ISSN 1063-7842, Technical Physics, 2016, Vol. 61, No. 8, pp. 1206–1208. © Pleiades Publishing, Ltd., 2016. Original Russian Text © A.P. Shaikin, I.R. Galiev, 2016, published in Zhurnal Tekhnicheskoi Fiziki, 2016, Vol. 86, No. 8, pp. 87–89.
GASES AND LIQUIDS
On the Effect of Temperature and the Width of the Turbulent Combustion Zone on the Ionization Detector Readings A. P. Shaikin* and I. R. Galiev Togliatti State University, Belorusskaya uk. 14, Togliatti, 445667 Russia *e-mail:
[email protected] Received November 26, 2015
Abstract—We have considered the functional dependence of the ionization detector readings (ion current) on the composition of the fuel–air mixture, adiabatic temperature, and the turbulent combustion zone width. Experiments on the engine show that, for an air excess factor of 0.75–1.15, the coincidence of the calculated and experimental data exceeds 90%. Our results can be used to predict and monitor the adiabatic temperature of the flame and the width of the turbulent combustion zone in the combustion changer of the internal combustion engine using the ionization detector. DOI: 10.1134/S1063784216080247
1. INTRODUCTON AND FORMULATION OF THE PROBLEM When designing, refining, and operating modern engines, it is necessary to ensure their compliance with toxicity regulations that show the tendency to gradually become increasingly stringent, as well as consumer needs, such as fuel consumption and noise level. Since the modification of the engine design is a time-consuming and expensive process that does not always produce the required results, the perfection of fuel combustion is considered by manufacturers during the last decade as the main way of solving problems associated with production of competitive articles [1]. However, the perfection of the combustion process remains a difficult problem because this process is complicated and has not been studied comprehensively. One of the approaches to solving this problem is the precision and low-inertia diagnostics of combustion of fuels in the engine and the selection of the optimal combustion regime by an electronic control system by varying controllable parameters. At present, optical methods and those based on the electrical conductivity of flame are used to diagnose the combustion process under the conditions of an internal combustion engine. In the optical diagnostic methods and an analysis of the combustion process (e.g., LaVision Flame Master), high-speed cameras (Phantom, Memrecam, and others) and high-frequency PIV lasers (like Litron) are used, which makes it possible to measure the characteristics of flame without affecting its hydrodynamic, thermal, and chemical structure. However, these methods have serious disadvantages, including the following: (i) their high prices (the cost of the equipment exceeds 70000 $US);
(ii) their application is labor-consuming and requires special skill; (iii) serious structural changes of the engine are required (installation of transparent walls, special windows, etc.). For this reason, optical methods are only used under laboratory conditions at stages far from the design and operation of the engine. The method based on the electrical conductivity of the flame is simpler and less costly [2, 3]. In this method, the combustion of the fuel in the combustion chamber of an engine is monitored by recording the ion current in the flame. Flame inspection sensors intended for indicating the presence or absence of the flame can successfully operate based on this principle for a long time. The disadvantage of the available flame inspection sensors is their inability to determine the flame characteristics and toxicity level of combustion products. Consequently, the electronic control system cannot perform a precision analysis of the efficiency of combustion of fuels and fails to ensure the most optimal regimes of this process. Since the main mechanism of the formation of charged particles at the hydrocarbon flame front is chemoionization, which is inseparably connected with the combustion of fuel, a change in the ion current, i.e., the signal from ionization detector (ID), indicates a change in the kinetics of chemical reactions and characterizes the process of combustion of fuel in the engine [4, 5]. This substantially extends the potentialities and field of application of IDs, which can be used not only to identify the presence (or absence) of the flame, but also to analyze the environmental and energy efficiency of the fuel combustion process. Thus, using this detector, the electronic system of controlling engine by varying its parameters (combus-
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tible mixture composition, time of injection of the fuel, etc.) will ensure the change in the running value of the ion current signal from the ID to the optimal value that corresponds to the effective combustion of the fuel. This will also make it possible to improve the models of combustion with insufficiently studied turbulence and can be used for actual combustion chambers without optical access. However, despite considerable advances in the application of IDs, it is necessary to know the effect of processes in the combustion chamber on the readings of inspection sensor in the combustion chamber of the engine, which are characterized by the adiabatic temperature of the flame and the width of the turbulent combustion zone, to further extend their potentialities, to achieve higher reliability, and to accurately process the signal. Thus, this communication aims to determine the effect of the adiabatic temperature of the flame and the width of the turbulent combustion zone on the readings of the ionization detector (i.e., the ion current). 2. DERIVATIVE OF BASIC RELATIONS Since the ID readings are affected by many other factors besides the temperature and width of the combustion zone (e.g., ID shape, size, and potential, as well as gasdynamic characteristics), analysis was carried out in relative quantities, viz., ratios of the parameters being analyzed to the parameters corresponding to the stoichiometric composition of the fuel–air mixture (FAM), to eliminate the effect of these factors:
I rel =
I (α= x ) , I (α=1)
(1)
where I(α = x) is the ion current for the current FAM composition [A], I(α = 1) is the ion current for the stoichiometric FAM composition [A], α is the excess air factor, and x is the running value of the excess air factor. In accordance with the traditional concepts of electrodynamics, the current determined by the motion of electrons to the positively charged ID is the product of the electron concentration by the electron charge, the velocity of motion, and the area of the ID surface. Then, expression (1) assumes the form
I rel =
n(α= x ) T(α= x ) , n(α=1) T(α=1)
(2)
where n is the electron concentration at the flame front, m–3, and T is the adiabatic temperature of the flame, K. It is well known that chemoionization at the hydrocarbon flame front is the main source of electrons, the concentration of which presumably depends of the chemoionization probability for carbon-containing molecules of fuel at the flame front with energy equal to the activation energy or exceeding it. Thus, the elecTECHNICAL PHYSICS
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tron concentration at the flame front is described by the formula
( )
Mg C KP (3) exp E , mS δ RT where M is the mass of the fuel in the combustion chamber, kg; m is the mass of a fuel molecule, kg; gC is the mass fraction of carbon in the fuel, K is the fraction of the fuel in the flame front, K ∈ [0; 1]; E is the activation energy, E = 7.1 kJ/mol [6], R is the gas constant, kJ/mol K; P is the chemoionization probability, P ∈ [0; 1]; S is the area of the turbulent flame front surface, m2; and δ is the width of the turbulent combustion zone, m. The final formula for the ion current was derived based on the following considerations: (i) since the ratio of ion currents in formula (1) is calculated for fuels with identical carbon concentrations, we have gC(α = x) = gC(α = 1); (ii) fraction K of fuel molecules in the flame front is determined by the diffusion coefficient which depends on temperature, D ~ T 3/2 [7]; consequently, K ~ T 3/2; (iii) we assume that for all FAM compositions, the chemoionization probability is the same, P(α = x) ≈ P(α = 1); (iv) the electron concentration is calculated for the volume of the flame front equal to the product of the area of the turbulent flame front surface by the width of the turbulent combustion zone; i.e., upon a change in the FAM composition and at constant turbulence intensity, only the value of δ changes. Thus, formula (2) becomes n=
I rel =
2 ⎛ ⎡ ⎤⎞ M relTrel exp ⎜ E ⎢ 1 − 1 ⎥ ⎟ . δ rel ⎝ R ⎣T(α=1) T(α= x ) ⎦ ⎠
(4)
3. EXPERIMENTAL RESULTS AND DISCUSSION To verify expression (4), which was derived theoretically, we experimentally determined the values of the quantities that appear in this expression, viz., the ion current, adiabatic temperature, and turbulent combustion zone width (Fig. 1). The experiments were carried out on a UIT-85 single-cylinder four-stroke engine operating on petrol. The rotation frequency of the crankshaft, i.e., turbulence intensity, was constant and equal to 600 min–1. The excess air factor varied from 0.75 to 1.25 within stable combustion of the FAM. The excess air factor was controlled by varying the mass of fuel in the FAM for a constant mass of air. Therefore, the relative mass of the fuel in expression (4) assumes the form (5) M rel = α −1. The experimental values of the adiabatic temperature of air-free flame were borrowed from [8]. To match the chosen values of temperature to our experi-
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Irel 1.4
Irel 1.5
1.0
1.0
0.6
0.5
0.2 0.75
0.85
0.95
1.05
1.15
1.25 α
Fig. 1. Dependences of experimental relative values of the ion current ( ), turbulent combustion zone width (◆), and adiabatic temperature of the flame (■) on the FAM composition (α).
0.85
0.95
1.05
1.15
1.25 α
Fig. 2. Comparison of experimental relative value of the ion current ( ) with calculated value (solid curve).
●
●
mental conditions, we introduced the following correction to this temperature:
T = T * + (Tc − T0*),
0 0.75
(6)
(7) Tc = Taε k −1, where asterisks mark the values of temperature borrowed from [8]; Tc is the temperature at the end of compression, K; T0* is the FAM temperature, K; Ta is the temperature at the end of admission; Ta = 323 K; ε is the compression ratio; and k is the polytropic exponent for compression, k = 1.382. A comparison of the calculated and experimental values of the ion current is illustrated in Fig. 2. It can be seen from Fig. 2 that, in the range of α ∈ [0.75; 1.15], good coincidence of the experimental results with the calculated data is observed. However, upon the further depletion of the FAM, the discrepancy between the calculated and experimental data increases; for example, for α = 1.2, the discrepancy attains 15%. This is probably due to the decrease in the chemoionization probability of thin mixtures compared to the stoichiometric composition; consequently, in the given case, P(α = 1.2) < P(α = 1). CONCLUSIONS Thus, as a result of our theoretical and experimental investigations, we have obtained a functional dependence that explains the influence of the FAM composition, adiabatic temperature of the flame, and the width of the turbulent combustion zone on the ion current, i.e., readings of the ionization detector. Our results can be used to predict and monitor the temperature and width of the turbulent combustion zone
from the ion current amplitude. This will improve the accuracy in the signal processing, increase the number of the characteristics of the flame being measured, and simplify and make less costly the existing methods of diagnostics and the investigation of the fuel combustion in engines using ionization detectors. ACKNOWLEDGMENTS This work was supported by the Foundation, which facilitates the development of small enterprises in science and technology (assignment no. 0010547), and by the state assignment (project no. 394). REFERENCES 1. G. P. Merker, C. Schwarz, and R. Teichmann, Combustion Engines Development. Mixture Formation, Combustion, Emissions and Simulation (Springer, London, 2012). 2. Z. Gao, X. Wu, H. Gao, and B. Liu, Int. J. Hydrogen Energy 35, 12 918 (2010). 3. A. Franke, Characterization of an Electrical Sensor for Combustion Diagnostics (Lund Inst. Technol., Lund, 2002). 4. I. S. Yasnikov, P. V. Ivashin, and A. P. Shaikin, Tech. Phys. 58, 1587 (2013). 5. A. P. Shaikin, P. V. Ivashin, and I. R. Galiev, Izv. Mosk. Gos. Tekh. Univ. “MAMI,” No. 3, 69 (2014). 6. W. C. Gardiner, Jr., Combustion Chemistry (Springer, New York 1984; Mir, Moscow, 1988). 7. J. Warnatz, U. Maas, and R. W. Dibble, Combustion. Physical and Chemical Fundamentals, Modeling and Simulation, Experiments, Pollutant Formation (Springer Berlin–Heidelberg, 2006; Fizmatlit, Moscow, 2003). 8. J. Stevens, Adiabatic Flame Temperature. http://heattransfer-thermodynamics.blogspot.ru/2015/06/adiabatic-flame-temperature.html.
Translated by N. Wadhwa TECHNICAL PHYSICS
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