The paper considers the processes taking place in the liquid phase: nucleation in liquids and liquid metals, electrical explosion of conductors and spark cavitation. It is established that all these processes are well described by the basic equation of cavitation, which is solved numerically by the Runge-Kutta method. For this purpose, a program in the Fortran programming language has been created, and a method for determining the time of appearance of cavitation nuclei by the method of numerical integration has been described. A mathematical model of homogeneous nucleation in the liquid phase was created. With the help of the created model, such parameters as the time of appearance of a cavitation bubble for various frequencies of external influence were calculated. The maximum amplitude and period of the natural oscillations of a bubble at various frequencies.
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In: K. E. Samouylov, L. A. Sevastianov, D. S. Kulyabov (eds.): Selected Papers of the IX Conference “Information and Telecommunication Technologies and Mathematical Modeling of High-Tech Systems”, Moscow, Russia, 19-Apr-2019, published at http://ceur-ws.org
1.
The theory of thermodynamic stability was developed by Gibbs in the last century.
Thermodynamic stability of a system is understood as the equilibrium of a system of
relatively small changes in its thermodynamic parameters, such as volume, pressure,
temperature, etc. For thermodynamic equilibrium of a system, it is necessary that its
internal energy be minimal. The condition of the positivity of the value of the second
derivative of the internal energy [
The boundary of the thermodynamic stability of the phase is the spinodal. The
position of the spinodal can be calculated from the thermal equation of state for liquids
and gases, the simplest of which is the Van der Waals equation. Spinodal consists
of two branches: steam and liquid. The spinodal of the vapor phase determines the
vapor saturation limit. The spinodal of the liquid phase determines the boundary of the
thermodynamic stability of the liquid [
Between the binodal (line of phase equilibrium) and the spinodal of the liquid phase lies the region of the metastable liquid, overheated or stretched. From the point of view of thermodynamics, a superheated liquid is essentially no diferent from a stretched one.
In the region of a metastable liquid, cavities filled with steam, gas, or their mixture (cavities) appear in the process of heating a liquid at constant pressure or in the process of decreasing pressure at a constant temperature.
The process of the formation and development of bubbles depends on the state of the liquid, including the presence of solid or gaseous impurities in it, and on the pressure in the liquid.
Thus, for nucleation in a liquid it must be stretched to a certain pressure, not
exceeding in absolute value the limits of the thermodynamic stability [
2.
The theory of nucleation was proposed in the works of V.P. Skripov and students [
Later it was shown that the implementation of unstable states is also possible for
liquid metals, which follows from experiments with an electric explosion of conductors [
Overheating cannot be arbitrarily large, since there is a boundary between the
thermodynamic stability of the phase and the spinodal. When approaching the spinodal,
the fluctuation mechanism of nucleation or homogeneous nucleation comes into play,
which ensures the rapid disintegration of the metastable phase [
Experiments have shown [
With this heat, the mass of metal evaporated through the surface the conductor and through the surface of vapor nuclei arising on the ready-made centers is insignificant, so the conductor remains in a liquid state up to the vicinity of the spinodal.
Overheating of a thermodynamically stable liquid above the spinodal point is impossible, since when approaching this point, an explosive boiling mechanism comes into play, caused by a high frequency of homogeneous nucleation of vapor nuclei.
The calculation shows that when approaching the spinodal, the frequency of homogeneous nucleation increases by 28 orders of magnitude, which ensures the explosive boiling of the superheated liquid metal. This process is the main factor determining the electrical explosion of conductors when they are heated by a microsecond current pulse. 4.
the cathode.
[
This phenomenon is observed in a vacuum diode with a cathode in the form of an edge and a flat anode. When a voltage pulse is applied to the diode, electrons are ejected from the tip through a tunneling mechanism; this phenomenon is called field emission. As the voltage pulse increases, the autoelectronic current increases and, when its density reaches a certain limit value, the tip of the tip explodes, which leads to an increase in the emission current 10 - 100 times. The process of microexplosions can be repeated many times, so as after the explosion of this tip, new microprotrusions are formed on
The electrical explosion mechanism of the tip is similar to the electrical explosion of
conductors; it is determined by the phase explosion of the superheated liquid metal [
In the case of an explosion of the edges, the probability of reaching unstable states of the liquid phase and its spinodal decomposition is greater than with the explosion of
A shock wave arises in the liquid, and a cavity filled with metal vapor forms around the microprotrusion. The cavity expands to a maximum radius, after which it makes damped oscillations.
With each compression of the cavity, the gas contained in it is heated. Such a
mechanism for the development of the spark cavitation process is confirmed by experimental studies.
Thus, if the developed interpretation of the observed efect in the process of spark
cavitation is fair, then with spherical cumulation of a suficiently strong shock wave in
the center of the spherical cavity in a certain short period of time, we should expect the
realization of the extreme state of matter [
5.
It can be said that all the processes described above, namely nucleation in liquids and in liquid metals, electrical conductor explosion and spark cavitation can be represented by the same model of homogeneous nucleation in the liquid phase.
We have created a mathematical model of homogeneous nucleation in the liquid
phase. It applies to all the processes described here and has already been used by us in
the case of spark cavitation [
The model is built on the basis of an equation describing the dynamics of a cavitation bubble: + 2 3 _2 = 1 [︃(︂ + 2 )︂ (︂ 0 0 )︂ 3 − 2 − 0 + (, 1) , ]︃ (1)
Here: 0 - radius of the nucleus at = 0; -radius of the nucleus at the next time instant ; -density of a liquid; - surface tension of the fluid; = 1 - adiabatic index for steam in the bud; - hydrostatic pressure in a liquid ( = ); - acceleration of the cavity wall; _ is the speed of movement of the cavity wall; 2 0 - Laplace pressure; 0 - amplitude of oscillations of the cavity, 1− the time of appearance of the first germ of homogeneous cavitation.
Time 1 is determined from the condition · 1 ∫︁ 0 exp ︂( )︂ 2 = 1.
Taking into account the geometric meaning of a definite integral (2), one can determine the point 1, by numerical method knowing that the area of the figure bounded by the function () on the interval [0, 1] should be equal to 1.
Part of the program of numerical integration for finding the time 1 is given below (see Listing 1). The program is written in a programming language Fortran.
Listing 1: Program for the numerical determination of the stretching time 1 print ∗ , ’ 1) ␣The␣ frequency ␣ o f ␣ the ␣ e x t e r n a l ␣ f i e l d ␣ (Hz) ; ’ print ∗ , ’ 2) ␣Molar␣mass␣ o f ␣ substance ␣mu␣ ( kg␣/␣mol ) ; ’ print ∗ , ’ 3) ␣ I n t e g r a t i o n ␣ step ␣ dt ␣ ( s ) . ’ read ( ∗ , ∗) aniu , amu , dt close ( 1 ) open ( 2 , f i l e = ’ amplit . tx t ’ ) write ( 2 , ∗) aniu , amu , dt 9 continue open ( 3 , f i l e = ’ t1− r e s . dat ’ ) open ( 4 , f i l e = ’ water − 1. dat ’ ) print ∗ , ’ ’ write ( ∗ , ∗) ’T␣A␣B␣L␣ I ␣C␣A ’ print ∗ , ’ ’ write ( ∗ , ∗) ’ External ␣ f i e l d ␣ frequency ␣= ’ , aniu , ’Hz ’ write ( ∗ , ∗) ’ temperature− time ␣ s t r e s s − pressure − c r i t . ␣ r a d i u s ’ write ( ∗ , ∗) ’T, ␣ grade . ␣C− t1 , ␣mks− P1 , ␣MPa− Ro , ␣nm ’ ak =1.3806581212 e− 23 ana =6.02213673636 e23 pi =3.141592654 w=2∗ pi ∗ aniu expo =2.7182818284590459 1 continue read ( 4 , ∗ ) tt , s i g , r o l , rov , pb , pa i f ( t t ) 13 ,13 ,12 12 b=( r o l / rov ) ∗ sqrt (2∗ s i g ∗ana /(amu∗ pi ) ) an=ana∗ r o l /amu akk=an ∗ 1 . 0 e − 06∗b a l =16∗ pi ∗( s i g ∗∗3) /(3∗ ak∗ t t ∗(1− rov / r o l ) ∗∗2) tau=pi /(2∗w) t0 =1.0e− 15 t2=tau ∗5 a i=0 aint=0 a l e v 1 =1/akk do 2 t=t0 , t2 , dt ptt=pa∗ sin (w∗ t ) pok=(− 1∗ a l ) /( ptt ∗ ptt ) pr=expo ∗∗pok aint=aint+pr ∗ dt r=aint− a l e v 1 k=k+1 i f ( r ) 7 ,7 ,8
p1=pb− ptt TEM=tt − 273.15 t1=t ∗ 1 . 0 e6 p11=p1 / 1 . 0 e6 aro =(2∗ s i g ) / ptt aro1=aro /(1. − rov / r o l )
We have created a program for the numerical solution of the cavitation equation in the Fortran programming language. It work is based on the Runge-Kutta method. The block diagram of the program is shown in the figure 1.
Initially, the main program asks for the values of external parameters, such as fluid temperature, oscillation frequency, and others. Then the main program refers to an array of tabular data for the values of surface tension, fluid viscosity, fluid pressure, vapor pressure at a given temperature. These tabular data are discrete values and do not always correspond to a given temperature. Therefore, the main program refers to auxiliary subroutine 1, which approximates or extrapolates the table data to a given point.
To calculate parameters such as the initial radius of the cavity, the pressure at which the first cavitation nucleus appears, the initial phase of external oscillations, the main program refers to subroutine 2, which calculates these values based on the data already calculated by subroutine 1.
Subroutine 3 then receives from subprogram 2 a task to calculate the time 1 during which the first cavitation nucleus appears in the fluid. The required tabular data is requested from subroutine 1. The result of the calculation is reported to the main program.
Having collected all the necessary data, the main program calculates the basic cavitation equation for the maximum amplitude of oscillations of the cavity.
Below is a part of the main program (see Listing 2) for the numerical solution of this system of equations, written in the programming language Fortran: C C C C
Listing 2: Program for the numerical solution of the equation (1) C C 2 1 5 4
Thus, in this paper we consider the processes taking place in the liquid phase, such as nucleation in liquids, nucleation in liquid metals, electrical explosion of conductors and spark cavitation.
Stretching time 1, mks Temperature, 0 = 100kHz = 250kHz = 600kHz 250 300 350 373
It is established that all the mentioned processes are well described by the basic equation of cavitation.
The indicated equation is solved numerically by the Runge-Kutta method. For this purpose, a program in the Fortran programming language has been created, a scheme of its work has been presented, and a method has been described for determining the time of appearance of cavitation nuclei using the numerical integration method.
The stretching time 1 for diferent frequencies of external influence is presented in Table 1, the maximum amplitude and period of natural oscillations of a bubble at diferent frequencies are given in Table 2.
The publication has been prepared with the support of the “RUDN University Program 5-100”.