BUILDING SCIENCE SERIES

Performance of Square-Edged

Orifices and Orifice-Target

Combinations as Air Mixers

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Performance of Square-Edged Orifices and

Orifice-Target Combinations 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.

Building Science Series 12

Issued November 24, 1967

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 15 cents
Contents

Page

1. Introduction 1

2. Test apparatus and procedure 2

2.1. Temperature measurements 2

2.2. Static pressure measurements ^ 3

2.3. Flow visualization 4

3. Description of mixers 5

4. Test conditions 5

6. Performance of mixers 6

5.1. Orifices 6

5.2. Orifice-target combinations 7

6. Conclusion 11

7. References 11
Performance of Square-Edged Orifices

and Orifice-Target Combinations as Air Mixers

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

A study was made at the National Bureau of Standards to determine the effectiveness of the square-edged orifice, or the orifice in combination with a target (circular baffle), for mixing an air stream which was initially nonuniform with respect to temperature. By

achieving uniformity of temperature at all points within the cross section of an air stream, instrumentation for measurement might be simplified and a more representative temperature

value obtained. Orifices having throat diameters of 8, 12, and 16 in were evaluated in a 24-in circular test duct to determine mixing effectiveness under selected test conditions

of temperature distribution and flow rate. Targets of 8, 12, and 16 in in diameter in combination with a 12-in orifice were also investigated undier similar conditions. Graphic material is presented which illustrates how the orifice and orifice-target

combinations perform as mixing devices under selected conditions. Results indicate that the 8-in (0.33 diam ratio) orifice effectively diminished the nonuniformity of temperature

but only at a high pressure drop across the orifice and that a distance of 4.5 duct diameters was required for mixing.

Key Words ; Diameter ratio, mixing effectiveness, square-edged orifice, temperature measurement,

temperature pattern.

1. Introduction

lj The National Bureau of Standards has undertaken a study to determine methods or processes

I by which nonuniformity of temperature m an air

I stream in a duct can be corrected by forced mixing. Since the enthalpy of an air water-vapor mix-

'! ture is related to its temperature and moisture

!i content, the flow of energy represented by the movi ing air stream can best be determined if the measI ured values of dry-bulb temperature and dewpoint

|f or wet-bulb temperature are truly representative

of the entire air stream. Under conditions where

' nonuniformity of velocity, temperature, and humidity exist, realistic averages of temperature and

humidity are very difficult to obtain. During lab- IJ oratory tests where moving air is involved, the

h measurement difficulties due to poor mixing can

j sometimes be compensated for by taking many

; temperature measurements throughout the cross

u section of the air stream. But even when a large

j number of measurements are taken, the observed

values must be evaluated and weighted, with reII spect to both temperature and velocity. For nonI uniform distributions of temperature and velocity, a method of weighting should be used for proper

representation of the average temperature of the

; total mass of air that is passing a measuring staj tion or stations.

I A mUch better method of determining the average air temperature is to produce a high degree of

uniformity of the air temperature through use of

a mixing device. Determination of the average

temperature could then be accomplished by making only a few measurements. If the mixing is good enough, one observation will suffice. A number of mixing devices are under investigation in this study. This paper, the second in a series of three on the apparatus and mixing devices, presents performance data for the square-edged orifice, and the combination of orifice and target

(circular baffle), two methods of mixing which

have been in use in laboratories for a number of

years. These methods mix the air principally by a jet

action. There has been considerable work done on

jet mixing, but most other investigators [1, 2, 3, 4] ^ have studied the mixing of the jet with the

surrounding air under unbounded conditions

whereas this study is concerned with mixing in a

duct. Theoretical and experimental work has been

done with both heated and unheated jets. In pre- vious investigations, the jets have been axially

symmetrical, whereas the present work deals with an unbalanced distribution of temperature Avithin

the air stream.

1 Figures in braclsets indicate the literature references at tlie end of this paper.

1
2. Test Apparatus and Procedure

2.1. Temperature Measurements

A test apparatus [5] was designed and built for

producing controlled temperature and humidity

conditions in an air stream to facilitate measure- ment of the effectiveness of air mixing devices. A

schematic layout of the apparatus is shown in fig- ure 1. A stream of air, maintained at a constant

and substantially uniform temperature, was drawn

into the apparatus with an inlet blower. The air

stream was conditioned to a desired pattern of

nonuniformity of temperature in that portion of

the apparatus shown in figure 1 as Section A-A.

The air stream was not conditioned with respect

to humidity for the tests covered in this paper.

Temperature measurements were made in a 24-in

diam duct at stations upstream and downstream

from a mixing device. These measurements

provided information about the temperature pat- tern upstream from the mixer and the resulting

distribution downstream. Conditions of nonuniformity at the upstream measuring station were

controlled and reproduced from test to test. A mixing device, in this case an orifice, was placed just

downstream from the location shown in figure 1 as Section B-B and the air stream of nonuniform

tempera;ture was forced through the mixer.

At selected distances downstream from the

mixer, measurements were made to determine the

temperature distribution as an indicator of the

effectiveness of the mixing device in producing

uniformity of temperature. By observing the temperature distribution in the duct at each of several

stations downstream from the mixer, the progress

of the mixing process could be evaluated and the

station of optimum mixing could be determined.

A statistical analysis was made of the upstream

and downstream temperature values to determine

the standard deviations at the respective stations.

For each of five sets of temperature determinations

recorded in a test, standard deviation values were computed from which mixing effectiveness was

calculated. The five effectiveness values were aver- aged to obtain the effectiveness for a particular

test. A value of effectiveness for a mixing device was calculated by subtracting from unity the ratio

of downstream to upstream standard deviation.

The functions, standard deviation and effective- ness, a;re shown below

SD.=[;^S(.,-.)=]"'

Effectivenesses D. )=(l-|^§^)x 100%

where:

a; individual measurements

i= arithmetic mean

n=number of measurements

S.D.D8= standard deviation downstream

S.D. us =standard deviation upstream.

QUADRANT

PARTITION

WATER

VftPOR Inlets

MEASURING

STATION « 2 EXHAUST

INSULATION '

CONDITIONED

AIR INLET

FiGUBE 1. Schematic of tH^ mixing apparatus which produces the test condition and houses the mixing devices to evaluated. be
The range of values was also found for the sets of upstream and downstream temperatures. An

alternate method for determining effectiveness,

based on the range, was figured t)y subtracting

from unity the ratio of downstream to upstream

ranges. This relationship is shown below:

EffectivenesS(Range)=(l-|^2|S)^^^^^^

i KangeD8= (Xmax —3^in) downstream

Rangeu8=(a:max— a^min) upstream

' This latter method based on the range is simpler

I to use but the results can be much more affected by

' variation of a single observation,

j Tests were performed under varied physical ar- rangements such as different orifice sizes' and

different longitudinal positions of the plane of

measurement downstream of the orifice. Tests were

I also performed under the following varied stream

conditions: different temperature patterns, flow

rates, and magnitudes of temperature difference.

Steady state conditions were maintained through1 out the system, a necessary condition for the above

I formulas to give a valid indication of effectiveness.

' The conditioning section of the 24-in square

! duct upstream of the mixing device was divided

I into four quadrants for producing distinct and

reproducible temperature distributions. The temj perature patterns were obtained by means of dif-

' ferent electrical energy inputs into the air streams

I in the quadrants. Most of the tests were performed

with a temperature difference between warm and

cool quadrants of approximately 3 deg F, with

j three of the quadrants kept at the higher temperaI ture and the fourth at the lower temperature,

j The temperature distribution in the cross-secI tional area of the air stream at each station was

j measured with 24 calibrated copper-constantan

[ thermocouples, fixed in similar patterns at each

station. Each thermocouple was soldered to a small

washer which, because of its mass, provided damping of small temperature fluctuations. Location of

the thermocouples at the upstream measuring sta- tion is shown in figure 2. The tubes which served

to aline the air stream as it issued from the quadrants toward the upstream measuring station can

be seen in the background of figure 2. The enlargement shows a typical assembly of the thermocouple

and its holder.

The measuring station upstream from the mixing device was always located at the same place in

I the test duct and was used to determine the temI perature distribution of the air as it left the four

ij quadrants of the duct. The downstream measuring

I station was movable from test to test ; thusj temperature distributions within the cross section of

the air stream could be taken at different distances

i downstream from the mixing device.

Figure 2. View of the upstream measuring station and

detail of the thermocouples and holders.

Periodically the thermocouples at each measuring station were checked to determine the magnitude of deviation between any two thermocouples

at a station. For this purpose the thermocouples were immersed in a container of water under iso- thermal conditions and the greatest deviation between any 2 of the 24 thermocouples at either

station was determined. These differences never

exceeded 0.7 fiV (equivalent to about 0.03 °F) and

are considered aaequate for this work. The indicating instrument was a precision manual potentiometer, providing resolution for comparison or

difference readings to the nearest 0.1 fiV.

2.2. Static Pressure Measurements

Static pressure measurements were taken at 4- in intervals along the length of the duct to determine the variation in pressure from the upstream

measuring station to the most remote location of

the downstream measuring station. Figure 3 shows

a characteristic static pressure profile for a squareedged orifice which illustrates the static pressure

regain and the location that has proved to be the

approximate location of optimum mixing.

It was found in this study that no appreciable

increase in mixing effectiveness was observed beyond the station where maximum regain occurred.

Static pressure regain for the orifice has been

studied and described in other publications [6]

and the data gathered in this study were in good

agreement with previous work.

3
I 1

1 - oT >—o-^ j

1

1

-

j^^PLANE OF ORIFICE

_ 1 APPROXIMATE LOCATION

OF OPTIMUM MIXING —

^ -J

—

STATIC PRESSURE -

RESAIN

1

1 1 1

1 1

DISTANCE DOWNSTREAM FROM THE ORIFICE PLATE (DUCT DIAMETERS)

Figure 3. Static pressure profile for the 0.50 diam ratio

orifice.

2.3. Flow Visualization

To gain knowledge of the pattern of flow as the

air passed through and on downstream from the

mixmg device, a method of flow visualization was

devised, using a smaller apparatus and dimensionally scaled orifices or orifices with targets, to

demonstrate the process of mixing. Smoke was

used to simulate conditions of nonuniformity

which were present in the large apparatus. By

proper lighting, the mixing process was clearly

visible through an 8 -in diam plastic duct making

it possible to study the stream qualitatively as it issued through the orifice in the form of a jet, and

to observe the entraining of the air around the jet.

In the case of the orifice, movement of the smoke

in the area surrounding the jet could be followed

in the low pressure region near the downstream

face of the orifice plate. In the case of the orifice- target combination, the movement of the air in the

wake of the target was observed. From these observations a qualitative determination of the ef- fectiveness of the mixers was gained.

Less definitive techniques of flow visualization were also used, such as observation of thread flut- ter, which served to give a general profile of intensity and direction of flow at a number of cross sec- tions along the length of the apparatus. Shown in

figure 4 are sketches of the apparent flow pattern

in the duct when using these techniques with the

/DIRECTION

( OF

AIR FLOW

-i > mrPOSITION OF DIRECTIONAL THREADS

0" 6" 12" 18" 24" 30^ 36^ 46^ 72" POSITION OF DIRECTIONAL THREADS

Figure 4. Flo^v patterns caused "by the orifice and orifice-target combinations.

(a) 12-In orifice (b) 12-in orifice in combination with a 12-in target

4:
orifice and orifice-target combination. The flutter and change in direction of the air flow is indicated

by multiple positions of the single thread used at each point in the cross section of the duct. These

patterns were consistent with those obtained

through flow visualization with smoke, but were

not as dramatic nor definitive in presentation of

detail and sharpness of outline.

3. Description of Mixers

The orifices used in these tests conform to the

specifications set forth for the "German Standard

Orifice 1939" [7]. This type of orifice has been the

subject of extensive research both in Germany

and the United States. Studies of the square-edged

orifice were made at the National Bureau of

Standards [8] to determine the flow characteristics,

such as compressibility, discharge coefficients, and

pressure distribution. Figure 5 shows the recom- mended dimensions of the square-edged orifice plate.

NOT GREATER THAN O.I d

(*- NOT GREATER THAN 0.02 d

diameters being 8, 12, and 16 in, respectively. The

throats were machined so that the leading edge

of each orifice was sharp and square. The downstream ed^e of each throat was beveled at a 45°

angle to give a throat thickness of 1/16 in. Each

of the orifices could be fitted into a housing which was securely attached to the duct wall, making

them readily interchangeable, and assuring that

the orifice throat was concentric with the duct.

The targets were also 8, 12, and 16 in in diameter and were made of 1/16-in steel plate. They

were supported from the duct wall by four struts providing a concentric placement within the duct.

The target could bs easily relocated at various

positions downstream from the orifice simply by

adjusting the compression of the struts against

the duct wall. Figure 6 shows the position of a tar- get in combination with the orifice.

NOT LESS THAN 30°

PiGUBE 5. Dimensions for "Oefman Standard

1939" {Pressure taps ommitted).

Orifice FLOW

The orifices were made of %-in thick aluminum plate. The diameter ratios of the orifice plates were 0.33, 0.50, and 0.67, with their throat

ORIFICE PLATE

—

H

r

i

-D4 —

DOWNSTREAM

MEASURING

STATION

Figure 6. Typical assembly of the orifice-target combinations.

4. Test Conditions

For determining the effectiveness of mixing devices as related to the temperature differences in an air stream, tests were performed using the ini- tial temperature patterns shown in fi^re 7. After

establishing steady operating conditions, the test for mixer effectiveness for a given set of conditions was conducted over a one-hour period with temperature readings taken on the quarter-hour. An

analysis of the five sets of readings gave standard

deviations as described earlier. Preliminary tests [5] 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 figuring effectiveness. When calculated by the standard deviation

method, a higher value of 17 percent was obtained

for this natural mixing because 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 the streams of

different temperature air. The static pressure profiles along the duct and

air flow rates were measured for each test, except

in the cases where only the measuring station was

i iooo III ooo

TEMPERATURE PATTERNS AT THE UPSTREAM MEASURING STATION (T>t)

FiGUBE 7. Mixing effectiveness of the three orifices as related to temperature pattern and orifice- duct diameter ratio d/D.

moved to provide variation in distance between

stations. Air flow rates were determined by means

of pitot-static probe traverses in a rectangular

duct located upstream from the inlet blower. The

relative humidity corresponded to ambient laboratory conditions at all times.
During the evaluation of the orifice mixers, the

following items were varied, one at a time

(1) Temperature pattern—as shown in figure 7. (2) Distance from orifice to downstream measing station.

(3) Magniture of temperature difference upstream of the mixer.

(4) Air flow rate. When the orifice-target combinations were studied the following parameters were varied in addition to those listed above for the orifices alone:

(1) Distance from the orifice to the target.

(2) Target size in combination with a particular orifice.

5. Performance of Mixers

5.1. Orifices

All values of mixing effectiveness presented are based on the standard deviation determination, ex- cept for the summary tables where results of both

methods are listed. The effectiveness of the orifices for the three temperature patterns is shown graphically in figure 7. A study of the bar graphs in

figure 7 indicates that the mixing effectiveness in- creased as the diameter ratio of the orifices decreased ; it increased as the interface area between

the cold and warm elements of the air stream in- creased ; and it increased as the mass or size of the

colder elements of the air stream decreased. The

data in figure 7 suggests that the orifice-duct diameter ratio may be the most significant of these three

parameters, and the interface area between the

cold and warm elements of next importance. Most

of the tests were performed with the temperature

pattern illustrated in the center of figure 7. Through flow visualization, in the smaller scaled

apparatus, a qualitative observation of the spreading and mixing of the jet with the surrounding

fluid was made. It was discernible in this apparatus using smoke as an indicator, that the smaller

orifices with diameter ratios of 0.33 and 0.50 provided better mixing than the orifice with a diameter ratio of 0.67. Measurements were then taken

in the large apparatus to indicate the temperature

distribution at selected distances downstream from

the mixer to determine in a quantitative manner

the extent of mixing at various points after the air

had passed through the orifice. Figure 8 illustrates the effect of varying the

magnitude of air temperature difference over the

range of 0.5 to 20.0 deg F at the mixer inlet as re- lated to mixing effectiveness. It is evident from the

findings that the effectiveness was not materially

affected by the variation in magnitude of the temperature difference. As a consequence the 3 deg

F difference was chosen for use in most tests be- cause it presented a measurable difference before

and after miking, and the condition was easily

controlled. This difference in temperature was sufficiently large compared with the magnitude of the

errors of the instrumentation to give reliable effec- tiveness values. The anomalous point on figure 8

for the 8-in orifice at 0.5 deg F temperature differ- ence between the quadrants was probably caused

by the greater ratio of measurement error to temperature difference for this test condition. No such

anomalies appear in the curves for any of the ori100

90

<n

UJ 70

1 1 1 1 1 1

1 -r

8" ORIFICE l0.33blAM. RATIO)

Q cT^ 12" ORIFICE 10.50 DIAM. RATIO)

~o-o 0 I6"0RIFICE 10.67 DIAM. RATIO)

o

1 1 1 1 1

1 1 0^ |i 3 5 10 15 20 25

TEMPERATURE DIFFERENCE BETWEEN QUADRANTS CF)

Figure 8. Effect of air temperature difference at mixer

inlet upon the performance of the orifices.

100

90

80

70

60

50

O

'»0

LJ

ll. U-UJ 30

20

10

8" ORIFICE 10.33 DIAM. RATIO)

,e"0R>F.CEC0.6°7D.AN,.RAT>0)

DISTANCE FROM ORIFICE TO DOWNSTREAM MEASURING STATION

^2 3.0 DUCT DIAMETERS

0 400 800 1200 1600 2000 FLOW RATE (CFM) Figure 9. Performance of the orifices over a selected range of flow rates.

fices at temperature differences larger than 1.5 deg F. Air flow rates were adjusted from 300 to 1400

cfm during the investigation. It is shown by figure

9 that there was less than 5 percent change in ef- fectiveness for the 0.67 and 0.50 diam ratio orifices and much less change for the 0.33 diam ratio orifice as the flow rate was varied over this range. The

Reynolds number in the throat of the orifices varied between 3 X 10* and 3 X 10^ for the range of

flows used. Plots of the effectiveness as a function

of Reynolds number give essentially the same re- lationship as shown m figure 9 for effectiveness versus flow rate.

6
Hi so >" 40lO B ORIFICE 0-1500 CFM

AI2"0RIFICE 0" 590 CFM

16" ORIFICE Q- 860 CFM

DISTANCE FROM ORIFICE TO DOWNSTREAM MEASURING STATION (DUCT DIAMETERS)

Figure 10. Mixer effectiveness the distance from the orifice to the measuring

station is varied.

Figure 10 shows the mixing effectiveness of the

three orifices in relation to the distance from the

plane of the orifice. These curves show that most

of the mixing was accomplished in the first three

duct diameters following the mixer. However, to achieve best mixing with an orifice, an orifice with

a diameter ratio of 0.50 or less should be used and

it should be placed at least 4 duct diameters ahead

of the point where uniformity is required.

It should be noted in figure 10 that the air flow

rates used for the three orifices were different and

that the flow rates were not proportional to orifice area. However, considering the small effect of air

flow rate on mixer effectiveness revealed in figure 9, the comparison made of the three orifices m fig- ure 10 for different air flow rates is reasonably

valid. That is, the relative positions of the three curves would he altered only slightly if all of the

data had been taken at the same air flow rate.

100

90 g 80

10z

Ul> 60

I-t

50

Ul 40

30

^16"

8 ORIFICE (0.33DIAM. RATIO) '"^lar ORIFICE (0.60DIAM. RATIO)

ORIFICE 10.67 DIAM. RATIO)

0 IjO 2.0 3.0 DIFFERENCE IN STATIC PRESSURE (INCHES WATER GAGE)

FiGUKE 11. PerforwMnce of the orifices as a function of pressure drop across the orifice.

Usually when using any type of mixing device,

in addition to the length required for mixing, an

important factor is the pressure drop across the

device. Figure 11 shows the effectiveness as related

to pressure drop for the three orifices. As shown

in figures 9 and 11, there was very little change in

performance of the 0.33 diam I'atio orifice over

the range of flow and pressure drop for these tests. The 0.50 and 0.67 diam ratio orifices did exhibit

small increases in effectiveness for increases in

flow rate and pressure drop. Thus, these figures

show, in the range of these tests, effectiveness depends largely on the diameter ratio rather than

the flow rate or pressure drop. Further, there appears to be a practical limit to the increase in

effectiveness that could be gained at the expense

of increased pressure losses by utilizmg orifices of

smaller than 0.33 diam ratio. Figures 12 and 13 illustrate the performance of

the 0.50 and 0.33 diam ratio orifices, respectively,

at several stations doAvnstream from the mixer

location. Shown in these figures to the left of the

plane of the orifice are the temperature pattern

and the temperature distribution upstream from

the mixing device showing an approximate temperature difference of 3 deg F, To the right of

the mixer are distributions which show progressive

improvement in uniformity of temperature as the

air passes down the duct. These illustrations are

consistent with the curves of figure 10 which show

mixing effectiveness as a function of distance from

the orifice to the downstream measuring station.

The "temperature profiles" shown in figures 12

and 13 for the various stations of observation

should not be regarded as the temperature dis- tribution along any diameter of the test duct, but

rather a combined linear presentation of the temperatures observed in the four quadrants of the

test duct. Tables 1(a), 1(b), and 1(c) are tabulations of the conditions under which each test was

run and the performance of each device under

these conditions.

5.2. Orifice-Target Combinations

The study of the orifice-target combinations was

carried out using the 0.50 diam ratio orifice in

combination with the different targets. Figure 14

7
/ T

/ QUAD. 2 T \

QUAD. 1 \

\ QUAD. 3 \ T

QUAD. 4 /

t /

AIR TEMPERATURE F

82.0 si.O edo 79.Q 78.0

QUAD. 3

QUAD 4

QUAD. 1 __L_

(590 CFM)

81.0 80.0 79.0 80.0 79.0 80.0 79.0 80.0 79.0

DISTANCE FROM ENTRANCE OF MIXING DEVICE TO THE DOWNSTREAM MEASURING STATION (DUCT DIAMETERS)

Figure 12. Temperature distribution of air stream at points upstream and downstream from the 0.50 diam ratio orifice.

AIR TEMPERATURE

/ T

/ QUAD. 2 T \

QUAD. 1 \

\ QUAD. 3 \ T

QUAD. 4 /

t /

82.0 81.0 80.0 79.0 78.0

QUAD. 2 >

QUAD. 3

2--

81.0 80.0 79.0 81.0 80.0 79.0 8L0 80.0 81.0 80.0 81.0 80.0

u

FLOW o (I500CFM)

0 12 3 4

DISTANCE FROM ENTRANCE OF MIXING DEVICE TO THE DOWNSTREAM MEASURING STATION (DUCT DIAMETERS)

FiGUKE 13. Temperature distribution of air stream at points upstream and douMstream from the 0.8S diam ratio orifice.

Table 1. SUMMARY OF TEST CONDITIONS AND PERFORMANCE OF THE SQUARE-EDGED ORIFICE

1(a) 0. 67 diameter ratio orifice (16-lnch orifice) Pressure

Drop Distance from

Test Temperature Flow Across Orifice to Temp. 7. Effectiveness _No^ Difference Rate Mixer Measuring Station Pattern* Range Std.Dev

(°F) (cfm) ( inT^W.G.) (in.) (Duct Diam.)

1 3.9 1700 .14 62 2.6 e 65 ,4 69.4

2 3.9 1700 .14 62 2.6 c 44.8 56.9

3 3.6 1700 .14 62 2.6 ^ 80.1 87.0

4 .6 860 .04 70 2.9 e 64.7 69.2

5 1.4 860 .04 70 2.9 e 63.6 68.8

6 10.9 860 .04 70 2.9 e 61.1 69.0

7 21.6 860 .04 70 2.9 e 58.3 68.7

8 3.8 860 .04 74 3.1 e 63.7 69.9

9 3.2 860 .04 66 2.7 e 60.7 65.9

10 3.8 860 .04 70 2.9 61.9 68.6

11 3.8 860 .04 45 1.9 t> 48.4 53.5

12 3.8 1270 .06 74 3.1 e 66.7 72.3

13 3.7 1400 .09 74 3,1 e 71 ,9 75.1

14 3.5 470 .01 74 3.1 e 66.3 72.2

15 3.3 300 74 3.1 t) 60.3 69.8

* Shaded area indicates a different temperature from unshaded area. 8
Table 1. SUMMARY OF TEST CONDITIONS AND PERFORMANCE OF THE SQUARE-EDGED ORIFICE—Con.

1(b) 0,50 diameter ratio orifice (12-lnch orifice)

Pressure

Drop Distance from Test Temperature Flow Across Orifice to Temp. % Effectiveness _No^ Difference

(°F) Race

(cfm) (

Mixer

in, W.G.) Measuring Station(In.) (Duct Diam.) Pattern* Range Std.Dev.

1 3 5 1260 .42 73 3 0 86 7 88.4

2 3 7 1260 .42 73 3 0 95 4 97.2

3 3 7 1260 .42 73 3 0 Q 76 0 84.8

4 6 590 .10 73 3 0 81 8 83.5

5 1 7 590 .10 73 3 0 83 7 85.4

6 11 4 590 .10 73 3 0 83 9 86.8

7 21 3 590 .10 73 3 0 83 0 86.6

8 3 6 590 .10 65 2 7 77 9 80.4

9 3 5 590 .10 57 2 4 72 ]L 74.3

10 3 9 590 .10 37 1 5 e 48 4 59.3

11 3 8 590 .10 73 3 0 e 82 0 85.1

12 3 4 980 .25 73 3 0 e 83 6 85.5

13 3 5 1200 .36 73 3 0 86 2 87.4

14 3 5 590 .10 88 3 7 e 90 1 92.0

15 3 6 590 .10 104 4 3 e 93 7 95.2

* Shaded area indicates a different temperature from unshaded area

1(c) 0. 33 diameter ratio orifice (8-inch orifice) Pressure

Test

No. Temperature Difference

(°F)

Fl ow Rate

(cfm)"

Drop

Across Mixer

(in. W.G.)

Distance from Orifice to Measuring Station

Tin.) (Duct Diam.)

Temp. Pattern*

% Effectiveness

Range ^td^ev

1 3.7 1100 1.37 74 3.1 94.2 95.0

2 3.4 1100 1.37 74 3.1 97.9 98.7

3 3.7 1100 1.37 74 3.1 91.1 94.6 4 3.9 850 .76 74 3.1 t) 93.8 95.1

5 3.6 1500 2.01 74 3.1 e 93.7 94.9

6 .6 1500 2.01 74 3.1 e 85.5 89.1

7 1.8 1500 2.01 74 3.1 r 93.3 94.8

8 10.6 1500 2.01 74 3.1 93.5 94.6

9 18.4 1500 2.01 74 3.1 e 93.6 94.7 10 3.6 1503 2.01 66 2.7 90.9 92.5

11 3.6 1500 2.01 58 2.4 t> 85.3 88.6 12 3.6 1500 2-01 50 2.1 79.9 85.5

13 3.5 1500 2 01 26 1.1 e 57,6 77.1 14 3.4 300 .10 74 3.1 e 95.5 96.3

15 3.6 1500 2 01 104 4.3 e 97.8 98.2 16 3,4 1500 2.01 88 3.7 96.2 97.1

*Shaded area indicates a different temperature from unshaded area.

9
DISTANCE BETWEEN ORIFICE AND TARGET HELD AT 24"-

0" 1000 CFM.

ORIFICE- TARGET COMB.

12"- 16" O 12"— 12" A I2°- 8" • 12° ORIFICE

DISTANCE FROM ORIFICE TO MEASURING STATION (DUCT DIAM)

FiGUEE 14. Performance of the combination orifice-target

mixer at selected points downstream and of the same orifice alone.

shows the performance of three combinations

under similar conditions. Tests were made to determine the performance of the orifice-target

combinations when the distance between the orifice and target was varied. Two series of tests were

performed, one in which the overall distance from

the orifice to the downstream measuring station was held constant and the other in which the dis- tance from the target to the downstream measuring station was constant. The results showed that

location of the target one duct diameter from the

orifice was a reasonable choice. Over the range of

distances used for the tests there was no improvement over the plain orifice for any of the combinations of the 0.50 diam ratio orifice with each

of the three targets. Figure 15 illustrates the magnitude of the pressure drop across the 0.50 diam

ratio orifice and the same orifice plus combinations

of the 8-5 12-, and 16-in targets. This shows that

there was a penalty of greater pressure drop without any accompanying increase in effectiveness.

0.4

01

Q = IOOO CFM

TYPES OF MIXING DEVICES

PiGUKE 15. Pressure drop across the orifice and orifice- target combinations for a flow rate of 1000

cfm.

Figure 16 illustrates the performance of the ori- fice-target combinations in terms of temperature

patterns just as figures 12 and 13 did for the orifices without the target. Since these initial investigations showed little or no improvement of the

orifice-target combinations over the plain orifice,

this phase of the study was curtailed. Table 2 is a summary tabulation of the conditions under which

the orifice-target combinations were tested and the

performance observed at each test condition.

/ T

/ QUAD, 2 T \QUAD. 1 \

\ QUAD 3 \ T

QUAD. 4 /

t /

AIR TEMPERATURE F SiO 8IX) 80.0 79.0 78.0

QUAD. 2 %

QUAD 3

81.0 80.0 ei.O 80.0

FLOW

( lOOOCFM)

DISTANCE FROM ENTRANCE OF MIXING DEVICE TO THE DOWNSTREAM MEASURING STATION (DUCT DIAMETERS)

Figure 16. Temperature distribution of air stream at points upstream and downstream from the mixing device {0.50

diam ratio orifice and 12-in target combination).

10
TABLE 2 SUMMARY OF TEST CONDITIONS AND PERFORMANCE OF THE ORIFICE -TARGET COMBINATIONS

Orifice-Target Combinations

D^ D3 Dlst. from Dlst. from

Test No

.

Temperature

Difference

Flow Target Diameter Orifice

to Target Target to Meas. Stat. Overall Distance

Temp.

Pattern*^'

7o Effectiveness

Range Std. Dev.

(cfm) ^ln. ) (Dj/D)^'

"(Duct

(in.) Dlam.) (in Vin \

.

;

(Duct Dlam.

)

(in ^ (Duct Dlam.)

j_ 3.3 1000 1

6

. 67 24 1.0 48 2 .

0

72 3 0 79.2 84 .

0

2 3.6 1000 16 . 67 24 1.0 84 3.5 108 4 5 94.1 95 .

6

3 3.5 1000 8 .33 24 1.0 84 3.5 108 4 5 m 84.4 87 .

9

4 3.5 1000 g .33 24 1.0 48 2.0 72 3 0 69.6 75 .

1

5 3.3 1000 1

2

.50 24 1.0 48 2.0 72 3 0 m 74.5 80.2

6 3.5 1000 12 .50 12 .5 48 2 .

0

60 2 5 m7 77.3 82.1

7 J.O 1 000 12 .50 36 1.5 48 2.0 84 3 5 78.5 82.3

8 3.6 1000 12 .50 48 2.0 48 2.0 96 4 0 82.3 85.4

9 3.5 1300 12 .50 24 1.0 60 2.5 84 3 5 e 83.0 85.7

10 3.6 1000 12 .50 24 1.0 60 2.5 84 3 5 e 82.6 86.3

11 3.4 1000 12 .50 24 1.0 84 3.5 108 4 5 e 90.4 92.3

12 3.5 1000 12 .50 36 1.5 72 3.0 108 4. 5 87.5 89.7 * D = diameter of duct ; Di = diameter of target. * Shaded area Indicates a temperature different from the unshaded area.

6. Conclusion

The results of this investigation showed that an

orifice having a diameter ratio of approximately

0.33 was consistently more effective over the dis- tance tested in reducing the thermal differences in an unmixed air stream. However, the mixing process was accompanied by a high pressure drop

across the mixer. If the pressure drop can be tol- erated, temperature nonuniformity could be re- duced to 3 or 4 percent of its origmal value, at a

distance of approximately 4.6 duct diameters

downstream from the orifice. The study indicated

that diameter ratio was the most important

parameter affecting the mixing effectiveness but

that the interface area between cold and warm

elements of the air stream and the size of the nonisothermal elements also had a bearing on the

mixing process. The performance of the orifice- target combinations showed no improvement over

the plain orifice even when using a target as large

as two-thirds of the duct diameter.

7. References

[1] S. CJorrsin, InTestigation of flow in an axlally symmetrical lieated jet of air, NACA ACR No. 3L23

(Dec. 1943).

[2] L. Helander and C. V. Jakowatz, Downward projection of heated air, ASHVE Res. Rpt. No. 1327,

ASHVE Trans. 54 (1948).

[3] V. Cleeves and L. M. K. Boelter, Isothermal and non- isothermal air-jet investigations, Chem. Engr. Progress [3] 43 (1947).

[4] A. M. Kuethe, Investigations of the turbulent mixing

regions formed by jets, J. App. Mech. [3] 2 (Sept

1935).

[5] T. K. Faison, J. C. Davis, and P. R, Achenbach, A

test apparatus for the study of forced air-mixing

devices, J. Res. NBS 70C (Engr. and Instr.) No. 1, 25 (1966).

[6] Fluid Meters—Their Theory and Application, 5th Ed.

1959, (American Society of Mech. Engineers, 29 W.

39th Street, New York, N.Y. )

.

[7] E. Ower, The measurement of air flow, 3d Ed. 1948,

(Chapman & Hall Ltd., 37 Essex St, W.C.2 London).

[8] H. S. Bean, B. Buckingham and P. S. Murphy, Dis- charge coefficients of square-edged orifices for measuring the flow of air, BS J. Res. 2, 561 (1929).

11

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