmetic mean,

S.D.Ds = average standard deviation

downstream,

S.D.us = average standard deviation

upstream.

The range of values was also found for the sets of upstream and downstream temperatures. An

alternate value for effectiveness, based on

range, was calculated by subtracting from

unity the ratio of the average downstream to upstream ranges. This relationship is shown in

eq (3) :

Effectiveness

where

(Range) (

1.0 -

Range I Range us )

X 100% (3)

Range DS = average (Xmax - aj^in)

downstream

Rangeus = average {x^^^ - Xn^in)

upstream.

This latter method of determining effective- ness, based on the range, is much simpler to

calculate but could be more influenced by a single temperature value. If, for instance, either

the maximum or minimum value were in error or differed considerably from the average value,

then the effectiveness based upon the range

would reflect this one value to a greater extent

than would the method based upon standard

deviation.

Since the air flow was in the turbulent re- gion for these tests, it was of interest to determine to what extent the air was mixed during

the simple process of flowing along the duct

with no mixing device in the duct. Earlier tests [1] with no mixing device in the duct showed

that mixing caused by the inherent turbulence

and duct configuration was less than 7 percent

when calculated by the range method of determining effectiveness. When calculated by the

standard deviation method, a higher value of 17

percent was obtained for the natural mixing be- cause this method takes into account all of the

observed values. Some mixing, which is not indicated by the range method, does occur at the

interface between streams of different temperature air.

4.1. Louvered Strip

Table 1 is a summary compilation of all of

the tests conducted on the louvered strip mixing device and gives details of the test conditions and resulting performance at these conditions. In all illustrations which relate effec5
TABLE 1. Summary of test conditions and performance of the louvered

strip mixer.

Test

No. FlowRate

Pressure

Drop Across

Mixer

Distance Between Mixing Elements

Distance from 2nd Element to Down- stream Meas . Station Overall

Distance

Temperature

Conditions

at Mixer Inlet Effectiveness

Temp

.

Diff. Temp

.

Pattern"

Range Std. Dev. CFM in. W.G. in

.

Duct

Diam., in

.

Duct

Diam.

in

.

Duct

Diam. deg F 7o %

1 1400 0.07 12 0.50 102 4 .2 5 114 4 . 75 3 .

6

90 .

1

92 .5

2 1400 0.07 24 1 . 00 90 3 . 75 114 4 . 75 3 .

6

91 .5 93 .

9

3 1400 0.07 36 1 .50 78 3.25 1 14 4 . 75 3 .

7

94 .

5

96 .

1

4 1400 0.07 48 2 . 00 66 2 . 75 1 14 4 . 75 3 .6 95 .

1

96 .2

5 1400 0.07 60 2 .50 54 2 .25 114 4 . 75 3 .6 94 .

7

96 .3

6 1400 0.07 72 3 . 00 42 1 . 75 114 4 . 75 3 .

6

93 .

8

96 .2

7 1400 0.07 48 2 . 00 48 2 . 00 96 4 . 00 3 .

6

94 .

0

95 .6

8 1400 0.07 48 2 . 00 36 1.50 84 3.50 3 .6 93 .

0

94 .

9

9 1400 0.07 48 2 . 00 24 1 . 00 72 3 . 00 3 .6 90.6 93 .

5

10 1400 0.07 48 2 . 00 66 0 "7 c: / . /5 1 14 4 . 75 98 .

3

99.0

11 1400 0.07 4 8 66 0 "7 c: 1 14 /. "7 C 6 .

5

Vl 97.6

12 1400 0.07 48 2 . 00 66 2 . 75 1 14 4 . 75 15 .5 98 .

8

99.2

13 1400 0.07 4 8 2 . 00 48 2 . 00 96 4 . 00 4 .

3

w 97 .3 98 .

5

14 1400 0.07 48 2 . 00 24 1 . 00 72 3 . 00 4 .2 W 94 .

5

96 .

6

15 1400 0.07 48 2 . 00 66 2 . 75 1 14 4 . 75 3 .5 84 .2 90.2

16 1875 0.15 48 2 .00 66 2 .75 114 4.75 4.8 o 97.4 98.5

17 1875 0.15 48 2 .00 66 2 .75 114 4.75 3.8 93.8 95.6

18 1000 0.04 48 2 .00 66 2 .75 114 4.75 3.5 94.9 96.5

19 1000 0.O4 48 2 .00 66 2.75 114 4.75 4.7 o 98.4 98.9

20 500 0.01 48 2 .00 66 2 .75 114 4.75 4.4 o 97.8 98.5

21 500 0.01 48 2 .00 66 2.75 114 4.75 3.5 • 94.2 95.6

* shaded area represents a temperature different from the unshaded area.

To assist readers interested in making use of the coherent system of SI units, the exact conversion factors to be used with the above table are Length 1 inch = 0.0254 meter > Temperature difference 1 deg F = 5/9 deg C = 5/9 K

Pressure 1 inch of water = 2.488 X 10' newton/meter

Flow rate 1 cubic foot/minute = 4.719 X 10-' meter Vsecond

tiveness to some other parameter, the standard

deviation method of determining effectiveness was used.

Three patterns of temperature nonuniformity were used to determine how mixer effec- tiveness was influenced by variation of the initial temperature distribution. The temperature

patterns are shown in figure 7. The right and

left patterns in figure 7 represent situations

where a maximum nonuniformity of 50 percent

warm and 50 percent cool air occurs. A pattern, which might be more common, of one6
90

f 80

70

50

90.2% I

1

96.2 7o

I

TEMPERATURE PATTERNS

(T and t represent areas at different temperatures)

Figure 7. Effectiveness of the louvered strip mixing

device for selected temperature patterns.

fourth of the stream at a different temperature

than the remaining three-fourths is shown in

the center of figure 7. From the results shown

by the three bar graphs in figure 7, it is evident that the lowest effectiveness occurred for

a pattern composed of 50 percent warm air and

50 percent cool air with the interface along the

diameter of the duct. The next most difficult pattern of nonuniformity to mix was that of

three-fourths of the stream at a different temperature than the remaining one-fourth. The

easiest pattern of nonuniformity to mix was

that of a concentric distribution. With the concentric pattern, the 99.0 percent effectiveness

of the mixer approached very nearly the limitation of the temperature-measuring system.

It was also of interest to determine how the

magnitude of an initial temperature difference

might affect the value of effectiveness. Tests

using the concentric temperature pattern were made by varying the temperature difference

between the warm and cool portions of the air

stream. Observations of temperature differ100

90

80

ences between portions of the unmixed stream

of 4.0, 6.5, and 15.5 deg F revealed no change

in the percentage effectiveness of the mixer. In studying all other parameters, a temperature

difference in the unmixed air stream of approximately 3 to 4 deg F was used for all mixers.Using two mixing elements in series, it was

desirable to know at what position the ele- ments should be located with respect to each

other for best mixing. Tests were made to determine the optimum placement by varying the

location of the second element with respect to the first element. Figure 8 shows a curve of ef- fectiveness as the distance between the mixing

elements was changed. As a result of space

limitations, the overall distance from the inlet

of the mixing device to the downstream measuring station was confined to 4.75 duct diameters. Because of the space limitation, the distance from the second mixing element to the

downstream measuring station varied as the

distance between mixing elements varied.

Under the conditions of the tests, the effective- ness increased gradually from 92.5 to 96.1 percent as the spacing was increased from 0.5 to

1.5 duct diameters; it remained constant at spacings greater than 1.5 duct diameters.

Figure 9 presents the effectiveness as a function of overall distance between the first mixer

element and the downstream measuring station

with the spacing between mixing elements held

constant at 2.0 duct diameters. Presented in

the figure is the performance of the mixer for

two temperature patterns, the concentric distri- bution and the quadrant distribution. As can

be seen, mixing effectiveness for the concentric

pattern was consistently the higher of the two.

Measurements were made over a selected

range of air flow corresponding to average air

stream velocities ranging from 150 to 600 fpm.

Figure 10 illustrates the negligible effect upon

the effectiveness of mixing as the flow or mean

velocity was varied over the above range.

OVERALL distance: 4.75 DUCT DIAMETERS

MEAN velocity: 450 FPM

0 1.0 2.0 3.0 DISTANCE BETWEEN MIXING ELEMENTS ( DUCT Dl AMETERS

)

The overall distance of 4.75 duct diameters from the inlet of the mixing device to the downstream station was held constant. Figure 8. Effectiveness of the louvered strip mixing device relative to the spacing

between mixing elements.

7
100

90

80

O CONCENTRIC TEMPERATURE PATTERN

^ QUADRANT TEMPERATURE PATTERN

(THREE QUADRANT'S WARM AND ONE QUADRANT COOL ) MEAN velocity; 450 FPM

2.0 3.0 5.0 DISTANCE FROM INLET OF MIXER TO DOWNSTREAM

MEASURING STATION (DUCT DIAMETERS)

The distance between the mixing elements was held constant at 2.0 duct diameters. Figure 9. Effectiveness of the louvered strip mixing

device as the overall distance from the inlet of the mixing device to the downstream measuring station was varied.

Measurements were also made to compare

the effectiveness of the mixer at two different

louver angle positions. The results illustrated

in figures 7 to 10 inclusive are for a position of

60° from the normal path of flow along the

axis of the duct. Using the quadrant temperature distribution pattern, the effectiveness decreased from 96 to 93 percent for the respective settings of 60° and 45° from the normal

path of flow. For the comparative tests, the flow

rate was 1400 cfm, the spacing between mixing

elements was 2.0 duct diameters, and the

downstream measuring station was 4.75 duct

diameters from the first mixing element.

The loss of static head due to the presence of a mixing device in the duct system was of in- terest because of the energy required and size

of equipment needed to move the air through

such devices. The pressure loss from the louvered strip mixer was relatively low, as is shown in the plot of figure 11. When the loss in

static pressure head is expressed in terms of

equivalent multiples of velocity head, the static

pressure drop is equivalent to approximately

seven velocity heads. For the other two mixers,

the louver-baffle and the concentric louver, the

equivalent velocity head multiples are 38 and

5, respectively.

4.2. Louver-Baffle

A summary of the test conditions and performance of the louver-baffle mixer is given in

table 2. The same three temperature patterns as previously described, having quadrant and annular distributions, were used in evaluating the

performance of the louver-baffle mixing device. Shown in figure 12 are bar graphs which

compare the performance for the three temperature patterns. By comparing figures 6 and 12,

it is seen that the effectiveness of this mixer is not as much affected by temperature distribution as was that of the louvered strip mixer.

From the curve of figure 13, it is seen that

there was little change in effectiveness beyond

the 3 deg F level of temperature difference. It

is possible that the computed values of effec- tiveness for temperature differences of less than 3 deg F at the mixer inlet (0.6 and 1.7

deg F) could have been influenced by the instrument error, since the ratio of the instrument error to the temperature gradient in- creases as the temperature gradient gets

smaller. At the downstream measuring station, an error of 0.01 deg F for an initial temperature difference of 0.5 deg F between quadrants

would cause an error of 2 percent in effective- ness. To reduce the influence of instrument

error upon the observed effectiveness, all other

tests were conducted at a temperature differ- ence of 3 deg F or higher.

Tests were conducted to determine at what

distance the two elements should be placed with

respect to each other and at what overall

length the mixer would be highly effective

(97% or higher). Figures 14 and 15, respectively, show the effect when the distance between elements is varied, and the effectiveness

100

90

80

O CONCENTRIC TEMPERATURE PATTERN

A QUADRANT TEMPERATURE PATTERN

(THREE QUADRANTS WARM AND ONE QUADRANT COOL

)

(OVERALL DISTANCE : 4.75 DUCT DIAMETERS)

100 200 300 400 500 600

MEAN AIR STREAM VELOCITY IN THE TWO FOOT DIAMETER DUCT (FPM)

Figure 10. Relation of mixer inlet velocity to effectiveness for the louvered strip

mixing device.

8
TABLE 2. Summary of test conditions and performance of the louver-baffle

mixer.

Test

No. Flow

Rate

Pressure

Drop Across

Mixer

Distance Between Mixing Elements

Distance from 2nd

Element to Down- stream Meas . Station

Overall

Distance

Temperature Conditiona

at Mixer Inlet Louver

Ffff.rr vfnnn

Temp. Dlff

.

Temp. Pattern*

Range std. Dev

.

Cfm in. W.G. in. Duct

Diam.

in. Duct

Diam.

in. Duct Diam. deg F Degree 7. 7.

1 1400 0.71 14 0.58 46 1.92 60 2 .50 3.3 60 94.6 95.5

2 1400 0.71 18 0.75 42 1.75 60 2 .50 3.3 60 95.9 96.7

3 1400 0.71 22 0.92 38 1.58 60 2.50 3.6 60 96.4 97.1

4 1400 0.71 26 1.08 34 1.42 60 2 .50 3.5 60 95.4 96.4

5 1400 0.71 22 0.92 38 1.58 60 2 .50 0.6 60 93.0 95.0

6 1400 0.71 22 0.92 38 1.58 60 2.50 1.7 60 94.6 96.0

7 1400 0.71 22 0.92 38 1.58 60 2.50 10.8 60 96.2 97.1

8 1400 0.71 22 0.92 38 1.58 60 2 .50 20.7 60 95.2 96.5

9 1400 0.71 6 0.25 42 1.75 48 2.00 3.6 60 91.5 93.8

10 1400 0.71 30 1.25 42 1.75 72 3.00 3.8 60 95.2 96.6

11 1400 0.17 22 0.92 38 1.58 60 2 .50 3.5 0 57.6 63.7

12 1400 0.25 22 0.92 38 1.58 60 2.50 3.5 30 57.8 67.6

13 1400 0.45 26 1.08 34 1.42 60 2 .50 3.2 45 83.3 88.2

14 1400 0.46 22 0.92 38 1.58 60 2.50 3.2 50 92.1 93.6

15 1400 0.54 22 0.92 38 1.58 60 2.50 3.3 ^y 55 92.5 94.6

16 1400 0.80 22 0.92 38 1.58 60 2 .50 3.5 65 94.4 95.6

17 300 0.02 22 0.92 38 1.58 60 2 .50 3.6 60 92.9 93.9

18 1000 0.21 22 0.92 38 1.58 60 2 .50 5.1 60 94.3 95.3

19 1400 0.71 22 0.92 26 1.08 48 2 .00 3.7 ^y 60 89.6 92.6

20 1400 0.71 22 0.92 30 1.25 52 2.17 3.7 60 91.6 93.5

21 1400 0.71 22 0.92 34 1.42 56 2 .34 3.7 60 94.0 95.0

22 1400 0.71 22 0.92 38 1.58 60 2.50 3.3 60 94.3 95.3

23 1400 0.71 22 0.92 42 1.75 64 2.67 3.9 60 96.3 97.0

24 1400 0.71 22 0.92 46 1.92 68 2 .84 4.1 60 96.4 96.9

25 1400 0.71 22 0.92 50 2.08 72 3.00 3.6 60 95.5 96.7

26 1400 0.71 22 0.92 54 2 .25 76 3.17 3.8 60 96.1 97.2

27 1400 0.71 22 0.92 12 0.50 34 1.42 3.5 60 75.8 83.3

28 1400 0.71 22 0.92 12 0.50 34 1.42 4.3 o 60 93.7 96.1

29 1400 0.71 22 0.92 24 1.00 46 1.92 4.3 o 60 97.4 98.3

30 1400 0.71 22 0.92 38 1.58 60 2 .50 4.4 o 60 98.4 98.9

31 1400 0.71 22 0.92 60 2 .50 82 3.42 4.3 o 60 98.3 99.0

32 1400 0.71 22 0.92 84 3.50 106 4.42 4.4 o 60 98.9 99.3

33 1400 0.71 22 0.92 38 1.58 60 2 .50 3.7 © 60 95.5 96.9

34 1400 0.71 34 1.42 42 1.75 76 3.17 3.7 60 95.2 96.3

35 1400 0.71 26 1.08 42 1.75 68 2 .84 3.6 60 94.6 95.7

36 1400 0.71 10 0.42 42 1.75 52 2.17 3.5 e 60 91.3 93.6

Shaded area represents a temperature different from the unshaded area. To assist readers interested in making use of the coherent system of SI units, the exact conversion factors to be

used with the above table are: Length 1 inch = 0.0254 meter Temperature difference 1 deg F = 5/9 deg C = 5/9 K

Pressure 1 inch of water = 2.488 X 10 ^ newton/meter ' Flow rate 1 cubic foot/minute = 4.719 X 10-' meter '/second

9
— —

. , 100 200 300 400 500 600

MEAN AIR STREAM VELOCITY IN TWO FOOT DIAMETER DUCT (FPM)

FlGXniE 11. Observed pressure drop for each of the three mixing devices for a

range of air flow.

TEMPERATURE PATTERNS

( T and t represent areas at different temperatures)

Figure 12. Effectiveness of the louver-baffle mixing

device for selected temperature patterns.

at various downstream locations after the air

stream has passed through the mixer. From

the data shown in figure 14, it was found that a distance of 0.92 duct diameter between ele- ments provided sufficient spacing to accomplish

good mixing. With the elements located 0.92

duct diameter apart, measurements were taken

to determine mixer performance as the overall

distance varied. For temperature patterns of

concentric and quadrant distributions, it was

found that, for overall distances greater than

2.5 duct diameters from the first mixing ele- ment to the downstream measuring station,

little improvement in mixing was accomplished. The maximum observed mixing effec- tiveness for the quadrant temperature pattern

and the concentric temperature pattern was

about 97 and 99 percent, respectively, for the

test conditions shown in figure 15.

100

90

80

QUADRANT TEMPERATURE PATTERN

(THREE QUADRANTS WARM AND

ONE QUADRANT COOL)

(OVERALL distance: 2.5 DUCT DIAMETERS,

MEAN velocity: 450 FPM)

10 15 20 25 TEMPERATURE DIFFERENCE BETWEEN PORTIONS OF THE AIR STREAM BEFORE MIXING (DEGREES F)

Figure 13. Effect of air temperature difference at mixer inlet upon the

effectiveness of the louver-baffle mixing device.

10
100

90 OVERALL distance: 2.50 DUCT DIAMETERS

MEAN VELOCITY : 450 FPM

80

0 0.5 1.0 1.5 DISTANCE BETWEEN MIXING ELEMENTS (DUCT DIAMETERS)

The overall distance of 2.5 duct diameters from the inlet of the mixing device to the downstream

measuring station was held constant. Figure 14. Effectiveness of the louver-baffle mixing device relative to the spaing

between mixing elements.

O CONCENTRIC TEMPERATURE PATTERN

A QUADRANT TEMPERATURE PATTERN

{THREE QUADRANTS WARM AND ONE

QUADRANT COOL)

MEAN velocity: 450 FPM

1.0 2.0 3.0 4.0 5.0 DISTANCE FROM INLET OF MIXER TO DOWNSTREAM MEASURING STATION

( DUCT DIAM ETERS )

The distance between the mixing elements was held constant at 0.92 duct diameter. Figure 15. Effectiveness of the louver-baffle mixing device as the overall distance from the inlet of the mixing device to the downstream measuring station was varied.

A QUADRANT TEMPERATURE PATTERN

( THREE QUADRANTS WARM AND

ONE QUADRANT COOL)

(OVERALL distance: 2.5 DUCT DIAMETERS , LOUVER SETTING; 60" )

100 200 300 400

MEAN AIR STREAM VELOCITY IN THE TWO FOOT DIAMETER DUCT (FPM)

Figure 16. Relation of mixer inlet velocity to effectiveness for the louver-baffle mixing device.

11
OVERALL distance: 2.5 DUCT DIAMETERS

DISTANCE BETWEEN MIXING ELEM ENT S : 0.92 DUCT DIAMETER MEAN VELOCITY : 450 FPM

0 10 20 30 40 50 60 70

LOUVER SETTING FROM NORMAL PATH OF FLOW (DEGREES)

Figure 17. Relation of the louver angle to effectiveness for the louver-baffle mixing device.

0.9

0.8

It 0.7 a/

Xo

z

0.6

Q.o

(Ea

0.5

Ul

(r 0.4

U)

v>

bJ

tc

0, 0.3 o

1- 0.2 DISTANCE BETWEEN MIXING ELEMENTS! 0 <

tt

1 DUCT DIAMETER

0. 1 MEAN velocity: 450 FPM

1 1 1 1 1 1

0 10 20 30 40 50 60 ' 70 LOUVER SETTING FROM NORMAL PATH OF FLOW (DEGREES)

Figure 18. Pressure drop for the louver-baffle mixing device at selected louver settings.

The louver-baffle mixer was found to be

slightly more sensitive to variation of flow rate than the louvered strip mixer. Over the velocity range from 100 to 450 fpm, as shown in

figure 16, the effectiveness increased approximately 3 percent as the mean stream velocity was increased over this range.

Along with placement of elements, temperature patterns, etc., tests to determine how the

effectiveness would change were made by varying the angular deflection of the air as it

passed through the mixer. Tests were made at various louver angle positions from 0°, or along

the axis of the duct, to 65° from the axis. As

shown in figure 17, the observed effectiveness was maximum at a setting of 60°. Beyond 60°

the pressure drop became prohibitively high

without an increase in effectiveness. Figure 18

shows a plot of the pressure drop across the

mixing device as the louver angle was changed

while the flow rate remained constant.

12
4.3. Concentric Louver

For the concentric louver mixing device, the

angular setting of the louvers in the outermost

area was selected to be 60° from the normal

path of fiow. At the 60° louver angle setting,

the pressure drop was determined for a given

rate of air flowing through the outer area. Then with the same flow rate,the angular setting of the louvers in the two inner areas was

adjusted to give a matching pressure drop for

the two inner areas and the outer area. The re- sulting angular setting for the two inner areas was 49° from the normal path of flow. All test were conducted at these louver angle settings. A

summary of test conditions and performance of

the concentric louver mixer is given in table 3. TABLE 3. Summary of test conditions and performance of the concentric

louver mixer.

Test

No. Flow

Rate

Pressure

Drop Across Mixer

Distance Between Mixing Elements

Distance from 2nd

Element to Down- stream Meas . Station Overall

Distance

Temperature

Conditions

at Mixer Inlet Effectiveness

Temp

.

Diff.

Temp.

Pattern"^ Range Std. Dev

.

CFM in. W .G. in

.

Duct

Diam.

in. Duct

Diam.

In

.

Duct

Diam. deg F X 7„

1 1400 0.06 12 U . J L UZ H . Z 3 LH- , 7^/ J 3 .

7

85 .

9

90 .

0

2 1400 0.06 24 1 .

0

90 3 . 75 11/. 3 .

7

93 .

8

96 .

0

3 1400 0.06 36 1 .5 78 J .Z 3

Zl 7=: 3 .

8

95 .

8

97.1

4 1400 0.06 A Q z . u DO 9 1^ 1 1 /l ± in- A 7S 3 .

7

97.1 98.

0

5 1400 0.06 60 2 .5 9 9 c; Z ,Z 3

11/. 3 .

9

95 .4 97.0

6 1400 0.06 72 3 .

0

1.75 1 1 /i /i 7 c, 3 .

7

93 .

1

95 .

1

7 1400 0.06 4 8 z . u o u 9Z

c: n . 3 U i UO 4 50 3 .

7

96 .2 97.3

8 1400 0.06 A ft z . u M- O 2 00 96 4 , 00 ft WD 95 .2 96 .

8

9 1400 0.06 48 2.0 36 1i . jU J . D u 3 .

8

#k

C7 94 .

9

96 .

5

10 1400 0.06 A ft z

9 . nu £. 1 00 1 z 3 . 00 7 93 .2 95 .

1

11 1400 0.06 48 9 n Z , U

1 9 u , _>u Ao nu Z9 . J^ nu 3 .

6

87 .2 91.9

12 1850 0.11 48 2.0 60 2.50 108 4.50 5.8 o 97.8 98.8

13 1850 0.11 48 2 .0 36 1.50 84 3.50 5.9 o 97.5 98.4

14 1850 0.11 48 2.0 12 0.50 60 2 .50 5.9 o 92 .9 94.9

15 1000 0.04 48 2.0 60 2 .50 108 4.50 3.8 © 96 .0 96.8

16 1850 0.11 48 2.0 60 2 .50 108 4.50 3.7 95.0 96.4

17 500 48 2.0 60 2 .50 108 4.50 4.0 96.9 97.7

18 1400 0.06 48 2.0 60 2 .50 108 4.50 3.7 95.2 97.1

19 1400 0.06 48 2.0 60 2 .50 108 4 .50 4.1 o 97.8 98.8

20 1400 0.06 48 2.0 60 2 .50 108 4 .50 5.8 o 98.5 99.1

21 1400 0.06 48 2.0 60 2 .50 108 4 .50 13.1 o 98.8 99.3

22 500 48 2.0 60 2 .50 108 4.50 5.6 o 98.4 99.1

shaded area represents a temperature different from the unshaded area.

To assist readers interested in making use of the coherent system of SI units, the exact conversion factors to be used with the above table are: Length 1 inch = 0.0254 meter Temperature difference 1 deg F = 5/9 deg C = 5/9 K

Pressure 1 inch of water = 2.488 X 10* newton/meter ° Flow rate 1 cubic foot/minute = 4.719 X 10-* meter '/second

13
The concentric louver mixer was subjected to air streams having the same nonuniform temperature distributions at the inlet as for the

other two mixing devices. The results of this

study are shown in figure 19. The bar graphs,

which relate the effectiveness of the mixer to the three temperature patterns, show that the

effectiveness is only slightly affected by the

temperature pattern of the air stream.

1

9 7.1 %

1

97.3 7o

1

© © © TEMPERATURE PATTERNS

( T and t represent areas at different temperotursE )

FiGtJRE 19. Effectiveness of the concentric louver mixing device for selected temperature patterns.

Using the temperature pattern shown in the

center of figure 19, tests were made to determine the effect of increasing the magnitude of

the temperature difference between portions of

the cross section of the air stream over the

range from 4 to 13 deg F. Results shown in

figure 20 indicate that there was no change in

performance within this range.

As in the case of the other mixers, which

have two elements in series, it was necessary to determine at what distance, with respect to each other, the mixing elements would best

perform. For this determination, the downstream measuring station was held in a fixed

position and the location of the second mixing

element with respect to the first was varied. The

results illustrated in figure 21 show that effec- tive mixing was gained when the elements were placed approximately two duct diameters

apart, but that very little mixing would be lost by reducing the spacing down to one duct

diameter. Tests using a spacing of two duct di- ameters between mixing elements showed that

the overall distance from the first mixing ele- ment to the point of maximum observed uniformity was 4.75 duct diameters. Figure 22

shows how the effectiveness varied as the overall distance was changed.

100

90

80

O CONCENTRIC TEMPERATURE PATTERN

(OVERALL distance: 4.75 DUCT DIAMETERS,

MEAN velocity: 450 FPM )

10 15 20

TEMPERATURE DIFFERENCE BETWEEN PORTIONS OF THE AIR STREAM BEFORE MIXING

C DEGREES F)

Figure 20. Effect of air temperature difference at mixer inlet upon the

effectiveness of the concentric louver mixing device.

The effect of velocity upon the mixing process was determined by varying the average

speed of the air stream over the range from

150 fpm to 600 fpm. The results plotted in

figure 23 show that the effectiveness of the concentric louver mixer is relatively unaffected by

the speed of the air stream.

4.3.1. Applications of the Concentric Louver Mixer

The concentric louver mixer has been used

successfullv in several test apparatuses at the

National Bureau of Standards in which uniformity of temperature of an air stream

greatly facilitated the purpose of the investigation. In one case it was used to mix the air

stream leaving a coil being tested in a psychrometric calorimeter apparatus for measurement

of the heat transfer characteristics of cooling

coils. After determining that the uniformity of

temperature downstream from the mixer was

good, only three thermocouples were used to obtain a representative sampling of the air

stream for the psychrometric determinations.

In the second case the mixer was used, in a study of the effect of thermal radiation upon

temperature-measuring sensors, to obtain a

condition of reasonably uniform air tempera14
^ 100

90

80

OVERALL DISTANCE ; 4.50 DUCT DIAMETERS

MEAN velocity: 450 FPM

0 1.0 2.0 3.0 DISTANCE BETWEEN MIXING ELEMENTS ( DUCT Dl A METERS

)

The overall distance of 4.5 duct diameters from the inlet of the mixing device to the downstream

measuring station was held constant. FiGXJKE 21. Effectiveness of the concentric louver mixing device relative to the spacing between mixing elements.

100 -

90

eo

O CONCENTRIC TEMPERATURE PATTERN

A QUADRANT TEMPERATURE PATTERN

(THREE QUADRANTS WARM AND ONE QUADRANT COOL ) MEAN velocity: 450 FPM

2.0 3.0 4.0 5.0 DISTANCE FROM INLET OF MIXER TO DOWNSTREAM

MEASURING STATION ( DUCT DIAMETERS ) The distance between the elements was held constant at 2.0 duct

diameters. Figure 22. Effectiveness of the concentric louver mixing

device as the overall distance from the inlet of the mixing device to the downstream measuring station was

varied.

ture at all points within the cross section of

the air stream prior to entering the heated portion of the duct. Figure 24 is a drawing of the

heated wall apparatus showing the mixer,

measuring stations, probe location, and heating

elements. The study was made by locating the sensors in a duct whose wall was heated to a higher temperature than the air which sur- rounded the sensors. Heat was applied to the

duct walls and, as the air passed downstream, an approximately symmetrical pattern of temperature from the duct wall to the sensors was achieved. At the point where the sensors were placed there existed a flow of air in

which temperature fluctuations were very

small. Error of measurement for temperature

sensors of thermocouples, a thermistor and a

resistance thermometer, due to the radiation

from the heated walls, was then determined

100

90

80

O CONCENTRIC TEMPERATURE PATTERN

A QUADRANT TEMPERATURE PATTERN (THREE QUADRANTS WARM AND

ONE QUADRANT COOL)

(OVERALL DISTANCE : 4.75 DUCT DIAMETERS)

100 200 300 400 500 600

MEAN AIR STREAM VELOCITY IN THE TWO FOOT DIAMETER DUCT (FPM)

FiGXJRE 23. Relation of mixer inlet velocity to effectiveness for the concentric louver mixing device.

15
METALLIC AIR MIXERS 1 INSULATION DUCT WALL

f // /Y / / /^/ ////// / /ILLLLU.LLLL

HEATED ACCESS DOOR WITH COPPER- CONSTANTAN THERMOCOUPLES THERMOCOUPLE

ARRAY NO.I- NO. 2

^zzzzzzzzzzz

-LIUJJJJ///A

PROBE

TO BLOWER A^D EXHAUST

THERMOCOUPLE

FOR MEASURING TEMPERATURE

OF AIR AT INLET

UNHEATED SECTION -MEASURING SECTION

THE VERTICAL DIMENSION IS EXAGGERATED FOR THE PURPOSE

OF CLARITY. DETAILS OF THE

PROBE ARE NOT SHOWN.

O RESISTANCE HEATER CABLES • COPPER-CONSTANTAN THERMOCOUPLES FOR MONITORING TEMPERATURE ON INSIDE OF DUCT WALL OR INSIDE OF ACCESS DOOR

+ CHROMEL P-CONSTANTAN THERMOCOUPLES

FOR MONITORING AIR MIXING (17 THERMOCOUPLES IN EACH ARRAY).

The performance of three types of air temperature sensing devices encased in the probe was evaluated using this heated wall apparatus. The air mixer location can be seen at the left of the illustration. Figure 24. A sketch of an apparatus in which the concentric louver mixing device was used to produce a uniform temperature in an air stream.

CF)

THERMOCOUPLE ARRAY

NCI

(°F) THERMOCOUPLE ARRAY

N0.2

(°F) THERMOCOUPLE ARRAY

NO. 3

\

t t

Due to the heat applied to the wall of the measuring section, the temperature of the air at measuring stations 2 and 3 was higher near the duct wall than at the center Figure 25. Temperature patterns at the three measuring stations in the heated wall apparatus, shown in figure 2^.

through an extensive investigation. Figure 25

shows temperature profiles at the three measuring stations when the duct wall at stations 2

and 3 was 50 deg F higher than the air stream

at the center of the duct.

Recently one of the mixers described in this

paper, the concentric louver, was proposed as a recommended mixing device in the ASHRAE

Standard 41-66 [5], Section on Temperature

Measurement of the Standard Measurements

Guide, Part I. The recommendation has been

incorporated in the new issue of the standard.

An appendix is included at the end of the

paper which gives detailed drawings of the

three mixing devices and discusses techniques

helpful in construction of the mixers.

16
1)4.4. Comparison of Other Air Mixer Designs

In the previous investigation on orifices and

j orifice-target combinations [1], it was determined that an orifice having a diamter ratio

(ratio of the orifice throat diameter to the duct

diameter) of approximately 0.33 was consistently more effective in reducing the thermal

differences in an unmixed air stream than orifices having diameter ratios of 0.50 and 0.67. However, the mixing process was accompanied

by a high pressure drop across the mixer. If the pressure drop can be tolerated, temperature nonuniformity could be reduced to 3 or 4

percent of its original value, at a distance of

approximately 4.5 duct diameters downstream

from the orifice. The study indicated that di-

; ameter ratio was the most important parame-

' ter affecting the mixer effectiveness, but that

j the interface area between cold and warm ele- ments of the air stream and size of the nonisothermal elements also had a bearing on the

; mixing process. The performance of the orij fice-target combinations showed no improve-

• ment over the plain orifice even when using a

• target as large as two-thirds of the duct diame-

' ter.During the course of the investigation of

mixing devices, a number of designs (including

the three reported herein) were evaluated using

I the flow visualization technique for observing

; the flow pattern, with smoke used as the indi-

, cator. Although observation of flow patterns is qualitative, good correlation between flow visualization and quantitative method of evaluation using temperature patterns was gained.

I This correlation indicates that flow visualization is a rather good but coarse method of evaluation. Listed below are a number of designs

which revealed relatively poor mixing characteristics.

j, 1. Screens. Layers of flat wire screen, having approximately 50 percent free area, placed

I in series over the area of the duct, through

1 which the air could flow, showed very little ef1 feet upon the air stream with respect to mixI ing. (Velocity profiles taken before and after a

1 number of layers of screen showed a distinct

improvement in the uniformity of the velocity

within the cross-sectional plane of the duct.)

It should be mentioned that for a temperature distribution having many small variations, the screen might be more effective, as the flow through the screen is in the form of

many small jets. For the distribution as described in this report, one quadrant different

from the other three, for example, screens provide little displacement of flow from one

part of the stream to another; thus, the general

effectiveness for screens was very limited.

2. Baffles.

a. Semicircular baffles were placed in se- ries approximately one duct diameter apart in

such a way that one-half of the area of the

round duct was blocked off by the first baffle. One duct diameter downstream, the second baffle was placed in a position corresponding to the space not occupied by the first baffle ; thus,

the air would pass through the restricted opening of the first baffle and strike the second baffle before passing through the second opening.

This method proved unsatisfactory (but better

than screens). It was evident that the patterns were not being fully broken up.

b. Single slats one-third of a duct diameter in width were placed along the diameter of

the duct and used as baffles. The baffles were

installed in series, the first horizontal and the

second vertical, approximately one duct diameter apart. The mixing was comparable to that

of the semicircular baffles but was not considered good.

c. Quartered baffles were used and showed some improvement over the above two types.

These baffles consisted of openings in the first and third quadrants through which the air

could pass, while the second and fourth quadrants were blocked off with metal plates. Approximately one duct diameter downstream,

another baffle was fixed with openings in the

second and fourth quadrants and the first and

third blocked off. Still the mixing was not con- sidered good, but an improvement.

3. Fans.

a. A multi-blade fan was placed in the air

stream and allowed to rotate as the air passed

through it. This method did not produce mixing, as the air was not displaced but traveled

essentially along its original path.

b. The same multi-blade fan was held in a

stationary position and the air was forced

through the openings between blades and

again little mixing occurred. The resulting ac- tion was that of swirling as the air moved

downstream. The pattern remained essentially

the same; there was simply an angular displacement as the air passed downstream.

c. Powered fans were not tested because of

the difficulty of mounting a moving mechanism

in the duct and because of the introduction of

unwanted heat.

17
Table 4. Comparative performance of mixing devices I

Louvered

Strip LouverBaffle Concentric

Louver

Orifice* Orifice*

(0.5 diam.

ratio)

(0.33 diam.

ratio)

Range of mixing effectiveness for differ- ent temperature patterns (percent)

90-99 97-99 97-99 85-97** 95-99**

Minimum distance between mixing ele- ments (duct diameters)

1.5 0.92 2.0

Distance required to obtain 97%

indicated effectiveness

4.75 3.0 3.8 *** 4.1

(duct diameters) 95% 3.5 2.3 3.0 4.2 3.3 Change in effectiveness for range of

velocity tested (percent)

1 3.2 1.3 3.3 1.4

Pressure drop through mixer (number

of equivalent velocity heads)

7 38 5 43 166

Reported in a previous publication (ref. [1]).

**The concentric temperature pattern was not con- sidered in the evaluation of the orifice.

***An effectiveness of 97 percent was not achieved

in the available length, 4.75 duct diameter, test duct.

1 I

5. Summary

Table 4 is a simplified presentation of comparative performance of the three mixers described in this paper plus the orifice as described in a previous publication [1]. From

these studies on mixing devices it was shown

that a poor distribution of temperature could

be effectively improved by either the louvered

strip, the louver-baffle, the concentric louver, or the 0.33-diameter ratio orifice. Each possesses good features and each has shortcomings. Consequently tradeoffs must be considered when selecting a mixer for a particular

application. The concentric louver, the louverbaffle, and the 0.33-diameter ratio orifice were

relatively unaffected by temperature pattern,

whereas the louvered strips showed some sensitivity to temperature pattern. As shown in

table 4, the resistance to flow for the louvered

strip and the concentric louver when described

in multiples of velocity head was 7 and 5, respectively, as compared to 38, 43, and 166 for

the louver-baffle, 0.50-, and 0.33-diameter

ratio orifices. The overall distance required to attain an effectiveness level of 95 percent

ranged from 2.3 to 4.2 duct diameters. A 97

percent effectiveness was obtained by all of the

mixers with the exception of the 0.50-diameter

ratio orifice. Through reference to table 4, it can be seen that the change in effectiveness

from 95 to 97 percent was accomplished in 0,7 to 0.8 duct diameter for the louver-baffle, the

concentric louver, and the 0.33-diameter ratio

orifice. The louvered strip mixer was less re- sponsive to the change and required a distance

of 1.25 duct diameters for the 2.0 percent in- crease in effectiveness.

Overall, the louver-baffle and the concentric

louver mixers were comparable in their mixing

effectiveness, with the tradeoffs being pressure

drop and distance required for mixing. The

mixing distance of the louver-baffle is approximately 0.8 of that required for the concentric

louver, but the pressure drop is some seven

times as high for the louver-baffle when compared to the concentric louver.

For all of the mixing devices, the magnitude

of the temperature differences in the cross sec- tion of the air stream does not seem to affect

the mixing effectiveness a great deal. As a word of caution, careful consideration should

be given in determining the average temperature of the mixed air stream when large temperature differences are encountered prior to mixing. With large initial temperature differ- ences, even with a high effectiveness, the temperature differences in the mixed portion of

the air stream could be significant in some applications.

'Si

titiiii

(iDJ

6. References

[1] Faison, T. K., Jr., Davis, J. C, and Achenbach,

P. R., Performance of square-edged orifices and

orifice-target combinations air mixers, Nat.

Bur. Stand. (U.S.), Bldg. Sci. Ser. 12 (Nov.

1967).

[2] Faison, T. K., Jr., Davis, J. C, and Achenbach,

P. R., A test apparatus for the study of forced air-mixing devices, J. Res. Nat. Bur. Stand.

(U.S.), 70C (Eng. and Instr.), No. 1 (Jan.-Mar.

1966).

18
lj[3] Temperature: Its Measurement and Control, Thermoelectric Thermometry 1, 202 (Reinhold Publ.

Corp., 1941).

[4] Wile, D. D., Air flow measurement in the laboratory. Refrigerating Engineering 53, No. 6, 515 (June 1947).

[5] ASHRAE Standard 41-66, Part I, Standard measurements guide, section on temperature measurements.

7. Appendix. Construction Techniques for Fabrication of Mixing Devices

Louvered Strip. This mixer was made of (24

gage) sheet metal and attachments between

members were made by soldering or with

screws. It is important that the outer band of

the mixer be very nearly circular. This was ac- complished by cutting a circular groove in a

flat sheet of plywood into which could be fitted a metal band to form the outer edge of the

mixing element. A metal strap approximately

d/6* wide and long was fitted into the

groove in the plywood and soldered to form the

circular band. Five additional grooves were cut

in the plywood to accept metal dividers which

ran parallel to and along the diameter. Four

dividers were fitted at equal distances parallel

to the one along the diameter, thus cutting the

circle into six parallel strips. Into each of these

strips was fitted louvers, set at 60° from the

axis of the duct on the downstream side but al- ternately, in adjoining strips, directing the

flow upward or downward. The louvers were

metal members, shaped to fit between the dividing strips. In figure 26 details are shown for

both the louvers and the band with its dividing

strips.

Two similar elements are required and are installed 90° out of phase; thus, if the first ele- ment directs the air vertically, the second ele- ment would direct the air horizontally.

"Since the mixers evaluated in this paper were 24 in in diameter,

the dimensions referred to, if in inches, will be based on this particular size, if a function of d, will be based on duct diameter, d

(any size).

Louver-Baffle. Again the circular band was made and held into place by a groove in

the sheet of plywood. Along the diameter of

the circle formed by the band, a metal strip was

attached. The strip was attached at both ends

to the circular band and was used to anchor

the baffles. Metal strips of d/Q width were cut and served as the baffles. The baffles were at- tached to the metal strip along the diameter of

the circle. The baffles were fitted so that they

extended orthogonally from the strip to the

band in an alternate fashion first to the right,

then to the left; thus, six baffles were fitted onto the circular band covering approximately

one-half of the area. Metal louvers were made

of sheet metal and fitted onto the baffles in the

positions as shown in figure 27. Small metal

tabs were used to attach the louvers to the baffles and were made of a heavier metal to assure

that the proper angle setting of the louvers was

maintained. Louvers were fixed over each half

of the circular area to direct the air flow to the

other side of the duct 60° from its normal path.

The two louvers nearest the center of each ele- ment were notched to permit flow of air

through the center region with no greater res- triction than that required for the other areas. The notched areas are shown in the rear view

of the element in figure 27. Two similar ele- ments are required and installed 90° out of

phase ; thus, if the first element directs the air

horizontally, the second element would direct

the air vertically.

6

60"V

1

d

10 AIR FLOW

BREAK U

SECTION B-B

LAY OUT OF DEFLECTING LOUVER (FRONT VIEW)

Figure 26. A detail of the construction of the louvered strip mixing device.

19
(FRONT VIEW) SECTION (REAR A- A VIEW)

Figure 27. A detail of the construction of the louver-baffle mixing device. ^SOLDERING TABS^

LOUVER SOLDERED

TO CIRCULAR BAND HERE

(FRONT VIEW) (SIDE VIEW) ENLARGEMENT OF A WITH PROJECTING VIEWS

OF ONE DEFLECTING LOUVER

Figure 28. A detail of the construction of the concentric louver mixing device. Concentric Louver. For this mixer three

grooves are neeiJeii to accept the three bands

having diameters of d, 2d/S, and d/3 concentrically spaced ; thus, providing a distance betw^een bands of d/6. The spaces betv^^een bands were fitted with louvers which direct the air

flow along the desired course. In the outer

area, the louvers were set at an angle of 60°

from, the normal path of flow and caused the

air to rotate around the axis of the duct in a

clockwise or counterclockwise direction depending upon the louver setting. The adjoining area was fitted with louvers set at an angle of

49° from the normal path and caused a rotation in the opposite direction from that in the

outer area. The central area is composed of

four louvers with the same angle setting of

49°, but in the same direction as those of the

outer area. The outer area was divided into 18

equal areas with the divisions being along the

radius. The adjacent area toward the center was divided into 12 equal areas and the central

portion divided into four parts. Figure 28

shows a detail of the concentric areas, divisions, and a louver pattern. The louvers are made from sheet metal and are custom shaped

for each of the three areas. The louvers should

not be made to fit too tightly between the

bands as the angle may have to be adjusted

after fabrication.

20

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