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| == '''[[Choked flow]]''' ==
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| The '''choked flow''' (often referred to as '''critical flow''') of a flowing [[gas]] is a limiting point which occurs under specific conditions when a gas at a certain [[pressure]] and [[temperature]] flows through a restriction<ref>A [[valve]], a [[convergent-divergent nozzle]] such as a [[de Laval nozzle]], an [[orifice plate]] hole, a leak in a gas pipeline or other gas container, a [[rocket engine]] exhaust nozzle, etc.</ref> into a lower pressure environment.
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| As the gas flows through the smaller cross-sectional area of the restriction, its linear [[velocity]] must increase. The limiting point is reached when the linear gas velocity increases to the [[speed of sound]] ([[sonic velocity]]) in the gas. At that point, the [[mass]] flow rate (mass per unit of time) of the gas becomes independent of the downstream pressure, meaning that the mass flow rate can not be increased any further by further lowering of the downstream pressure. The physical point at which the choking occurs (i.e., the cross-sectional area of the restriction) is sometimes called the ''choke plane''. It is important to note that although the gas velocity becomes choked, the mass flow rate of the gas can still be increased by increasing the upstream pressure or by decreasing the upstream temperature.
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| The choked flow of gases is useful in many engineering applications because, under choked conditions, valves and calibrated orifice plates can be used to produce a particular mass flow rate. Choked flow in a [[de Laval nozzle]] as used in a [[rocket engine]] can be accelerated to [[supersonic]] linear velocities.
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| In the case of liquids, a different type of limiting condition (also known as choked flow) occurs when the [[Venturi effect]] acting on the liquid flow through the restriction decreases the liquid pressure to below that of the liquid [[vapor pressure]] at the prevailing liquid temperature. At that point, the liquid will partially "flash" into bubbles of vapor and the subsequent collapse of the bubbles causes [[cavitation]]. Cavitation is quite noisy and can be sufficiently violent to physically damage valves, pipes and associated equipment. In effect, the vapor bubble formation in the restriction limits the flow from increasing any further.<ref>[http://www.fisherregulators.com/technical/sizingcalculations/ Scroll to discussion of liquid flashing and cavitation]</ref><ref>[http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf Search document for "Choked"]</ref>
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| ===Conditions under which gas flow becomes choked===
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| All gases flow from upstream higher pressure sources to downstream lower pressure environments. Choked flow occurs when the ratio of the absolute upstream pressure to the absolute downstream pressure is equal to or greater than:
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| :<math>(1)</math> <font style="vertical-align:+15%;"><math>\big[(k+1)/2 \big]^{\,k/(k-1)}</math></font>
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| where <math>k</math> is the [[specific heat ratio]] of the discharged gas (sometimes called the [[isentropic expansion factor]] and sometimes denoted as <math>\gamma</math> ).
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| For many gases, <math>k</math> ranges from about 1.09 to about 1.41, and therefore the expression in '''(1)''' ranges from 1.7 to about 1.9, which means that choked velocity usually occurs when the absolute upstream vessel pressure is at least 1.7 to 1.9 times as high as the absolute downstream pressure.
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| ''[[Choked flow|.... (read more)]]''
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| ! style="text-align: center;" | [[Choked flow#References|notes]]
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| {{reflist|2}} | | {{reflist|2}} |
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| | </small> |
Latest revision as of 09:19, 11 September 2020
After decades of failure to slow the rising global consumption of coal, oil and gas,[1] many countries have proceeded as of 2024 to reconsider nuclear power in order to lower the demand for fossil fuels.[2] Wind and solar power alone, without large-scale storage for these intermittent sources, are unlikely to meet the world's needs for reliable energy.[3][4][5] See Figures 1 and 2 on the magnitude of the world energy challenge.
Nuclear power plants that use nuclear reactors to create electricity could provide the abundant, zero-carbon, dispatchable[6] energy needed for a low-carbon future, but not by simply building more of what we already have. New innovative designs for nuclear reactors are needed to avoid the problems of the past.
(CC) Image: Geoff Russell Fig.1 Electricity consumption may soon double, mostly from coal-fired power plants in the developing world.
[7] Issues Confronting the Nuclear Industry
New reactor designers have sought to address issues that have prevented the acceptance of nuclear power, including safety, waste management, weapons proliferation, and cost. This article will summarize the questions that have been raised and the criteria that have been established for evaluating these designs. Answers to these questions will be provided by the designers of these reactors in the articles on their designs. Further debate will be provided in the Discussion and the Debate Guide pages of those articles.
- ↑ Global Energy Growth by Our World In Data
- ↑ Countries, organizations, and public figures that have reconsidered their stance on nuclear power are listed on the External Links tab of this article.
- ↑ Pumped storage is currently the most economical way to store electricity, but it requires a large reservoir on a nearby hill or in an abandoned mine. Li-ion battery systems at $500 per KWh are not practical for utility-scale storage. See Energy Storage for a summary of other alternatives.
- ↑ Utilities that include wind and solar power in their grid must have non-intermittent generating capacity (typically fossil fuels) to handle maximum demand for several days. They can save on fuel, but the cost of the plant is the same with or without intermittent sources.
- ↑ Mark Jacobson believes that long-distance transmission lines can provide an alternative to costly storage. See the bibliography for more on this proposal and the critique by Christopher Clack.
- ↑ "Load following" is the term used by utilities, and is important when there is a lot of wind and solar on the grid. Some reactors are not able to do this.
- ↑ Fig.1.3 in Devanney "Why Nuclear Power has been a Flop"
