Nonconvective radiators of single or double-active surface design are analyzed for surface temperature distribution; also for view factors in the case of fin-and-tube geometry. Methods and examples of maximizing heat rejection per unit weight are given.
The experience gained from the operation of a commercially available turbojet engine laboratory system is described. This system, the Turbine Technologies, Ltd. Mini-Lab TM , is suitable for use in undergraduate mechanical and aeronautical engineering laboratories. Key turbojet engine performance parameters can be computed from the data measured during test runs. The use of this system provides an excellent opportunity for students to apply the principles of thermodynamics.The Mini-Lab TM was acquired by the Mechanical Engineering Department of Loyola Marymount University (LMU) during the fall semester of 1999. It was checked out and interested faculty members were trained in the use of the system. The system was installed in the Thermal Sciences Laboratory at LMU and approved for operation by the university's Environmental Health and Safety Officer. The installation included providing the necessary utilities, building a baffled intake manifold for sound suppression and building a double-walled exhaust manifold for exhaust gas expulsion, thermal protection and sound suppression.The Mini-Lab TM includes the SR-30 turbojet engine, the auxiliary subsystems required for the operation of the engine, controls, a safety enclosure, the instrumentation needed to acquire the experimental data and the data acquisition interface. The engine consists of a conical diffuser, a centrifugal compressor, a reverse flow annular combustor, an axial flow turbine and a converging conical exhaust nozzle. The system has been used in LMU's senior mechanical engineering laboratory for the past two years and for demonstrations during open house type events. Engine speed, various pressures and temperatures, fuel flowrate and thrust are measured. Using these measured data, thermodynamic relationships, and property data, the following performance parameters can be determined: compressor, turbine and exhaust nozzle adiabatic efficiencies; fuel-air ratio; air mass flowrate; engine thermal efficiency; specific thrust; and thrust specific fuel consumption. In addition, the thrust can be computed from exhaust nozzle data and compared with the measured thrust.
Engineering students encounter the Otto cycle in their first course in thermodynamics (usually during the sophomore year). This cycle is the theoretical basis for the spark ignition (SI) internal combustion engine (ICE). The traditional analysis (the air-standard analysis) of the Otto cycle is a static thermodynamic analysis that cannot be used to predict the dynamic performance of a SI ICE. Given sufficient information, the work per cycle for a particular engine can be computed. However, by making three simple modifications, the air-standard analysis can be extended to include a computation of the dynamic performance of a SI ICE. The first of these modifications is the selection of representative values of specific heats and specific heat ratios for the working fluid during each process. This improves the accuracy of the analysis. The second is an equation relating the heat release during combustion to pertinent engine parameters (the fuel-air ratio and the compression ratio). The third is the inclusion of an equation for the volumetric efficiency of the engine as a function of engine speed. This incorporates into the analysis the single most significant loss and results in performance that is dependent on engine speed. The resulting analysis predicts the dynamic performance (power and torque as a function of engine speed) of contemporary SI ICE engines with reasonable accuracy. Most importantly, this analysis can be easily understood and conducted by engineering students in their first thermodynamics course. Students have used this analysis, with excellent results, to analyze typical engines for a variety of applications (various types of passenger cars, pick-up trucks, SUV's, Formula 1 vehicles and, even, "monster" trucks).
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