Heat transfer process in gas-liquid two-phase flows is accompanied by the presence of a moving and deforming phase interface. Specifically, during boiling process vapour bubbles rapidly form at the solid-liquid interface, detach from the surface when they reach a certain size, and attempt to rise to the free surface of the liquid. According to the bulk fluid motion, boiling is classified as pool boiling, which is under quiescent fluid conditions, or flow boiling, which is under forced-flow conditions.
2.1.1 Pool Boiling
Pool boiling refers to boiling along a heated surface submerged in a large volume of quiescent liquid (Naterer, 2002). As shown in Figure 2.1, pool boiling arises under two types of conditions: electrical heating and thermal heating. With electrical heating, the heat flux can be calculated based on measurements of the applied current and voltage. Thus the heat flux is an independent variable, whereas temperature is a dependent variable. However, in thermal heating, the surface temperature can be set independently of the heat flux. Figure 2.1 also illustrates that in pool boiling any liquid motion is due to free convection and mixing induced by bubble growth and detachment from the heated surface.
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Figure 2. 1: Electrical and thermal heating (Naterer, 2002)
The study of pool boiling was pioneered by Nukiyama (1966) who used electrically heated nichrome and platinum wires immersed in liquids in his experiments. Nukiyama noticed that boiling takes different forms, depending on the value of the wall superheat ΔTsup (=TW-Tsat), which is the temperature difference between the heater surface and the saturation temperature of the liquid. Four distinct boiling regimes are identified: natural convection boiling, nucleate boiling, transition boiling, and film boiling. These regimes are illustrated on Nukiyama‘s boiling curve in Figure 2.2, which is a plot of boiling of heat flux q versus the wall superheat ΔTsup.
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Natural convection (up to A): Free single-phase natural convection occurs from the heated surface to the saturation liquid without formation of bubbles.
Nucleate boiling (A-C): Bubbles nucleate, grow and depart from the heated surface, and further coalesce, mix, and ascend as merged jets or columns of vapour, as wall superheat increases.
Transition boiling (C-D): An unstable (partial) vapour film forms on the heating surface, and conditions oscillate between nucleate and film boiling.
Film boiling (beyond D): A stable layer of vapour forms between the heated surface and the liquid, and blocks the liquid from contacting the surface.
Among these four boiling regimes, nucleate boiling is the most desirable one in practice because high heat transfer rates can be achieved in this regime with relatively small values of ΔTsup, typically under 30 °C for water. During nucleate boiling, vapour bubbles start forming at cavities along the heated surface where a gas or vapour phase already exists. The liquid in microlayer, which is a thin layer underneath the bubble, extract heat from the surface and evaporate. Due to the continuous heating and liquid evaporation, the vapour bubbles keep growing and expanding until the buoyancy force is large enough to lift the bubbles from the cavities. During this process, bubbles ascend and carry away the latent heat of evaporation, while liquid between the bubbles continues to absorb heat by natural convection from the surface (Figure 2.3).
Figure 2. 3: Bubble grow and departure on an active site (Li et al., 2014a). At large values of ΔTsup, the rate of evaporation at the heater surface reaches such high values that bubbles grow rapidly and eventually merge together. Consequently, a
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large fraction of the heated surface will be covered by bubbles, making it difficult for the liquid to reach the heated surface and wet it. Thus, the heat flux increases at a lower rate with increasing ΔTsup, and reaches a maximum at point C in Figure 2.2. The heat flux at this point is the critical heat flux (qmax, CHF). Nukiyama (1966) noticed that when the power applied to the nichrome wire immersed in water exceeded qmax even slightly, the wire temperature jumped suddenly to the melting point of the wire (1500 K) and burnout occurred beyond his control. Therefore, point C on the boiling curve is also called the burnout point. In the design of boiling heat transfer equipment, it is extremely important for the designer to have a good knowledge of the critical heat flux to avoid the danger of burnout.
2.1.2 Flow Boiling
Flow boiling is the boiling process where the fluid is forced to move in a heated pipe (internal flow boiling) or over a surface (external flow boiling) by external means such as a pump as it undergoes a phase-change process.Since there is no free surface for the vapour to escape during internal flow boiling (two-phase flow), the consequent mixing of the liquid and vapour phase make it more complicated in nature and strongly influence the boiling heat transfer. Therefore, flow boiling heat transfer is closely related to the two-phase flow structure of the evaporating fluid. And it exhibits characteristics of both convection and pool boiling. Commonly observed flow structures are defined as two-phase flow patterns. The flow patterns encountered in co- current upflow of gas and liquid in a vertical tube are shown in Figure 2.4.
Bubbly flow: small discrete bubbles in the continuous liquid phase with various shapes and sizes.
Slug flow: with increasing the gas fraction, larger bubbles formed due to collision and coalescence.
Churn flow: with increasing the velocity, the flow becomes unstable and the liquid travels up and down in an oscillatory fashion.
Annular flow: a thin film of liquid on the wall with the gas as the continuous phase in the centre of the tube.
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Figure 2. 4: Flow patterns in vertical upflow: (a) bubbly flow; (b) slug flow; (c) churn flow; (d) annular flow.
The different stages encountered in flow boiling in a heated tube are illustrated in Figure 2.5 together with the variation of the heat transfer coefficient along the tube. Initially, the liquid is subcooled and forced convection dominates the heat transfer to the liquid. Then the bubbles‘ formation and detachment from the heated surface of the tube, and the sequent draft into the mainstream gives the fluid flow a bubbly appearance. With the fluid heated further, the size of the bubbles increase gradually and eventually approach the pipe diameter due to bubble coalescence. The slug of vapour occupy up to half of the volume in the tube until the liquid mainly flows as a film along the walls and the core of the flow consists of vapour only. This is the annular-flow regime, and very high heat transfer coefficients are realized in this regime.
In pool boiling, the vapour flow is largely buoyancy driven. In contrast, forced flow boiling involves bulk motion of the liquid and buoyancy effects. Thus the heat transfer coefficient is less dependent on heat flux than in pool boiling, while its dependence on the local vapour quality appears as a new and important parameter. Both the nucleate and convective heat transfer mechanisms must be taken into account to predict heat transfer data in the flow boiling regime. The local flow parameters such as void fraction, bubble velocity, bubble size and interfacial area concentration become critical to the prediction of heat transfer in flow boiling.
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Figure 2. 5: Two-phase flow regimes in vertical pipe flow (Naterer, 2002).
How to improve the critical heat flux and the heat transfer coefficient has always been a hot topic in the research of boiling heat transfer. For pool boiling, since the fluid in bulk flow field is almost stationary, the focus is on the heated surface where evaporation and convection mostly occur. Techniques such as sintering, brazing, and flame spraying, which can modify the characteristics of the heated surface have been developed rapidly and numerously to build porous structures on the heated surface and enhance nucleation (Pais and Webb, 1991). Bubble coalescence and interactions between the vapour columns can also affect total heat transfer by changing the convective flow of liquid returning to the heating surface. For flow boiling, as previously mentioned, the heat transfer is closely related to the two-phase flow structure of the evaporating fluid. As the use of nanofluids instead of pure liquids can significantly enhance the boiling heat transfer, a detailed and systematic literature review of experimental findings of gas-nanofluid bubbly flows is needed, in order to develop a comprehensive model.