Literature Review: Design of micro gas turbine engine for MAVs

Micro gas turbine (MGT) engine is different from gas turbine (GT) engines and is not assumed as a smaller version of GTs. MGTs have advantages related to distributed energy systems, low environmental impact and for its low cost in operations and maintenance. MGTs have gained worldwide attention because they are well suited in service sector, aviation and in small industrial applications. MGTs have found their usage in applications such as unmanned aerial vehicles and in micro aerial vehicles (MAV). MGTs have low gas mass flow-rate which results in its smaller size and rotational speed (Renzi et al., 2014). This implies if the machine is small, its rotational speed is high. There are two operation modes in MGT namely non-cogeneration and cogeneration. Non-cogeneration mode of MGT is used only in electricity production application. Cogeneration is the combined production of both electrical and thermal energy (Jain et al., 2015). In this review report, the design aspects of the micro gas turbine engine are explored for its use in micro aerial vehicles.

Decuypere & Verstraete, (2005) explained different general uses of micro gas turbine engine from the perspective of potential users. The authors highlight the use of MGT in energy production for portable devices and in the propulsion of vehicles. It is important to note the main advantage of micro gas turbine engines is it's high energy density of fuel-based systems. MGTs also provide benefits such as system redundancy and reliability along with flexibility in its operations. In the area of vehicle propulsion, MGTs are used in the propulsion of micro aerial vehicles (MAV) and in the propulsion of unmanned vehicle or small manned aircraft in the method known as distributed propulsion (Isikveren et al., 2014). MAVs are used in different applications such as in environmental monitoring and for other observation purposes. Usually, MAVs have dimensions of less than 6 inches and have the capacity to carry a miniaturised payload, a communication link which is equipped with a fully autonomous system for navigation. The weight of MAVs is around 50 grams and hence MAVs are used in covert military operations (MacAllister et al., 2013).

Nascimento et al., (2016) provides a detailed and rigorous analysis of micro gas turbine engine performance for its further use in many application areas. Micro gas turbines have the characteristics of variable rotation, reliability, simplicity, high-frequency electric alternator, ease of installation and maintenance, and so on.The authors evaluated the performance of micro- turbines at variable loads to understand its efficiency and usage. The performance of gas turbine was tested for natural gas and liquid fuel Capstone micro-turbines and results presented for the partial and full load. The performance evaluation intends to show the comparison of micro gas turbines are better than internal combustion engines in terms of efficiency in terms of emissions (CO and NOx) when fueled by natural gas. The authors’ highlight that micro gas turbines have the potential for cheap large scale manufacturing.

Caresana et al., (2017) provided an overview of MGTs as the critical function of heat recovery in improving the energy competitiveness of technology. Cogeneration (the generation of heat and electricity) can be considered as a native application of MGTs. While there are many advantages in terms of flexibility and efficiency, the authors state that MGTs are highly sensitive to the production of electrical power in ambient temperature and electrical efficiency. However, with a technique known as a fogging system, the dependence of ambient temperature is prevented which is preferred in many applications. The authors analyse two options towards increasing electrical efficiency. They are organic Rankine cycles and STIG (steam injected gas) configuration. The Rankine cycles method is easier to apply as it does not need changes to the design of MGT, but replacement of recover boiler with organic vapor generator is needed. Rankine cycles technology is also developed and available in markets for its low-temperature heat recovery applications. Conversely, in STIG configuration the redesign of the combustion chamber and revisions of both control and housing systems are needed. However, both these technologies improve electrical efficiency in MGTs.

Studies related to the micro gas turbine engine were explored. Czarnecki & Olsen (2017a), explained the design of miro gas turbine engine by analyzing different parameters. The parameters selected for numerical investigation include vaneless diffuser height coefficient, diffuser blade design angle, and efficiency for design and fuel consumption (isentropic efficiency). The entire analysis was made using ANSYS CFX to identify the sensitivity of the design. The design of the micro jet engine is derived from two main designs namely, MW54 developed by Wren Turbines (Parish et al., 2000) and KJ-66 engine (Volponi, 2014). In this article, the analysis is provided by influencing some modifications in GT60 turbonet by equipping it with KKK OEM 5324-123-2017 compressor wheel (Czarnecki & Olsen, 2017b). In this study, numerical simulations were presented to show the results of design. Usually, in this level of design, the industry standard of a κ-ε model for turbulence was applied. However, the authors propose that this model needs to be compared with a real engine for its performance. Also, the use of computational fluid dynamics was used or its advantage over analytical method because the designer can also rate the design from its internal behaviour which is possible in this 3-dimensional environment. The analysis falls short of overcoming the numerical inaccuracies found in CFD modeling. In this article, the reader is provided with a conceptual stage of gas turbine engine behavior and the design has been rated for engine performance, the efficiency of design and fuel consumption indicators (Czarnecki & Olsen, 2017a).

Elbanhawi et al., (2017) explain the utility of micro air vehicles for its benefits in terms of low cost in a variety of commercial systems. MAVs are popular due to miniaturisation of flight control systems and advances in propulsion and energy storage technologies. MAVs are used in applications such as environmental monitoring, communications relay and so on. They are dependent on the operator in the loop and do not have autonomous operations. The authors provide a review of MAV operations in complex environments and provide the limitations in MAV, which are attributed to computational power, and energy storage. This paper provides further insights to enable MAVs to operate autonomously in urban environments.

Devi (2015), presented an experiment to study the design and analysis of turbine blades in a micro gas turbine engine using data from literature surveys. MGT engines have a combustor which is the main component and the author emphases that this design must be simple and robust in construction. In MGT, burnt gases from the combustor will pass through the turbine blades and hence the design and production of turbine blades are highly crucial. In order to achieve high optimal efficiency the shape, blade angle and blade size must be considered in the design. The design must also consider compatibility with the combustor and must be based on aerodynamic considerations. In this study, the author considered design data from existing literature survey and using these values the flow parameters for the engine are derived to analyse the increased momentum thrust. The turbine design was made using CATIA software and data exported for analysis into the FLUENT software. The results from this experiment showed desired performance through estimated designs. Increased thrust was achieved by using velocity and pressure at different angles of attacks. However, this analysis falls short of overcoming complications in blade design.

Hassanalian & Abdelkefi (2017) made a review of classifications, applications and design challenges in drones. Drones are unmanned aerial vehicles and have capacities to fly thousands of kilometers or smaller drones can fly in limited spaces. The authors classify drones based on their characteristics as MAVs, NAV (Nano air vehicle), VTOL (vertical take-off and landing), and so on. In the case of MAVs, the authors explain propulsion options that include MGTs (Radmanesh et al., 2012). Here, it must be noted that MGT is lightweight and smaller compared to other propulsion systems. The use of MGTs is highlighted as the propulsion system in the design of MAVs in this report, mainly for its high efficiency and flexibility for use in MAVs (Hassanalian & Abdelkefi, 2017).

Oppong (2016) presented a detailed study to show the performance of BMT 120 KS micro gas turbine engine. The MGT was evaluated for its component matching of engine elements using compressor and turbine characteristic maps and engine’s performance was analysed. This study was made to have a better understanding of the benefits and limitations of engine components and their impact on performance. MGT engines and the engine presented in this study have inter-related components with non-linear characteristics. It is important, to note the overall engine performance is achieved by the contribution made by individual components. The investigation is done by matching components of the BMT engine with modified compressor and turbine stages. In order to have an improved component matching of the engine, reviews related to the analytical and numerical analysis of the engine was done. The review of BMT 120 KS thermodynamic scale by was done by assessing the engine’s previous evaluations related to theoretical and experimental data. This study was conducted to validate thermodynamic performance of the MGT engine using simulation software. The software used was GasTurb for modeling and simulating the baseline engine and Flownex SE was used for analysis. The analysis included the modified compressor and turbine components for different engine configurations to finally suggest improvements in preliminary turbine stage design for the chosen engine.

Dsouzaa et al., (2016) state that MAV due to their small size will not be able to achieve accurate aerodynamic flying characteristics. The authors also mention that MAVs do not have adequate testing methods and have low Reynold’s number. In order to overcome these complexities, the authors propose the use of sub-sonic open circuit micro sealed wind tunnel. They present the development of wind tunnels which were gathered and analysed from literature from earlier decades. Using data from secondary sources, the authors suggest a fabricated micro model of the tunnel which can be used to test the fundamental aerodynamics of MAV flying at low-speed and having low Reynold’s number. This article highlights various possibilities to study and understand in-flight properties of very small aerial vehicles at low speeds with low aspect ratio. It must be noted the design process of MAV is driven by the wing shape and its aerodynamics. These two features are determined from the experimental data obtained from the wind tunnel test.

According to (Singh & Singh, 2013) the theoretical design procedure for MAV will include

• Determining the components and take off weight
• Evaluating cruise velocity
• Analysing the lift coefficient needed for a given area to maintain flight level
• Applying wind tunnel data to know predictions of drag coefficient and lift
• coefficient for different platform profiles
• Calculating the angle of attack required to achieve the lift coefficient for changing wing area and aspect ratio (this include different planform shapes)
• Selecting wing plan from the shape
• Selecting aspect ratio suited for different applications

Since the technology of MAV is emerging and demands are increasing, the demand for wind tunnels has also increased. Therefore, it has become customary to first develop a model prototype wind tunnel prior to designing a full-scale wind tunnel. The prototype will help the developer overcome problems in the design cycle. It is important to note the low-cost wind tunnel testing will focus on testing aspects such as flow stability and uniformity to achieve maximum wind speed for testing various MAVs (Dsouzaa et al., 2016).

In summary, it may be noted that the design of micro gas turbine engines will play a role for its use in propelling micro aerial vehicles. It was noted that the design of MAVs can play a role in influencing the design of MGT and vice versa. However, the benefits derived from MGT and MAV in terms of cost, operational efficiency and flexibility are highlighted by most of the authors, from the literature explored.