Flow patterns, heat transfer and pressure drop for evaporation and condensation of R134A in microfin tubes

Saturated flow boiling and convective condensation experiments for oil-free Refrigerant-134a have been carried out with two microfin tubes as well as with a smooth one. All the tubes have the same outer diameter of 9.52 mm; they are horizontally operated and are heated/cooled by a water stream. Both the microfin tubes are characterized by sharp fins (apex angle of 40°) alternating with two different heights whereas the fin number is different, namely, 54 and 82, respectively. Evaporation tests are carried out at a nominal temperature of 5°C, for a mass flux ranging from about 100 to 340 kg/(m2s), inlet quality between 0.25 and 0.70, and quality change varying from 0.10 to 0.70, whereas for the condensation tests the nominal temperature is 35°C, the mass flux varies between 100 and 440 kg/(m2s), the inlet quality ranges from 0.75 to 0.10 and the quality change from 0.10 to 0.70. The paper reports on average heat transfer coefficient and pressure drop. Furthermore, a visual inspection was carried out to characterize how fins affect the flow pattern transitions. Possible modifications of some flow pattern maps, developed for horizontal smooth tubes in evaporation and condensation, are suggested to extend their use to microfin tubes. Finally, the paper presents comparisons between experimental data and estimates obtained by recent correlations specifically proposed for these tubes

Experimental Characterization of a Small Cogenerative Plant at Full and Partial Loads

This paper presents the results of an experimental analysis performed on a small cogenerative plant equipped with a microturbine as a prime mover. The aim of the experimental campaign was to verify behavior and performances of the machine at both full and partial load. Attention was particularly paid to the energy saving indexes defined in the AEEG 42/02 Directive (Italian Authority for Electrical Energy and Gas) and in the more strict EU Directive 2004/8/EC. The results of the experimental campaign show that the cogenerative plant can easily exceed the minimum values of the energy saving index imposed by the current directives during operation at full load. On the contrary, very accurate design of the plant and prediction of the load profiles are required to get acceptable results if operation at partial load becomes necessary

Simplified Thermal Model of a Stacked Ball Grid Array Package

Simplified integrated approaches to thermal modeling of electronics packages call for fast and easy thermal models. However, this is often in contrast with the complexity of the thermal problem to be solved, which prevents the application of simplified approaches. After a review of the available modeling strategies, steady state thermal analysis of a typical stacked ball grid array package based on a thermal resistance network is presented. The physical model is discussed in detail, together with its advantages and limitations. The results have been compared with detailed 3D-simulations performed by means of a commercial code: for thermal design purposes, the accuracy is satisfactorily, leading a prediction error of the junction temperature lower than 10%

A detailed characterization of viscous oil-water flows downward sudden contractions in horizontal pipes

Two-phase flows of viscous oil and water through singularities such as sudden area contractions/expansions have been taken into limited consideration in the relevant scientific literature. Nevertheless, they play a role of primary importance in industrial systems, for instance, but not only, in the exploitation of oil wells and pipelines. The proposed work is based on the comparison of photographic images of the flow patterns taken from three points of view, i.e. upper, lower and frontal, thanks to a couple of mirrors ???45° inclined with respect to the horizontal plane. Oil-water flow regimes have been observed both upward and downward of five horizontal test sections with area contractions d/D = 40/50, 30/50, 30/40, respectively. The observed structures of the oil-water interface, especially for core-annular flows, has suggested also detecting flow patterns in a 30 mm straight pipe for sake of comparison. Actually, the shape of the oil-core interface appears significantly influenced by the sharp-edged area change as well as by the expected momentum variation

Characterization of the critical transition from annular to wavy-stratified flow for oil-water mixtures in horizontal pipes

The transition from annular to wavy-stratified oil-water adiabatic flow within horizontal pipes is experimentally analysed and a semi-empirical model is proposed. The transition is referred to as critical because it occurs suddenly, giving rise to a sharp and strong increase in the pressure drop due to the contact of the high-viscosity oil with the pipe wall. This could lead to a dangerous accident in pipelines. Experimental runs were performed on eight test sections of both Plexiglas® and Pyrex® pipes with internal diameters ranging from 21.5 mm to 50 mm, using tap water and oil with viscosity about 880 times higher than that of water. On the basis of pressure drop measurement and flow pattern visualization, the transition boundary between annular and wavy-stratified flow was analytically determined and compared with flow pattern maps