ICAS 2000 CONGRESS
THE EFFECT OF WATER INGESTION ON THE OPERATION OF THE GAS TURBINE ENGINE
Imre SÁNTA Department of Aircraft and Ships Technical University of Budapest H-1521 Budapest, Hungary
Keywords: water ingestion, mathematical modelling, compressor map, operating line, surge, flameout.
Abstract 1. Analyses of processes occur in the engine Gas turbine engines often operate in such mete- When the effects of the air water content on the orological circumstances, when the air entered processes taking place in the compressor is ex- the intake of the engine has significant amount amined, here are two fundamental cases to be of water content. The ingestion of water by gas distinguished: turbine engines changes the process which takes - water is present in vapour phase, places in the compressor, turbine, combustion - water is present in liquid and vapour phase in chamber, turbine and nozzle. In unfavourable the air to be ingested by the compressor. conditions an engine flameout is also possible. The two cases induce processes essentially In this paper, the effect of water ingestion different from each other in the compressor, as on the engine operation is analysed. It explores well as in the power-plant elements placed be- the physical content of the processes occurred hind it in the flow. It should be noted that prior in the engine and deals with the mathematical to the inlet duct, the saturated or nearly saturated modelling of them. humid air free of liquid drops nevertheless may The method of examination is based on the contain condensed drops of liquid in the cross- application of the mathematical model of the sectional area prior to compressor. This conden- engine. Finally the results of investigations are sation process is initiated in the case of proper demonstrated. condensation cores (e.g. grains of dust), and due to the release of heat of vaporisation, it will in- Introduction crease the stagnation temperature prior to com- pressor thus reducing (with control rule During training flights, at touch-and-go ma- n = const.) the corrected number of revolution noeuvre performed in heavy rain, one of the of the first stages, since (in case gas constant R engines of a 2-engine aircraft (AN-26) - imme- and specific humidity x are identical) diately after touching - stopped. After a short period of time at the same manoeuvre both en- gine of an another AN-26 aircraft also stopped. = T0 ncorr n * (1) Restart of the engines a few minutes later was T1 successful. It was heavy raining and on the runway where - n - is the relative physical number of water was gathered in puddles within the region revolution, T0 - is the temperature of correction, of touching. The engine stop occurred at the T* - is the stagnation temperature at the inlet of same part of the runway. 1 The paper analyses the processes caused by the compressor or compressor section. the water ingestion. In case water is contained in vapour phase across the inlet cross-sectional area of the com-
524.1 Imre SÁNTA
ϕ ≤ * pressor, i.e. the relative humidity is 1, the m T κ κ −1 δ ! 1 = − δ − − λ2 + effect of water content will manifest itself * 0.5 R 2 ln 1 1 p ()κ −1 κ + 1 through the change in the physical parameters of 1 κ + 1 the humid air as mixture of gases [1],[5]. By λ2 −1 κ − 1 applying the method of small deviations, from + 1 δκ []κ + − ()κ − λ2 the change of adiabatic exponent δκ and specific 2 1 1 1 u gas constant δR at λ = 1 = const. (where λ u1 c where 1=c1/c1cr; 1cr the change in pressure-ratio by: u1 and c1cr – peripheral and critical velocities respectively) can be determined the changes in κ 2 the corrected mass flow rate, pressure ratio and δπ * = 4 B − c efficiency [1],[5]. ()()κ − 1 2 κ + 1 2 + B With yielded values can be calculated on κ * κ ()1 + B κ −1 − π the basis of the following relationships, as de- − ln()1 + B − c δκ scribed here: ()κ − ()κ + π * 1 1 c The gas constant of humid air by: where R + x R = a 1 v κ −1 Rh.a (2) 1 1 + x B = π * κ − 1 1 c η* c its adiabatic exponent by: the change in efficiency by:
κ −1 1 + * * * c pa x1c pv π κ π κ 4κ 1 −η κ = (3) δη* = c ln c + c − + − − c * κ 2 * c pa x1c pv Ra x1Rv Bη ()κ − 1 ()2 + B η c ()1 + B κ −1 c κ where * κ − 1 ()1 + B κ −1 − π − c δκ Ra , and Rv - is the gas constant of dry κ ()κ + 1 1 η*π * κ air and water vapour, B c c cpa, cpv, cpw - the specific heat at constant pressure of dry air, water vapour and water, The deviations calculated according to the x , x - the specific humidity and water 1 above relationships, in the case of compressor content at the inlet, respectively, the formula of map having the form: π * η* = []()λ which are: c , c f q ,ncorr ϕ p m should be interpreted with the corrected number x = v x = ! w (4) − 1 of revolution: p1 pv m! air Here, κ pv - the partial pressure of water vapour, p1 - T R a ϕ 0 a κ + 1 pressure at the inlet of the compressor, - the = a (5) ncorr n κ * h.a relative humidity, m! w - the water mass flow rate, T R 1 h.a κ + h.a 1 m! a - air mass flow rate. The change in the corrected mass-flow rate on the basis of relationship: taking into consideration the variation of the new gas characteristics, too. (Here subscripts of a and h.a. symbolise parameters according to dry air and humid air respectively).
524.2 THE EFFECT OF WATER INGESTION ON THE OPERATION OF THE GAS TURBINE ENGINE
At low temperatures, pv and x are small, so The evaporation process itself, due to its that according to our calculations, the effect of complex character, cannot be exactly calculated. vapour-content on the compressor characteris- Calculations can be carried out only on the basis tics will be negligibly small [2],[5]. of appropriate measurement results. However, At higher temperatures ( T > 300 K) and on the basis of steam tables it can be determined also with a relative humidity of about ϕ =1 , in a simple way that - in our case - with a corre- only the change of the gas constant will be con- sponding pressure of p2 ≈ 8 bar, the volume of siderable, the change of κ will be negligible due the superheated steam will increase to the 280- to the adjacent values of the adiabatic exponents 290-fold value of water-volume, depending on of water vapour and dry air. temperature [2]. With all this taken into consideration, the Due to the increase in volume involved by corrected number of revolution will be reduced the phase-change of water, ever smaller amount due to the increase in the humidity of inlet air. of air can get into the burner with a constant fuel As a consequence of those said above, and tak- mass-flow rate. This increase of volume results ing into consideration relationship (5), both the in the increase in the volume flow-rate of the mass of inlet air and the pressure ratio of com- vapour-air mixture, and as a consequence, in the pression decreases (downward motion on the increase of axial velocity ca in the final stages of operating-line can be observed). the compressor. The change of the axial veloc- In case water is contained in the air also in ity, in this way, will modify the velocity trian- the form of water drops (e.g. preliminary con- gles of the final stages so that the angle of attack densation in the inlet duct, or take-off and will decrease, then the permeability of these landing in the rain, or flight at law altitude, re- stages will also decrease, which will result in the spectively), it will evaporate partially in the reduction of the volume of ingested air, the compressor. Consequently, in the downstream increase in the angle of attack of the first stages stages of compressor, the partial pressure of and the reduction in the pressure ratio of the vapour will increase relative to that of dry air, compressor. As a consequence of this, the oper- π * ating-point will move towards the surge line on which involves additional reduction c and the compressor map. m (due to the change in composition). At the ! 1 With the gas-turbine aircraft engines, a same time, due to the amount of the heat ab- considerable part of the ingested water in liquid sorbed by evaporation, the temperature of air phase can be transferred into the burner, as our will also reduce. experiences show, and there it will be evapo- This, in turn, results in the increase of the rated. This means that at the outlet cross- corrected number of revolutions of the compres- sectional area of the burner, the gas-steam mix- sor stages concerned (controlling rule is n = ture will contain more steam than at the inlet π * const.), which involves the increase of c . As a area, i.e. the outlet mass flow-rate of the gas- result of this double effect, according to our steam mixture will be higher than the inlet one. examinations, if a small amount of water is In case the temperature of the burner is evaporated in the compressor, its pressure ratio kept constant, the mass-flow rate of the gas- will generally decrease, while in case of the steam mixture to pass through the turbine will evaporation of a large amount of water, its pres- decrease due to the considerable increase of the sure ratio may increase. When analysing the volume flow-rate (thermal throttling). process, taking place in the compressor, we (At a burner-temperature of T3 ≈ 1060 K, should examine the very important circumstance and at a pressure of ~ 8 bar [2], the volume of that the volume of water will increase to its the superheated steam is about 400-fold value of multiple value in the course of evaporation. the water volume).
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This reduction in the mass flow-rate will, In case we should like to increase the rpm in turn, bring about a further reduction in the or keep it constant because of the change in the amount of ingested air, as well as in the pressure angle of attack of the propeller, the torque of the ratio of the compressor in the first stages, the turbine, or its power should be increased. The increase in the angle of attack, the approach of excess power δ = η − − ∆ the operating-point to the surge-line, and the P PT m Pc P (10) enrichment of the fuel-air mixture. or With a determined air-water ratio, due to δ P = P η − P − P − ∆ P (11) the over-enrichment of the combustion-mixture, T m c pr the combustion will be extinct, or else the com- required e.g. by the change in the angle of attack pressor will get into an unstable operational of the propeller can be attained through the duty. It can be detected by the occurrence of one stocking of excess fuel, - in case of constant of the following phenomena as a function of the cross-sectional areas of flow - through the in- properties of the relevant engine, as well as the crease of the turbine inlet temperature as com- cause of defective operation: pared to that of the steady-state operational duty. - combustion will be extinct due to the over- The increase of the turbine inlet tempera- enrichment of the combustion-mixture brought ture will involve the modification of the oper- about by the abrupt reduction of the ingested air ating-line plotted on the compressor perform- amount and the pressure ratio of the compressor ance. (It is assumed that the map of the com- - the rpm cannot be increased due to the in- pressor is unchanged during the transient proc- crease of the turbine inlet temperature esses). By increasing the turbine inlet tempera- - vibration effecting the whole power-plant, ture, the operating-line will move to the left, * * and a high fluctuation in the pressure and in the towards the greater T3 / T0 = const. lines, and as amount of air will occur leading to flame- a result, towards the surge-line (Fig. 1). As a separation in the burner. consequence of the changes in temperature and in the co-operational duty between the turbine 2. Processes taking place in the engine during and compressor, the pressure ratio of the com- an abrupt increase of power pressor and the ingested amount of air will also be modified. The increase in temperature as The motion of equation of the rotor in the tur- compared to the previous steady-state phase will bojet engine can be written in the following result in thermal throttling in the burner, and form: due to it, an instantaneous reduction in the in- η − − ∆ = π 2 θ dn PT m Pc P 4 n (6) gested amount of air is involved, while the pres- dt sure ratio of the compressor will increase. The in the turboprop it will be reduction of the amount of air and the excess dn P η − P − P − ∆ P = 4π 2nθ (7) fuel intake will bring about the enrichment of T m c pr dt the combustion-mixture. Consequently, the where limitation imposed on the speed-up, or the PT - the power developed by the turbine over- power-increase is marked by reaching the surge coming the friction and driving the compressor, line, but in the course of the process, the en- propeller and the auxiliary equipment,Θ - the richment of the combustion-mixture can also inertial moment of the rotor, n- rotational speed. occur, which, in turn, can lead to the extinction In the steady-state operational duty equations of combustion. are: We should like to note that this process - due to the shortage of the few data available – η − − ∆ = PT m Pc P 0 (8) can not be calculated even with approximate η − − − ∆ = accuracy. PT m Pc Ppr P 0 (9)
524.4 THE EFFECT OF WATER INGESTION ON THE OPERATION OF THE GAS TURBINE ENGINE
3. Analyses of the joint effect of abrupt the operating-point the more approached the power-increase and the atmospheric condi- surge-line. In the meanwhile, to cover the tions. power-requirement raised by the increase of the angle of attack of the propeller, excess fuel had The most probable cause of the stalling of en- to be charged into the burner. This, in turn, re- gines should be looked for - with the knowledge sulted in an increase of temperature in the of circumstances - in the joint occurrence of the burner, thermal throttling and thus further de- special (accelerating or power-increasing) op- crease in the amount of ingested air. The oper- erational duty, on the one hand, and the weather ating-point approached the more the surge-line. conditions prevailing during the flight training, At the same time, due to the thermal throttling on the other. and the steam volume developed in the course The detailed analysis of the two effects as of the phase-change of water, the relatively performed separately from each other will con- smaller amount of air ingested into the burner firm this assumption. Such situations may occur formed an over-enriched mixture with an in- during a touch-and-go manoeuvre performed in creasing amount of fuel, and as a consequence, heavy rain, and on the runway, water gathered in combustion became extinct. puddles within the region immediately prior to In the last analysis, we can state that the landing. stalling of the engine was caused with great The liquid phase contained in the ingested probability by the joint effect of the great air due to rain, as well as a part of amount of the amount of water ingested into the compressor, liquid splashed up and entered the compressor at and the abrupt power-increase taking place at the touching the runway and during running was the same time. This is characterising feature of evaporated in the compressor, while the remain- the given type of engines. der (greater part) of it in the burner. Both cases of evaporation involved the re- duction of the air amount delivered by the com- 4. Modelling of processes pressor. In the final stages of the compressor the Analyses of processes occur in gas turbine en- mass of steam-air mixture was increased due to gines - among other methods - is possible by evaporation, as well as the volume flow-rate mathematical model, described in [3], [4]. was also increased to a considerable extent. Model built up by this way is the basis of inves- Here the increase of the volume flow-rate re- tigation of effect of water ingestion on the proc- sulted in the increase of axial velocity, and - at esses in the engine too. In mathematical model the same time - the reduction of the angle of such operational modes we should take into attack. Thus, the blades became more and more account characteristic of the processes men- the obstacle of flow, and this, in turn, led to the tioned above reduction of the air amount ingested into the The process occurred in compressor as a compressor, and to the increase of the angle of result of vaporisation becomes a cooled poly- attack in the first stages. The operating-point tropic one, instead of adiabatic. The water is was shifted towards the surge-line, the pressure- considered as a separate parallel stream. It ratio of the compressor was reduced. means, the specific work required for compres- The steam generated in the burner in- sion can be determined by equation creased additionally the steam-content of the zx 2 = 1 ()− ()* − * + 1in u gas-steam mixture and its volume at the same wc c p c n T2 T1 1 + x 1 + x 2 time so that the given flow area of the turbine 1in 1in proved to be narrow to ensure the flow of steam (12) through it. Due to it, the amount of ingested air where continued to decrease, the angle of attack of the first compressor stage continued to increase, and
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Fig.1.
524.6 THE EFFECT OF WATER INGESTION ON THE OPERATION OF THE GAS TURBINE ENGINE
Fig.2.
524.7 Imre SÁNTA
n − κ count by proportional decrease of the cross- c = c (13) n v n − 1 sectional areas downstream the place of vapori- m! air RT + c - is the specific heat of air at constant volume; mwater vwater v A p ! n - is the mean polytropic exponent of the proc- sation. α = new = m RT κ Aold ! air + ess, - is the adiabatic exponent of air; u – is m! water vvapour the tip peripheral velocity for the stage; z – p number of stages where the water in liquid state (17) [5]. where Aold ,Anew - original and new cross-sectional ar- = m! w x1in at the inlet of given compressor sec- eas; T,p -static temperature and pressure of air at m! a the given cross-sectional area; vvapour ,vwater - tion. specific volume of vapour and water respec- These expressions should be applied to that tively. section of multispool compressor, where the The results of calculations by mathematical vaporisation takes place. model are demonstrated in Fig. 1 and Fig. 2. The mean polytropic exponent can be de- Fig.1 shows the twin-spool engine behaviour at termined from the heat balance of ingested wa- steady operational modes in case of water in- ter and air in compressor. The heat absorbed by gestion. Diagrams in Fig.2 show a twin-spool water formed from friction of compression pro- engine accelerating process in case of increasing cess and polytropic heat. water content on original compressor maps. The diagrams confirm the shifting of oper- − n f 1 κ n * * f ating points and operating lines to the surge line. RT π n f − 1 − − 1 c κ − − 1 n f 1 Summary n−1 − c T * π * n − 1 = x ()i* − i* n 1 c 1in wout win The behaviour of the aircraft gas turbine engine changes, due to water ingestion by the engine. (14) This paper examines the origin and characteris- where tics of these changes. It can be stated, that while * − * - the change in stagnation enthalpy of iwout iwin the ingestion of water in vapour phase hardly water in the given compressor section including modifies, the increase of water content in liquid phase change. phase shifts operating line both in steady and ln π * transient duties to surge line. In case of accel- n = c (15) f κ −1 eration or power increase of the engine the in- π * κ * gestion of water may cause the flame out of the ln π − ln1 + c c η combustion chamber. s
Enthalpy balance for the combustion chamber References [1] Litvinov J.A., Borovik V.O. Characteristiki i ek- spluatacionnaya svojstva aviacionnuh TRD, Moscow, c T* + fH η + x ()i* − i* = pa 2 L b 1in win wout (16) 1979. = ()+ + * [2] Raznjevic. Hõtechnikai táblázatok., Budapest, 1975. 1 x1in f c pmix T3 [3] Santa I. Non-linear mathematical thermal models of where c pmix - the isobaric specific heat of the gas turbine engines and their application in opera- air- vapour mixture. tion, ICAS Proceedings 1990 Stockholm, Sweden, pp The effect of increase in volume flow rate 2264-2270. [4] Santa I. Transient Analysis of Gas Turbine Power due to vaporisation, should be taken into ac- Plants at different cases of Deterioration, Third Mini
524.8 THE EFFECT OF WATER INGESTION ON THE OPERATION OF THE GAS TURBINE ENGINE
Conference on Vehicle Dynamics, Identification and Anomalies. T.U. Budapest, 1992. [5] Walsh P.P., Fletcher P. Gas turbine performance,1 st edition, Blackwell Science Ltd. and ASME, 1998.
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