Petra Dančová

Heat and Mass Transfer Caused by a Laminar Channel Flow Equipped With a Synthetic Jet Array

Low Reynolds number laminar channel flow is used in various heat/mass transfer applications such as cooling and mixing. A low Reynolds number implies a low intensity of heat/mass transfer processes since they rely only on the gradient diffusion. To enhance these processes, an active flow control by means of synthetic (zero-net-mass-flux) jets is proposed. The present study is experimental, in which a Reynolds number range of 200―500 is investigated. Measurement has been performed mainly in air as the working fluid by means of hot-wire anemometry and the naphthalene sublimation technique. Particle image velocimetry (PIV) experiments in water are also discussed. The experiments have been performed in macroscale at the channel cross sections (20 × 100) mm and (40 × 200) mm in air and water, respectively. The results show that the low Reynolds number channel flow can be influenced by an array of synthetic jets. The effect of synthetic jets on the heat transfer enhancement is quantified. The stagnation Nusselt number is enhanced by 10―30 times in comparison with the nonactuated channel flow The results indicate that the present arrangement can be a useful tool for heat transfer enhancement in various applications, e.g., cooling and mixing.

Modelling of air flow rate in significantly flattened rounded rectangular ventilation ducts

Paper presents new mathematical model for air flow velocity distribution in rounded rectangular ducts and its experimental verification. In papers [1, 2] an mathematical model based on modified Prandtl equation for power power-law velocity profile was determined. It works very well for smaller cross sections. During the study of larger cross sections new phenomena in flowing air have been observed, it forced the search for a new model. The new model is based on a rounded rectangular division into two parts: slot and rounded square.

Comparison of digital holographic interferometry and constant temperature anemometry for measurement of temperature field in fluid

The presented paper shows possibility of using digital holographic interferometry (DHI) for temperature field measurement in moving fluids. This method uses a modified Twymann-Green setup having double sensitivity instead of commonly used Mach-Zehnder type of interferometer in order to obtain sufficient phases change of the field. On the other hand this setup is not light efficient as Mach-Zehnder interferometer. For measurement of the fast periodical phenomenon is not necessary to use always the high speed camera. One can consider this field to coherent phenomenon. With employing one digital camera synchronized to periodic field and external triggered one can capture whole period of the phenomenon. However the projections form one viewing direction of asymmetrical temperature field maybe misguided. Hence for sufficient examination of the asymmetrical field one should capture a large number of the phenomenon’s projections from different viewing directions. This projections are later used for 3D tomographic reconstruction of the whole temperature field and its time evolution. One of the commonly used method for temperature field measurement in moving fluids is hot wire method - constant temperature anemometry (CTA). In contrast to whole field measurement of DHI it is an invasive point temperature measurement method. One of the limiting factor of using CTA in moving fluids is frequency of temperature changes. This changes should not exceed 1 kHz. This limitation could be overcome by using of optical methods such as DHI. The results of temperature field measurement achieved by both method are compared in the paper.