The effect of initial pressure on detonation propagation across a mixture

Abstract

This study determines the effect of the initial pressure on the propagation of a Chapman–Jouguet detonation wave from a stoichiometric C₃H₈/O₂ mixture (donor) to a stoichiometric C₃H₈/air mixture (acceptor). Depending on the initial pressure ratio in the donor and the acceptor, the result can be a smooth transmission, a re-initiated detonation wave, or a transmitted shock wave. When the donor is divided into a driver donor and a driven donor, the degree of overdrive in a driven donor varies with the donor pressure ratio. There must be a greater degree of overdrive in the driven donor for re-initiation of a detonation wave in the acceptor, particularly if the initial pressure in the driven donor is lower than the Chapman–Jouguet pressure in the acceptor. The bi-dimensional effect is also another major factor.

Introduction

In applications where a pre-detonator is used in pulse detonation engines, the propagation of a detonation wave from a hydrocarbon–oxygen mixture (donor) to a hydrocarbon–air mixture (acceptor) is of interest. In a highly sensitive mixture, a detonation wave can be initiated via the deflagration-to-detonation (DDT) process. Previous studies^1,2 have found that when an incident detonation wave propagates into a less sensitive mixture, there are several modes of wave transmission, depending on the properties of the two mixtures. Initiation energy is another important parameter. ³ The longer the donor (less effect from the rarefaction wave), the higher the degree of overdrive in the acceptor. The wave propagation is characterized as detonation decay, decay and re-initiation, detonation transition, and critical detonation transition. Engebretsen et al. ⁴ also noted that the strength of the incident shock is critical in detonation re-initiation, particularly when there is a low-sensitivity mixture in the acceptor. Haloua et al. ⁵ showed that the detonation propagation mode varies with the initial pressure of the mixture in the acceptor. As the initial pressure decreases, a stable detonation wave may switch into a shuttered detonation wave, a galloping detonation wave, or a failure. The tube or channel size also plays an important role for detonation propagation. ⁶ When a detonation wave propagates from a large tube into a narrow channel, there is a velocity deficit. As the width of the channel decreases, the stable detonation wave may transit to a galloping detonation wave or a failure. A study by Li et al. ⁷ also noted that a transmitted overdriven detonation wave occurs when there is a strong incident overdriven detonation wave. Krivosheev and Penyaz’kov ⁸ found that the value of the critical pressure for successful wave transmission is reduced by a factor of 3–5 when there is an overdriven detonation wave.

In some previous studies, the incident detonation wave was not in a stable condition; for example, an overdriven detonation wave approaching the Chapman–Jouguet (CJ) state. In this study, a CJ detonation wave is formed in a stoichiometric C₃H₈/O₂ mixture in the donor or in the driver donor when the donor is divided into two sections. An overdriven detonation wave is generated in the driven donor and the degree of overdrive varies with the donor pressure ratio. The incident detonation wave then propagates into a stoichiometric C₃H₈/air mixture at a fixed initial pressure. The effect of the initial pressure on the propagation of an incident detonation wave is determined. In a tube of finite size, a curved detonation wave propagates downstream. Because of the curvature, the streamline of the burned products behind the detonation wave turns at a small angle toward to the tube wall. Flow expansion results in a decrease in pressure and a reduction in the degree of overdrive. ⁹ This bi-dimensional effect, which affects detonation propagation, is also studied.

Experimental setup

The experimental setup is shown in Figure 1. The smooth, aluminum 6061-T6 tube with an inner diameter of 50.8 mm is 1219.2 mm in length. The donor and the acceptor are 762 and 457.2 mm, respectively. For a stoichiometric C₃H₈/O₂ mixture at 1 atm, Li et al. ¹⁰ demonstrated that the run-up distance for DDT (x_DDT) is approximately 150 mm from the closed end of a smooth tube with an inner diameter of 50.8 mm. An overdriven detonation is observed and this approaches a CJ detonation at x/x_DDT ≈ 2, where x is the streamwise distance from the closed end. ⁷ Li et al. ¹¹ also studied the diaphragm effect (t = 25–100 µm in thickness) on detonation propagation across a mixture. Quenching of the incident detonation wave near the interface is observed and the presence of the diaphragm means that a longer distance is required for the re-initiation of a detonation wave in the acceptor. It was also found that the propagation velocity of a detonation wave when t ≤ 50 µm is similar to that for mixtures that are separated by a slide gate valve (no diaphragm). Therefore, a thin LUMIRROR Mylar diaphragm (t = 30 µm) is located at x = 762 mm for this study. This separates the donor (C₃H₈/O₂) and the acceptor (C₃H₈/air at 1 atm). A CJ detonation wave forms ahead of the diaphragm, which is denoted for Case A. To study the propagation of an overdriven detonation from a C₃H₈/O₂ mixture to a C₃H₈/air mixture, the donor with C₃H₈/O₂ is divided into a driver donor and a driven donor (Case B). Another Mylar diaphragm (t = 30 µm) is located at x = 584.2 mm. Notably, the driven donor is 177.8 mm in length, in which the overdriven detonation is relatively stable from the preliminary tests. Prior to each run, the tube is evacuated to 20 Pa. The C₃H₈ and O₂ are then injected into the tube and the equivalence ratio of the mixtures was determined by the partial pressure method. For Case A, the initial pressure of the donor, p_d, ranges from 0.5 to 2.25 atm. The cell width is 1.87–0.32 mm ¹² and the ratios for the tube diameter and cell width are 27–158. For Case B, the initial pressures in the driver donor, p_d1, and the driven donor, p_d2, are 0.5–2.0 and 0.027–0.5 atm, respectively. The donor pressure ratio (p_d* = p_d1/p_d2) ranges from 1 to 33. The acceptor that is filled with the stoichiometric C₃H₈/air has an initial pressure, p_a, of 1 atm for all test cases. It is also noted that the form of diaphragms is not planar at different pressures in the tube sections and some time is required for its destruction. To ensure the homogeneity of the mixtures in the donor and the acceptor, two circulation pumps are used for approximately 3 min. The filling pipes are mounted near both end of the donor to ensure homogeneity of mixtures and the concentration of the mixtures is calibrated by gas chromatography (CHINA chromatography Model 9800 GC/TCD). The ignition system consists of a transformer (May & Christe z201402e2, 220–14,000 V) and an electric spark at the closed end and the maximum effective discharge energy is estimated to be 0.5 J.

Figure 1.

Experimental setup: 1. donor (Case A) or driver donor (Case B); 2. donor (Case A) or driven donor (Case B); 3. acceptor; 4. piezoelectric pressure transducers; 5. spark electrodes; 6. Mylar; 7. Mylar (only for Case B); 8. circulation pump; and 9. vacuum pump.

The velocities of the shock and detonation waves were measured using time-of-flight, by pressure sensors. Piezoelectric pressure transducers (PCB 113B22) were installed on the tube wall (x = 3.81–30.48 cm). The rise time for the transducers is 1 µs. Sensors were mounted on the donor to characterize the propagation of the incident detonation wave (overdriven or CJ detonation). The wave velocity in the acceptor was also measured. The propagation speed of the pressure wave was estimated to be u = ΔL/(Δt ± e_t), where Δt and ΔL are the propagation time for a wave past two pressure sensors and the spacing between them (=3.81 cm), respectively. The uncertainty in the propagation time for a detonation past two pressure sensors is equal to $e_t=\sqrt{e_{\tau }^2+e_r^2}$ , where e_τ corresponds to the error that is due to the actual wave front spreading over a small time interval, instead of a step rise, and e_r is the inherent rise time in the pressure transducers. ¹³ The signals from the pressure transducers were acquired simultaneously and stored by NI-PXI 6133 high-speed data acquisition modules at a typical sampling rate of 1 MHz. The uncertainty in the speed of the pressure wave speed is approximately 6.3%.

Results and discussion

Case A: CJ detonation

For a detonation shock tube, a self-sustained detonation wave in a driver tube (or the donor) is attenuated because there is a reflected rarefaction wave. This results in direct transmission, re-ignition, or a transmitted shock wave in a driven tube (or the acceptor). In this study, the experiments were conducted using a stoichiometric C₃H₈/O₂ mixture at p_d = 0.5–2.25 atm. The p_a value for a stoichiometric C₃H₈/air mixture or air in the acceptor was 1 atm. It is expected that the pressure wave for a C₃H₈/air mixture that does not undergo chemical reaction propagates at the same speed as that of the air in the acceptor. The variations in the speed of the pressure wave versus p_d near the diaphragm (x* = 0.44) are shown in Figure 2. It is seen that the pressure wave propagates at a greater speed when there is an increase in p_d. At p_d = 1.75–2.25 atm, the speed of the pressure wave in a C₃H₈/air mixture is greater than that for air (ΔV ≈ 140 m/s), indicating there is a chemical reaction from C₃H₈/O₂ to C₃H₈/air at higher p_d.

Figure 2.

The speed of the pressure wave at x* = 0.44 for a C₃H₈/air mixture or air in the acceptor.

When there is a C₃H₈/O₂ mixture in a donor that is 762 mm in length, a CJ detonation wave is formed ahead of the diaphragm. ⁷ It is also known that a C₃H₈/air mixture is less sensitive than a C₃H₈/O₂ mixture for the same initial conditions. Therefore, both the velocity, V_CJ, and pressure, p_CJ, of a CJ detonation wave in a C₃H₈/O₂ mixture are greater than those for a C₃H₈/air mixture. Therefore, when an incident CJ detonation wave propagates from a C₃H₈/O₂ mixture to a C₃H₈/air mixture, the transmitted wave is an overdriven detonation wave. The velocity of the transmitted pressure wave in the acceptor is shown in Figure 3. The origin in the abscissa corresponds to the location of the diaphragm and the relative distance from the diaphragm x* (=x/d) is normalized by the tube diameter. In the diagram, x* denotes a location that is equidistant from the two pressure transducers. The value of V_CJ for a C₃H₈/air mixture at 1 atm is also shown, for reference. When p_d = 2 atm, the incident detonation wave is smoothly transmitted to the acceptor and an overdriven detonation wave propagates downstream. When the value of p_d is reduced (=1.0 and 0.75 atm), there is quenching of the detonation near the interface and a re-initiated detonation wave with a lower degree of overdrive (V* = V/V_CJ) forms at locations further downstream. When p_d = 0.5 atm, there is no transmission of the incident detonation wave. Instead, a transmitted shock wave (or a quasi-detonation wave) is observed within the limited length of the acceptor.

Figure 3.

The effect of p_d on the propagation of a pressure wave in the acceptor.

Case B: overdriven detonation

When the donor is divided into a driver donor and a driven donor, V* in the driven donor is dependent on p_d*. Figure 4 shows the distribution of the speed of the pressure wave in the acceptor. p_d1 ranges from 0.5 to 2 atm and p_d2 = 0.5 atm (p_d* = 1, 2, and 4). Note that p_d* = 1 corresponds to Case A, where p_d = 0.5 atm, in which a quasi-detonation wave (no re-initiated detonation) propagates downstream within the limited length in this study. When p_d* = 2 and 4 (V* = 1.04 and 1.09, respectively), the incident overdriven detonation is successfully transmitted to the acceptor (x* = 1.27) either by re-initiation (p_d* = 2) or by smooth transmission (p_d* = 4). At further downstream locations, the value of V* increases as p_d* increases. For a fixed value of p_d1 (=1 atm), a decrease in p_d2 also results in an increase in p_d*. Note that the value of p_d2 is less than that for p_a. When p_d* = 6, 21, and 33, V* = 1.18, 1.41, and 1.53 (V = 2681, 3175, and 3402 m/s), respectively. The corresponding peak driven donor pressure is 12.31, 6.12, and 4.56 atm. The distribution of the speed of the pressure wave in the acceptor is shown in Figure 5. The pressure of the incident overdriven detonation is less than the value of p_CJ for a C₃H₈/air mixture in the acceptor. Quenching of detonation is seen near the interface and no smooth transmission is observed in all three test cases. When p_d* = 6 and 21, re-initiated overdriven detonation waves form at x* = 2.02 and 1.27, and then decay to a CJ wave at locations further downstream. Therefore, even though the incident overdriven detonation is not directly transmitted to a detonation wave in the acceptor, there is transmission by re-initiation for greater values of V* or p_d*.

Figure 4.

The effect of p_d* on the propagation of a pressure wave in the acceptor when p_d2 = 0.5 atm.

Figure 5.

The effect of p_d* on propagation of a pressure wave in the acceptor when p_d1 = 1.0 atm.

The effect of a decrease in p_d1 on the propagation of an incident overdriven detonation wave is also of interest. In Figure 6, p_d1 is fixed at 0.75 atm and the value of p_d* ranges from 2.0 to 15 (p_d2 = 0.05–0.375 atm). When p_d* = 2.0 and 4.5, V* = 0.95 and 1.06, respectively. The incident overdriven detonation wave is not transmitted to the acceptor, or a shock wave is transmitted. With the value of p_d* increases (=7.5; V* = 1.18 or V = 2681 m/s; the peak driven donor pressure = 8.3 atm), there is no instantaneous initiation of a transmitted detonation wave. Instead, a transmitted shock forms and then a re-initiated detonation is observed at x* ≈ 4.27. At locations further downstream, V* approaches the CJ value. When p_d* = 9.5 and 15.0 (V* = 1.25, and 1.33; V = 2822 and 2998 m/s, respectively), the re-initiation occurs further upstream, that is, x* = 2.02 and 1.27, respectively. The corresponding peak driven donor pressure is 8.03 and 6.33 atm. Therefore, even when there is a relatively lower pressure in both the driver and the driven donors, an increase in V* (or higher p_d*) results in re-initiation of the detonation in the acceptor. It is also noted that the re-initiated detonation appears to be unstable. There is a repeated process of decay, transition, and decay (or longitudinal pulsations), which may correspond to a spinning detonation.^7,14 A study by Austin and Shepard ¹⁵ demonstrated that the cell width of a stoichiometric C₃H₈/air mixture at 1 atm is 51.3 mm and that this value varies with the value of V*. ¹⁶ In this study, the tube diameter (=50.8 mm) is similar to the cell width and this partially accounts for the unstable detonation wave in the acceptor.

Figure 6.

The effect of p_d* on propagation of a pressure wave in the acceptor when p_d1 = 0.75 atm.

The “shock wave amplification through coherent energy release (SWACER)” theory, which is similar to the induction time gradient theory that was developed by Zeldovich et al., ¹⁷ provides a qualitative description of DDT.^18,19Figure 6 shows that a detonation wave is re-initiated in the acceptor when p_d1 = 0.75 atm and p_d* = 15.0. The pressure profiles are shown in Figure 7. It is seen that the peak pressure of the transmitted shock wave increases continuously as it propagates downstream. The maximum peak pressure (≈52 atm) is observed at x* = 3.14. At locations further downstream locations (x* = 3.89 and 4.64), the amplitude of the peak pressure (≈34 atm) is similar, which demonstrates an overdriven detonation that corresponds to shock amplification and slows down to a CJ condition. When p_d* = 7.5, a detonation wave is also initiated in the acceptor and the pressure profiles are shown in Figure 8. It is seen that the shock wave propagates downstream and the peak pressures are 21–25 atm at x* = 0.89–3.14, which corresponds to a transmitted shock wave. At x* = 3.89, the increase in the initial pressure is approximately the same, following a second peak pressure of 50 atm. A high peak pressure (or a detonation wave with a higher value for V*, as shown in Figure 6, is observed at x* = 4.64. A localized explosion probably occurs at x* = 3.89–4.64, resulting in larger local pressure and velocity than those for p_d* = 15.0. The second peak pressure at x* = 3.89 corresponds to the upstream propagation of a retonation wave, so the re-initiation originates from the heated unburned mixture behind the shock wave. When p_d* = 9.5, the propagation pressure wave is similar to that for p_d* = 7.5. This implies that a re-initiated detonation wave corresponds to a local explosion that originates near the turbulent flame, or the SWACER mechanism, depending on the value of p_d*.

Figure 7.

The pressure profiles for the sensors in the acceptor when p_d1 = 0.75 atm and p_d* = 15.0.

Figure 8.

The pressure profiles for the sensors in the acceptor when p_d1 = 0.75 atm and p_d* = 7.5.

Both the pressure and the velocity of the burned products in the donor are dominant elements in the transmission of an incident detonation wave that propagates into the acceptor. The normalized wave velocity, S*, and the pressure, p*, are shown in Figure 9, where $S^{\ast }=(V_{DW,d2}/V_{CJ,a})$ and $p^{\ast }(=p_{DW,d2}/p_{CJ,a})$ are, respectively, the velocity and pressure of an incident detonation wave in the driven donor, normalized by the CJ values of the C₃H₈/air mixture in the acceptor. For all test cases, the value of S* is >1 (an overdriven detonation wave). The solid symbols denote smooth transmission or re-initiation and the hollow symbols correspond to transmitted shock waves. When p* ranges from 2.0 to 2.5, the pressure of an incident detonation wave is greater than the CJ pressure for the C₃H₈/air mixture in the acceptor, so smooth transmission is observed. A decrease in the value of p* (=1–2) results in quenching of the detonation near the interface between the driven donor and the acceptor. A re-initiated detonation is observed at locations further downstream. When p* ranges from 0.6 to 1.0, a re-initiated detonation wave or a transmitted shock is observed. Transmission of an incident detonation wave depends on the values of both S* and p*. An incident detonation wave may not be sustained in the acceptor for lower values of S*. For lower values of p* (<0.5) and higher values of S* (>1.5), re-initiation of detonation waves is observed, for which the value of p*^0.5S* is approximately a constant. This demonstrates that the velocity of an incident detonation wave plays an important role in the re-initiation of a detonation wave, particularly for lower values of p*.

Figure 9.

The pressure and velocity ratios when there is transmission of a detonation wave across a mixture.

Bi-dimensional effect on the degree of overdrive in the donor

As previously mentioned, a CJ detonation wave forms ahead of the first diaphragm for Case B and the value of V* in the driven donor is dependent on p_d*. According to the Hugonoit equation, the mechanical balance of a flow means that the particle velocity in the driver donor must be equal to that of the driven donor; that is, $u_{p,d1}=u_{p,d2}$ . A sketch of the wave propagation is shown in Figure 10. In laboratory coordinates, the particle velocity ( $u_{p,d2}=u_1-u_2=(1-({\nu }_2/{\nu }_1))u_1$ , $u_1={\upsilon }_1\sqrt{(p_2-p_1)/({\upsilon }_1-{\upsilon }_2)}$ ) for an overdriven detonation is given on the left-hand side of equation (1), where p is the pressure, $\nu$ is the specific volume, $\gamma$ is the specific heat ratio, and u is the velocity in the stationary coordinate. Subscripts 1 and 2, respectively, denote the state of the unburned mixture and burned product. The right-hand side of equation (1) corresponds to $u_{p,d1}$ , for which the properties of the CJ state ( $p_{\mathrm{CJ}}$ , $u_{\mathrm{CJ}}$ , and ${\gamma }_{\mathrm{CJ}}$ can be calculated using STANJAN code ²⁰

$$\left(1-\frac{{\upsilon }_2}{{\upsilon }_1}\right){\upsilon }_1\sqrt{\frac{p_2-p_1}{{\upsilon }_1-{\upsilon }_2}}={\left(\frac{p_{CJ}}{p_2}\right)}^{\frac{1}{{\gamma }_{CJ}}}{u}_{CJ}$$

(1)

where $v_2=[q_c+(({\gamma }_1)/({\gamma }_1-1))p_1{\upsilon }_1+1/2(p_2-p_1){\upsilon }_1]/[(({\gamma }_2)/({\gamma }_2-1))p_2+1/2(p_2-p_1)]$ and $q_c$ is the difference in enthalpy between State 1 and State 2.

Figure 10.

A sketch of wave propagation in a driver donor and a driven donor.

The bi-dimensional effect in a tube of finite size may result in a decrease in V*, when a curved detonation wave propagates downstream. ⁹ The non-dimensional modified pressure, P, is given as $p_{\mathrm{DW},d1}/p_{\mathrm{CJ},d1}$ , where $p_{\mathrm{DW},d1}$ is the modified pressure of a detonation wave in the driver donor. In Desbordes’ study (d = 52 mm), the value of p_d1 for a stoichiometric C₂H₂/O₂ mixture was approximately 1 atm and p_d* = 2–8. It was found that P = 0.7. Using equation (1), the calculation of P (=0.5–1.0) is shown in Figure 11. When p_d2 = 0.1 and 0.25 atm, the value of P is approximately 0.6, although the data are slightly scattered. A decrease in p_d2 (=0.05 atm) results in a reduction in the value of P (=0.5), except when p_d* = 15–25, where P ranges from 0.5 to 0.6. For the CJ theory, P = 1 and the induction distance is assumed to be 0. However, according to the ZND model, there is a longer induction distance when there is an increase in the cell width for a given mixture. At an initial pressure of 0.125 atm, the cell widths for stoichiometric C₂H₂/O₂ and C₃H₈/O₂ mixtures are 2²¹ and 8 mm, ¹² respectively. When there is a decrease in the initial pressure, the cell widths for a stoichiometric C₃H₈/O₂ mixture at 0.1 and 0.05 atm, respectively, are estimated to be 12 and 27 mm. ¹² Therefore, the discrepancy between Desbordes’ data and the present findings corresponds to the scale of the induction distance (or cell width). This demonstrates that the bi-dimensional effect is dependent on the values of both p_d* and p_d2. In other words, the value of V* decreases when the cell width increases or when there is a longer induction distance.

Figure 11.

The degree of overdrive versus pressure ratio for a stoichiometric C₃H₈/O₂ mixture.

Conclusion

An experimental study is conducted to investigate detonation propagation from a stoichiometric C₃H₈/O₂ mixture (donor) to a stoichiometric C₃H₈/air mixture (acceptor, p_a = 1 atm). For Case A, there is direct transmission, re-ignition, or a transmitted shock wave. A greater initial pressure in the donor (p_d ≥ 0.5 atm or p_d/p_a ≥ 0.5) is required for successful transmission. For Case B, the donor is divided into a driver donor and a driven donor. When there is a fixed pressure in the driven donor (p_d2 = 0.5 atm), a CJ detonation wave is formed in the driver donor. The degree of overdrive in the driven donor (V* = 1.04–1.53) varies with the initial donor pressure ratio (p_d* = 2–33). When the initial driver donor pressure (p_d1 = 0.75 atm) is fixed, there is a transmitted shock and a re-initiated detonation is observed at locations further downstream, for p_d* = 2–15. In a tube of finite size, the bi-dimensional effect results in a lower degree of overdrive than the theoretical value, particularly when there is a decrease in p_d2. It is found that an incident overdrive detonation at high pressure is required for smooth transmission. Re-initiated detonation waves are observed through a local explosion or possibly a SWACER mechanism, which is dependent on the value of p_d*. The velocity of an incident overdriven detonation wave also plays an important role. Therefore, even for lower values of the initial pressure in both the driver and the driven donors, a detonation wave in the acceptor can be re-initiated when there is a higher degree of overdrive in the driven donor.