Laboratory Experience With A Model Jet Turbine

This paper describes the experience gained from the operation of a JetCat model turbojet engine as part of an undergraduate mechanical engineering program. The engine was remotely controlled from a laptop using Jettronic for Windows software for the serial interface. Engine speed, fuel consumption, and exhaust gas temperature were measured using the software and the thrust was determined from a digital force gauge and compared with calculations based on different readings. Students designed the turbine mount and a safety enclosure for the engine. The use of this engine has been a low cost alternative to other commercially available turbojet laboratory systems. Introduction It is now 65 years since the first successful flight using a jet turbine in the Heinkel He 178 aircraft 1 . Since then, modern turbo-jets have been developed to a high level of sophistication. During the last 15 years, model aircraft builders have also developed fully functional scale versions of jet turbines 2-4 . In recent years the Turbine Technologies SR30 turbojet engines have been used in mechanical engineering laboratories 5-7 . Another available laboratory system is the Powertek axial flow gas turbine engine. Our choice was to purchase a lower cost model aircraft engine kit that included all the necessary auxiliary equipment. In this study, a turbojet laboratory system was set up in the undergraduate manufacturing course, and used for labs and demonstration purposes in fluid mechanics and applied thermodynamics 8 . The laboratory set up consists of a JetCat P-70 model jet engine and subsystems required for operation. The turbojet engine with a weight of only 1.2 kg and a diameter of 94 mm produces a maximum thrust of around 70 N at 120,000 rpm. The idle rpm is 35,000. The instrumentation provided with the engine includes a temperature sensor at the exhaust exit and a speed sensor. Furthermore, an electronic control unit (ECU) simplifies start up and assures safe operation by maintaining turbine rpm within certain limits. A support unit (GSU) was connected to the ECU for monitoring parameters such as temperature, rpm, and fuel pump voltage. By using a RS-232 serial P ge 938.1 Proceedings of the 2004 American Society for Engineering Education Annual Conference & Exposition Copyright ©2004, American Society for Engineering Education interface and Windows software, the ECU was linked to a laptop computer for download of operational data. The software also allowed the user to start and stop the turbine and to set desired rpm, see figure 1. Fig. 1. Jettronic for Windows software used to control the jet turbine Engine thrust was measured using Transducer Techniques HFG-45 digital force gauge, see figure 2. The engine could operate using deodorized kerosene, 1-K kerosene or JetA1 fuel. The fuel was mixed with 5 % Aeroshell 500 turbine oil and Coleman Powermax fuel was used as starting gas during initial startup driven by an electric motor positioned at the intake of the turbine. Fig. 2. Test stand including engine and digital force gauge Students designed and fabricated the basic engine mount based on four linear bearings for movement. The entire mount was screwed to a removable plate attached to a cart. A P ge 938.2 Proceedings of the 2004 American Society for Engineering Education Annual Conference & Exposition Copyright ©2004, American Society for Engineering Education safety system was installed by enclosing the engine with polycarbonate. Two fans were added at the top to blow along the polycarbonate in order to ensure that it would not be overheated, see figure 3. Fig. 3. Engine test stand showing polycarbonate enclosure and cooling fans. Theory The properties that can be measured and calculated using this system are listed in Table 1. The thrust measured by the force gauge was compared with theoretical results based on the turbine’s outlet triangle and the mean peripheral speed u related to the mean diameter of the turbine wheel dm = 0.0553m and the rotational speed n.