Browsing by Author "Gamil, Abdelaziz A. A."

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    Assessment of numerical radiation models on the heat transfer of an aero-engine combustion chamber
    (Elsevier, 2020-11-02) Gamil, Abdelaziz A. A.; Nikolaidis, Theoklis; Lelaj, Indrit; Laskaridis, Panagiotis
    Thermal radiation is the most dominant type of heat transfer inside the combustion chamber, which can directly affect the temperature distributions at the combustor walls. This paper provides a comprehensive analysis of the effects of two radiation models on the flame and liner-walls temperatures. A combustion chamber used in the Rolls-Royce-RB-183 turbofan engine was examined in this study by integrating a solid combustor model with the numerical fluid domain. The results indicated that the implementation of radiation models shrinks the flame peak-temperature and altered temperature distribution across the liner. The Discrete Ordinates Method (DOM) estimated a 10% higher temperature at the front part of the liner compared to the non-radiation model and 15% less than the P-1 radiation method. After the dilution zone, the DOM and P-1 models estimated respectively 15% and 25% reduction in the liner temperature compared to the non-radiation combustor. The radiation models have also down predicted the flame temperature by 200 K and more than 200 K for DOM and P-1 case respectively. The results also showed that the emissivity value had minimal effects on the combustor temperature distribution. The DOM considered being more accurate to estimate the combustor wall and flow temperatures compared to the P-1 radiation method
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    Assessment of surface roughness effects on micro axial turbines
    (ASME, 2021-01-11) Gamil, Abdelaziz A. A.; Nikolaidis, Theoklis; Teixeira, Joao Amaral; Madani, S. H.; Izadi, Ali
    Surface roughness significantly affects the aerodynamics and heat transfer within micro-scale turbine stages. It results in a considerable increment in the blade profile loss and leads consequently to sizeable performance reductions. The provision of low roughness surfaces in micro gas turbine stages presents challenges on account of the small (mm scale) sizes, manufacturing complexity and associated costs. The axial turbine investigated in this study is fitted to Samad Power’s TwinGen domestic micro combined heat and power unit. The micro gas turbine has a compressor pressure ratio of 3, 1200K turbine inlet temperature and a rotational speed of 170,000 rpm. This paper presents a numerical assessment of the effects of varying the surface roughness on the performance and heat transfer of the micro turbine. The surface roughness was uniformly distributed on the NGV and rotor blades. The results showed that increasing the surface roughness from 3 microns to 6, 20, and 100 microns resulted in a reduction in stage total efficiency of 0.8%, 4% and 12% respectively as well as a comparable decrease in output power (0.7%, 3.6%, and 11% respectively). The turbine temperature was also observed to be very sensitive to surface roughness and a temperature increase of some 5% at the rotor hub and over 4% increment in the blade tip surface was observed for 100 microns when compared to the 3 microns surface roughness case. The findings of this paper highlight the adverse effects of the surface roughness on the micro-turbine performance and temperature distribution as well as the importance of careful consideration of wall roughness during the design and manufacturing stages
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    Effect of mist/steam uniformity on heat transfer characteristics in unconfined jet impingement
    (Elsevier, 2020-12-17) Alhajeri, Hamad M.; Almutairi, Abdulrahman; Alenezi, Abdulrahman H.; Gamil, Abdelaziz A. A.; Al-Hajeri, M. H.
    This paper presents the numerical investigations of the injection of mist into an unconfined cooling steam jet to analyze the effects on the corresponding heat transfer characteristics. The infusion of mist into an confined cooling steam jet demonstrates a remarkable improvement in the heat transfer specifications as described in numerous previous studies. Therefore, the current research contributes to the knowledge by focusing on the uniformity configurations of the injected second phase to the unconfined regions and the influence of the surrounding environment on the mist injected. Several variables govern the physical model of the mist concentration and jet to plate distance, which is included in three-dimensional incompressible Navier-Stokes flow with the discrete phase model for the multiphase flow regime. The simulation was conducted over a range of common working Reynolds numbers. The results show that controlling the uniformity of the injected mist will redefine the cooling characteristics. The accumulated mist near the edge leads to the heat transfer enhancement at H/D = 2, 4, and 6. The surrounding environment had a significant influence on the droplet behavior at H/D = 8, thus causing a heat transfer reduction of 8%. The droplet Stock number can determine the droplet performance on the target wall and introducing mist generates a third heat transfer peak that results in an average increase of 38% for 1% mist.
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    Effects of mist fractions on heat transfer characteristics in a rotating roughened cooling passage
    (Elsevier, 2019-07-30) Alhajeri, Hamad M.; Alenezi, Abdulrahman H.; Almutairi, Abdulrahman; Alhajeri, M. H; Gamil, Abdelaziz A. A.
    This paper investigates the effects of the mist fractions on heat transfer characteristics applied on a rotating U-channel with inclined ribs at an angle of 45°. This study has been conducted on five different mist percentages from 1 to 5 with a 1% increment at each step, and all of the cases have been investigated for Reynolds number values of 5000, 10000, 25000 and 40000. The numerical results obtained from the application of RNG k-ε turbulence model with enhanced-wall function were in good agreement with the experimental data of the smooth and ribbed channels both with and without mist addition. Results also demonstrated a 300% increase in the flow temperature difference and a noticeable increase in Nusselt number at each bend region and at the downstream of the leading edge around 25% and 110% respectively, at high mist fraction (5%); when compared with the case where only air was used. With %5 mist addition, the convective efficiency also reaches approximately 69%.
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    Flow structure and heat transfer of jet impingement on a rib-roughened flat plate
    (MDPI, 2018-06-13) Alenezi, Abdulrahman H.; Almutairi, Abdulrahman; Alhajeri, Hamad M.; Addali, Abdulmajid; Gamil, Abdelaziz A. A.
    The jet impingement technique is an effective method to achieve a high heat transfer rate and is widely used in industry. Enhancing the heat transfer rate even minimally will improve the performance of many engineering systems and applications. In this numerical study, the convective heat transfer process between orthogonal air jet impingement on a smooth, horizontal surface and a roughened uniformly heated flat plate is studied. The roughness element takes the form of a circular rib of square cross-section positioned at different radii around the stagnation point. At each location, the effect of the roughness element on heat transfer rate was simulated for six different heights and the optimum rib location and rib dimension determined. The average Nusselt number has been evaluated within and beyond the stagnation region to better quantify the heat transfer advantages of ribbed surfaces over smooth surfaces. The results showed both flow and heat transfer features vary significantly with rib dimension and location on the heated surface. This variation in the streamwise direction included both augmentation and decrease in heat transfer rate when compared to the baseline no-rib case. The enhancement in normalized averaged Nusselt number obtained by placing the rib at the most optimum radial location R/D = 2 was 15.6% compared to the baseline case. It was also found that the maximum average Nusselt number for each location was achieved when the rib height was close to the corresponding boundary layer thickness of the smooth surface at the same rib position.
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    Numerical investigation on heat transfer performance and flow characteristics in a roughened vortex chamber
    (Elsevier, 2020-02-26) Alhajeri, Hamad M.; Almutairi, Abdulrahman; Alenezi, Abdulrahman H.; Gamil, Abdelaziz A. A.
    In this study, an investigation of a vortex chamber was carried out to gain a full understanding of the nature of the vortex flow and the cooling capability inside the chamber. The paper discusses the effects on flow and heat transfer rates when the inside surface of the vortex chamber was roughened by adding flow turbulators to its wall. The turbulators took the shape of a rib with a square cross-section, the dimension of which varied between 0.25 mm and 2.00 mm. The paper also presents the results of a comparative investigation of jet impingement and vortex cooling on a concave wall using different parameters, such as the total pressure loss coefficient, Nusselt number and thermal performance factor, to evaluate the cooling effectiveness and flow dynamics. Furthermore, the entropy generation in swirl flow with the roughened wall was assessed over a wide range of Reynolds numbers. In this study, an investigation of a vortex chamber was carried out to gain a full understanding of the nature of the vortex flow and the cooling capability inside the chamber. The paper discusses the effects on flow and heat transfer rates when the inside surface of the vortex chamber was roughened by adding flow turbulators to its wall. The turbulators took the shape of a rib with a square cross-section, the dimension of which varied between 0.25 mm and 2.00 mm. The paper also presents the results of a comparative investigation of jet impingement and vortex cooling on a concave wall using different parameters, such as the total pressure loss coefficient, Nusselt number and thermal performance factor, to evaluate the cooling effectiveness and flow dynamics. Furthermore, the entropy generation in swirl flow with the roughened wall was assessed over a wide range of Reynolds numbers. The results show that surface roughness considerably influences the velocity distribution, heat transfer patterns and pressure drop in the vortex chamber. The highest thermal performance factor takes place at rib heights of 0.25 mm and 0.50 mm with a low Re number. Further increase in rib height has an adverse impact on thermal performance. At a Reynolds number lower than 50,000, it is highly recommended to use roughened vortex cooling to obtain the best thermal performance.