Applied Mechanics and Materials Vol. 787

Abstract: Concentrated solar collectors have high efficiency as compared to flat plate and evacuated tube solar collectors. Cavity receivers are mainly used on the parabolic dish concentrators and tower type concentrator systems. The heat transfer surfaces of cavity receiver are composed by coiled metal tube. Heat transfer fluid flows in the internal spaces of coiled metal tube, and the external surfaces would absorb the highly concentrated solar energy. This paper explains the thermal performance of parabolic dish concentrator system with hetero-conical cavity receiver. The experimental analysis was done during the month of April 2014 on clear sunny days at Chennai [Latitude: 13.08^oN, Longitude: 80.27^oE] to study its thermal performance.

Abstract: R134a is the most widely used refrigerant in domestic refrigerators. It must be phased out soon according to Kyoto protocol due to its high global warming potential (GWP) of 1300. It has been proposed that by previous experimental investigations, R290/R600a is the best alternate ozone friendly refrigerant mixture for R134a. But due to its high flammability this mixture is not completely substituted as an alternate working fluid in our country. In the present work, performance investigation has been made with eco-friendly refrigerant mixture (composed of R123 and R290 in the ratio of 70:30 by weight) in a refrigerator. This mixture has a very low ozone depletion potential (ODP) and has zero GWP value. Also this mixture has low flammability index when compared with R290/R600a. Continuous running tests were performed under different ambient temperatures (23°C, 28°C, 32°C and 36°C).The ON OFF and DOOR OPEN test results showed that ON TIME percentage of the compressor for the mixture (HCFC123/HC290) is less than that for HCM (R290/R600a). Also the power consumed by mixture (HCFC123/HC290) is lesser by 15% than that of R290/R600a and due to its low flammability property it can be a valuable replacement for the R290/R600a refrigerant mixture.

Abstract: This paper presents the heat transfer and friction coefficient correlations for triangular plain fin surfaces of plate fin compact heat exchanger. It will be prohibitively expensive and time consuming to fabricate heat exchanger cores and conduct experiments over reasonable ranges of all the geometric variables. In contrast, it is relatively easy and cost effective to carry out a parametric study through numerical simulation and derive acceptable correlations for use in industry. A numerical model has been developed for the triangular plain fin of plate fin heat exchanger. The CFD analysis is carried out using FLUENT 12.1, Colburn factor j and fanning friction factor f are calculated for different Reynolds numbers. These values are compared with the available literature data of j and f factors. The correlations have been expressed in terms of two separate equations over the low and high Re regions along with dimensionless geometric parameters.

Abstract: TIn this present study, the forced convection heat transfer performance of different fluids, namely, Al₂O₃-water, and CuO-water nanofluid has been studied experimentally in an automobile radiator. Three different concentration of nanofluid in the range of 0-1.0 vol.% have been prepared by addition of Al₂O₃ and CuO nanoparticles into water. The test fluid flow rate can be varied in the range of 2 l/min to 5 l/min to have a fully turbulent regime. Obtained results demonstrate that the fluid circulating rate can improve the heat transfer performance. The heat transfer performance of CuO-water nanofluid was found better than the other heat transfer fluids. Furthermore, the Nusselt number is found to increase with the increase in the nanoparticle concentration and nanofluid velocity.

Abstract: Wind energy is one of the most significant renewable energy sources in the world. It is the only promising renewable energy resource that only can satisfy the nation’s energy requirements over the growing demand for electricity. Wind turbines have been installed all over the wind potential areas to generate electricity. The wind turbines are designed to operate at a rated wind velocity. When the wind turbines are exposed to extreme wind velocities such as storm or hurricane, the wind turbine rotates at a higher speed that affects the structural stability of the entire system and may topple the system. Mechanical braking systems and Aerodynamic braking systems have been currently used to control the over speeding of the wind turbine at extreme wind velocity. As a novel approach, it is attempted to control the over speeding of the wind turbine by aerodynamic braking system by providing the chord wise spacing (opening). The turbine blade with chord wise spacing alters the pressure distribution over the turbine blade that brings down the rotational speed of the wind turbine within the allowable limit. In this approach, the over speeding of the wind turbine blades are effectively controlled without affecting the power production. In this paper the different parameters of the chord wise spacing such as position of the spacing, shape of the spacing, width of the spacing and impact on power generation are analyzed and the spacing parameters are experimentally optimized.

Abstract: A numerical investigation has been carried out in the present work to study flow characteristics and thermal performance of a chevron type brazed plate heat exchanger (PHE). A local element-by-element analysis utilizing e-NTU method is employed for simulating the heat exchanger. In this approach, Nusselt number is expressed in terms of friction factor which in turn, is given as a function of chevron angle of the heat exchanger. Water has been used as cold and hot fluid. Effect of fluid flow rate and inlet temperatures on the heat duty, overall heat transfer coefficient, fluid outlet temperatures and pressure drop have been studied. When compared with literature results of intermating plate heat exchangers, chevron type plate heat exchanger gives greater heat transfer enhancement, though with increased pressure drop.

Abstract: In recent years there has been a growing attention towards use of solar energy. Advantages of photovoltaic (PV) systems employed for harnessing solar energy are reduction of greenhouse gas emission, low maintenance costs, fewer limitations with regard to site of installation and absence of mechanical noise arising from moving parts. However, PV systems suffer from relatively low conversion efficiency. Therefore, maximum power point tracking (MPPT) for the solar array is essential in a PV system. The nonlinear behaviour of PV systems as well as variations of the maximum power point with solar irradiance level and temperature complicates the tracking of the maximum power point. This paper reviews various MPPT methods based on three categories: offline, online and hybrid methods. Design of a PV system in a encoding environment has also been reviewed here. Furthermore, different MPPT methods are discussed in terms of the dynamic response of the PV system to variations in temperature and irradiance, attainable efficiency, and implementation considerations.

Abstract: Garments plays a vital role among the basic needs of human life, hence there are lots of demand and production of dyestuffs every year to colour the fabrics. In the dyeing process 10 to 20% of the dyes are wasted as effluent. Dye laden wastewater is treated by biological or chemical treatment which is found to be inadequate and expensive. In this study, an attempt has been made to prepare an activated carbon from agro-waste based material, Corn Cobs which are abundantly available in Tamil Nadu. Base activation of Corn Cobs was done with ZnCl₂ solution for 12 hours and carbonized at 350º C for 2 hours. A kinetic experimental study was carried out for Congo red dye removal. In the preliminary studies both the Commercial Activated Carbon (CAC) and Corn Cobs Activated Carbon (CCAC) with various initial adsorbate concentration of 50, 65, 75, 100 and 125 ppm were considered. For further studies on the effect of various parameters such as the contact time and adsorbent dosage, initial concentration of adsorbate was chosen as 65 ppm. The other experimental parameters such as temperature and initial pH were kept at 27±0.5°C and 6.8±0.2 respectively. The experimental results of both CAC and CCAC were compared. The CCAC showed a removal of 97.8% at an adsorbent dosage of 1.2 g, whereas the CAC showed a removal of 98.1% at a dosage of 0.3 g. The result shows that CCAC has abundant potential for the removal of dye.

Abstract: Air pre-heater and economizer are heat transfer surfaces in which air temperature and water temperature are raised by transferring heat from other media such as flue gas. Hot air is necessary for rapid combustion in the furnace and also for drying coal in milling plants. So an essential boiler accessory which serves this purpose is air pre-heater. The air pre-heater is not essential for operation of steam generator, but they are used where a study of cost indicates that money can be saved or efficient combustion can be obtained by their use. The decision for its adoption can be made when the financial advantages is weighed against the capital cost of heater. The efficiency of the boiler increases with the increase in the temperature of the combustion air used in the furnace. This is achieved by the increased temperature of the flue gas in the air preheater and economizer zone. This paper deals with the different ways to obtain the maximum heat from the flue gas travelling through the air preheater and the economizer zone to improve the boiler efficiency.

Abstract: NACA 0012 and NACA 4415 were discussed in Part 1 of the paper to study the capabilities of the airfoil blades by considering the effect of cambered airfoil blade on self-starting of vertical axis wind turbine. The numerical studies are carried out to identify self-starting capability of the airfoil using CFD analysis by studying the flow field over the vertical axis wind turbine blades. In this Part 2 paper, detailed numerical results of asymmetrical NACA 0018 and cambered airfoil NACA 63415 are presented. The lift force generated and the rotor torque induced varies with angle of attack. Based on the contours of static pressure and velocity distribution as well as based on the torque induced in the flow field over blade profiles, NACA 0018 is found to be better compared to cambered airfoil. Even though the lift force for cambered airfoils are higher, based on the rotor torque values, the wind turbine with asymmetrical airfoil blades NACA 0012 is better by 9.80% compared with NACA 4415 and 21.73% compared with NACA 63415. Self-starting issue can be addressed by proper selection of NACA blade profiles. By comparing the four airfoil blades in Part 1 and Part 2 of the papers, the asymmetrical NACA 0012 is found to be most suitable airfoil for self-starting the vertical axis wind turbine (VAWT).