In what follows the literature relevant to the present study is briefly reviewed. Pool boiling heat transfer is a process of vigorous heat transfer resulting from latent heat exchange associated with liquid-to-vapor phase change in a quiescent liquid. Nukiyama [3] conducted a pioneering pool boiling experiment in 1934 and arranged the experimental heat transfer data as a form of the wall superheat versus the heat flux, which is known as the “boiling curve”
today. After that, the pool boiling heat transfer research has received considerable attention.
The state of the art cooling technologies for handling heat dissipation in microelectronic equipments have been developed extensively over the past 30 years. Several products were released including Air-Cooled Modules, High Thermal Conduction Modules, and Liquid-Cooled Modules, as discussed by Bar-Cohen [4].
In an early attempt to improve pool boiling heat transfer by using a micro-configured surface, Miller et al. [5] found that vapor retention could be a function of the scale and geometry of the micro-configurations. They pointed out that the relation between the stability of the potential nucleation sites and the micro-configuration size and geometry required further investigation, so that the size and the site density of the cavities could be optimized for boiling heat transfer enhancement.
Slightly later a few studies have been carried out to examine the influences of the surface fabricated microstructures on the pool boiling heat transfer. These include boiling of FC-72 on micro-porous surfaces with particle coating tested by Chang and You [6] and by Vemuri and
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Kim[7], adding micro-porous pin-fins and in the meantime coating particles to the surface investigated by Rainey and You [8,9], and fabricating micro-pin-fins and submicron-scale roughness on the surfaces by Honda et al. [10] and Wei et al. [11]. The study of Rainey and You [8, 9] concluded that the microporous coating can significantly enhance the boiling heat transfer performance over the pin-finned surfaces. In examining the pool boiling on the micro-pin-fin surfaces, Honda et al. [10] and Wei et al. [11] noted that the boiling curves were characterized by a very small increase in the wall superheat could cause a large increase in the heat flux. And increasing the fin height was found to provide better heat transfer in the nucleate boiling regime and result in a higher critical heat flux. Nucleation site interaction in pool boiling on an artificial surface was investigated by Zhang and Shoji [12] and by Yu et al.
[13]. The hydrodynamic interaction can be also influenced by some factors, such as the liquid properties, subcooling, system pressure. The study of Yu et al. [13] concluded that the critical heat flux was dependent on the cavity density. The evaporation/boiling heat transfer regimes in the capillary wicking structures were identified and discussed by Li et al.[14] and Li and Peterson[15]. Anderson and Mudawar [16] reported that microstructures in the forms of fins, studs, grooves and vapor-trapping cavities on the boiling surface significantly shifted the boiling curve toward lower superheats while increasing the incipience excursion. Their results also suggest that the maximum boiling heat flux is a function of surface geometry and orientation but independent of the initial conditions, surface roughness, or the presence of large artificial cavities. Intending to augment boiling heat transfer, O’Connor and You [17]
painted silver flakes on the boiling surface. Their experimental data show that the incipience boiling superheats are 70-85% lower and the nucleate boiling superheats are 70-80% lower than the bare surface. Besides, the critical heat flux is increased by 109%. O’Connor et al. [18]
then compared two methods of generating surface microstructures, “spraying” and “painting”, for pool boiling heat transfer enhancement. They noted that the incipient boiling superheat showed 33-55% reduction for the sprayed alumina and 63-85% reduction for the painted
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diamond. The enhancement in the critical heat flux can be up to 47% for the sprayed alumina and 103% for the painted diamond microstructures. Chang and You [19] further studied the effects of coating different sizes of the diamond particles on the pool boiling heat transfer performances. They classified the coating thickness into two groups. For coatings thinner than 100 μm, increasing the coating thickness would generate a higher active nucleation density.
But for coatings thicker than 100 μm, a further increase in the coating thickness does not always enhance the pool boiling heat transfer. They attributed this result to higher impedance for liquid-vapor exchange channels and higher thermal resistance for the thicker coating. Jung and Kwak [20] investigated the effects of submicron-scale roughness on the subcooled boiling heat transfer over a boiling surface anodized in DMF (dimethylforamide) and HF (hydrofluoric acid). Both surface treatments were found to increase the effective boiling area and served for increasing the nucleation sites and hence showed considerable enhancement in the boiling heat transfer. The critical heat flux also increases linearly. Honda and Wei [21]
reviewed recent advances in enhancing boiling heat transfer from electronic components immersed in dielectric liquids through the use of surface microstructures and concluded that most of the surface microstructures were effective in decreasing the wall superheat at the boiling incipience. The nucleate boiling heat transfer also can be improved and the critical heat flux is raised. Rainey and You [22] and Rainey et al. [23] respectively studied the effects of the orientation and pressure on the pool boiling heat transfer from microporous surface. Their data show that nucleate boiling performance increases slightly for the surface inclined from 0(horizontal) to 45 and then decreases for the inclination angle ranging from 90 to 180. Moreover, for the plain and microporous surfaces increases in boiling performance and critical heat flux and decrease in the incipience wall superheat were noted as the pressure increased.
Chou et al. [24] arranged several grooved patterns on surfaces intending to enhance boiling heat transfer of distilled water. Their experimental data reveal that the radial grooved pattern has the best enhanced boiling heat transfer performance and the spiral or concentric
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grooved pattern has poorer boiling heat transfer coefficient. The worst performance is noted for the grid or the spotted grooved pattern. All grooved patterns they investigated have better heat transfer performance than the plain surface and the denser groove is better than the sparser one for the same patterns.
Hasegawa et al. [25] covered a heat pipe with a woven screen to investigate the associated boiling characteristics and burnout phenomena. Their results disclose that the additional screen produces two opposite effects of inhibiting and enhancing the boiling heat transfer. Tsay et al. [26] explored pool boiling heat transfer enhancement by covering the boiling surface with a screen in distilled water. They found that the screen coverage could raise bubble generation frequency and enhance the boiling heat transfer. But the screen can also cover some nucleation sites and hence may retard the boiling heat transfer. They also noted that the boiling heat transfer became poorer at lowering the liquid level. They concluded that covering the heated surface with a screen could augment the pool boiling heat transfer if the mesh size was comparable with the bubble departure diameter. In boiling of methanol and HFE-7100, Liu et al. [27] pointed out that placing a fine mesh layer on the boiling surface enhanced nucleate boiling heat transfer at low wall superheat (∆T𝑠𝑎𝑡<10K) but an opposite trend resulted at a high superheat (∆T𝑠𝑎𝑡>10K). They also reported that the heat transfer in nucleate boiling always became worse with a coarse mesh on the boiling surface when compared with that on a smooth surface. Moreover, Franco et al. [28] used dielectric refrigerant R141b to investigate enhancement in the boiling heat transfer performance by covering the heated surface with wire meshes. The boiling heat transfer coefficient was noted to increase significantly, especially at relatively low heat fluxes. They also found that the wire mesh coverage on the heating surface resulted in slower transition to steady film boiling. In studying the effects of the wall superheat and the mesh layer covering on boiling heat transfer, Kurihara and Myers [29] tested several working fluids including water, acetone, n-hexane, carbon tetrachloride, and carbon disulfide. They found that active nucleation sites on the
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heating plate increased due to the mesh covering and the boiling heat transfer coefficient was proportional to the one-third power of the bubble column numbers at high numbers.
Shi et al. [30] investigated pool boiling heat transfer in liquid saturated particle bed and fluidized bed of distilled water. The tests were conducted for glass beads, steel ball, find sand and 𝐴𝑙2𝑂3 particles. They showed that boiling heat transfer could be enhanced greatly by adding the solid particles into the liquid whether in the fixed or fluidized particle bed. The boiling heat transfer enhancement is closely related to the particle size (𝑑𝑝=0.5, 1.0 and 2.0 mm), initial bed depth (𝐻𝑝=3.0, 6.5, 9.5 and 13.4 mm) and heat flux applied. The best heat transfer enhancement is 120% for the particle diameter 𝑑𝑝=1.0 mm and bed height 𝐻𝑝=9.5 mm. A similar study was conducted by Matijevic et al. [31] using lead spheres to cover a heating surface. The spheres were packed as closely as possible into a single layer. They noted that boiling heat transfer from the heating surface to water could be enhanced substantially by the metallic spheres (d=3.0, 3.5, 3.6, 4.0 and 4.5 mm), and the smaller spheres resulted in a better enhancement of boiling heat transfer.
Heat transfer enhancement by employing nanofluids has become very popular recently.
In nanofluids a very large number of nano particles (diameters smaller than 100 nm) are added into a working fluid which is considered to significantly increase thermal conductivity of the fluid. Wen and Ding [32] reported an enhancement of boiling heat transfer coefficient for about 40% with alumina water based nanofluids. On the other hand, by using the same nanoparticles in the same fluid, Bang and Chang [33] found that the boiling heat transfer coefficient deteriorated for about 20% when the nanoparticles are added.
Some active techniques to enhance boiling heat transfer were also proposed in the literature. Jeong and Kwon [34] found that the CHF augmentation in pool boiling of water due to ultrasonic vibration was closely related to its effects on the process of bubble generation and its departure. They noted that the rate of increase in CHF for downward facing surface ranged from 87~126% as the water subcooling varied from 5 to 40℃. Cipriani et al. [35] imposed
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electric field on pool boiling of FC-72 over a heated platinum wire (d=0.1 and 0.2 mm)and found that the boiling heat transfer was strongly influenced by the presence of the electric field at a low wire superheat. An increase of the boiling heat transfer coefficient up to 400% was encountered with the maximum applied voltage. But it is almost unaffected by electrical force at high wire superheat. Through heated surface vibration, Navruzov et al. [36] demonstrated that boiling heat transfer of ethanol could be substantially enhanced at low imposed heat flux.
The amplitude of the surface vibration is found to be a governing parameter for heat transfer enhancement at low-frequency vibrations. Besides, the vibration of the heat transfer surface significantly alters the heat transfer process both in subcooled boiling and in free convection.
The single-phase heat transfer curves are 70-80% above the basic curve at increasing heat loads.