1,721,084 research outputs found
Transient laminar natural convection along rectangular channels
No full textAn analytical approach is proposed to investigate the transient behaviour of a Newtonian, single-phase fluid in natural laminar convection, in a rectangular open-ended duct. The continuity, momentum and energy equations, with the classical Boussinesq approximation, are solved using a twofold sine Fourier transform and the Laplace transform. The unsteady state is due to a step variation of the temperature in the four walls of the duct, which can assume four different, uniform arbitrary values. Considering hydrodynamically developed flow and uniform wall temperatures (UWTs), the velocity and temperature of the fluid are given as series containing two spatial co-ordinates and the time. Some plots show the transient evolution of the velocity and temperature distribution. Then the induced volumetric flow rate, the exchanged power, the mixing cup temperature, and the average Nusselt number are evaluated, as a function of time, emphasising the influence of the duct aspect ratio and the irrelevance of the channel height
The role of the viscous dissipation in microchannels
No full textMany experimental works appeared in the last decade in the open literature dealing with forced convection through microchannels. These experimental results evidenced that for channels having a
hydraulic diameter less than 1 mm the conventional theory can no longer be considered as able to predict pressure drop and convective heat transfer coefficients. This conclusion seemed to be valid for both gas and liquid flows. Sometimes the authors justified this conclusion by invoking new micro-effects, e.g. electrostatic interaction between the fluid and the walls or scaling effects (axial heat conduction, viscous forces, conjugate heat transfer, wall roughness and so on). In this paper the
role of the viscous dissipation in fluids flowing through heated microchannels will be analyzed by using the conventional theory. It will be presented a correlation between the Brinkman number and the Nusselt number for silicon and microchannels. It will be demonstrated that the effects of the viscous dissipation tend to become very important for liquid flows when the hydraulic diameter of the microchannel is less than 70 μm. The role of the cross-section aspect ratio on the viscous dissipation will be highlighted. The main goal of this work is to demonstrate that the problem of heat transfer enhancement in microdevices cannot be solved by indefinitely reducing the
microchannel dimensions because the viscous dissipation effects shall offset the gains of high heat transfer coefficients associated with a reduction in the channel size
Greenhouse gas reduction and primary energy savings via adopting of a fuel cells hybrid plant in a hospital”
No full textThe Kyoto agreement, expressing great concern about global climate change, has stated emissions of
greenhouse gases, especially CO2 from fossil fuel use, need to be reduced. According to this, existing plants
have been required to cut emissions; moreover, there has been a greater emphasis on adopting efficient
systems in order to reduce the energy losses. Among high efficiency technologies, fuel cells appear to be the
most promising for their high efficiency and their very low environmental impact.
This paper first reviews the state-of-the-art of fuel cells systems, then simulates the operation of a hybrid
fuel cells plant in a ‘‘typical hospital’’ analysing how it could optimize the hospitals energetic requirements.
Hospitals and sanitary structures are normally characterized by considerable energy demands not often
suitable with resolute energy retrofit strategies.
Both the considerable primary energy savings and the pollutant emissions reduction, achieved upgrading
conventional systems to a fuel cell hybrid plant, have the potential to prompt national boards to support
their business development, as long as they achieve a consolidated market penetration
The thermal entrance length problem for slug flow in rectangular ducts
No full textASME JOURNAL OF HEAT TRANSFE
Laminar heat transfer between parallel plates as the limiting solution for the rectangular duct
No full text- …
