Investigation on Scaling of Gas Turbine Engines for Drone Propulsion

Investigation on Scaling of Gas Turbine Engines for Drone Propulsion G. Jims John Wessley Assistant Professor Department of Aerospace Engineering Karunya University, Coimbatore, Tamilnadu, India Swati Chauhan PG Student Department of Aerospace Engineering Karunya University, Coimbatore, Tamilnadu, India ABSTRACT The development of micro engines by downscaling contemporary gas turbine engines is one of the promising means of developing a viable propulsion system for small aircrafts, UAVs and drones. Though the idea of downscaling is promising, the drawbacks and challenges associated with the process necessitates enormous amount of research and innovation in terms of design, manufacturing and testing. This paper presents the detailed investigation on the need for developing micro engines and the associated benefits and challenges. A model for development of micro engines with the various steps involved is also presented. The outcomes of this research will be very useful for researchers and industrialists in overcoming the practical difficulties and venturing into new potential areas of development in micro engines to power future UAVs and drones. Keywords Micro gas turbine engines, Scaling of engines, drone Propulsion, unmanned aerial system, Propulsion INTRODUCTION The encouraging use of drones and UAVs in non-military applications such as civil and commercial markets has resulted in tremendous economic and technological advancements during the recent past. A strenuous effort by researchers to evolve new expertise in the areas of computation, propulsion, communication, materials, sensors and payloads have paved the way for significant development of this technology. However, this technology has not matured to its fullest as most the developments are under way and sill in the developmental / experimental phase. The propulsion system in a UAV consists of an energy input, a mechanism to convert the energy to mechanical energy and thereby produce the thrust force required. Most of the modern unmanned aircrafts use battery-powered propulsion and occasionally fuel cell powered systems while the use of solar power is in its primitive stage. Reciprocating engines used in MALE UAVs produce very high power output and the high rotational speeds generate high-frequency noise. The small measures involved in miniaturization of turbine engines and the high value of thrust developed makes the micro turbo engines an attractive alternative source for drone propulsion [1]. However, the use of micro gas turbines to propel drones and UAVs have proven to have a huge potential and advantageous over its counterparts. There are also a few areas of concern that requires further investigation and research so that micro gas turbines can be a commercially viable alternate source of power for the future drones. This paper brings out the detailed investigation on the feasibility of using micro gas turbines, the advantages and challenges involved in the scaling down of existing gas turbines to suit real-time application in UAV and drone propulsion. 48 G. Jims John Wessley, Swati Chauhan

LITERATURE BACKGROUND The use of small UAVs in performing civil and commercial operations like photography, wild life research, agriculture surveying and mapping and others necessitate high endurance, low noise and emissions and extended mission ranges. Most of the researches focused on improving UAV and drone capabilities depend on improving the propulsion system as the mass of the power plant and its specific fuel consumption has a significant effect on the range and performance of the UAV [2]. Unlike manned drones, the UAV and drones have very specific requirements like long endurance, high power to weight ratio, high altitude of flight etc., The small gas turbine engines have small number of moving parts, low noise, multi-fuel capabilities and high energy density potential. The works of researchers in the similar front have been presented in this section. A comparison of the UAV propulsion systems [2] is given below in Table 1. Table 1. Comparison of various UAV propulsion systems The detailed study on the scaling limitations of micro engines by Joseph Griebel [3], brings out the scaling issues in micro engines that are of primary concern and suggests feasible solutions offering promising technological advancements that could lead to realistic and practical developments of micro gas turbine engines. Onur Tuncer and Ramiz Omur Icke [4], performed reverse engineering studies on a micro turbojet engine and obtained the parameters of this engine in-line with the performance of existing engines. The study suggests that the use of reverse engineering procedures reduces the overall turnover time of a new design which does not require the designing to originate from scratch. The outcomes of this study can be considered as a benchmark for designing more prospective advanced turbojet engines as the performance parameters estimated analytically matches closely with the manufacturers specifications. The investigations of scaling laws for combustion engine performance by Brown e.al [5], have shown increased performance in small combustion engines. The study also attempts to evaluate the different outcomes based on different methods of quantifying the scaling laws and tries to unify the results and outcomes. The study points out the need for thorough understanding of true nature of combustion processes like fuel-air mixing, ignition, flame propagation, heat release and heat loss before attempting to develop miniature scale engines. Further, the study also reveals that the heat loss due to incomplete combustion is the major factor of concern in miniature scale engines. 49 G. Jims John Wessley, Swati Chauhan

Derek et.al [6], studied the influence of vibratory stresses on a scaled rotor blade of a single-stage high pressure turbine. The rotor blade scaling was performed by keeping the rotor solidity, the ratio of the tip clearance height and the blade height, and the axial gap size as constant. The study demonstrates that the blade scaling effects were essentially negligible on the time-averaged turbine performance but remarkable on the unsteady surface pressure fluctuations occurring near the leading edges and towards the root of the blades. Juntung et.al [7], performed computational investigations on Reynolds number and wall heat transfer effects on the performance of a NASA sage 35 compressor. Results show that the efficiency of the miniaturized compressor decreases due to decrease in pressure ratio and Reynolds number. However, the optimization of compressor blade geometry is suggested to avoid high entropy regions in the miniaturized structure. More energy loss through wall is observed for compressor at small-scale, which leads to more cooling of compressor blades thereby increasing the overall efficiency of the compressor. Sandeep Kumar et.al [8], carried out studies on redesigning the combustion chamber of a small-scale turbojet engine and charted the design, manufacturing and operational aspects of the combustion chamber. The study reveals that turbine inlet section has to be improved to avoid thermal stresses arising due to hot air stagnation. Also, the air distribution system in the combustion chamber has to be improved to reduce improper burning of fuel and the pollution due to incomplete combustion. The work on investigating the theoretical limits of scaling-down internal combustion engines by Sher et.al [9], presents a simple algebraic equation that shows the inter relationships between the pertinent parameters that constitutes the lower possible miniaturization limits of IC engines. SCALING OF GAS TURBINE ENGINES Scaling is a common technique to define larger or smaller geometries with similar characteristics. In these lines, scaling of a high performance large gas turbine engine does not necessarily result in a good micro gas turbine engine. The reduction of scale has several effects on the performance and construction of the turbine. Scaling of established gas turbine engines has lots of advantages over designing and building an engine from scratch. Some of the advantages are [10] : a) High energy density potential b) Increased redundancy and reliability c) Increased operational flexibility However, the miniaturization of gas turbine engines is not that easily achievable, as mere scaling down of individual components does not yield a good optimal scaled design. There are few challenges to be addressed through ardent research before this could be a reality in the near future. The important factors to be considered while scaling of gas turbine engines are discussed below. 1. Downscaling Effects The square cube law generally used to scale down engine and its components states that when an object undergoes a proportional decrease in size, its new length is inversely proportional to the sizing factor, its new area inversely to the square of the sizing factor and its new volume is inversely proportional to the cube of the sizing factor. Because of this, the new area to volume ratio becomes misappropriate as the overall size of the engine is decreased. Dimensional analysis shows that the power developed by a gas turbine is proportional to the density of the gas, the fifth power of the diameter and the third power of the rotational speed which means the size of the turbine and its rotational speed are inversely proportional. 2. Large change in Reynolds Number The large change in Reynolds number in a downscaled engine is of major concern. The Reynolds number decreases down linearly to several orders in a scaled engine so that the Reynolds number of the flow in a scaled engine is much lower than a conventional gas turbine engine. With the prevailing laminar flow, the viscous fictional losses are estimated to be higher and the mixing of fuel and air in the combustor is expected to be slower thus reducing the efficiency and power density of the engine. 50 G. Jims John Wessley, Swati Chauhan

3. Heat Transfer Due to the increase in surface to volume ratio in a downscaled engine, the heat losses from the engine may be more causing a reduction in the efficiency of the engine. At very small sizes, the heat losses from the engine can exceed the heat generated, because of which ignition of the fuel is affected. Another heat transfer issue arises with the internal heat transfer between the hot turbine and the cold compressor which are a two different extreme temperatures of operation. Due to the flow of heat flux in the micro engine, the flow in the compressor absorbs heat and the flow in the turbine looses heat. This leads to incomplete flame propagation which affects the overall efficiency of the micro engine. 4. Geometrical restrictions related to manufacturing The major challenge in manufacturing a downscaled engine is the inability to maintain the same level of relative accuracy in turbine and compressors as in the larger ones. The roughness and the materials has a negative impact on the efficiency with decreasing dimensions. Further, the efficiency of scaled compressor is very sensitive to change in tip clearances and gap-to-blade height ratio. 5. Tribology factors Another major area of concern is the increase in the friction, wear and stiction factors within the downscaled engine. Due to the inability to bring out sealing rings suitable to the downscaled engine, it is essential to permit friction between moving parts of the cylinder to avoid chamber leakages thereby increasing the friction resulting in resonance and instability. MODELLING OF MICRO GAS TURBINES Figure 1 : Process of downscaling gas turbine engines 51 G. Jims John Wessley, Swati Chauhan

The primitive step in modeling a micro gas turbine engine is to select a suitable engine for the downscaling process. The selection of suitable reference engine is done based on three important parameters like a) type of propulsion system b) UAV type on which the engine is to be used c) Mission requirements of the UAV. The main challenge of the downscaling process is to preserve the characteristics of the thermodynamic cycle even in the reduced flow conditions in the scaled engine. The thermodynamic performance of the engine depends mainly on the turbine inlet temperature, compressor pressure ratio and component efficiencies. The design processing of downscaling of gas turbines is summarized in Figure 1. The preliminary micro gas turbine can be designed from the reference model chosen by applying the appropriate non-dimensional parameters. The inlet mass flow is directly proportional to the second power of the diameter and roughly to the output power of the engine. Hence, mass flow is scaled first to obtain the required power output by suitably changing the diameter. The relation between the mass flow rate of air (m a), the diameter (D) and the power output (P) is given by the equation shown in Eq (1) below. (1) The next step is to identify the scaling factor (n) based on the expected shaft output power (P Desired) and the reference engine power (P REF) at specified engine cycle conditions. The relation used to determine (n) is given in Eq (2). (2) The required mass flow rate of the desired engine is estimated using the Eq (3). * n (3) Further, component efficiencies can be calculated with standard laws and correlations, including the size of the engine as a variable and determining the change in performance of the engine with change in dimensions. The component efficiencies are influenced by factors like rotor/ impeller sizes, velocity ratio, ratio of rotor tip speed to theoretical spouting velocity, exit flow coefficient and clearance gaps. The empirical relation used to obtain the efficiency of the compressor ( c) is given by Eq (4). The turbine efficiency ( t) of the scaled engine can be estimated using the equation Eq (5) as shown below. (4) Where n= scaling factor, R= gas constant and =index of compression. The weight of the new scaled engine needs to be estimated as the integration of this engine to the UAV will impact the aerodynamic performance of the UAV. The empirical relation used to obtain the specific weight of the turbine is given by equation Eq (6). (6) Further, Gas turbine Simulation Program (GSP) can be used to predict the engine performance of the scaled engine in terms of efficiency and power output. (5) 52 G. Jims John Wessley, Swati Chauhan

CONCLUSIONS A detailed study on the downscaling of gas turbine so as to propel UAVs and drones is performed. The attainment of a micro gas turbine is may not be possible by creating a miniaturized copy of the larger ones. However, the intervention of new materials and new manufacturing techniques is sure to make these micro systems a competent and viable option to power UAVs and drones in the near future. REFERENCES [1] Dutczak, J. 2016. Micro turbine engines for drones propulsion. IOP Conference Series : Material Science and Engineering 148 (2016), 1-10. [2] Anna Marcellan. 2015. An exploration into the potential of microturbine based propulsion systems for civil unmanned and Aerial vehicles. Technical Report. Delft University of Technology. [3] Joseph. C.P. Griebel. 2010. Scaling limitations of Micro Engines. Undergraduate Research Journal at UCCS. Volume 3.2 (October 2010), 1-11. [4] Onur Tuncer and Ramiz Omur Icke. 2011. Reverse engineering of a micro turbojet engine. International Aerospace conference AIAC-2011-092, Turkey (September 2011). 1-13. [5] S. Brown, S. Menon, C. Hagen. Investigation of scaling laws for combustion engine performance. Western States Section of the Combustion Institute (Fall 2015) Meeting Provo, UT. 1-16. [6] Derek Lasiwka, Dongil Chang and Stavros Tavoularis. Effect of rotor blade scalinhg on gas turbine performance. Sixth international symposium on turbulence and shear force phemonena. (June 2009). South Korea. 803-808. [7] Junting Xiang, Jorg Uwe Schluter and Fei Duan. 2014. Computational analysis of the scaling effects on the performance of an axial compressor. International Journal of Mechanical, Aerospace, Mechatronic and Manufacturing Engineering, Volume 8. No. 5. 869-875. [8] Sandeep Kumar Singh and Mondal, S.S. 2014. Redesigning of the combustion chamber for small scale static hrus turbojet engine. International Journal of Scientific and Engineering Research. Volume 5, Issue 1. 934-941. [9] Sher, E. and Sher, I. 2011. Theoretical limits of scaling-down internal combustion engines. Chemical Engineering Science. Volume 66, Issue 3, 260-267. [10] Decuypere, R. Verstraete, D. (2005) Micro Turbines from the standpoint of potential users. Micro Gas Turbines. 15.1-15.14. 53 G. Jims John Wessley, Swati Chauhan