HEFAT2002 1 ST international Conference on Heat Transfer, Fluid Dynamics and Thermodynamics 8-10 April 2002, Kruger National Park, South Africa KS1 INSIGHT INTO MECHANICSMS AND REVIEW OF AVAILABLE MODELS FOR CRITICAL HEAT FLUX (CHF) IN POOL BOILING Satish G. KANDLIKAR Department of Mechanical Engineering Rochester Institute of Technology Rochester, NY 14623 [email protected] ABSTRACT Greek Symbols Critical Heat Flux (CHF) has been a subject of interest for boiling equipment designers from design as well safety as viewpoints. Although considerable data exists in literature, a fundamental knowledge of the mechanisms responsible for the CHF phenomenon is needed to develop better predictive methods and efficient enhancement strategies. A critical assessment of the CHF models available in the literature is presented here, highlighting the needs for future research in this area. φ - angle of inclination of the tube with horizontal ψm - cavity mouth angle, degrees µL – liquid viscosity, Ns/m2 θ - contact angle, degrees ρ L , ρ G - liquid and vapor density, kg/m3 σ - surface tension, N/m 1. INTRODUCTION NOMENCLATURE Dc – cavity mouth diameter, m g – acceleration due to gravity, m2/s h – heat transfer coefficient, W/m2K h L – single phase heat transfer coefficient with liquid phase, W/m2K iLG – latent heat of vaporization, J/kg k – thermal conductivity, W/mK K – contstant in eq. (4) Na – number of active cavities, number/cm2 Nas – number of available cavities, number/cm2 q – heat flux, W/m2 q C – critical heat flux, W/m2 Tsat – saturation temperature, K ∆Tsat - wall superheat, K ∆Tsat,ONB - wall superheat at ONB, K u- flow velocity, m/s v LG – specific volume difference between vapor and liquid, m3/kg Critical Heat Flux, or CHF, represents the maximum heat flux that can be dissipated by nucleate boiling. Any further increase in the wall superheat leads into transition boiling mode with deterioration in the accompanying heat transfer rate. In heat flux controlled systems, CHF represents the point of discontinuity where an increase in the heat flux causes a rapid temperature excursion in the wall temperature, terminating nucleate boiling and leading into film boiling mode of heat transfer. As early as in 1888, Lang [1] recognized through his experiments with high pressure water that as the wall temperature increased beyond a certain point, it resulted in a reduction in the heat transfer rate in a nucleate boiling system. However, it was Nukiyama [2] who realized that the “maximum heat transmission rate” might occur at relatively modest temperature differences. Drew and Mueller [3] present an excellent summary of the historical development in this area. A recent survey of the critical heat flux literature is presented by Kandlikar [4]. 2. THERMAL AND HYDRODYNAMIC CONDITIONS PRIOR TO CHF required for the bubble to grow beyond the cavity opening, (same as ∆Tsat ,ONB ), was obtained as The models proposed in the literature are generally based on the physical phenomena that are supported by experimental evidence. A brief overview of the thermal and hydrodynamic conditions existing on the heater surface prior to reaching CHF is presented in this section. 2.1 Nucleation and Bubble Dynamics Bubble Characteristics over the Nucleate Boiling Range: The bubble characteristics during nucleate boiling was investigated with a high-speed motion camera by Gaertner [5]. He identified different regions based on these characteristics as illustrated in Fig. 1. In the initial phase of nucleate boiling, discrete bubbles are formed over isolated cavities. As the heat flux increases, the bubbles depart in rapid succession forming vapor columns as shown in Fig. 1(b). Subsequently, the neighboring vapor columns merge into a large bubble. This large bubble is referred to as a vapor patch by Zuber [6], and as a hovering bubble by Haramura and Katto [7]. The thin liquid film existing under the large bubble is referred to as the macrolayer. Existence of such macrolayer has been confirmed by Kirby and Westwater [8] and Yu and Mesler [9]. Further increase in the heat flux results in a broader coverage of the heater surface with the large bubbles as shown in Fig. 1(c). Subsequently, the critical heat flux is reached, beyond which point, the degradation caused by the large vapor coverage offsets the increase in the heat transfer due to rapid bubble growth and efficient heat transfer in the macrolayer. The focus of the present discussion is on the exact mechanism that causes the transition from the region depicted in Fig. 1(c) to the CHF condition. Nucleation Site Density: The key elements of pool boiling are bubble nucleation and bubble growth followed by coalescence with neighboring bubbles, and collapse or departure from the heater surface. For a given wall superheat, the range of active cavity sizes is determined by the cavity size distribution on the heater surface. At high heat fluxes, a large number of small diameter cavities are activated. The size and shape of a cavity governs its nucleation characteristics. Griffith and Wallis [10] postulated that as a bubble grows in a cavity, it comes at the mouth of the cavity and assumes a hemispherical shape. This state also represents the point where the radius of curvature of the interface is a minimum. The cavity is assumed to be active when a bubble grows past this condition. Employing the Clasius-Clapeyron equation for the pressure difference between the vapor inside the bubble and the surrounding liquid, the wall superheat Figure 1. Bubble characteristics in different regions of nucleate boiling, Gaertner, [5]. ∆Tsat,ONB = 4σ Tsat ρV i LG Dc (1) where Dc is the cavity mouth diameter, m. It is seen that the wall superheat to activate a cavity varies inversely with the cavity diameter. Smaller cavities are activated at higher wall superheats. Subsequently, Hsu [11] and Sato and Matsumura [12] considered the temperature profile in the liquid and suggested that the hemispherical bubble on a cavity will grow if the liquid temperature at the tip of the bubble was above the saturation temperature corresponding to the vapor pressure inside the bubble. Assuming that cavities of all sizes are available, the following expression for the wall superheat at the onset of nucleate boiling, ONB, was obtained. ∆Tsat,ONB 4σ Tsat v LG h L = k L i LG k L i LG ∆Tsub 1 + 1 + 2σ Tsat v LG h L (2) Hsu, and Sato and Matsumura also obtained an expression for the range of active cavity radii. In deriving the above equation, the liquid temperature at the bubble tip was calculated from the single-phase convection heat transfer coefficient in the liquid phase. As boiling progresses, the actual heat transfer coefficient increases progressively. It is therefore expected that nucleation will progress toward smaller cavity sizes more rapidly with an increase in the wall superheat. Indeed, the nucleation site density increases rapidly with an increase in the wall superheat. Nucleation is also facilitated by the availability of vapor from neighboring sites. Following a mechanistic approach, Wang and Dhir [13, 14] derived the following relationship between the nucleation site density and the wall superheat. N s ( sites / cm2 ) = 5.8 ×10 −5 Dc −5. 4 (3) value of wall superheat. The conditions near the wall are therefore very crowded with nucleating bubbles. 3. CHF MODELS CONSIDERATIONS • • • • • • Since the wall superheat is inversely proportional to the cavity diameter, it follows that the nucleation site density varies 5 .4 ∆Tsat . Dhir [15] presented a plot showing this relationship for different values of contact angle (θ ) and cavity mouth opening angle (ψ m ). Figure 2 shows such a plot for a contact as angle of 18° and cavity mouth angles below 90°. It can be seen that the number of active cavities increases quite dramatically for smaller cavity diameters (on the order of a few micrometers). Relating the active site density information to the critical heat flux condition, it becomes clear that a large number of cavities, with very small diameters, are activated at the high ON HYDRODYNAMIC As the heat flux is increased to the critical heat flux value, vapor generation rate increases and presents an increasing resistance to the liquid flowing toward the heating surface. The hydrodynamic models of CHF are based on the instability encountered at the liquid-vapor interface. The exact location of the interface where the instability sets in is somewhat different in different models presented in literature: • Figure 2 Active nucleation site density variation with cavity mouth diameter, Dc , for a contact angle θ =18° and cavity mouth angle ψ m<90° , Nas – number of available cavities, Na – number of active cavities, Liaw and Dhir [26]. BASED Kutateladze [16, 17] – destruction of stability of twophase flow near the heater surface Borishanskii [18] – liquid flowing coaxially in a countercurrent manner Zuber [19] – instability of a vapor patch over the heater surface, CHF modeling from the transition boiling side Chang [20] – considers a force balance on a bubble attached to a vertical surface, relative velocity between the rising bubble and the liquid reaches a critical value at CHF Moissis and Berenson [21] – instability in the vertical vapor columns surrounded by the counterflow of liquid flowing toward the heater surface Katto and Yokoya [22] and Haramura and Katto [7] – instability of vapor columns formed in the macrolayer, dryout of the macrolayer Sefiane et al. [23] – instability at the interface of an evaporating meniscus at the base of a bubble Perhaps not so surprisingly, all the above formulations, with the exception of Sefiane et al.’s model (which presents a set of equations), lead to a similar expression for the critical heat flux as originally obtained by Kutateladze [17]. This shows that all the instability models, irrespective of their physical model, utilize the same functional groups in representing the instability criterion for the liquid-vapor interface. A brief description of these models is given below. Kutateladze [16, 17] proposed that the meaning of bubble generation and departure was lost near the critical heat flux condition, which is essentially a hydrodynamic phenomenon with the destruction of the stability of the two-phase flow existing close to the heating surface. Critical condition is reached when the velocity in the vapor phase reaches a critical value. Following a dimensional analysis, he proposed the following correlation. qC′′ i LG ρ G 0. 5 [σ g ( ρ L − ρ G )]1 / 4 =K (4) The value of K was found to be 0.16 from the experimental data. Borishanskii [18] modeled the problem by considering the two-phase boundary in which liquid stream flowing coaxially with gas experiences instability. His work lead to the following equation for K in the Kutateladze’s equation, Eq. (4) ρL σ 3/ 2 K = 0.13 + 4 2 1/2 µ [g (ρ L − ρ G )] −0. 4 (5) Although viscosity appears in eq. (5), its overall effect is quite small on CHF. to 0.138. Simplifying the analysis further, Zuber proposed a value of K=0.131. Chang [20] performed a force balance on a single bubble, close to its departure condition on a vertical surface. He obtained the vapor rise velocity from an expression developed for gas bubbles in a pool of liquid. The resulting departure bubble radius and relative velocity between the two phases were employed in defining the Weber number. Chang postulated that this Weber number reached a critical value at the CHF condition. This resulted in the same expression as eq. (4), with K=0.098. The CHF for vertical surfaces was taken to be 75 percent of the value predicted for a horizontal surface. Moissis and Berenson [24] developed a model based on the interaction of the continuous vapor columns with each other. The maximum heat flux is then determined by introducing Taylor-Helmholtz instability for the counterflow of vapor flow in columns and liquid flow between them as shown in Fig. 4. L V L V V Figure 4 Moissis and Berenson’s [24] model with countercurrent flow of liquid (L) between vapor columns (V) adjacent to the heated wall. Figure 3 Instability of a liquid-vapor interface over a heated surface leading to vapor jets, Zuber [19]. Zuber [19] also postulated that vapor patches form and collapse on the heater surface as CHF is approached. According to Zuber, “In collapsing, ... [as] the vapor-liquid interface of a patch approaches the heated surface, large rates of evaporation occur and the interface is pushed violently back.” Zuber considered the dynamic effects of vapor jets to be important and proposed that the Taylor and Helmholtz instabilities are responsible for the CHF condition. Figure 3 shows a heater surface covered with vapor after it has reached the critical heat flux value. The progression from (a) through (f) shows how the smooth interface experiences instability and creates vapor jets and mushrooms. In Fig. 3(c), the liquid fingers extend to the wall and are pushed back resulting in vapor jets as shown in Fig. 3(f). Using the stability criterion of a vapor sheet, Zuber obtained an equation similar to that of Kutatelazde [17], but the value of constant K ranged from 0.157 Figure 5 Macrolayer evaporation model with vapor jet instability within the macrolayer, Haramura and Katto [7]. Haramura and Katto [7] refined an earlier model proposed by Katto and Yokoya [22] and proposed that the heat transfer is related to formation and evaporation of a macrolayer under a hovering vapor bubble as shown in Fig. 5. An expression for the hovering period was obtained using the force balance between the buoyancy and inertia forces. Complete evaporation of liquid in the macrolayer within the hovering time leads to the critical heat flux condition. However, the thickness of the macrolayer is unknown in this formulation. Haramura and Katto considered the merger of the adjacent vapor stems to result in pushing the bulk liquid away from the hovering bubble. Combining with Zuber’s model for the critical heat flux, Haramura and Katto obtained an expression for the thickness of the macrolayer. This approach combines the hydrodynamic and thermal aspects in a single model. Sefiane et al. [23] considered the recoil forces in their analysis of the liquid-vapor interface instability in the contact line region. The rapid evaporation in this region generates a recoil force due to the unbalanced momentum caused by the velocity difference between the liquid and vapor phases at the interface. This recoil force was assumed to create an instability condition at the interface as shown in Fig. 6. A set of equations was developed for predicting the interface motion and the accompanying heat flux. Vapor Liquid Stable evaporating interface Perturbed evaporating interface θ Figure 6 A liquid-vapor interface near the contact line region perturbed by the recoil forces, Sefiane et al. [23]. The models presented in this section are derived mainly on the basis of hydrodynamic considerations. One of the major drawbacks of these models is that the contact angle is not considered to be a relevant parameter. However, it is well known that the contact angle has a major influence on the CHF. Gaertner’s [25] experiments clearly showed that the CHF value was dramatically reduced for a surface coated with a nonwetting plastic (very high contact angle). The bubbles coalesced and formed a vapor film underneath the liquid. Liaw and Dhir [26] observed a similar trend over a contact angle range of 0 to 108° for water boiling on a vertical plate. Although Sefine et al.’s [23] analysis considers the contact angle effect, the instability of the interface alone does not seem to be a sufficient condition for CHF condition. Drying and rewetting with considerable instability are constantly occurring during the boiling process. A review of CHF models based on thermal considerations is presented in the next section. 4. CHF MODELS CONSIDERATIONS BASED ON THERMAL The CHF models based on hydrodynamic considerations presented in Section 3 consider the instability of the liquidvapor interface that somehow prevents the liquid from reaching the heater surface. In Zuber’s model, he considers the CHF from the transition boiling side. A vapor film covers the heated surface, and the instability of the interface causes liquid wavelets to approach the heater surface. The rapid evaporation of these wavelets causes the vapor to get violently pushed back in the form of uniformly spaced jets. Although this describes the transition boiling phenomenon, the initiation of the critical heat flux condition is not entirely clear. Haramura and Katto provide an explanation of how this condition is arrived at by proposing the macrolayer evaporation theory. They postulate that the CHF is reached when the macrolayer evaporates completely. Using the Zuber’s correlation, they derived an expression for the initial macrolayer thickness. The hovering bubble in Haramura and Katto’s model represents the vapor patch in the Zuber’s model. Haramura and Katto’s model thus provides an explanation for the formation of the macrolayer (and vapor patch in Zuber’s model), but it does not provide an answer to the mechanism leading to the CHF condition. The questions not addressed in the hydrodynamic models are the liquid supply to the wall and the expansion of the hovering vapor bubble. Further, the effect of contact angle on CHF is not included. As seen from Gaertner’s [25] photographic study and Liaw and Dhir’s [26] experimental investigation, CHF reduces significantly as the contact angle increases. This leads us to believe that the CHF is more intimately linked to the liquidvapor-solid interactions at the wall, where thermal effects are important and need to be included in the modeling of CHF. Sefiane et al. [23] present a model that addresses the contact line region and the effect of contact angle to some extent. This model includes the recoil forces that arise due to the momentum change as liquid at the interface in the contact line region evaporates into vapor. The higher vapor velocity results in an unbalanced momentum, which provides a recoil force at the interface. However, the instability of this interface initiated by the recoil forces is assumed to lead to CHF. The exact sequence of events that lead to the CHF condition following the instability condition still remains unanswered. 5. A NEW MODEL BASED ON CONTACT LINE MOVEMENT CRITERION Critical heat flux phenomenon occurs at the heater surface. It occurs at speeds that are in the microseconds range as we see the bubble growth and collapse information generated through studies on the inkejet printers. With this is mind, although the hydrodynamic conditions occurring at the interface of the hovering bubble provide answers to the occasional contacts of liquid with the heater surface, they are not able to describe the mechanism responsible for the critical heat flux condition. The motion of the interface at the contact line holds the key to whether the vapor will push the liquid away, or the liquid front will rewet the heater and release the bubble that is covering the heater surface. A new model presented in the following sections to include the effects of contact angle and the recoil forces in determining the movement of the interface at the contact line. 5.1 Macrolayer Evaporation The agglomeration of vapor in the vicinity of the wall restricts the flow of liquid toward the heater surface. The macrolayer under the bubble continues to evaporate. As seen from Fig. 2, the number of active sites is very large at high wall temperatures encountered near CHF. These sites are present in the thin film as well. Presence of a thin film above nucleating cavities would cause the bubbles to open up at the interface thereby increasing the bubble frequency. Heat conduction through the macrolayer is also quite efficient as the interface of the film is at saturation temperature due to evaporation of liquid. However, the limited availability of the liquid in the macrolayer leads to its eventual dryout. Figure 7 shows a high speed photograph of a liquid droplet boiling on a glass plate. The picture is taken with a high speed video camera looking from the underside of the glass plate. The initial glass temperature is 170°C. A bubble is formed in the liquid as identified by the four bright spots in Fig. 7(a). In the next frame, 1 ms later, a dryout front appears in the center of the bubble. The dryout front continues to expand in frame 7(c) where it almost reaches the left edge of the bubble base. In subsequent frames, the dryout front stops expanding and is seen to recede causing the rewetting of the base. This happens because the glass temperature has dropped considerably from its initial high temperature. In this case, the dryout front could not push the liquid-vapor interface further under the liquid because of the lower surface temperature. 5.2 Model Description Kandlikar [4] presented a model that incorporates the effects of contact angle and recoil forces. Figure 8 shows a force balance on a vapor bubble growing on a horizontal heated surface. The interface is considered to be cylindrical in shape with a unit length. A force balance is performed on the left half of the cylinder. The surface tension forces act at the top of the bubble, parallel to the heated wall, Fs,2, and at the heater surface, along the interface in the contact line region, at an angle equal to the contact angle θ. Fs,1 represents the component of the surface tension force along the heater surface. The hydrostatic pressure difference between the top and the bottom of the bubble introduces a force due to gravity. The inertia forces are neglected. A n additional force, FM, due to recoil is introduced to represent the recoil force due to change in momentum as liquid evaporates into vapor with a higher velocity. FG FS,2 Db FM Hb θ (a) Figure 7 Development of a dryout front in the macrolayer under a bubble inside an impinging droplet, viewed from the bottom of a heated glass surface, successive frames 1 ms apart, (a) a vapor bubble identified by four bright reflections, (b) formation of a dryout front, (c) dryout front reaches the edge of the bubble, (d)-(f) liquid rewets the dry spot. Heater surface A FS,1 Figure 8 Force balance on a bubble experiencing a high evaporation rate in the vicinity of contact line region, Kandlikar [4]. The heat transfer from the bubble is considered to cover an area of twice the bubble diameter over the heater surface. Critical heat flux condition is reached when the recoil force overcomes the combined surface tension and hydrostatic forces along the heater surface. The final expression for the critical heat flux is given by: CHF, W/m2 1,000,000 Figure 9 shows a comparison of the above model with the experimental data from Lienhard and Dhir [27]. Kutateladze [27] correlation is also shown in Fig. 9 for comparison. A contact angle value of 45° is used. It can be seen that the Kandlikar model given by eq. (6) predicts the data quite well over the entire range of pressure. The effect of contact angle on CHF was studied by Liaw and Dhir for water on a vertical plate. Figure 10 shows a comparison of the Kandlikar model and Kutateladze model with their data. The Kutateladze correlation does not account for the contact angle variation. The Kandlikar model is able to correctly predict the CHF trend seen in the experimental data. Lienhard and Dhir (1973) Experimental data Kutateladze (1951) Distilled Water Present model Horizontal Plate Liquid-Vapor Interface 100,000 10 20 30 40 50 Vapor Liquid Pressure, kPa Figure 9 Variaton of CHF with pressure for distilled water boiling on a horizontal plate; comparison of present model, Kandlikar [4], (using contact angle of 45 degrees) and Kutateladze correlation with Lienhard and Dhir [27] data. Motion of the interface A (a) 10,000,000 Water Vertical Plate CHF, W/m2 Evaporating Vapor Liquid-Vapor Interface 1,000,000 Liquid Liaw and Dhir (1986) Experimental data Kutateladze (1951) Vapor Evaporating vapor pushing the interface in the contact line region Present model A 100,000 0 20 40 60 80 100 120 Contact Angle, Degrees (b) Figure 10 Effect of contact angle on CHF for water boiling on a vertical plate; comparison of present model and Kutateladze correlation with Liaw and Dhir [26] data. 1 + cos θ q′C′ = iLG ρ G1/ 2 16 2 π + (1 + cosθ ) cosφ π 4 1/ 2 [σ g( ρ L − ρ G )]1/ 4 (6) This model includes the contact angle, θ, and the orientation angle of the heater with horizontal, φ. Figure 11 A schematic representation of the changes in the interface profile due to recoil forces, Kandlikar and Steinke [28]. The interface geometry in the contact line region determines the contact angle of the receding interface. The shape of the interface in this region has been investigated by Kandlikar and Steinke [28]. They obtained high-speed videos of the side views of impinging liquid droplets on a heated surface. The liquid drop spreads and recoils following the impact. The dynamic advancing and receding contact angles were measured during the spread and the recoil phases respectively. Figure 11 shows the contact angle variations with the heater surface temperature. It is seen that the dynamic receding contact angle undergoes a step change at a surface temperature around 135150°C and attains the same value as the dynamic advancing contact angle. The change in the dynamic receding contact angle is believed to be due to recoil forces resulting from high evaporation rates in the contact line region. A schematic representation of this behavior is illustrated in Fig. 12. The contact angle increases due to recoil forces as the interface recedes into the liquid, thereby allowing the vapor to cut under the bulk of the liquid. This phenomenon has been termed as the vapor cutback phenomenon by Kandlikar and Steinke [28]. 2. 3. 4. 5. 6. CONCLUDING REMARKS 6. The hydrodynamic models of the CHF phenomenon are based on the instability of the interface as originally proposed by Kutateladze [17], and then refined by Zuber [19] and Moissis and Berenson.[21]. The formation of the macrolayer and the vapor patch implicit in the Zuber’s model was explained by the macrolayer dryout model by Haramura and Katto [7]. The major concerns in the hydrodynamic models are: (a) the contact angle effect on CHF is not considered, (b) the actual mechanisms of CHF and transition boiling are not clearly explained. As the heater surface temperature increases, many small diameter (a few micrometers) cavities are activated. As a result, the macrolayer under a vapor bubble evaporates rapidly and provides a high heat transfer coefficient. The dryout spot under a bubble has been observed by Kandlikar and Steinke [28] through the underside of a heated glass plate. The dryout front reached the edge of the bubble, but did not cause further expansion of the bubble as the heater surface temperature was not high enough. The vapor recoil forces and the associated receding contact angle at the edge of the bubble (in the contact line region) have been identified to play an important role in the motion of the interface near CHF. The contact angle is seen to change at temperatures near CHF condition. The theoretical model developed by Kandlikar [4] based on thermal considerations is able to correctly predict the effect of contact angle and orientation on CHF. Future research is needed to obtain additional data on the effect of contact angle on the CHF. 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