Department of Mechanical Engineering
https://open.metu.edu.tr/handle/123456789/113
Makina Mühendisliği Bölümü2020-07-06T10:14:54ZImpacts of inhomogeneous clamping force on local performance and liquid water formation in polymer electrolyte fuel cells
https://open.metu.edu.tr/handle/123456789/28620
Impacts of inhomogeneous clamping force on local performance and liquid water formation in polymer electrolyte fuel cells
Mehrtash, Mehdi; Tarı, İlker; Yesilyurt, Serhat
A two dimensional, half-cell, non-isothermal, multi-phase model of a polymer electrolyte fuel cell (PEFC) is developed. The model accounts for the acting clamping force on the cell with accompanying effects on gas transport properties and contact resistances. Spatial variations of anisotropic structural and physical properties of gas diffusion layers (GDLs) in both in-plane and through-plane directions are considered. Designed mechanistic model is compared and validated with the experimental data for voltage-current characteristics and channel-rib current density distribution for the first time. Significant changes are observed in local gas and water concentrations as well as current density profiles with respect to cell compression and humidity ratios of entrant gases. Compression exacerbates the liquid saturation under the rib in consequence of porosity and permeability reduction. Under compression, phase change rate increases in the cell; degree of supersaturation under the channel escalates leading to higher condensation rate while degree of undersaturation under the rib increases leading to higher evaporation rate.
2017-07-01T00:00:00ZInfluence of oxygen concentration on the sooting behavior of ethanol droplet flames in microgravity conditions
https://open.metu.edu.tr/handle/123456789/28603
Influence of oxygen concentration on the sooting behavior of ethanol droplet flames in microgravity conditions
Yozgatlıgil, Ahmet; Park, Seul-Hyun; Choi, Mun Young; Kazakov, Andrei; Dryer, Frederick L.
The influence of oxygen (O-2) concentration and inert on the sooting and burning behavior of large ethanol droplets under microgravity conditions was investigated through measurements of burning rate, flame temperature, sootshell diameter, and soot volume fraction. The experiments were performed at the NASA Glenn Research Center (GRC) 2.2 s drop tower in Cleveland, OH. Argon (Ar), helium (He), and nitrogen (N-2) were used as the inerts and the O-2 concentration was varied between 21% and 50% mole fraction at 2.4 atm. The unique configuration of spherically symmetric droplet flames enables effective control of sooting over a wide range of residence time of fuel vapor transport, flame temperature, and regimes of sooting to investigate attendant influences on burning behavior of droplets. For all inert cases, soot volume fraction initially increased as a function of the 02 concentration. The highest soot volume fractions were measured for experiments in At environments and the lowest soot volume fractions were measured for the He environments. These differences were attributed to the changes in the residence time for fuel vapor transport and the flame temperature. For the He inert and N2 inert cases, the soot volume fraction began to decrease after reaching a maximum value. The competition between the influence of residence time, rate of pyrolysis reactions, and soot oxidation can lead to this interesting behavior in which the soot volume fraction varies non-monotonically with increase in O-2 concentration. These experiments have developed new understanding of the burning and sooting behaviors of ethanol droplets under various O-2 concentrations and inert substitutions. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
2007-01-01T00:00:00ZRealization of human gait in virtual fluid environment on a robotic gait trainer for therapeutic purposes
https://open.metu.edu.tr/handle/123456789/28578
Realization of human gait in virtual fluid environment on a robotic gait trainer for therapeutic purposes
Ertop, Tayfun Efe; Yuksel, Tolga; Konukseven, Erhan İlhan
Patients with disorders such as spinal cord injury, cerebral palsy and stroke can perform full gait when assisted, which progressively helps them regain the ability to walk. A very common way to create assistive effects is aquatic therapy. Aquatic environment also creates resistive effects desired for strength building. In this study, realization of a virtual fluid environment on a robotic gait trainer is presented as an alternative method. A model was created to determine torques and forces acting on the human body while performing gait in a fluid environment. The developed model was implemented on a robotic gait trainer. By adjusting the virtual fluid model parameters, precise control over assistive and resistive effects during gait was achieved without enforcing any pre-defined gait pattern. The real-time gait phase information required by the fluid model to determine torques was provided with a developed algorithm which only uses kinematic gait data. Experiments with healthy subjects were done using the robotic gait trainer to verify the gait phase algorithm, and to compare gait characteristics obtained in virtual land and water environments with the literature. Additional experiments were performed with the robotic system to assess effects of changing fluid model parameters to healthy subject gait characteristics. The results show that force and torque effects of virtual fluid environment on robotic gait trainer were achieved. The gait phase algorithm was shown to provide smooth transition between phases. Also, significant changes in gait characteristics were observed by modifying fluid model parameters.
2018-07-01T00:00:00ZNumerical solution of solidification in a square prism using an algebraic grid generation technique
https://open.metu.edu.tr/handle/123456789/28558
Numerical solution of solidification in a square prism using an algebraic grid generation technique
Dursunkaya, Zafer; Odabasi, G.
The solidification of an infinitely long square prism was analyzed numerically. A front fixing technique along with an algebraic grid generation scheme was used, where the finite difference form of the energy equation is solved for the temperature distribution in the solid phase and the solid-liquid interface energy balance is integrated for the new position of the moving solidification front. Results are given for the moving solidification boundary with a circular phase change interface. An algebraic grid generation scheme was developed for two-dimensional domains, which generates grid points separated by equal distances in the physical domain. The current scheme also allows the implementation of a finer grid structure at desired locations in the domain. The method is based on fitting a constant arc length mesh in the two computational directions in the physical domain. The resulting simultaneous, nonlinear algebraic equations for the grid locations are solved using the Newton-Raphson method for a system of equations. The approach is used in a two-dimensional solidification problem, in which the liquid phase is initially at the melting temperature, solved by using a front-fixing approach. The difference of the current study lies in the fact that front fixing is applied to problems, where the solid-liquid interface is curved such that the position of the interface, when expressed in terms of one of the coordinates is a double valued function. This requires a coordinate transformation in both coordinate directions to transform the complex physical solidification domain to a Cartesian, square computational domain. Due to the motion of the solid-liquid interface in time, the computational grid structure is regenerated at every time step.
2003-12-01T00:00:00Z