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The paper presents the aerodynamic performance of an axial reversible fan for high air temperatures, obtained by applying the similarity theory and numerical flow simulations. This analysis was performed for the possibility of installing the existing structure of an impeller of a reversible fan in a jet fan for the longitudinal ventilation of a tunnel. The direction of forced flow in tunnels changes, and it is always towards low atmospheric pressure, i.e. in the direction of natural air-flow. These fans often work with high air temperatures in the event of a fire. The paper shows the performance of fans for high operating air temperatures, up to 400 °C. The results were obtained through numerical simulations of the flow in an original design fan, whose performance was experimentally determined for normal atmospheric operating conditions (p = 1.2 kg/m3). Performances are given for two fan speeds. Unnecessary corrections of the existing impeller design for use in jet fans for tunnel ventilation were also considered. [ABSTRACT FROM AUTHOR]

This paper considers viscous fluid flow in a slot between two parallel plates which start inclining with respect to the horizontal line. The lower plate was heated and had non-homogeneous temperature distribution while the upper plate was cooled and with homogeneous temperature distribution. The spatially periodic temperature distribution was gradually applied at the lower plate, after which the plates were slowly inclined in the positive-counterclockwise direction, and the fields of vorticity, stream function, and temperature are presented for different values of the angle of inclination. We used the vorticity-stream function formulation of Navier-Stokes equations, Fourier-Galerkin, and Chebyshev collocation method for numerical simulation of 2-D viscous fluid flow. We carried out numerical simulation using our in-house MATLAB code for subcritical uniform Rayleigh number, Rauni, and periodic Rayleigh number, Rap, on the lower plate. An accurate numerical scheme was developed to capture the full time-dependent behavior here. The interest lied in how the intensities of the vortexes and convection rolls changed as the inclination angle was increased with respect to time. Convection rolls rotating in the clockwise direction expanded and the rolls rotating in the counterclockwise direction shrank and their centers moved closer to the lower wall. Thermal drift appeared between them when the inclination angle started increasing. [ABSTRACT FROM AUTHOR]

Extensive research has been carried out with regard to the composition of the exhaust gases of Diesel engines in operation with biodiesel in relation to the operation with the conventional diesel fuel. Producing biodiesel from different raw materials and different technological biodiesel production processes can result in different individual physical and chemical characteristics of fuel. Generally, it can be said that the use of biodiesel (and mixtures) reduces the overall toxicity of the exhaust gases in relation to the operation of the engine with diesel fuel, and this is a significant environmental potential of biodiesel as a fuel for Diesel engines. However, there is a diversity of research results, due to different factors. The paper reviews and summarizes the relevant literature on the mentioned research that can contribute to the explanation of these effects. It also points to the need for a very careful selection of biodiesel for use as a Diesel engine fuel. [ABSTRACT FROM AUTHOR]

The paper presents a numerical simulation of two-phase turbulent flow in straight horizontal channels of pneumatic transport with non-circular cross-section. For the granular flow simulation, we have chosen the flow of solid particles of quartz, flour, and ash in the flow of air, which is transporting fluid. During the modeling of the flow, the transported solid particles are reduced to spherical shapes. A correction of the stress model of turbulence is performed by taking into account the influences of the induction of secondary flows of the second order in the gas phase. The full Reynolds stress model was used for modeling the turbulence, and the complete model is used for the turbulent stresses and turbulent temperature fluxes. All numerical experiments were conducted for the same initial flow conditions and a single uniform grid was adopted for all numerical experiments. The flow is observed in a straight channel of a square cross-section and dimensions of sides of 200 mm and the length of 80 Dh. During the simulation, the fineness of the numerical grid was also tested, and the paper shows results of the numerical grid of the highest resolution beyond which the fineness does not influence the obtained results. The paper offers graphics of velocities of the solid particles transported by the transporting fluid (air) along the channel. [ABSTRACT FROM AUTHOR]

Cavitation problems occurs in water flow where the local pressure reaches saturated vapour pressure, due to the sudden increase of local velocity values, forming bubbles and different forms of flow discontinuity. Water flow in the Venturi nozzle is the typical example of potentially cavitation flow. Due to the large importance of Venturi nozzles in engineering practice, it is important to determine their cavitation characteristics, such a cavitation number and loss coefficient of the nozzle. In case we are unable to obtain cavitation characteristics of the nozzle experimentally, there is the possibility to conduct numerical simulations of the flow in the Venturi nozzle. The objective of this study is to present the possibilities of obtaining cavitation characteristics of Venturi nozzles by applying numerical simulations, and to determine the optimal numerical modelling of flow in the case of Venturi nozzle. [ABSTRACT FROM AUTHOR]

The paper presents results of an experimental investigation performed under controlled laboratory conditions, in order to identify real causes of variation in the rotational speed of an axial fan that can be noticed after a certain period following the start of operation. The theoretical background important for understanding the observed phenomenon is given and the hypothesis is proposed which states that the temperature rise in motor windings and a consequential rise of their resistances are responsible for such specific behavior of the axial fan. The experiment was performed on a standard test rig with a small industrial axial fan, driven by a three-phase induction motor with the rated power of 1.5 kW. The experimental procedure is described in details, measured and calculated results are presented and appropriate conclusions are given. The obtained experimental results confirm the proposed hypothesis. [ABSTRACT FROM AUTHOR]

As the global demand for energy grows, numerous studies in the field of energy efficiency are stimulated, and one of them is certainly the use of pumps in turbine operating mode. In order to reduce time necessary to determine pump characteristic in turbine operating mode problem was studied by computational fluid dynamics approach. The paper describes various problems faced during modeling (pump and turbine mode) and the approaches used to resolve the problems. Since in the majority of applications, the turbine is a pump running in reverse, many attempts have been made to predict the turbine performance from the known pump performance, but only for best efficiency point. This approach does not provide reliable data for the design of the system with maximum energy efficiency and does not allow the determination of the head for a wide range of flow rates. This paper presents an example of centrifugal norm pump operating in both (pump and turbine) regime and comparison of experimentally obtained results and computational fluid dynamics simulations. [ABSTRACT FROM AUTHOR]

Low-pressure axial fan impellers designed according to the principle of equal specific work of all elementary stages have blades whose profile near the fan hub is under a significantly larger inclination angle than at the impeller periphery. In order to minimize the spatial curvature of the fan blades and the fan hub length, impeller blades of low-pressure axial fans can be designed with different specific work of elementary stages, so that the specific work of elementary stages is smaller at the hub than at the periphery. This paper presents the operating characteristics of a low-pressure reversible axial fan with straight blade profiles, de-signed with different specific work of elementary stages. The fan was tested on a standard test rig, with air intake loading on the suction side of the fan. [ABSTRACT FROM AUTHOR]

The paper presents the design and operating characteristics of a model of reversible axial fan with only one impeller, whose reversibility is achieved by changing the direction of rotation. The fan is designed for the purpose of providing alternating air circulation in wood dryers in order to reduce the consumption of electricity for the fan and increase energy efficiency of the entire dryer. To satisfy the reversibility of flow, the shape of the blade profile is symmetrical along the longitudinal and transversal axes of the profile. The fan is designed with equal specific work of all elementary stages, using the method of lift forces. The impeller blades have straight mean line profiles. The shape of the blade profile was adopted after the numerical simulations were carried out and high efficiency was achieved. Based on the calculation and conducted numerical simulations, a physical model of the fan was created and tested on a standard test rig, with air loading at the suction side of the fan. The operating characteristics are shown for different blade angles. The obtained maximum efficiency was around 0.65, which represents a rather high value for axial fans with straight profile blades. [ABSTRACT FROM AUTHOR]