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Effects of an Electric Field on Nucleate Pool Boiling and Bubble Behavior on a Horizontal Wire
Published in John C. Chen, Yasunobu Fujita, Franz Mayinger, Ralph Nelson, Convective Flow Boiling, 2019
Y. C. Kweon, M. H. Klm, H. J. Cho, I. S. Kang, S. J. Kim
When boiling heat transfer occurs, vapor bubbles can be affected by the electrophoretic force due to the polarized charges on their surface and the dielectrophoretic force due to the difference of the permittivity between bubbles and the dielectric fluid. The relative magnitude of two electric forces depends on the electric field strength, the electrical conductivity and the permittivity. Induced polarized charge which is built up on the bubble surface is related to the dielectric relaxation time and the bubble rising time. If the bubble rising time is much less than the dielectric relaxation time, there are no polarized charges on the bubble surface. For vapor bubbles rising at a finite depth in the boiling fluid, the rising time of vapor bubbles is much less than the charge relaxation time. Therefore, the polarized charges on the surface of vapor bubbles may be negligible. Also, since the permittivity of vapor bubbles is less than that of the boiling fluid, vapor bubbles will be driven toward the region of the lower electric field. The faster departure of vapor bubbles will promote the instability of the thermal layer on the heating surface and disturb the bulk fluid more effectively, thus the thermal resistance on the heating surface is reduced and nucleate boiling heat transfer will be enhanced.
Lumped Capacity Transient Heat Transfer
Published in Randall F. Barron, Gregory F. Nellis, Cryogenic Heat Transfer, 2017
Randall F. Barron, Gregory F. Nellis
The cooldown problem arises because film boiling invariably occurs when a cryogenic fluid is brought suddenly in contact with a surface that is initially at a much higher temperature. As discussed in Chapter 7, film boiling occurs when a large temperature difference exists between the heated surface and the boiling fluid. In this boiling regime, the heating surface is completely blanketed by a stable film of vapor, which has a relatively low thermal conductivity. The more favorable nucleate boiling regime occurs at smaller values of the temperature difference. Nucleate boiling is associated with individual vapor bubbles forming various nucleation sites on the heating surface that is otherwise covered with liquid. The heat flux (or heat transfer coefficient) is relatively high in the nucleate boiling regime because of the agitation of the liquid adjacent to the heating surface that is caused by the bubbles. As soon as nucleate boiling is initiated, the mass will cool very rapidly; however, the surface must be cooled to approximately 35°F or 20°C above the fluid saturation temperature before nucleate boiling is initiated for most cryogenic fluids. The boiling regimes are illustrated in Figure 3.8 for pool boiling of liquid nitrogen. Figure 3.8a illustrates the heat flux as a function of the surface-to-fluid temperature difference, and Figure 3.8b illustrates the same information in terms of the heat transfer coefficient. Note that the maximum heat transfer coefficient in nucleate boiling is almost 100 times larger than the minimum heat transfer coefficient in film boiling.
Quenching Phenomena
Published in G. F. Hewitt, J. M. Delhaye, N. Zuber, Post-Dryout Heat Transfer, 2017
R. A. Nelson, K. O. Pasamehmetoglu
Berenson’s early investigation of transition boiling (Berenson, 1960) often is quoted as demonstrating the effect of surface roughness on transition boiling (see Fig. 40). However, a closer look at Fig. 40 leads to the question of whether the data are reflecting a change in transition boiling or if a change is occurring in nucleate boiling. Because of the modeling approach of connecting the maximum and minimum points, which is implied by the figure, this effect is not fully clear. More recently, Chowdhury and Winterton (1984) have noted that the effect of surface roughness is limited to the nucleate boiling region. The change in nucleate boiling is a result of the number of nucleation sites and cavity geometry, rather than the surface roughness, which has a tendency to reflect these factors.
Nucleate pool boiling thermal management systems of hydrazine reduced graphene oxide (H-rGO) nanofluids with rough surface
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2023
Kamatchi Rajaram, Mohan Raman, Kalaimegam Dhanapal, Suresh Muthusamy, Ponarun Ramamoorthi, Murugesan Govindasamy, Om Prava Mishra
Nucleate boiling is an efficient method that offers twofolds of heat transfer coefficient (HTC) than single-phase heat transfer and hence, it is used in chemical reactors, refrigeration cycles, re-boilers, solar collectors, etc. Over last two decades, a numerous experimental studies have been conducted with rough and enhanced heating surfaces for improving HTC of different base fluids. The studies related to rough surface showed that contact angle and surface roughness are the major factors that affect HTC (Hong, Imadojemu, and Webb 1994; Tong et al. 1990). Kandlikar and Steinke (Kandlikar and Steinke 2001) investigated surface roughness of copper and stainless steel with contact angles of water. Stainless steel exhibits an inverse proportion between contact angle and surface roughness, whereas direct proportion is noticed for copper. Although a number of literature reports available on contact angle, it is still difficult to understand the boiling phenomena. Nucleate boiling with smooth and rough surface is studied in detail by McHale and Garimella (2010). They reported new correlations on nucleation sites to be developed by considering surface roughness.
Heat Transfer Characteristics of CO2 in a Horizontal Tube under Subcritical and Supercritical Pressures
Published in Heat Transfer Engineering, 2023
Chengrui Zhang, Bingtao Hao, Liangyuan Cheng, Jinliang Xu, Qingyang Wang
As shown in Figure 8a and b, under subcritical pressure, the magnitude of the wall temperature peak increases with increasing heat flux, as expected. The heat transfer coefficient in the low enthalpy region increases with heat flux, which is because nucleate boiling dominates this regime and higher heat flux enhances bubble dynamics and improves nucleate boiling heat transfer. Similar behavior has been observed in Ref. [38, 46–48]. The sudden rise of the wall temperature due to departure from nucleate boiling shifts toward smaller enthalpy value with increasing heat flux, due to the more efficient bubble generation promoting vapor film formation. In the large enthalpy region (after the wall temperature peak), the heat transfer coefficient is almost independent of heat flux, which can be attributed to the dominance of mist flow in this regime.
Experimental studies of the pool boiling heat transfer enhancement capability and mechanisms of the micro-pillared surfaces on copper substrate
Published in Experimental Heat Transfer, 2022
Linsong Gao, Lv Jizu, Yubai Li, Yang Li, Dongdong Gao, Lin Shi, Minli Bai
Boiling could take a large amount of heat from a surface to overcome energy barriers during the liquid-vapor phase change [1, 2]. Boiling can mainly be divided into two regimes: nucleate boiling and film boiling. The nucleate boiling, as an efficient heat transfer enhancement regime, which provides a high heat transfer rate with the small wall superheat in the quasi-steady state, is widely utilized in industrial practices such as boiler, desalination, and air conditioner. In the nucleate boiling regime, isolated bubbles nucleate from the heated surface. In this regime, the process can be regarded as a combination of convection and evaporation processes [3] and the main heat transfer is used for the bubble growth process [4]. So, an important strategy to enhance pool boiling heat transfer is to improve the bubble nucleation sites [5]. The generation of bubbles from the heated surface is related to the surface structure [6]. Based on this mechanism, the surface with carefully designed microscale structures has the potential of a better pool boiling heat transfer performance. The pool boiling performance is characterized by its heat transfer coefficient (HTC) and critical heat flux (CHF).