School of Mechanical Engineering Seminar
Wednesday, January 7, 2015 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Condensation in parallel pipes with common headers

Rachel Ofir

MSc Student of Prof. Yehuda Taitel and Prof. Dvora Barnea

In heat exchangers the motive fluid is usually distributed in multiple parallel channels in order to enhance heat transfer by increasing the contact area and at the same time decrease the equipment size. Uniform flow distribution is required to obtaining high performance in heat exchangers. However maldistribution may occur under certain condition, primarily in evaporating systems.

In this work condensation within a parallel pipes system is examined. The studied system is an air cooled condenser comprised of a number of parallel pipes (horizontally oriented) with common headers located at the entrance to the pipes and at exit of the pipes.

Hot n-Pentane flows through the parallel pipes system. Air flows across the pipes and heat is rejected from the hot n-Pentane to the ambient air. In this thesis a model for predicting flow rate distribution in an air cooled condenser is presented.

You are invited to attend a lecture

By

Iddo Amit

PhD student under the supervision of Prof. Yossi Rosenwaks

Department of Physical Electronics, School of Electrical Engineering

Tel Aviv University

On the subject:

Functional Imaging of In-situ and Ex-situ Doped Silicon Nanowires

Semiconductor Nanowire-based electronics offers the opportunity to achieve tight control over a precisely defined component, often comprised of the nanowire (NW) itself, which in turn, enables the enhancement of the device performance. Currently, one of the main challenges facing the fabrication process of NW-based electronics is the formation of well-defined doping profiles which are crucial for the fine-tuning of the device behavior.

However, the conventional doping mechanism, where dopants are introduced in-situ during growth, results in both axial and radial inhomogeneous doping profiles which stem from a surface doping mechanism known as vapor-solid (VS) doping. Moreover, in interfaces between different doping types, a region of diffuse boundaries is formed rather than an abrupt transition between the two segments.

We use quantitative Kelvin probe force microscopy (KPFM) and nano-probe scanning Auger spectroscopy to measure both the longitudinal and the radial doping distribution in doped Si nanowires (SiNWs). Our findings shed light on the underlying mechanisms that produce these inhomogeneities by studying P doping profiles of axially modulation-doped SiNWs. We find that both the VLS and the VS mechanisms result in radially inhomogeneous doping, specifically, a lightly doped core surrounded by a heavily doped shell structure. By designing a modulated doping profile, the effects of the two mechanisms can be distinguished. We also discuss the influence of the reservoir effect that significantly broadens the axial doping junctions.

These results are compared to measurements conducted on monolayer contact doped (MLCD) NWs. This ex-situ doping process takes advantage of the precision of electron beam lithography as well as the selectivity and controllability of chemical monolayer formation to produce tailor-made dopant profiles by post-growth doping through the surface.

24 December 2014, at 14:00,

Room 031, Engineering Labs Building

You are invited to attend a lecture

by

Omer Shlomovits

(MSc. student under the supervision of Prof. Moshe Tur)

School of Electrical Engineering, Tel-Aviv University, Tel-Aviv 69978, Israel

Depletion and noise issues in fiber-optic Brillouin amplification

In the Stimulated Brillouin Scattering (SBS) effect, two frequency-shifted counter-propagating lightwaves, called signal and pump, interact to induce an acoustic wave, which later functions as a light scatterer, thus coupling the optical fields to each other. Provided that the frequency offset between the two lightwaves matches a particular resonance frequency, characteristic of the fiber, called the Brillouin Frequency Shift, amplification of the lower frequency lightwave, the signal, occurs as it propagates.

SBS has been a favorable mechanism for its robustness, simplicity of implementation and low pump power in standard fibers at room temperature. For this reasons the effect is widely used for numerous applications, including distributed sensing of temperature and strain, fiber lasers, optical memories and more. This work spans two topics, aiming at a common purpose of getting a better insight into the Brillouin amplification process while adjusting the existing models one step closer to the physical world.

In the first part of the talk, we will discuss the impact of laser phase noise on the Brillouin gain. It is shown that insufficient coherence of laser sources used in Stimulated Brillouin amplification setups not only leads to reduced gain but also to source-induced gain fluctuations (noise), which may affect performance. We will show that under commonly encountered conditions, even a relatively coherent source, having a linewidth of a 1 MHz, has enough phase noise to become the dominant contributor to the observed Brillouin gain fluctuations.

In the second part, we will focus on gain and polarization vector-properties of the SBS interaction in the presence of birefringence for the case of a strong enough signal to cause pump depletion. We determine that maximum and minimum gains are achieved for the same input states of polarization as in the undepleted case. Under normal conditions, the signal and pump power distributions along the fiber of the two input polarizations, which exhibit maximum and minimum gains, can be approximately deduced from a much simpler scalar formulation.

Tuesday, December 23, 2014, at 11:00

Room 011, EE- KITOT building

***** Seminar *****

Gilad Wallach

M.Sc. student under the supervision of Prof. Yael Hanein

Glutamate Mediated Astrocytic Filtering of Neuronal Activity

Neuron-astrocyte communication is an important regulatory mechanism in various brain functions. The complexity and role of this communication are not yet fully understood and several recent models have been proposed. In this work we built, characterized and optimized a new in vitro experimental setup suitable for the investigation of the neuro-glia communication pathway. The electrical activity of neuron-astrocyte cultures on multi electrode arrays was mapped using  imaging while neurons were selectively activated using underlying micro-electrodes. Using this setup, we demonstrate, for the first time, the existence of a frequency dependent, glutamate-mediated signaling mechanism between neurons and astrocytes. By modifying the stimulation frequency and number of spikes, we revealed a clear onset of astrocytic activation at neuron firing rates around 3 Hz. Astrocytic activation by neurons, as evidenced by astrocytic intracellular free calcium ion concentration  transients, was abolished with glutamate receptor blockers, validating the glutamate-dependence of this neuron-to-astrocyte pathway. These experimental results are compared with a biophysical model of neuron-astrocyte intercellular signaling developed by Berry and colleagues, which further support a glutamate mediated frequency dependent neuro-astrocyte communication. The frequency dependent neuro-astrocytes communication presented here may play an important, task specific role in brain function.

Tuesday, 16 December 2014 at 11:00, Room 011, Kitot Building.