M.Sc Thesis


M.Sc StudentKam Ori
SubjectOn the Operation of a Microjet Engine's Vaporizer under
Steady-State Conditions
DepartmentDepartment of Aerospace Engineering
Supervisor PROFESSOR EMERITUS Yeshayahou Levy
Full Thesis textFull thesis text - English Version


Abstract

Fuel system injection using vaporizers found a wide application in small jet engines. The vaporizers have a simple design, however, the processes involved in the fuel preparation for combustion are highly complicated. They include disintegration of liquid jet under co-flowing air, impingement of liquid jet on hot wall, formation and motion of liquid film, droplet splashing and fuel evaporation occurring over different two-phase flow regimes. The objective of the presented study is to investigate these processes and develop physical models to evaluate the fuel evaporation rate. Throughout the presented study, the following achievements were obtained. A global correlation to evaluate the breakup length of liquid jet under co-flowing air was developed. This allows us to distinguish between the liquid jet stability before impinging the wall during engine operation in Idle and Maximum power conditions. Additionally, the splash mechanism of a wall-jet impingement event was found to be completely different depending on whether the liquid jet is continuous or disintegrated. For both cases, two different models were developed based on measured data of the liquid splash ratio. A major part of the study was based on a dedicated experimental system that was developed to evaluate the fuel evaporation rate in a tube that simulates a fuel vaporizer. The tube was subjected to a crossflow of hot exhaust gasses, generated by burning a kerosene-air mixture in a swirl stabilized combustion chamber. Liquid evaporation measurements were performed by comparing the liquid mass flow at the outlet with respect to that at the inlet. The liquid and gas phases were separated at the tube outlet using an insulated cyclone separator. Based on the kerosene evaporation measurements, a new model was developed to evaluate the internal two-phase convective heat transfer. The evaporation performance of the vaporizer was found to be very sensitive to the inlet fuel to air mass flow ratio. Hence, besides altering the vaporizer geometry, by modifying the fuel to air mass flow ratio, one can easily control the vaporizer performance, improve the combustion efficiency and the global engine performance.

In future work, combining commercial CFD codes with the tools achieved in this study will enable the development of a method for an optimal fuel vaporizer’s design.