Jets discharging to atmosphere

Jets discharging

to atmosphere

Keith Moodie and Bruce C. R. Ewan* Health and Safety Executive, Buxton SKI 7 9JN, UK

Explosion

and Flame Laboratory,

RLSD,

Harpur

Hill,

Consideration is given to the range of jets that may arise from the safety or accidental venting of the contents of pressurized liquid or gas storage vessels. Attention isfocussed on the momentum dominated region of the discharge, and the present analytical description of gaseous jets with a view to safety analysis is reviewed. Data are presented for a number of two-phase jets of superheated Freon 11, including temperatures, velocities, mass fractions and droplet sizes, and these are shown to be strongly influenced by the liquid/vapour non-equilibrium in the near field and the subsequent evaporating nature of the liquid phase and the accompanying air entrainment. The experimental needs in these complex two-phase systems are discussed with a view to obtaining a better understanding of the source terms. Some recent developments using laser fluorescence as a selective probe are presented. These are shown to have the potential to resolve some of the properties of the individual liquid and vapour phases. The case of liquid temperature is examined. (Keywords: toxicity; wind speed; jets)

The work of the HSE has been concerned over several years with the need to be able to predict the effects of material releases under pressure, with a view to laying down standards for vessel construction. siting and operating procedures. The sources of such releases are widespread and can arise during the transport of liquefied gases under pressure; the heating of stored liquids in vessels due to fires; and from the occurrence of runaway reactions in the chemical industry. High pressure jets of material may be involved at any stage of the above events, and their evolution depends very much on the nature of the source and material properties. Three areas of mass tlow can be identified: l

l

l

The flow of material from its source within the vessel along a release path to an exit plane. The evolution of the jet under momentum dominated conditions within the atmosphere. The transport of the material under the influence of buoyancy and local wind forces.

The present discussion centres around the momentum dominated region, although the ability to describe this region depends strongly on understanding the preceding release history. The following types of jet may arise depending on the thermodynamic state of the fluid and the geometry of the release process: l

A sicgle-phase gaseous expanded, under-expanded

jet. which may be or over-expanded.

fully

Received 3 October 1989 Presented at the FIW Int. Conf. on Loss of Conmnment. London. UK 09504230~90/010068-0993.00 @ 1990 Butterworth & Co. (Publishers) 68

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Ind., 1990, Vol3, January

l

l

A single-phase liquid jet. which may have varying degrees of superheat. A two-phase jet, which may be fully expanded or under expanded and have varying degrees of superheat.

A single-phase gaseous jet may arise from the direct storage of high pressure gases or from the venting of the gaseous phase above a pressurized liquid. A single-phase liquid jet. however. can only arise due to a release path from within the liquid space and, in the case of low superheat of only a few degrees. can give rise to a large degree of liquid dropout with subsequent pool evaporation behaviour. The main interest here therefore is that of a high momentum jet giving rise to a large degree of atomization through aerodynamic forces, or a large degree of superheat, where a large proportion of material can be transferred into the gaseous state within the momentum dominated region. A two-phase jet is likely to be the more common occurrence when liquids are present under pressure. In the case of venting from above the liquid space. a two-phase jet is likely to arise when the liquid level swells due to depressurization and foaming. Even in the absence of swell. the release of a saturated vapour at pressure will give rise to considerable cooling up to and beyond the exit, with resulting condensation. When venting occurs from within the liquid space for a liquid that is considerably superheated with respect to the ambient pressure. then evaporation along the release path occurs giving rise to two-phase flow, which may be choked. In this case, a strongly two-phase source exists, which has characteristics different from the low superheat case above. Some of these jet types are well

Jets discharging understood, of theoretical

while others are still very much development.

to atmosphere:

K. Moodie and B. C. R. Ewan

FLOW

in a state

Fully expanded gaseous jets Many experimental and theoretical studies have dealt with the evolution of velocity and concentration decay in round gaseous jets. These are known to be characterized by a core region extending to around 20-40 jet diameters, beyond which the jet has self similar properties in the radial variation of velocity and concentration. The starting point for most of the early treatments has been that of incompressible jets, and the application of conservation of total momentum over the jet area in these, leads to a centreline velocity decay within the developed region which has a hyperbolic form iJ m

_

kA ‘I2 z+a

-?_ = exp(-[kr)(c

= exp(-[&I’) u, where r is the radial position. Examples of data on these radial variations have been published ‘,c. For the developed region, the values of b, and bH are linear functions of axial distance, and average values over several studies have been reported6 b,, = 0.107 x Z

and

bH = 0.126

x

Z

For sonic and supersonic jets, consideration must be given to the compressible nature of the flow and the above relations for velocity and concentration are less satisfactory. Several workers7-y have carried out analyses for this case and in particular the study of Kleinstein” has demonstrated that inclusion of the compressible eddy viscosity leads to an expression for the centreline velocity and concentration = I-

exp

1

KP,“zZ

Fi

i

[ PC I

-RI

x =

1

where F, and Fi can refer to velocity (U,) or concentration (0,), and the subscript refers to the jet exit property as before. X, is a non-dimensional core length, given the value 0.7. The constant K is derived from the analysis. and for the velocity has the value 0.074, while for concentration Kz equals 0.104. A number of studies have explored the velocity range over which this equation is applicable and Warren I” has recommended a Mach number dependence for the constant K such that

Shock structure around unexpanded

gaseous jet exit

K = 0.08(1 - 0.16 M,) where

u, where (I is the axial velocity, Z is the distance from the exit, A is the exit area, and subscripts m and j refer to the local centreline and jet exit values, respectively. The constant k can be calculated as 7.46, and many experimental studies can be optimized’-’ to around a value of 7. By the same type of analysis, the radial variation of velocity and concentration (8) has a Gaussian form such that

F 2

Figure 1

M, is the exit Mach number

Underexpanded

of the jet.

gaseous jets

For stagnation pressures that are high enough. the exit velocity is sonic and the exit static pressure is higher than the back pressure. The relation between stagnation and exit pressures is given by the isentropic expansion relations: Y/C :‘- I ) 2 2 P,, ~ PC = PII y+-l [ [ Y+l where superscripts e and 0 refer To exit and stagnation properties, respectively. Values of PC/P,,, T,/T,, and p,/po are given in Table I for different values of y. It can be seen that for only small stagnation to ambient pressure ratios the exit pressure will be above ambient. giving an underexpanded jet. The behaviour of the jet immediately following release in this case is well known I’ and is shown in Figure I. For exit pressure ratios above 2, the jet expansion immediately beyond the exit usually results in a normal shock or Mach disc within a few diameters of the exit. From the viewpoint of jet development for assessment of the dispersion problem, the distances involved in the initial shock structure are relatively small. and can be incorporated into a correction to the origin position of the jet. Recent work” has investigated the extent to which the established fully expanded gaseous jet theories could be applied to the underexpanded case. This has involved a study of the axial velocity decay from a number of gaseous materials at different pressures and temperatures using laser Doppler anemometry. Jets from 6.3 to 25.4 mm diameter were used with air,

P,=

Table 1 releases Y

1.20 1.40 1.67

Ratio

I:(yI, 1

1

of exit to

stagnation

properties

for sonic gaseous

PeIPo

TeITo

PJPO

0.564 0.528 0.487

0.909 0.833 0.749

0.620 0.634 0.649

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K. Moodie and 6. C. R. Ewan

helium and Freon 12 at stagnation pressures up to 20 bar and release into ambient air. The experimental arrangement is shown in Figure 2. It was found from this work that the Kleinstein model for fully exp?nded jets could be modified to account for the observed velocity decay by making certain reasonable assumptions about the jet expansion at the exit and applying mass conservation. It followed from this that the underexpanded jet at the exit gives rise to an equivalent fully expanded and near sonic jet at some distance downstream, with diameter and density related to the exit pressure ratio as follows: rn 1112 D L1

,J

‘

I.5 l-3

. 1

.

12.7 . 12-7 06.3’ +

6-3

mnl

.

JET .

AT 10 am-*,R .5.5-

”

.‘,

”

”

”

6-5

-

-

”

-

HECI”” FREON

12

c

where subscript a refers to ambient conditions. This gives rise to a modified velocity dependence in the Kleinstein velocity equation given by

u

2 = ui

1 -

[&I

exp -

where

I1 I/?

Q = 0.08(1 - O.l6M,)

E

+ cq ‘Y The application of this correlation to the range of gaseous cases is shown in Figures 3a and 36. Since the Kleinstein analysis is also applicable to concentration decay, the modified correlation can also be applied to available data on concentration. This is available through the work of Birch et al. I3 on ethylene jets. and a comparison of the appropriate correlation for concentration and their data is shown in Figure 4. It has been shown I? that for the range of densities, jet diameters and distances studied, a hyperbolic decay law gives a good fit to the data.

05

0.3

0.1

Figure 3 Decay of centreline axial velocity for several gaseous materials at different exit diameters and source pressures. Solid curve is the modified Kleinstein equation for underexpanded jets

Two phase jets A quantitative description of the evolution of the full range of two phase jets in terms of the velocities and concentration of the two phases is not yet available. A fundamental requirement in any jet analysis procedure will be the starting conditions for the jet. These will include for each phase, the temperatures. velocities and concentration as well as the effective dimensions of the

MEASURING VOLUME

Figure2 Mobile LDA geometry used velocity decay in jets over 10 m distance

70

AXIAL

for

measuring

axial

J. Loss Prev. Process Ind., 1990, Vol3, January

CORRELATION

PARAMETER

0.104

Figure 4 Decay of centreline concentration for underexpanded ethylene jet into air from the data of Birch ~a/.~. Solid curve is the modified Kleinstein equation for concentration

Jets discharging

to atmosphere:

K. Moodie and B. C. R. Ewan

jet. This is a considerable task and involves the solution of the flow equations along the release path for the fluid up to the exit plane. A large class of cases involve the release of superheated liquids, and this is a class which has received the greatest analytical and experimental study over a number of years. In almost all cases some degree of evaporation occurs along the release path upstream of the exit, which can give rise to choking in the same way as for gaseous releases, wherein mass flow rat’e becomes independent of any further drop in external pressure. The models available to describe this problem range from analytical types that embody a series of simplifying assumptions regarding the thermodynamic equilibrium between the phases and inter-phase slip, to those which set out to solve the conservation equations for mass, momentum and energy for each phase’“-“. The object of these codes is to evaluate the individual phase properties at the exit plane including velocity, temperature, void fraction and pressure. In practice, much of the attention has focussed on that of mass flow rate for which the codes are best suited, and greater uncertainty exists for the other variables. For the large class of critical flows of this kind, certain exit properties are expected to apply. The work of Richter’” in particular. has demonstrated that the vapour phase can have a much greater velocity than the liquid, but that its temperature can be considerably lower. These relative differences are also a function of the release path length and stagnation conditions. The visual appearance of two extremes of release are shown in Figures Sa and 5h for Freon 11. In the first case the stagnation pressure is low giving rise to a jet that is just sub-critical, while in the second. a critical flow exists in which the exit pressure is above ambient, as can be seen by the immediate expansion beyond the exit. The immediate exit region is of considerable interest, since there exists in this region an area of expansion and acceleration and also of intense interaction between the two phases. At the same time the liquid phase can be considerably superheated, and will be undergoing a rapid phase change. Droplet sizing Part of the overall evolution of the two phase jet system involves droplet evaporation, and an impoitant input parameter of any predictive algorithm must involve the condition of the liquid phase around the jet source. In addition, such algorithms will then set out to predict the subsequent evaporation behaviour, and the measurement of droplet sizes then becomes a useful parameter for validation purposes. Two regimes of evaporation behaviour are important for the superheated liquid releases of particular interest. These involve the region immediately beyond the exit, followed by the development and entrainment region. In the immediate region. there exists the newly emerged liquid, which can be highly superheated with respect to the local pressure. Liquid break-up due to

Figure5 Freon 11 release from straight pipes: a, source pressure is 2.5 bara and flow is close to critical; b, source pressure is 6 bara and flow is critical and underexpanded at exit

aerodynamic forces and Rayleigh instabilities have been well documented. Evidence exists, however, that for high degrees of superheat, the liquid phase may undergo shattering. According to the work of Bushnell and Gooderum Ix, such shattering occurs when the liquid temperatre is above the local saturation temperature by a minimum amount given by Tsh

-

Ts,,

&= T\h

where 0.07 < E -C 0.1. This condition

is likely to be met

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by many regimes of liquid storage, and the shattering process will continue until the associated evaporative temperature drop has sufficiently reduced the liquid temperature. There is some evidence of this presented in Figure 6, which shows droplet size measurements for a number of Freon 11 releases from two lengths of pipe at two source pressures. These measurements were carried out using a laser diffraction sizer, and show the peak sizes in the weight distribution versus distance from the jet exit as measured across the jet diameter. It can be seen that for higher source pressures, and hence liquid superheat at the exit, smaller peak drop sizes are found, and this is consistent with more vigorous shattering. The inflence of pipe length, which shows an increase in peak drop size with shorter pipes for the same pressure, is less easily explained. Critical flow codes indicate that the void fraction for the shorter lengths can be considerably less, and hence the degree of fragmentation prior to exit of the liquid may lead to different atomization behaviour. The size results and trends are consistent with reported literature values” for water and in particular for Freon 11 at temperatures of 324K and 340 K, where measured sizes were respectively 55 pm and 36 pm. The change in measured peak sizes with distance from the exit is most likely a droplet evaporation effect arising through faster evaporation of the smaller droplets, particularly around the jet edges. Further evidence for the phenomenon of shatterx during the combustion of ing has been documented liquid droplets. In this case, the interior temperature of the droplet rises until the excess internal pressure causes sudden break-up. Temperature measurements These can yield a useful insight into the processes occurring within the jet and have been measured for a range of Freon 11 jets. In addition to using a conventional bare thermocouple, a shielded type, such as that described by Shearer et al. 11, has been used. A scheme of this type is shown in Figure 7, in which the copper/constantan junction wires are 2.5 ym in diameter. While the bare thermocouple gives an average temperature weighted by the mass fractions of the respective phases, the shielded thermocouple is designed to prevent liquid droplets from impinging on the junction and in this way gives information biased toward the gas phase. Figures 8 and 9 show the temperature fields for a Freon jet at a stagnation pressure of S bar absolute and 78°C releasing from a 4 mm diameter pipe. In both cases, the general features are the same, with locations close to the jet and centreline having higher temperatures than points at greater axial distances or greater radii. It should be noted that predictions from the critical flow code for this condition” indicate that liquid temperatures at the exit are around 60°C. Furthermore, the equilibrium liquid temperature at atmospheric

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20 20

40

60 80 D&tance Porn exti - cm

P

Figure 6 Variation of peak droplet sizes in weight distribution for Freon 11 jet spray field. Pipe diameter is 4 mm: A, 40 mm length at 3 bar source pressure; 0, 120 mm length at 3 bar source pressure; I, 40 mm length at 6 bar source pressure; +, 120 mm length at 6 bar source pressure

. FLOW

Figure 7 Schematic prevent impingement andexposedbead

diagram of shielded thermocouple of liquid phase, showing cut-away

used to section

Figure6 Temperature in the spray field of Freon 11 jet obtained using shielded thermocouple. Pipe length is 120 mm, 4 mm diameter at source pressure of 5 bar

Jets discharging

to atmosphere:

K. Moodie

and 6. C. R. Ewan

fraction reasons, then it would be expected to reflect the bulk or average liquid temperature. The vapour temperature, however, is expected to be closer to that of the droplet surface temperature and the difference between the two curves may give some indictation of the temperature difference between the liquid bulk and surface.

Figure g Temperature in the spray field of Freon 11 iet obtained using bare thermbcouple. Pipe length is 120 mm. 4 mm diameter at source prkwre of 5 bar

pressure for Freon 11 is 24°C. It is clear therefore that the strongly negative temperatures of around -25°C. seen at large distances and radii, can only arise from droplet evaporation, and that the implied vapour pressures are well below atmospheric at these locations. Since the static pressure is atmospheric. the balance is made up by air, and it is clear that the temperature field is closely related to the entrainment field. The possible implication is therefore that the evaporation process is rapid, and takes place for thermodynamic reasons to maintain vapour equilibrium which is continually depleted by air dilution. Figure 10 shows the same comparsion for the temperature on the jet axis. Of particular interest is the initially low shielded thermocouple temperature, which rises within the first 30 mm to a maximum. This is consistent with the view that vapour has a much lower temperature than the liquid at the exit, due to choking. and the initial rise can be explained by the rapid partial liquid phase change creating higher temperature vapour. The subsequent decay of the temperature curves shows the ‘vapour measurement’ to be 3-5°C below that of the bare thermocouple. If the bare thermocouple is considered to be biased toward the liquid for mass

Mass fraction measurements in the evaporating jet The ultimate experimental objective in studying evaporating sprays must be to have a contour map of the mass fractions of the various liquid and vapour components. This is not yet possible due to the equilibrium changes that take place with physical sampling. However, some useful behaviour can be studied by making total measurements of entrained air and jet material. This can be achieved with the simple collection device shown in Figure II. which consists of a 3 mm diameter sample probe connected to an evacuated collection vessel which is cooled with dry ice. This has been used with Freon 11 jets to obtain the ratios of the total Freon 11 to air versus location in the jet. The mass ratio results are shown in Figure 12 for the complete field. As expected, the ratio is very high near the axis and close to the jet exit, and tends to zero at the edges and

u

Figure 11 Arrangement of evacuated collection vessel cooled with dry ice for measuring total Freon 11 and air in the spray

D/STAIVCEFROMWT-

mm

Figure 10 Comparison of the temperatures on the jet axis for the Freon 11 release at 5 bara using the two different thermocouples

Figure 12 Mass ratio of total Freon 11 and air collected in the length = 120 mm, release. Pipe spray field of pipe diameter = 4 mm, source pressure = 5 bara

J. Loss

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K. Moodie and B. C. R. Ewan the liquid phase is likely to be limited by vapour concentration gradients from the liquid surface into the bulk vapour space or by heat transfer from within the droplets to their surface. Jet velocity

Figure 13 Calculated mass ratio of Freon 11 and air for example of Figure 72, based on the measured liquid temperature results and assumption of equilibrium with the bulk vapour at large distances. Although the local concentration of Freon 11 vapour cannot be predicted at present, certain consequences follow if the vapour pressure is the saturation value, since the partial pressure of the air then becomes determined. To gain some insight into the extent of overall equilibrium, a predicted mass ratio was calculated based on the assumption of equilibrium between liquid droplets and their vapour pressure. This can be done by enthalpy conservation. The enthalpy per unit mass at the pipe exit is known from the critical flow algorithm”. In addition, since the temperature field is known for the spray, it follows that enthalpy equivalence can only arise for a particular proportion of vapour in the unit mass, and an appropriate contribution from the air component. The collected volume can then be determined from the known saturation vapour pressure and the air mass calculated as the complement. Results for the complete field are shown in Figure 13. Compared with the measured values, it can be seen that except very close to the jet and in the the predicted Freon ll/air values are centreline, consistently larger than the measured mass ratios. This is an indication that the dilution process plays a dominating role in influencing the bulk vapour concentration for this case, and that the rate of evaporation of

measurements

While conventional two beam laser Doppler anemometry can be applied routinely to single-phase flows, the requirement that very few scattering particles be present in the measurement volume excludes its use in dense two-phase flows. This means that only the smaller axial jets and fan type sprays can be accessed by this means. An alternative laser anemometry system that can be used for denser sprays is shown in Figure 14. This originated from the work of Smeets” and consists of a single beam system in which the receiving optics are coupled to a phase sensitive Michelson interferometer. The scattered wavelength shift due to the velocity of the spray is converted to a real time voltage. The system is particularly suited to higher velocities, and has a lower limit of around 2 m s-l. Its principle advantage is that the electronics operate best with a strong and continuous scattered signal as provided by the denser sprays. Even for these, however. lack of clarity becomes a. problem in limiting the regions of access as does the fall off in signal associated with the edges of jets. Figures 15 and 16 show the axial velocity on the centreline and full axial velocity, respectively, for a number of Freon 11 jets. An interesting initial observation concerns the velocity close to the exit. The critical pipe release code referred to earlier predicts exit vapour and liquid velocities. For the 7 bar example, the predicted liquid exit velocity is around 24 m SK’ and as can be seen, the centreline velocity at 100 mm from the exit is given as 61 m s-l. The predicted exit velocity for vapour is much greater at 118 ms-‘, and the results suggest that a strong interaction between vapour and liquid may be tending to equalize the velocities. As

t \

Figure 14 Schematic diagram of single beam laser anemometer that can be used for dense sprays. Operation based on fast feedback electronics detecting wavelength shift through phase sensitive optics

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Figure 15 Axial velocity on centreline for Freon 11 spray field from release pipe 120 mm long and 4 mm diameter at various source pressures: A, 7 bar; W, 5 bar; 0.4 bar; +, 3 bar

Jets discharging

Figure 16 of release bar

Axial velocity field for Freon 11 spray for single case length 120 mm, diameter 4 mm, source pressure 5

would be expected, velocity decay rates are very much slower than for gas jets and at 250 diameters still have 50% of their initial velocity. The radial variation of axial velocity also indicates that a Gaussian profile is only achieved at around 60-70 diameters, which is three to four times longer than for gases. New optical diagnostic developments The development of novel non-intrusive optical methods of measurement in dense sprays seems to be the only way forward to isolate the individual phase properties, which is necessary to support theoretical developments. Many of the conventional optical methods are not expected to be easily applied due to the contribution of the other phase to the signal. For infrared vapour absorption, for example, the liquid phase will contribute in the same region of the absorption spectrum, while the application of Raman techniques for temperature are not suited to low temperature regimes. An alternative laser technique, which has gained prominence in the past decade has been that of laser induced fluorescence. Being a fluorescence technique, the scattered signal strengths are much greater than for Raman type experiments, offering the possibility of easier identification against the strong liquid Rayleigh scattering, and in addition, the availability of tunable dye lasers and frequency doubling into the U.V. region means that many molecular electronic transitions are accessible. In general terms, to implement a fluorescence technique, one either uses the fluorescence properties of the liquid, or adds a small amount of fluorescent tracer from which one observes the fluorescence spectrum, when excited by radiation in the U.V.. In many cases the resulting fluorescence spectrum is a sensitive function of the molecular environment in the liquid, and thus provides a means of studying interactions. In particular, many spectra demonstrate a temperature dependence. This arises through solvent relaxation or diffusion effects and normally has its greatest effect in certain temperature regions. Because solvent relaxation effects tend to be fast, the influence

to atmosphere:

K. Moodie and B. C. R. Ewan

of temperature on the spectrum due to this mechanism tends to occur at rather low temperatures, except for viscous host liquids (eg. glycerol). A choice must therefore be made of the best fluorophore for the host liquid and temperature range of interest. A more generally applicable fluorescent phenomenon originates from a large number of studies, which demonstrate the principle of excimer and exciplex formation in liquids. In this case, a second additive molecule is used in the host liquid and referred to as the quencher. The function of the quencher is to react with the excited state of the first additive or monomer forming an excited state complex or exciplex. Many exciplex combinations will simply lose their excited state energy by collision or internal decay processes. Some show fluorescent activity and demonstrate a fluorescence spectrum that is separated from that of the parent monomer. partly due to the binding energy of the complex. This is an important aspect of the process since the reaction product can be uniquely identified from the reactants within the liquid. The useful feature of these reactions is that they occur in the liquid phase, and one therefore has a means of studying this phase alone. As with the single fluorescent additive experiment, some of these combinations demonstrate fluorescent emission which is temperature dependent, and therefore provide a method of monitoring the liquid phase, which is less dependent on liquid physical properties and more related to the chemical kinetics, over which one has better control. This temperature phenomenon is quite widely reportedz3-zh and has been demonstrated as a diagnostic method for liquid temperature measurement by Melton2h-31 with several host liquids and monomer/quencher pair materials. These studies have a particular relevance to some of the heavier hydrocarbons in the 1.50-300°C range, but have demonstrated the principle of identifying the individual phase components of a two phase mixture by means of their different fluorescence activity. By this means, there is some expectation that vapour phase concentrations may be accessible. Investigation of monomer and quencher combinations Fluorescence in general is affected by the host liquid, and in the simplest case may show a shift of the spectrum for different solvents. Some solvents will completely quench the fluorescence and this is more likely for more polar solvents. It is clear therefore that those already reported, particularly those in the temperature work of Melton, may not be suitable for the polar host liquids such as Freon 11, and part of the present study has been to identity and investigate suitable fluorescent combinations for the lower boiling point liquids of interest. A suitable pairing for exciplex generation in Freon 11 was found to be that of triphenylamine and 9cyanophenanthrene, and a typical fluorescence spectrum is shown in Figure i7. The main feature of this

J. Loss Prev. Process Ind., 1990, Vol3, January

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K. Moodie and 19.C. R. Ewan at least on the gas phase spectra. This opens up the possibility of comparing spray results in inert gases with a normal air environment as an additional means of studying air entrainment.

References I Corrsin. S. and Uberoi. M. S. in ‘Further experiments on the flow and heat transfer in a heated turbulent air jet’. NACA Report 9X8. 1950 2 Crow. S. C. and Chamoagne. F. H. J. Fluid Merh. 1971.48.547 3 Wygnanski. 1. and Fiedler. H. J. Nuid Mrch. IYhY. 38.577 4 Becker. H. A., Hottel. H. C. and Williams. G. C. J. Fluid Mech. 19h7.30.285

OL 250

300

350

400

450

WAVELENGTH Figure 17 Fluorescence 0.2% 9-cvanophenanthrene emission around 450 nm

500

of fluorescence spectra 9CNP solution in Freon 11 Temperature 60

Wavelength Inm) 380 0.73 475 0.64 lntensitv ratio 0.87

600

spectrum of 0.5% triphenylamine, in Freon 11 showing ‘exciplex’

Table 2 Intensities

Ph,N/O.P%

550

- nm

at two lines for 1% (“C)

15.5

-0.5

0.46 0.99 2.15

0.16 0.96 6.00

spectrum is that two emission bands are present, one at shorter wavelength due to the parent species, and one peaking around 450 nm due to the excited state complex. Investigation of the temperature dependence of this particular case has shown a significant shift in the spectra, and this feature is summarized in Table 2 for two particular wavelength regions of the spectrum at 380 nm and 475 nm. It can be seen that the intensity ratio of the emission at 380 and 475 nm varies by a factor of around 7 between -0.5 to 60°C. The above observations indicate that if the ratio can be determined to an accuracy of around 0.1, then the liquid temperature can be measured to an accuracy of 1°C. Another point worth noting is the fact that oxygen acts as a quenching molecule, and would be expected to have an influence

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