Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology Discussion of the Problem on Designing the Global Database for Different Kinds of Quenchants NIKOLAI KOBASKO IQ Technologies Inc, Akron, USA and Intensive Technologies Ltd, Kyiv, Ukraine [email protected] Abstract: To make computer simulation for heat treating industry possible, engineers need a database for cooling capacity of different kinds of quenchants. Unfortunately, there is no such database available for engineers and computer programmers yet. As a rule, investigators use standard Inconel 600 probe or silver probe with one thermocouple at the core to measure cooling capacity of quenchants. It is shown that Inconel 600 probe can provide average effective heat transfer coefficients. Silver probes can be used to measure the heat transfer coefficients during full film boiling and to measure critical heat flux densities. It is shown that critical heat flux densities must be measured to predict heat transfer modes during quenching and optimize the process of cooling. It is stated that heat transfer coefficients should be calculated on the basis of solving inverse problem based on testing real steel parts with thermocouples near the surface. The possibility of use the first kind of boundary condition in the paper is widely discussed. Such approach simplifies significantly calculations and can be combined with the third kind of boundary condition. Key – Words: - Quenchants, Cooling capacity, Initial and critical heat flux densities, Shock and nucleate boiling, Global database, Designing. boiling point, immediately. 1 Introduction During quenching of steel parts four heat transfer modes can be observed, which have been investigated by Prof. Tensi and other scientists [1, 2]. The first mode is characterized by moving film blanket along the surface of steel part. Film and nucleate boiling, and also convection can act simultaneously on the surface (see Fig. 1). The second mode is characterized by simultaneous transition from film boiling to nucleate boiling and then to convection. The third mode is characterized by existing the local film boiling on the surface of steel parts. The fourth mode differs from mentioned above modes by periodical appearing and disappearing of film boiling (see Fig. 2). The first mode was investigated by many authors [1, 2, 4, 5]. In this paper the second mode is considered and widely discussed. During quenching of steel parts the process of the second mode can be developed by two scenarios (see Fig. 3). When hot steel part is immersed into cold liquid, the convection takes place initially because there are no bubbles yet. When liquid in the boundary layer is heated to ISBN: 978-960-474-268-4 then boiling process starts Fig. 1 The first type of heat transfer during quenching in water at 60oC [1]. Fig. 2 Periodical changes of heat flux density during quenching of a cylindrical probe in some polymer water solutions [3]. 117 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology Fig. 4 Typical stages of cooling, shown as the logarithm of temperature difference against time: 1, when full film boiling is eliminated; 2, when full film boiling exists. Fig. 3 Two possible boiling processes that may occur during quenching, depending on critical heat flux densities. Unfortunately, it is impossible to investigate shock boiling and measure real heat transfer coefficients by use standard probe since shock boiling is extremely short and standard probe doesn’t provide surface temperature during quenching which is needed to calculate heat transfer coefficients. However, standard probe allows to measure average effective heat transfer coefficients. The authors [7, 8] developed a method for measuring average heat transfer coefficients and made thousands of experiments to measure duration of nucleate boiling process on the basis of noise control system [6]. Some results of experiments are provided in Table 1. As one can see from Table 1, the average generalized Biot number BiV is almost the same against the size of samples. For example, when diameter of probe changes from 6 mm to 40 mm, average generalized Biot number remains equal to average value 1.05. Duration of nucleate boiling process depends significantly on diameter of probe. The transition temperature Ttr also depends on size of the probe. It should be noted that during developed transient nucleate boiling process surface temperature changes insignificantly, i.e. Tsf = TS + ∆ξ ≈ const . Here Tsf is the surface Due to very high initial heat flux density, the thousands of tiny vapor nucleus appear which oscillate at a high frequency and make noise which can be measured by noise control system [6]. This is so called shock boiling which follows after “convection” (see Fig. 3). When at the end of shock boiling the heat flux density is less than the first critical heat flux, film boiling is absent and tiny vapor nucleus grow and start to produce vapor babbles which departure with frequency 67 – 76 Hz when cooling by water or water salt solutions. In the first scenario the film boiling is absent. In this case initial heat flux density is less than the first critical heat flux density. The second scenario consideres the passway when the initial heat flux density is higher as compared with the first critical heat flux. The transient full film boiling appears first and then nucleate boiling switches by film boiling which in due time passes to convection (Fig. 3). During nucleate boiling process the surface temperature of steel parts mantains at the level of boiling point. Many theoretical investigations and many experimental data consider the process of quenching like a heat transfer problem with the third kind of boundary condition. For instance, Grossmann factor H is used in heat treating industry [1, 2, 7]. The standard cylindrical probe 12.5 mm in diameter was developed and instrumented with one thermocouple at the core to measure cooling rate when quenching it in different quenchants (standarsd ASTM D6200). ISBN: 978-960-474-268-4 temperature during transient nucleate boiling process; TS is the saturation temperature (the boiling point of liquid); ∆ξ is average overheat of liquid in a boundary layer which causes the boiling process. The average overheat can be measured as ∆ξ = ϑI + ϑII 2 (see Table 2). It means that in many cases the boundary condition of the first kind can be 118 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology used instead of nonlinear third kind of boundary condition. transfer coefficient during nucleate boiling can be Table 1 Generalized Biot number (BiV,), Kondratjev number (Kn), and temperature transition from boiling to convection versus size of cylindrical probes. where ∆T is overheat; 3< m< 10/3. evaluated by equation [8, 9, 10]: α nb = C∆T m −1 Ttr ,o C Dia, mm 6 20 30 40 BiV 1.07 1.1 1.15 1.2 Kn 0.56 0.565 0.58 0.59 150 180 210 220 Table 2 Overheat of the boundary layer at the beginning of developed nucleate boiling ( ϑI ) , at the end of boiling ( ϑII ), and duration of nucleate boiling Fig. 5 Liscic-Nanmac steel probes: (a) first version of probe [1]; (b) accurate thermocouple furnishings; (c) second version of probe [2]. process ( τ nb ) versus diameter of cylindrical probe. Diameter of ϑI , oC cylinder, mm 6 14.3 20 9.96 30 8.82 40 8.1 ϑII , oC τ nb , s 3.66 3.66 3.66 3.66 1.40 11.9 24.0 39 Liscic – Nanmac probe can be used to design global database. However, the problem remains opened since it is not enough clear how to transfer data received by Liscic – Nanmac probe to the very big steel parts like rollers, rotors or discs of turbines. This problem is discussed below. In contrast to standard probe, Liscic – Nanmac probe (see Fig.5) allows to investigate accurately boiling processes and measure heat flux densities, heat transfer coefficients versus surface temperature or time. As is well known, real heat transfer coefficient during transient nucleate boiling process changes significantly with changing of boundary layer overheat and doesn’t depend on size of probe and thermal properties when overheat is fixed [9, 10]. In Table 1 average effective heat transfer coefficients are provided which simplify calculations. Here effective heat transfer coefficients are considered because heat treating industry deals with the effective heat transfer coefficients, and rarely real heat transfer coefficients are used. To be on one page, one must analyze both real heat transfer coefficients (HTC) and effective HTC. The real heat ISBN: 978-960-474-268-4 2 Mathematical model for transient nucleate boiling process investigation For understanding processes of cooling connected with steel quenching let us firstly consider cooling of classical bodies like plate, cylinder and sphere. The differential equation of heat conductivity at symmetric statement of the problem for solids has the form ∂ 2T (r ,τ ) j − 1 ∂T (r , τ ) ∂T (r , τ ) (1) = a + 2 ∂τ r ∂r ∂r ( τ > 0; 0 < r < R j = 1,2,3 for a plate, cylinder and sphere correspondingly) with the boundary conditions (2), initial conditions (3) and condition of symmetry (4): 119 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology ∂T β m + (T − TS ) m = 0 λ ∂r r =R ; T (r ,0) = T0 ; ∂T (0, τ ) =0. ∂r Table 3 Kondratjev form coefficients K = (3) bodies of a simple configuration (results of analytical calculations) (4) Shape of steel part An approximate analytical solution for mathematical model (1) – (4), in the case of regular thermal condition, was received by authors in 1979 [11] which is very simple and can be written as: τ − τ ir = ϑ 2mR 2 ln Ι . ( j + 1)( j + 5) a ϑ This solution is true for nucleate boiling process when within the interval ϑI - - ϑII surface temperature maintains at the level of boiling point and changes very slowly with the time (see Table 2). For example, during quenching of cylinder (40 mm in diameter ) surface temperature changes from 108.1oC to 103.7oC during 39 seconds. In many cases engineers and scientists, who work in heat treating industry, consider the process of quenching as the process which should be governed by heat conductivity Eq (1) and third kind of boundary condition (6): 2.467 Cylinder of radius R R2 5.784 Square prism with equal sides of L 2R 2 5.783 4.935 π2 4R 2 3π 2 7.403 R2 9.87 π2 According to the universal correlation Kn = ψBiV , generalized Biot number also remains the same with the size changes of the samples [12], i.e. B iV = idem . (9) For a cylinder, when j = 2, Kondratjev number Kn (7) is equal Kn = Here K is Kondratjev form factor [12] which for different shapes of steel parts is provided in Table 3. By equating (5) and (7) and taking into account Table 3, one can receive 2 b π2 Sphere (6) K ϑ ln Ι . aKn ϑ R2 , m2 b 4R2 Cube Consume that it can be done, then, according to the regular thermal condition theory [12], it is another equation true: τ − τ ir = K= Slab (5) ∂T α ∂r + λ (T − Tm ) = 0 . r =R R2 for b (2) 3, 21 = 0.545; for a sphere, when j = 2mb Kondratjev number Kn is equal 16 Kn = = 0.487 . Taking into account universal mb correlation [12] 2 R 2mR = abKn ( j + 1)( j + 5)a Kn = or (Bi V Kn = ( j + 1)( j + 5) = idem . 2mb ISBN: 978-960-474-268-4 BiV 2 ) + 1.437BiV + 1 0.5 , one can evaluate generalized effective Biot numbers for cylinder, and sphere which are equal correspondently 1.05 and (8) 120 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology 0.84. Calculations coincide well experiments presented in Table 1. with the 3 Inverse problem in quenching and boiling processes investigation To understand the transient nucleate boiling process more deeply, let’s consider accurate experiments of French [13] which are presented in Table 4 below. Fig. 6 Depiction of how thermocouples were arranged and accurately flattened to the wall of spheres and polished by French [13]. Fig. 7 Heat flux density vs. time during immersion of heated spherical steel samples (875oC) into 5% water solution of NaOH at 20 oC: a) - 6.35mm; b) 12.7 mm; c) - 25.4 mm. Table 4 Cooling time in seconds of spherical steel probes cooled from 875oC in 5% NaOH water solution at 20oC (moving at 0.914 m/s), according to French [13]. Dia, mm 6.35 12.7 25.4 63.5 120 180 500oC 0.043 0.058 0.055 0.065 0.09 0.10 150oC 0.69 0.60 0.82 0.59 0.95 1.15 As we can see from Table 4, surface temperature of samples drops from 875oC to 150oC for less than 1 second and this time is almost the same for different diameters. More information and more experimental data are provided in publications of French [13]. Using these experimental data and our software IQLab [14], the initial heat flux densities, HTC , and Kn were calculated which are presented in Fig. 7, Fig. 8, Fig. 9, and Fig. 10. ISBN: 978-960-474-268-4 Fig. 8 Heat flux density vs. time during immersion of heated spherical steel samples ( 875oC) into 5% water solution of NaOH at 20 oC: a) – 63.5mm; b) 120 mm; c) - 180 mm. 121 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology Table 5 Coefficients Ω depending on properties of quenchants at 20 oC [15] As one can see from Fig. 10 , the average Kondratjev number Kn is equal to 0.15 + 0.95 = 0.55 . In this case generalized 2 Quenchant Ω Kn = Water, 20oC 4.17 30-50% CaCl2 4.78 5 – 12% NaOH 3.6 Biot number BiV is equal to 1.05 that agrees well with Table 1. Also author [7] reported that average Kn, during quenching of cylindrical specimens in 12% NaOH water solutions, was 1.1 when their temperature was 20oC . 6 – 8% NaNO3 3.76 5 Duration of transient nucleate boiling process and its characteristics Note: Initial temperature is fixed at 850oC The transient nucleate boiling and self – regulated thermal processes were investigated since 1968 and their results were published in [7]. Duration of self – regulated thermal process differs insignificantly from the time of transient nucleate boiling (within 0.5 – 1 second). The notion of self- regulated thermal process was proposed in 1968 [7] and it means that wall temperature of steel part is kept at the level of saturation point varying insignificantly. The real heat transfer coefficients during nucleate boiling process are very high and can reach 200 – 250 kW/m2K (see Fig. 9). The effective Kondratjev number Kn is linear function of Fourier number during transient nucleate boiling process (Fig. 10). The equation for determining the duration of transient nucleate boiling (self-regulated thermal process) was firstly received by generalization of experimental data and then derived from the analytical equation and has the form [15]: Fig. 9 Shock and nucleate boiling heat transfer coefficients versus time for a sphere of 38.1 mm in diameter quenched from 875oC in a 5 % aqueous NaOH solution at 20oC [15]. τ nb = Ω k F kW D2 , a (9) where value Ω depends on initial temperature of a steel part and condition of cooling. For initial temperatures 850oC it can be within 3.6 – 4.17 (see Table 5) [15]. As known, before quenching in liquid media, steel parts are heated to high temperature 800oC – 900oC. Coefficient k F depends Fig. 10 Kondratjev number Kn versus Fourier number Fo suitable for cylinders 20, 30, and 40 mm quenched in 5% water alkaline solution [15]. ISBN: 978-960-474-268-4 on configuration of steel part. For plate- shaped form 122 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology From the above considerations follow that there is no universal correlation between heat transfer coefficient and surface temperature of the tested sample. At least three curves can exist depending on value of the critical heat flux density. For instance, in some cases during quenching film boiling can prevail. During intensive quenching film boiling is completely eliminated. And also pure convection can prevail [8]. And also three modes can exist simultaneously on the surface. k F = 0.1013; for cylinder – shaped form k F = 0.0432; for spherical – shaped form k F = 0.0253; kW is dimensionless coefficient which depends on liquid flow velocity. For motionless liquid kW = 1. For high flow velocity of liquid which prevents nucleate boiling kW = 0. That is why for different condition we have 0 ≤ kW ≤ 1. D is thickness of the component: diameter of cylinder, sphere or thickness of the plate; a is thermal diffusivity of a material. Using the average results of experiments from Table 6 Discussion 0 1 (BiV = 0.92, Kn = 0.516, and Ttr = 190 C ) and Cooling capacity of quenchants should be characterized by many parameters which includes critical heat flux densities, initial heat flux densities, heat transfer coefficients, and boiling temperature of vaporizable liquids [16, 17]. To predict what kind of heat transfer mode will appear during immersion of steel parts into liquid, initial heat flux densities should be compared with the first critical heat flux which is one of the most important parameter of the quenchant (see Table 6). generalized equation (10) , we obtain Eq (11): 2 BiV T −T K , + ln 0 m T − Tm aKn 2.095 + 3.867 BiV τ = (10) 2 850 − 20 D 190 − 20 a , τ nb = 0.0432 ×1.94 0.33 + ln τ nb = 3.72k F kW D2 , a (11) Table 6 The first critical heat flux density for water and water salts and alkaline solutions at 20oC [8] Quenchant at 20oC Critical heat flux densities, qcr1 (MW/m2) 12 % NaCl water 13 solution 5 % NaOH water 15 solution 12 % NaOH water 15 - 16 solution Water 6.9 – 7.0 Investigations show that during quenching of steel samples in 5-12% alkaline and salt water solutions film boiling is absent; and surface temperature drops very rapidly almost to boiling temperature and then maintains at the level of boiling point. The equation (9) is proposed for calculating duration of transient nucleate boiling process suitable for the second type of heat transfer mode. When convection increases significantly, duration of nucleate boiling process where kF = 0.0432; Ω = 3.72 . 850 − 20 Ω = 1.94 × 0.33 + ln = 3.72 , 190 − 20 1 D2 = 1.94 . K= , Kn 23.132 0 K = k F D 2 = 0.0432D 2 ; Ttr = 190 C . The average transition temperature from nucleate boiling to convection (Ttr) is taken from Table 1. As we can see, Eq (11) is similar to Eq (9). This equation was received early analytically by author [15] which is widely used at intensive quenching technologies designing and recipes development. Note that BiV = 0.92 when Kn = 0.516. That is why for cylinders 2 BiV . For our particular case kW = 1 . = 0.33 2.095 + 3.867 BiV ISBN: 978-960-474-268-4 123 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology calculations. There is a great interest to solve inverse problem for hyperbolic heat conductivity equation and use it for shock boiling investigation. The new methods of solving of such inverse problem are discussed in literature [18 – 21]. Unfortunately, there is no appropriate DATABASE for cooling capacity of quenchants to be widely used by engineers and metallurgists which could accelerate solving environment problem because optimizing the process of quenching in many cases leads to switching of oils with plain water or neutral water salt solutions. decreases and in some cases convection can prevail. In this case just existing dimensionless equation can be used to calculate heat transfer coefficients during convection. If transient nucleate boiling prevails, the first kind of boundary condition can be used, taking into account that temperature maintains at the boiling point. The full film boiling, and local film boiling should be eliminated because full film boiling doesn’t provide suitable hardness and high compressive residual stresses at the surface of steel parts. The local film boiling causes the high distortion of steel parts during quenching. That is why the critical heat flux densities should be measured and included into global database to serve engineers and metallurgists in heat treating technologies development. Especially important are investigations connected with the initial period of cooling where shock boiling takes place. Shock boiling can increase the first critical heat flux density due to creation of thousands of tiny bubbles which remove very quickly heat from the steel part surface. Fig. 7 supports this point of view because during quenching of small spherical probes in 5% alkaline water solution film boiling is absent in spite of high initial heat flux which is equal to 23.5 MW/m2 (see Fig. 7). It means that the first critical heat flux density during shock boiling should be at least 25 MW/m2. Table 6 provides critical heat flux densities received by conventional method of measurement. Summary 1. Cooling capacity of quenchants should include critical and initial heat flux densities, heat transfer coefficients, and boiling temperature of vaporizable liquids. 2. There is a big difference between real heat transfer coefficient and effective heat transfer coefficient which should be taken into account during computer simulation. 3. When initial heat flux density exceeds the first critical heat flux, full film boiling starts. When initial heat flux is less than qcr1, film boiling is absent. In this case surface temperature of steel parts drops rapidly almost to saturation temperature and a rather long time maintains at the level of boiling point of liquid. 4. A correlation for evaluating duration of transient nucleate boiling process is analyzed and explained from different points of view. 5. If film boiling is absent, the first kind of boundary condition can be used instead of third kind of boundary condition using proposed correlation. 6. For intensive quenching processes, when convection prevails, existing dimensionless correlations can be used for evaluation HTC. 7. Developed approach significantly simplifies computer simulations. With increase the size of probes the initial heat flux decreases and becomes less than the first conventional critical heat flux density (see Fig. 8 and Table 6). This comparison shows that it is easy to eliminate film boiling when quenching large steel parts. It should be noted that real heat transfer coefficients are evaluated as a ratio q/(Tsf – Ts) and effective heat transfer coefficients as q/(Tsf – Tm). Since Tsf – Tm >> Tsf – Ts , real heat transfer coefficients have a very high value (see Fig. 9). As one can see, there is a big difference between real heat transfer coefficient and effective heat transfer coefficient which should be taken into account during generalization results of experiments and ISBN: 978-960-474-268-4 124 Recent Advances in Fluid Mechanics, Heat & Mass Transfer and Biology [12] Kondratjev, G. M., Regulyarnyi Teplovoy Rezhim (Regular thermal mode), Gostekhizdat, Moscow, 1954. [13] H.J.French, The Quenching of Steels, American Society for Steel Treating, Cleveland, OH, 1930. [14] V.V.Dobryvechir, N.I.Kobasko, E.N.Zotov, W.S.Morhuniuk, Yu.S.Sergeyev, Software IQLab, ITL, Kyiv, Ukraine, www.itl.kiev.ua [15] N.I. Kobasko, Transient Nucleate Boiling as a Low of Nature and a Basis for Designing of IQ Technologies, Proc. of the 7th IASME/WSEAS Int. Conference on Heat Transfer, Thermal Engineering and Environment (THE’09), Moscow, Aug. 20 – 22, 2009, pp. 67 – 76. [16] N.I. Kobasko, Current State of the Problem and Principal Criteria to Evaluating the Cooling Capacity of Quenching Media, Metal Science and Heat Treatment, Vol. 38, Nos. 1–2, 1996, pp.49–55. [17] B. Liscic, Critical Heat-Flux Densities, Quenching Intensity and Heat Extraction Dynamics During Quenching in Vaporizable Liquids, Proceedings of the 4th International Conference on Quenching and the Control of Distortion, Beijing, May 20–23, 2003, pp. 21– 28. [18] Sh. E. Guseynov, Methods of the Solution of Some Linear and Nonlinear Mathematical Physics Inverse Problems, Doctoral thesis, University of Latvia, Riga, 2003. [19] Sh.E.Guseynov, N.I.Kobasko, On One Nonlinear Mathematical Model for Intensive Steel Quenching and its Analytical Solution in Closed Form, Proc. of the 5th WSEAS Int. Conf. on Heat and Mass Transfer (HMT'08) , Acapulco, Mexico, January 25-27, 2008. [20] N.I. Kobasko, and Sh. E. Guseynov, Initial Heat Flux Densities and Duration of Nonstationary Nucleate Boiling During Quenching, Proceedings of the 5th WSEAS International Conference on Heat and Mass Transfer (HMT ’08), Acapulco, Mexico, January 25–27, 2008, pp. 104–109. [21] T. Bobinska, M.Buike, A.Buikis, Hyperbolic Heat Equation as Mathematical Model for Steel Quenching of L-Shape Samples, Part 2. (Inverse Problem), Proceedings of the 5th IASME/WSEAS Int. Conf. on Continuum Mechanics, Fluids, Heat. Cambridge, UK, WSEAS Press, 2010, pp. 21-26. References: [1] B.Liscic, H.M.Tensi, W.Luty, Theory and Technology of Quenching, Springer - Verlag, Berlin, New York, 1992, 484 p. [2] B.Liscic, H.M.Tensi, L.C.F. Canale, G.E.Totten, Quenching Theory and Technology, Second Edition, CRC Press, New York, 2010, 709 p. F.A.Krivoshei, On the [3] N.I.Kobasko, Mechanism of Temperature and Heat Flow Oscillations in Cooling Metallic Specimens in Aqueous Solutions of Polymers, Dokl. Akad. Nauk Ukr., No. 11, 1994, pp. 90–94. [4] N.I.Kobasko, N.P.Timchenko, Cinematographic investigation of the cooling process of alloys in aqueous polymeric solutions, Metal Science and Heat Treatment, 28 (10), 1986, pp. 725 – 729.N. [5] H. J. Vergana-Hernandez, and B. HernandezMorales, A Novel Probe Design to Study Wetting Front Kinematics During Forced Convective Quenching, Experimental Thermal and Fluid Science, Vol. 33, No. 5, 2009, pp. 797–807. [6] A.A.Moskalenko, N.I.Kobasko, L.M.Protsenko, O.V. Rasumtseva, Acoustical System Analyzes the Cooling Characteristics of Water and Water Salt Solutions, Proc. of the 7th IASME / WSEAS International Conference on HEAT TRANSFER, THERMAL ENGINEERING AND ENVIRONMENT (HTE '09), Moscow, Aug. 20 – 22, 2009, pp. 117 – 122. [7] N.I.Kobasko, Thermal Processes in Quenching of Steel, Metal Science and Heat Treatment, Vol. 10, No. 3, 1968. [8] N.I.Kobasko, M.A.Aronov, J.A.Powell, G.E.Totten, Intensive Quenching Systems: Engineering and Design, ASTM International, Newburyport, 2010, 252 p. [9] V.I.Tolubinsky, Heat Transfer at Boiling, Naukova Dumka, Kyiv, 1980, [10] S.S.Kutateladze, Heat Transfer at condensation and Boiling, Mashgiz, Moscow, 1952. [11] N.I.Kobasko, G.I. Zhovnir, Analytical Method of Solving Thermal Problems for Steel Quenching, Visnyk of Academy of Sciences of Ukrainian SSR, No. 12, 1979, pp. 41–50. ISBN: 978-960-474-268-4 125
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