film boiling regime 4

[yppzb]

I have a question about quenching a copper sphere into liquid nitrogen and thus it undergoes subcooled boiling heat transfer. The question is when I coated a petroleum jelly on the sphere, it acutally increased the rate of cooling (but the jelly acts like an insulator). why would this happen?

Does it have anything to do with the production of vapor layer? Thanks

 

[Montemayor]

yppzb:

Your question is a good one because it relates to the basic mechanism of liquid vaporization - as explained in such classic texts as Kern's "Process Heat Transfer".

The most favorable mechanism for boiling is nucleate boiling, where the formation of individual bubbles on the heat transfer surface actually create turbulance and very high heat transfer rates, resulting in a super heat transfer coefficent. However, as in all such "perfect" worlds, there is a trade off: this just doesn't happen with all surfaces and those surfaces where it does occur are prone to a short life. So-called "slick" surfaces such as highly polished chrome, Teflon, (& perhaps even a vaseline -or petroleum jelly film) promote nucleate surfaces. DuPont tried to use the favorable characteristics of this effect many years ago by sponsoring the fabrication and application of Teflon tube heat exchangers (they had a monopoly on Tefon production). Nucleate boiling has always been an "El Dorado" for all heat exchanger designers. As we write, there may be engineers in many parts of the world still searching for the perfect heat transfer surface that promotes and maintains a nucleate boiling surface. If you talk to old Dinosour engineers like me, you'll walk away dis-enchanted that this type of perfect surface will never be achieved. Boiling coefficients, especially pool boiling ones, are difficult to predict and maintain. The phenomena of "vapor binding" is one problem that still haunts a lot of installations and designs - together with the inevitable contamination of the heat transfer surface by process impurities. This effect of nucleate boiling defeated when there is a predominance of vapor at the tube wall because of the high rate of heat throughput, and very little liquid actually contacts the hot tube wall. The amount of vapor formed at the tube wall actually serves as a gas resistance to the passage of heat into the liquid and reduces the film coefficent for vaporization as the temperature differnce increases. The net result is that we have to resort to applying an empirical maximum heat flux to each pool vaporization application. Kettle reboilers are notorious for exhibiting this phenomenom. You will recall from basic heat transfer that the best insulators are, in reality, gas films. Basically all industrial insulation materials rely basically on trapped, static gas pockets.

To address your specific query, I would venture to say that the acceleration of heat transfer under the petroleum jelly, while very pronounced, will only last for a certain time - after which, the heat transfer will "crash" and undergo a vapor binding effect.

This has been my experience with this type of heat transfer.

 

[yppzb]

This really help. Thanks a lot. I have read many of the Journals and text books. Almost none of them give me some useful insight. Thanks again.

 

[pennpoint]

BINGO! Montemayor
A star for you. You layed it out very clearly to me.

Best Regards

pennpoint

 

[25362]

Constructing on the clear interpretation by Art Montemayor, one may add on the mechanism of boiling liquids, as follows:

Bubble formation is influenced by two main factors: the finish of the surface and the wetting properties of the liquid. A high surface tension against the hot wall, results in smaller spherical vapor bubbles that have an insulating effect. To avoid this, wetting agents are added to certain liquids. The jelly in this case, may also have enhanced the wetting properties of liquid nitrogen.

The vaporization of the liquid is done in two distinct steps. At first, most of the heat is transferred to the liquid adjacent to the heat transfer surface where it attains temperatures slightly above saturation, creating tiny bubbles that leave the solid surface.

Then, as stroboscopic photographs on various boiling cases have revealed, the volume of vapor bubbles increases from 100 to 4500 % after these leave the heated surface; i.e. most of the vaporization into the vapor bubbles takes place after the bubbles have started to rise, as heat of vaporization is extracted from the liquid.

Vapor bubbles start at specific sites. A rough surface will have more sites where bubbles can start than a polished surface, and therefore have a higher coefficient of heat transfer (HTC). This fact led designers to use finned surfaces on evaporators for some simple, clean liquids that do not deposit scale, as the refrigerants R11 and R115.

In most cases, however, smooth surfaces are preferred, to reduce the risk of scale formation, and to make cleaning easier. For example, in evaporators for spent sulphite liquor, polished surfaces are used.

The HTC of a boiling liquid increases with increasing temperature difference [Δ]t to a maximum value, at a critical temperature difference [Δ]t_c, and decreases again for higher [Δ]t's.

The increase is due to the increased number of bubbles giving more interface between vapor and liquid, and increased agitation from the rising bubbles. The decrease at [Δ]t's > [Δ]t_c is due, as Art Montemayor so clearly explained, to the insulating or blanketing effect of gas at the heat transfer surface.

As a corollary: in a previous thread, dealing with kettle reboilers, it was explained that at present, no simple, general equation is available to calculate the HTC for boiling liquids; existing equations give only orders of magnitude and depend on the physical properties of the compound.

Industrial evaporators are generally designed for temperature differences between the wall and the boiling liquid well below [Δ]t_c, and for heat fluxes as recommended by practice and appropriate sources such as Kern, Process Heat Transfer, McGraw-Hill, 1950.