Everything about Critical Heat Flux totally explained
Critical heat flux describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat
water), which suddenly decreases the efficiency of
heat transfer, thus causing localised overheating of the heating surface.
NOTE: the
Critical heat flux for
ignition is the lowest thermal load per unit area capable of initiating a
combustion reaction on a given material (either
flame or
smoulder ignition).
Description
When
liquid coolant undergoes a change in
phase due to the
absorption of heat from a heated solid surface, a higher
transfer rate occurs. The more efficient heat transfer from the heated surface (in the form of
heat of vaporization plus
sensible heat) and the motions of the bubbles (bubble-driven
turbulence and
convection) leads to rapid mixing of the
fluid. Therefore,
boiling heat transfer has played an important role in
industrial heat transfer processes such as
macroscopic heat transfer
exchangers in
nuclear and fossil power plants, and in microscopic heat transfer devices such as heat
pipes and
microchannels for cooling
electronic chips.
The use of boiling is limited by a condition called
critical heat flux (CHF), which is also called as
boiling crisis or departure from
nucleate boiling (DNB). The most serious problem is that the boiling limitation can be directly related to the physical
burnout of the materials of a heated surface due to the suddenly inefficient heat transfer through a
vapor film formed across the surface resulting from the replacement of liquid by vapor adjacent to the heated surface.
Consequently, the occurrence of CHF is accompanied by an inordinate increase in the surface temperature for a surface-heat-flux-controlled system. Otherwise, an inordinate decrease of the heat transfer rate occurs for a surface-temperature-controlled system. This can be explained with
Newton's law of cooling:
where
represents the heat flux,
represents the
heat transfer coefficient,
represents the wall temperature and
represents the fluid temperature. If
decreases significantly due to the occurrence of the CHF condition,
will increase for fixed
and
while
will decrease for fixed
.
Use
The understanding of CHF phenomenon and an accurate prediction of the CHF condition are important for safe and economic design of many
heat transfer units including
nuclear reactors,
fossil fuel boilers,
fusion reactors, electronic chips, etc. Therefore, the phenomenon has been investigated extensively over the world since
Nukiyama (
1934) first characterized it. In particular, a large amount of significant work has been done during the last four decades with the development of water cooled
nuclear reactors. Now many aspects of the phenomenon are well understood and several reliable
prediction models are available for conditions of common interests.
A number of different terms are used to denote the CHF condition: departure from nucleate boiling (DNB), liquid film dryout (LFD),
annular film dryout (AFD), dryout (DO), burnout (BO), boiling crisis (BC), boiling transition (BT), etc. DNB, LFD and AFD represent specific mechanisms which will be introduced later.
DO means the disappearance of liquid on the heat transfer which properly describes the CHF condition; however, it's usually used to indicate the liquid film dryout from
annular flow. BO, BC and BT are phenomenon-oriented names and are used as general terms. The CHF condition (or simply the CHF) is the most widely used today, though it may mislead one to think as if there exists a criticality in the heat flux. The terms denoting the value of heat flux at the CHF occurrence are CHF, dryout heat flux, burnout heat flux, maximum heat flux, DNB heat flux, etc.
The term peak pool boiling heat flux is also used to denote the CHF in pool boiling.
Further Information
Get more info on 'Critical Heat Flux'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://critical_heat_flux.totallyexplained.com">Critical heat flux Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
