development of a particle generator for combined particle...

15th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 05-08 July, 2010

Development of a particle generator for

combined particle image thermo- and velocimetry in air

Daniel Schmeling1*, Johannes Bosbach1, Claus Wagner1

1: German Aerospace Center, Institute of Aerodynamics and Flow Technology, Göttingen, Germany,

*[email protected] Due to the Prandtl number dependence of thermally driven flows there is a strong demand for simultaneous acquisition of instantaneous temperature and velocity fields in air or gas flows. We report on combined Particle Image Thermo- and Velocimetry of air flows, which provides the requested properties. A feasibility study was performed successfully in the past showing the applicability of this promising measurement technique (Bosbach et al. 2007 and Schmeling et al. 2010b). The present paper is focused on the experimental advancement in developing an improved particle generator, which is needed to continuously produce tiny unencapsulated thermochromic liquid crystals. Phase Doppler Anemometry is applied in order to characterize the produced droplets. The particle generator, which utilizes the principle of Rayleigh breakup (Rayleigh 1878), is presented and its functionality is demonstrated. A strong dependency of the diameter of the produced droplets on the Reynolds number and the oscillation frequency of the installed piezoelectric oscillator is observed. With switched off piezoelectric oscillator or low extruding velocities broad size distributions are generated. Within a certain parameter range, which still has to be studied in more detail, the production of monodisperse droplets has been achieved. However, the diameters of the particles are larger by a factor of three in diameter as expected by pure Rayleigh breakup. A reason might be the straight upwards oriented extruding direction, which leads to a thin fluid layer on the aperture plate. Thermochromic liquid crystals forming agglomerates on a black plate, which was moved through the stream of droplets, show a brilliant, temperature depending play of colours. 1. Introduction Combination of Particle Image Velocimetry (PIV) and Particle Image Thermography (PIT) is well known in experiments in liquids (Dabiri 2009). By using thermochromic liquid crystals (TLCs) as tracer particles for simultaneous PIT and PIV one can measure instantaneous temperature and velocity fields simultaneously at the same location. The intention of our work is to adapt this promising measurement technique to air flows, which are important, e.g. for indoor climatisation (Linden 1999), cooling of electronic packaging (Baskaya et al. 2005), heat exchangers (Webb 1980) or aircraft cabin ventilation (Kühn et al. 2009). Due to the Prandtl number (Pr) dependence of the flow properties in thermal flows, the accomplishment of convection experiments with air or other gases as working fluid is of great interest. In some cases the dependence of the flow conditions on the Prandtl number is not yet fully understood. Recent experiments on Rayleigh-Bénard convection at a Prandtl number of 0.67 reveal partially great differences to experiments accomplished at Prandtl numbers of 4-5, clarifying the need for measurements in fluids with , like e.g. air (Ahlers et al. 2009). 7.0Pr ≈The main advantage of this technique is the ability to acquire the whole temperature distribution non-intrusively, i.e. without integrating thermocouples or Resistance Temperature Detectors (RTDs). Beside the fact that such probes might disturb the flow, only point wise measurements or average fields obtained by spatial scanning can be measured. Another important issue is the response time, which can be of the order of milliseconds for TLCs and thus allows for high time resolution: Ireland and Jones (1987) accomplished measurements of the response time of encapsulated TLCs embedded in a 10 µm layer on a surface and found a temperature response time

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of 3 ms. Nevertheless, the thermal reaction time of the generated unencapsulated TLC particles as well as the exact following behaviour have to be studied in detail. Moreover, by simultaneous acquisition of instantaneous temperature and velocity information the understanding of the underlying physical processes, e.g. the interaction between buoyancy and inertia forces can be improved. The feasibility of this new measurement technique has been proved by Bosbach et al. (2007) and Schmeling et al. (2010b) in a cubic Rayleigh-Bénard convection cell with the dimensions of 300 x 300 x 300 mm3 and air as working fluid. Instantaneous velocity and temperature fields were recorded simultaneously at Rayleigh numbers of 1.2 x 107 < Ra < 4.7 x 107. Thereby, the Rayleigh

number is defined by νκ

β⋅

⋅∆⋅⋅=

3HTgRa , with g the acceleration of gravity, β the thermal

expansion coefficient, ∆T the temperature difference between the top and bottom plates of the cell, H the height of the cell, κ the thermal diffusivity of the fluid and ν its kinematic viscosity. Depending on different TLC types (crystal types: R20C6, R20C13 and R20C20, Hallcrest) were used as tracer particles. Even though it was already possible to detect the main flow structures like e.g. thermal plumes and large scale structures, many important technical problems have not been solved so far. Due to a broad and mostly unknown size distribution the sinking velocities and the colour play of the TLCs differ from particle to particle. Furthermore, the particle generation was not continuously and therefore measurements could only be conducted in closed cavities. Therefore, an improved particle generator is currently under development, which can continuously produce tiny monodisperse, unencapsulated TLC particles.

T∆

With the new generator PIT/PIV measurements will be conducted in thermal and mixed convection using a double shutter colour CCD-camera. A rectangular enclosure heated from below and cooled from above will be the first system to be investigated (Schmeling et al. 2010a and Westhoff et al. 2010). Therein, heat transport processes, caused by thermal plumes or externally driven flows, will be studied. For the first time the dynamics of the velocity field of a thermal plume can be correlated with its temperature field experimentally in air. Furthermore, the dynamics of large scale structures in correlation with the instantaneous temperature contributions will be analysed. After this introduction the present paper is continued with a section summarising the fundamentals of the TLCs as tracer particles for combined PIT and PIV. The experimental advancement in developing an improved particle generator is addressed in the following section including the analysis of the produced droplets by means of Phase Doppler Anemometry (PDA). A short conclusion, which includes the outlook, finishes the paper. 2. TLCs as Tracer Particles for Combined PIT and PIV In order to use TLCs as tracer particles for combined PIT and PIV, of e.g. mixed convection, the particles have to fulfil some specific requirements: They have to provide a temperature depending reflection of different wavelengths, a good following behaviour, a high light scattering efficiency, a fast response time and a long lifetime. Furthermore, they have to be continuously producible and must be inert to the surrounding fluid, which is air in our case. Based on the review paper of Dabiri (2009) some of the fundamentals of TLCs are described in the following. It is well known that the colour of a TLC droplet depends not only on its temperature and on the spectral characteristics of the light source but furthermore on, e.g. the angle between the incident illumination and the observation direction, the background light as well as the age of the droplet. Due to the angular dependence a local calibration provides a far better accuracy than a global one. Another important parameter is the size of the droplets; hence the produced particles shall have a sharp size distribution and also the absolute size of the TLCs has to be adjustable. The minimal/maximal possible size of the TLCs depends on two factors: On the one hand the TLCs should be large enough to provide their temperature depending reflection of different wavelengths

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(play of colours) while on the other hand they must provide best possible following characteristics. The usable size range has to be identified and a calibration of the temperature depending hue-value (Dabiri 2009) for the resulting particles has to be performed. Thereby the hue-value h is, besides saturation and value, one of the three parameters of the HSV colour space as depicted in Figure 1.

Figure 1. The HSV-colour space, source: Wikipedia [URL: http://commons.wikimedia.org/wiki/File:HSV_cone.jpg].

H can be calculated from the RGB-values (red: r, green: g, blue: b) (Gil at al. 2006) by:

⎪⎪⎪

⎩

⎪⎪⎪

⎨

⎧

=+−−

=+−−

=−−

=

),,max(,4),,min(),,max(

),,max(,2),,min(),,max(

),,max(,),,min(),,max(

),,(

bgrbifbgrbgr

gr

bgrgifbgrbgr

rb

bgrrifbgrbgr

bg

bgrh (1)

The parameters saturation and value can be used to correct artefacts like black or white pixels of the image. All colour information, which is necessary to calculate the temperature, is expressed with the single hue-parameter, hence this parameter is used to calculate the temperature from the colour image. 3. Improved Particle Generator Due to the strong white light reflection by the shell, encapsulated TLCs cannot be used for PIT in air flows. Therefore a new particle generator, which can continuously create tiny unencapsulated TLCs with a very sharp size distribution, is developed. It uses the Rayleigh instability (Rayleigh 1878) for the production of monodisperse acetone-TLC droplets (Brenn et al., 1996). The acetone-TLC mixture is extruded through an aperture plate with a hole by a slight over pressure. A frequency controlled piezoelectric oscillator impresses a wavelength to the fluid jet forcing it to

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break up into monodisperse droplets. In a following drying unit the acetone vaporises and pure TLC particles remain. By varying the mixing ratio the size of the dried TLC particles can be adjusted. The droplet generation process is characterized by the following parameters: aperture plate thickness h, hole diameter D, pressure difference between nozzle and ambience as well as frequency and voltage of the piezoelectric oscillator. The corresponding important fluid

mechanical parameters are the Reynolds number

p∆

pf pU

µρ⋅⋅

=DuRe and the Ohnesorge number

σρµ

⋅⋅=

DOh . Thereby, describes the mean extruding velocity, the diameter of the

aperture hole,

u D

ρ the density, µ the dynamic viscosity and σ the surface tension. According to the theory of v. Ohnesorge (1936) the pair of parameters (Re, Oh) describes the jet break up characteristics. Figure 2 shows a sketch of the particle generator. Pressure reservoir, valves, accessories and the nozzle are connected with an acetone resistant PTFE (polytetrafluoroethylene, Teflon) tube. A slight overpressure up to 1.5 bar can be applied to the nozzle that contains a piezoelectric oscillator (diameter of the ceramic layer 9 mm, thickness 0.22 mm resonance frequency 5 kHz) as well as an aperture plate of stainless steel with 2.0=h mm and a 50=D µm hole. In the measurements described in the following the voltage of the piezoelectric oscillator was kept constant at the maximal value of 10 V. For pure Rayleigh decay the generated droplets have about twice the size of the hole in the aperture plate (Brenn et al. 1996).

Figure 2. Sketch of the particle generator. Scanning electron microscope exposures at a magnification of 2000x of two aperture plates with laser drilled holes in their centres can be seen in Fig. 3.

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Figure 3. Scanning electron microscope exposures of the aperture plate at a magnification of 2000x, 'Da 1' denotes the

diameter and 'Db 1' the area of the holes. Aperture plate #1 (left), aperture plate #2 (right). Both holes have a nominal diameter given by the manufacturer of 40 µm but their actual diameters are approximately 47.4 µm (# 2) and 51.6 µm (# 1). Such deviations may lead to difficulties in the production of monodisperse TLC particles of well defined size, because the mixing ratio between acetone and TLCs has to be adjusted for each aperture plate. Whether a particle size deviation of 10% already causes measurable colour deviation still has to be tested. For the first investigations a TLC particle diameter of 25≈TLCD µm was striven for in order to guarantee that the TLCs are not too small to show their play of colours. Therefore, the volume mixing ratio of acetone to TLC was set to 63:1. As TLCs R20C20 (LCR Hallcrest) crystals, which have a red starting point at °C and a colour active temperature range of K, were used. Hence, the crystals are colourless for

20≈T 20≈∆T20<T °C and °C and show their play of colours

in the range of 40>T

CTC °<<° 4020 . However it is well known that the finite particle size may have an impact on the colour play (Dabiri 2009). Hence, deviations from the colour active temperature range have to be expected. The generated droplets were extruded in an extractor hood wherein the acetone part of the droplets can vaporise safely. The droplets were observed with a Nikon D70s colour CCD camera with Nikkor mm lenses and an external flash in front of a black wall. The aperture was set to . Figure 4 shows the nozzle of the particle generator and the jet of droplets.

50=f8.1=F

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Figure 4. Nozzle of the particle generator with the extruding jet of acetone-TLC droplets. The extruding jet decaying into single droplets is shown in Fig. 4, demonstrating the functionality of the particle generator. A very rough optical estimation of the droplet size, by measuring the dimension in pixel in Fig. 4, yields a mean droplet diameter of approximately 300 µm. This size is unexpectedly high and reveals that a closer analysis of the generator parameters is necessary. Nevertheless, if the extruding jet decays always into droplets with a diameter of µm, an adjustment of the mixing ratio between acetone and TLCs could already guarantee TLC particles of the requested size.

300≈D

In order to further analyse the diameter of the produced acetone-TLC droplets Phase Doppler Anemometry (PDA) measurements were conducted for various pressure differences and frequencies. Besides the particle diameter the horizontal and vertical velocity components were measured by means of Laser Doppler Anemometry (LDA). Due to the high price of the TLCs and their negligible influence on the flow properties during the droplet formation process, pure acetone droplets were produced for the first analysis. The corresponding results are presented and discussed in the following. Thereby a gauge for the monodispersity is given by the standard deviation of the diameter Dσ divided by the mean diameter of the droplets D . Monodisperse particle are given

for values 10.0<DDσ .

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(a) kHz. (b) 0=f 5=f kHz.

Figure 5. Particle diameter histograms of 100,000 measured acetone droplets with a bin width of approximately 3.6

µm; piezoelectric oscillator switched off (a) and piezoelectric oscillator excited with a frequency of 5 kHz (b). Figure 5 presents two particle diameter histograms measured with the same aperture plate and at the same pressure difference to the surrounding air of approximately 250 mbar. Both histograms are generated from the data of 100,000 acetone droplets measured at a distance of approximately 70 mm from the aperture plate. The droplets were extruded straight upwards and the mean droplet velocities of m/s and m/s lead to 49.4=u 45.4=u 548Re = and 543Re = for frequencies of the piezoelectric oscillator of kHz and 0=f 5=f kHz, respectively. Thereby, the Reynolds number

was calculated with 41.0=ρµ mm2/s, the aperture diameter as characteristic length and the mean

droplet velocity, measured by means of LDA, as characteristic velocity. As one can see, a bell-shaped curve of the diameter distribution around the maximum at 275=D µm for kHz (see

Fig. 5 (a)) with

0=f

15.0=DDσ turns into a diameter distribution with one main maximum at

µm and a side maximum at 258=D 324=D µm for 5=f kHz (see Fig. 5 (b)). The dispersity of

this distribution is 18.0=DDσ and therefore less monodisperse than (a). We think that the side

maximum arises from inelastic droplet collisions since two droplets with a diameter of 258=D µm have almost exactly the same volume as one droplet with a diameter of µm. According to the theory of Lord Rayleigh (1878) and the experiments of e.g. Brenn et al. (1996), the diameter of the droplets should be about twice the diameter of the aperture hole, which means that monodisperse droplets with a diameter

324=D

100≈D µm should be produced: The Ohnesorge number is constant for all cases and amounts to 01.0=Oh . Hence, the Rayleigh break up of the extruding jet should dominate the decay for , according to the theory of v. Ohnesorge (1936). A reason that our droplets have a diameter of

3102Re ⋅<258=D µm might be the fact, that they are extruded

straight upwards and a thin fluid layer accumulates on top of the aperture plate. Thus, the droplets are not formed out of a jet with the diameter of the aperture hole, but out of a jet which extrudes from a fluid layer over the aperture plate. Measurements with the jet extruding horizontally or straight downwards will be performed soon.

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(a) , 194Re = 426=D µm, 16.0=D

Dσ . (b) 450Re = , 364=D µm, 15.0=DDσ .

(c) , 543Re = 279=D µm, 18.0=D

Dσ . (d) 616Re = , 316=D µm, 09.0=DDσ .

(e) , 723Re = 354=D µm, 31.0=D

Dσ . (f)

Figure 6. (a)-(e) Particle diameter histograms different Reynolds numbers (different extruding velocities), 10,000

acetone droplets were measured for each plot and the bin width is approximately 5.8 µm;. The frequency of the piezoelectric oscillator was fixed at kHz. (f) Circles: mean diameter (left y-axis), crosses: dispersity 5=f

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⎟⎠

⎞⎜⎝

⎛D

Dσ (right y-axis), black line at dispersity = 0.10, plotted for different Reynolds numbers (cases (a)-(e)).

The influence of the Reynolds number, varied by adjusting the extruding velocity via the pressure difference, on the size distribution is shown in Fig. 6. All measurements were conducted with a constant frequency of the piezoelectric oscillator of 5=f e results are summarized in Tab

194= (Fig. 6 (a)) and 450Re = (Fig. 6 (b)) the particle diameter distributions are broad while at (Fig. 6 (c)) a sharp maximum at

kHz. Th1. At Re

543Re = 258=D µm with a side maximum at

324=D µm develops. This case was discussed in detail above. The higher DDσ value

to the previous cases is caused by the lower mean droplet diameter, the standard itself is lower compared to the previous cases (see Tab. 1). At 616Re = (Fig. 6 (d)) and

723Re = (Fig. 6 (e)) diameter dis h a sharp maximum at 312=D µm and

the case (d) leading to a d

compareddeviation

tions witµm, respectively, were found. Thereby, the diameter distribution has aller variance in

ispersity of

tribu335=D a sm

09.0=DD

σ and therefore monodisperse droplets. In the

case (e) the dispersity of 31.0=DDσ is quite large even though rp maximum is detected.

This is caused by droplets with the extremely large diameters ( 600>D µm) orly A calculation excluding these very large ( %5.4

a sha

, e po in the figure.

which ardissolved ≈ of the total number) and

the very small ( %8.0≈ of the total number) droplets yields to 12.0=DDσ for

100 µm 600<< D µm. Anyhow, it is shown that the particle generator is able to produce monodisperse droplets with the parameters presented in case (d). Figure 6 (f) shows the mean diameters (circles, left y-axis) and the dispersities (crosses, right y-axis) for all different Reynolds numbers (cases (a) to (e)). Thereby, mean diameter and dispersity were calculated of all 10,000

easured droplet diameters. The black linm e at disper the limit of sity = 0.10 indicatesmonodispersity. Only in the case (d) with 616Re = monodisperse droplets are produced. Table 1

es the bser result Table 1: R the particle e lo is or different Reynolds numbers. Re [-]

[ u

[m/s

conclud o ved s.

diametesults of r and ve city stat tics fp∆

mbar] ] uσ

[m/s] D

[µm] Dσ

[µm] DDσ

Maat:

Side peak at: [µ ]

ximum [µm] m

194 105 59.1 0.10 426 69.0 0.16 broad 450 215 69.3 0.16 364 55.4 0.15 broad 543 250 45.4 0.14 279 49.0 324 0.18 258 616 315 05.5 0.13 316 27.8 0.09 312 monodisperse723 400 93.5 0.24 354 108 0.31 335

In further investigations droplets with an acetone-TLC mixture were sprayed on a black plate, which was moved through the droplet jet. As a consequence, the acetone-TLC droplets formed larger agglomerates on the plate. After vaporisation of the acetone pure TLC particles remained on the plate. The resulting TLC particles were investigated with an optical stereoscopic microscope

eiss SteREO Discovery.V20) at a magnification of 150x. The development of some colour changing particles on the plate indicates that the process of producing TLC droplets by solving the TLC slurry in acetone is working.

(Z

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(a) t=t0.

(b) t=t0+31 s.

(c) t=t0+72 s.

Figure 7. Microscope exposures of TLC droplets at a magnification of 150x.

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Figure 7 presents three microscope exposures of TLC droplets at different points in time. Thereby, the first exposure was made at time t0 and the following exposures were made 31 s and 72 s later. The agglomerate of droplets had a diameter of 400≈D µm and was heated only by the light source of the microscope. The colour change (see Fig. 7 (a) to (c)) towards the blue (higher hue-values) clearly reveals two aspects: First, the remaining droplets consist of TLCs and second, the light source is warming the droplet. Especially the first aspect is important for the further development, because we can be sure that the extruded jet contains TLCs. Combined with the decay of the jet into single droplets the problem of generating TLC particles is mostly solved. Furthermore, the measured size of a single particle of the agglomerate is presented in Fig. 7 (b). This measurement reveals a radius of 4.93 µm adding up to a particle diameter of approximately 10 µm. It has to be studied yet, if such small TLC particles show their colour play also when they are not agglomerated. Nevertheless, the fact that such small particles still provide such a brilliant colour play gives hope that TLC particles with sizes smaller than 10 µm may be applicable for combined PIT and PIV. This is desirable because a smaller particle size will improve the following characteristics. 4. Conclusions Advancements of the adaptation of combined Particle Image Thermography and Particle Image Velocimetry to air flows by using thermochromic liquid crystals are presented. A new particle generator was developed, which is able to continuously create small particles of TLCs. These temperature sensitive crystals shall be used as tracer particles for combined PIT and PIV in air in the future for experimental set-ups with continuous air exchange. Phase Doppler Anemometry measurements of the produced droplets show on the one hand the ability of the new generator to produce monodisperse droplets and on the other hand the strong dependence of the droplet diameter statistics on the frequency of the piezoelectric oscillator and the extruding velocity (Reynolds number). The unexpected large size of the droplets might be caused by a thin fluid layer, which was formed on the aperture plate. Further measurements with a slurry of acetone and TLCs demonstrated that the produced droplets contain TLC particles. The crystals which remained after the acetone vaporised revealed the characteristic color play of TLCs indicating that the generator is able to produce TLC tracer particles Next steps are further precise measurements of the acetone-TLC droplet and the TLC particle diameters by means of Phase Doppler Anemometry (PDA) as well as measurements with the droplet jet extruding horizontally or straight downwards. Further compulsive calibration of the TLCs has to be performed Acknowledgment We would like to thank G. Brenn for helpful discussions. Furthermore, we would like to thank F. Schlenkrich and S. Seyffarth of the Institute of Material Physics of the University of Göttingen for making the scanning electron microscope exposures of the aperture plates. References Ahlers G, Bodenschatz E, Funfschilling D, Hogg J, (2009) Turbulent Rayleigh-Bénard convection for a Prandtl number of 0.67. J Fluid Mech 641:157-167 Baskaya S, Erturhan U, Sivrioglu M, (2005) Experimental investigation of mixed convection from an array of discrete heat sources at the bottom of a horizontal channel. Heat Mass Transfer

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42:56-63 Bosbach J, Kühn M, Czapp M, Grabinski N, Westhoff A, Wagner C, (2007) Measurement techniques for large scale convective air flow. In: Proceedings of 8th ONERA-DLR Aerospace Symposium, Göttingen, Germany Brenn G, Durst F, Tropea C, (1996) Monodisperse sprays for various purposes – their production and characteristics. Part Part Syst Charact 13:179-185 Dabiri D, (2009) Digital particle image thermography / velocimetry: a review. Exp Fluids 46:327- 338 Gil P, Torres F, Ortiz F, Reinoso O, (2006) Detection of partial occlusions of assembled components to simplify the disassembly tasks. Int J Adv Manuf Technol 30:530-539 Ireland P, Jones T, (1987) The response time of a surface thermometer employing encapsulated thermochromic liquid crystals. J Phys E Sci Instrum 20:1195-1199 Kühn M, Bosbach J, Wagner C, (2009) Experimental parametric study of forced and mixed convection in a passenger aircraft mock-up. Building and Environment 44:961-970 Linden P, (1999) The fluid mechanics of natural ventilation. Annu Rev Fluid Mech 31:201-238 Lord Rayleigh JWS, (1878) On the instability of jets. Proc Lond Math Soc 10:4-13 Schmeling D, Westhoff A, Kühn M, Bosbach J, Wagner C, (2010a) Flow structure formation of turbulent mixed convection in a closed rectangular cavity. Notes on Numerical Fluid Mechanics and Multidisciplinary Design (NNFM), 112 (in press) Schmeling D, Czapp M, Bosbach J, Wagner C, (2010b) Development of combined particle image velocimetry and particle image thermography for air flows. In: Proceedings of 14th International Heat Transfer Conference, Washington D.C., USA v. Ohnesorge W (1936) Die Bildung von Tropfen an Düsen und die Auflösung flüssiger Strahlen. ZAMM 16:355-358 Webb R, (1980) Air-side heat transfer in finned tube heat exchangers. Heat Transfer Engineering 1:33-49 Westhoff A, Bosbach J, Schmeling D, Wagner C, (2010) Experimental study of low-frequency oscillations and large-scale circulations in turbulent mixed convection. Int J Heat Fluid Flow (in press)

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development of a particle generator for combined particle...

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