ISSN Vol.03,Issue.11 June-2014, Pages:

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1 ISSN Vol.03,Issue.11 June-2014, Pages: Thermal Analysis of a Regenerative Gas Turbine Cycle for Various Turbine Efficiencies DEEP PRAKASH SINGH 1, DR. MOHAMMAD TARIQ 2, VIJENDRA KUMAR KUSHWAHA 3 1 PG Scholar, Dept of Mechanical Engineering, SSET, SHIATS-DU, India. 2 Asst Prof, Dept of Mechanical Engineering, SSET, SHIATS-DU, India 3 Asst Prof, Dept of Mechanical Engineering, SSET, SHIATS-DU, India. Abstract: The effect of a regenerative heat exchanger in a gas turbine is analysed using a regenerative Brayton cycle model, where all fluid friction losses in the compressor and turbine are quantified by an isentropic efficiency term and all global irreversibility s in the heat exchanger are taken into account by means of an effective efficiency. For a fixed ratio of the compressor inlet temperature to the turbine inlet temperature and in terms of the isentropic and regenerator efficiencies explicit results has been presented for the behaviour of the maximum efficiency, efficiency at maximum power, maximum power and power at maximum efficiency as well as for the behaviour of the pressure ratios required for maximum efficiency and maximum power. Regenerative gas turbine engine cycle is presented that yields higher cycle efficiencies than simple cycle operating under the same conditions. The power output, efficiency and specific fuel consumption are simulated with respect to operating conditions. The analytical formulae about the relation to determine the thermal efficiency are derived taking into account the effected operation conditions (ambient temperature, compression ratio, regenerator effectiveness, compressor efficiency, turbine efficiency and turbine inlet temperature). Model calculations for a wide range of parameters are presented, as are comparisons with simple gas turbine cycle. The power output and thermal efficiency are found to be increasing with the regenerative effectiveness, and the compressor and turbine efficiencies. The efficiency increased with increase the compression ratio to 15, then efficiency decreased with increased compression ratio, but in simple cycle the thermal efficiency always increase with increased in compression ratio. The increased in ambient temperature caused decreased thermal efficiency, but the increased in turbine inlet temperature increase thermal efficiency. Keywords: Turbine Efficiency, Turbine Inlet Temperature. I. INTRODUCTION The development of the gas turbine is a source of great pride to many engineers worldwide and, in some cases takes on either industry sector fervor (for instance the aviation versus land based groups) or claims that are tinged with pride with one s national roots. People from these various sectors and subsectors can therefore get selective in their reporting. So for understanding the history of the gas turbine, one would have to read several different papers and select material written by personnel from the aviation, and landbased sectors. At that point, one can fill in the gaps. What follows therefore are two different accounts of the gas turbine s development. Neither of them is wrong. The first of these presents an aircraft engine development perspective [7]. In the original 19th-century Brayton engine, ambient air is drawn into a piston compressor, where it is compressed; ideally an isentropic process. The compressed air then runs through a mixing chamber where fuel is added, an isobaric process. The heated (by compression), pressurized air and fuel mixture is then ignited in an expansion cylinder and energy is released, causing the heated air and combustion products to expand through a piston/cylinder; another ideally isentropic process. Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement. II. MATERIALS AND METHODS A. Analysis of the Ideal Cycle The Air Standard cycle analysis is used here to review analytical techniques and to provide quantitative insights into the performance of an ideal-cycle engine. Air Standard cycle analysis treats the working fluid as a calorically perfect gas, that is, a perfect gas with constant specific heats evaluated at room temperature. In Air Standard cycle analysis the heat capacities used are those for air. In the present work the heat capacities for air and gas are taken as per their chemical constituents. Regeneration involves the installation of a heat exchanger (recuperator) through which the turbine exhaust gases pass. If the regenerator is well designed (i.e., the heat exchanger effectiveness is high and the pressure drops are small) the efficiency will be increased over the simple cycle value. However, the relatively high cost of such a regenerator must also be taken into account. Regenerators are being used in the gas turbine engines of the M1 Abrams main battle 2014 SEMAR GROUPS TECHNICAL SOCIETY. All rights reserved.

2 DEEP PRAKASH SINGH, DR. MOHAMMAD TARIQ, VIJENDRA KUMAR KUSHWAHA tank of Desert Storm fame, and in experimental gas turbine automobiles. Regenerated gas turbines increase efficiency 5-6% and are even more effective in improved part-load applications. Inlet air cooling and intercooling are two important methods for rising power output of gas turbine cycles. Many researchers have focused on three different inlet air cooling methods including the evaporative cooling with or without pre-compressed air and refrigeration cooling. However, gas turbine intake air cooling may cause a small decrease in efficiency because a lot of fuel is needed to bring compressor exhaust gas equal to the same gas turbine entry temperature. recuperator, which is taken as percentage of inlet pressure. The enthalpy of air entering the combustion chamber is given by energy balance equation ε_rc=((t_(rc,g) )_i-(t_(rc,g) )_e)/((t_(rc,g) )_i- (T_(rc,a) )_i ) (2) m _(rc,g).c_(p,g).ε_rc.{(t_(rc,g) )_in-(t_(rc,g) )_out m _(rc,a).c_(p,a).{(t_(rc,a) )_out-(t_(rc,a) )_in } (3) B. Thermodynamic analysis of various components The thermodynamic properties of air and products of combustion are calculated by considering variation of specific heat and with no dissociation. The curve fitting the data is used to calculate specific heats, specific heat ratio, and enthalpy of air and fuel separately from the given values of temperature. Mixture property is then obtained from properties of the individual component and fuel air ratio (FAR). Following equations are used to calculate the specific heats of air and gas as a function of temperatures. If T_a 800 c_pa=( ^3)-( T_a )+( ^(- 4).T_a^2)+( ^(-7).T_a^3 )-( ^(- 10).T_a^4 ) (1) In the above equations, T stands for gas or air temperature in deg K and t=t/ Regenerative Gas Turbines It has been shown in many publications that the efficiency of the Rankine cycle could be improved by an internal transfer of heat that reduces the magnitude of external heat addition, a feature known as regeneration. It was also seen in previous literatures that this is accomplished conveniently in a steam power plant by using a heat exchanger known as a feed water heater. 2. Regenerator A regenerator is modeled as a gas-to-gas counter flow heat exchanger (Fig 1). In this, heat is exchanged between compressed air coming out of the compressor and the hot gas exiting the gas turbine after expanding before entering the HRSG. The advantage of using a recuperator is that, some heat is added to the compressed air in the recuperator itself before it enters into the combustion chamber, so the same turbine inlet temperature is achieved as that when no such recuperator is employed, with lower fuel consumption hence the efficiency of the plant increases. The following are the assumptions for modeling of a recuperator. A concept of effectiveness of the recuperator is introduced to account for its inefficiencies. There is a pressure drop in the streams passing through the Figure 1 Schematic and temperature- heat transfer surface area for a counter-flow surface type recuperator. Figure 1 shows a gas turbine with a counter flow heat exchanger that extracts heat from the turbine exhaust gas to preheat the compressor discharge air to T5 ahead of the combustor. As a result, the temperature rise in the combustor is reduced to T_4. T5 a reduction reflected in a direct decrease in fuel consumed. The actual heat transfer rate to the air is c_p (T _5- T' _2), and the maximum possible rate is c_p (T' _4- T' _2) Thus the regenerator effectiveness can be written as ε_rc=(t_5- T' _2)/( T' _4- T' _2 ) (4) It is seen that the combustor inlet temperature varies from T2 to T4 as the regenerator effectiveness varies from 0 to 1. The regenerator effectiveness increases as its heat transfer area increases. Increased heat transfer area allows the cold fluid to absorb more heat from the hot fluid and therefore leave the exchanger with a higher T_5. C. Gas Turbine Analysis with Regeneration The exhaust gas temperature at the exit of the heat exchanger may be determined by applying the steady-flow energy equation to the regenerator. Assuming that the heat exchanger is adiabatic and that the mass flow of fuel is negligible compared with the air flow and noting that no shaft work is involved. Fig. 3 shows the T-S diagram for regenerative gas turbine cycle. The actual processes and ideal processes are represented in dashed line and full line respectively. The compressor efficiency (η_c), the turbine efficiency (η_t) and effectiveness of regenerator (heat exchanger) are considered in this study. These parameters in terms of temperature are defined as in (1) [8]:

3 Thermal Analysis of a Regenerative Gas Turbine Cycle for Various Turbine Efficiencies η_c=(t_2^'-t_1)/(t_2-t_1 ) (5) η_t=(t_4-t_5)/(t_4-t_5^' ) (6) Where, turbine inlet temperature (TIT) = T4.The net work is expressed as in (6) i.e. W_net=W_t-W_c (10) W_net c_pg T_4 η_t [1-1/(r_p^((γ_g-1)/γ_g ) )] -c_pa T_1 [(r_p^((γ_a-1)/γ_a )-1)/η_c ] (11) In the combustion chamber, the heat supplied by the fuel is equal to the heat absorbed by air, Hence, Q_ad=c_pg [T_4-T_1 (1-ε) (1+(r_p^((γ_a-1)/γ_a ) -1)/η_c )-ε T_4 [1-η_t (1-1/(r_p^((γ_g-1)/ γ_g ) ))]] (12) Power output is given by: P m _a W_net (13) Fuel to air ratio is given by Fig. 2 Schematic of a Regenerative gas turbine. FAR=(C.V.)/Q_ad (14) And Specific Fuel consumption SFC=(C.V.)/(AFR W_net ) (15) Thermal Efficiency is given by η_th=w_net/q_ad (16) Figure 3 T-s representation of Regenerative gas turbine cycle. Regenerative effectiveness is given by ε=(t_3-t_2)/(t_5-t_2 ) (7) The work required to run the compressor is expressed as in (2): W_c=c_pa T_1 [(r_p^((γ_a-1)/γ_a )-1)/η_c ] (8) The work developed by turbine is then rewrite as in (5): W_t c_pg T_4 η_t [1-1/(r_p^((γ_g-1)/γ_g ) )] (9) TABLE 1: INPUT DATA S. No. Components Values PA (Atm. Pressure) R ETAT ETAC ETAP ETAGEN RG ETAM TRS DOR C ETACL TBL CCL MA ETAB CV Ambient Temperature OPR TIT bar 287 kj/kgk kg kj/kg (K) (K)

4 DEEP PRAKASH SINGH, DR. MOHAMMAD TARIQ, VIJENDRA KUMAR KUSHWAHA Work Ratio: Work ratio is also very important parameter for the selection of a gas turbine cycle. The work ratio is given by the following equation: WR=W_net/W_t (17) Specific Fuel Consumption: Specific fuel consumption is defined as the fuel consumed by the combustor per unit network of the cycle; If the mass of fuel consumed is given in kg/s and the network developed is in kw then the specific fuel consumption will be calculated in kg/kwh. SFC=(3600 m_f)/w_net (kg/kwh) (18) Net Power: Net power available is calculated by the following equation. Net Power m_a w_net η_gen (19) If mass of air flow is given in kg/s and net power is given in kj/kg then the net power will be calculated in kw. efficiency. On increasing the turbine efficiency, the heat requited to combustor also increases. On increasing the regenerative effectiveness, the heat required to combustor decreases for a given value of turbine efficiency. III. RESULTS AND DISCUSSION In the following paragraphs the results of the present work have been discussed with the help of graphs. The results are based on the software developed in C++ and afterwards graphs have plotted with the help of menu driven software Origin 50. The graphs are plotted for various parameters for different compressor, turbine efficiency, turbine inlet temperature, overall pressure ratios and regenerative effectiveness. The results have been given the thermal efficiency, compressor work, specific fuel consumption and power developed etc. of the gas turbine cycle. The various turbine polytropic efficiency has been considered for the various output values. Figure 5 Compressor work with regenerative effectiveness. Figure 5 represents the compressor work with different regenerative effectiveness for different turbine efficiency. On increasing the turbine efficiency, the compressor work increases. On increasing the regenerative effectiveness, the compressor work also increases for a given value of turbine efficiency. Figure 6 represents the thermal efficiency with different regenerative effectiveness for different turbine efficiency. On increasing the turbine efficiency, the thermal efficiency increases. On increasing the regenerative effectiveness, the thermal efficiency also increases for a given value of turbine efficiency but the rate of increase is less as compared to the previous graph. Figure 4 Heat input to combustor with regenerative effectiveness. Figure 4 represents the heat input to the combustor with different regenerative effectiveness for different turbine Figure 6 Thermal efficiency with regenerative effectiveness.

5 Thermal Analysis of a Regenerative Gas Turbine Cycle for Various Turbine Efficiencies Figure 7 Mass of fuel required in combustor with regenerative effectiveness. Figure 7 represents the mass of fuel required in the combustor with different regenerative effectiveness for different turbine efficiency. On increasing the turbine efficiency, the mass of fuel required in the combustor also increases. On increasing the regenerative effectiveness, the mass of fuel required in the combustor decreases for a given value of turbine efficiency. Figure 8 represents the air rate with different regenerative effectiveness for different turbine efficiency. On increasing the turbine efficiency, the air rate decreases. On increasing the regenerative effectiveness, air rate increases for a given value of turbine efficiency. Figure 9 represents the specific power with different regenerative effectiveness for different turbine efficiency. On increasing the turbine efficiency, the specific power increases. On increasing the regenerative effectiveness, specific power decreases for a given value of turbine efficiency. Figure 9 Specific Power with regenerative effectiveness. Figure 10 represents the thermal efficiency with turbine inlet temperature (TIT) for different turbine efficiency. On increasing the turbine efficiency, thermal efficiency of the cycle has also been increases. On the other hand, increase in the turbine inlet temperature also results in the increase in the thermal efficiency of the gas turbine cycle. Figure 10 Thermal Efficiency with Turbine inlet temperature. Figure 8 Air rate with regenerative effectiveness. Figure 11 represents the mass of fuel required in the combustor with turbine inlet temperature (TIT) for different turbine polytropic efficiency. On increasing the turbine efficiency, fuel required in the combustor has also been increases. On the other hand, increase in the turbine inlet temperature also results in the increase in the fuel required in the combustor of the gas turbine cycle.

6 DEEP PRAKASH SINGH, DR. MOHAMMAD TARIQ, VIJENDRA KUMAR KUSHWAHA Figure 11 Mass of fuel required in combustor with Turbine inlet temperature. consumption are simulated with respect to operating conditions. The analytical formulae about the relation to determine the thermal efficiency are derived taking into account the effected operation conditions (ambient temperature, compression ratio, regenerator effectiveness, compressor efficiency, turbine efficiency and turbine inlet temperature). Model calculations for a wide range of parameters are presented, as are comparisons with variable compressor efficienciy of gas turbine cycle. The power output and thermal efficiency are found to be increasing with the regenerative effectiveness, and the compressor efficiency. The efficiency increased with increase the compression ratio to 15, then efficiency decreased with increased compression ratio, but in simple cycle the thermal efficiency always increase with increased in compression ratio. The increased in ambient temperature caused decreased thermal efficiency, but the increased in turbine inlet temperature increase thermal efficiency. V. REFERENCES [1] S. Minnet, Clean energy: The heart of a sustainable energy future for Europe, in: Proceeding of the Sixth International Conference on Technologies and Combustion for Clean Environment, Lisbon, [2] [3] F.F. Huang, L. Wang, Thermodynamic study of an indirect fired air turbine cogeneration system with reheat, ASME Journal of Engineering for Gas Turbine and Power 109 (1987) [4] R.P. Allen, J.M. Kovacik, Gas turbine cogeneration principles and practice, ASME Journal of Engineering for Gas Turbine and Power 106 (1984) Figure 12 Net power with Turbine inlet temperature. Figure 12 represents the net power developed with turbine inlet temperature (TIT) for different turbine polytropic efficiency. On increasing the turbine efficiency, net power developed has also been increases. On the other hand, increase in the turbine inlet temperature also results in the increase in the net power developed of the gas turbine cycle. IV. CONCLUSION The regenerative gas turbine power plant has been analyzed for various parameters. The most important parameter which has been covered in this work is the various polytropic efficiencies of compressor. The gas turbine regeneration package is designed to increase the efficiency of gas turbines with exhaust gas recirculation. Regenerative gas turbine engine cycle is presented that yields higher cycle efficiencies than simple cycle operating under the same conditions. The power output, efficiency and specific fuel [5] J.W. Baughn, R.A. Kerwin, A comparison of the predicted and measured thermodynamic performance of a gas turbine cogeneration system, ASME Journal of Engineering for Gas Turbine and Power 109 (1987) [6] P.J. Dechamps, Advanced combined cycle alternatives with latest gas turbines, ASME Journal of Engineering for Gas Turbine and Power 120 (1998) [7] Gas Turbine World, vol. 33 (6), Pequot Publishing Inc., Southport, CT, [8] The History of Aircraft Gas Turbine Development in the United States, St. Peter, J., Published IGTI, ASME, [9] Mohammad H. Albeirutty et al., (2004), Heat transfer analysis for a multistage gas turbine using different bladecooling schemes, Applied Thermal Engineering 24, [10] A. Poullikkas, An overview of current and future sustainable gas turbine technologies,

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