Wall roughness effects in laminar flows: an often ignored though significant issue

Gloss, D.; Herwig, Heinz

doi:10.1007/s00348-009-0811-6

Cited by 38 publications

(19 citation statements)

References 14 publications

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“…This, however, is also predicted in our SLA results; for a detailed discussion, see [45]. With the same kind of analysis, we could show that, contrary to the common belief, there is also a definite roughness effect in laminar pipe and channel flows; see [47][48][49].…”

Section: Straight Pipes and Channelssupporting

confidence: 78%

How to Determine Losses in a Flow Field: A Paradigm Shift towards the Second Law Analysis

Herwig

Schmandt

2014

Entropy

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Assuming that CFD solutions will be more and more used to characterize losses in terms of drag for external flows and head loss for internal flows, we suggest to replace single-valued data, like the drag force or a pressure drop, by field information about the losses. These information are gained when the entropy generation in the flow field is analyzed, an approach that often is called second law analysis (SLA), referring to the second law of thermodynamics. We show that this SLA approach is straight-forward, systematic and helpful when it comes to the physical interpretation of the losses in a flow field. Various examples are given, including external and internal flows, two phase flow, compressible flow and unsteady flow. Finally, we show that an energy transfer within a certain process can be put into a broader perspective by introducing the entropic potential of an energy.

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Section: Straight Pipes and Channelssupporting

confidence: 78%

How to Determine Losses in a Flow Field: A Paradigm Shift towards the Second Law Analysis

Herwig

Schmandt

2014

Entropy

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“…Although the flow regime in this study is laminar, however, Gloss and Herwig [40] showed that the roughness effect is not negligible in laminar flow. In addition, in 1936, Wenzel indicated that the wettability of liquid increases with increasing the surface roughness [41].…”

Section: Effect Of Surface Roughnessmentioning

confidence: 65%

Nanofluids effects on the evaporation rate in a solar still equipped with a heat exchanger

et al. 2017

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In this paper, the performance of a solar still equipped with a heat exchanger using nanofluids has been studied both experimentally and theoretically through three key parameters, i.e., freshwater yield, energy efficiency and exergy efficiency. First, experiments are performed on a set-up, which is mainly composed of two flat plate solar collectors connected in series, and a solar still equipped with a heat exchanger. After heated in the collectors, the nanofluid enters the heat exchanger installed in the solar still basin to exchange heat with brackish water. The research question is to know how much the effect of nanofluids on the evaporation rate inside the solar desalination system is. The experiments are conducted for different nanoparticle volume fractions, two sizes of nanoparticles (7 and 40 nm), two depths of water in the solar still basin (4 and 8 cm), and three mass flow rates of nanofluids during various weather conditions. It is found that the weather conditions (mainly the sun radiation intensity) have a dominant influence on the solar still performance. To discover the effects of nanofluids, a mathematical model is developed and validated by experimental data at given weather conditions. The results reveal that using the heat exchanger at temperatures lower than 60 o C is not advantageous and the corresponding yield is smaller than that of solar still without the heat exchanger; although in such a case, using 2 nanofluids as the working fluid in the heat exchanger can enhance the performance indices about 10%. At higher temperatures (e.g. 70 o C), the use of heat exchanger is beneficial; however, using nanofluids instead of water can augment the performance indices marginally i.e. just around 1%. In addition, it is found that in high temperatures using SiO 2 /water nanofluids, which have a lower effective thermal conductivity than that of Cu/water nanofluids, provides higher performance indices.Keywords: Nanofluids; Solar desalination; Heat exchanger; Freshwater yield 1-IntroductionNowadays, "Nano" and "Energy" have been two hot keywords, not only in the scientific community but also in our daily life. During recent decades, researchers have attempted to apply nanotechnology to various energy and power systems such as electric generators, fuel cells, batteries, and solar cells [1][2][3][4][5]. Nanotechnology has also been implemented to enhance the heat transfer potential of common liquids like water and oil to ameliorate the efficiency of thermal systems; this can be done through adding solid nanoparticles (particles with a size of 1-100 nm) to the liquids. The mixture of nanoparticles and conventional liquids is named "nanofluid" [6].Despite some limitations such as relatively high preparation cost and stability issues, extensive attempts have been made to develop the applications of nanofluids in energy systems such as solar energy based devices [7][8][9][10][11][12][13], cooling and thermal management of electronic equipment [14,15], grinding and drilling, absorption systems, medicine...

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“…Anyway, the fluid flows under laminar conditions. It follows that as long as the amplitude of the roughness is much smaller the thickness of the film, then the small protuberances plays no significant role in determining the flow resistance [33]. By introducing the scalingf = (ũ f / )f , we obtain the dimensionless equation that relates the bottom evolution to the calcite flow, namely…”

Section: (B) Chemical Modelmentioning

confidence: 99%

Thin-film-induced morphological instabilities over calcite surfaces

2015

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Precipitation of calcium carbonate from water films generates fascinating calcite morphologies that have attracted scientific interest over past centuries. Nowadays, speleothems are no longer known only for their beauty but they are also recognized to be precious records of past climatic conditions, and research aims to unveil and understand the mechanisms responsible for their morphological evolution. In this paper, we focus on crenulations, a widely observed ripple-like instability of the the calcite–water interface that develops orthogonally to the film flow. We expand a previous work providing new insights about the chemical and physical mechanisms that drive the formation of crenulations. In particular, we demonstrate the marginal role played by carbon dioxide transport in generating crenulation patterns, which are indeed induced by the hydrodynamic response of the free surface of the water film. Furthermore, we investigate the role of different environmental parameters, such as temperature, concentration of dissolved ions and wall slope. We also assess the convective/absolute nature of the crenulation instability. Finally, the possibility of using crenulation wavelength as a proxy of past flows is briefly discussed from a theoretical point of view.

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Wall roughness effects in laminar flows: an often ignored though significant issue

Cited by 38 publications

References 14 publications

How to Determine Losses in a Flow Field: A Paradigm Shift towards the Second Law Analysis

How to Determine Losses in a Flow Field: A Paradigm Shift towards the Second Law Analysis

Nanofluids effects on the evaporation rate in a solar still equipped with a heat exchanger

Thin-film-induced morphological instabilities over calcite surfaces

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