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Film boiling collapse in a solid hot sphere immersed in subcooled forced convection
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Film boiling collapse in a solid hot sphere immersed in subcooled forced convection ⁎
Peiwei Nia, Zhi Wena,b, Fuyong Sua,b, , Jie Huanga, Xunliang Liua,b, Guofeng Loua,b, Ruifeng Doua,b a b
School of Energy and Environmental Engineering, University of Science & Technology Beijing, Beijing 100083, China Beijing Key Laboratory of Energy Saving and Emission Reduction for Metallurgical Industry, University of Science & Technology Beijing, Beijing 100083, China
H I GH L IG H T S
film on a hot sphere immersed in forced convection is stripped away sequentially. • The dissipation at support area and inertial force of liquid affect the stripping. • Heat rupture position of the film moves to support area as flow rate increases. • Initial quantity of initial rupture points increases as subcooling degree of liquid raises. • The • The rupture speed of vapor film increases as subcooling degree of liquid raises.
A R T I C LE I N FO
A B S T R A C T
Keywords: Forced convection film boiling Visualization experiment Rupture process Flow rate of the subcooled liquid Liquid subcooling degree
An experimental study of forced convection film boiling was conducted, on a stainless steel sphere with diameter of 30 mm with different coolant velocities. A visualization research on the motion of the vapor/liquid interface and the rupture process has revealed a detailed mode for vapor film rupture, which relates to the motion of a wetting boundary on the hot surface of a sphere. The rupture sequence of the vapor film on different monitor points in the sphere was obtained by the analysis of surface temperatures and heat fluxes during the process. In addition, the influence factors of vapor film rupture were also studied. It is concluded that, in forced convection, the rupture process of vapor film on the surface of the sphere is affected by the temperature distribution of the sphere; liquid flow velocity, cooling effect of the cylindrical support pipe and liquid subcooling degree also have significant impacts on it.
1. Introduction The film boiling process occur under many circumstances, such as metal quenching, special chemical catalytic synthesis reactions and wet treatment of blast furnace slag [1]. Blast furnace slag, as the main byproduct of blast furnace ironmaking, has a huge output each year. In the process of wet treatment of blast furnace slag, the slag above 1400 °C is crushed by subcooled fluid and condensed, forming a large number of fragments, including small slag particles and large slag blocks. When high temperature slag particles contact with subcooled scouring slag water, a layer of gas film will be formed on the surface. The breakdown of gas film can not only promote the further breakage of slag particles [2], but also promote the condensation of slag particles. In the cooling process of the fragments, complex crystallizations [3] exist
⁎
at different crystallization temperatures [4]. Therefore, it is necessary to study the heat transfer phenomena in the process of wet slag treatment in order to improve the added value of slag particles. Slag blocks can be regarded as solid metal spheres, and the boiling vapor film rupture during water cooling of slag fragments can be regarded as the film rupture process on the surface of hot spheres. As early as 1756, Leidenfrost firstly observed the film boiling phenomenon of the subcooled liquid after contacting with the hot wall. At the beginning of the contact, as the surface temperature of the wall is much higher than the liquid temperature, many bubbles are generated on the wall and join together that a layer of vapor is formed and coat the surface of the wall. This vapor film decreases the temperature drop rate of the wall surface. A large number of researchers have studied the film boiling
Corresponding author at: School of Energy and Environmental Engineering, University of Science & Technology Beijing, Beijing 100083, China. E-mail address: [email protected] (F. Su).
https://doi.org/10.1016/j.applthermaleng.2019.114630 Received 6 June 2019; Received in revised form 28 October 2019; Accepted 2 November 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Peiwei Ni, et al., Applied Thermal Engineering, https://doi.org/10.1016/j.applthermaleng.2019.114630
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Nomenclature
R Ra Re Tsat Tsphere Tsup
Rh λ
Bi
Biot number,
cp D h m Nu Pr q
specific heat capacity of stainless steel, kJ/(kg·° C) diameter of the sphere, m heat transfer coefficient, kW/(m2·° C) mass of the sphere, kg Nusselt number Prandtl Number heat flux, kW/m2
radius of the sphere, m roughness average parameter, μm Reynolds number saturation temperature of the liquid, °C temperature of the sphere, °C subcooling temperature of spheres, °C
Greeks λ
thermal conductivity, kW/(m·° C)
over the surface of the hot sphere immersed in the subcooled liquid by using the experimental method. Jouhara and Axcell [17] et al. proposed a new model of vapor film rupture by experimental study of transient film boiling. The formation of vapor film after solid-liquid contact and the movement of rewet front on the surface of the hot sphere were recorded. Gylys [18] used the experimental method to put copper, stainless steel and aluminum spheres with a certain degree of superheat into cold water to study the influence of their thermal conductivity, heat capacity, density and liquid subcooled degree on the change of the surface temperature. It is concluded that the minimum temperature on the surface of the sphere that keeps the vapor film steady depends on the heat capacity of the material, the thermal conductivity and the subcooled degree of the liquid. Through the analysis of existing researches, there are still some noteworthy problems in the experiment:
phenomenon of different heated objects immersed in subcooled liquids. Nusselt first established a theoretical model of film condensation heat transfer on a vertical wall [5], Bromley [6] et al. studied the strong convection film boiling process of vertical surface and horizontal cylinder on the basis of Nusselt. Stevens [7] and Walford [8] et al. carried out an experimental study on the transition process from film boiling to nuclear boiling when high-temperature copper spheres moving rapidly in water. The characteristics of the vapor film collapse and the process of rewetting were revealed. Dhir [9] studied the quenching process of spheres with different materials in natural and forced flow experimentally and theoretically. They found that sphere size plays an important role in causing premature partial contact which makes the heat transfer coefficient increase and may result in a pulsating film in subcooling film boiling. Dreitser [10] presented experimental investigations of convective heat transfer enhancement for cryogenic liquid flow in channels and introduced problems of heat transfer enhancement in channels for film boiling. Ebrahim [11] studied the influences of liquid subcooling, surface conditions and material properties on the film pool boiling heat transfer using metal rods with vertical quenching experiments. The study on film boiling process of subcooled liquids around heated spheres mainly include three aspects: (1) pool film boiling; (2) natural convection film boiling; (3) forced convection film boiling. Research methods are usually by combining relevant experimental conclusions for empirical equations of heat transfer coefficient and other parameters correlated with mass and heat transfer. Motte and Bromley [12] analyzed the process of forced convection film boiling theoretically, they proposed a general expression of convective heat transfer coefficient for high temperature solid wall immersed in subcooled liquids. Wilson [13] analyzed forced convection film boiling around the surface of a hot sphere theoretically, and ordinary differential equations describing the film thickness were obtained. Epstein and Hauser [14] studied the film boiling process over the stagnation zone of the surface of the sphere in forced convection, and got the detailed expression of vapor film thickness and the area and empirical correlations between Re, Pr and Nu. Liu [1], etc. did a lot of experimental researches about film boiling processes of spheres with various sizes, temperatures, materials immersed in liquids with different velocities, phases and subcooled temperature. Empirical equations of heat transfer coefficient were proposed by comparing the analysis of those experiments. With the development of the high-speed video technology, many researchers adopt visualization experiments to study the film boiling process of subcooled liquids around the surface of a sphere in detail in recent years. The research directions include the formation and crushing processes of the vapor film, the minimum maintaining surface temperature of the vapor film, other influencing factors, the effect of surface roughness on the rupture process, etc. Sher [15] et al. studied the film boiling collapse in solid spheres immersed in a subcooled liquid and found that the surface of the sphere would experience a boiling process similar to that of a “golf ball”. In this case, the rewetting process is an explosive event occurred in the whole surface. Freud [16] et al. identified the minimum temperature needed to maintain the vapor film
(1) From the perspective of the distribution of the vapor film over the surface of the sphere, Walfort [8], Stevens and Witte [7], Dix and Orozco [20] et al. studied the portion where the vapor film was produced on the sphere. The following laws can be driven from their research conclusions: all of the conclusions on the shape of vapor film at the front (inflow) part of the sphere are that the front of the sphere is covered by an unbroken and undulating vapor film. However, there is no accepted conclusion about the shape of the rear (outflow) vapor film. There are many reasons for this situation. First, the formation and maintenance of the vapor film at the rear part depend not only on the temperature of the rear wall surface of the sphere, but also on the flow state of the liquid. Because the liquid around the sphere will appear stratification and eddy phenomenon at the rear part [21]. Secondly, the formation of the rear vapor film is also significantly affected by the support mode of the sphere. The heat loss through the support tube on the upper part of a sphere was evaluated by Yoon [22] et al. The results show that the heat loss through the support tube is non-negligible. This support mode has been widely used in many researches. (2) According to the Leidenfrost phenomenon, the generation of boiling vapor film will hinder heat transfer from solid to liquid, so that the convection heat transfer coefficient (HTC) on the solid surface is low in the process of film boiling. The HTC between solid and fluid would increase sharply with the rupture of vapor film. The rupture of the boiling vapor film is very important for the boiling process. Most of the researches have focused on the rupture process of boiling vapor film formed by static subcooled liquid or natural convection subcooled liquid [7] around a hot sphere. Based on experimental results, Sher [15] et al. obtained the rupture process of boiling vapor film over a high-temperature sphere with uniform material and smooth surface immersed in static subcooled liquid. The phenomenon of “golf ball” on the surface of the sphere indicates that the rupture of the vapor film in static subcooled liquid occurs simultaneously on the whole surface of the sphere. However, few studies have focused on the rupture process of boiling vapor film formed by forced convection subcooled liquid around a hot sphere. Although Liu et al. studied the film boiling of a high2
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temperature sphere in flowing liquid, more detailed analysis is still necessary for the process of film breaking in forced convection subcooled liquid. (3) After the high-temperature sphere contacts the subcooled liquid, a large number of bubbles will generate on the contact surface, the departure diameter of the bubble and the frequency of bubble generation are related to the size of the active nucleation site on the surface of the sphere. The roughness of the surface of the sphere determines the size and density of active nucleation sites [23]. After the 1950s, people began to pay attention to the influence of rough surface on boiling heat transfer process. For example, the bubble motion between parallel plates with rough surface was studied by Bankoff [23] et al. The investigation showed the influence of liquid density, surface tension, contact angle and rough surface structure on bubble motions. Mchale [24] et al. conducted a detailed study on the boiling nucleation center. They measured bubble departure diameters, growth periods, ebullition periods, and void fraction above the surface are obtained from high-speed graphic visualizations. They also compared correlations of heat transfer coefficient and bubble ebullition characteristics with different measures of surface roughness. Surface roughness is also important for the rupture process of the vapor film. The heat transfer resistance between solids and fluids decreases sharply during the rupture of the vapor film, and the location with high roughness is often the initial position of the vapor film rupture process. The mean roughness of the solid surface determines the duration of film boiling directly. Bogacz [25] investigated the effect of the stainless steel surface roughness on incrustation of MgSO4·7H2O from aqueous solutions and heat transfer resistance. (4) Various ways have been adopted by researchers to dip the sphere into the subcooled liquid. Aziz and Hewitt [19] dropped the sphere free in still water to observe the film boiling process. Liu [1] dipped the sphere into water and heated it to a specific temperature in a steady-state flow. Jouhara [17] et al. fixed the heated sphere by a tube hanging in an empty chamber and studied the film boiling process of the subcooled liquid around the sphere by filling the chamber with liquid and lifting the liquid level rapidly above it. Yoon [22] et al. supported the sphere with a hollow tube connect to the top of it, and fall the sphere to the designated position below the liquid level to study the heat transfer process of static film boiling. The dip method has an impact on flow regime of the liquid around the sphere when first entering the liquid. It also influences the initial subcooled temperature of the surface of the sphere, especially when the subcooling degree is high.
In the present study, a visual experiment was designed for the research of transient rupture process during the forced convection film boiling around a hot sphere immersed in the subcooled liquid. The rupture sequence of the vapor film was recorded by a high-speed camera and analyzed through surface temperatures and heat fluxes during the process. The factors that influence the rupture process of the vapor film, such as liquid flow velocity, cooling effect of the cylindrical support pipe and liquid subcooling degree, were also studied. 2. Experiment The specimens used in this experiment are all stainless steel solid spheres with 30 mm diameter. The distance between the thermocouple and the inner contact surface of the sphere is 1 mm from the sphere surface. The subcooling degrees were 35.6 °C, 42.3 °C, 54.9 °C and 63.9 °C, the flow speeds of the liquid were set to be 0.035 m/s , 0.056 m/s and 0.098 m/s . The schematic of the experimental device is shown in Fig. 1. The subcooled liquid is pumped out of the cistern by a suction pump and flow into the water channel through the pressure maintenance pump, ball valve and electromagnetic flowmeter. The flow velocity of the liquid can be adjusted by the ball valve after the pressure maintenance pump. The liquid level in the water channel is adjusted by another ball valve connecting the water outlet of the channel. Since the diameters of inlet and outlet of the water channel are smaller than the truncation size of the middle section of the water channel, two expansion areas are set near the inlet and outlet. A control device for the experimental sphere is arranged above the water channel, which enables the sphere to reach the same position at the same dropping speed and remain stationary in repeated experiments. The internal structure of the sphere is shown in Fig. 1(c). There were three holes which extend to different positions on the inner surface of the sphere. Three K-type thermocouples contacting the inner surfaces of the sphere through the holes respectively (0–400 °C, the measuring deviation is 1.6%; 400–1000 °C, the measuring deviation is 0.4%) were used to monitor the temperatures of front, bottom and rear parts of the sphere during the test. As shown in Fig. 1(c), the corresponding numbers of the three thermocouples were respectively: 1-rear part, 2bottom, 3-front part. The thermocouples converged and connected with the data-acquisition device through a stainless steel tube (the outer diameter of the tube is 5 mm) supporting the sphere on the upper part. The data acquisition frequency was 40 Hz. The small hole inserted into the thermocouple was sealed with high-temperature resistant adhesive, so as to prevent the liquid from infiltrating into causing errors in the experiment.
Fig. 1. Schematic of the experimental system. 3
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Experiment process is as follows: The sphere with its support tube was heated in a furnace to over 800 °C. Then the support tube was fixed in the control device. When the temperature of the sphere dropped to 800 °C, the motion mechanism on the control device was started to immerse the sphere in the liquid with a constant flow rate. The sphere was remained stationary till the end of boiling process. The high temperature which the sphere is heated to may result in a layer of oxides generate on its surface. Based on the conclusions of Sher [15] et al., the effect of this oxide layer can be ignored on the heat transfer process of the sphere. However, the oxide layers will affect the roughness of the surface of spheres. Therefore, in each experiment, the oxide layer was removed before the sphere was heated by the furnace. A 1500 objective sandpaper was used to remove the oxide layer, so that the initial roughness of the surface of the sphere was always maintained at about Ra0.15[26]. 3. Results and discussion According to the previous experimental studies in static film boiling, it can be found that the vapor film rupture is caused by the decrease of surface temperature of the sphere. The process of liquid evaporation is slower than that of vapor liquefaction on the vapor-liquid interface of the vapor film, so that the thickness of the vapor film is reducing until the liquid in contact with the sphere directly. In static boing film rupture, this process occurs simultaneously at various points on the surface of the sphere. It can be regarded that the whole vapor film covered the sphere burst at the same time. However, when the rupture process starts in forced convection film boiling, the temperature of the sphere is higher than the minimum heat flux temperature in static film boiling. The rupture process of forced convection vapor film recorded by the high-speed camera is shown in Fig. 2. The initial temperature of the sphere was 800 °C, and the velocity of the liquid with subcooling degree of 35.6 °C was 0.035 m/s . When the liquid flow rate is 0.035 m/s , the boiling vapor film strips away from the surface of the sphere. During the 8 s after rupture started, image results were captured every 1 s. The subcooled liquid flowed in from the left side of the sphere, and then flowed out through the right side. It can be seen from Fig. 2 that, different from the overall rupture mode in static film boiling, the rupture process of forced convection vapor film starts from the front stagnation point of the sphere, and a circle of obvious wetting boundary appears in the vapor film on the surface of the sphere. The rupture of the vapor film produces a large number of tiny bubbles on the wetting boundary. Inside the wetting boundary, the surface of the sphere is directly in contact with the subcooled liquid, and the outside part of the sphere surface is still covered by a stable vapor film. As the crushing process progresses, the
Fig. 3. (a) Stainless steel sphere temperature vs. time for different parts on the sphere. (b) Stainless steel sphere heat flux vs. temperature for different parts on the sphere. The flow rate is 0.035 m/s.
Fig. 2. The rupture process of forced convection vapor film, the liquid flow rate is 0.035 m/s . 4
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positions along the direction of liquid flow on the surface of the sphere. It can be concluded that the rupture sequence of the vapor film is consistent with the flow direction of the liquid along the sphere. Fig. 3(b) shows that after immersed in the subcooled liquid, the higher temperature of the sphere causes evaporation of the surrounding liquid. A stable vapor film was formed over the surface of the sphere. And so the heat flux through the spherical wall is decreased. As can be seen from Fig. 3(a), the vapor film remains stable over the surface of the sphere for a long time. Due to the low heat flux, the cooling range of the sphere in this process is small. After the vapor film rupture, with the disturbing effect of a large number of tiny bubbles caused by it, the heat transfer process between the sphere and the liquid is greatly promoted, and the heat flux through the surface of the sphere increases rapidly. The variation trend of heat flow density with temperature in the boiling process shown in Fig. 3(b) is consistent with the experimental conclusions from Liu [1], Sher [15], Yagov [27] and other researchers. As can be seen from Fig. 2 (1), when the critical point of vapor film rupture is reached, the vapor film covering the inlet flow surface of the sphere (3-front part) is the thinnest, followed by the vapor film covering the back flow surface (1-rear part), and the film on the bottom is the thickest (2-bottom). It can also be concluded from Fig. 3(a) and Fig. 3(b) that in the process of film rupture, the inlet surface of the sphere has the lowest temperature and the highest heat flux and the back flow surface has the highest temperature and the lowest heat flux. Because the fluid inertia force around the front stagnation point of the
wetting boundary expands along the flow direction of the fluid on the surface of the sphere until the boiling vapor film is completely detached from the surface of the sphere. When a stable vapor film is formed over the sphere in our experiment, the HTC value is lower than 250 W/(m2·° C) , the Biot number approximate lower than 0.1, therefore the lumped capacitance method can be applied for heat transfer calculation. The specific solution equation is shown as follows:
q=
h=
msphere cp dTsphere 2 πDsphere dt
q q = ΔTsup Tsphere − Tsat
(1)
(2)
The surface temperature curves of different parts on the sphere immersed in subcooled liquid is shown in Fig. 3(a). The corresponding images of Fig. 2 are also plotted near the approximate point. Since the support pipe was located on the upper part of the sphere, no temperature measuring point was arranged on the top inner surface of the sphere. It can be seen from Fig. 3(a) that the temperature variation regularities are similar to those drawn by Yagov [27]. The differences lie in that the distribution of temperature measuring points of Yagov’s specimens describes the variation of temperatures in different positions along the direction of diameter inside the sphere, while those in this experiment describes the variation of temperatures in different
Fig. 4. The rupture process of forced convection vapor film, (a) the liquid flow rate is 0.056 m/s. (b) The liquid flow rate is 0.098 m/s. 5
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but did not affect the process of vapor film rupture.
sphere is the greatest, therefore the vapor film on the flow surface is the thinnest. It is also the reason why the process of film rupture near the surface of the sphere starts at the front stagnation point in forced convection. The generation of wetting boundary and the stripping rupture mode of vapor film are caused by the stability of the vapor film due to the high surface temperature in the area still covered by the vapor film on the sphere. The inertial force of the liquid is the driving force of the film splitting rupture. The temperature of the wetted spherical surface decreases rapidly due to direct contact with the subcooled liquid. It can be seen that the decrease of temperature on the surface of the sphere is not the main factor affecting the rupture of the vapor film in forced convection. The flow around the sphere is closely related to the process of vapor film rupture. The existence of the wetting boundary on the sphere surface changes the shape of the vapor film, but at the same time ensures the integrity and leakproofness of the film. The motion of the wetting boundary is the main reason for the sequence of vapor film rupture on the surface of the sphere. Based on Yoon's research conclusion, it can be seen that the support tube connected with the sphere facilitates the heat loss of the sphere at the connecting area, and the promotion cannot be ignored. It can also be seen from Fig. 2 that during the film boiling process, the vapor film next to the support tube on the top of the sphere was acting violently,
4. Analysis of influencing factors for steam film rupture The vapor film rupture in static film boiling is caused by the decrease of the surface temperature. In the case of forced convection, the main influencing factors of the rupture process of vapor film formed by forced convection subcooled liquid around a hot sphere are need to get further studies. 4.1. Liquid flow rates As shown in Fig. 4, the image results are captured every 1 s after 8 s from the beginning of the rupture of the vapor film on the sphere surface at the liquid flow rate of 0.056 m/s and 0.098 m/s. The subcooled liquid flows in from the left side of the sphere, and then flows out through the right side. As can be seen from Fig. 4(a), when the flow rate of the subcooled liquid increased to 0.056 m/s, the rupture mode of the vapor film is similar to that at low flow rate. However, the starting position of the rupture process changes from the front stagnation point to the position next to the junction of the support tube on the inlet flow surface of the sphere.
Fig. 5. Stainless steel sphere temperature vs. time for different parts on the sphere and stainless steel sphere heat flux vs. temperature for different parts on the sphere. The diameter of the sphere is 30 mm, the subcooling degree of the liquid is 35 °C. (a) The flow rate is 0.056 m/s. (b) The flow rate is 0.098 m/s. 6
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Fig. 4(b) presents that when the flow rate is 0.098 m/s, the heat dissipation of the supporting rod is strengthened with the increase of the flow rate, there are two initial positions of the vapor film rupture. The position next to the junction of the support tube on the sphere is still the initial. The other is the front stagnation point. The phenomena at the vapor-liquid interface at the rupture position is similar to microbubble Emission Boiling (MEB), which promotes the heat transfer process. As shown in Fig. 4(b) (1), the rupture at the end of the period at the front lag point occurs between Fig. 4 (b) (2) and Fig. 4 (b) (3). The processes of vapor film rupture motions initiated by two rupture positions merge at the front stagnation point and interferes with each other. In addition, the change of the starting position of the rupture also changes the sequence of film rupture process. As can be seen from Fig. 5(a), the wetting boundary first passes through the front part, then through the rear part, and finally through the bottom. The image of heat flux vs. temperature shows that the change of the sequence also affects the heat flux at temperature measuring points. The reason for the above change lies in the existence of the support tube and the enhancement of the cooling process of the sphere with the increase of the flow rate. As mentioned before, after the sphere is immersed in the forced convection subcooled liquid, the vapor film at the front stagnation point on the flow surface becomes thinner due to the action of the fluid inertia force. At the same time, the temperature of the sphere near the junction is reduced due to the cooling effect of the support tube. When the flow velocity is low, the liquid inertial force is small, so is the total heat lost by the support tube. The cooling effect of the support tube does not affect the vapor film rupture before the burst of the film on the front stagnation point. Therefore, the sphere temperature can still maintain the stability of the vapor film near the support tube after the rupture occurs on the front stagnation point. As the flow velocity increases, the heat carried away by the liquid through the support tube increases. The cooling effect of the support tube on the top of the sphere is enhanced. Although the liquid inertia force acting on the front stagnation point also increases, but the cooling effect of the support tube is still stronger than the influence of fluid inertia force, therefore, the rupture of the vapor film on the position next to the junction of the support tube on the inlet flow surface of the sphere starts earlier than that on the front stagnation point. Since the vapor film at the front stagnation point also ruptures under the action of the liquid inertia force, the motion process of the wetting boundary changes again as shown in Fig. 5(b), leading to the change of the rupture sequence of the vapor film. The movement process of the wetting boundary first passes through the front part of the sphere, then the bottom, and finally the rear part. It can be seen that when the vapor film begins to rupture, the cooling effect of the support tube on the vapor film at the connection area on the sphere is still stronger than the effect of the liquid inertia force even though the raise of the flow rate increases the liquid inertia force. The vapor film at the front stagnation point has been crushed before the wetting boundary reaches. Since the front stagnation point of the flow surface is affected by the liquid inertial force, the liquid velocity has the most significant effect on the film rupture on this point. Fig. 6 shows the change of heat flux on the front part of the sphere during the cooling process at different flow rates. As can be seen from Fig. 6, the compression speed of the vapor film is increased with increasing the velocity of the subcooled liquid. Therefore, the rupture time is also brought forward (The initial temperature of the sphere are all 800 °C). However, the film rupture at the bottom and rear part of the sphere is significantly affected by the motion development of the wetting boundary. The cooling effect of the support tube also affects the expansion mode of the wetting boundary. When the flow rate is 0.098 m/s, the variation of heat flux vs. temperature is special. The heat flux does not increase sharply with the rupture of the vapor film, but appears a slow rising process. This case
was not proved in this experiment. It can be inferred that it was caused by the interaction between the fluid inertia force and the interference caused by the merging of two rupture motions at the front stagnation point. In addition, the rupture position next to the junction of the support tube appears earlier, as shown in Fig. 4(b) (2). The temperature on the surface of the sphere decreases rapidly after the film breakdown. Because of the thermal conductivity inside the solid sphere, the temperature on the sphere surface which is still covered by the vapor film at the front stagnation point would also decrease, and the maximum heat flux value during the film breakdown would also be affected. 4.2. Water subcoolings Fig. 7(a) shows the rupture process of forced convection vapor film when the liquid flow rate is 0.035 m/s and the subcooling degree is 42.3 °C. Under this subcooling degree, the motion of the wetting boundary when the vapor film is rupturing is similar to that when the liquid subcooling degree is 35.6 °C. The rupture process of the vapor film develops to the bottom of the sphere between 4 and 5 s, and the rear part between 7 and 8 s. The initial rupture position of the vapor film layer is also consistent with the 35.6 °C subcooling degree. Due to the liquid high subcooling degree, the amount of bubbles generated by the surface area contacting with the liquid directly during the rupture process is less, the volume of the bubble at the vapor-liquid interface is smaller, and the vapor-liquid interface is clearer. When the liquid flow rate is constant and the subcooling degree is raised to 54.9 °C, the resulting images is shown in Fig. 7(b). The local temperature difference of the sphere is increased during the process of hot sphere immersing in the liquid. The influence of the flow velocity during the rupture process of the vapor film is relatively weakened. The initial rupture position is located near the position where the sphere first contacts the liquid. Between 2 and 3 s after the rupture process occurs, the vapor film on the front stagnation point of the sphere start rupturing. The bubble volume on the vapor-liquid interface is further reduced and the interface is clearer. The rate of the film rupture increases, and the maintaining time of the film is shortened. Fig. 7(c) shows the image of the film collapse at the sphere surface when the liquid flow rate is 0.035 m/s and the degree of subcooling is 63.9 °C. Due to the increase of the liquid subcooling degree, the boiling vapor film on the surface of the sphere fluctuates sharply during the maintenance process of the vapor film. Several fracture points in the vapor film appear during the rupture process. As shown in Fig. 7(c) (2), the initial contact area of the sphere with the liquid and the front stagnation point are still the earlier positions where the film rupture
Fig. 6. Stainless steel sphere heat flux vs. temperature for the front part on the sphere under the condition of different flow rates. 7
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Fig. 7. The rupture process of forced convection vapor film. (a) The subcooling is 42.3 °C. (b) The subcooling is 54.9 °C. (c) The subcooling is 63.9 °C.
Under this degree of subcooling, the rupture rate of the vapor film is the fastest. Fig. 8 shows the changes in heat flux with temperature for the bottom and backflow surfaces of the sphere under different liquid
occurs. It can be inferred that the velocity of the fluid, the liquid subcooling degree and the immersing method of the sphere are still the main factors affecting the rupture process of the vapor film, even though the fluctuation of the film also affects the thickness of the film. 8
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5. Conclusions In this work, the phenomenon of film boiling process and stripping processed of hot sphere immersed in subcooled liquid with forced convection are studied by visualization experiments. By the analysis of sphere surface temperatures and heat fluxes during the film boiling and rupture process, the rupture sequence of the vapor film on different monitor points of the sphere during the stripping process is studied. The influence factors, such as cooling effect of the cylindrical support pipe, liquid flow velocity and liquid subcooling degree, which relate to the stability maintenance of the vapor film are analyzed. The flow speeds of the liquid were 0.035 m/s , 0.056 m/s and 0.098 m/s and the subcooling degrees were 35.6 °C, 42.3 °C, 54.9 °C and 63.9 °C. Under different flow rates of the liquid, the heat dissipation at the support point of the sphere and the inertial force of the liquid flow have a significant effect on the stripping and rupturing process of the vapor film on the surface of the sphere. The initial rupture position of the film transfers to the support point as the flow rate increases. The motion of the wetting boundary changes correspondingly. When the liquid flow rate is constant, the initial rapture temperature of the vapor film raises, the rupture speed of the vapor film increases, the maximum heat flux on the surface of the sphere and its corresponding temperature also increase by increasing the subcooling degree of liquid. The rupture process of the vapor film on the surface of the sphere develops from a single-point rupture process at the front stagnation point to a two-point rupture process at the front stagnation point and initial contact area of the sphere with the liquid, and finally becomes a multi-point one. Due to the complexity of the vapor film rupture process, merely changing the degree of subcooling of the liquid does not completely control other factors that affect the rupture process of the vapor film on the surface of the sphere. To sum up, in forced convection, the rupture process of vapor film on the surface of the sphere is affected by the temperature of the sphere; liquid flow velocity, cooling effect of the cylindrical support pipe, liquid subcooling degree and the immersing method of the sphere into the liquid also have significant impacts on it. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Fig. 8. Stainless steel sphere heat flux vs. temperature for the bottom (a) and the rear (b) parts on the sphere in different liquid subcooling degrees. The liquid flow rate is 0.035 m/s.
Acknowledgements
subcooling. It can be seen that, the corresponding temperatures of rupture starting point increase gradually as the liquid subcooling degree increases. When the subcooling degree is higher than 54.9 °C, the duration of the vapor film on the sphere is shorter. The hindrance of the vapor film to the heat exchange process is reduced and the film fluctuation is severe before the rupture process started. At the same time, the maximum heat flux on the surface of the sphere and its corresponding temperature also increase as the liquid subcooling degree raises. Under the condition that the liquid flow rate remains the same and the sphere falls in the same way, only changing the subcooling degree of the liquid cannot completely control other factors affecting the rupture process of the vapor film on the surface of the sphere. With the increase of the subcooling degree, the rupture process of the vapor film on the surface of the sphere develops from a single-point rupture process at the front stagnation point to a two-point rupture process at the front stagnation point and initial contact area of the sphere with the liquid, and finally becomes a multi-point one.
This work was supported by the National Key R&D Program of China [No. 2016YFC0401201] and National Key R&D Program of China [2018YFB0605900]. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114630. References [1] C. Liu, T.G. Theofanous, Film boiling on spheres in single- and two-phase flows, J. Office Sci. Tech. Inform. Tech. Rep. (2000), https://doi.org/10.2172/764210. [2] B. Kim, M.L. Corradini, Modeling of small-scale single droplet fuel/coolant interactions, Nucl. Sci. Eng. 98 (1988) 16–28 https://doi.org/10.13182/NSE88-A23522. [3] C. Fredericci, E.D. Zanotto, E.C. Ziemath, Crystallization mechanism and properties of a blast furnace slag glass, J. Non-Cryst. Solids 273 (2000) 64–75, https://doi.org/ 10.1016/S0022-3093(00)00145-9. [4] A.A. Francis, Conversion of blast furnace slag into new glass-ceramic material, J. Eur. Ceram. Soc. 24 (2004) 2819–2824, https://doi.org/10.1016/j.jeurceramsoc. 2003.08.019.
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[5] W. Nusselt, The surface condensation of water vapour, J. VDI Z. 60 (1916) 541–546. [6] L.R.A. Bromley, N.R. Leroy, J.A. Robbers, Heat transfer in forced convection film boiling, J. Ind. Eng. Chem. 45 (1953) 2639–2646. [7] J.W. Stevens, L.C. Witte, J.W. Stevens, et al., Destabilization of vapor film boiling around spheres, J. Int. J. Heat Mass Transf. 16 (1973) 669–670, https://doi.org/10. 1016/0017-9310(73)90231-7. [8] F.J. Walford, Transient heat transfer from a hot nickel sphere moving through water, J. Int. J. Heat Mass Transf. 12 (1969) 1621–1625, https://doi.org/10.1016/ 0017-9310(69)90096-9. [9] V.K. Dhir, G.P. Purohit, Subcooled film-boiling heat transfer from spheres, J. Nucl. Eng. Des. 47 (1978) 49–66, https://doi.org/10.1016/0029-5493(78)90004-3. [10] G.A. Dreitser, Heat transfer enhancement in channels for film boiling of cryogenic liquids, J. Appl. Therm. Eng. 25 (2005) 2512–2521, https://doi.org/10.1016/j. applthermaleng.2004.11.035. [11] S.A. Ebrahim, C. Shi, F.B. Cheung, et al., Parametric investigation of film boiling heat transfer on the quenching of vertical rods in water pool, J. Appl. Therm. Eng. 140 (2018) 139–146, https://doi.org/10.1016/j.applthermaleng.2018.05.021. [12] E.I. Motte, L.A. Bromley, Film boiling of flowing subcooled liquids, J. Ind. Eng. Chem. 49 (1954) 1921–1928, https://doi.org/10.1021/ie50575a043. [13] S.D.R. Wilson, Steady and transient film boiling on a sphere in forced convection, J. Int. J. Heat Mass Transf. 22 (1979) 207–218, https://doi.org/10.1016/00179310(79)90144-3. [14] M. Epstein, G.M. Hauser, Subcooled forced-convection film boiling in the forward stagnation region of a sphere or cylinder, J. Int. J. Heat Mass Transfer 23 (1980) 179–189, https://doi.org/10.1016/0017-9310(80)90195-7. [15] I. Sher, R. Harari, R. Reshef, et al., Film boiling collapse in solid spheres immersed in a sub-cooled liquid, J. Appl. Therm. Eng. 36 (2012) 219–226, https://doi.org/10. 1016/j.applthermaleng.2011.11.018. [16] R. Freud, R. Harari, E. Sher, Collapsing criteria for vapor film around solid spheres as a fundamental stage leading to vapor explosion, J. Nucl. Eng. Des. 239 (2009) 722–727, https://doi.org/10.1016/j.nucengdes.2008.11.021. [17] H. Jouhara, B.P. Axcell, Film boiling heat transfer and vapour film collapse on
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spheres, cylinders and plane surfaces, J. Nucl. Eng. Des. 239 (2009) 1885–1900, https://doi.org/10.1016/j.nucengdes.2009.04.008. J. Gylys, R. Skvorcinskiene, L. Paukstaitis, et al., Film boiling influence on the spherical body’s cooling in sub-cooled water, J. Int. J. Heat Mass Transf. 95 (2016) 709–719, https://doi.org/10.1016/j.ijheatmasstransfer.2015.12.051. S. Aziz, G.F. Hewitt, D.B.R Kenning, Heat transfer regimes in forced-convection film boiling on spheres, J. Begel House Inc. (1986). https://doi.org/10.1615/ihtc8.700. D. Dix, J. Orozco, Film boiling heat transfer from a sphere in natural and forced convection of Freon-113, J. Exp. Heat Transf. 3 (1990) 129–148, https://doi.org/ 10.1080/08916159008946382. M. Ozgoren, A. Okbaz, S. Dogan, B. Sahin, Investigation of flow characteristics around a sphere placed in a boundary layer over a flat plate, J. Exp. Therm. Fluid Sci. 44 (2013) 62–74, https://doi.org/10.1016/j.expthermflusci.2012.05.014. S.H. Yoon, H.C. No, Film boiling heat transfer of a hot sphere in a subcooled liquid pool considering heat loss through its support rod, J. Nucl. Eng. Des. 325 (2017) 97–106, https://doi.org/10.1016/j.nucengdes.2017.10.003. S.G. Bankoff, Entrapment of vapor in the spreading of a liquid over a rough surface, J. AIChE J. 4 (2010) 24–26, https://doi.org/10.1002/aic.690040105. J.P. Mchale, S.V. Garimella, Nucleate boiling from smooth and rough surfaces – Part 2: Analysis of surface roughness effects on nucleate boiling, J. Exp. Therm. Fluid Sci. 44 (2013) 439–455, https://doi.org/10.1016/j.expthermflusci.2012.08.005. W. Bogacz, M. Lemanowicz, M.H. Al-Rashed, et al., Impact of roughness, wettability and hydrodynamic conditions on the incrustation on stainless steel surfaces, J. Appl. Therm. Eng. 112 (2017) 352–361, https://doi.org/10.1016/j. applthermaleng.2016.10.076. H.H. Huang, C.T. Ho, T.H. Lee, et al., Effect of surface roughness of ground titanium on initial cell adhesion, J. Biomol. Eng. 21 (2004) 93–97, https://doi.org/10.1016/ j.bioeng.2004.05.001. V.V. Yagov, M.A. Lexin, A.R. Zabirov, O.N. Kaban’kov, Film boiling of subcooled liquids. Part I: Leidenfrost phenomenon and experimental results for subcooled water, J. Int. J. Heat Mass Transf. 100 (2016) 908–917, https://doi.org/10.1016/j. ijheatmasstransfer.2016.02.049.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Film boiling collapse in a solid hot sphere immersed in subcooled forced convection ⁎
Peiwei Nia, Zhi Wena,b, Fuyong Sua,b, , Jie Huanga, Xunliang Liua,b, Guofeng Loua,b, Ruifeng Doua,b a b
School of Energy and Environmental Engineering, University of Science & Technology Beijing, Beijing 100083, China Beijing Key Laboratory of Energy Saving and Emission Reduction for Metallurgical Industry, University of Science & Technology Beijing, Beijing 100083, China
H I GH L IG H T S
film on a hot sphere immersed in forced convection is stripped away sequentially. • The dissipation at support area and inertial force of liquid affect the stripping. • Heat rupture position of the film moves to support area as flow rate increases. • Initial quantity of initial rupture points increases as subcooling degree of liquid raises. • The • The rupture speed of vapor film increases as subcooling degree of liquid raises.
A R T I C LE I N FO
A B S T R A C T
Keywords: Forced convection film boiling Visualization experiment Rupture process Flow rate of the subcooled liquid Liquid subcooling degree
An experimental study of forced convection film boiling was conducted, on a stainless steel sphere with diameter of 30 mm with different coolant velocities. A visualization research on the motion of the vapor/liquid interface and the rupture process has revealed a detailed mode for vapor film rupture, which relates to the motion of a wetting boundary on the hot surface of a sphere. The rupture sequence of the vapor film on different monitor points in the sphere was obtained by the analysis of surface temperatures and heat fluxes during the process. In addition, the influence factors of vapor film rupture were also studied. It is concluded that, in forced convection, the rupture process of vapor film on the surface of the sphere is affected by the temperature distribution of the sphere; liquid flow velocity, cooling effect of the cylindrical support pipe and liquid subcooling degree also have significant impacts on it.
1. Introduction The film boiling process occur under many circumstances, such as metal quenching, special chemical catalytic synthesis reactions and wet treatment of blast furnace slag [1]. Blast furnace slag, as the main byproduct of blast furnace ironmaking, has a huge output each year. In the process of wet treatment of blast furnace slag, the slag above 1400 °C is crushed by subcooled fluid and condensed, forming a large number of fragments, including small slag particles and large slag blocks. When high temperature slag particles contact with subcooled scouring slag water, a layer of gas film will be formed on the surface. The breakdown of gas film can not only promote the further breakage of slag particles [2], but also promote the condensation of slag particles. In the cooling process of the fragments, complex crystallizations [3] exist
⁎
at different crystallization temperatures [4]. Therefore, it is necessary to study the heat transfer phenomena in the process of wet slag treatment in order to improve the added value of slag particles. Slag blocks can be regarded as solid metal spheres, and the boiling vapor film rupture during water cooling of slag fragments can be regarded as the film rupture process on the surface of hot spheres. As early as 1756, Leidenfrost firstly observed the film boiling phenomenon of the subcooled liquid after contacting with the hot wall. At the beginning of the contact, as the surface temperature of the wall is much higher than the liquid temperature, many bubbles are generated on the wall and join together that a layer of vapor is formed and coat the surface of the wall. This vapor film decreases the temperature drop rate of the wall surface. A large number of researchers have studied the film boiling
Corresponding author at: School of Energy and Environmental Engineering, University of Science & Technology Beijing, Beijing 100083, China. E-mail address: [email protected] (F. Su).
https://doi.org/10.1016/j.applthermaleng.2019.114630 Received 6 June 2019; Received in revised form 28 October 2019; Accepted 2 November 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Peiwei Ni, et al., Applied Thermal Engineering, https://doi.org/10.1016/j.applthermaleng.2019.114630
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Nomenclature
R Ra Re Tsat Tsphere Tsup
Rh λ
Bi
Biot number,
cp D h m Nu Pr q
specific heat capacity of stainless steel, kJ/(kg·° C) diameter of the sphere, m heat transfer coefficient, kW/(m2·° C) mass of the sphere, kg Nusselt number Prandtl Number heat flux, kW/m2
radius of the sphere, m roughness average parameter, μm Reynolds number saturation temperature of the liquid, °C temperature of the sphere, °C subcooling temperature of spheres, °C
Greeks λ
thermal conductivity, kW/(m·° C)
over the surface of the hot sphere immersed in the subcooled liquid by using the experimental method. Jouhara and Axcell [17] et al. proposed a new model of vapor film rupture by experimental study of transient film boiling. The formation of vapor film after solid-liquid contact and the movement of rewet front on the surface of the hot sphere were recorded. Gylys [18] used the experimental method to put copper, stainless steel and aluminum spheres with a certain degree of superheat into cold water to study the influence of their thermal conductivity, heat capacity, density and liquid subcooled degree on the change of the surface temperature. It is concluded that the minimum temperature on the surface of the sphere that keeps the vapor film steady depends on the heat capacity of the material, the thermal conductivity and the subcooled degree of the liquid. Through the analysis of existing researches, there are still some noteworthy problems in the experiment:
phenomenon of different heated objects immersed in subcooled liquids. Nusselt first established a theoretical model of film condensation heat transfer on a vertical wall [5], Bromley [6] et al. studied the strong convection film boiling process of vertical surface and horizontal cylinder on the basis of Nusselt. Stevens [7] and Walford [8] et al. carried out an experimental study on the transition process from film boiling to nuclear boiling when high-temperature copper spheres moving rapidly in water. The characteristics of the vapor film collapse and the process of rewetting were revealed. Dhir [9] studied the quenching process of spheres with different materials in natural and forced flow experimentally and theoretically. They found that sphere size plays an important role in causing premature partial contact which makes the heat transfer coefficient increase and may result in a pulsating film in subcooling film boiling. Dreitser [10] presented experimental investigations of convective heat transfer enhancement for cryogenic liquid flow in channels and introduced problems of heat transfer enhancement in channels for film boiling. Ebrahim [11] studied the influences of liquid subcooling, surface conditions and material properties on the film pool boiling heat transfer using metal rods with vertical quenching experiments. The study on film boiling process of subcooled liquids around heated spheres mainly include three aspects: (1) pool film boiling; (2) natural convection film boiling; (3) forced convection film boiling. Research methods are usually by combining relevant experimental conclusions for empirical equations of heat transfer coefficient and other parameters correlated with mass and heat transfer. Motte and Bromley [12] analyzed the process of forced convection film boiling theoretically, they proposed a general expression of convective heat transfer coefficient for high temperature solid wall immersed in subcooled liquids. Wilson [13] analyzed forced convection film boiling around the surface of a hot sphere theoretically, and ordinary differential equations describing the film thickness were obtained. Epstein and Hauser [14] studied the film boiling process over the stagnation zone of the surface of the sphere in forced convection, and got the detailed expression of vapor film thickness and the area and empirical correlations between Re, Pr and Nu. Liu [1], etc. did a lot of experimental researches about film boiling processes of spheres with various sizes, temperatures, materials immersed in liquids with different velocities, phases and subcooled temperature. Empirical equations of heat transfer coefficient were proposed by comparing the analysis of those experiments. With the development of the high-speed video technology, many researchers adopt visualization experiments to study the film boiling process of subcooled liquids around the surface of a sphere in detail in recent years. The research directions include the formation and crushing processes of the vapor film, the minimum maintaining surface temperature of the vapor film, other influencing factors, the effect of surface roughness on the rupture process, etc. Sher [15] et al. studied the film boiling collapse in solid spheres immersed in a subcooled liquid and found that the surface of the sphere would experience a boiling process similar to that of a “golf ball”. In this case, the rewetting process is an explosive event occurred in the whole surface. Freud [16] et al. identified the minimum temperature needed to maintain the vapor film
(1) From the perspective of the distribution of the vapor film over the surface of the sphere, Walfort [8], Stevens and Witte [7], Dix and Orozco [20] et al. studied the portion where the vapor film was produced on the sphere. The following laws can be driven from their research conclusions: all of the conclusions on the shape of vapor film at the front (inflow) part of the sphere are that the front of the sphere is covered by an unbroken and undulating vapor film. However, there is no accepted conclusion about the shape of the rear (outflow) vapor film. There are many reasons for this situation. First, the formation and maintenance of the vapor film at the rear part depend not only on the temperature of the rear wall surface of the sphere, but also on the flow state of the liquid. Because the liquid around the sphere will appear stratification and eddy phenomenon at the rear part [21]. Secondly, the formation of the rear vapor film is also significantly affected by the support mode of the sphere. The heat loss through the support tube on the upper part of a sphere was evaluated by Yoon [22] et al. The results show that the heat loss through the support tube is non-negligible. This support mode has been widely used in many researches. (2) According to the Leidenfrost phenomenon, the generation of boiling vapor film will hinder heat transfer from solid to liquid, so that the convection heat transfer coefficient (HTC) on the solid surface is low in the process of film boiling. The HTC between solid and fluid would increase sharply with the rupture of vapor film. The rupture of the boiling vapor film is very important for the boiling process. Most of the researches have focused on the rupture process of boiling vapor film formed by static subcooled liquid or natural convection subcooled liquid [7] around a hot sphere. Based on experimental results, Sher [15] et al. obtained the rupture process of boiling vapor film over a high-temperature sphere with uniform material and smooth surface immersed in static subcooled liquid. The phenomenon of “golf ball” on the surface of the sphere indicates that the rupture of the vapor film in static subcooled liquid occurs simultaneously on the whole surface of the sphere. However, few studies have focused on the rupture process of boiling vapor film formed by forced convection subcooled liquid around a hot sphere. Although Liu et al. studied the film boiling of a high2
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temperature sphere in flowing liquid, more detailed analysis is still necessary for the process of film breaking in forced convection subcooled liquid. (3) After the high-temperature sphere contacts the subcooled liquid, a large number of bubbles will generate on the contact surface, the departure diameter of the bubble and the frequency of bubble generation are related to the size of the active nucleation site on the surface of the sphere. The roughness of the surface of the sphere determines the size and density of active nucleation sites [23]. After the 1950s, people began to pay attention to the influence of rough surface on boiling heat transfer process. For example, the bubble motion between parallel plates with rough surface was studied by Bankoff [23] et al. The investigation showed the influence of liquid density, surface tension, contact angle and rough surface structure on bubble motions. Mchale [24] et al. conducted a detailed study on the boiling nucleation center. They measured bubble departure diameters, growth periods, ebullition periods, and void fraction above the surface are obtained from high-speed graphic visualizations. They also compared correlations of heat transfer coefficient and bubble ebullition characteristics with different measures of surface roughness. Surface roughness is also important for the rupture process of the vapor film. The heat transfer resistance between solids and fluids decreases sharply during the rupture of the vapor film, and the location with high roughness is often the initial position of the vapor film rupture process. The mean roughness of the solid surface determines the duration of film boiling directly. Bogacz [25] investigated the effect of the stainless steel surface roughness on incrustation of MgSO4·7H2O from aqueous solutions and heat transfer resistance. (4) Various ways have been adopted by researchers to dip the sphere into the subcooled liquid. Aziz and Hewitt [19] dropped the sphere free in still water to observe the film boiling process. Liu [1] dipped the sphere into water and heated it to a specific temperature in a steady-state flow. Jouhara [17] et al. fixed the heated sphere by a tube hanging in an empty chamber and studied the film boiling process of the subcooled liquid around the sphere by filling the chamber with liquid and lifting the liquid level rapidly above it. Yoon [22] et al. supported the sphere with a hollow tube connect to the top of it, and fall the sphere to the designated position below the liquid level to study the heat transfer process of static film boiling. The dip method has an impact on flow regime of the liquid around the sphere when first entering the liquid. It also influences the initial subcooled temperature of the surface of the sphere, especially when the subcooling degree is high.
In the present study, a visual experiment was designed for the research of transient rupture process during the forced convection film boiling around a hot sphere immersed in the subcooled liquid. The rupture sequence of the vapor film was recorded by a high-speed camera and analyzed through surface temperatures and heat fluxes during the process. The factors that influence the rupture process of the vapor film, such as liquid flow velocity, cooling effect of the cylindrical support pipe and liquid subcooling degree, were also studied. 2. Experiment The specimens used in this experiment are all stainless steel solid spheres with 30 mm diameter. The distance between the thermocouple and the inner contact surface of the sphere is 1 mm from the sphere surface. The subcooling degrees were 35.6 °C, 42.3 °C, 54.9 °C and 63.9 °C, the flow speeds of the liquid were set to be 0.035 m/s , 0.056 m/s and 0.098 m/s . The schematic of the experimental device is shown in Fig. 1. The subcooled liquid is pumped out of the cistern by a suction pump and flow into the water channel through the pressure maintenance pump, ball valve and electromagnetic flowmeter. The flow velocity of the liquid can be adjusted by the ball valve after the pressure maintenance pump. The liquid level in the water channel is adjusted by another ball valve connecting the water outlet of the channel. Since the diameters of inlet and outlet of the water channel are smaller than the truncation size of the middle section of the water channel, two expansion areas are set near the inlet and outlet. A control device for the experimental sphere is arranged above the water channel, which enables the sphere to reach the same position at the same dropping speed and remain stationary in repeated experiments. The internal structure of the sphere is shown in Fig. 1(c). There were three holes which extend to different positions on the inner surface of the sphere. Three K-type thermocouples contacting the inner surfaces of the sphere through the holes respectively (0–400 °C, the measuring deviation is 1.6%; 400–1000 °C, the measuring deviation is 0.4%) were used to monitor the temperatures of front, bottom and rear parts of the sphere during the test. As shown in Fig. 1(c), the corresponding numbers of the three thermocouples were respectively: 1-rear part, 2bottom, 3-front part. The thermocouples converged and connected with the data-acquisition device through a stainless steel tube (the outer diameter of the tube is 5 mm) supporting the sphere on the upper part. The data acquisition frequency was 40 Hz. The small hole inserted into the thermocouple was sealed with high-temperature resistant adhesive, so as to prevent the liquid from infiltrating into causing errors in the experiment.
Fig. 1. Schematic of the experimental system. 3
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Experiment process is as follows: The sphere with its support tube was heated in a furnace to over 800 °C. Then the support tube was fixed in the control device. When the temperature of the sphere dropped to 800 °C, the motion mechanism on the control device was started to immerse the sphere in the liquid with a constant flow rate. The sphere was remained stationary till the end of boiling process. The high temperature which the sphere is heated to may result in a layer of oxides generate on its surface. Based on the conclusions of Sher [15] et al., the effect of this oxide layer can be ignored on the heat transfer process of the sphere. However, the oxide layers will affect the roughness of the surface of spheres. Therefore, in each experiment, the oxide layer was removed before the sphere was heated by the furnace. A 1500 objective sandpaper was used to remove the oxide layer, so that the initial roughness of the surface of the sphere was always maintained at about Ra0.15[26]. 3. Results and discussion According to the previous experimental studies in static film boiling, it can be found that the vapor film rupture is caused by the decrease of surface temperature of the sphere. The process of liquid evaporation is slower than that of vapor liquefaction on the vapor-liquid interface of the vapor film, so that the thickness of the vapor film is reducing until the liquid in contact with the sphere directly. In static boing film rupture, this process occurs simultaneously at various points on the surface of the sphere. It can be regarded that the whole vapor film covered the sphere burst at the same time. However, when the rupture process starts in forced convection film boiling, the temperature of the sphere is higher than the minimum heat flux temperature in static film boiling. The rupture process of forced convection vapor film recorded by the high-speed camera is shown in Fig. 2. The initial temperature of the sphere was 800 °C, and the velocity of the liquid with subcooling degree of 35.6 °C was 0.035 m/s . When the liquid flow rate is 0.035 m/s , the boiling vapor film strips away from the surface of the sphere. During the 8 s after rupture started, image results were captured every 1 s. The subcooled liquid flowed in from the left side of the sphere, and then flowed out through the right side. It can be seen from Fig. 2 that, different from the overall rupture mode in static film boiling, the rupture process of forced convection vapor film starts from the front stagnation point of the sphere, and a circle of obvious wetting boundary appears in the vapor film on the surface of the sphere. The rupture of the vapor film produces a large number of tiny bubbles on the wetting boundary. Inside the wetting boundary, the surface of the sphere is directly in contact with the subcooled liquid, and the outside part of the sphere surface is still covered by a stable vapor film. As the crushing process progresses, the
Fig. 3. (a) Stainless steel sphere temperature vs. time for different parts on the sphere. (b) Stainless steel sphere heat flux vs. temperature for different parts on the sphere. The flow rate is 0.035 m/s.
Fig. 2. The rupture process of forced convection vapor film, the liquid flow rate is 0.035 m/s . 4
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positions along the direction of liquid flow on the surface of the sphere. It can be concluded that the rupture sequence of the vapor film is consistent with the flow direction of the liquid along the sphere. Fig. 3(b) shows that after immersed in the subcooled liquid, the higher temperature of the sphere causes evaporation of the surrounding liquid. A stable vapor film was formed over the surface of the sphere. And so the heat flux through the spherical wall is decreased. As can be seen from Fig. 3(a), the vapor film remains stable over the surface of the sphere for a long time. Due to the low heat flux, the cooling range of the sphere in this process is small. After the vapor film rupture, with the disturbing effect of a large number of tiny bubbles caused by it, the heat transfer process between the sphere and the liquid is greatly promoted, and the heat flux through the surface of the sphere increases rapidly. The variation trend of heat flow density with temperature in the boiling process shown in Fig. 3(b) is consistent with the experimental conclusions from Liu [1], Sher [15], Yagov [27] and other researchers. As can be seen from Fig. 2 (1), when the critical point of vapor film rupture is reached, the vapor film covering the inlet flow surface of the sphere (3-front part) is the thinnest, followed by the vapor film covering the back flow surface (1-rear part), and the film on the bottom is the thickest (2-bottom). It can also be concluded from Fig. 3(a) and Fig. 3(b) that in the process of film rupture, the inlet surface of the sphere has the lowest temperature and the highest heat flux and the back flow surface has the highest temperature and the lowest heat flux. Because the fluid inertia force around the front stagnation point of the
wetting boundary expands along the flow direction of the fluid on the surface of the sphere until the boiling vapor film is completely detached from the surface of the sphere. When a stable vapor film is formed over the sphere in our experiment, the HTC value is lower than 250 W/(m2·° C) , the Biot number approximate lower than 0.1, therefore the lumped capacitance method can be applied for heat transfer calculation. The specific solution equation is shown as follows:
q=
h=
msphere cp dTsphere 2 πDsphere dt
q q = ΔTsup Tsphere − Tsat
(1)
(2)
The surface temperature curves of different parts on the sphere immersed in subcooled liquid is shown in Fig. 3(a). The corresponding images of Fig. 2 are also plotted near the approximate point. Since the support pipe was located on the upper part of the sphere, no temperature measuring point was arranged on the top inner surface of the sphere. It can be seen from Fig. 3(a) that the temperature variation regularities are similar to those drawn by Yagov [27]. The differences lie in that the distribution of temperature measuring points of Yagov’s specimens describes the variation of temperatures in different positions along the direction of diameter inside the sphere, while those in this experiment describes the variation of temperatures in different
Fig. 4. The rupture process of forced convection vapor film, (a) the liquid flow rate is 0.056 m/s. (b) The liquid flow rate is 0.098 m/s. 5
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but did not affect the process of vapor film rupture.
sphere is the greatest, therefore the vapor film on the flow surface is the thinnest. It is also the reason why the process of film rupture near the surface of the sphere starts at the front stagnation point in forced convection. The generation of wetting boundary and the stripping rupture mode of vapor film are caused by the stability of the vapor film due to the high surface temperature in the area still covered by the vapor film on the sphere. The inertial force of the liquid is the driving force of the film splitting rupture. The temperature of the wetted spherical surface decreases rapidly due to direct contact with the subcooled liquid. It can be seen that the decrease of temperature on the surface of the sphere is not the main factor affecting the rupture of the vapor film in forced convection. The flow around the sphere is closely related to the process of vapor film rupture. The existence of the wetting boundary on the sphere surface changes the shape of the vapor film, but at the same time ensures the integrity and leakproofness of the film. The motion of the wetting boundary is the main reason for the sequence of vapor film rupture on the surface of the sphere. Based on Yoon's research conclusion, it can be seen that the support tube connected with the sphere facilitates the heat loss of the sphere at the connecting area, and the promotion cannot be ignored. It can also be seen from Fig. 2 that during the film boiling process, the vapor film next to the support tube on the top of the sphere was acting violently,
4. Analysis of influencing factors for steam film rupture The vapor film rupture in static film boiling is caused by the decrease of the surface temperature. In the case of forced convection, the main influencing factors of the rupture process of vapor film formed by forced convection subcooled liquid around a hot sphere are need to get further studies. 4.1. Liquid flow rates As shown in Fig. 4, the image results are captured every 1 s after 8 s from the beginning of the rupture of the vapor film on the sphere surface at the liquid flow rate of 0.056 m/s and 0.098 m/s. The subcooled liquid flows in from the left side of the sphere, and then flows out through the right side. As can be seen from Fig. 4(a), when the flow rate of the subcooled liquid increased to 0.056 m/s, the rupture mode of the vapor film is similar to that at low flow rate. However, the starting position of the rupture process changes from the front stagnation point to the position next to the junction of the support tube on the inlet flow surface of the sphere.
Fig. 5. Stainless steel sphere temperature vs. time for different parts on the sphere and stainless steel sphere heat flux vs. temperature for different parts on the sphere. The diameter of the sphere is 30 mm, the subcooling degree of the liquid is 35 °C. (a) The flow rate is 0.056 m/s. (b) The flow rate is 0.098 m/s. 6
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Fig. 4(b) presents that when the flow rate is 0.098 m/s, the heat dissipation of the supporting rod is strengthened with the increase of the flow rate, there are two initial positions of the vapor film rupture. The position next to the junction of the support tube on the sphere is still the initial. The other is the front stagnation point. The phenomena at the vapor-liquid interface at the rupture position is similar to microbubble Emission Boiling (MEB), which promotes the heat transfer process. As shown in Fig. 4(b) (1), the rupture at the end of the period at the front lag point occurs between Fig. 4 (b) (2) and Fig. 4 (b) (3). The processes of vapor film rupture motions initiated by two rupture positions merge at the front stagnation point and interferes with each other. In addition, the change of the starting position of the rupture also changes the sequence of film rupture process. As can be seen from Fig. 5(a), the wetting boundary first passes through the front part, then through the rear part, and finally through the bottom. The image of heat flux vs. temperature shows that the change of the sequence also affects the heat flux at temperature measuring points. The reason for the above change lies in the existence of the support tube and the enhancement of the cooling process of the sphere with the increase of the flow rate. As mentioned before, after the sphere is immersed in the forced convection subcooled liquid, the vapor film at the front stagnation point on the flow surface becomes thinner due to the action of the fluid inertia force. At the same time, the temperature of the sphere near the junction is reduced due to the cooling effect of the support tube. When the flow velocity is low, the liquid inertial force is small, so is the total heat lost by the support tube. The cooling effect of the support tube does not affect the vapor film rupture before the burst of the film on the front stagnation point. Therefore, the sphere temperature can still maintain the stability of the vapor film near the support tube after the rupture occurs on the front stagnation point. As the flow velocity increases, the heat carried away by the liquid through the support tube increases. The cooling effect of the support tube on the top of the sphere is enhanced. Although the liquid inertia force acting on the front stagnation point also increases, but the cooling effect of the support tube is still stronger than the influence of fluid inertia force, therefore, the rupture of the vapor film on the position next to the junction of the support tube on the inlet flow surface of the sphere starts earlier than that on the front stagnation point. Since the vapor film at the front stagnation point also ruptures under the action of the liquid inertia force, the motion process of the wetting boundary changes again as shown in Fig. 5(b), leading to the change of the rupture sequence of the vapor film. The movement process of the wetting boundary first passes through the front part of the sphere, then the bottom, and finally the rear part. It can be seen that when the vapor film begins to rupture, the cooling effect of the support tube on the vapor film at the connection area on the sphere is still stronger than the effect of the liquid inertia force even though the raise of the flow rate increases the liquid inertia force. The vapor film at the front stagnation point has been crushed before the wetting boundary reaches. Since the front stagnation point of the flow surface is affected by the liquid inertial force, the liquid velocity has the most significant effect on the film rupture on this point. Fig. 6 shows the change of heat flux on the front part of the sphere during the cooling process at different flow rates. As can be seen from Fig. 6, the compression speed of the vapor film is increased with increasing the velocity of the subcooled liquid. Therefore, the rupture time is also brought forward (The initial temperature of the sphere are all 800 °C). However, the film rupture at the bottom and rear part of the sphere is significantly affected by the motion development of the wetting boundary. The cooling effect of the support tube also affects the expansion mode of the wetting boundary. When the flow rate is 0.098 m/s, the variation of heat flux vs. temperature is special. The heat flux does not increase sharply with the rupture of the vapor film, but appears a slow rising process. This case
was not proved in this experiment. It can be inferred that it was caused by the interaction between the fluid inertia force and the interference caused by the merging of two rupture motions at the front stagnation point. In addition, the rupture position next to the junction of the support tube appears earlier, as shown in Fig. 4(b) (2). The temperature on the surface of the sphere decreases rapidly after the film breakdown. Because of the thermal conductivity inside the solid sphere, the temperature on the sphere surface which is still covered by the vapor film at the front stagnation point would also decrease, and the maximum heat flux value during the film breakdown would also be affected. 4.2. Water subcoolings Fig. 7(a) shows the rupture process of forced convection vapor film when the liquid flow rate is 0.035 m/s and the subcooling degree is 42.3 °C. Under this subcooling degree, the motion of the wetting boundary when the vapor film is rupturing is similar to that when the liquid subcooling degree is 35.6 °C. The rupture process of the vapor film develops to the bottom of the sphere between 4 and 5 s, and the rear part between 7 and 8 s. The initial rupture position of the vapor film layer is also consistent with the 35.6 °C subcooling degree. Due to the liquid high subcooling degree, the amount of bubbles generated by the surface area contacting with the liquid directly during the rupture process is less, the volume of the bubble at the vapor-liquid interface is smaller, and the vapor-liquid interface is clearer. When the liquid flow rate is constant and the subcooling degree is raised to 54.9 °C, the resulting images is shown in Fig. 7(b). The local temperature difference of the sphere is increased during the process of hot sphere immersing in the liquid. The influence of the flow velocity during the rupture process of the vapor film is relatively weakened. The initial rupture position is located near the position where the sphere first contacts the liquid. Between 2 and 3 s after the rupture process occurs, the vapor film on the front stagnation point of the sphere start rupturing. The bubble volume on the vapor-liquid interface is further reduced and the interface is clearer. The rate of the film rupture increases, and the maintaining time of the film is shortened. Fig. 7(c) shows the image of the film collapse at the sphere surface when the liquid flow rate is 0.035 m/s and the degree of subcooling is 63.9 °C. Due to the increase of the liquid subcooling degree, the boiling vapor film on the surface of the sphere fluctuates sharply during the maintenance process of the vapor film. Several fracture points in the vapor film appear during the rupture process. As shown in Fig. 7(c) (2), the initial contact area of the sphere with the liquid and the front stagnation point are still the earlier positions where the film rupture
Fig. 6. Stainless steel sphere heat flux vs. temperature for the front part on the sphere under the condition of different flow rates. 7
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Fig. 7. The rupture process of forced convection vapor film. (a) The subcooling is 42.3 °C. (b) The subcooling is 54.9 °C. (c) The subcooling is 63.9 °C.
Under this degree of subcooling, the rupture rate of the vapor film is the fastest. Fig. 8 shows the changes in heat flux with temperature for the bottom and backflow surfaces of the sphere under different liquid
occurs. It can be inferred that the velocity of the fluid, the liquid subcooling degree and the immersing method of the sphere are still the main factors affecting the rupture process of the vapor film, even though the fluctuation of the film also affects the thickness of the film. 8
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5. Conclusions In this work, the phenomenon of film boiling process and stripping processed of hot sphere immersed in subcooled liquid with forced convection are studied by visualization experiments. By the analysis of sphere surface temperatures and heat fluxes during the film boiling and rupture process, the rupture sequence of the vapor film on different monitor points of the sphere during the stripping process is studied. The influence factors, such as cooling effect of the cylindrical support pipe, liquid flow velocity and liquid subcooling degree, which relate to the stability maintenance of the vapor film are analyzed. The flow speeds of the liquid were 0.035 m/s , 0.056 m/s and 0.098 m/s and the subcooling degrees were 35.6 °C, 42.3 °C, 54.9 °C and 63.9 °C. Under different flow rates of the liquid, the heat dissipation at the support point of the sphere and the inertial force of the liquid flow have a significant effect on the stripping and rupturing process of the vapor film on the surface of the sphere. The initial rupture position of the film transfers to the support point as the flow rate increases. The motion of the wetting boundary changes correspondingly. When the liquid flow rate is constant, the initial rapture temperature of the vapor film raises, the rupture speed of the vapor film increases, the maximum heat flux on the surface of the sphere and its corresponding temperature also increase by increasing the subcooling degree of liquid. The rupture process of the vapor film on the surface of the sphere develops from a single-point rupture process at the front stagnation point to a two-point rupture process at the front stagnation point and initial contact area of the sphere with the liquid, and finally becomes a multi-point one. Due to the complexity of the vapor film rupture process, merely changing the degree of subcooling of the liquid does not completely control other factors that affect the rupture process of the vapor film on the surface of the sphere. To sum up, in forced convection, the rupture process of vapor film on the surface of the sphere is affected by the temperature of the sphere; liquid flow velocity, cooling effect of the cylindrical support pipe, liquid subcooling degree and the immersing method of the sphere into the liquid also have significant impacts on it. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Fig. 8. Stainless steel sphere heat flux vs. temperature for the bottom (a) and the rear (b) parts on the sphere in different liquid subcooling degrees. The liquid flow rate is 0.035 m/s.
Acknowledgements
subcooling. It can be seen that, the corresponding temperatures of rupture starting point increase gradually as the liquid subcooling degree increases. When the subcooling degree is higher than 54.9 °C, the duration of the vapor film on the sphere is shorter. The hindrance of the vapor film to the heat exchange process is reduced and the film fluctuation is severe before the rupture process started. At the same time, the maximum heat flux on the surface of the sphere and its corresponding temperature also increase as the liquid subcooling degree raises. Under the condition that the liquid flow rate remains the same and the sphere falls in the same way, only changing the subcooling degree of the liquid cannot completely control other factors affecting the rupture process of the vapor film on the surface of the sphere. With the increase of the subcooling degree, the rupture process of the vapor film on the surface of the sphere develops from a single-point rupture process at the front stagnation point to a two-point rupture process at the front stagnation point and initial contact area of the sphere with the liquid, and finally becomes a multi-point one.
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