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== '''[[Choked flow]]''' ==
== '''[[Economics]]''' ==
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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. 
The term '''economics''' refers both to an intellectual discipline and to a profession.  


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.
The intellectual discipline of economics is an attempt to gain an  understanding of  the processes that govern the production, distribution  and consumption of [[wealth (economics)|wealth]], and to use that understanding to assist in the prediction of the consequences of economic activities. It uses the methodology of [[science]] and can be considered to be a science insofar as it produces testable propositions (see [[/Tutorials#Economics as a science|economics as a science]]), although some branches of the subject are widely considered to be normative (see [[/Tutorials#Normative economics|normative economics]]). Like other sciences, it is subject to a continuing process of revision.


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.
The profession of economics includes academics<ref>For a light-hearted look at the life of an academic economist, see Axel Leijonhufvud's ''Life among the Econs''[http://www.kysq.org/diss/LifeamongtheEcon.pdf]</ref>  who construct, develop and teach economic theory, and practitioners who use economic theory  to make forecasts or to advise upon political, commercial and regulatory decisions. Its most influential application is to the management of the economy. Mistaken decisions in that context can do more damage than in most others.


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>
===The methodology of economics===
The traditional methodology of economics has been  first to formulate a theory, and then to examine how far it provides  operationally useful conclusions. Its pioneers have often adopted an  [[instrumentalism|instrumentalist]] approach:  basing  a theory on arbitrary axioms - such as consistently rational human behaviour - and then advocating its acceptance solely  on the grounds that it had  provided  operationally useful results. That methodology has  proved to be vulnerable to changing conditions, however, and there  has recently been a tendency to move away  from an exclusively axiom-based  approach  towards a greater recognition  of observed behaviour.  Among the  techniques that have been coming into use for that purpose are those of [[Philosophy of economics#Behavioural economics|behavioural economics]] and [[neuroeconomics]].


===Conditions under which gas flow becomes choked===
''[[Economics|.... (read more)]]''
 
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:
 
:<math>(1)</math> &nbsp; &nbsp; <font style="vertical-align:+15%;"><math>\big[(k+1)/2 \big]^{\,k/(k-1)}</math></font>
 
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> ).
 
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.
 
''[[Choked flow|.... (read more)]]''


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Revision as of 21:52, 27 April 2012

Economics


The term economics refers both to an intellectual discipline and to a profession.

The intellectual discipline of economics is an attempt to gain an understanding of the processes that govern the production, distribution and consumption of wealth, and to use that understanding to assist in the prediction of the consequences of economic activities. It uses the methodology of science and can be considered to be a science insofar as it produces testable propositions (see economics as a science), although some branches of the subject are widely considered to be normative (see normative economics). Like other sciences, it is subject to a continuing process of revision.

The profession of economics includes academics[1] who construct, develop and teach economic theory, and practitioners who use economic theory to make forecasts or to advise upon political, commercial and regulatory decisions. Its most influential application is to the management of the economy. Mistaken decisions in that context can do more damage than in most others.

The methodology of economics

The traditional methodology of economics has been first to formulate a theory, and then to examine how far it provides operationally useful conclusions. Its pioneers have often adopted an instrumentalist approach: basing a theory on arbitrary axioms - such as consistently rational human behaviour - and then advocating its acceptance solely on the grounds that it had provided operationally useful results. That methodology has proved to be vulnerable to changing conditions, however, and there has recently been a tendency to move away from an exclusively axiom-based approach towards a greater recognition of observed behaviour. Among the techniques that have been coming into use for that purpose are those of behavioural economics and neuroeconomics.

.... (read more)