Droplet sizing and infrared temperature measurements in superheated sprays

Transcription

1 8 th European Symposium on Aerothermodynamics for Space Vehicles Droplet sizing and infrared temperature measurements in superheated sprays G. Lamanna (1), P. Rack, Y. Khafir, H. Fulge, S. Fasoulas, G. Gréhan, S. Saengkaew, B. Weigand, J. Steelant (1) Institut für Thermodynamik der Luft- und Raumfahrt (ITLR) Universität Stuttgart Pfaffenwaldring 31, Stuttgart 1

2 Content! State-of-the-Art! Thermal field by DIT! Atomisation by GRT! Summary & Outlook 2

3 State- of- the- art: Atomisa2on Cleary et al. measurements Courtesy of H. Witlox et al. J. of Hazardous Materials, 12: , 2007 R p = P sat ( T P inj ) x/d = 100 Proposed cumulative size distribution for fully flashing sprays: 0.5( D SMD) 1 v(d) = e 3.5 modified Rosin-Rammler distribution 3

4 State- of- the- art: Atomiza2on Progress:! Measurement as close as possible to the nozzle (limited by the high optical densities)! To obtain a complete statistical description of the size distribution function! To identify a model for the size distribution function x/d = 6.7 x/d = 0 x/d = 100

5 State- of- the- art: Temperature! Early attainment of self-similarity in velocity and temperature profiles T T am T inj T am = v x v m 5 H. Kamoun et al., Experimental flashing jet thermal characterisation by non intrusive optical technique Space propulsion 2012, Bordeaux, 2012

6 State- of- the- art: Temperature Axial Temperature: Acetone p am = 8 kpa Axial Temperature: Ethanol T inj = 373 K Key-points:! Flash boiling is confined in the region x/d < 20! Downstream turbulent mixing and the vaporisation process modulate the decay of axial temperature (and velocity)! Similar conclusions can be found in Vetrano et al., Exp Fluids 5:1573 (2013) 6

7 State- of- the- art: Temperature Key-point:! In recent years, considerable progress has been made in the characterization of the thermal field in superheated jets! However, the assessment of evaporation models based on experimental data still requires careful examination on how to compare the data 51 (a) T( C) 3 1 Tm Ts T( C) 39 Tc 37 TLIF t(ms) 7 Courtesy of Perrin et al., Exp Fluids (2015) 56:29, DOI /s

8 Thermal field by DIT Integral radiation balance I Cam1 = ε Medium I Medium + "# ( 1 ε Medium )I Back1 $ % +σ Medium I tot I Cam (( ε Medium ) I Back 2 ) σ Medium tot 2 = ε MediumI Medium I Fluid medium Reflection/Scattering Differential operation ε Medium = 1 I I Cam2 Back 2 I I Cam1 Back1 Camera Thanks to the differential operation, the determination of the emissivity is NOT affected by any optical disturbance Meas. Sci. Technol. 17 (2006)

9 Thermal field by DIT T DIT = T CAM ( 1 ε spray )T Back ε spray Epsilon [ ] Temperature [K] Meas. Sci. Technol. 17 (2006)

10 Thermal field by DIT What does the DIT method measure? T DIT = T CAM ( 1 ε spray )T Back ε spray Epsilon [ ] Temperature [K] Meas. Sci. Technol. 17 (2006)

11 Thermal field by DIT In differential terms, the radiative heat transfer equation can be written as 1 β slab di slab ds + I slab = I ε,slab [T(s)] Transmitted radiation Source: Thermal radiation Eq. A A formal integral solution of Eq. A was proposed by Sharkov I ( δ) = I 0 exp δ δ 0 ( ) + I ε,slab exp( δ slab )dδ δ = β slab s 0 ds I Cam1 = " #( 1 ε Spray )I Back1 $ % +ε SprayI Spray Optical Depth Meas. Sci. Technol. 17 (2006)

12 Thermal field by DIT σε Spray T DIT δ = σε slab T slab (δ) exp( δ slab )dδ 0 T DIT is NOT a physical temperature, rather an equivalent temperature such that it reproduce the complete thermal radiation emitted by the spray. Droplet Temp. Emissivity (ideal case) Droplet Temp.

13 Thermal field by DIT σε Spray T DIT δ = σε slab T slab (δ) exp( δ slab )dδ 0 T DIT is Key-point: NOT a physical temperature, rather an equivalent temperature such T that it reproduce DIT cannot be used for validation of evaporation models, the complete thermal because it does not reflect the effective liquid temperature, radiation emitted by the spray. rather only a weighted average. Emissivity (ideal case) Can we reconstruct the 3D temperature field? Droplet Temp. Droplet Temp.

14 Reconstruc2on of the Spray Temperature σε Spray T DIT δ = σε slab T slab (δ) exp( δ slab )dδ 0 ( ) δ = ln 1 ε spray measured 1st Abel transform ε slab (0) =1 exp[ β slab (0)ds] σε Spray T DIT measured Required Optical Depth 2nd Abel transform δ slab (0) = δ DIT 2 T slab (0) = f slab (0) σε slab (0)exp"# δ slab ( 0) $ % Source: Wikipedia Abel Inversion Fulge et al. (2011), 2 nd AIAA Plasmadynamics and Lasers Conf., Hawaii, AIAA

15 Reconstruc2on of the Spray Temperature Application of the Abel reconstruction algorihms to actual sprays revealed the following problems 1. Separation of the spray from the background 2. Verification of the Abel transform 3. Scattering effects are included by applying the Kubelka-Munk approach Estimated scattered fraction T LIF T Abel T DIT

16 Experimental Techniques: GRT θ rainbow = f!" n( T ), d mean # $ Global rainbow thermometry superimposes the rainbow signals from all droplets in measurement volume Ripple- structure disappears Meas. Sci. Technol. 17 (2006)

17 Experimental Techniques: GRT! Mean diameters and width of size distribu;on are determined from the con2nuous distri- bu;ons.

18 Experimental Techniques: GRT Mean Diameter Arithmetic/linear mean Sauter Mean Diameter Median d th percentile d 10 Size Distribution Differential Size Distribution (DSD) Cumulative Undersize Distribution (CUD) 90 th percentile d 90 Deviation variable Half Width Half Maximum Standard Deviation Span d 90 d 10 Differential Size Distribution (DSD) CUD

19 Experimental Techniques: GRT Mean Diameter Arithmetic/linear mean Sauter Mean Diameter Median d 50 Key-point: 10 th percentile d th percentile d 90 Deviation variable Half Width Half Maximum Size Distribution Differential Size Distribution (DSD) How to use these data to derive information on the efficiency of the atomisation condition under fully flashing condition? Standard Deviation How can be best model to size distribution function? Cumulative Undersize Distribution (CUD) How to chose the reference parameters? CUD: F(x) =1 e ( x λ ) k λ = scale parameter Differential Size Distribution (DSD) k = shape parameter Span d 90 d 10 CUD

20 Experimental Techniques: GRT! Comparison of the different mean diameters with SMD as a reference:

21 Experimental Techniques: GRT x/d = 0 R p Key-point:! SMD decreases with increasing superheat R p, in conformity with literature results (e.g. Cleary et al.)

22 Experimental Techniques: GRT x/d = 6.7 Key-point:! A better atomisation is observed in the transition regime! Secondary break-up due to nucleation within drops is not observed.

23 Experimental Techniques: GRT x/d = 6.7! Aerodynamic instabilities cause a rather homogeneous drop size distribution log-normal (alternatively upper-limit function)! Random bubble bursting results in a broader size distribution and larger drop sizes! The enhanced evaporation rate (due to superheated conditions) is mainly responsible for the creation of a finely atomised spray in the downstream region (i.e. x/d > 0)

24 Experimental Techniques: GRT R p x/d = 6.7 Key-points:! The shape of the size distribution changes between the transition and fully flashing regime! The Weibull function is the only suitable choice to model effectively the shape variation in superheated sprays.

25 Experimental Techniques: GRT Key-points:! The Weibull function is the only suitable choice to model effectively the shape variation in superheated sprays. x/d = 6.7! The shape of the size distribution changes varies also along the spray axis! In fully flashing sprays, the enhanced evaporation induces a narrowing of the size distribution function

26 Conclusions & Outlook! For the temperature field, two complementary techniques have been developed! Flashing is less efficient than aerodynamic-driven atomization, as it leads to larger SMDs immediately after disintegration! Superheated evaporation leads rapidly to a finely atomised spray! The Weibull is the only suitable choice to model the shape variation from the transition to the fully flashing regime! Database on droplet size is currently being extended to obtain a statistically relevant database for model development! For an accurate 3D reconstruction of the temperature field, scattering effect are currently incorporated following the Kubela-Munk approach 26

27 Acknowledgments! The financial support of ESA-ESTEC is gratefully acknowledged Multi-Phase Aspects in Propulsion Systems - Contract No /09/NL/EM 27

28 Thank You 28

29 Thank You 29