What Is Detonation?

Detonation is also called detonation. It is a chemical reaction transmission process with a large amount of energy release. The front of the reaction zone is a shock wave moving at supersonic speed, which is called detonation wave. After the detonation wave sweeps, the medium becomes the detonation product of high temperature and high pressure. The system capable of detonation can be a gas phase, liquid phase, solid phase or gas-liquid, gas-solid and liquid-solid mixed phase system. The liquid and solid phase detonation systems are usually called explosives.

Chinese name: Detonation
Foreign name: detonation

Also known as: Knock
Nature: Chemical reaction transmission process
usually: The liquid and solid phase detonation system is called an explosive.

Introduction to Detonation

In the early 1880s, French physicists M. Betello, P. Vieret, E. Malal, and H.-L. Le Charlie conducted experiments on flame propagation. They ignited one end of a tube filled with a combustible gas mixture and found that the flame usually propagates at a low speed of several centimeters to several meters per second, but in some special cases, this slow burning process can be transformed into a high-speed special burning process They call this phenomenon detonation. It was later discovered that solid and liquid explosives can also detonate.

The detonation process is not only a hydrodynamic process, but also a complex chemical reaction kinetic process. The two influence each other and are coupled with each other. Detonation is also accompanied by thermal, light, and electrical effects. When detonation interacts with the surrounding medium, shock waves or stress waves will be generated in the surrounding medium, pushing the object to move and causing the object to be destroyed. People usually associate combustion (ie deflagration) with detonation. The most obvious difference between detonation and combustion lies in the different propagation speeds. When burning, the flame propagation speed is on the order of 10 to 10 meters / second, which is less than the speed of sound in the burning material; and the detonation wave propagation speed is on the order of 10 to 10 meters / second, which is greater than the speed of sound in the material. For example, the stoichiometric hydrogen and oxygen mixture burns at a speed of 10 m / s under normal pressure, while the detonation speed is about 2820 m / s. The chemical reaction process in detonation releases energy at high speed. Therefore, the power of detonation is very large, and the power of high-efficiency explosives per square centimeter of detonation wave front is as high as 10 watts. This feature makes detonation a unique way of energy conversion. The study of detonation usually includes the initiation of detonation, the structure of detonation waves and the interaction between detonation and surrounding media. ^[1]

Detonation

Spark discharges or shock waves are usually used to detonate gas mixtures, and detonators and detonators are used to detonate the grains. In the process of initiation, the two problems of shock to detonation transition (SDT for short) and detonation to detonation transition (DDT for short) are important topics in the current detonation research. A large number of studies have shown that there are two types of initiation processes: For homogeneous systems in the gas phase, liquid phase (excluding bubbles and impurities), and solid phase (single crystal), the material behind the initial shock wave front is heated as a whole and chemical reactions occur. And turned into a detonation where the longest heating time, that is, the earliest impact. The detonation wave propagates in the medium that has been impacted and becomes an overpressure detonation. This overpressure detonation wave catches up with the initial shock wave front and develops into a steady detonation. For liquid and solid heterogeneous systems, the process of shock initiation is complicated. The initial shock wave interacts with the medium at the density discontinuity in the heterogeneous system to form a hot spot, a chemical reaction occurs, and energy is released to strengthen the initial shock wave and strengthen the shock wave. It interacts with the medium where the density is discontinuous, forming a hot spot with a higher temperature, which causes more explosives to decompose and emit more energy. In this way, the shock wave is continuously strengthened and directly transferred to the steady detonation.

Detonation structure

The structure of detonation waves is a major problem in detonation research. DL Chapman put forward the simplest detonation wave structure theory in 1899 and E. Ruggey in 1905, and later called the CJ theory. In the 1940s, .B.Zelidovich, J.von Neumann, and W.Dulin each independently established a model of the internal structure of the detonation wave, which is referred to as the ZND model.

CJ theory reduces the detonation wave to an impact compression discontinuity surface, and the chemical reaction on it is completed instantaneously. The initial and final parameters on both sides of the discontinuity surface can be connected by three conservation laws: mass, momentum and energy. Together, the following three equations can be obtained by transformation:

Where p is the pressure; v is the specific volume (v = 1 / , is the density); e is the specific internal energy; u is the particle velocity; D is the velocity of the detonation wave, that is, the detonation velocity; state. The second formula is a straight line in the p - v plane, usually called the Rayleigh line; the third formula is called the Xugoniu equation, which is a curve in the p - v plane, called the Xugoniu line (Figure 2 ). All state points that satisfy the conservation relationship with the initial state ( p 0, v 0) are on this line. The equation of state of the detonation product can be written as: e = e ( p , v ).

Figure 2 Xugon New Line and CJ point of detonation wave

There are five unknowns in the four equations. To determine the detonation parameters with a single value, you must find the fifth equation. To this end, Chapman and Ruggey proposed the famous hypothesis (called the CJ hypothesis or CJ condition): the state of the stable detonation product corresponds to the tangent point J of the Xugoniu line and the Rayleigh line, which is the CJ point (Figure 1), the detonation velocity DJ at this point is a minimum value, it can be proved that there is the following relationship at point J :

D _J = u _J + c _J ,

In the formula, c is the speed of sound, and the subscript " J " represents the value of point J. The BA segment of the curve in Figure 1 corresponds to the detonation process, the AE segment does not correspond to any actual process, and the EF segment corresponds to the deflagration process. Some properties of detonation and deflagration waves are shown in Table 1.

Table 1 Properties of detonation and deflagration waves

p- graph

process

Du ₀

entropy

Detonation wave

Detonation

Weak detonation

Strong detonation

CJ Detonation

Constant volume detonation

> D _J

D _J

> c ₀ (speed of sound)

> c

c _J

Smallest

Deflagration wave

EJ JF

Deflagration

Strong deflagration

Weak deflagration

CJ deflagration

Constant pressure deflagration

<D _J

D _J

<c ₀

> c

c _J

maximum

Using the CJ theory to calculate the actual detonation system, it is generally possible to obtain results similar to the experimental detonation velocity values, which shows that the CJ theory is basically correct. However, the detonation pressure and density values obtained by precise measurement of gas-phase detonation are about 10% to 15% lower than those obtained using the CJ theory, and the Mach number measured for the detonation product is about 10 higher than the calculated CJ value. % To 15%. This shows that CJ theory is an approximation theory. In addition, the detonation of explosives actually has a reaction zone with a certain width, and the width of some reaction zones is quite large. Therefore, it is not appropriate to consider the detonation wave as just a strong discontinuity. This shows that the internal structure of detonation waves must be further studied.

The ZND model detonation wave has a two-layer structure: the first layer is a shock wave propelled by supersonic speed, and the next layer is a chemical reaction zone. The shock wave still acts as a strong discontinuity, and the detonating material is instantly compressed to a high temperature and high density state, and then a chemical reaction is started until the end of the reaction zone reaches the CJ state, and the effects of viscosity and heat conduction are ignored in the reaction zone. Figure 2 shows the ZND model (the pressure distribution below). In addition to the equations of state of the initial substances, detonation products, and their mixtures, a reaction rate equation must be established:

In the formula, is the degree of chemical reaction, which is called the reaction progress; t is the time; is the reaction rate, which is a function of the pressure p , temperature T and . In the ZND model, = 0 corresponds to the initial state of the reaction zone, that is, the state after the shock wave; = 1 corresponds to the final state of the reaction zone. The points in the reaction zone are in a state of thermodynamic equilibrium.

From the Rayleigh line, Xugoniu line and the reaction rate equation, the spatial or temporal distribution of each parameter in the reaction zone can be obtained in principle. Little is known about the kinetics of chemical reactions in such rapid processes as detonation.

Detonation Experimental Study

In the 1920s, when C. Campbell and DW Woodhead studied the detonation characteristics of gas mixtures, they first observed the existence of periodic disturbances in detonation waves. When a detonation wave propagates in a circular tube, if a thin layer of silver powder or smoke carbon is coated on the inner wall of the tube, the trace of the spiral line can be observed, so this type of detonation is called a spiral detonation. At first, spiral detonation was thought to be an unstable phenomenon only when it approached critical conditions. However, a large number of experimental studies in the past 20 years found that gas detonation wave fronts have complex three-dimensional structures. In the flat tube, a very regular cell structure can be observed on the plane of the soot layer. A similar structure was observed in the extended spherical detonation. Cell structures similar to gas phase detonations were also found in the detonation of liquid and solid explosives. The observed cell size is 1 to 2 orders of magnitude larger than the width of a normal detonation reaction zone. Experiments show that, in addition to the waves traveling along the direction of detonation wave propagation, there are also weak transverse shock waves that make periodic pulsations. Figure 3 shows the structure of the detonation wave front. Based on the results of a large number of experimental studies so far, it can be assumed that detonation waves generally have an unsteady, non-planar and multi-wave head cell structure. The CJ state is just an average state of macro thermodynamics, it is a comprehensive manifestation of a large number of micro or quasi-micro states. Therefore, although the CJ assumption cannot reflect the complex detonation wave structure, it is still a good approximation. The mechanism of the shear wave is still unknown.

Figure 3 Schematic diagram of the attack wave front structure

Detonation interaction

When the detonation wave interacts with the surrounding medium, a shock or stress wave is generated in the medium, pushing the object to move, causing stratification and fragmentation. Detonation is a high-speed energy conversion method and a means of generating dynamic extra-high pressure. The pressure value can reach 103 Gigapascals. Figure 4 shows the shock insulation lines of some materials and the reflection Xugon Newlines of explosive detonation products. The intersection point of the two determines the initial intensity of the shock wave generated on the surface of the material when the plane detonation wave is striking. If the detonation wave acts on the surface of the material instead of vertically, that is, the detonation wave slips into the incident (Figure 5),

The initial motion direction of the detonation product is parallel to the surface of the material, and the intensity of the shock generated in the material is much smaller. Table 2 shows the comparison of the shock wave pressure of B explosive in two cases.

Shaped charge is the use of high-pressure explosive detonation to deform the metal drug-shaped cover to produce a metal jet moving at a high speed of several kilometers per second. It has a strong penetrating effect and is used in all kinds of armor breaking arms.

Figure 4 Schematic diagram of detonation wave slip incidence

Table 2 Shock wave pressure generated by explosives in contact with explosives

materials

Shock wave pressure (Jippa)

Normal incidence

Slip incidence

copper

iron

aluminum

Plexiglass

water

48.6

47.5

36.0

22.0

19.5

20.5

19.6

18.9

10.5

12.1

Detonation data processing

The detonation process has high pressure (condensed phase detonation can reach the order of 10 Gigapascals), high temperature (103 Kelvin), and short duration (on the order of microseconds), and must have appropriate testing technology; Numerical calculation techniques are increasingly used in detonation research.

Detonation test

The commonly used instruments for detonation testing are high-speed cameras, X-ray flash cameras, high-resolution time measuring instruments and pulse oscilloscopes. Currently: Rotary mirror high-speed cameras with a frame frequency of 2 × 10 ^ 7 frames / second, a scanning speed of 20-60 mm / microsecond, and an image quality resolution of 25 lines / mm; X-ray flash camera with megavolt and flash time of 20 nanoseconds; multi-channel time measuring instrument with time resolution of nanoseconds; . In addition, laser interference velocimetry, laser holography and pulsed laser spectroscopy have also been gradually applied to detonation testing.

Numerical calculation of detonation

The first is the calculation of CJ steady detonation, and the second is the numerical simulation of the initiation process and the interaction between the detonation and the inert medium. A necessary prerequisite in the calculation is to know the equation of state of the detonation products. So far, it is not possible to obtain the equation of state of condensed-phase detonation products only from the molecular structure and basic physical and chemical properties of explosives without relying on detonation data. The BKW state equation, LJD state equation, JWL state equation, and JCZ state equation currently cited are all semi-empirical equations. Nevertheless, the numerical calculation of detonation has provided a lot of useful knowledge, helped to solve many engineering and technical problems, and facilitated the in-depth study of the nature of detonation. For example, the FORTRAN BKW calculation program proposed by CL MADER uses the minimum free energy method to calculate the equilibrium composition of multiple chemical reaction products. It can not only calculate explosives composed of 10 chemical elements, 20 gases, and 5 solids The CJ detonation parameters of the mixed system can also be calculated for the Xugoniu line and the isentropic line, giving a quantitative description that is roughly consistent with the actual situation. In recent years, great progress has been made in the numerical simulation of detonation. One-, two-, and three-dimensional fluid mechanics calculation programs have been developed to simulate the interaction of detonation products with inert media, and interact with chemical kinetic equations and the constitutive of explosive The equation (see the constitutive relationship) and the detonation product state equation are combined to obtain a model of uniform and non-uniform explosive initiation process, which can predict the initiation characteristics of a certain explosive under specific conditions.

Detonation

Detonation can occur in gas-phase, condensed-phase (liquid or solid), and multi-phase (mixed-phase) systems.

Gas phase detonation

At the same pressure, if the initial temperature of the gas mixture increases and the density decreases, the detonation speed decreases; while at the same temperature, if the pressure increases and the density increases, the detonation speed increases. Doping nitrogen or other inert gases in the gas mixture will reduce the detonation velocity and detonation pressure. Table 3 lists the measured detonation parameters of some gas mixtures at room temperature and 1 atmosphere. For gas mixtures, detonation can occur only within a certain concentration range. Outside this concentration range, the same conditions cannot cause detonation. This concentration range is called the detonation limit. Table 4 lists the detonation limits for some gas mixtures. Within the range of the concentration limit, the detonation speed changes with the concentration, and the change of different gas mixtures is different, and some of the maximum or minimum values of the detonation speed occur.

Table 3 Measured detonation parameters of some gas mixtures (288 K, 101325 Pa)

Explosive mixture

Initial density (g / cm)

Explosive heat (kilojoules / kg)

Energy density (kilojoules / meter)

Burst speed (m / s)

Detonation pressure (Mpa)

2H ₂ + O ₂

2CO + O ₂

CH ₄ + 2O ₂

CH ₄ + 2O ₂ + 7.52N ₂

C ₃ H ₅ + 5O ₂

C ₃ H ₅ + 5O ₂ + 18.8N ₂

C ₂ H ₂ + 2.5O ₃ + 9.4N ₂

0.51 × 10

1.24 × 10

1.13 × 10

1.17 × 10

1.44 × 10

1.25 × 10

1.21 × 10

13284

6353

10048

2763

10048

2797

3412

6775

7878

11354

3232

14469

8496

4129

2820

1750

2280

1540

2320

1730

1870

2.07

2.33

3.04

1.57

3.33

1.86

2.01

Table 4 Detonation limits of some gas mixtures

Gas mixture

Detonation limit (percent of fuel)

fuel

Oxidant

Cap

Lower limit

H ₂

C ₂ H ₂

CO + H ₂

O ₂

air

O ₂

air

O ₂

air

18.3

3.5

4.2

17.2

Detonation condensed phase detonation

Detonation in the liquid or solid phase. Condensed-phase detonation systems are often called explosives. The detonation speed of explosives increases with the increase of the charge density, and generally has a linear relationship. Table 5 lists the measured detonation parameters for some liquid and solid explosives. The detonation speed of the explosive also increases with the increase of the diameter of the charge, generally:

Where R is the diameter of the grain; D _R is the detonation velocity when the diameter of the grain is R ; D is the detonation velocity corresponding to an infinite diameter grain; K is a constant. For various explosive columns, there is a minimum diameter, called the critical diameter. For example, the critical diameter of an RDX / TNT (65/35) explosive with a density of 1.71 g / cm is 4 mm without a shell. When the diameter is smaller than the critical diameter, the detonation wave cannot propagate steadily. This is because the side sparse wave (ie, expansion wave) is introduced into the chemical reaction zone during the detonation of the grain. The critical diameters of elementary explosives and mixed explosives decrease with the decrease of the explosive particle size and the increase of the initial density of the charge. Industrially used drug packs are also affected by drug pack diameter and charge density.

Table 5 Measured detonation parameters of some explosives

Explosive name

Code

status

Initial density (g / cm)

Explosive heat * (MJ / kg)

Energy density (kJ / cm)

Burst speed (m / s)

Detonation pressure (Gpa)

Nitromethane

Nitroglycerin

Trinitrotoluene

2,4,6-trinitrobenzidine (tetrad)

Pentaerythritol tetranitrate (Taian)

1,3,5-trinitro-1,3,5-triazacyclohexane (melanin)

1,3,5,7-tetranitro-1,3,5,7-tetraazaoctane (Octolin)

TNT

Tetryl

PETN

RDX

HMX

liquid

solid

1.135

1.60

1.634

1.714

1.77

1.767

1.890

4.44

6.19 (calculated value)

4.27

4.56

5.73

5.94

5.73

5.04

9.90

6.96

7.82

10.14

10.50

10.83

6320

7700

6930

7640

8290

8700

9110

13.0

25.3

19.1

26.8

34.0

33.8

38.7 (calculated)

Multi-phase detonation

Including detonation in gas, liquid, solid two-phase systems or three-phase systems. For example, aerosols mixed with liquid fuel and air, coal powder, and metal powder and air mixtures can all detonate under certain conditions. The so-called fuel air explosive (FAE) is a two-phase detonation system. Ethylene oxide or other fuels are dispersed in the air by an explosive method to form clouds, and then a detonation device is used to detonate the clouds. Detonation waves and shock waves formed in the air can have a sufficiently strong destructive effect. This principle has been put to practical use in weapons. Detonation of a mixture of metal powder, pulverized coal, and grain powder equivalent to air often causes great disasters and is an important subject in safety research.

Detonation engine

For gasoline engines, when the mixture (full mixture of air and fuel) enters the combustion chamber during the intake stroke, the piston compresses it during the compression stroke. After the spark plug ignites the high-pressure mixture, the pressure generated by its combustion is converted into Engine power.

The main causes of knocking are as follows

First, the ignition angle is too early: In order to make the piston get the power immediately after entering the power stroke after the compression top dead center , the ignition is usually advanced before the piston reaches the top dead center (because it takes a period of time from ignition to complete combustion). However, too early ignition will cause most of the oil and gas to be burned when the piston is still in the compression stroke. At this time, the unburned oil and gas will undergo extreme pressure to spontaneously ignite, causing knocking. 2. Excessive carbon deposits in the engine : Excessive carbon deposits in the combustion chamber of the engine will not only increase the compression ratio (high pressure), but also generate high temperature hot spots on the surface of the carbon deposits, causing the engine to knock. Third, the engine temperature is too high: the engine is too hot, the intake air temperature is too high, or the engine cooling water circulation is poor, will cause the engine high temperature and knock. Fourth, the air-fuel ratio is incorrect: an excessively lean fuel-air mixture ratio will increase the combustion temperature, and an increase in the combustion temperature will cause the engine temperature to increase, and of course, it is prone to knocking. Fifth, the fuel octane value is too low: the octane number is an indicator of fuel anti-knock. The higher the octane number, the stronger the anti-knock performance. An engine with a high compression ratio has a high pressure in the combustion chamber. If fuel with low knock resistance is used, knocking is likely to occur.

How to know the impact of knock and knock

Knocking in English is Detonation, which means knocking, so the engine will knock when knocking. The slightly discontinuous knock sound is quite crisp, a bit similar to tapping the triangle iron. During severe and continuous knocks, the engine will make a "mile-mile-mile" sound, and the engine will be obviously weak at this time. At present, in order to squeeze the engine's maximum performance and reduce fuel consumption, many car manufacturers usually set the ignition angle of the common speed range in advance, so some engines will inevitably have a slight explosion when the load is between 2000 and 3000 rpm. Shock, however, a slight knock will not have much impact on the engine, and the owner should not worry too much. However, if there is a knock due to a problem with the engine, such as severe carbon deposition or poor heat dissipation, this knock is usually very serious. If a continuous and severe knock occurs at high speed and high load, it will take less than one minute to The spark plug and piston will be melted, and even the cylinder and engine body will blow through severely.

Knock sensor

The fastest and most effective way to suppress knocking is to delay the ignition advance angle and reduce the combustion pressure. Therefore, the operating principle of the knock sensor is that when the engine knock is detected, the ignition advance angle is delayed to the ignition timing that will not be knocked. When the engine does not knock, the ignition is slowly restored in advance. . The knock sensor uses an acceleration sensor to measure the acceleration change of the engine, that is, vibration. When the engineer adjusts the knock sensor, the knock vibration mode is written into the ECU. Once the knock sensor detects the vibration mode, the ECU determines the engine knock, and then delays the ignition advance angle. At present, more advanced knock sensors can even determine which cylinder is knocking, and individually retard the ignition advance angle for this cylinder.