Flow and heat transfer characteristics of turbulent swirling impinging jets [thesis]

Numerous industrial applications rely on impinging jets to impart convective heat and mass transfer in processes ranging from the cooling of electronic devices and gas turbine blades to drying of paper and food products. Conventionally, non-swirling impinging jets have been employed, but some studies have shown that inducing swirl allows better control of uniformity and improved convective fluxes. A better understanding of the underlying physical mechanisms that lead to such behaviour warrants deeper insights into the flow and heat transfer characteristics of impinging jets, both swirling and non-swirling. Whilst important to achieve, the flow field of an impinging jet is already quite complex even before the addition of swirl which, in free (not impinging) jets, induces vortex breakdown and other instability modes. The addition of swirl to impinging jets thus has the potential to affect the transient and steady-state convective behaviour, both of which are crucial in industrial applications. This study features experimental and numerical investigations of incompressible turbulent impinging air jets that utilize aerodynamically generated swirl. The research focuses on the velocity field, upstream near the nozzle exit plane as well as further downstream, and the way in which it affects heat transfer at the impingement plane, both under transient and steady-state conditions. Boundary conditions at the nozzle exit were measured using Constant Temperature Anemometry. The surface temperature distribution of a thin foil heater, which forms the impingement surface cooled by the ambient temperature jet, was measured using infrared thermography for a range of Reynolds numbers (Re=11,600-35,000), swirl numbers (S=01.05), and impingement distances (H/D=2-6). The effects of different inflow conditions for non-swirling and weakly swirling impinging jets were also simulated (numerically) using ANSYS Fluent (version 16.2). Particle Image Velocity was utilized to resolve the flow field, over low (S=0.30) and higher (S=0.74) swirl over a range of Reynolds numbers (Re=11,60035,000) and nozzle-to-plate distance (H/D=2 and 4). Whilst the use of non-intrusive infrared thermography has been widely reported in studies of the steady-state heat transfer behaviour of impinging jets, an image processing methodology to resolve the time-dependant (transient) convective heat transfer behaviour was lacking. In this context, a MATLAB based method was developed to quantify the role of various impinging jet parameters on the time to reach steady-state. The effect of spatial discretization, image resolution, and the threshold value of time-dependent Nusselt number, on the time to reach steady-state, was also analysed. The role of various operating (Re, S) and geometric conditions (H/D) on the temporal evolution of turbulent impinging jets was also resolved. By applying the innovative image processing methodology developed, results show that for non-swirling jets, transient heat transfer characteristics at some conditions (H/D=4) are distinct if compared to others (H/D=2 and 6) and that the heat transfer distribution over the impingement plate changes significantly over a small interval of time. For swirling jets, the peak Nusselt number shifts to the wall jet region as the intensity of the swirl increases. Two correlations (no-to-low swirl, moderate-to-high swirl) are proposed to predict the time needed to reach a steady-state for Re=35,000. Computational Fluid Dynamics was then used to resolve the role of various (upstream) nozzle exist conditions (velocity profiles) on the emerging heat transfer characteristics at the impingement plane. Results showed that under some conditions (S=0.31, uniform velocity profile) a small recirculation zone, stabilised on the impingement plane, affects the heat transfer compared to other tested velocity profiles. This study also gave valuable insights on the impact of using (simple) geometric inserts to generate for swirl into impinging jets, a method widely used for its simplicity. Results showed that this can fundamentally perturb the results unlike the use of aerodynamic swirl which relies on tangential air ports. For the experimentally measured flow field, vortex breakdown is observed for two of conditions (Re=11,600 and 24,600 at S=0.74) out of the six tested. Impingement affects the position, shape, and strength of the vortex breakdown. For Re=24,600, impingement significantly affects (shape and position) the recirculation bubble when compared to impingement at Re=11,600. Heat transfer characteristics at high swirl are compared with low swirling impinging jets. The vortex breakdown (at high swirl) affects the impingment heat transfer and showed comparatively uniform heat transfer distribution in contrast to low swirling impinging jets. Vortex breakdown significantly deteriorates stagnation zone heat transfer and the Nusselt number peak occurs in the wall jet region. Benefits derived from this study include identifying impingement conditions that allow quicker stabilisation of heat transfer (shorter transients) as well as an improved understanding for the role of impingement on the upstream and downstream velocity field and heat transfer characteristics

Impacts of Social Networking Sites on Apparel Purchasing Behavior of Female Consumers

This study examines the impact of social networking sites on the apparel purchasing behavior of female consumers. The study's primary goal is to identify the difference between female university students' socio-demographic characteristics and apparel purchasing habits. The study employed a cross-sectional research design vis-à-vis the survey method to carry out this research. By choosing the purposive sampling technique, data is collected from 500 female respondents through a self-administered questionnaire. The theoretical framework of this study is based on the uses and gratification theory. To analyze the difference between different demographic variables, t-test and one-way ANOVA were applied. The results revealed a significant difference in mean scores of younger and older women, rural and urban residents, and junior and senior students.  Moreover, it is concluded that online advertisements through social networking sites greatly influence the apparel purchasing habits of young students compared to their older fellows. The findings suggest that apparel manufacturers and companies should adopt social media marketing strategies to promote their products.&nbsp

Relevance of Environmental, Institutional and Economic Developments for ASEAN-6

Economic Growth is influenced by large number of factors and there is an ongoing debate on the extent to which some of these strong and more quantifiable variables are effective. In order to comprehensively discuss the mainstream determinants of Economic Growth explored through past literature, the study chose to focus on ASEAN-6 countries encompassing Singapore, Malaysia, Indonesia, Thailand, Philippines and Vietnam. By considering three decades from 1985 to 2014 period and dividing it into multiple time span, intention was to evaluate the performance of ASEAN region before and after Asian Financial Crisis, Dot-Com bubble and Global Economic Crisis to reveal the most important drivers of economic performance. Our empirical results suggest that ASEAN-6 region performed better after Post-Global Economic Crisis period due to its better economic integration both intra-regionally and with other economies like China, EU. We find that Through Pooled OLS method along with Random Effect and Fixed Effect as robustness techniques to judge the effectiveness of trade liberalization, financial deepening, environment quality, foreign direct investment and 5 components of institutional quality (IQ), empirical findings have reported both trade openness and C02 emissions to have the most profound and consistent role in fostering the productivity of the region studied throughout. However, financial deepening in the region failed to significantly impact the economic growth except for 2002-2006 period when it negatively affected the growth. Net FDI inflows proved to be insignificant throughout except for pre-Asian financial crisis period. While IQ index played unimportant role for ASEAN-6 region during last 3 decades, IQ indicators like better voice and accountability enhanced growth throughout, regulatory quality during post Asian and Global Economic periods considered, good government effectiveness and control of corruption damaged the economic activity during period 1997-2001. ASEAN-6 region is hence recommended to continue with free trade notion, promote environment friendly technology, look and improve key institutional indicators for the region, and strengthen financial systems, as means to enhance its role as contributor to the global economy

Effect of cutting parameters and tool geometry on the performance analysis of one-shot drilling process of AA2024-t3

Drilling is an important machining process in various manufacturing industries. High-quality holes are possible with the proper selection of tools and cutting parameters. This study investigates the effect of spindle speed, feed rate, and drill diameter on the generated thrust force, the formation of chips, post-machining tool condition, and hole quality. The hole surface defects and the top and bottom edge conditions were also investigated using scan electron microscopy. The drilling tests were carried out on AA2024-T3 alloy under a dry drilling environment using 6 and 10 mm uncoated carbide tools. Analysis of Variance was employed to further evaluate the influence of the input parameters on the analysed outputs. The results show that the thrust force was highly influenced by feed rate and drill size. The high spindle speed resulted in higher surface roughness, while the increase in the feed rate produced more burrs around the edges of the holes. Additionally, the burrs formed at the exit side of holes were larger than those formed at the entry side. The high drill size resulted in greater chip thickness and an increased built-up edge on the cutting tools

In vitro storage of synthetic seeds: Effect of different storage conditions and intervals on their conversion ability

In vitro derived shoots of olive cv. Moraiolo were employed in synthetic seeds preparation by alginate encapsulation, and then stored in artificial endosperm solution at cold (4°C) and room storage (21 ± 2°C) conditions in interaction with different storage intervals of 0, 15, 30, 45 and 60 days to evaluate the comparative regrowth and conversion capacity of synthetic seeds. Cold stored synthetic seeds were superior in terms of their regrowth capacity than that of room stored ones for all the growth parameters studied. A promising degree of interaction was observed between 4°C and 45 days of storage interval for regrowth percentage as well as for shoot and root development. Moreover, an ascending trend was recorded in conversion potential with an increase in storage intervals up to 45 days (S3) whereas there was a declining trend after that up to 60 days (S4). Moreover plantlets regenerated from synthetic seeds, with 4 - 6 fully expanded leaves and well developed root system were successfully acclimatized under ex vitro conditions. The protocol can be used for germplasm exchange of woody trees and preparation of synthetic seed.Keyword: Synthetic, seed, olive, encapsulation, storage, conversionAfrican Journal of Biotechnology Vol. 9(35), pp. 5712-5721, 30 August, 201

Comparative Neurological and Behavioral Assessment of Central and Peripheral Stimulation Technologies for Induced Pain and Cognitive Tasks

Pain is a multifaceted, multisystem disorder that adversely affects neuro-psychological processes. This study compares the effectiveness of central stimulation (transcranial direct current stimulation—tDCS over F3/F4) and peripheral stimulation (transcutaneous electrical nerve stimulation—TENS over the median nerve) in pain inhibition during a cognitive task in healthy volunteers and to observe potential neuro-cognitive improvements. Eighty healthy participants underwent a comprehensive experimental protocol, including cognitive assessments, the Cold Pressor Test (CPT) for pain induction, and tDCS/TENS administration. EEG recordings were conducted pre- and post-intervention across all conditions. The protocol for this study was categorized into four groups: G1 (control), G2 (TENS), G3 (anodal-tDCS), and G4 (cathodal-tDCS). Paired t-tests (p < 0.05) were conducted to compare Pre-Stage, Post-Stage, and neuromodulation conditions, with t-values providing insights into effect magnitudes. The result showed a reduction in pain intensity with TENS (p = 0.002, t-value = −5.34) and cathodal-tDCS (p = 0.023, t-value = −5.08) and increased pain tolerance with TENS (p = 0.009, t-value = 4.98) and cathodal-tDCS (p = 0.001, t-value = 5.78). Anodal-tDCS (p = 0.041, t-value = 4.86) improved cognitive performance. The EEG analysis revealed distinct neural oscillatory patterns across the groups. Specifically, G2 and G4 showed delta-power reductions, while G3 observed an increase. Moreover, G2 exhibited increased theta-power in the occipital region during CPT and Post-Stages. In the alpha-band, G2, G3, and G4 had reductions Post-Stage, while G1 and G3 increased. Additionally, beta-power increased in the frontal region for G2 and G3, contrasting with a reduction in G4. Furthermore, gamma-power globally increased during CPT1, with G1, G2, and G3 showing reductions Post-Stage, while G4 displayed a global decrease. The findings confirm the efficacy of TENS and tDCS as possible non-drug therapeutic alternatives for cognition with alleviation from pain

A comprehensive design and optimization of an offset strip-fin compact heat exchanger for energy recovery systems

Energy recovery in conventional thermal systems like power plants, refrigeration systems, and air conditioning systems has enhanced their thermodynamic and economic performance. In this regard, compact heat exchangers are the most employed for gas to gas energy recovery because of their better thermal performance. This paper presents an economic optimization of a crossflow plate-fin heat exchanger with offset strip fins. A detailed software-based numerical code for thermal, hydraulic, economic, and exergy analysis is developed for three fin geometries. Genetic Algorithm, parametric, and normalized sensitivity analyses are used to discover the most influential parameters to optimize the total cost. The parametric study showed that with the increase of mass flow rates and plate spacing, outlet stream cost and operating cost increased due to the rise in pressure drops. Finally, the optimization reduced the operational cost by ∼78.5%, stream cost by ∼64.5%, and total cost by ∼76.8%

Düşük ve yüksek frekanslı titreşimle çalışan mikro akışkan jetlerinde yapı-akış ilişkisinin incelenmesi

Thesis (M.A.)--Özyeğin University, Graduate School of Sciences and Engineering, Department of Mechanical Engineering, June 2015.Synthetic jets are being investigated last several years. Researchers have been interested in its unique applications for a wide range of flow control to thermal management of electronics applications. Synthetic jets are made up of actuators such as piezoelectric, magnetic, or linear piston technology etc. Piezoelectric synthetic jets are investigated and found relationship between its structural and flow parameter like deflection, frequency and resonance with velocity and heat transfer enhancement. In order to find a relationship some numerical and experimental techniques are being used. For experimental result and fully-automated experimental setup made using high end equipment like Laser Vibrometer, Hotwire anemometer and 3D robot. Four different type of measurement were made on synthetic jet actuators deflection, heat transfer, velocity measurements and power consumption. The heat transfer enhancement factor of each of these jets with respect to natural convection is measured over a 25.4x25.4 (mm) vertical heater. Finally, power consumption of the low and high frequency synthetic jets were measured and compared. It is found that disk deflection and operating frequency are directly related to heat transfer enhancement factor, if the Helmholtz frequency of a cavity has no effect on the performance of a jet. For numerical analysis commercially available software are used like Ansys and Comsol. Eigen frequency and transient deflection is modeled using Comsol 4.0 and different mode shapes and deflection measurements using PZD module of Comsol. A 3-D computational fluid dynamics model was constructed using Ansys CFX to determine the flow and temperature fields of a meso-scale central orifice synthetic jet at a nozzle-to-target surface spacing of y/d = 2, ReDj = 1400 and f = 500 Hz. Unlike the majority of previous computational studies, rather than specifying the boundary conditions at the nozzle, the flow inside the synthetic jet device was solved by specifying the time dependent boundary conditions on the vibrating diaphragm and utilizing the moving mesh technique. Local surface pressure and heat transfer coefficient distributions were determined and discussed. It was found that the pulsating flow at the nozzle exit for a round jet generates vortex rings and these rings seem to have some considerable effects on the target surface profiles. Finally figure out the design parameters for Synthetic jet actuator for the optimum performance.Son zamanlarda sentetik jetlerin uygulanması üzerine araştırmalar yapılmaktadır. Sentetik jetlerin kendilerine özel sağladıkları geniş akış kontrolü ile elektroniklerde termal performansın artırılması konusunda araştırmacıların ilgisini çekmektedir. Sentetik jetler basit olarak piezo elektrik yada magnet gibi tetikleyiciler ile yapılmaktadır. Geliştirilen piezo elektrik sentetik jetler için yapısal ve akış parametreleri (bükülme, frekans, akış hızına göre rezonans, ve ısı atımında performans artışı gibi) arasında bir ilişki elde edilmiştir. Özellikle ısı atımında performans artışı hedeflenen sentetik jetlerde ki bu etkin parametreler arasındaki ilişkinin en doğru şekilde kurgulanabilmesi için sayısal ve deneysel methodlar kullanılmıştır. Deneysel methodların uygunlanması için lazer titreşim cihazı, sıcak kablo ile hız ölçer ve 3 boyutlu robot gibi üst düzey cihazlar eşliğinde deneysel sistem kurulmuştur. Sentetik jetlerde bükülme, ısı atımı, akış hız ölçümü ve enerji tüketimi olmak üzere dört tür hesaplama yapılmıştır. Yapılan jetlerin ısı atım performans faktörü, daha önceden kayıpları hesaplanmış olan 25.4x25.4 (mm)'lik dikey bir ısıtıcı yüzeyinden hesaplanmıştır. Son olarakta , düşük ve yüksek frekansta çalıştırılan jetlerin enerji tüketimleri hesaplanmış ve kıyaslanmıştır. Sayısal method Ansys ve Comsol programları ile yapılmıştır. Zamana bağımlı bükülme testleri Comsol da yapılırken, 3 boyutlu akış ve termal dağılım simülasyonları ( Jet nozulu ile ısıtıcı yüzeye arası başluk oranı y/d = 2, ReDj = 1400 ve f = 500 Hz için) Ansys CFX'te yapılmıştır. Bölgesel basınç ve ısı atım katsayısı elde edilmiş ve titreşimli akışların (pulsating flow) jet nozul çıkışında vorteks halkası oluşturduğu ve bu vorteksin önemsenecek boyutta sonuca etkisi gözlemlenmiştir. Sonuç olarak bir sentetik jet tetikleyicisinin performans sağlayabileceği dizayn parametreleri ortaya çıkartılmıştır