Detonation interaction with a diffuse interface and subsequent chemical reaction
Summary (2 min read)
1 Introduction
- When a detonation wave propagating in a gaseous combustible mixture reaches a concentration boundary, a complex interaction results between the detonation and interface between the two gases.
- In the present study, the interface is a composition gradient between the combustible and non-combustible mixtures.
- The authors have studied this case and the results will be presented in a companion paper.
- It is possible for combustion to occur in the turbulent mixing zone (TMZ) by choosing a combustible mixture such that the combustion products are incompletely oxidized.
2 Experimental setup
- The experiments were carried out using the GALCIT Detonation Tube (GDT) [1, 2] , which is 7.
- Figure 2 is a view of the test section illustrating the location of the end flange of the GDT, the sliding valve assembly, and the test section.
- Detonation velocities were measured to within 5% of the Chapman-Jouguet speed.
- The diffuse interface was made with a gravity current (GC).
- In the PLIF experiments, the combustible mixture in the GDT was replaced with an acetone-helium mixture of matched density.
3 Results and discussion
- The two layer situation was created by allowing a long time to elapse between the creation of the gravity current and detonation initiation (see discussion below).
- As the detonation exits the gravity current, what remains is a transmitted shock followed by the TMZ (c).
- The ture is colored yellow for visibility and flows below the less dense nitrogen gas.
- This translates to a leading shock wave that is for the most part perpendicular to the top and bottom surface along with a reflected trailing shock that is possibly due to slight imperfections in the interface shape.
- 4c except that the gravity current occupies half the height of the test section thus changing the curvature of the leading wave.
3.1 Composition gradient
- Experiments indicate the gravity current heads are out of the field of view and the unsteady motion is minimized.
- The composition gradient is estimated from the gravity current measurements [12, 20] which were carried out specifically to determine the role that the composition profile plays.
- The detonation shape velocities are obtained from the images using EQUATION and considering the detonation to have zero thickness.
- The wave angle β (see Fig. 1 ) is determined numerically from the wave shape.
3.2 Experimental impulse: the role of secondary combustion
- The impulse is calculated from experimental pressure traces to quantify the amount of secondary burning that occurs in the TMZ.
- The integration is carried out using the two-point Newton-Cotes method and the results reported in MPa s.
- The integration of the pressure trace starts at the arrival of the incident shock wave and terminates upon the arrival of pressure disturbances from the GDT approximately 5 ms later.
- The impulse has an abrupt change in slope at the arrival of the incident and reflected shocks.
- It was found that with all factors being equal, the impulse was 1-5% higher with oxygen versus nitrogen as the test gas.
3.2.1 Time scale of combustion
- This section addresses the chemical reaction time scale of the partially oxidized combustion products (CO and H 2 ) and the test gas.
- The initial temperature before the reaction takes place is a weighted average of the detonation products and shocked test gas temperatures and determined during the frozen composition mixing.
- The shear layer displacement thickness changes from being negative to positive causing the surrounding fluid to be displaced and generating compression waves.
- Given the simplicity of the model, only order of magnitude agreement can be expected at best.
- The fact that theory estimates impulses on the same order as the experimental findings indicates that secondary combustion in the TMZ is a plausible explanation for the impulse increment.
4 Conclusion
- The authors have experimentally studied detonation propagation along diffuse interfaces.
- The authors have observed the refraction of the detonation wave through the interface and subsequent creation of a turbulent mixing zone from the remains of the interfacial region.
- By using a fuel-rich ethylene-oxygen combustible mixture and an oxidizing test gas, the authors were able to observe and quantify the effects of combustion inside the turbulent mixing zone.
- The main consequence of the mixture composition gradient through the diffuse interface was to cause the detonation wave to curve: eventually the reaction zone decouples from the leading shock wave when the combustible mixture becomes sufficiently dilute.
- The authors attribute the wave curvature to the decrease in lead shock velocity with increasing dilu-.
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Citations
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Cites background from "Detonation interaction with a diffu..."
...Lieberman and Shepherd [97] investigated detonation interaction with a diffuse interface between two mixture layers....
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...Thank you Prof. Joseph E. Shepherd for inviting me to work in your group and for opening doors for the future....
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18 citations
Cites background from "Detonation interaction with a diffu..."
...For larger systems compared to the detonation cell size (such as those considered in experiments [89, 90]), where many triple points can form and interact, the influence of channel size would be less pronounced....
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...Experiments have shown that the sizes of detonation cells vary spatially in relation to the variation in fuel concentration [89] and that the shape of the detonation wave is curved in these systems [89, 90]....
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...The shape of the detonation front was similar for all three gradients considered, and did not exhibit the curved structure observed in experiments [89, 90]....
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References
1,487 citations
"Detonation interaction with a diffu..." refers methods in this paper
...The increase in pressure is related to the velocity increase using the acoustic equation [13]...
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393 citations
"Detonation interaction with a diffu..." refers methods in this paper
...The thickness of the diffuse interface, δc, was estimated from the thickness of the region of vorticity obtained from digital particle image velocimetry (DPIV) [9, 20] measurements from the water channel experiments and then re-scaled for the gas phase experiments....
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220 citations
214 citations
"Detonation interaction with a diffu..." refers background in this paper
...It has been shown [18] that TMZ growth can increase by a factor of six after the re-shock event....
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204 citations
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Frequently Asked Questions (15)
Q2. What is the effect of the shear layer displacement thickness on the surrounding fluid?
the shear layer displacement thickness changes from being negative to positive causing the surrounding fluid to be displaced and generating compression waves.
Q3. How many impulses are used to calculate the increment?
A reference impulse of 1000 kg·m−1·s−1 obtained from impulse measurements at pressure transducer P4 is used to calculate the increment.
Q4. What is the main consequence of the mixture composition gradient through the diffuse interface?
The main consequence of the mixture composition gradient through the diffuse interface was to cause the detonation wave to curve: eventually the reaction zone decouples from the leading shock wave when the combustible mixture becomes sufficiently dilute.
Q5. What is the effect of the curved wave on the reaction zone structure?
For a detonation modeled as an ideal one-dimensional discontinuity with no affect of curvature on reaction zone structure, the normal component of the curved wave will correspond to the local Chapman-Jouguet detonation velocity.
Q6. What is the purpose of the sliding valve?
The sliding valve, actuated by a falling mass, was designed to completely isolate both the combustible mixture and test gas, as well as to open sufficiently fast to control the formation of the gravity current.
Q7. What is the effect of the rapid increase in induction time with decrease in shock velocity?
The rapid increase in induction time withdecrease in shock velocity results in the decoupling of the reaction zone from the shock and the formation of a gap between the transmitted shock and the turbulent mixing zone.
Q8. How did the authors measure the effects of combustion in the turbulent mixing zone?
By using a fuel-rich ethylene-oxygen combustible mixture and an oxidizing test gas, the authors were able to observe and quantify the effects of combustion inside the turbulent mixing zone.
Q9. What is the effect of the vorticity thickness of the interface?
It is evident that the vorticity thickness of the interface is not uniform and exhibits a sinuous shape as a result of the turbulent flow structure present at the interface.
Q10. What is the color of the ethylene-oxygen combustible mixture?
In these experiments the φ = 2.5 ethylene-oxygen combustible mixture is colored yellow for visibility and flows below the less dense nitrogen gas.
Q11. What is the effect of a curved detonation wave on the reaction zone?
When the detonation propagation direction is perpendicular to the mixture gradient, a curved detonation wave results that ultimately decouples into a shock wave and turbulent mixing zone (TMZ), shown in Fig. 1b.
Q12. What is the constant of proportionality C?
The constant of proportionality C is varied from a value of 1 to 10 to account for interface growth after the shock reflection off the end-wall.
Q13. What is the velocity of the left (u+p) edges of the fluid element?
The velocity of the left (δu−p ) and right (δu+p ) edges of the fluid element are combined to obtain an expression for the growth rate of the fluid elementdx∗ dt = δu + p −δu−p .
Q14. What was the optical viewport used for the experiments?
Visualization for the experiments, using a schlieren system [1], was made through an optical viewport (BK7 or quartz windows) that could be arranged in two separate positions.
Q15. What are the details of the interaction between the detonation and the interface?
The details of this interaction are dependenton the mixture compositions, the relative geometry of the detonation and interface, and the characteristic thickness of the interface.