Session: 01-05 Engine Performance and Cycle Design III

Paper Number: 153051

Optimum Pressure Ratio in a Turbojet Engine: Mathematical Expression to Determine the Pressure Ratio Required for Optimization of the Specific Thrust 

With the resurgence of interest in blended wing body aircraft, flight test vehicles are being designed to perform scaled model testing of the concept. Consequently, there is a demand for high-efficiency turbojet engines in the sub-1000 N-thrust range. Since very few engines have been manufactured specifically for these flight test vehicles, RC scale/hobby engines have been the solution of choice. These engines are famously known for their low efficiency; therefore, further improvement is required. The pressure ratio of a turbojet engine plays a major role in its efficiency and performance. The effects of pressure ratio have been explored fairly extensively and often require an iterative process because of the complexity of the equations and the number of variables. This publication investigates the derivation of a mathematical expression to determine the optimum pressure ratio to maximize the specific thrust of a turbojet engine. Mathematical expressions in place of iterative processes accelerate the time it takes to perform an optimization. Mathematical expressions for the optimum total pressure across the compressor to maximize the specific thrust are developed for two scenarios. In the first scenario, the engine components, including the compressor, combustor, and turbine, are assumed to possess isentropic efficiencies of one hundred percent. In the second scenario, these efficiencies are not assumed to be perfect, and a pressure loss exists across the combustor. These expressions are obtained by differentiating the net specific thrust with respect to the total compressor pressure ratio. The results are validated by calculating the net specific thrust for varying pressure ratios and then manually identifying the optimum value. The results of this exercise serve as a starting point for selecting total compressor pressure ratios in a clean sheet turbojet design.

Presenting Author: Jedidiah C. Nevo Carleton University

Presenting Author Biography: Jedidiah Nevo is a mechanical engineering graduate student at Carleton University. Having finished his bachelor's degree in Aerospace engineering, he decided to specialize in aircraft propulsion. Jedidiah's thesis is centered on the improvement of the combustor in a typical small gas turbine engine to maximize engine performance, consequently aiding in its implementation in flight test vehicles for blended wing body aircraft. Jedidiah has previously completed an internship in the Aerospace Research Centre of Canada's National Research Council and also worked on gas turbine design for his capstone project at the end of his undergraduate studies. Jedidiah hopes to nurture his skills and secure a career in the field of aircraft propulsion.

Authors:

Jedidiah C. Nevo Carleton University
Karl Y. Kurniawan Carleton University
Henry Saari Carleton University
Hamza M. Abo El Ella Carleton University