A fast modelling tool for plate heat exchangers based on depth-averaged equations

In this study, the depth-averaged flow and energy equations for plate heat exchangers are presented. The equations are derived by integrating the original 3D flow and energy equations over the height of the gap between the bottom and top plates. This approach reduces the equations from 3D to 2D but still takes into account the frictions on the surfaces and heat transfer through the plates. The depth-averaging reduces the elapsed time of CFD simulations from hours to minutes. Thus, it is very practicable modelling method in real time design work. 2D CFD simulations with depth-averaged equations are compared with full 3D models for five different corrugation angles and corrugation lengths. The simulation results show that the 2D model predicts with relatively good accuracy the profile of the pressure drop and the temperature change as a function of the corrugation angle and the function of the corrugation length. In order to get more extensive information about the significance of the different geometry parameters on the efficiency of the heat exchanger, we simulated 30 different geometries with the fast 2D model. The results suggest that the temperature change is not as sensitive for the geometrical modifications as the pressure drop.     

Muscle activity patterns and spinal shrinkage in office workers using a sit–stand workstation versus a sit workstation

Reducing sitting time by means of sit–stand workstations is an emerging trend, but further evidence is needed regarding their health benefits. This cross-sectional study compared work time muscle activity patterns and spinal shrinkage between office workers (aged 24–62, 58.3% female) who used either a sit–stand workstation (Sit–Stand group, n = 10) or a traditional sit workstation (Sit group, n = 14) for at least the past three months. During one typical workday, muscle inactivity and activity from quadriceps and hamstrings were monitored using electromyography shorts, and spinal shrinkage was measured using stadiometry before and after the workday. Compared with the Sit group, the Sit–Stand group had less muscle inactivity time (66.2 ± 17.1% vs. 80.9 ± 6.4%, p = 0.014) and more light muscle activity time (26.1 ± 12.3% vs. 14.9 ± 6.3%, p = 0.019) with no significant difference in spinal shrinkage (5.62 ± 2.75 mm vs. 6.11 ± 2.44 mm). This study provides evidence that working with sit–stand workstations can promote more light muscle activity time and less inactivity without negative effects on spinal shrinkage.

Practitioner Summary: This cross-sectional study compared the effects of using a sit–stand workstation to a sit workstation on muscle activity patterns and spinal shrinkage in office workers. It provides evidence that working with a sit–stand workstation can promote more light muscle activity time and less inactivity without negative effects on spinal shrinkage.     

An allostratigraphic approach to subdivide fine-grained sediments for urban planning

Bulletin of Engineering Geology and the Environment - Fine-grained sediments in southern Finland were deposited during the retreat of the continental Fennoscandian ice-sheet at around...     

浏览器市场竞争激烈

竞争出成绩,自从业界巨人Netscape和Microsoft就研制最佳Web浏览器公开进行角逐以来,我们就只有坐山观虎斗的份了。在最新一回合比赛中,这两家公司都推出了比它们以前版本更好的目前最流行、功能最强大的测览器:Netscape的Navigator 3.0和Microsoft的Internet Explorer3.0。 从宣传材料上看,这两种产品非常相似。它们部支持大量相同的新的Internet技术、控制和插入件(plug-in)。但仔细研究后会发现二者之间有很多差异。Microsoft不懈的努力会在将来对Netscape构成严重威胁,但到目前为止,Netscape的产品仍是目前最好的浏览器,虽然优势已不那么明显。