During heat leak to the compensation chamber (CC) in LHP, is it possible for vapor to have leaked to CC? In a recent paper on bubble dynamics inside an idealized porous wick by Ram Ranjan et al.(2012) they observed columns of vapor rising from the hot wall and porous wick interface (which was being fed by capillary action from the side). They considered symmetry bc's on the left hand side, the pressure outlet on the right hand side and the top face of the porous wick.
Has anyone ever noticed vapor leakage to the CC through the wick, or does vapor grow on the inner face (of cylindrical wick) or on the upper face (of a flat grooved evaporator having CC on top of it) due to bubble nucleation by heat conduction through the wick?
Is it due to very fine pore as against in heat pipes,thereby causing low permeability of LHP wick, that vapor can't penetrate wick ?
I am just confused by results of Ranjan et al(2012) " Bubble Dynamics during Capillary-Fed Nucleate Boiling in Porous Media, 13th IEEE ITHERM conference proceedings ". As they have reported presence of vapor columns extending from wick-wall interface to the wick-vapor interface inside a vapor chamber(they are having flat wick without any grooves), though they used pore size in the range of 200 micrometer to 400 micrometer which is very coarse unlike LHP's wick.
Thanks for your comment. I appreciate it. I have readed your paper on "HOMOGENEOUS NUCLEATION AND THE HEAT-PIPE BOILING LIMITATION" which is very insightful paper.
With Kind personal Regards,
Bharat
Dear Mr.Bharat bhushan Sharma,
Sorry for the late reply and thanks for your interest in my work.
Your question is truly excellent. However, the phenomena occured in the fine porous wick are not really like the one as modelling in your attached paper:
(1)In fine porous wick, due to the high capillary force, the boiling mechanism will be severely depressed and the evaporation and it evaluation will be main mechanism of heat transfer, especially after the startup process. This statement is based on our visualization, given as the wick is not submerged in the liquid.
(2) for a flat type LHP, if adoping our configuration and with a much lower thermal conductivity of the wick, the influencet of thermal leak on the phase change in the CCis not so heavy, but it really exists and this thermal leak degrade the performance. However, compared to the mass evaporation in the wicked channels, the boiling in the CC is occasional if the heating power is not so large.
I wish my response will be helpful to you.
Regards,
Ji Li
Yes your and Dr. Mishkinis answer has cleared my doubt. Thanks for the reply.
Best Regards,
Bharat
Dear Dr Li & Dr. Mishkinis,
I hope Dr. Li is assuming that vapor removal channels are not flooded by his statement "given as the wick is not submerged in the liquid. ". In that case I agree that vapors may not penetrate the wick however I was going through classic work of Ku.(Operating characteristics of LHP) which though is quite old but still they observed a possibility of actual vapor penetration to the core through wick, in case the vapor is not present initially in core, wick and also vapor removal channels are also flooded with liquid. I am quoting his words as
" In situation 3, both the vapor grooves and the evaporator core are flooded with liquid. The saturation state exists elsewhere in the loop where liquid and vapor phases coexist at the ambient temperature. As a heat load is applied to the evaporator, the liquid in the vapor grooves must be superheated before nucleation can be initiated. Since the heat leak from the evaporator to the compensation chamber is small, the compensation chamber temperature remains virtually unchanged as the evaporator is being heated. At the boiling incipience, vapor could penetrate the evaporator wick if the superheat is high enough. Vapor can also be generated inside the evaporator core after the boiling inception due to the high thermal conductivity of the metal wick. ". Thus accordingly there is a possibility of vapor leakage in case of high heat loads. Now my question is did you considered the case of fully flooded evaporator ? If yes, then what were your observations and if no, are you aware of any work where this study has been done ? And please tell me your opinion in this case as well.
Regards,
Bharat
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