Numerical simulations were performed to determine the heat transfer coefficient of a perforated plate with square arranged cylindrical perforations. Three parameters were varied in the study: plate porosity, pitch Reynolds number and working fluid, while perforation diameter and plate thickness were constant. The Reynolds number was varied in the range from 50 to 7000, and porosity in the range from 0.1 to 0.3. As working fluids, helium, air or carbon-dioxide were set, respectively. The Nusselt number was correlated in the function of the Reynolds number, the Prandtl number, and the pitch-to-diameter ratio. The comparison with other correlations is given at the end of the paper. The difference was found to be acceptable.

In this paper, a fuzzy controller is proposed for wind turbine control. A model is analyzed and combined with a stochastic wind model for simulation purposes. Based on the model, a fuzzy control of wind turbine is developed. Wind turbine control loop provides the reference inputs for the electric generator control loop in order to make the system run with maximum power. Since the wind speed involved in the aerodynamic equations is a stochastic variable, whose effective value cannot be measured directly, a wind speed estimator is also proposed.[Projekat Ministarstva nauke Republike Srbije, br. TR33036/2011: Development of new meteorological mast for turbulence parameters characterization and br. TR35005/2011: Research and development of new generation wind turbines of high-energy efficiency

The results of the experimental investigations of fluid flow and heat transfer in laboratory experimental shell-and-tube heat exchanger are presented in this paper. Shell-and-tube heat exchanger is with one pass of warm water on the shell side and two passes of cold water in tube bundle. Shell-and-tube heat exchanger is with 24×2 tubes (U-tube) in triangle layout. During each experimental run, the pressure drops and the fluid temperatures on shell side, along the shell-and-tube heat exchanger (at positions defined in advance) have been measured. The special attention was given to the investigation of the segmental baffles number influence of the shell-and-tube heat exchanger effectiveness.

Article Highlights• Applied method -Energy and exergy analysis • Paper goal -Application of energy and exergy analysis of hot water boiler • The obtained results: largest energy loss -flame pipe, largest exergy -leaving flue gasses • Investigation of possibilities of design modification to increase reliability and availability • Proposing and analyzing more reliable solution AbstractIn engineering practice, exergy can be used for technical and economic optimization of energy conversion processes. The problem of increasing energy consumption suggests that heating plants, i.e., hot water boilers, as energy suppliers for household heating should be subjected to exergy and energy analysis. Heating plants are typically designed to meet energy demands, without the distinguished difference between quality and quantity of the produced heat. In this paper, the energy and exergy analysis of a gas fired hot water boiler is conducted. Energy analysis gives only quantitative results, while exergy analysis provides an insight into the actually available useful energy with respect to the system environment. The hot water boiler was decomposed into control volumes with respect to its functional components. Energy and exergy of the created physical model of the hot water boiler is performed and destruction of exergy and energy loss in each of the components is calculated. The paper describes the current state of energy and exergy efficiency of the hot water boiler. The obtained results are analyzed and used to investigate possibilities for improvement of availability and reliability of the boiler. A comparison between the actual and the proposed more reliable solution is made.Improving energy efficiency as well as energy saving represents one of the major problems of modern developed countries worldwide. Hot water boilers, common in district heating systems, should be subjected to reliability, safety and efficiency research [1]. The efficiency of a hot water boiler has a large impact on thermal performance in district heating systems. In order to improve efficiency, heat transfer from flue gases to water is increased to reduce energy losses Correspondence: M.N. Todorović, Faculty of Mechanical Engineering,

The purpose of thermal comfort is to specify the combinations of indoor space environment and personal factors that will produce thermal environment conditions acceptable to 80% or more of the occupants within a space. Naturally ventilated indoors has a very complex air movement, which depends on numerous variables such as: outdoor interaction, intensity of infiltration, the number of openings, the thermal inertia of walls, occupant behaviors, etc. The most important mechanism for naturally ventilated indoors is the intensity of infiltration and thermal buoyancy mechanism. In this study the objective was to determine indicators of thermal comfort for children, by the CFD model based on experimental measurements with modification on turbulent and radiant heat transfer mathematical model. The case study was selected on school children of 8 and 9 years in "France Presern" primary school in Belgrade. The purpose was to evaluate the relationships between the indoor environment and the subjective responses. Also there was analysis of infiltration and stack effect based on meteorological data on site. The main parameters that were investigated are: operative temperature, radiant temperature, concentration of CO 2 , and air velocity. The new correction of turbulence and radiative heat transfer models has been validated by comparison with experimental data using additional statistical indicators. It was found that both turbulence model correct and the new radiative model of nontransparent media have a significant influence on CFD data set accuracy.