Applied Mechanics and Materials Vol. 831

Abstract: The present paper offers an analysis of heat transfer and fluid flow in two phase thermosyphon loop with minichannels. A one-dimensional model of two-phase flow and heat transfer in a closed thermosyphon loop with minichannels was examined. The created general model is based on mass, momentum, and energy balances in the evaporators, rising tube, condensers and the falling tube. The separate two-phase flow model is used in calculations. The numerical results obtained for the selected heater and cooler using the general model of thermosyphon loop indicate that the mass flux increases with increasing length of the heated section and decreases with increasing length of the cooled section of the loop. It was found that the heat transfer coefficient for flow boiling and flow condensation in the steady state increases with increasing heat flux in the heater and cooler with minichannels, respectively. The design and configuration of heaters and coolers has a considerable impact on the efficiency of thermosyphon loop. These factors make it possible to optimize the computer processor cooling.

Abstract: Effective techniques for cooling electronic devices must deal with increasing heat loads associated with higher heat flux density. Many conventional cooling techniques like fan are reaching the limits of their effectiveness and shape. The novel method of heat transfer enhancement is synthetic jet. In this paper experimental results are presented. Synthetic jet actuator consist of STX 6.5 inch speaker installed in metacrylate chamber with circular orifice. The actuator was powered with signal from amplifier. The signal consist of basic sinusoidal function and THD noise added with some amplitude. The root mean square of signal voltage was constant 4V. The properties of synthetic jet were measured using constant temperature thermo-anemometer. Instantaneous velocity of air was measured in the orifice center and compared with input signal. Additional RMS and average velocity of air were measured. Measurement revealed that input signal of synthetic jet generator can contain some noise without effect on RMS and average velocity of air in the orifice. The THD less than 1% does not cause negative effect on synthetic jet fluid velocity.

Abstract: Horizontal-axis wind turbines are sophisticated technical constructions that require modern design methods for further improvement and higher economic output. Thorough understanding of the whole turbine and its components is indispensable for this aim. The technological development in the last decades indicates the progress in this area and shows the potential of optimizations due to the improvement of rotor blade aerodynamics by additional aerodynamic devices. In this project the effect of an aeroelastic flap mounted on the trailing edge of a rotor blade on energy yield was investigated. For this purpose a flexible flap of fiberglass reinforced plastic for the profile at 70% of the rotor blade span has been designed for the trailing edge and analyzed using a 2-way fluid-structure-interaction simulation. In the first step the simulation is reduced to a two-dimensional investigation using the cross-section at 70% diameter of the rotor blade. Compared to an unmodified profile, the aeroelastic flap increases the lift-to-drag-ratio of the profile in an angle of attack range between 3.5° to 9.5°. The aeroelastic flap has a positive effect on the glide ratio, similar to a non-elastic, static flap.

Abstract: In this paper the wind tunnel at the University of Applied Sciences Stralsund and few construction details as well as the calibration of the nozzle are presented. Furthermore some approaches to visualization and measurement of velocity fields of a simple body of vehicle are shown. The wake structure behind of simple car models was investigated experimentally using Particle Image Velocimetry (PIV). Furthermore the visualization of flow field by the fog probe systems supports the evaluation of the flow and drag behavior.

Abstract: In our study we examined the behaviour of thin liquid layers at curved solid edges experimentally and numerically by coating investigations based on the laser-induced fluorescence technique (LIF) and by numerical film simulations based on the Volume-of-Fluid multiphase flow model (VOF), respectively. The main motivation was to find optimal combinations of influencing quantities to reduce the so-called Fat-Edge effect. Therefore, we performed a study of these influencing quantities, in which application parameters like edge curvature radii of the solid substrates and application layer thicknesses as well as determining liquid properties like viscosity and surface tension have been varied. Results are described qualitatively at corresponding Fat-Edge shapes and quantified by suitable Fat-Edge parameters, which had to be identified and selected.We can show that adverse and appropriate influencing parameter combinations exist, which generate conspicuous and less distinctive Fat-Edges, respectively. The experimental findings and proportionalities regarding Fat-Edge shapes and dimensions are found to be physically plausible. Furthermore, an order of significance of the influencing quantities has been established.Eventually, we could derive a dimensionless quantity by dimensional analysis, which describes the Fat-Edge effect. Thus, the Fat-Edge effect may also be described by the application of similarity theory and a corresponding dimensionless number.

Abstract: In the presented work the effective thermal conductivity of steel was received from the simple experiment in which the change of phase metrology-matter was applied [1]. As the metrology-matter gallium was used.The change of phase process plays role of the heat source. During the change of phase period the temperature is constant. A quasi-stationary temperature fields allows to use the stationary model of heat transfer calculations. The model is a cylindrical rod of finite length losing heat out one face to melting-substance and by radiation from surface, respectively. All process measurement was carried out in vacuum. Measurement of thermal conductivity quasi-stationery method is practiced at the Department of Thermodynamics at Rzeszow University of Technology. This method has application in measurement of thermal conductivity of good conductors.

Abstract: Enhancement of efficiency of defrosting process of freight cars is a very important problem in any of countries characterized by severe or temperate climates. In the present article, the calculation methodology of air parameters, that is of energy carrier, in defrosting room is presented. Heat between the particular media is transferred only by convection. Changes of volumetric flow rates and heat fluxes in characteristic points of defrosting room in the function of volumetric flow rate of supplied air are presented in the work.

Abstract: In this work, Nusselt number and friction factor are calculated numerically for turbulent pipe flow (Reynolds number between 6000 and 12000) with constant heat flux boundary condition using nanofluids. The nanofluid is modelled with the single-phase approach and the simulation results are compared with experimental data. Ethylene glycol and water, 60:40 EG/W mass ratio, as base fluid and SiO2 nanoparticles are used as nanofluid with particle volume concentrations ranging from 0% to 10%. A prior turbulence model evaluation of k-ε-, k-ω- and k-ω-SST-model revealed substantial deviations between the tested models and resulted in applying the k-ω-SST-model for the simulation. Nusselt number predictions for the nanofluid are in agreement with experimental results and a conventional single-phase correlation. The mean deviation is in the range of 5%. Friction factor values show a mean deviation of 1.5% to a conventional single-phase correlation, however, they differ considerably from the nanofluid experimental data.

Abstract: Solar collectors is one of the technologies absorbing energy from solar beam and utilizing it for heating purposes, displacing the need to burn fossil fuels. There are many ways to improve effectiveness of the solar collectors [1,2]. Recent method to absorb more heat from the solar beam is to modify thermal characteristics of the working fluid. For this purpose one can use nanofluids, i.e. suspensions of metallic or nonmetallic nanoparticles in a base fluid [3].

Abstract: This study is focused on experimental investigation of a selected type of brazed plate heat exchanger (PHEx). The main aim of the paper was to experimentally check the ability of nanofluids to enhance the performance of PHEx. A typical water-Al₂O₃ nanofluid was tested and compared to that of the base fluid, i.e. water. Nanoparticles were tested at the concentration of 0.1% and 1% by weight. Impact of the 1 day and 3 days break of operation of the tested PHEx on its performance was of particular interest. Pressure drop in all runs was measured as well. The Wilson approach was applied in order to estimate heat transfer coefficients for the PHEx passages. It was observed, that addition of nanoparticles resulted in deterioration of an overall heat transfer coefficient for the selected PHEx and tested conditions, i.e. temperature range and Reynolds number. Moreover, substantial increase of pressure drop was recorded after each break of operation of the tested PHEx.