MOTIVATION

The relatively poor understanding of gravity effects on multiphase flow and phase-change heat transfer has been identified as one of the primary obstacles to reliable design of space based hardware and processes such as heat exchange, cryogenic fuel storage and transportation, and electronic cooling. This lack of understanding has led to the use of lower power components in space-based systems since higher power systems requiring the use of two-phase flow was considered to be risky. If gravity effects on two-phase flows can be quantified, the size and weight of these systems can be reduced, lowering the cost of launching the components and space based systems. Experiments to date have shown that stable, subcooled pool boiling on flat plates in microgravity environments is possible, although usually with reductions in heat transfer coefficients up to 50% compared with earth gravity values. Relatively little experimental data is available regarding the local heat transfer rates under and around the bubbles as they grow and depart from the surface. Such data can provide much needed information regarding the relevant wall heat transfer mechanisms during the bubble departure cycle by pinpointing when and where in the cycle large amounts of heat are removed.

Microgravity effects on critical heat flux levels is another very important area that must be addressed if boiling is to be used reliably as a heat removal mechanism, and is another area in which little quantitative information is available. By conducting tests in microgravity as well as lunar and martian gravity, it is possible to assess the effect of buoyancy on the overall boiling process and determine the relative magnitude of effects with regards to other “forces” and phenomena such as Marangoni forces, liquid momentum forces, and microlayer evaporation.

The objectives of the this work are to determine the boiling heat transfer mechanisms in microgravity at various bulk fluid subcoolings and pressures, and to compare these mechanisms to those at normal, martian, and lunar gravity levels. It is hypothesized that coalescence becomes the primary bubble removal mechanism in microgravity, and causes the formation of a primary bubble. If this bubble is attached to the surface, it causes local dryout to occur, and heat can only be transferred from the surface by the smaller bubbles surrounding the primary bubble. This small-scale boiling behavior is similar across gravity levels, and it is hypothesized that boiling curves in microgravity can be obtained from knowledge of earth gravity data and the size of the primary bubble in microgravity.

Low gravity environments have been produced using 1). Drop towers. 2). Parabolic Aircraft, 3). Sounding Rockets, and 4). Space flight. We have obtained boiling data using the KC-135 Aircraft and a Sounding Rocket, and are designing an experiment for the International Space Station that is expected to fly within the next few years. Some of the advantages and disadvantages of each of these platforms is summarized below:

  Advantages Disadvantages
Drop Towers Many drops per day can be made, high quality microgravity (a/g<0.0001) possible Microgravity time limited to 10 s.
Parabolic Aircraft Up to forty 25 second low-gravity period per flight with two flight per day possible Poor quality gravity (a/g<0.02).
Sounding Rocket High quality microgravity (a/g<1E-6 possible), minutes of microgravity time per flight Expensive–hundreds of thousands to millions of dollars per flight.
Space Flight High quality microgravity (a/g<1E-6 possible), months to years of microgravity time possible Expensive–millions of dollars per flight, few flight opportunities.

RESULTS

The primary effect of low gravity on boiling is the formation of a primary bubble on the surface that results in a dry patch as seen on Figure 1 for various subcoolings and wall superheats. A large primary bubble forms whose size is on the order of the heater array (2.7 mm in this case). This bubble formed from the coalescence of smaller bubbles that grew on the surface after transition into low-g during the parabolic flight. The primary bubble increases in size with increasing bulk fluid temperature and wall superheat, and is fed by smaller satellite bubbles that surround it. For a given bulk temperature, the size of the satellite bubbles decreases with increasing superheat since the size to which they are able to grow is limited by the increasing size of the primary bubble. The satellite bubbles themselves are surrounded by even smaller bubbles which feed them.

The time averaged heat transfer distribution on the wall is also shown on Figure 2. Very little heat transfer is associated with the primary bubble. Much higher heat transfer rates are associated with the rapid growth and coalescence process of the satellite bubbles. A significant amount of the surface dries out before CHF. CHF occurs when the dry spot size grows faster than the increase in heat transfer outside the primary bubble.

A quicktime movie of the bubble behavior in the transition between low-g and high-g, and the heat flux distribution can be downloaded by clicking here (2.7 MB).

 

Figure 1 (264 KB)

Figure 2 (52 KB)

Boiling curves

Boiling curves for low-g, earth gravity, and high-g (1.7 g) for various subcoolings are shown on Figure 3. Little effect of subcooling is seen in the nucleate boiling regime for the 1 g and high-g data, which is consistent with the observations of previous researchers. The low-g data tend to follow the 1 g and high-g data for DTsat<20 °C, but drop below them for higher superheats as the primary bubble causes dryout over a successively larger fraction of the heater. CHF for the low-g curves are significantly lower than those for the 1 g and high-g data.

If we sample the time resolved data only when we have boiling on the surface (we exclude the dry patch under the primary bubble), then we can obtain the heat flux associated with the smaller satellite bubbles. The results are shown on Figure 4, which includes data obtained in 1-g and 1.8 g environments. It is seen that the heat flux collapses onto a single curve, indicating that the small scale boiling is independent of subcooling and gravity level. This suggests that if one is able to predict the extent of the dry area in low-g, then one can predict the low-g boiling curve from earth or high gravity boiling heat flux data.

 

Figure 3 (8 KB)

Figure 4 (36 KB)

Some papers documenting these results are

1). Kim, J., Yaddanapuddi, N., and Mullen, J.D., (2001) "Heat Transfer Behavior on Small Horizontal Heaters During Saturated Pool Boiling of FC-72 in Microgravity", Microgravity Science and Technology, Vol. XII, pp. 116-127.

2). Kim, J., and Benton, J.F., “Subcooled pool boiling heat transfer at various gravity levels”, International Journal of Heat and Fluid Flow, Volume 23, No. 4, pp. 497-508, August 2002.


3 ). Kim, J., Benton, J.F., and Wisniewski, D., “Pool Boiling Heat Transfer on Small Heaters: Effect of Gravity and Subcooling”, International Journal of Heat and Mass Transfer, Vol. 45, No. 19, pp. 3921-3934, 2002.

4). Henry, C.D. and Kim, J. “Heater size, subcooling, and gravity effects on pool boiling heat transfer”, International Journal of Heat and Fluid Flow, Vol. 25, No. 2, pp. 262-273, 2004.

5). Henry, C.D., Kim, J., “Thermocapillary Effects on Low-G Pool Boiling From Microheater Arrays of Various Aspect Ratio”, Microgravity Science and Technology, XVI, pp. 170-175, 2005.

6). Henry, C.D., Kim, J., Chamberlain, B., and Hartmann, T.G., “Heater aspect ratio effects on pool boiling heat transfer under varying gravity conditions”, Experimental Thermal and Fluid Science, Vol. 29, No. 7, pp. 773-782, 2005.

Reviews of microgravity effects on boiling and a summary of the worldwide activites are given below:

1). DiMarco, Paolo, “Review of reduced gravity boiling heat transfer: European Research”, Invited review paper for Japan Society of Microgravity Application Journal, Vol. 20, No. 4, pp. 252-263, 2003.

2). Kim, J., “Review of reduced gravity boiling heat transfer: US Research”, Invited review paper for Japan Society of Microgravity Application Journal, Vol. 20, No. 4, pp. 264-271, 2003.

3). Ohta, H., “Review of reduced gravity boiling heat transfer: Japanese Research”, Invited review paper for Japan Society of Microgravity Application Journal, Vol. 20, No. 4, pp. 272-285, 2003.

This work was sponsored by NASA 's Office of Biological and Physical Resarch.