Makine Mühendisliği Bölümü Yayın Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/263

Browse

Filters

2017 - 201922020 - 20222

Settings

Has files: NoPublisher: search.filters.publisher.Turkish Soc thermal Sciences Technology

Search Results

Now showing 1 - 4 of 4
  • Article
    Citation - WoS: 1
    Numerical Analysis of Pulsating Circular Impinging Laminar Jet on a Planar Disc
    (Turkish Soc thermal Sciences Technology, 2017) Kahroba, Mitra; Türkoğlu, Haşmet; Turkoglu, Hasmet; Makine Mühendisliği
    In this study, the flow and heat transfer characteristics of pulsating circular air jets impinging on a flat surface were numerically analyzed. The jet velocity pulsated in time. The objective of the work is to investigate the influence of the jet Reynolds number, pulsation amplitude and pulsation frequency on the rate of heat transfer from the target hot surface. For the analysis, a computer program, based on the control volume method and SIMPLE algorithm, was developed. Laminar flow with the time averaged jet Reynolds numbers between 300 and 700 were analyzed. The pulsation amplitude is ranged between 0.0V(0) (steady jet) and 0.8V(0) (m/s) (V-0 is period averaged jet velocity), and the frequency is ranged between 1 and 6 Hz. The nozzle-to-plate distance was kept constant at H/d=3. From the simulation results, it was observed that at any instant of the pulsation period, the local Nusselt number is maximum at the stagnation point, and it decreases along the plate. This decrease in the local Nusselt number is not monatomic as in the steady jet cases. It has local maximum and minimum values (fluctuations) due to the moving recirculating flow regions along the bottom plate. At low frequencies, the time (period) averaged stagnation point Nusselt numbers are lower than the corresponding steady jet Nusselt numbers. However, with the increasing frequency, the stagnation point Nusselt number increases and become higher than the steady jet Nusselt number.
  • Article
    Experimental Study and Theoretical Investigation of High Temperature Proton Exchange Membrane Fuel Cell Micro-Cogeneration Application
    (Turkish Soc thermal Sciences Technology, 2018) Yapıcı, Ekin; Devrim, Yilser; Ozgirgin Yapici, Ekin; Makine Mühendisliği
    In this study, a house hold micro-cogeneration system is designed using high temperature proton exchange membrane (HTPEM) fuel cell. HTPEM type fuel cells gain the highest interest lately, due to their advantages in terms of increasing efficiency and power quality, reducing harmful emissions and flexibility of operation with respect to the other fuels. The micro-cogeneration system involves producing both electrical energy and hot water and/or vapor together in an economical way, utilizing single fuel (HTPEM fuel cells) for household applications. During the operation of the fuel cell, for high efficiency and stable power production, the access heat of the stack should be removed constantly and the temperature of the stack should be held stable. Heat recovered from the designed innovative cooling system is used for acquiring energy for heating water. This way, thermal efficiency is almost doubled compared to simple cycle. In the scope of this study, 225 W HTPEM fuel cell stack is designed and tested at 160 degrees C operation temperature with hydrogen gas and air. During operation, for homogenous distribution of temperature among the cells, for a short start up period leading to a fast required steady state temperature and for constantly removing the access heat produced in the cell, the cell stack is cooled by using a cooling fluid (Heat Transfer Oil 32- Petrol Ofisi). Selection of insulation material type and thickness for the cell stack is done using natural convection and radiation loss calculations. For the most efficient operating conditions, micro-cogeneration system water inlet and exit temperatures, water and cooling fluid flow rates, convenient pipe diameter and pump power calculations are done to finalize the design. With the cogeneration system designed during the studies, by recovering the access heat of the insulated HTPEM cell stack, district water with initial temperature of 15-20 degrees C is heated around 50 degrees C. Data gathered during studies indicate that fuel cell micro-cogeneration application is highly viable.
  • Article
    Citation - WoS: 5
    Citation - Scopus: 5
    Ann and Anfis Performance Prediction Models for Francis Type Turbines
    (Turkish Soc thermal Sciences Technology, 2020) Aylı, Ülkü Ece; Ayli, Ece; Ulucak, Oguzhan; Makine Mühendisliği
    Turbines can be operated under partial loading conditions due to the seasonal precipitation fluctuations and due to the needed electrical demand over time. According to this partial working need, designers generate hill chart diagrams to observe the system behavior under different flow rates and head values. In order to generate a hill chart, several numerical or experimental studies have been performed at different guide vane openings and head values which are very time consuming and expensive. In this study, the efficiency prediction of Francis turbines has been performed with ANN and ANFIS methods under different operating conditions and compared with simulation results. The obtained results indicate that it is possible to obtain a hill chart using ANFIS method instead of a costly experimental or numerical tests. ANN and ANFIS parameters which effect the output, have been optimized with trying 100 different cases. 75% of the numerical data set is used for training and 25 % is used for validation as testing data. To asses and compare the performance of multiple ANN and ANFIS models several statistical indicators have been used. Insight to the performance evaluation, it is seen that ANFIS can predict the efficiency distribution with higher accuracy than the ANN model. The developed ANFIS model predicts the efficiency with 1.41% mean average percentage error and 0.999 R-2 value. To the best of the author's knowledge, this is the first study in the literature that ANN and ANFIS are used in order to predict the efficiency distribution of the turbines at different loading conditions.
  • Article
    Citation - WoS: 2
    Citation - Scopus: 2
    Analysis of Heat Transfer Enhancement in Tubes With Capsule Dimpled Surfaces and Al2o3-Water Nanofluid
    (Turkish Soc thermal Sciences Technology, 2022) Ibrahim, Mahmoud Awni A. Haj; Turkoglu, Hasmet; Yapici, Ekin Ozgirgin; Haj Ibrahim, Mahmoud Awni A.
    This study aims to numerically investigate and evaluate the enhancement of heat transfer by new capsule dimples on tube surfaces for flow of water and Al2O3-water nanofluid with different concentrations, under uniform surface heat flux. The originality of this work lies in combining two passive heat transfer enhancement methods such as geometrical improvements and nanofluids together. Capsule dimples with different depths were considered. Al2O3- water nanofluid was modeled as a single-phase flow based on the mixture properties. The effects of dimple depth and nanoparticle concentrations on Nusselt number, friction factor and performance evaluation criteria (PEC) were studied. Numerical computations were performed using ANSYS Fluent commercial software for 2000-14000 Reynolds number range. It was found that when laminar, transient and fully developed turbulent flow cases are considered, increase in the dimple depth increases the Nusselt number and friction factor for both pure water and Al2O3-water nanofluids cases. Also, the friction factor increases as dimple depth increases. Results show that increase in PEC is more pronounced in the laminar region than in the transition region, it starts to decrease for turbulent flows. For nanofluid, PEC values are considerably higher than pure water cases. The variation of PEC for capsule dimpled tubes are dependent on flow regimes and dimple depths. Increasing the nano particle volume concentration and dimple depth in laminar flows increase the PEC significantly.