- Email: [email protected]
The observed influence of surface obstructions on the airflow pattern within livestock buildings
J. agric. Engng Res. (1976) 21, 33-39
The Observed Airflow
Influence Pattern
of Surface within
Obstructions
Livestock
on the
Buildings
J. M. RANDALL*; V. A. BATTAMS* Ability to predict the efficiency of a livestock ventilation system requires a knowledge of the airflow pattern within the building. This paper describes a practical method for determining the interactions between an airflow pattern and surface obstructions of rectangular cross-section. A required flow pattern may be obtained by designing a suitable obstruction or the influence of an obstruction on a flow pattern may be evaluated. Purlins are a common form of obstruction and are sited where they may have a large effect on the flow pattern. A range of sizes was used to provide coefficients of linear regressions of air speed approaching the purlin against the angle of deflection of the air jet. 1.
Introduction
Qualitative rules governing the ways in which air moves in livestock buildings have been established.’ These rules, however, apply only to simple arrangements and there are many factors influencing them which have not yet been quantified. One of the most important of these is the deflection of air paths caused by obstructions which project from otherwise smooth wall, floor or ceiling surfaces. Obstacles such as troughs, solid pen divisions, feeding equipment, lamps, purlins and the stock themselves may have a marked effect on the airflow pattern. Some of the ways in which solid pen divisions and feeding troughs influence air movement have been investigated,’ but the effects of small obstacles are not known. Many ventilation systems produce a jet of air beneath the ceiling in buildings with structural members such as purlins which project from an otherwise smooth surface. The present paper evaluates the influence of these purlins on airflow patterns. One approach used by King* is to study the fine structure of the jet and the ways in which velocity profiles are modified by obstructions in order to predict the re-attachment distance. The method is restricted to jets projected horizontally with the upper edge of the inlet formed by a horizontal ceiling surface. The results show that with obstructions like purlins it is possible to determine when a jet will become detached from a ceiling but prediction of the angle of departure or the distance before re-attachment cannot be achieved with reliability. Further, a detailed knowledge of the velocity profile of the jet is assumed and such data are usually difficult to acquire for a livestock building either by measurement or calculation. Factors which still have to be accounted for before this detailed procedure can be adopted are the effects of air recirculation within an enclosure and the effects of temperature and buoyancy on the jet behaviour. Walker3 investigated the way in which a series of small ceiling irregularities modify the velocity profile near the surface. Jet entrainment, which is of importance inside an enclosure, is assessed but no definite conclusions can yet be drawn as to the reliability of predicting the velocity profile. The air speed profiles of jets may be calculated using some empirical formulae2-5 but this process requires more information than is usually available to the building designer. It is, therefore, the intention of this paper to present data which will permit the ready calculation of adequate values in practice. Thus all that needs to be known is the approximate speed of the jet at a fixed distance in front of the purlin for the likely effects on the airflow pattern to be predicted for many practical designs. ‘Farm Received
Buildings
Department,
I1 September
1974;
National
Institute
accepted
in revised
of Agricultural
Engineering,
form
1975
17 April
33
Wrest
Park,
Silsoe,
Bedford
INFLUENCE
34 2.
OF
SURFACF
OHS’1‘RIIC‘TIONS
Experimental procedure
The full-scale section of a livestock building used in this work has been described in detail elsewhere.6 The section was 7.35 m wide representing a typical span which may accommodate two pig pens and three passages. It was arranged with solid walls, 1.05m high, to form a feeding passage at each side and with a central wall to form two pens (Fig. I). The depth of the section (3.1 nl) represented the length of one pen and the height was 2.14 m to the eaves and 2.74 m to the ridge. An insulated shell connected to an air conditioner and enclosing the side walls and roof allowed the temperature outside the section to be controlled at about 19LC. Ventilating air was drawn from the shell by a propeller fan and the temperature inside the section varied according to the fan speed
(b)
(d)
(f)
Fig. I. Airflow patterns showing the effects of the presence of purlins (dimensiotrs in metres)
selected. A larger fanwas used for air entryat the eaves than for entry at the ridge. The heat released from the stock was represented by 28 simulated pigs’ resulting, for some of the lower fan speeds, in a higher temperature in the section than would be experienced in practice. A purlin was attached to the ceiling midway between the eaves and the ridge. The effects of purlins extending 25, 53, 78, 106, 159 and 300 mm beneath the otherwise smooth ceiling were studied. All were 51 mm thick, extended over the full depth of the building section and were mounted at. right angles to the slope of the ceiling. Using liquid film bubble? illuminated by a narrow beam of light, photographs were taken through a transparent wall of the building and sketches made of the airflow patterns with particular regard to the deflections caused by the purlins.
J.
M.
RANDALL;
V.
A.
35
BATTAMS
In order to provide a relatively simple procedure for use in intensive livestock buildings a single point was chosen for measuring the air speed approaching the purlin. This point had to be as near to the purlin as possible to allow for purlins which might be close to the inlet and yet be sufficiently far away for the purlin to have a negligible effect on the measured speed. It seemed likely that the relative size of the purlin and the distance of the maximum speed from the ceiling would influence the angle of deflection of the jet. In livestock buildings the ventilating air jets are commonly directed parallel to a surface and can be regarded as half jets with the maximum speed close to the surface.*, 8 This characteristic of a jet is frequently referred to as the Coander effect9 The measuring position should therefore be sufficiently close to the surface to be representative of the jet velocity. Using these considerations the speed was measured with a heated thermocouple anemometerlO positioned 300 mm from the purlin on the upstream side and 150 mm from the ceiling. Fan speeds were selected to cover the widest possible range of air speeds approaching the purlin and 10 readings of each speed were taken, from which the means and standard deviations were calculated.
Airspeed
at themeasuring
points
(m/s1
Fig. 2. Relationships between ventilation rate and air speed at the measuring points for air entering at the ridge (0) or at the eaves (0). Typical ventilation rates: ~ - -, minimum; __ -, maximum
The jet, on entering the enclosure from either the ridge or the eaves through a slot 41 mm wide and extending for the full depth of the section tended to follow the slope of the ceiling All angles were measured with reference to this slope and at 150 mm from it. The direction of the jet after deflection was determined by observing only the path of those bubbles which passed close to the anemometer. A record was made of the point at which the path of these bubbles intersected a vertical scale set downstream of the purlin. Ten observations, estimated to the nearest 40 mm, were made from which the means and standard deviations were calculated. The angle of deflection (6) was then measured using a protractor on a scale diagram.
36
INFLUENCE
3.
OF
SURFACF,
OBS’rRI’(“TiONS
Results and discussion
The relationship between ventilation rate and air speed at the appropriate measuring points (Fig. 2) were different due to the slope of the ceiling: the jet entering from the ridge was directed horizontally at the same level as the purlin whilst the one entering horizontally at the eaves was below the purlin. Typical maximum and minimum ventilation rates are 0.9 and 0.1 ms:s respectively for the 28 pigs. With air entering at the eaves the larger fan allowed the normal maximum ventilation rate to be exceeded up to 1.8 m3/s. With eaves entry and an air volume below 0.2 m”js the jet always fell as it entered the building setting up a clockwise rotation above the stock (Fig. l(a)). This rotation resulted in the air returning beneath the ceiling and breaking away from it at about the mid-point. Hence the positioning influence on the air-flow pattern. This result of any purlin here (Fig. l(b)) had an insignificant
159 60
c
5 106
I
1
0
0.2
0.4 Airspeed
Fig. 3. The relationshigs
n
0.6
0.8
1.0
at the measuring
1.2 points
1.4
1.6
7.8
(m/s)
between air speed, angle of jet deflection andpurlin or at the eaves ( 0, L)
size with air entering at the ridgr (e,
A)
demonstrated that purlins need not have an effect on air distribution, although in this case the position was fairly critical limiting its use in practical designs. For air entering at the eaves with the purlin absent and above a volume of 0.2 m3/s the pattern was always as shown in Fig. l(c) and with entry at the ridge for all air volumes as in Fig. I(d). The experimental results for the angle of jet deflection, air speed, air direction and purlin size were plotted together with a line fitted by linear regression for each size of purlin (Fig. 3). The broken lines show the range of the standard error of each regression line; the probability of an observation lying within this range is 68%. The lower ends of the lines show the speeds below which no permanent deflection was observed. The line of best fit was calculated using the observations for both eaves and ridge entry as all the data are closely grouped, although in some cases
J.
M. RANDALL;
37
V. A. BATTAMS TABLE
I
Coefficients of the linear regressions using the speed at the selected measuring point with the purlin present (l), with the purlin absent (2)
-
I
Purlin size (mm)
(1) or (2) (1) or (2) (1) (1) (1) $I (2)
159 106 300
errors Correlation coejkient
a. 6% per mlsl
-
25 53 78 106 159 300
Standard
I
(deg pzr m/s)
-_______(1) or (2)
Coefficients
_
no uermanent deflelztion 17.14 4.46 1.56 I.29 16.94 16.06 1.31 16.49 30.02 16.16 35.80 0.93 6.07 67.96 0.57 16.67 30.68 1.35 1.09 17.27 36.95 4.82 69.24 0.46 -
1.97 1.42 1.21 0.90 0.66 1.19 0.92 0.55
0.869 0.929 0.862 0.932 0.903 0.855 0.912 0.906
-
Minimum speed (m/s)
0.66 0.55 0.40 0.37 0.34 0.40 0.37 0.34
-
the individual lines of best fit for each direction of entry would be marginally different but within the limits of experimental error. The coefficients of the linear regressions of the equation
(where B is the angle of the jet deflection and V the speed in m/s at the measuring point), were calculated together with the correlation coefficients and the standard errors in a and b (a,, ub) (Table I) (denoted by (1) in column 1). The last column shows the approximate speed below which it may be assumed that there was no permanent jet deflection due to the purlin and for which the linear regression is not applicable. Below these speeds and at all times for the 25 mm purlin the flow patterns shown in Figs I(c) and I(d) were unchanged by the presence of the purlins because the jet became reattached to the ceiling. Above these speeds the corresponding flow patterns are drawn in Figs Z(e) and Z(J), the extent of the deflection caused by the purlin varying with the air speed at the measuring point. The profiles of air speed at the measuring point without a purlin present (Fig. 4) show that the maximum speed was within the selected measuring distance of 150 mm. However, the distance 800
600 z -“, 500 .E .z c 400 E ii 300 s .Y .; 200
0
0.2
0.4
Airspeed
Fig. 4.
Air speed profiles
0.6
at 300
mm
for air entering
06
1.0
ua~tream
1.2 of thepurlin
, .4
1.6
posltlon
l-6 with
2.0 the purlln
at the ridge (0, A) or at the eaves measuring position (- - -)
2.2
2.4
:
akeent(m/s)
(0,
A)
in relation
to the standard
II\;FLUENCE
38
OF
SUKl:A(‘I~
OHS I-KI!(‘IIONS
from the ceiling of this maximum speed increased as the size of the purlin was increased. l-‘or air entering at the ridge, purlins of size 159 and 300 mm produced an increase in the speed at the measuring point by displacing the maximum value (Fig. 5). For air entering at the eaves the 106 and 159 mm purlins increased the speed, whilst above 0.7 m/s the 300 mm purlin caused a decrease by displacing the maximum speed further than 150 mm for the ceiling. The presence of purlins of size 78, 53 and 25 mm did not influence the measured speed. In many design problems, it is likely that only the speed without a purlin present can be estimated and that lit tie is known of the shape of the profile. To establish the effects of using only speeds without the purlin the coefficients of linear regression were recalculated (Table I) for the three larger purlins (denoted by (2) in column 1). The change in the values of the coefficients are relatively small and for practical purposes either speed may be used for calculating the deflection. This is not unexpected since reference to F&S. 3 and 4 shows that for the largest purlins which cause the greatest chanpe in measured speed, the variation of deflection with speed is the least.
0.4-
0
0.2
0.4
0.6 Airspeed
Fig. 5. The e&cts of 300 mm (0,
0),
0,8 without
1.0 purlin
1.2
14
1.6
(m/s)
159 mm (A, A) and 106 mm (m, U) purlins ONair speed ut tl7e measuring The air speed is unchanged (- - --) for all other pwlirl sizes
points for air entering at the ridge and eaves respertively.
From the method of measurement used it might be assumed that the results apply only to an approximately isothermal situation and that practical non-isothermal situations will modify the results of the correlation procedure. In a livestock building an air throughput corresponding to 0.8 m3/s in the building section would be called for with an outside temperature of about 18‘ C resulting in an internal temperature of about 20” C. Thus the patterns of flow described would apply directly to a real situation with outside temperatures above 18” C. Lower temperatures corresponding to the lower air speeds would reduce the buoyancy of the incoming jet causing deflections greater than those recorded and thus the calculated angles should be considered to be minimum values. Practical use of these results, which are derived from the common arrangements of jets issuing from continuous slot inlets, may be extended to some other forms of inlet. For example a continuous row of holes in a duct form discrete jets which in entraining air spread at an angle of about 2.5 degrees.“’ l2 At some distance from the duct, depending on the size and separation of the holes,
J.
M.
RANDALL;
V.
A.
39
BATTAMS
the individual jets coalesce to form a continuous jet.
In other buildings each pen may have a hopper inlet which is only one third of the pen width. If the spread of 25 degrees causes the individual jets to coalesce before reaching the obstacle then the resulting jet will be of a form
similar to the ones used in the experiments. If they have not combined then their spread will often be sufficient for the correlations to be used if the reference speed is measured in the vertical plane through the centre of the inlet. The use of this value will indicate the maximum deflection of the centre line of the jet and this is probably adequate for the purpose of building design. 4.
Conclusions
Purlins projecting beneath a ceiling surface may completely change the pattern of air flow in a mechanically ventilated building. With a sloping ceiling this change is more likely to occur with air entering horizontally from the ridge than with air entering horizontally from the eaves. For air jets entering horizontally through slots beneath a ceiling, the angle of deflection of a jet due to a regular obstruction such as a purlin may be calculated using a linear regression, provided that the air speed at 150 mm from the surface 300 mm upstream of the purlin can be measured or estimated. Acknowledgements The authors wish to thank J. C. Hawkins and G. A. Carpenter for their advice and encouragement.
REFERENCES
Randall, J. M. The prediction of airflow patterns in livestock buildings. J. agric. Engng Res., I975 20 (2) 199-215 2 King, F. C.; White, G. M.; Walker, J. N. The effect of surface obstructions on air wall jets. A.S.A.E. Paper No. 70-403, 1970 3 Walker, J. N.; White, G. M. Znfhrence of ceiling surface irregularities on air jets. Trans. A.S.A.E., 1973 16 (1) 145-147 4 Wilson, J. D.; Esmay M. L.; Persson S. Wall jet velocity and temperature profiles resulting from a ventilation inlet. Trans. A.S.A.E., 1970 13 (1) 77-81 5 Albright, L. D.; Scott, N. R. The low speed non-isothermal wall jet with applications to ventilation. A.S.A.E. Paper 72-913, 1972 6 Carpenter, G. A.; Moulsley, L. J.; Randall, J. M. Ventilation investigations using a section of a livestock building and visualisation by bubbles. J. agric. Engng Res., 1972 17 (4) 323-331 7 Owen, J. E.; Randall, J. M.; Carpenter, G. A. A simulated pig for use in ventilation studies. N.I.A.E. Departmental Note FB/168/3020, 1971 8 Baturin, V. V. Fundamentals of Industrial Ventilation. Oxford: Pergamon Press, 1972 9 Borque, C.; Newman, B. G. Re-attachment of a two dimensional incompressible jet to an adjacent flat plate. Aeronaut. Q., 1960 XI 201-230 lo Randall, J. M.; Wilmshurst, T. A multiple head, low velocity, omnidirectional heated thermocouple anemometerfor use in ventilation studies. N.I.A.E. Departmental Note FB/271/3020 and FB/413/3020, 1973 ” Farquharson, I. M. C. The ventilating air jet. J. Inst. Heating Ventilating Engrs, 1952 19 449469 ‘* Owen, J. E. Air jets for ventilation. Fm Bldgs Dig., 1973 8 (2) 10-12 ’
The Observed Airflow
Influence Pattern
of Surface within
Obstructions
Livestock
on the
Buildings
J. M. RANDALL*; V. A. BATTAMS* Ability to predict the efficiency of a livestock ventilation system requires a knowledge of the airflow pattern within the building. This paper describes a practical method for determining the interactions between an airflow pattern and surface obstructions of rectangular cross-section. A required flow pattern may be obtained by designing a suitable obstruction or the influence of an obstruction on a flow pattern may be evaluated. Purlins are a common form of obstruction and are sited where they may have a large effect on the flow pattern. A range of sizes was used to provide coefficients of linear regressions of air speed approaching the purlin against the angle of deflection of the air jet. 1.
Introduction
Qualitative rules governing the ways in which air moves in livestock buildings have been established.’ These rules, however, apply only to simple arrangements and there are many factors influencing them which have not yet been quantified. One of the most important of these is the deflection of air paths caused by obstructions which project from otherwise smooth wall, floor or ceiling surfaces. Obstacles such as troughs, solid pen divisions, feeding equipment, lamps, purlins and the stock themselves may have a marked effect on the airflow pattern. Some of the ways in which solid pen divisions and feeding troughs influence air movement have been investigated,’ but the effects of small obstacles are not known. Many ventilation systems produce a jet of air beneath the ceiling in buildings with structural members such as purlins which project from an otherwise smooth surface. The present paper evaluates the influence of these purlins on airflow patterns. One approach used by King* is to study the fine structure of the jet and the ways in which velocity profiles are modified by obstructions in order to predict the re-attachment distance. The method is restricted to jets projected horizontally with the upper edge of the inlet formed by a horizontal ceiling surface. The results show that with obstructions like purlins it is possible to determine when a jet will become detached from a ceiling but prediction of the angle of departure or the distance before re-attachment cannot be achieved with reliability. Further, a detailed knowledge of the velocity profile of the jet is assumed and such data are usually difficult to acquire for a livestock building either by measurement or calculation. Factors which still have to be accounted for before this detailed procedure can be adopted are the effects of air recirculation within an enclosure and the effects of temperature and buoyancy on the jet behaviour. Walker3 investigated the way in which a series of small ceiling irregularities modify the velocity profile near the surface. Jet entrainment, which is of importance inside an enclosure, is assessed but no definite conclusions can yet be drawn as to the reliability of predicting the velocity profile. The air speed profiles of jets may be calculated using some empirical formulae2-5 but this process requires more information than is usually available to the building designer. It is, therefore, the intention of this paper to present data which will permit the ready calculation of adequate values in practice. Thus all that needs to be known is the approximate speed of the jet at a fixed distance in front of the purlin for the likely effects on the airflow pattern to be predicted for many practical designs. ‘Farm Received
Buildings
Department,
I1 September
1974;
National
Institute
accepted
in revised
of Agricultural
Engineering,
form
1975
17 April
33
Wrest
Park,
Silsoe,
Bedford
INFLUENCE
34 2.
OF
SURFACF
OHS’1‘RIIC‘TIONS
Experimental procedure
The full-scale section of a livestock building used in this work has been described in detail elsewhere.6 The section was 7.35 m wide representing a typical span which may accommodate two pig pens and three passages. It was arranged with solid walls, 1.05m high, to form a feeding passage at each side and with a central wall to form two pens (Fig. I). The depth of the section (3.1 nl) represented the length of one pen and the height was 2.14 m to the eaves and 2.74 m to the ridge. An insulated shell connected to an air conditioner and enclosing the side walls and roof allowed the temperature outside the section to be controlled at about 19LC. Ventilating air was drawn from the shell by a propeller fan and the temperature inside the section varied according to the fan speed
(b)
(d)
(f)
Fig. I. Airflow patterns showing the effects of the presence of purlins (dimensiotrs in metres)
selected. A larger fanwas used for air entryat the eaves than for entry at the ridge. The heat released from the stock was represented by 28 simulated pigs’ resulting, for some of the lower fan speeds, in a higher temperature in the section than would be experienced in practice. A purlin was attached to the ceiling midway between the eaves and the ridge. The effects of purlins extending 25, 53, 78, 106, 159 and 300 mm beneath the otherwise smooth ceiling were studied. All were 51 mm thick, extended over the full depth of the building section and were mounted at. right angles to the slope of the ceiling. Using liquid film bubble? illuminated by a narrow beam of light, photographs were taken through a transparent wall of the building and sketches made of the airflow patterns with particular regard to the deflections caused by the purlins.
J.
M.
RANDALL;
V.
A.
35
BATTAMS
In order to provide a relatively simple procedure for use in intensive livestock buildings a single point was chosen for measuring the air speed approaching the purlin. This point had to be as near to the purlin as possible to allow for purlins which might be close to the inlet and yet be sufficiently far away for the purlin to have a negligible effect on the measured speed. It seemed likely that the relative size of the purlin and the distance of the maximum speed from the ceiling would influence the angle of deflection of the jet. In livestock buildings the ventilating air jets are commonly directed parallel to a surface and can be regarded as half jets with the maximum speed close to the surface.*, 8 This characteristic of a jet is frequently referred to as the Coander effect9 The measuring position should therefore be sufficiently close to the surface to be representative of the jet velocity. Using these considerations the speed was measured with a heated thermocouple anemometerlO positioned 300 mm from the purlin on the upstream side and 150 mm from the ceiling. Fan speeds were selected to cover the widest possible range of air speeds approaching the purlin and 10 readings of each speed were taken, from which the means and standard deviations were calculated.
Airspeed
at themeasuring
points
(m/s1
Fig. 2. Relationships between ventilation rate and air speed at the measuring points for air entering at the ridge (0) or at the eaves (0). Typical ventilation rates: ~ - -, minimum; __ -, maximum
The jet, on entering the enclosure from either the ridge or the eaves through a slot 41 mm wide and extending for the full depth of the section tended to follow the slope of the ceiling All angles were measured with reference to this slope and at 150 mm from it. The direction of the jet after deflection was determined by observing only the path of those bubbles which passed close to the anemometer. A record was made of the point at which the path of these bubbles intersected a vertical scale set downstream of the purlin. Ten observations, estimated to the nearest 40 mm, were made from which the means and standard deviations were calculated. The angle of deflection (6) was then measured using a protractor on a scale diagram.
36
INFLUENCE
3.
OF
SURFACF,
OBS’rRI’(“TiONS
Results and discussion
The relationship between ventilation rate and air speed at the appropriate measuring points (Fig. 2) were different due to the slope of the ceiling: the jet entering from the ridge was directed horizontally at the same level as the purlin whilst the one entering horizontally at the eaves was below the purlin. Typical maximum and minimum ventilation rates are 0.9 and 0.1 ms:s respectively for the 28 pigs. With air entering at the eaves the larger fan allowed the normal maximum ventilation rate to be exceeded up to 1.8 m3/s. With eaves entry and an air volume below 0.2 m”js the jet always fell as it entered the building setting up a clockwise rotation above the stock (Fig. l(a)). This rotation resulted in the air returning beneath the ceiling and breaking away from it at about the mid-point. Hence the positioning influence on the air-flow pattern. This result of any purlin here (Fig. l(b)) had an insignificant
159 60
c
5 106
I
1
0
0.2
0.4 Airspeed
Fig. 3. The relationshigs
n
0.6
0.8
1.0
at the measuring
1.2 points
1.4
1.6
7.8
(m/s)
between air speed, angle of jet deflection andpurlin or at the eaves ( 0, L)
size with air entering at the ridgr (e,
A)
demonstrated that purlins need not have an effect on air distribution, although in this case the position was fairly critical limiting its use in practical designs. For air entering at the eaves with the purlin absent and above a volume of 0.2 m3/s the pattern was always as shown in Fig. l(c) and with entry at the ridge for all air volumes as in Fig. I(d). The experimental results for the angle of jet deflection, air speed, air direction and purlin size were plotted together with a line fitted by linear regression for each size of purlin (Fig. 3). The broken lines show the range of the standard error of each regression line; the probability of an observation lying within this range is 68%. The lower ends of the lines show the speeds below which no permanent deflection was observed. The line of best fit was calculated using the observations for both eaves and ridge entry as all the data are closely grouped, although in some cases
J.
M. RANDALL;
37
V. A. BATTAMS TABLE
I
Coefficients of the linear regressions using the speed at the selected measuring point with the purlin present (l), with the purlin absent (2)
-
I
Purlin size (mm)
(1) or (2) (1) or (2) (1) (1) (1) $I (2)
159 106 300
errors Correlation coejkient
a. 6% per mlsl
-
25 53 78 106 159 300
Standard
I
(deg pzr m/s)
-_______(1) or (2)
Coefficients
_
no uermanent deflelztion 17.14 4.46 1.56 I.29 16.94 16.06 1.31 16.49 30.02 16.16 35.80 0.93 6.07 67.96 0.57 16.67 30.68 1.35 1.09 17.27 36.95 4.82 69.24 0.46 -
1.97 1.42 1.21 0.90 0.66 1.19 0.92 0.55
0.869 0.929 0.862 0.932 0.903 0.855 0.912 0.906
-
Minimum speed (m/s)
0.66 0.55 0.40 0.37 0.34 0.40 0.37 0.34
-
the individual lines of best fit for each direction of entry would be marginally different but within the limits of experimental error. The coefficients of the linear regressions of the equation
(where B is the angle of the jet deflection and V the speed in m/s at the measuring point), were calculated together with the correlation coefficients and the standard errors in a and b (a,, ub) (Table I) (denoted by (1) in column 1). The last column shows the approximate speed below which it may be assumed that there was no permanent jet deflection due to the purlin and for which the linear regression is not applicable. Below these speeds and at all times for the 25 mm purlin the flow patterns shown in Figs I(c) and I(d) were unchanged by the presence of the purlins because the jet became reattached to the ceiling. Above these speeds the corresponding flow patterns are drawn in Figs Z(e) and Z(J), the extent of the deflection caused by the purlin varying with the air speed at the measuring point. The profiles of air speed at the measuring point without a purlin present (Fig. 4) show that the maximum speed was within the selected measuring distance of 150 mm. However, the distance 800
600 z -“, 500 .E .z c 400 E ii 300 s .Y .; 200
0
0.2
0.4
Airspeed
Fig. 4.
Air speed profiles
0.6
at 300
mm
for air entering
06
1.0
ua~tream
1.2 of thepurlin
, .4
1.6
posltlon
l-6 with
2.0 the purlln
at the ridge (0, A) or at the eaves measuring position (- - -)
2.2
2.4
:
akeent(m/s)
(0,
A)
in relation
to the standard
II\;FLUENCE
38
OF
SUKl:A(‘I~
OHS I-KI!(‘IIONS
from the ceiling of this maximum speed increased as the size of the purlin was increased. l-‘or air entering at the ridge, purlins of size 159 and 300 mm produced an increase in the speed at the measuring point by displacing the maximum value (Fig. 5). For air entering at the eaves the 106 and 159 mm purlins increased the speed, whilst above 0.7 m/s the 300 mm purlin caused a decrease by displacing the maximum speed further than 150 mm for the ceiling. The presence of purlins of size 78, 53 and 25 mm did not influence the measured speed. In many design problems, it is likely that only the speed without a purlin present can be estimated and that lit tie is known of the shape of the profile. To establish the effects of using only speeds without the purlin the coefficients of linear regression were recalculated (Table I) for the three larger purlins (denoted by (2) in column 1). The change in the values of the coefficients are relatively small and for practical purposes either speed may be used for calculating the deflection. This is not unexpected since reference to F&S. 3 and 4 shows that for the largest purlins which cause the greatest chanpe in measured speed, the variation of deflection with speed is the least.
0.4-
0
0.2
0.4
0.6 Airspeed
Fig. 5. The e&cts of 300 mm (0,
0),
0,8 without
1.0 purlin
1.2
14
1.6
(m/s)
159 mm (A, A) and 106 mm (m, U) purlins ONair speed ut tl7e measuring The air speed is unchanged (- - --) for all other pwlirl sizes
points for air entering at the ridge and eaves respertively.
From the method of measurement used it might be assumed that the results apply only to an approximately isothermal situation and that practical non-isothermal situations will modify the results of the correlation procedure. In a livestock building an air throughput corresponding to 0.8 m3/s in the building section would be called for with an outside temperature of about 18‘ C resulting in an internal temperature of about 20” C. Thus the patterns of flow described would apply directly to a real situation with outside temperatures above 18” C. Lower temperatures corresponding to the lower air speeds would reduce the buoyancy of the incoming jet causing deflections greater than those recorded and thus the calculated angles should be considered to be minimum values. Practical use of these results, which are derived from the common arrangements of jets issuing from continuous slot inlets, may be extended to some other forms of inlet. For example a continuous row of holes in a duct form discrete jets which in entraining air spread at an angle of about 2.5 degrees.“’ l2 At some distance from the duct, depending on the size and separation of the holes,
J.
M.
RANDALL;
V.
A.
39
BATTAMS
the individual jets coalesce to form a continuous jet.
In other buildings each pen may have a hopper inlet which is only one third of the pen width. If the spread of 25 degrees causes the individual jets to coalesce before reaching the obstacle then the resulting jet will be of a form
similar to the ones used in the experiments. If they have not combined then their spread will often be sufficient for the correlations to be used if the reference speed is measured in the vertical plane through the centre of the inlet. The use of this value will indicate the maximum deflection of the centre line of the jet and this is probably adequate for the purpose of building design. 4.
Conclusions
Purlins projecting beneath a ceiling surface may completely change the pattern of air flow in a mechanically ventilated building. With a sloping ceiling this change is more likely to occur with air entering horizontally from the ridge than with air entering horizontally from the eaves. For air jets entering horizontally through slots beneath a ceiling, the angle of deflection of a jet due to a regular obstruction such as a purlin may be calculated using a linear regression, provided that the air speed at 150 mm from the surface 300 mm upstream of the purlin can be measured or estimated. Acknowledgements The authors wish to thank J. C. Hawkins and G. A. Carpenter for their advice and encouragement.
REFERENCES
Randall, J. M. The prediction of airflow patterns in livestock buildings. J. agric. Engng Res., I975 20 (2) 199-215 2 King, F. C.; White, G. M.; Walker, J. N. The effect of surface obstructions on air wall jets. A.S.A.E. Paper No. 70-403, 1970 3 Walker, J. N.; White, G. M. Znfhrence of ceiling surface irregularities on air jets. Trans. A.S.A.E., 1973 16 (1) 145-147 4 Wilson, J. D.; Esmay M. L.; Persson S. Wall jet velocity and temperature profiles resulting from a ventilation inlet. Trans. A.S.A.E., 1970 13 (1) 77-81 5 Albright, L. D.; Scott, N. R. The low speed non-isothermal wall jet with applications to ventilation. A.S.A.E. Paper 72-913, 1972 6 Carpenter, G. A.; Moulsley, L. J.; Randall, J. M. Ventilation investigations using a section of a livestock building and visualisation by bubbles. J. agric. Engng Res., 1972 17 (4) 323-331 7 Owen, J. E.; Randall, J. M.; Carpenter, G. A. A simulated pig for use in ventilation studies. N.I.A.E. Departmental Note FB/168/3020, 1971 8 Baturin, V. V. Fundamentals of Industrial Ventilation. Oxford: Pergamon Press, 1972 9 Borque, C.; Newman, B. G. Re-attachment of a two dimensional incompressible jet to an adjacent flat plate. Aeronaut. Q., 1960 XI 201-230 lo Randall, J. M.; Wilmshurst, T. A multiple head, low velocity, omnidirectional heated thermocouple anemometerfor use in ventilation studies. N.I.A.E. Departmental Note FB/271/3020 and FB/413/3020, 1973 ” Farquharson, I. M. C. The ventilating air jet. J. Inst. Heating Ventilating Engrs, 1952 19 449469 ‘* Owen, J. E. Air jets for ventilation. Fm Bldgs Dig., 1973 8 (2) 10-12 ’