THERMAL ANALYSIS OF COOLING EFFECT ON GAS TURBINE BLADE

Transcription

1 IJRET: International Journal of Research in Enineerin and Technoloy eissn: pissn: THERMAL ANALYSIS OF COOLING EFFECT ON GAS TURBINE BLADE Amjed Ahmed Jasim AL-Luhaibi 1, Mohammad Tariq 2 1 Technical collae Kirkuk, Fuel and Enery Enineerin Department, Foundation of Technical Education, Iraq 2 Assistant Professor, Department of Mechanical Enineerin, SSET, SHIATS-DU, Allahabad Abstract Performance of a as turbine is mainly depends on various parameters e.. ambient temperature, compressor pressure ratio, turbine inlet temperature etc. The most important parameter to increase the life of the turbine blade is the coolin of the blade, which is necessary after reachin a certain temperature of the ases passin throuh the blades. Various types of coolin models are available for a turbine blade coolin. The power output of a as turbine depends on the mass flow rate throuh it. This is precisely the reason why on hot days, when air is less dense, power output falls off. This paper is to analyze the film coolin technique that was developed to cool ases in the initial staes of the turbine blades, where temperature is very hih (>1122 K). It is found that the thermal efficiency of a cooled as turbine is less as compare to the uncooled as turbine for the same input conditions. The reason is that the temperature at the inlet of the turbine is decreased due to coolin and the work produced by the turbine is slihtly decreased. It is also found that the power consumption of the cool inlet air is of considerable concern since it decreases the net power output of as turbine. In addition, net power decreases on increasin the overall pressure ratio. Furthermore, the reviewed works revealed that the efficiency of the cooled as turbine larely depends on the inlet temperature of the turbine and previous research said that the temperature above 1123K, require coolin of the blade. Keywords: Gas turbine, Turbine blade coolin, film coolin technique, Thermal Efficiency *** INTRODUCTION In a bid to remain at the forefront of technoloical development as well as a technical expert to United States industry, NASA identified the need for an improved desin process within the civilian aero enine industry, in hopes of improvin their market share, reducin time to market, and minimizin research costs [2]. Areas of interest included, but were not limited to, hih temperature materials, advancin turbine analysis techniques, and improvin the overall enine desin and analysis process. The latter interest called for the impact assessment of enine component technoloies from the micro to system levels [2]. A ood example of the need for this type of analysis comes from determinin the required service life of a turbine blade, which is limited by the exit temperature from the combustor and the material properties that in turn, limits the performance of the as turbine. Ideally, the enine would operate at a hih enouh temperature to achieve the hihest possible thrust ratin [3], while at the same time maintainin an economic service life. Currently, to address the issue of exit temperature, modern turbines utilize a cooled turbine blade to improve the possible rotor inlet temperature, and this necessary coolin flow has a stron impact on the turbine efficiency [4, 5]. By improvin coolin technoloy for a as turbine blade it is possible to increase the combustor exit temperature sufficiently, therefore achievin ood improvement in turbine efficiency and thrust [6]. However, this improvement does not prove viable when considerin the complete process. The increase in coolin flow to the turbine blades and vanes takes bleed air away from the compressor, reducin its on efficiency. This detrimentally affects the efficiency of the whole system, such that the improvements in the turbine are eclipsed. Bein able to track all these whilst lookin at a particular component within an aircraft as turbine, is obviously very desirable, especially at the early staes of a desin. A successful desin process incorporates flexibility and freedom at the early conceptual staes and continuin as far into the desin as possible, savin time and money in fixin problems that would have arisen had an investiation not taken place. Lookin into technoloies to improve an enine, one needs to provide useful benchmarks from which comparisons can be made. If a superior product is oin to be produced, analysis of the areas affected by the new product needs to be considered. If this is not the case a lot of money could be invested in desins that in the end prove to be impractical. A new technique has been reviewed up to date that was developed to cool inlet air to as turbine [9]. The techniques includin the mechanical chillers, media type evaporative coolers and absorption chillers have been reviewed. It is found that the power consumption of the cool inlet air is of considerable concern since it decreases the net power output of as turbine. Experimental tests to investiate the film coolin performance of converin slot hole (console) rows on the turbine blade have been performed [10]. Film coolin effectiveness of each sinle hole row is measured Volume: 03 Issue: 03 Mar-2014, 603

2 IJRET: International Journal of Research in Enineerin and Technoloy eissn: pissn: under three momentum flux ratios based on the wide-band liquid crystal technique. Measurements of the coolin effectiveness with all the hole rows open are also carried out under two coolant mainstream flux ratios. Film coolin effectiveness of cylindrical hole rows on the same blade model is measured as a comparison. The thermodynamic performance of MS9001 as turbine based coeneration cycle havin a two-pressure heat recovery steam enerator (HRSG) for different blade coolin means have compared. Internal convection coolin technique, film coolin and transpiration coolin techniques, employin steam or air as coolants, are considered for the performance evaluation of the cycle[11]. The analysis is useful for power plant desiners to select the optimum compressor pressure ratio, turbine inlet temperature, fuel utilization efficiency, power-to-heat ratio, and appropriate coolin means for a specified value of plant specific work and process heatin requirement. The as turbine plant performance and the effects of turbine coolin have been presented [8]. The thermal efficiencies are determined theoretically, assumin air standard (a/s) cycles, and the reductions in efficiency due to coolin are established; it is shown that these are small, unless lare coolin flows are required. The theoretical estimates of efficiency reduction are compared with calculations, assumin that real ases form the workin fluid in the as turbine cycles. 2. METHODOLOGY 2.1 Open Gas Turbine Cycle enters the turbine where it expands to ambient pressure and produces work. Fi 2 T-s representation of a as turbine enine 2.2 Gas Model The thermodynamic properties of air and products of combustion are calculated by considerin variation of specific heat and with no dissociation. Table containin the values of the specific heats aainst temperature variation have been published in many references. Followin equations are used to calculate the specific heats of air and as as a function of temperatures. For T a 800 the value of cpa will be calculated from the followin equation. c pa = T a Ta Ta Ta4 (1) In the above equations, T stands for as or air temperature in de K and t = T 100 Fi. 1: Schematic for an open as-turbine cycle Fresh air enters the compressor at ambient temperature where its pressure and temperature are increased. The hih pressure air enters the combustion chamber where the fuel is burned at constant pressure. The hih temperature (and pressure) as Compressors are subjected to various aerodynamic losses which are accounted here by introducin the concept of polytropic efficiency. For a iven polytropic efficiency, pressure ratio and bleed point pressure, the temperature and other variables for each stream are calculated. The mass and enery balances yield the compressor work, as iven below Mass balance: m a,i = m a,e + m cl (2) Volume: 03 Issue: 03 Mar-2014, 604

3 IJRET: International Journal of Research in Enineerin and Technoloy eissn: pissn: Enery balance: W c = m a,e. h a,e + m cl. h cl m a,i. h a,i (3) Where determination of m cl (mass of the coolin air) extracted from the compressor, is iven after turbine coolin flow calculation. p 2 = p 1 p 2 p 1 (4) T 2 T 1 = T 1 η c p 2 p 1 T 2 T 1 = p 2 p 1 γ a 1 γ a 1 (5) γ a 1 η pc.γ a (6) Where p 2 is the overall pressure ratio of compressor and η p c 1 and η pc are isentropic and polytropic efficiencies of the compressor, respectively. 2.3 Combustion Chamber The purpose of the combustion system of a as turbine enine is to increase the thermal enery of a flowin as stream by combustion which is an exothermic chemical reaction between the hydrocarbon fuel and oxyen in the air-stream. Inflowin air is diffused at the entrance of the burner. Kerosene is the standard aviation fuel but the hihly distilled forms of diesel used make little difference in performance compared to kerosene and hence diesel is used here for the analysis. The mass and enery balances for combustor, with suffixes i and e denotin its inlet and outlet sections, are iven by Mass balance: m = m a,i + m f,cc (7) Enery balance: η cc. m f,cc. LCV f = m,e. h,e m a,i. h a,i (8) The pressure at combustor exit is iven by (Δp cc =2% pressure drop has been assumed in the combustor) p cc,e = p cc,i Δp cc (9) The fuel to air ratio (FAR) is calculated as, In this equation T e is the turbine inlet temperature, T i is exit temperature of compressor, η cc is the combustion efficiency of the main combustion chamber, normally taken between 0.98 to 0.99 and LCV f is the lower calorific value of the fuel taken as kj/kk assumin fuel as diesel. Values of specific heat of air and ases are to be calculated from the as model. 2.4 Cooled Gas Turbine Turbine produced power to drive enine compressor due to expansion of as stream apart from the excess work developed at the shaft of the turbine. A eneral model that represents the as turbine with turbine blade coolin has been developed. The model is intended for use in cycle analysis applications. Aerodynamic losses and hence the inefficiency of as turbine is taken care by considerin polytropic efficiency of turbine. T i T e = p i p e η pt. γ t 1 γ t (11) Pressure ratio, the temperature and other variables for each stream are calculated for a iven polytropic efficiency. W t is the work developed by turbine, and η m is the mechanical efficiency which is used to account for the losses due to windae, bearin friction, and seal dra and is defined as η m = mechanical power output mechanical power input Mass and enery balances yield the turbine work, as iven below. Mass balance: Where m,i = m,e + m cl (12) m,i = m a,i + m f,cc (13) and m cl represents the sum of air coolant flow rates to the stae (stator and rotor). Enery balance: m,i. h,i + m cl,i. h cl,i = m,e. h,e + W T (14) Mass of coolant required per k of as flow is iven by followin equations as explained in [8]. m cl m = R c trans (15) m f,cc m a = c p.t e c pa.t i η cc.lcv f c p.t e (10) Where η iso film = 0.4 Volume: 03 Issue: 03 Mar-2014, 605

4 Thermal Efficiency Thermal Efficiency Thermal Efficiency Mass of Coolant required per k of Exhaust as (m c '/m ' ) IJRET: International Journal of Research in Enineerin and Technoloy eissn: pissn: R c trans = T,i-T bl. c p,. 1- η iso film ε T bl T cl,i. c p,cl (16) Work Ratio: Work ratio is also very important parameter for the selection of a as turbine cycle. The work ratio is iven by the followin equation: WR = W net W t (17) m ' c /m ' for 1st stator m ' c /m ' for 1st rotor m ' c /m ' for 2st stator m ' c /m ' for 2st rotor Specific Fuel Consumption: Specific fuel consumption is defined as the fuel consumed by the combustor per unit net work of the cycle; If the mass of fuel consumed is iven in k/s and the net work developed is in kw then the specific fuel consumption will be calculated in k/kwhr. SFC = 3600 m f W net (k/kwhr) (18) Net Power: Net power available is calculated by the followin equation Fi 4 Variations of m c / m with TIT Net Power = m a w net η en (19) If mass of air flow is iven in k/s and net power is iven in kj/k then the net power will be calculated in kw Thermal Efficiency No Turbine Coolin 3. RESULTS The results are based on the software developed in C++ and afterwards raphs have plotted with the help of menu driven software Oriin Turbine Inlet Temperarture (K) 0.36 Fi 5 Variations of Thermal Efficiency with TIT OPR = 30 Ta = 300 K Thermal Efficiency with Turbine Blade Coolin 0.22 Thermal Efficiency (No Turbine Blade Coolin) Thermal Efficiency (With Turbine Blade Coolin) Fi 3 Variations of Thermal Efficiency with TIT Fi 6 Variations of Thermal Efficiency with TIT Volume: 03 Issue: 03 Mar-2014, 606

5 Net Power Specific Fuel Consumption Work Ratio IJRET: International Journal of Research in Enineerin and Technoloy eissn: pissn: Fi 7 Variations of Work Ratio with TIT Fiure 3 shows the variations of the thermal efficiency for various turbine inlet temperature (TIT) at a iven values of atmospheric pressure, atmospheric temperature and overall pressure ratio (OPR). It seems from the raph that the efficiency increases with the increase in TIT for both type of cycles i.e. for cooled turbine blade and uncooled turbine blade. It has been observed that the thermal efficiency of the uncooled turbine blade is slihtly better that the one with cooled turbine blade. It is due to fact that the temperature at the inlet of the turbine is decreases. Fiure 4 shows the variations of m c / m with TIT. The mass of coolant required is calculated as the fraction of the exhaust ases. It has been observed from the fiure that the mass of coolant required is limited up to first stator only at low turbine inlet temperature but as lon as the temperature increases, the coolin required is in later staes as well. As shown in the fiure that for TIT 1800 and above, the coolin is must for at least two staes. That is two stators and two rotors Fi 8 Variations of Specific Fuel Consumption with TIT The variations of the raphs are based on the input values taken and the thermodynamic laws overned those equations. The parametric analysis of the as turbine cycle of the present work, film coolin has been taken for the coolin of the turbine blade to increase the life and better performance Fiure 5 represents the variations of Thermal Efficiency with TIT for an uncooled turbine blade for various overall pressure ratios. The thermal efficiency increases on increasin the OPR for hiher rane of TIT. At lower TIT, the thermal efficiency is hih for low OPR and it has little variation for the entire rane of TIT. Fiure 6 represents the variations of Thermal Efficiency with TIT for a cooled turbine blade for various overall pressure ratios. Almost same variations have been found in this raph also. Fiure 7 shows the variations of Work Ratio with TIT for various overall pressure ratios. For a iven value of OPR, the work ratio increases on increasin the TIT. At a iven TIT, the work ratio decreases on increasin the OPR. Fiure 8 shows the variations of Specific Fuel Consumption (SFC) with TIT for various Overall pressure ratios. SFC decreases on increasin the TIT for a iven OPR. At low OPR, this chane is in a limited rane but at hiher OPR it varies in wide rane. Fiure 9 shows the variations of Net Power with TIT. Net power increases on increasin the value of TIT. It has been observed that the net power decreases on increasin the OPR for a iven value of TIT. Fiure 10 represents the variations of Thermal Efficiency for an uncooled turbine with TIT for various ambient temperatures. Thermal efficiency increases on increasin the TIT for a iven value of ambient temperature. At low ambient temperature the thermal efficiency is hiher compared to hih ambient temperature. Fiure 11 shows the variations of Thermal Efficiency of a cooled as turbine with TIT for various ambient temperatures. It also has the same effect as the uncooled turbine blade does. The difference in that, the uncooled as turbine has more efficiency that a cooled as turbine blade for a iven ambient temperature. 0.1 Fi 9 Variations of Net Power with TIT Volume: 03 Issue: 03 Mar-2014, 607

6 Specific Fuel Consumption Thermal Efficiency without TBC Thermal Efficiency with TBC Thermal Efficiency Without TBC IJRET: International Journal of Research in Enineerin and Technoloy eissn: pissn: Thermal Efficiency without turbine blade coolin Fiure 12 shows the variations of SFC with TIT for various ambient temperatures. SFC decrease continuously on increasin the TIT. It decreases steeply in the low rane of TIT but at the hiher rane of TIT, this variation is slihtly low. It also reflects that the maximum value of SFC is for hih ambient temperature. Fiure 13 represents the variations of mass of coolant required per k of as (m c/m ) with ambient temperatures. The mass of coolin air required is increases on increasin the ambient temperature. In the first stae of turbine, the mass of coolant required is increases continuously with the ambient temperature but in the second stae of turbine the variation of coolant required is almost constant with the ambient temperatures. Fi 10 Variations of Thermal Efficiency uncooled turbine with TIT Thermal Efficiency with turbine blade coolin 0.12 (Mass of Coolant Required) m ' c /m' st Stator 1st Rotor 2nd Stator TIT=1500 K Atmospheric Temperature (K) Fi 13 Variations of (m c/m ) with Ambient Temperatures Fi 11 Variations of Thermal Efficiency of cooled Turbine with TIT 0.33 TIT=1500 K Overall Pressure Ratio Fi 14 Variations of Thermal Efficiency of uncooled turbine with OPR 0.3 Fi 12 Variations of SFC with TIT Fiure 14 represents the variations of thermal efficiency of an uncooled turbine with OPR for various ambient temperatures. It has been observed that, the thermal efficiency first increases and then decreases on increasin the OPR for a iven value of ambient temperature. The thermal efficiency is hiher for low ambient temperature and also the thermal efficiency for this Volume: 03 Issue: 03 Mar-2014, 608

7 Mass of Coolant required (m ' c /m' ) Thermal Efficiency with TBC IJRET: International Journal of Research in Enineerin and Technoloy eissn: pissn: rane of ambient temperature is continuously increases on increasin the OPR TIT=1500 K Overall Pressure Ratio method approach have already been published in various literatures. In the present wok the film coolin has been taken for the calculation of the blade coolin. It describes how the coolin flow in each chordwise strip may be calculated, assumin a particular internal coolin eometry. Comparisons with a simplified method of calculation, assumin constant blade temperature alon the span, indicate broad areement. The new method presented should then ive more reliable evaluations than the semi-empirical methods, particularly for the study of innovative cycles, where conventional fluids and operatin conditions cannot be assumed in the performance calculations. It is hoped that such full development of the code will provide a useful desin tool for the turbine desiner faced with the necessity of blade coolin. Table 1: Input parameters for calculation Fi 15 Variations of Thermal Efficiency of cooled blade with OPR m ' c /m' for 1st Stator m ' c /m' for 1st Rotor m ' c /m' for 2st Stator TIT=1500 K Overall Pressure Ratio Fi 16 Variations of (m c/m ) with OPR Fiure 15 shows the variations of thermal efficiency of a cooled blade as turbine with OPR for various ambient temperatures. In this case, the natures of the curves are almost similar to the previous raph. The difference is that, efficiency is decreases for the entire rane of the OPR after a sliht increase at the low rane of OPR. Fiure 16 shows the variations of mass of coolant required (m c/m ) with OPR for a iven value of ambient temperature and turbine inlet temperature. In the first stae of turbine, the mass of coolant required is increases on increasin the OPR but for the second stae the mass of coolant is required are decreases. 4. CONCLUSIONS The as turbine power plant has been analyzed for various parameters. The most important parameter which has been covered in this work is the new method of calculatin the coolant flow requirements for as turbine blades especially at hiher temperatures. There are various types of coolin S. No. Parameter Value 1 Atm. Press bar 2 Turbine Efficiency 90% 3 Compressor Efficiency 90% 4 Mechanical Efficiency 99% 5 Mass of air flow 1 k/s 6 Turbine Inlet Temp K 7 Overall Pressure Ratio REFERENCES [1]. Koff, B., Gas Turbine Technoloy Evolution: A Desiner's Perspective. Journal of Propulsion and Power, (4): p [2]. Tai, J. and e. al, University Research Enineerin Technoloy Institute (URETI) on Aeropropulsion and Power Technoloy 2006, Georia Institute of Technoloy. [3]. Winstone, M., The Contribution of Advanced Nickel Alloys to Future Aero-Enines. Journal of Defence Science (4): p. F65-F71. [4]. Gauntner, J., Alorithm for Calculatin Turbine Coolin Flow and the Resultin Decrease in Turbine Efficiency, N.L.R. Center, Editor. 1980, NASA: Cleveland, OH. [5]. Esar, J., R. Colladay, and A. Kaufman, An Analysis of the Capabilities and Limitations of Turbine Air Coolin Methods, N.L.R. Center, Editor. 1970, NASA: Cleveland, OH. [6]. Torbidini, L. and A. Massardo, Analytical Blade Row Coolin Model for Innovative Gas Turbine Cycle Evaluations Supported by Semi-Empirical Air-Cooled Blade Data Journal of Enineerin for Gas Turbines and Power, : p [7]. Vittal, S., P. Hajela, and A. Joshi, Review of Approaches to as turbine life manaement, in 10th AIAA/ISSMO Multidisciplinary Analysis and Optimization. 2004, AIAA: Albany, NY. [8]. Horlock, J. H., Watson, D. T., and Jones, T. V., 2001, Limitations on Gas Turbine Performance Imposed by Lare Turbine Coolin Flows, ASME J. En. Gas Turbines Power, 123, pp Volume: 03 Issue: 03 Mar-2014, 609

8 IJRET: International Journal of Research in Enineerin and Technoloy eissn: pissn: [9]. Thamir K. Ibrahim1*, M. M. Rahman1 and Ahmed N. Abdalla2 Improvement of as turbine performance based on inlet air coolin systems: A technical review International Journal of Physical Sciences Vol. 6(4), pp , 18 February, 2011 [10]. Cun-lian Liu et al., Film coolin performance of converin slot-hole rows on a as turbine blade, International Journal of Heat and Mass Transfer 53 (2010) [11]. Sanjay Onkar Sinh and B.N. Prasada, Comparative performance analysis of coeneration as turbine cycle for different blade coolin means, International Journal of Thermal Sciences 48 (2009) BIOGRAPHIES Amjed Ahmed Jasim AL-Luhaibi born on 8th Au 1988 and completed B. Tech. from Technical collae / Kirkuk in fuel and enery enineerin department, Foundation of Technical Education, Iraq. Currently pursuin M. Tech. in Thermal enineerin, Mechanical enineerin department Sam Hiinbottom Institute of Ariculture, Technoloy and Sciences (Formerly Allahabad Aricultural Institute-Deemed University) Dr. Mohammad Tariq, Assistant Professor, Department of Mechanical enineerin, SSET, Sam Hiinbottom Institute of Ariculture, Technoloy and Sciences (Formerly Allahabad Aricultural Institute-Deemed University), Allahabad, UP, India. I have more than 14 years of academic experiences. About 20 articles/ papers have already been published in various journals. More than 20 M. Tech. students have been supervised. 5 students are doin Ph.D. under my supervision. Volume: 03 Issue: 03 Mar-2014, 610