We study the gas flow processes in ultra-tight porous media in which the matrix pore network is composed of nanometre- to micrometre-size pores. We formulate a pressure-dependent permeability function, referred to as the apparent permeability function (APF), assuming that Knudsen diffusion and slip flow (the Klinkenberg effect) are the main contributors to the overall flow in porous media. The APF predicts that in nanometre-size pores, gas permeability values are as much as 10 times greater than results obtained by continuum hydrodynamics predictions, and with increasing pore size (i.e. of the order of the micrometre), gas permeability converges to continuum hydrodynamics values. In addition, the APF predicts that an increase in the fractal dimension of the pore surface leads to a decrease in Knudsen diffusion. Using the homogenization method, a rigorous analysis is performed to examine whether the APF is preserved throughout the process of upscaling from local scale to large scale. We use the well-known pulse-decay experiment to estimate the main parameter of the APF, which is Darcy permeability. Our newly derived late-transient analytical solution and the late-transient numerical solution consistently match the pressure decay data and yield approximately the same estimated value for Darcy permeability at the typical core-sample initial pressure range and pressure difference. Other parameters of the APF may be determined from independent laboratory experiments; however, a pulse-decay experiment can be used to estimate the unknown parameters of the APF if multiple tests are performed and/or the parameters are strictly constrained by upper and lower bounds.

Gas flow in ultra-tight shale strata

Meeting future world energy needs while addressing climatic changes has led to greater strain on conventional power sources. One of the viable sustainable energy sources is wind. But the installation large scale wind farms has a potential impact on the climatic conditions, hence a decentralized small scale wind turbines is a sustainable option. This paper presents review of on different types of small scale wind turbines i.e., horizontal axis and vertical axis wind turbines. The performance, blade design, control and manufacturing of horizontal axis wind turbines were reviewed. Vertical axis wind turbines were categorized based on experimental and numerical studies. Also, the positioning of wind turbines and aero-acoustic aspects were presented. Additionally, lessons learnt from various studies/countries on actual installation of small wind turbines were presented.

A review on small scale wind turbines

This paper presents a review on the performance of Savonius wind turbines. This type of turbine is unusual and its application for obtaining useful energy from air stream is an alternative to the use of conventional wind turbines. Simple construction, high start up and full operation moment, wind acceptance from any direction, low noise and angular velocity in operation, reducing wear on moving parts, are some advantages of using this type of machine. Over the years, numerous adaptations for this device were proposed. The variety of possible configurations of the rotor is another advantage in using such machine. Each different arrangement of Savonius rotor affects its performance. Savonius rotor performance is affected by operational conditions, geometric and air flow parameters. The range of reported values for maximum averaged power coefficient includes values around 0.05–0.30 for most settings. Performance gains of up to 50% for tip speed ratio of maximum averaged power coefficient are also reported with the use of stators. Present article aims to gather relevant information about Savonius turbines, bringing a discussion about their performance. It is intended to provide useful knowledge for future studies.

A review on the performance of Savonius wind turbines

http://www.iust.ac.ir/files/ee/kalantar_921e9/ISI_Journals/ISI%20[18].pdf

Dynamic behavior of a stand-alone hybrid power generation system of wind turbine, microturbine, solar array and battery storage

This paper deals with the evolution of infrared (IR) thermography into a powerful optical tool that can be used in complex fluid flows to either evaluate wall convective heat fluxes or investigate the surface flow field behavior. Measurement of convective heat fluxes must be performed by means of a thermal sensor, where temperatures have to be measured with proper transducers. By correctly choosing the thermal sensor, IR thermography can be successfully exploited to resolve convective heat flux distributions with both steady and transient techniques. When comparing it to standard transducers, the IR camera appears very valuable because it is non-intrusive, it has a high sensitivity (down to 20 mK), it has a low response time (down to 20 μs), it is fully two dimensional (from 80 k up to 1 M pixels, at 50 Hz) and, therefore, it allows for better evaluation of errors due to tangential conduction within the sensor. This paper analyses the capability of IR thermography to perform convective heat transfer measurements and surface visualizations in complex fluid flows. In particular, it includes the following: the necessary radiation theory background, a review of the main IR camera features, a description of the pertinent heat flux sensors, an analysis of the IR image processing methods and a report on some applications to complex fluid flows, ranging from natural convection to hypersonic regime.

/pdf/infrared-thermography-for-convective-heat-transfer-56a0bwe7om.pdf

Infrared thermography for convective heat transfer measurements

Separation of conduction and convection heat transfer effects for a metal foamed flat plate impinged by a circular jet

The purpose of the present work is the experimental investigation on the extinguishment of diesel container fires by a water-mist system with base injection through multiple nozzles placed symmetrically at various positions around the rim of the container. Experimental parameters selected are the orifice diameter of the nozzle, the number of nozzles, position of nozzles (angle of orientation, radial and vertical distance of the nozzle from the edge of the container), and injection pressure. Nozzles used in the present work are pressure-swirl nozzles with Z-type swirl-insert with solid-cone spray. Desirable properties of the system such as the non-clogging operation of the nozzle and uniform mass flux density in the spray are tested for the nozzles. The optimized solution is provided considering water consumption and the physical stability of the fuel surface in the container. Displacement of oxygen is observed as a predominant extinction mechanism due to the effective application of spray in the base region of the fire. The present work provides the parametric study of the extinction establishing the feasibility of the water mist system with a base injection for extinguishing tank fires. This may serve as guidelines for the preliminary design of the system with the extension for larger tank fires.

Performance of water mist system with base injection in extinguishing small container fires

An experimental investigation is performed to study the effect of jet-to-plate spacing and Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth semi-cylindrical concave surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 11000 and 28000 and jet-to-plate spacing between 0.5 to 8 nozzle diameters. Experiments were carried out for two relative curvatures (D/d) of 4.4 and 6 by changing the jet tube diameter. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Effect of curvature on heat transfer distribution is studied and compare with the flat plate results. Curvature results in decrease of Nusselt number compared to flat plate values.

Experimental Study on Local Heat Transfer Distribution between Smooth Semi-Cylindrical Concave Surface and Impinging Air Jet from a Circular Straight Pipe Nozzle

The effect of channel orientation and rib pitch-to-height ratio on the pressure drop distribution in a rib-roughened channel is an important issue in turbine blade cooling. The present investigation is a study of the overall pressure drop distribution in a square cross-sectioned channel, with rib turbulators, rotating about an axis normal to the free stream. The ribs are configured in a symmetric arrangement on two opposite surfaces with a rib angle of 90∘ to the
mainstream flow. The study has been conducted for three Reynolds numbers, namely, 13 000, 17 000, and 22 000 with the rotation number varying from 0–0.38. Experiments have been carried out for
various rib pitch-to-height ratios (P/e) with a constant rib
height-to-hydraulic diameter ratio (e/D) of 0.1. The test
section in which the ribs are placed on the leading and trailing
surfaces is considered as the base case (orientation angle=0∘, Coriolis force vector normal to the ribbed
surfaces). The channel is turned about its axis in steps of
15∘ to vary the orientation angle from 0∘ to
90∘. The overall pressure drop does not change
considerably under conditions of rotation for the base case.
However, for the other cases tested, it is observed that the
overall pressure drop increases with an increase in the rotation
number for a given orientation angle and also increases with an
increase in the orientation angle for a given rotation number.
This change is attributed to the variation in the separation zone
downstream of the ribs due to the presence of the Coriolis
force—local pressure drop data is presented which supports this
idea. At an orientation angle of 90∘ (ribs on the top and
bottom surfaces, Coriolis force vector normal to the smooth
surfaces), the overall pressure drop is observed to be maximum
during rotation. The overall pressure drop for a case with a rib
pitch-to-height ratio of 5 on both surfaces is found to be
the highest among all the rib pitch-to-height ratios covered in this
study with the maximum increase in the overall pressure drop being
as high as five times the corresponding no-rotation case at the
highest rotation number of 0.38 and 90∘ orientation
angle.

/pdf/effect-of-channel-orientation-and-rib-pitch-to-height-ratio-a0t9vy8vu4.pdf

S.V. Prabhu

Papers

Separation of conduction and convection heat transfer effects for a metal foamed flat plate impinged by a circular jet

Performance of water mist system with base injection in extinguishing small container fires

Experimental Study on Local Heat Transfer Distribution between Smooth Semi-Cylindrical Concave Surface and Impinging Air Jet from a Circular Straight Pipe Nozzle

Effect of Channel Orientation and Rib Pitch-to-Height Ratio on Pressure Drop in a Rotating Square Channel with Ribs on Two Opposite Surfaces

Influence of metal foam thickness on the conduction and convective heat transfer for a flat plate with metal foam impinged by a rectangular slot jet