Heat transfer boiling

When a surface is exposed to a liquid and is maintained at a temperature above the saturation temperature of the liquid, boiling may occur, and the heat flux will depend on the difference in temperature between the surface and the saturation temperature. When the heated surface is submerged below a free surface of liquid, the process is referred to as pool boiling. If the temperature of the liquid is below the saturation temperature, the process is called subcooled, or local, boiling. If the liquid is maintained at saturation temperature, the process is known as saturated, or bulk, boiling. 

In nucleate boiling, bubbles are created by the expansion of entrapped gas or vapor at small cavities in the surface. The bubbles grow to a certain size, depending on the surface tension at the liquid-vapor interface and the temperature and pressure. Depending on the temperature excess, the bubbles may collapse on the surface, may expand and detach from the surface to be dissipated in the body of the liquid, or at sufficiently high temperatures may rise 

Experiments have shown that the bubbles are not always in thermodynamic equilibrium with the surrounding liquid i.e., the vapor inside the bubble is not necessarily at the same temperature as the liquid. Considering a spherical bubble as shown in Fig. 9-4, the pressure forces of the liquid and vapor must be balanced by the surface-tension force at the vapor-liquid interface. The pressure force acts on an area of nr2, and the surface tension acts on the interface length of 2irr. The force balance is 

suppose we consider a bubble in pressure equilibrium, i.e., one which is not growing or collapsing. Let us assume that the temperature of the vapor inside the bubble is the saturation temperature corresponding to the pressure pv. If the liquid is at the saturation temperature corresponding to the pressure ph it is below the temperature inside the bubble. Consequently, heat must be conducted out of the bubble, the vapor inside must condense, and the bubble must collapse. This is the phenomenon which occurs when the bubbles collapse on the heating surface or in the body of the liquid. In order for the bubbles to grow and escape to the surface, they must receive heat from the liquid. This requires that the liquid be in a superheated condition so that the temperature 

Pressure force = irr2 pv - Pf) Surface tension force 2 nr o 

1) For AT = (Ts — Ttoii) 6 K (H2O, Ibar), we are in the region of convective boding, and a increases only slightly with temperature (a rs./ 

2) For 6 K A T 30 K, we have nucleate boding, and a strongly increases (0 AT ), which is the result of the intensification of the Uquid circulation by steam bubbles. 

3) For AT 30 K, we get film boiling. The bubbles become so numerous that the surface is partially blanketed with them. If the surface temperature continues to increase, the heat transfer decreases due to the insulating effect of the steam blanket. Thus, this region is characterized by an increase in AT but nevertheless a decrease in heat flux. 

4) A further increase in AT then causes total film boiling (AT 100 K), and the steam completely blankets the heat transfer surface. The heat transfer coefficient is then approximately constant, and the heat flux again increases in proportion to AT 

In practice, the transition from nucleate boiling to film boiling occurs suddenly, and the temperature difference rapidly rises. The heat flux associated with that point of transition is commonly denoted as the critical heat flux, which is an important critical parameter in many applications. 

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The final correlation for the overall boiling heat-transfer coefficient in pipes or channels (20) is a direct addition of the macroscopic (mac) and microscopic (mic) contributions to the coefficient ... 

Figure 10-98. Levy correlation for boiling heat transfer equation. (Used by permission Levy, S. ASME paper no. 58-HT-8, 1958. American Society of Mechanical Engineers. All rights reserved.)...

Process the side boiling heat transfer coefficient ... 

Levy, S., Generalized Correlation of Boiling Heat Transfer, Paper No. 58-HT-8, presented AlChE-ASME Joint Heat Transfer Conference, Chicago, Aug. 1958, pub. ASME Journal of Heat Transfer (1959). 

Bergles, A. E. and W. M. Rohsenow, The Determination of Eorced-Convection Surface-Boiling Heat Transfer, Trans, of the ASME Journal ofiHeat Transfier, (1963), Paper No. 63-HT-22. 

Chen, J. C., A Correlation for Boiling Heat Transfer to Saturated Fluids in Convection Flow, Heat Trans. Div. ASME Conference, Boston (1963), Paper No. 63-HT-34. 

Rohsenow, W. M., Nucleation with Boiling Heat Transfer, Heat Trans. Div. ASME, Conference, Detroit, Ml, May (1970), Paper No. 70-HT-18. 

Vachon, R. I., G. H. Nix, G. E. Tanger, and R. O. Cobb, Pool Boiling Heat Transfer from Teflon-Coated Stainless Steel, ASME Journal Heat Trans., V. 91, Aug. (1969) p. 364. 

Mercer, A. D., Laboratory Research in the Development and Testing of Inhibited Coolants in Boiling Heat Transfer Conditions , in Engine Coolant Testing Slate of the Art 1979. ASTM STP 705, W. H. Ailor, ed., 53-80 (1980)... 

B2. Barnett, P. G., The scaling of forced convection boiling heat transfer, AEEW-R.134 (1963). 

B7. Barte, D. R., Bankoff, S. G., and Colahan, W. J., Summary of conference on bubble dynamics and boiling heat transfer held at the Jet Propulsion Laboratory, June 14th-15th, 1956, JPL Memo. No. 20-137, Calif. Inst. Tech., Pasadena. 

B16. Berenson, P. J., Transition boiling heat transfer, Natl. Heat Transfer Conf, 4th, A.l.Ch.EIASME, 1960, Preprint No. 18. 

E2. Ellion, M. E., A study of the mechanism of boiling heat transfer, JPL Memo. No. 20-88, Calif. Inst. Technol., Pasadena, California (1954). 

H7. Hines, W. S., Forced convection and peak nucleate boiling heat transfer characteristics for hydrazine flowing turbulently in a round tube at pressures to 1000 psia, Rept. No. 2059, Rocketdyne, Canoga Park, California (1959). 

Ivey, H. J., Acceleration and the critical heat flux in pool boiling heat transfer, Proc. Inst. Mech. Engrs. (London) (1962). 

N2. Noel, M. B., Experimental investigation of the forced convection and nucleate boiling heat transfer characteristics of liquid ammonia, JPL Tech. Rept. No. 32-125 (July 1961). 

Salt, K. J., and Wintle, C. A., Design and operation of a transistorized bridge-type detector for burnout in boiling heat transfer experiments, AEEW-R.330, H.M. Stationery Office, London (1964). 

Stock, B. J., Observations on transition boiling heat transfer phenomena, ANL-6175 (1960). 

T3. Tong, L. S., Boiling Heat Transfer and Two-Phase Flow. Wiley, New York, 1965. 

Mostinski, I.L. Brit. Chem. Eng. 8 (1963) 580. Calculation of boiling heat transfer coefficients, based on the law of corresponding states. 

Chapter 7 deals with the practical problems. It contains the results of the general hydrodynamical and thermal characteristics corresponding to laminar flows in micro-channels of different geometry. The overall correlations for drag and heat transfer coefficients in micro-channels at single- and two-phase flows, as well as data on physical properties of selected working fluids are presented. The correlation for boiling heat transfer is also considered. 

Tzanand YL, Yang YM (1990) Experimental study of surfactant effects on pool boiling heat transfer J Heat Transfer 112 207-212... 

Wu WT, Yang YM (1992) Enhanced boiling heat transfer by surfactant additives. In Pool and External Flow Boiling. ASME, New York, pp 361-366... 

Wu WT, Yang YM, Maa JR (1995) Enhancement of nucleate boiling heat transfer and depression of surface tension by surfactant additives. J Heat Transfer 117 526-529... 

Yang YM, Maa JR (2003) Boiling heat transfer enhancement by surfactant additives. In Proceedings of the 5th International Conference Boiling Heat Transfer, ICBHT, Montego Bay, Jamaica, 4-8 May 2003... 

Yen T-H, Shoji M, Takemura F, Suzuki Y, Kasagj N (2006) Visualization of convective boiling heat transfer in single micro-channels with different shaped cross-sections. Int J Heat Mass Transfer 49 3884-3894... 

To the best of our knowledge there is a paucity of data in the literature on gas-liquid (without boiling) heat transfer in micro-channels. Hetsroni et al. (2003a) studied this matter in a test section, that contains 21 parallel triangular micro-channels of d/h = 130 pm. A scheme of the test section used in that work is shown in Fig. 5.13 (see Sect. 5.4). 

Celata GP (ed) (2004) Heat transfer and fluid flow in micro-channels. Bergel, New York Chen JC (1966) Correlation for boiling heat transfer to saturated fluids in convective flow. Ind Eng Chem Process Des Dev 5 322-329... 

The detail experimental study of flow boiling heat transfer in two-phase heat sinks was performed by Qu and Mudawar (2003b). It was shown that the saturated flow boiling heat transfer coefficient in a micro-channel heat sink is a strong function of mass velocity and depends only weakly on the heat flux. This result, as well as the results by Lee and Lee (2001b), indicates that the dominant mechanism for water micro-channel heat sinks is forced convective boiling but not nucleate boiling. 

The convective and nucleate boiling heat transfer coefficient was the subject of experiments by Grohmann (2005). The measurements were performed in microtubes of 250 and 500 pm in diameter. The nucleate boiling metastable flow regimes were observed. Heat transfer characteristics at the nucleate and convective boiling in micro-channels with different cross-sections were studied by Yen et al. (2006). Two types of micro-channels were tested a circular micro-tube with a 210 pm diameter, and a square micro-channel with a 214 pm hydraulic diameter. The heat transfer coefficient was higher for the square micro-channel because the corners acted as effective nucleation sites. 

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