Boštjan Drobnič

A Numerical Analysis of the Local Anomalies in a Natural-Draft Cooling Tower

This paper presents experimental and numerical analyses of the aero-thermodynamic characteristics of a natural-draft cooling tower. The influence of local technical faults in the fill and rain region on the cooling towers performance was estimated. Measurements of the air velocity and the temperature above the droplet eliminators showed a noticeable non-uniformity of both parameters. This is caused by a non-uniform airflow resistance and heat-transfer rate within the fill and rain region. Based on these measurements, a commercial CFD model was customized with additional relations describing the heat- and mass-transfer, as well as the airflow resistance in individual regions of the cooling tower. The results of a 3D numerical simulation of the cooling tower are the temperature and velocity distributions within the entire cooling tower. A comparison of nominal and actual cooling tower operation shows regions with unfavorable air temperatures or velocities. Thus, the inefficiently operating areas of the cooling towers cross-section can be identified. These areas cause non-homogeneous aero-thermodynamic characteristics and have an influence on the integral characteristics of the cooling tower. A sample calculation of an actual cooling tower shows the usefulness of the method when it comes to improving the cooling towers performance. The improvement can be achieved by modifying the fill resistance and water distribution.

The Characterization of Pulverized-Coal Pneumatic Transport Using an Array of Intrusive Electrostatic Sensors

Fan/impact mills are commonly applied in the grinding and dilute-pneumatic transportation of lignite or brown coals with high moisture contents to the furnaces in large steam boilers. Each of the two to eight mills feeds pulverized coal into two or more burner nozzles. An online detection of the pulverized-coal mass flow distribution among the burners is vital for the control of the combustion process. Knowing the distribution, measures for the redistribution of the coal or, alternatively, for the adjustment of the combustion air flow according to the actual distribution can be employed. Determining the characteristics of a gas-solid two-phase flow using an electrostatic principle is a promising online method of measurement because it is robust and inexpensive. Furthermore, due to their better spatial sensitivity, rod sensors are more suitable for large rectangular ducts related to fan/impact mills than ring-, pin-, or arc-shaped sensors. Sets of 1-D and 2-D electrostatic sensor arrays with a corresponding data acquisition system were employed to determine the mass flow distribution in the cross section of the duct that feeds the pulverized lignite to the four burner nozzles. Various operating regimes for the fan/impact mill were tested. The time series of the signals from the electrostatic sensors were analyzed statistically. It was shown that the skewness, kurtosis, and autocorrelation time delay at the characteristic value can indicate different grinding qualities of the coal.

Numerical Modeling of Heat Transfer and Flow Phenomena in an Axial Rotating Rotor Cascade

This paper presents a study of heat transfer in a rotating axial rotor. The study was done on the basis of computational fluid dynamics simulations and validated with an infrared thermocamera experimental setup. The influence of full turbulence vs. Menters boundary layer method was studied in terms of heat transfer and flow phenomena. Additionally it was found out that a thin paint layer has a major influence on heat transfer phenomena and acts as insulation. For comparison of numerical results with experimental, a model of the actual rotor was established. Results were found to be in good agreement, so the effect of rotation on heat transfer was studied. Results are presented in terms of Nusselt number distribution and an empirical model of heat transfer.