Novel Cloud Chamber Design for ‘Transition Range’ Aerosol Combustion Studies

0957±5820/01/$10.00+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 79, Part B, July 2001

NOVEL CLOUD CHAMBER DESIGN FOR `TRANSITION RANGE’ AEROSOL COMBUSTION STUDIES L. R. J. CAMERON and P. J. BOWEN Division of Mechanical Engineering and Energy Studies, Cardiff University, Cardiff, UK

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ombustion of liquid fuel sprays is employed in a diverse range of industrial applications. Aerosol combustion characteristics differ to those exhibited by gaseous combustion primarily due to the heterogeneity of the unburnt aerosol mixture. Simpli®ed models of aerosol burning rate have identi®ed possible laminar burning velocity enhancement for aerosol fuel=air systems comprising droplet sizes within the so-called combustion `transition range’Ðtypically understood to be 5±15 mm for mono-disperse aerosols. However, burning velocity enhancement has not been validated to date, and hence understanding of aerosol combustion mechanisms is limited for conditions of considerable practical importance. This paper describes the methodology utilized to design and commission an integrated cloud chamber=combustor capable of systematically producing quasi-monodisperse droplet mists within the combustion `transition range’ for the ®rst time. The principle of Wilson’s cloud chamber is used to produce quasi-monodisperse ethanol aerosol clouds, facilitating systematic cloud generation across the combustion `transition range’. The Malvern Mastersizer XTM, operating in transient mode, is used to examine droplet growth, ®nal droplet size and mono-dispersity, while Particle Image Velocimetry is used to con®rm pre-ignition quiescence. The creation of quasi-monodisperse, quiescent mists comprising droplets within the combustion `transition range’ facilitates aerosol combustion studies (e.g. ¯ame propagation, ignition energies, etc.) of importance to explosion hazard quanti®cation as well as other industrial applications such as internal combustion engines. Keywords: aerosol; combustion; `transition range’.

INTRODUCTION

explosion with potentially devastating consequences. Since the beginning of the industrial revolution there has been an ever increasing awareness regarding explosion hazards present in industrial environments, especially those associated with the production or re®ning of coal, oil or gas, where the substance and products themselves are potentially hazardous. Recently, explosion hazard research has been concerned with assessing the in¯uence of primary variables on the severity of the consequent event. Mathematical models have been developed to progress towards robust quanti®cation of vapour explosions, but none have yet been published for aerosol explosions. Under certain conditions it has been proposed that the ignition of a fuel aerosol may result in an explosion of greater severity than that of an `equivalent’ gaseous explosion, though this has yet to be validated2. As early as 1960, Williams 3 presented a 1-D model for ¯ame propagation through aerosol mixtures in premixed (< 8 mm) mode and diffusion (> 15 mm) mode. Williams gave an early warning of the dif®culties of predicting ¯ame propagation between these modes, and introduced the phrase `transition range’ (8 mm < d < 15 mm). Polymeropoulos4 predicted a signi®cant burning rate enhancement for mono-disperse aerosol fuel-air mixtures comprising droplets in the range 5±15 mm (see Figure 1). No experimental data has yet been provided for ¯ame propagation within the

Combustion of liquid fuel sprays (or `aerosols’) is of considerable technological importance to a diverse range of engineering applications. Spray combustion was initially used in the 1880s as a powerful method of burning relatively involatile liquid fuels. This methodology is based on the creation of very small fuel droplets in order to increase the surface area per unit mass, and hence increase the rates of heat and mass transfer1. A burning spray differs from the combustion of a premixed, ¯ammable gaseous mixture in that it is by de®nition heterogeneous. Processes involving the controlled combustion of liquid fuel sprays have been a major factor in technological advances this century. From the controlled combustion of fuels within automotive Internal Combustion (IC) engines and within gas turbine aero-engines, to the burning of fossil fuels in power stations for electrical power generation, it is apparent that combustion is still by far the most widespread and most ¯exible method of energy production. Further interest concerning aerosol combustion has been provided recently by renewed interest in G-DI (Gasoline Direct Injection) engines and in-¯ight re-ignition of gas turbine aircraft engines. The combustion processes mentioned above are carefully controlled. However, unintentional ignition of a ¯ammable fuel vapour or vapour=droplet mixture can result in an 197

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cloud production to fuel mist generation in the laboratory, a general background into aerosol generation is now presented, followed by the introduction of several previous cloud chamber designs. Atmospheric Clouds

Figure 1. Aerosol laminar burning velocity predictions.

so-called droplet `transition range’ to appraise the predictions of Polymeropoulos and others since. The lack of physical understanding concerning aerosol explosions and ¯ame propagation through droplet mists has led to research in this area. The aim of the research presented in this paper is to provide a facility that allows systematic study of fundamental aerosol burning characteristics and their underlying mechanisms, hence facilitating progress towards model validation and in particular, quanti®cation of aerosol explosion hazards. The design of a novel cloud-chamber=combustor, used for both gaseous and two-phase combustion, is presented in this paper. This unique apparatus allows the creation of quasi-monodisperse fuel aerosols with droplet sizes systematically traversing the combustion `transition range’Ð droplet sizes ranging from 5±15 micronsÐdepending upon the particular control parameters chosen.

DESIGN CRITERIA AND BACKGROUND Design Criteria The primary aim of the cloud chamber apparatus is to produce quiescent, quasi-monodisperse fuel mists with constituent droplet sizes traversing the combustion `transition range’ (5±15 mm) for a ®xed overall air-to-fuel ratio. Ideally, independent control of relative fuel phases should be sought also. After aerosol generation, it is intended to ignite the fuel aerosols in order to analyse the various modes of ¯ame propagation. In particular, burning rate enhancement due to the presence of fuel droplets within the transition range could then be systematically investigated. The primary requirements of a facility for studying and quantifying aerosol ¯ame propagation have been speci®ed and discussed in detail elsewhere2. A suitably designed cloud chamber was considered capable of ful®lling all the necessary criteria and the most appropriate cloud generation technique compared to alternative methods of liquid fuel atomization (e.g. injectors). Ranging from atmospheric

A two-stage process of heterogeneous nucleation is responsible for the creation of water droplets comprising atmospheric clouds. First, excess water vapour condenses on to a multitude of tiny particles suspended in the air, thereby forming droplet embryos. The embryos then increase in size to form a cloud composed of small water droplets. The growth of these droplets tends to oppose further increase in the supersaturation, which usually reaches a peak less than 1% before decreasing5. Droplet nucleation sites can vary widely in nature and include sub-micron soil particles, clay dust, smoke and salt particles from the oceans. In the absence of foreign particles and ions, the process of homogeneous nucleation, which requires higher supersaturation for droplet condensation, creates the droplet embryos. The thermodynamic aspects of homogenous nucleation and critical droplet embryo size have been evaluated by Carey6. re =

2s (RTv =×l)ln[P× =Psat (T× )] ± P× + Psat (T× )

(1)

Using equation (1), the critical droplet embryo size for a water droplet created by laboratory cloud chamber apparatus is in the order of 1 10 - 9 m. This is too small to be measured by any droplet sizing instrument currently available and equates to a very low number of atoms. Interestingly, atmospheric clouds exhibit the precise characteristics required for controlled aerosol combustion studies. Droplet sizes, size distributions, turbulence intensities and homogeneity are all compatible with the fuel aerosol generation studies performed by Cameron7. Wilson’s Cloud Chamber C.T.R. Wilson was fascinated by the curious optical phenomena as the sun shone through the clouds whilst at the Ben Nevis observatory in September 1894. This inspired him to work towards a method for creating clouds within his laboratory. Generating clouds by expansion of moist air in the spirit of Coulier8 and Aitken9±10, the apparatus Wilson devised in 1895 he called a `Cloud Chamber’ (see Figure 2)11. The expansion of moist air in Wilson’s Cloud Chamber causes a temperature drop, whereby the mixture becomes saturated or even supersaturated with water vapour. When the saturation temperature of the mixture is reached, the air can no longer contain the quantity of water vapour, and droplet formation begins as the vapour condenses. Although Wilson did not undertake rigorous quantitative analysis, his qualitative summary accurately describes the outcome of his investigations, which in the main still holds true today. The principle of operation is common to all cloud chambers, and relies upon a rapid pressure drop to reduce the temperature of the mixture and cause condensation of the vapour. Wilson’s ®ndings describe some interesting facts relating to aerosol formation that are summarized below and used as part of the design process in this paper: Trans IChemE, Vol 79, Part B, July 2001

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Table 1. Analysis of Hayashi’s apparatus.

Minimum volume, cc Maximum volume, cc Maximum stroke, m Expansion time, s Expansion speed, ms Droplet SMD, mm

Figure 2. Schematic of Wilson’s cloud chamber.

° ° ° °

repeated expansions when experimenting with non®ltered air are to be avoided; condensation nuclei do not have to be present in order for aerosol generation; the expansion ratio must be greater or equal to 1.25:1 in mixtures that contain ®ltered air in order to generate droplets by the homogeneous nucleation process; droplet sizes are primarily in¯uenced by the expansion ratio and expansion rate. Hayashi’s Cloud Chambers

The aerosol-generating apparatus developed by Hayashi et al.12±14 is based on the principle of Wilson’s Cloud Chamber11 for the purpose of characterizing aerosol burning rates. Hayashi and co-workers experimented with three variations of cloud chamber, each producing quasi-monodisperse fuel aerosol clouds. The key difference between the three designs was the change in droplet SMD produced. All three designs relied upon the operation of a mechanical piston-cylinder arrangement to expand a saturated fuel vapour=air mixture to generate aerosol clouds. The so-called `Type A’ apparatus was able to create extremely ®ne mists containing mono-sized droplets between 4±7 mm SMD, depending upon the expansion ratio. The rig developed later, in 1976, was capable of producing quasi-monodisperse aerosols with SMD ranging from 18±25 mm. Hayashi continued to generate aerosols comprising `large’ dropletsÐrelative to the transition rangeÐwith SMD of approximately 30 mm using his `Type C’ design. It is worth noting that there were no laser-based particle sizing techniques available at this time. Hence, the droplet sizes quoted by Hayashi et al. have to be interpreted bearing in mind the accuracy of the elementary sizing analysis employed, with relatively few droplet samples in the distributions presented. Subject to this proviso, Hayashi’s apparatus appears to have worked well in producing quasi-monodisperse mists. Ironically, no droplet clouds were produced within the transition range. Table 1 details the range of droplet sizes Trans IChemE, Vol 79, Part B, July 2001

1

Type A

Type B

Type C

800 1000 0.01768 0.01 1.77 4±7

667 1000 0.02944 0.50 0.0588 20±24

2333 3500 0.0580 3.00 0.019 30±34

created using the apparatus of Hayashi. Based on this data, and subject to the droplet sizing accuracy, the authors have previously hypothesized2 that the cloud chamber methodology could be modi®ed to produce droplet clouds that systematically traverse the combustion `transition range’ for a ®xed air-to-fuel ratio. This paper aims to substantiate this hypothesis through the design, construction and commissioning of a new cloud chamber, capable of providing the required control parameters for combustion studies of fuel aerosols comprising mean droplet sizes less than, within and above the transition range. DESIGN The methodology of Hayashi et al. has been utilized in order to develop an aerosol generator capable of producing droplets with SMDs within the combustion `transition range’. A brief analysis has been performed on Hayashi’s apparatus in order to aid the design and development of a rig that will be capable of producing droplets less than, within and greater than the elusive 5±15 mm size range. Analysing Hayashi’s data, it would appear that in order to obtain the sizes traversing the transition range, expansion ratios should be in the order of 1.25:1 (the minimum from Wilson’s work) to 1.50:1, whilst expansion speeds should be in the order of 0.1±1.0 ms - 1. These criteria were set as the `target’ control range for the kinematic control parameters of the rig. Another essential control parameter is the initial temperature before expansion, which is governed by the liquid fuels to be considered together with the total air-to-fuel ratio required. Cloud Chamber Design Although a spherical geometry is desirable for explosions in fully-con®ned vessels, a cylindrical geometry is chosen due to the relative simplicity of the mechanical expansion process. The cylinder was manufactured from BS1502-316S31 stainless steel drilled hollow bar, which was machined to an internal diameter of 119 mm and an external diameter of 160 mm. The choice of cylinder diameter was determined by the limitations of the droplet sizing technique employed, an issue discussed in more detail later. The bore was then ground to an internal diameter of 120 mm, with a surface roughness of approximately 1 mmÐa requirement of the seal type used. A stainless steel piston was designed and manufactured with an external diameter of 119 mm and peripheral grooves to accommodate custom-made PTFE `spring-energized’ seals. These provided a gas-tight interface with the cylinder bore. A considerable advantage of utilizing this design of seal is that no lubrication is required. A pneumatic actuator connected to the piston provided control of the piston and its

CAMERON AND BOWEN

Figure 3. CAD drawing detailing cloud chamber apparatus.

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retraction velocity. Several aluminium tubes were produced to alter the stroke length of the actuator between 1.25:1 and 1.50:1, depending upon the initial volume of the chamber. The initial volume of the chamber can be set to either 800 cc or 1000 cc, providing an additional control parameter which may be in¯uential (Figure 3 presents a CAD drawing of the apparatus). Pneumatic Actuator Analysis As shown previously, the retraction velocity strongly in¯uences droplet size generated from a cloud chamber. Tests were undertaken on a pneumatic actuator with two inch bore and 75 mm stroke to determine the range of retraction velocities attainable. The method devised to determine retraction speed employed a linear potentiometer, the output from which is directly proportional to displacement. The potentiometer had a maximum displacement of 100 mm, and had its sliding contact connected to a `spring-return’ rod to allow the motion of the piston to be precisely tracked. An additional potentiometer was connected in parallel to allow for a zero adjustment when the pneumatic actuator was in its fully extended position. The voltage applied was altered to achieve a sensitivity of one Volt per centimetre (1.0 Vcm - 1) along the entire stroke of the piston. A storage oscilloscope displayed a voltage-time history for the duration of the actuator stroke. By accurately measuring the gradient of the voltage-time trace it was possible to determine the retraction speed of the pneumatic actuator. In order to take friction and mass loading into account, the measurement was performed on the assembled cylinderpiston apparatus. The actuator was supplied with compressed air regulated to pressures between 0.4±0.7 MPa. Results showed that a maximum retraction speed of 0.375 ms - 1 was attainable, while lower speeds could be obtained by regulating the operating pressure supplied to the actuator. Whilst the range of retraction speeds provided by this pneumatic system is within the range proposed by the design study, it does not traverse the full design range proposed, and hence, it was thought, that this may have to be modi®ed to facilitate increased speeds later. However, in light of the uncertainty regarding accuracy of droplet sizing from previous studies, it was considered reasonable to progress with this reduced parameter control range at this stage of development, in the knowledge that increased retraction speeds could be sought later if necessary. Laser and Optical Access Requirements Optical access that does not signi®cantly attenuate or degrade the transmission of light is required for the various diagnostic techniques proposed to measure the variables of the problem. The particular access requirements are now discussed for photographic and laser diagnostic equipment. Photographic=visual access Aerosol cloud visualization is an essential requirement for high speed cine photography, Particle Image Velocimetry (PIV) and Schlieren photography. A large ®eld of view is advantageous for planar optical techniques, and so a circular end-window spanning the diameter of the cloud chamber Trans IChemE, Vol 79, Part B, July 2001

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was included in the design to facilitate a full ®eld of view of the cloud chamber. Malvern Mastersizer XTM access The Malvern Mastersizer XTM is an optical diagnostic technique requiring line-of-sight access in order to perform quantitative sizing analysis of aerosol clouds. Diametrically placed windows facilitate line-of-sight access through the chamber. The windows should be of suitable dimensions to allow non-restricted laser beam entry, and in addition, should not limit the collection of scattered light by the Fourier transform lens on the optical receiver. This latter requirement is most important as analysis of the diffracted light determines the droplet size distribution. PDA access The Phase Doppler Anemometry (PDA) method for droplet sizing requires two windows: one for laser beam access and the other for the receiving optics. The receiving optics collect scattered light at an angle of between 65±72 degrees from the transmission probe axis for ®rst order refraction dominance for hydrocarbon fuels15. The location of windows, therefore, is very important if PDA is to be used as a diagnostic tool. PIV and LIF access Particle Image Velocimetry (PIV) relies upon a CCD camera being placed orthogonal to a laser light-sheet in order to analyse droplets illuminated by the light-sheet. Laser Induced Fluorescence (LIF) also utilizes a similar equipment con®guration requiring laser sheet access as well as orthogonal camera access16. Two windows are again necessary here as access for the light-sheet and CCD camera are both system requirements. Window Design Fused quartz windows were chosen over conventional glass windows due to their superior optical qualities. It was decided after analysing the optical access requirements to include a large circular end window, allowing visual observation and photography, and two diametrically opposed side windows, allowing access for Malvern, PIV and PDA. The mechanical properties of fused quartz are much the same as other glasses. The material is extremely strong in compression, with a design compressive strength in excess of 1.09 109 Pa. Surface ¯aws drastically reduce the inherent strength of fused quartz, with the tensile strength in particular being affected. The design tensile strength for fused quartz with good surface quality is in excess of 48 MPa, although in practice this value is greatly reduced for safety considerations17. Using a factor of safety of seven (SMAX = 7.0 MPa) and a maximum pressure of 1.0 MPa, the thickness of the é120 mm circular end window and 60 mm 40 mm rectangular side windows were determined: t=

PMAX r2 2:28SMAX

1=2

(2)

These were found to be equal to 15 mm and 7.5 mm respectively. For further contingency, these dimensions were increased to 25 mm and 12 mm respectively. 1.0 mm

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thick PTFE gaskets were manufactured especially, and used to provide gas-tight seals for each window.

Controlling the temperature of the cloud chamber is essential for ensuring that the fuel vapour-air mixture is saturated prior to expansion and subsequent aerosol generation. A custom-made `heating jacket’ allowing laser and operational access surrounds the apparatus and controls the air temperature within the cloud chamber. A temperaturecontroller regulates the chamber temperature from ambient to 373 K, by means of a k-type thermocouple positioned within the combustion chamber. This temperature control allows aerosol generation and subsequent combustion investigations to be performed on a variety of fuels. The apparatus was left for 24 hours in order to achieve a steady-state air temperature before any experiments were undertaken. In the present study, the fuel is ethanol, although hydrocarbon fuels such as n-octane and n-decaneÐrepresentative fuels for automotive studiesÐcould also be used.

cloud chamber. Experiments were carried out at both 297 K and 307 K, in order to investigate the effect of temperature on aerosol formation, with the cloud chamber volume set at either 800 cc or 1000 cc before expansion. The ®nal volume of the cloud chamber was dependent upon the expansion ratio, which varied from 1.25:1 to 1.50:1. The air supply to the pneumatic actuator was kept constant at 0.7 MPa, which generated a retraction speed of 0.375 ms - 1. The Malvern Mastersizer XTM employs a laser diffraction technique to deduce particle sizes within aerosols. When a collimated beam of monochromatic light falls upon a particle, a diffraction pattern is formed whereby the scattering angle is representative of the particle’s size. The scattered light is focused through a Fourier transform lens onto a photosensitive detector, from which the droplet distribution is derived. Droplet sizing measurements were analysed every ten milliseconds by operating the Malvern Mastersizer XTM particle sizer in transient mode. Hence, the transient aspects of droplet growth and aerosol formation could be analysed. Particle Image Velocimetry (PIV) was employed to con®rm pre-ignition quiescence within the cloud chamber a short time after aerosol cloud generation.

Fuel Concentration Control

RESULTS AND DISCUSSION

In order to control the total equivalence ratio, highly accurate ( 1 mLitre) micropippettes were employed to inject liquid fuel into the cloud chamber apparatus prior to expansion. To determine the correct amount of fuel to inject for a stoichiometric mixture, the stoichiometric chemical balance for combustion of ethanol in air was utilized:

The pre-ignition system variables required to adequately characterize a fuel aerosol cloud for systematic combustion studies have been previously speci®ed7. These comprise temporally resolved particle size, equivalence ratio (ideally phase resolved), quiescence and homogeneity. It was proposed that the Malvern Mastersizer XTM particle sizer operating in transient mode would be suitable for resolving droplet size distributions, mono-dispersity and mean size. However, ®rst the inherent errors associated with diffraction based particle sizingÐas utilized within the Malvern systemÐare considered and quanti®ed in relation to the cylindrical geometrical design.

Cloud Chamber Temperature Control

C2 H5 OH(g) + 3fO2(g) + 2CO2(g) + 3H2 O(g) +

79 N g±! 12 2(g) 3(79 21)N2(g)

(3)

With a volumetric air-to-fuel ratio of 14.285 and an initial cloud chamber volume of 800 cc, the volumes occupied by the fuel vapour and air respectively can be determined: Vf = 52.336 cc, Va = 747.664 cc. The corresponding quantity of liquid fuel can be easily calculated knowing the number of moles of fuel, the fuel’s molar volume and its liquid density. Other equivalence ratios can easily be determined by performing similar calculations (see Table 2). DESIGN APPRAISAL The objectives of the design were to generate quasimonodisperse, quiescent fuel aerosols with constituent droplet sizes traversing the combustion `transition range’ for fuel concentrations traversing the stoichiometric concentration. A series of experiments involving the expansion of saturated ethanol vapour-air mixtures were conducted in order to examine the aerosol generating capabilities of the Table 2. Temperature and liquid fuel (ethanol) requirements for various equivalence ratios. Equivalence ratio Temperature, ¯ C Liquid fuel volume, mL

LEAN 0.6 20 82

0.8

1.0

1.2

1.4

1.6

! RICH 1.8 2.0

22

24

26

28

30

32

34

109

136

163

190

218

245

272

Vignetting and Obscuration Vignetting, inherent in laser diffraction droplet sizing devices, was analysed during the design of the cloud chamber, and it was found that its in¯uence could be minimized to a tolerable level by appropriate choice of geometric parameters7. The vignetting phenomenon occurs when the particle ®eld is located beyond the `cut-off point’ of the Fourier transform lens. When vignetting occurs, the lens does not collect the light scattered from the smaller particles, and effectively causes the mean droplet size to arti®cially increase. In order to quantify the severity of vignetting, experimental investigations were conducted within a similar rig using water mists. Malvern measurements con®rmed the onset of vignetting with minor sizing discrepancies ( + 2 mm) occurring at distances greater than 150 mm from the Fourier-transform lens (see Figure 4). Extremely ®ne aerosols, representative of the lower end of the combustion `transition range’ (1±8 mm SMD), were used in order to accentuate the vignetting effect, and hence present a conservative assessment of this particular source of error. Droplets larger than the test aerosol selected are not affected by vignetting as much, due to the smaller angle of diffraction. Trans IChemE, Vol 79, Part B, July 2001

NOVEL CLOUD CHAMBER DESIGN FOR `TRANSITION RANGE’ COMBUSTION STUDIES

Prior to expansion, the mixture within the cloud-chamber apparatus is one of saturated ethanol vapour and air. This is identi®ed on Figures 5±7 by the numerical values of zero and one for the droplet SMD and relative vapour fraction respectively. Droplet nucleation is observed to occur approximately 50±75 ms from the beginning of expansion, depending upon expansion variables, and is identi®ed by the discontinuous increase in gradient on the mean-droplet=time graph. This is coupled with an associated decrease in relative vapour fraction, as some of the fuel vapour condenses to produce droplet embryos. It is interesting to note that if a droplet embryo is de®cient of just one molecule from its critical size i.e. sub-critical, it will rapidly evaporate preventing any further droplet growth. The resulting droplet growth is similarly shown by the steep droplet SMD-time curve. Eventually, the dynamic relationship between the fuel vapour, fuel droplets and temperature reaches an equilibrium state, and the droplets no longer increase in size. A long plateau region of the droplet SMD-time curve is indicative of this equilibrium state, and has a duration dependent upon the droplet SMD produced. Most importantly, during the dynamic equilibrium state, pre-ignition conditions are ideal: droplet diameter, liquid and vapour fuel fractions seem relatively constant. The relative vapour fraction measurement provided only qualitative information regarding the vapour equivalence ratio during the transition from a fuel vapour-air mixture to that of a fuel aerosol-vapour-air mixture. The relative vapour fraction is determined from liquid fuel concentration values, calculated by the Malvern instrument during sizing analysis, and knowledge of the total equivalence ratio. It is noticeable that in some cases the relative vapour fraction has a value less than zero. This erroneous diagnostic indicates that the collective amount of fuel present in the liquid fuel droplets, generated by the expansion process, exceeds the total amount of fuel available within the apparatus, which is clearly incorrect. The relative fuel vapour fraction cannot, therefore, be used as an absolute measure of the amount of fuel vapour present during aerosol formation, and should only be used as a qualitative indicator. This emphasises the need for the development of other techniques to quantify fuel vapour fraction, such as Laser Induced Fluorescence or Near Infra Red absorption techniques, as suggested previously2.

Variation in Droplet Sauter Mean Diameter as a Function of Distance from Lens

Distance from Fourier Lens/millimetres

Figure 4. Vignetting quanti®cation results.

Another measurement error commonly associated with the sizing analyses of ®ne droplet mists is that of high obscuration, due to the in¯uence of multiple scattering under these conditions. Obscuration values are quoted in percentage terms and relate to the amount of laser light absorbed by the particle ®eld. For accurate particle sizing analysis, obscuration values between 11±30% are recommended, although correction formulae have been proposed for aerosols where the obscuration assumes higher values. Problems concerning obscuration were not encountered throughout the sizing experiments reported in this paper, as typical concentrations were always within the recommended range. Measured droplet size distributions using the Malvern system, therefore, are taken to be suf®ciently accurate to distinguish between the various modes of combustion; above, below and within the transition range.

Transient Droplet Sizing Operating the Malvern Mastersizer XTM instrument in transient mode, it has been shown possible to identify droplet nucleation and growth trends, and to quantify droplet size during aerosol generation within the cloud chamber. Transient Malvern measurements are presented for stoichiometric ethanol aerosols comprising 5 mm, 9 mm and 14 mm SMD droplets in Figures 5(a), 6(a) and 7(a).

100 ER = 1.0; Ti = 24 deg C; Vi = 800 cc; Vf = 1200 cc

0.8

12.0

0.6 0.4 0.2 0.0

6.0

-0.2

4.0

-0.4

2.0

-0.6

0.0 0

100

200

300

400

500

600

Time / milliseconds

700

800

900

80 70

Relative Vapour Fraction

14.0 Droplet diameter (D32) / microns

1.0

8.0

Volume %

90

16.0

10.0

60 50 40 30 20 10

-0.8 1000

0

(a)

1.0

10.0 Particle Diameter (µm.)

Figure 5. 4 mm ethanol aerosol: (a) transience; (b) droplet size distribution.

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100.0

(b)

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Droplet diameter (D32) /microns

Relative vapour Fraction

ER = 1.0; Ti = 24 deg C; Vi = 1000 cc; Vf = 1400 cc

Time/milliseconds

Figure 6. 9 mm ethanol aerosol: (a) transience; (b) droplet size distribution.

ER = 1.0; Ti = 34 deg C; Vi = 800 cc; Vf = 1000 cc

Particle Diameter

Figure 7. 15 mm ethanol aerosol: (a) transience; (b) droplet size distribution.

Monodispersity Figures 5(b), 6(b) and 7(b) provide quantitative droplet size information 250 milliseconds after the beginning of expansion, and after the droplet growth phase. Although the droplets are not perfectly monosized, quasi-monodisperse stoichiometric ethanol aerosol clouds are clearly shown by the Malvern Mastersizer XTM measurements. Comparing the monodispersity of aerosol clouds produced by the cloud-chamber apparatus to those created by other researchers18, the droplet size distributions exhibit good monodispersity to droplet size distributions invariably bounded by 5 mm of the SMD value. The detailed analysis presented here is consistent with that of Hayashi et al.12±14 who also indicated aerosol clouds with narrow droplet size distributions. Hence, in this respect, the clouds generated are also ideal for fundamental combustion studies.

and quantitatively by analysing the individual vector plots. Figure 8 displays an instantaneous representation of the ¯ow®eld within the cloud chamber immediately after aerosol formation. From analysis of the PIV ¯ow®eld

Aerosol Quiescence and Homogeneity Pre-ignition conditions are appraised using PIV. The ¯ow®eld vector plots generated by PIV are used to analyse the quiescence of the ethanol aerosols. Preignition conditions can be described both qualitatively

Figure 8. Instantaneous PIV vector plot.

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NOVEL CLOUD CHAMBER DESIGN FOR `TRANSITION RANGE’ COMBUSTION STUDIES maps, it is shown that the expansion process generates negligible turbulence. The ¯ow®eld is indicative of droplets settling due to gravity, and does not exhibit many randomly orientated vectors that would be associated with turbulence effects. A signi®cant difference in droplet diameters generated by the cloud chamber technique would result in a wide spread of velocities and, therefore, a non-uniform ¯ow®eld. The PIV images, therefore, provide further con®rmation of the monodispersity indicated by the Malvern measurements. CONCLUSIONS This paper discusses the development and appraisal of novel facilities previously proposed2 as part of a strategy to progress towards quanti®cation of aerosol explosion hazards. A novel, integrated cloud-chamber=combustor, capable of producing quasi-monodisperse aerosols in the combustion `transition range’, has been designed and commissioned to facilitate future fundamental aerosol combustion studies; this type of facility was ®rst requested by modellers over 40 years ago. The functionality of the rig is based upon the same principle as Wilson’s cloud chamber. Aerosol cloud characterization has been performed for a variety of conditions using the Malvern Mastersizer XTM particle sizing instrument, and Particle Image Velocimetry (PIV). In order to verify the droplet sizing results, a rigorous examination of the vignetting phenomenon was undertaken on water mists contained within a geometry of similar dimensions to that of the cloud chamber combustor. After a systematic investigation, it was concluded that the errors induced as a result of vignetting for the chosen geometrical con®guration were not signi®cant in the context of the proposed application. Operating the Malvern in transient mode, droplet sizing was performed every ten milliseconds, which permitted analysis of droplet growth. Constant droplet size was attained shortly after expansion, which then persisted in a pseudo-steady state for approximately one second. In addition, PIV characteristics indicate a quiescent mixture during the pseudo-steady period, hence an ideal environment for aerosol combustion studies and in particular, aerosol burning characteristics. Ethanol droplet size distributions created within the new cloud chamber exhibited extremely good monodispersity, and by varying the rig control parameters, aerosol clouds representative of all three modes of aerosol combustion could be generated : premixed `gaseous’ combustion, `transition range’ aerosol combustion and droplet-droplet combustion. The fuel vapour component remains the only outstanding parameter required for the complete quanti®cation of preignition conditions within the aerosol fuel system. Inverting the diffraction based concentration measurement to provide relative fuel fractions proved inappropriate, providing qualitative information only. For future vapour phase quanti®cation, laser-based techniques previously2 suggested are likely to be required. Finally, it is worth noting that the new cloud chamber with associated diagnostic facilities appears to provide characteristics suitable for other areas of fundamen-

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tal research, including studies of droplet nucleation and growth processes, and the interaction between water mists and propagating ¯ames. NOMENCLATURE g PMAX Psat(Tv) Pv R r re SMAX Tv t vl s

acceleration due to gravity, m s - 2 maximum pressure, N m - 2 saturation pressure at temperature Tv, N m vapour equilibrium pressure, N m - 2 universal gas constant, J mol - 1 K - 1 unsupported window radius, m droplet embryo radius, m design tensile strength, N m - 2 vapour temperature, K window thickness, m speci®c volume, m3 kg - 1 surface tension, N m - 1

2

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ADDRESS Correspondence concerning this paper should be addressed to Dr P.J. Bowen, Division of Mechanical Engineering and Energy Studies, Cardiff University, Queens Building, Newport Road, PO Box 925, Cardiff CF24 0YF, UK. E-mail: [email protected] The manuscript was received 8 December 2000 and accepted for publication after revision 10 May 2001.