School of Mechanical Engineering Seminar
Monday, January 19, 2015 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

ACTIVE FLOW CONTROL AS A FLUIDIC FENCE PREVENTING TIP STALL ON SWEPT BACK WINGS

Israel J. Wygnanski

Th is limited by flow separation. Although some principles of active separation control in two-dimensional flow are well known, their application to aircraft has not been realized partly because of wing sweep-back and partly because of the weight and complexity of the systems providing the flow control. Sweeping jet actuators overcame many of the objections stated above because they do not possess moving parts and if used in small numbers they require little mass flow and energy to operate. This is the case on swept back, high aspect ratio wings where the spanwise boundary layer flow has to be reduced in order to avoid tip stall. This observation is rooted in the application of boundary layer theory to yawed airfoils of infinite span. The concept that is commonly referred to as the “Independence Principle” was considered inapplicable to turbulent flows for more than sixty years, and it cannot be rigorously applied to finite, tapered wings. Nevertheless the precept is sufficiently robust to be reduced to practice as it broadly suggests that the spanwise flow contributes to separation although its dominance is not an indicator of stall. Experimental results at multiple scales supporting these observations will be presented and discussed.e size and complexity of an airplane wing is determined by the maximum lift that it can generate, mostly for takeoff and landing purposes, and this

School of Mechanical Engineering Seminar
January, May 5, 2014 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Solar electricity with photon-enhanced thermionic emission (PETE)

Prof. Abraham Kribus

School of Mechanical Engineering, Faculty of Engineering

Tel Aviv University, Israel

The efficiencies of photovoltaic and thermal conversion of solar energy are both limited by thermodynamics, and by the complexity required in real conversion devices. A novel third alternative of photon-enhanced thermionic emission (PETE) offers an intriguing synergy of direct photonic conversion together with thermal conversion, and theoretically may reach efficiency exceeding 70%. However, realizing this conversion path requires a major multi-disciplinary effort in semiconductor physics, surface science, thermal science, and systems engineering.
A PETE converter consists of a semiconductor cathode with a bandgap that is matched to the solar spectrum, and an anode separated by a vacuum gap. Illuminating the cathode with concentrated solar radiation increases the conduction band electron population and reduces the energy barrier to electron emission by two mechanisms: increasing temperature due to heating by thermalization, and photo-generation that increases the electron concentration above its equilibrium level. Coating the cathode with a material that lowers the electron affinity can further reduce the barrier to emission. Hence, PETE devices utilize both photonic and thermal processes for energy conversion. The anode receives electrons from the cathode, and is heated by thermalization of these electrons. This waste heat is removed from the anode by an external heat exchanger. It is possible to use this heat to drive a secondary thermal cycle (e.g., heat engine or thermoelectric converter) to generate additional electricity and increase the overall device efficiency. A theoretical analysis of the ideal performance limits shows that under 1,000 suns, efficiency of the two-stage PETE and thermal conversion can exceed 70%, higher than ideal PV cells and higher than ideal thermal conversion. Practical implementation, however, is not easy. Several major challenges related to materials and device engineering will be discussed. Much further research, as well as engineering ingenuity, are needed to resolve both the materials issues and the device level design challenges, on the path to realize the promise of PETE conversion.