ISSN 1068-7998, Russian Aeronautics (Iz.VUZ), 2015, Vol. 58, No. 1, pp. 205–209. © Allerton Press, Inc., 2015. Original Russian Text © A.I. Gur’yanov, O.A. Evdokimov, Sh.A.Piralishvili, S.V. Veretennikov, R.E. Kirichenko, D.G. Ievlev, 2015, published in Izvestiya VUZ. Aviatsionnaya Tekhnika, 2015, No. 2, pp. 65---69.

AIRCRAFT AND ROCKET ENGINE DESIGN AND DEVELOPMENT

Analysis of the Gas Turbine Engine Combustion Chamber Conversion to Associated Petroleum Gas and Oil a

a

a

a

b

A. I. Gur’yanov , O. A. Evdokimov , Sh. A. Piralishvili , S. V. Veretennikov , R. E. Kirichenko , b and D. G. Ievlev a

Rybinsk State Aviation Technical University, ul. Pushkina 53, Rybinsk, 152934 Russia b OAO NPO Saturn, pr. Lenina 163, Rybinsk, 152903 Russia e-mail: [email protected] Received April 24, 2014

Abstract—We performed the computational and analytical estimation of parameters and formulated the recommendations for conversion of the combustion chamber of the E70/8 RD engine combustion chamber to associated petroleum gas and oil. DOI: 10.3103/S1068799815020117 Keywords: combustion chamber, combustion, gas turbine engine, associated petroleum gas, oil, airfuel ratio.

Gas turbine engines (GTE) and power plants as very compact, powerful, economical, and maneuverable ones are one of the main types of power drives for marine engineering, air transport, and energetics. Modification of its operation conditions and application field led to necessity of providing multifuel GTE and possibility of using alternative kinds of fuel. In addition, discovery and development of new hydrocarbon power fields, Russian policy in the field of power- and resource-saving technologies pose a problem of using oil and associated petroleum gas (APG) as a fuel for gas turbine engines. In this case, a necessary condition for its realization consist in providing requirements on power density, thermodynamic efficiency, combustion efficiency, emissions of air pollutants [1–4]. A most justified and economical way of GTE conversion to APG and oil consist in thermophysical analysis of combustion chamber and engine operation, making recommendations on conditions, restrictions, opportunities and fundamental problems of conversion to alternative fuel. FORMULATION OF THE PROBLEM The object of investigation is the E 70/8 RD GTE combustion chamber operating by the dual-fuel scheme on base kinds of fuel, namely, natural gas (State Standard 5542–87) and diesel fuel (State Standard 305–82) with the possibility of continuous transition between them. Fuel combustion is realized according to the RQL (Rich burn – Quick quench – Lean burn) – scheme with the integral value of airfuel ratio α burn = 0.5 after the burner and the known air mass flow separating function.

ANALYSIS OF THE RESULTS The main integral parameter characterizing the GTE efficiency is the thermodynamic efficiency that can be defined in the general form as ηeff = N eff N therm , where Neff is the effective power of gas turbine engine; Ntherm is the thermal power of the combustion chamber. For providing the required value of ηeff within engine

conversion to alternative fuel, it should be necessary that the values of Neff and Ntherm remain constant. 205

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Fulfillment of such conditions can be provided by the constant product Gprod Lturb, where Gprod is the mass flow of the combustion products at the flame tube outlet; Lturb is the turbine work. The compressor work Lcomp depends on the initial thermodynamic parameters of the working fluid, number of stages and its design. Value of Lcomp is not directly related to the fuel applied in GTE cycle. The turbine work, in turn, is defined by total enthalpy of gases and, as a consequence, by the total temperature at the combustion * chamber outlet Tcomb . Within the framework of the a rich-burn, quick-quench lean-burn (RQL) combustion chamber used in the base E70/8RD engine and the conditions of saving the combustion efficiency ηcomb and emissions of air pollutants, it is necessary to provide that the mass averaged temperature in primary “rich” combustion * zone Tburn remain constant. Therefore, the main conversion conditions can be formulated as follows

G prod = const ⎫ ⎪ * Tcomb = const ⎪ ⎪ * Tburn = const ⎬ . (1) ⎪ N therm = const ⎪ * πcomp = const ⎪ ⎭ According to the mass conservation law, the mass flow of combustion products is equal to the sum of flame tube air mass flow and fuel mass flow, G prod = Gair + G fuel . This value does not depend on the kind of alternative fuel applied and under conditions (1) is defined as (2) G fuel = N therm ( Qcomb ηcomb ) . The lowest combustion heat Qcomb , entering into Eq. (2), is determined by the composition of alternative fuel and its physico-chemical properties. These properties for oil and APG vary over a wide range depending on the field and its type (oil field; oil and gas field; oil gas condensate field). The most expedient version of the analysis is to consider the physico-chemical properties for some fields in composition range indicated in table. Table Component CH4 C2H6 C3H8 iC4H10 CO2 N2 H2S iC5H12

Min, % (by volume) 20 1.5 0.1 0.04 0.04 0.2 0.003 0.02

Max, % (by Min, % (by Max, % (by Component volume) volume) volume) 92 iC6H14 0.03 14 16 iC7H16 0.03 7 21 C6H6 0.01 0.1 7 C7H16 0.04 1.6 4 iC8H18 0.01 4.5 1.4 C7H8 0.003 0.1 0.005 iC9H20 0.003 0.9 12 iC10H22 0.001 0.2

Element C H S W N O – –

Min, % (by mass) 77 11 0.01 0.01 0.001 0.005 – –

Max, % (by mass) 87 14.5 6 0.1 1.8 0.35 – –

Volume content of heavy hydrocarbons in APG (gases with molecular mass more than propane one, first of all, isomers of butane, pentane, hexane, heptane, and octane) fundamentally influences the thermophysical properties of fuel. This influence for different fields is shown in Fig. 1. The APG density of some compositions due to a sufficiently small content of heavy hydrocarbons is 3 close to that of natural gas ρ ≈ 0.7 kg/m that consists of about 95% methane. Increase of Cheavy CΣ up to

the value Cheavy CΣ = 0.2 leads to rising the APG density by 1.8 – 2.0 times. The quotient Cheavy CΣ of end stages of separation of some oilfields can reach a value of Cheavy CΣ = 0.7 that leads to the density rise by three and more times relative to the natural gas density. It determines difference in values of combustion heat, stoichiometric ratio, and its nonlinear dependence on composition (Fig. 2). RUSSIAN AERONAUTICS

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ρ, kg/m 3

L0

2

16

2

15

1

1

14

0

0.25

0.5

0.75 Cheavy CΣ

40000 45000 50000 Qcomb , kJ/kg

Fig. 1.

Fig. 2.

Oil composition and its physico-chemical properties can be described by only the element composition unlike the APG produced in the gas phase. It is caused the fact that oil as a liquid is a mixture of more than 1000 individual liquid substances and heteroatomic organic compounds. Incompressibility of oil as a liquid defines the linear stoichiometric ratio dependence on combustion heat (Fig. 2). The magnitude of the lowest combustion heat per mass unit of fuel for the composition range varies in the range 39 to 44 MJ/kg, and for APG varies in the range 48.3 to 52.7 MJ/kg. These dependences show that engine conversion to alternative fuel within the system of equations (1) leads to a necessity of air redistribution along the flame tube and change in the air-fuel ratio after a burner α burn . The dependence α burn on oil and APG composition as a function of the combustion heat Qcomb is shown in Fig. 3. A unique solution of the problem is possible by introducing an overflow valve into the combustion chamber design and modifying the control system to provide a possibility of air-fuel control not only by fuel mass flow, but also by air mass flow. Similar technical solutions have been worked through and successfully used in practice by the world's leading manufacturers of low-emission multifuel combustion chambers. The simplest and most trivial way of transition is to choose the average air-fuel ratio for oil and APG compositions applied as shown in Fig. 3. This will lead to deviations of integral operation parameters of combustion chamber from the calculated values for the mode selected. In order to simplify the α burn control as a factor that defines the transition values, it is expedient to use the stoichiometric ratio L0 instead of the combustion heat Qcomb . Despite the fact that the values of L0 for all the compositions of oil and APG are not critical to bring to the condition N therm = Qcomb G fuel hcomb = const , they provide with the almost linear and convenient dependence for air-fuel ratio as a function of APG composition (Fig. 4, pos.2). It is caused by physical meaning of the stoichiometric ratio L0 . Conversion of combustion chamber to oil allows using any of the dependences Figs. 3, 4, pos. 1). α burn

α burn

3

0.49

2

4

0.49

2 0.47 0.45

38000

0.47

1

5

41000 44000 47000

Qcomb , kJ/kg

Fig. 3.

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L0

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As opposed to oil, when the linear dependence (Fig. 3, pos. 1) can be used to select α burn , the use of APG leaves no possibility to make such a conclusion (Fig. 3, pos. 2). Therefore, in order to define α burn , it is necessary to perform the purposeful calculation for every composition applied. This imposes certain restrictions in the case of continuous transition from one alternative fuel to another. For example, it is necessary to save almost constant the value of α burn for oil and APG compositions marked by transition curve 3 (Fig. 3). At the same time, it is necessary to vary α burn for maintaining a constant thermal heat in the case of transition scenario 4 (Fig. 3). Moreover, it should be considered that the nominal transition curve can have both ascending (Fig. 3, pos. 5) and descending character (Fig. 3, pos. 4) relative to α burn . These conversion conditions considered allow us to define the values of air (Figs. 5 and 6, pos. 2) and fuel (Figs, 5 and 6, pos. 1) mass flows feeding in the flame tube through the burner, cooling holes and dilution zone and forming a total flow of gas entering the turbine (Figs. 5 and 6, pos. 3). An analysis of these dependences for APG shows that increasing the combustion heat leads to monotonous rising of total mass flow through the flame tube (Fig. 5, pos. 3). A similar curve for oil as an alternative fuel (Fig. 6, pos. 3) is linear due to the linear dependence of the stoichiometric ratio of the combustion heat (Fig. 2) for the whole range of element compositions. The analogous dependence for APG is nonlinear that determines uncompensated decrease of fuel mass flow and growth of air and combustion product mass flows. G fuel , kg/s

Gair , G prod kg/s

1 0.49

G fuel , , kg/s

Gair , Gprod , kg/s

3

0.61

23.80

1

24.4 0.59

23.60

3 0.47

48000

24

2 49000

50000

51000

Qcomb , kJ/kg

Fig. 5.

0.57

23.40

2

39000

40000

41000 42000

Qcomb , kJ/kg

Fig. 6.

The procedure of using these dependences for the conversion of the combustion chamber to alternative fuel is as follows. For compositions of oil and APG selected, we define the combustion the heat, mass and volume stoichiometric ratios L0 and V0 . Using the values obtained and dependences in Figs. 5 and 6, the fuel mass flow corresponding to the condition N therm = const formulated in (1) and expression (2). * * = const and Tburn = const allows defining the air The calculated value of G fuel and conditions Tcomb

mass flows through the burner and the flame tube (Fig. 5). For this the equation system of heat balance for “rich” combustion zone and flame tube outlet are composed . Solution of these equations with respect * * and Tcomb for the base and alternative fuels allows obtaining the values of α comb , α burn and Gair to Tburn providing the condition G prod = const in system (1). System (1) is closed by the condition π*comp = const that is defined by the engine cycle thermodynamics and coordinated operation of the high pressure turbine and compressor. The dimensionless air mass flows through the burner versus the combustion heat and stoichiometric ratio for both alternative fuels (pos. 1— oil, pos. 2—APG) are shown in Figs. 7 and 8. The nature of these dependences are defined by peculiarities of the influence of fuel composition on its physico-chemical properties.

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Gair burn 0.14

Gair burn

2 1

0.14

1 0.12

35000

2

0.12

40000

45000

Qcomb , kJ/kg

Fig. 7.

13

14

15

16

L0

Fig. 8.

Change of APG combustion heat as compared with natural gas is associated with the change of its composition and influence on the pollutant emission, thermal power, and combustion stability. Appearance of heavy fractions in APG increases significantly the probability of transition from combustion to vibrating mode accompanied by the emergence of self-oscillation of thermogasdynamic parameters and destruction of GTE elements at resonance frequencies of the natural oscillations. Application of non-stoichiometric combustion schemes worsens the problem. Thus, engine conversion to the alternative gaseous fuel should be based on the Wobbe attitude as WI alt WI bas =

Qcomb alt Qcomb bas

(r

fuel bas

rfuel alt ) . The most advisable way is satisfy the simplex WI alt WI bas ≈ 1 that 3

defines conformity of the fuel composition and nozzle pressure difference in the base and alternative cases. If using some APG compositions, we cannot provide this condition, then the alternative solution should be based on ballasting the fuel by inert impurities, proportion of which is selected from the condition WI alt WI bas ≈ 1 . If conversion of the combustion chamber from diesel fuel to oil does not allow obtaining the combustion heat no more than 42 MJ/kg and there is no possibility of geometry changes, then the problem solution also should be based on ballasting oil by inert impurities, for example, nitrogen, combustion products (CO2, H2O), steam supplied to the nozzle to reduce oil viscosity and increase its spray quality. The necessary level of ballasting for engine E 70/8 RD should be selected to obtain the value of combustion heat no more than 42 MJ/kg. Changing thermogasdynamic parameters of the combustion chamber considered for engine conversion to alternative fuel leads to a necessity of estimating its aerodynamic efficiency using integral characteristics defining total engine efficiency. Thus, we obtained the calculated dependences and proposed sequence of their application for conversion of the E 70/8 RD combustion chamber to APG and oil. We justified the operation conditions on thermal power of combustion chamber, air-fuel ratio, combustion temperature that define necessary emission, combustion efficiency and efficiency of the thermodynamic cycle of the gas turbine engine. REFERENCES 1. Piralishvili, Sh. A. and Gur’yanov, A.I., Fizika protsessov goreniya (Physics of Combustion Processes), Rybinsk: RGATA, 2010. 2. Evdokimov, O. A. and Gur’yanov, A.I., Investigation of Burnout Dynamics in Combustion Chambers and Burners of Energy, Vestnik RGTA im. P.A. Solov’eva, 2013, no. 4, pp. 36–42. 3. Gur’yanov, A. I., Emission Characteristics of Combustion in Swirling Gas Dynamic Counter Flows, Teplovye Protsessy v Tekhnike, 2013, no. 1, pp. 2–6. 4. Lefebvre, H. and Ballal, D.R., Gas Turbine Combustion, West Lafayette: Purdue University, 1998. 5. Fomin, G. S. and Fomina, O.N., Neftegazovaya entsiklopediya mezhdunarodnykh standartov (Oil and Gas Encyclopedia of International Standards), Moscow: Protektor, 2012. RUSSIAN AERONAUTICS Vol. 58

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