Experimental study on combined buoyant-thermocapillary flow along with rising liquid film on the surface of a horizontal metallic mesh tube

Abstract

Temperature distribution and variation with time has been considered in the analysis of the influences of the initial level of immersion of a horizontal metallic mesh tube in the liquid on combined buoyant and thermo-capillary flow. The combined flow occurs along with the rising liquid film flow on the surface of a horizontal metallic mesh tube. Three different levels of immersion of the metallic mesh tube in the liquid have been tested. Experiments of 60 min in duration have been performed using a heating metallic tube with a diameter of 25 mm and a length of 110 mm, sealed outside with a metallic mesh of 178 mm by 178 mm, and distilled water. These reveal two distinct flow patterns. Thermocouples and infrared thermal imager are utilized to measure the temperature. The level of the liquid free surface relative to the lower edge of the tube is measured as angle q. The results show that for a smaller q angle, or a low level of immersion, with a relatively low heating power, it is possible to near fully combine the upwards buoyant flow with the rising liquid film flow. In this case, the liquid is heated only in the vicinity of the tube, while the liquid away from the flow region experiences small changes in temperature and the system approaches steady conditions. For larger q angles, or higher levels of immersion, a different flow pattern is noticed on the liquid free surface and identified as the thermo-capillary (Marangoni) flow. The rising liquid film is also present. The higher levels of immersion cause a high temperature gradient in the liquid free surface region and promote thermal stratification; therefore the system could not approach steady conditions.

Keywords

rising liquid film / combined flow / thermo-capillary flow / buoyant flow / metallic mesh tube / horizontal tube

Introduction

Liquid film dynamics and its evaporation process on the surface of a horizontal metallic tube have been extensively studied on different solid interface topographies [¹–⁶]. The major focus of previous studies has been on the various phenomena directly associated with the solid interface geometry, as well as their respective impacts on the liquid film heat and mass transfer efficiency. The enhanced heat and mass transfer characteristics of the thin liquid film formed are often highlighted in these studies. In particular, the rising of liquid film on the surface of a horizontal metallic mesh tube has proven to be one of the most efficient methods of rising liquid film evaporation [²]. It is possible for the liquid film to rise and cover the entire surface of the horizontal metallic mesh tube. The velocity of the rising film is mainly affected by the diameter of the smooth horizontal tube, as well as the gap between the tube and the metallic mesh [¹]. However, to the best of the authors’ knowledge, there is a lack of research on the influences of the liquid free surface level relative to the lower edge of the horizontal metallic mesh tube, or level of immersion. The liquid level or the level of immersion of the tube (heat source) is closely connected to the nature of the combined buoyant and thermo-capillary flow that can occur in the liquid. This can highly affect the performance of the rising liquid film flow. The combined flow may occur in different forms, inducing different currents patterns in the bulk liquid.

Independent of the rising liquid film flow and its mechanisms, there is the combination of buoyant and thermo-capillary flow that may occur in the liquid container. The buoyant flow is induced by density changes in the fluid due to the temperature gradient in the vertical direction [⁷], whereas the thermo-capillary flow is induced by temperature gradient along the horizontal direction [⁷] due to the surface tension gradient in the liquid free surface. Both flows are driven by the temperature gradients present in the system, thus the analysis of the flow development can be based, but not limited to the temperature distribution and variations in the system. The two flows can occur simultaneously and interact with each other. Recently, the combination of buoyant and thermo-capillary flow has been extensively studied theoretically, numerically and experimentally [⁷–¹⁵]. A variety of flow patterns being generated in the process and their respective evolutions, in addition the existence of an active and stagnant region in the liquid have been reported. Furthermore, most of these studies have been conducted in rectangular liquid containers or cavities and the results reported present some consistency with each other. However, to the best of the authors’ knowledge, until this moment, no available studies of this particular subject associated with the study of rising liquid film on the surface of horizontal metallic mesh tube have been performed. Therefore, the results of this present study can only be partially substantiated by previous studies related to the combined buoyant and thermo-capillary flow. The work presented by Lee et al. [⁹] about the combined thermo-capillary and natural convection using localized heating is the closest one related to this present work. However, the rising liquid film is not part of their study.

Mei et al. [¹] have studied the rising liquid film on the surface of horizontal metallic mesh tube. They have shown that the rising liquid film flow is driven by the capillary pressure and circumferential temperature difference in the presence of evaporation in the surface of horizontal metallic mesh tube. Additionally, they have discovered that the velocity of the rising liquid film increases with a decrement of the diameter of the smooth-tube and the gap between the tube and the metallic mesh. The formation of the rising liquid film on the surface of horizontal metallic mesh tube is influenced by the circumferential temperature difference.

Mei et al. [²] have studied the rising liquid film induced by rewetting and heat transfer on the surface of a fluted helix horizontal tube. They have identified three possible driving forces contributing to the rising liquid film formation and heat transfer: the capillary pressure induced by the geometry structure of the helix channel, the driving force induced by circumferential temperature, and the driving forces induced by movements of the bubbles. In their opinion, these three kind of driving forces would thus work together to induce the coupling effect. Experiments on a fluted helix horizontal tube sealed with metallic mesh have also been performed by Mei et al. [²]. The fluted helix horizontal tube sealed with metallic mesh have demonstrated better results than the ones performed solely on fluted helix horizontal tube. They have also found that the increment of the number of metallic mesh homogenizes the rising liquid film on the surface of horizontal metallic mesh tube, thus enhancing heat transfer.

About thermo-capillary flow, a comprehensive review of experimental study on the subject has been given by Schatz et al. [⁸]. Lee et al. [⁹] have studied the combined thermo-capillary and natural convection in rectangular containers where the fluid is heated by a thin wire placed along the free surface. They have reported two distinct regions, including an active flow region and a nearly stagnant region beneath the active one. They have also stated that steady two-dimensional flow field becomes oscillatory and three-dimensional in the presence of a certain temperature difference. Kang et al. [⁷] have analyzed the surface deformation and flow pattern on buoyant and thermo-capillary convection. Imposing a temperature difference between the two sides of the liquid layer, they have shown that the stability of the convection is dependent on a critical value of the temperature difference. Furthermore, they have also reported temperature oscillations with small amplitudes.

Different initials levels of immersion of the tube in the liquid or even slight changes in the course of events, combined with other factors, are susceptible of changing the flow field conditions thus heat and mass transfer mechanisms. Hence, a study of the influences of liquid level on the temperature distribution and variation in the system is conducted and presented in this paper.

Description of the phenomena

The phenomenon of rising liquid film on the surface of horizontal metallic mesh tube can be described as follows. When a metallic mesh tube is heated to the boiling temperature of the liquid, the liquid on the bottom of the horizontal metallic mesh tube can migrate upwards spontaneously along the metallic mesh tube and a so called rising liquid film is formed [¹], as shown in Fig. 1. Additionally, while the liquid migrates upwards, evaporation will simultaneously occur and induce a rewetting flow [¹]. The need to slightly immerse the metallic mesh tube in the liquid is evident when the overheating of the tube can affect the formation and stability of the rising liquid film. This can occur if the lower edge of the tube is kept at the same level as the liquid free surface. Immersing the horizontal tube in the liquid will trigger the boiling process on the downward facing immersed surface of the metallic mesh tube [¹⁶,¹⁷], which can generate distinct flow patterns due to the temperature gradient introduced into the system. These flow patterns can be identified as characteristics of natural convection (Buoyant Flow) and thermo-capillary (Marangoni) convection, in addition to the rising liquid film flow. The magnitude and flow characteristics of the combined and each particular flow are dependent on the level of immersion of the tube in the liquid.

Given the nature of the system in question, a portion of the surface area of the metallic mesh tube remains immersed, while the other portion of the surface area remains above the free surface level and it is covered by the flowing liquid film. The level of immersion is defined by the level of the free surface relative to the lower edge of the metallic mesh tube, measured as angle q, and also interpreted as the level of immersion in the scope of this paper. The level of immersion is limited to half of the volume of the metallic mesh tube, and above the referred level the evaporation rate decreases considerably and therefore, holds no value for this study.

Focusing on the immersed portion of the surface area, it is most likely that the boiling process associated with the heat and mass transfer mechanism is subcooled boiling, or local boiling. Nevertheless, depending on the physical boundaries of the liquid container and other factors, the boiling process can change over time from subcooled boiling to saturated boiling, depending on the conditions under which it occurs. Saturated boiling is mostly undesired for this practice and it is avoided. However, in this paper, the boiling process is not considered.

Experimental apparatus and procedure

Experiments with the metallic mesh tube initially immersed at three different levels are performed. All the experiments have the same procedures and are performed under atmospheric pressure. The experimental facilities schematized in Fig. 2 are built in order to perform the experiments. The experimental facilities are mainly composed of three major parts, including the rising liquid film flow section, the infrared thermal imager section, and the temperature measurement section.

The rising liquid film flow section, shown in Fig. 3, is consisted of a metallic mesh tube, a DC transformer with an adjustable output voltage from 0 to 220 V connected to the metallic mesh tube, a cubic liquid container, and a support system. The stainless steel smooth metallic tube is 25 mm in diameter, 110 mm in length, and 1.5 mm in thickness. The smooth stainless steel tube is packed inside with an electric heating unit and sealed outside with a 178 mm by 178 mm stainless steel metallic mesh. The gap between the surface of the smooth metallic tube and the metallic mesh is 0.2 mm. The metallic mesh tube is heated with an average heat flux density of 8297 W/m². The cubic liquid container is made of regular glass material with an exterior dimension of 290 mm and 5 mm in thickness. Inside the liquid container is placed a convenient support system which is built in accordance with the needs of the physical system, to hold the metallic mesh tube horizontally and fix all the thermocouples in place. The thermocouples utilized to obtain the circumferential temperature distribution (CTD) are inserted 0.5 mm into the surface of the smooth metallic tube in a radial direction. Holes with 1 mm in diameter and 0.5 mm in depth are drilled on the surface of the smooth metallic tube in order to allocate the thermocouples.

The support system is mainly made of the acrylic glass (PMMA) material, capable of withstanding the system maximum working temperature, with the exception of a metallic arm holding the heating tube in horizontal position. The support system also includes a ring made of the same PMMA material to support the thermocouples. It is 255 mm in internal diameter, 265 mm in external diameter, and 5 mm in thickness. The ring is designed to be large enough in order not to interfere with liquid flow and fairly fix the thermocouples position. Distilled water is used as the fluid.

The infrared thermal imager section is merely composed of a mobile infrared thermal imager fixed at an adequate position to measure and visualize liquid free surface temperature distribution. The infrared camera is positioned about 300 mm above the upper edge of the tube and 250 mm away from the middle section of the tube, with an inclination angle of approximately 56°, centred with the position of the tube. This procedure is to avoid the interference from the vapour as the thermal images of the liquid free surface are taken. The infrared thermal imager is focused inside the perimeter of the liquid free surface, to avoid interference of the background temperature in the measurement scale. Nonetheless, the background temperature is set accordantly before each experiment. The infrared thermal imager is calibrated to measure the temperature of the water surface, by setting the emissivity ϵ to 0.93 for water material, according to the emissivity material list of the infrared thermal imager manufacturer. The captured thermal images are later processed using the software of the imager. Figure 4 gives the details of the infrared thermal imager position.

The temperature measurement section is consisted of two sets of thermocouples, an Adam data collector system, and a computer. One set of five thermocouples is utilized to measure the circumferential temperature of the smooth metallic tube. Another set of five thermocouples is utilized to measure the temperature of the liquid in predetermined positions. Figure 2 demonstrates the position of all thermocouples which measure the temperature with the same time stamp. T-type thermocouples with a probe having a diameter of 1 mm and a length of 300 mm are chosen. The thermocouples are connected to the Adam data collector system which is connected to a computer, collecting and storing the temperature variations with time stamps.

The liquid is divided into three levels and marked as levels q1, q2, and q3 which correspond to the approximate values of q angles 0°, 45°, and 90° respectively. A relatively low average heat flux density is applied to the surface of the horizontal metallic mesh tube. Therefore, the DC transformer is set up to output 90 V. The experiments are sequentially performed from level q1 to level q3.

The metallic mesh tube is centred relative to the liquid free surface. Prior to the experiments, and upon first usage, the metallic mesh tube is aged under the same conditions as those of the experiments. The tube is continuously heated with the DC transformer set to the indicated input voltage. The indicated average heat flux density q is given by Eq. (1). The experiment is run for 60 min, and between each experiment the liquid and the metallic mesh tube are allowed to cool down and reach a steady condition.

(1)q={\displaystyle \frac{U^{\mathrm{2}}}{R\mathrm{\pi }DL}},

where U is the power source voltage (V), R is the resistance of the metallic tube (W), D is the diameter of the smooth-tube (m), and L is the length of the metallic tube (m).

Given the largeness of the liquid free surface and the low evaporation rate predicted, the reference liquid free surface level is easily contained within oscillation amplitudes less than 0.5 mm. Small amounts of liquid are periodically injected at the corner of the liquid container, by means of a pipe connected to the bottom of the liquid container. The water level is constantly monitored throughout the experiments. The liquid is injected with a syringe and the same amount of liquid injected is considered in the measurement of the average evaporation rate.

CTD is obtained with thermocouples equally distanced by 45° along the tube wall from the lower edge to the upper edge. CTD is measured with thermocouples T1, T2, T3, T4 and T5 corresponding to the temperatures measured at the circumferential positions 0°, 45°, 90°, 135°, and 180° respectively. The distance between the lower edge of the tube and the bottom of the liquid container is 260 mm. The liquid temperature is measured with thermocouples T6, T7, T8, T9, and T10. The physical system is assumed to be completely symmetrical; hence only one half of the surface of the smooth metallic tube and liquid volume is tested.

The thermocouples are placed intentionally and distributed in vertical and horizontal directions relative to the position of the lower edge of the metallic mesh tube. The positions the thermocouples are arranged into is intended for the temperature measurement of the bulk and active regions of the liquid for flow analysis. The planes of the section in which each of the two sets of thermocouples is placed, are displaced 10 mm along the length of the tube to avoid a high concentration of thermocouples in the same section. Zones of potential flow field changes and stagnation zones in the liquids can be detected using temperature readings. The liquid temperature is obtained and processed separately, thus the flow condition below the liquid free surface can be analysed. The analysis is based on respective temperature readings, regardless of the usage of average temperature for further conclusions. Table 1 provides the details of the position of the thermocouples in the liquid.

Results and discussion

The measured temperatures in the liquid, the CTD of the smooth metallic tube, and the thermal images of the liquid free surface region are considered in the analysis of the temperature variations and the influences of the initial level of immersion of the metallic mesh tube in the liquid.

Temperature oscillations are detected in the measurements of temperatures with the thermocouples placed in the liquid and along the surface of the tube. Temperature oscillations are detected prior to the experiment, in the measured temperature of the liquid, and during the development of the flows. Oscillations in temperature measurements have also been reported by Kang et al. [⁷]. Figure 5 illustrates the oscillations found in the temperature measured by the thermocouples placed along the surface of the tube, at all three levels of immersion. The oscillations appear in the measurement, with increasing amplitude, after the rising liquid film flow is triggered, suggesting instabilities in the thin film region. The immersed region of the tube and thick film region present small amplitudes in the oscillations. The oscillation in the temperature measurement can have various causes, including the oscillatory behaviour in the atmospheric conditions affecting the evaporation rate, the surface deformation caused by the thermo-capillary effect [⁷] in the liquid free surface, the instabilities causing oscillatory flow due to the buoyancy and thermo-capillary forces [⁹], the presence of the rising liquid film flow, and any other unknown source of noise. Figure 6 gives the spatial and temporal variations of the smooth-tube wall temperature.

Lower level of immersion

For the lowest level of immersion experimented, or level q1, the highest values of temperature registered in the liquid is given by T6, as shown in Fig. 7. T6 is positioned near the lower edge of the metallic mesh tube, which is in the buoyant flow region. The remaining measured temperatures in the bulk liquid present small variations over time tending to steady conditions. According to the temperature distribution in the bulk liquid, as displayed in Fig. 8, the liquid near the lower edge of the horizontal tube must be flowing upwards towards the free surface. The liquid flows along the immersed surface of the metallic mesh tube, due to buoyant force causing a temperature gradient in that area.

Under the specified circumstances, a high temperature gradient will be present at the liquid free surface area in the vicinity of the tube, which can cause a thermo-capillary (Marangoni) effect in the liquid free surface, accordantly to the position of the heat source (tube), induced by the surface tension gradient. A convective motion can be produced, resulting in a shear stress at the liquid free surface, thus thermo-capillary flow occurs. In this kind of system, the thermo-capillary flow is usually combined with the buoyant flow. This is a result of the inevitable immersion of a portion of the metallic mesh tube in the liquid, even at zero degree of immersion and caused by the wetting phenomenon, where a meniscus is formed. The flow patterns and the temperature field in the bulk liquid characteristic of combined buoyant and thermo-capillary flow should be present in the system. However, the thermal images depicted in Fig. 9 show the evolution of the surface temperature over time, in which a slight and negligible increase in the surface temperature is observed. Therefore, an important conclusion can be drawn where the buoyant flow must be strongly combined with the rising liquid film flow on the upper surface of the tube. In this way, the liquid free surface can have such temperature distribution.

As the velocity of the rising liquid film is dependent on the circumferential temperature difference [¹], it should reach maximum values at levels of immersion in which the circumferential temperature difference is more accentuated. Figure 10 exhibits the average CTD for different levels of immersion. For a low level of immersion, or level q1, the circumferential temperature difference is more accentuated than that for higher levels of immersion, or levels q2 and q3. It decays with the increase of the level of immersion; hence the maximum flow rate should be obtained at level q1. However, the measured average evaporation rate reveals that the maximum evaporation rate can be obtained somewhere between the lowest and higher levels of immersion, given that the heating power of the tube is kept constant. Under tested circumstances, the maximum evaporation rate is obtained at 45° of immersion, with an evaporation rate of 62.5 mL/h. This accents the presence and the influences of the buoyant flow in the rising liquid film flow. The lowest evaporation rate is obtained at 90° of immersion, with an evaporation rate of 40 mL/h. Finally, an evaporation rate of 50 mL/h is obtained at 0° of immersion. A comparison of all the evaporation rates at all levels of immersion indicates that the variation of the evaporation rate with the change of the level of immersion is non-linear. At low heat flux densities, the increment of the level of immersion of the metallic mesh tube has a significant impact on the evaporation rate, thus on the flow rate (rewetting). Oppositely, the decrement of the level of immersion below the lowest tested level, level q1, should augment the evaporation rate as well, because of the increase in the CTD positively affecting the rising liquid film flow by increasing the evaporation surface.

Further research on this particular case is conducted with an experiment, for flow visualization purposes, which is performed under different conditions, but with the same fluid and level of immersion and utilized distilled water mixed with a blue colour dye to observe the fluid flow. The result in Fig. 11 demonstrates that in the presence of rising liquid film flow, with an adequate mass flow rate, it is possible to obtain a fluid flow directed radially towards the immersed surface of the metallic mesh tube. Subsequently, the liquid rises and flows above the liquid free surface level in the form of a liquid film on the surface of the metallic mesh tube.

The result of the latter experiment can explain the low temperature variation on the liquid free surface observed in the former experiment. However, the slight change in the surface temperature, observed in the former experiment, could be caused by the non-uniform heat flux densities along the length of the tube, as shown by the thermal images. Consequently, in some areas the resulted rising liquid film flow might not be sufficiently strong. Changes in the air temperature around the experimental facility, due to the experiment itself, can also be added to the causes for the slight change in temperature on the liquid free surface. Given the temperature distribution and variation, and flow conditions, it is reasonable to assume that the system could reach steady conditions under these circumstances.

Higher levels of immersion

At higher levels of immersion, including levels q2 and q3, the measured temperature in the bulk liquid and liquid free surface, diverges from the ones observed at a lower level of immersion, or level q1, which suggests changes in the flow field. Figure 12 show the time variation of the temperatures measured in the active and bulk regions of the liquid, for immersion levels q2 and q3 respectively. As observed in the lower level of immersion, T6 also has the highest values of temperature measured in the liquid, which belongs to the active region of the liquid near the lower edge region of the tube. Comparing the temperature profile of the three levels of immersion, it is evident that there are some changes in the temperature profile in the buoyant flow region.

Following the evolution of the average value of the oscillating temperature of all three cases, it can be noticed that the temperature profile develops from an apparent parabolic profile to an apparent linear profile. The changes in the temperature profiles highly suggest changes in the flow field. From these results, it can be assumed that the flow conditions change with the increase of the level of immersion.

The temperature measured by T9 and T10 in the liquid show a salient increase occurring approximately 15 min after the start of the experiment at all levels of immersion, which aggravates at higher levels of immersion. This may be caused by some common development of the flows at all levels of immersion, as the experiment period represents the transient state.

Previously, the results obtained for lower levels of immersion show that it is possible to obtain a nearly full combination of the buoyant flow with the rising liquid film flow. Thus, this overcomes the thermo-capillary effects on the liquid free surface that could weaken or disrupt this particular combination. However, for higher levels of immersion, the temperature measurements in the liquid show that the liquid flow may develop differently at distinct levels of immersion, or be susceptible to changes during the course of events. This assumption can be validated by the thermal images, shown in Figs. 13 and 14, in which the thermo-capillary effect induced by the imposed temperature gradient at the liquid free surface, which induces the surface tension gradient, is noticeable by the convection of the fluid along the interface over short period of time. The temperature measured in the bulk liquid along the extension of the liquid free surface, at respective time stamps, substantiate that the hot layer of the fluid is confined at the liquid free surface region (thermal stratification). The DT between the liquid free surface and the bulk liquid is considerable.

In contrast with the first case, a reasonable cause for the occurrence of thermo-capillary flow on the liquid free surface could be the weakening of the rising liquid film flow with the increase of the level of immersion at certain degrees of immersion. In the same sense, the buoyant flow overcomes the rising liquid film flow in magnitude. As the level of immersion increases, the accumulation of heated liquid in the vicinity of the metallic mesh tube increases. The reason for this is that the rising liquid film flow rate is insufficient to remove the majority of the heated liquid volume from that area. Therefore, a rather stronger convective motion due to the thermo-capillary effect will be produced, resulting in a shear stress at the liquid free surface. The thermo-capillary flow produced transports the heat towards the boundaries of the liquid container parallel to the length of the tube.

In the terminal phase of the experiments, it is expected that the combined flows are in an advanced phase of development, whether the system approaches steady conditions or not. Henceforth vortices can be found either in steady conditions or travelling inside the bulk liquid. The highest temperature gradient observed is in the vicinity of the tube and at the liquid free surface. If the average heat flux density on the surface of the metallic tube is kept constant, the magnitude will be dependent on the initial and sub-sequential level of immersion, which defines the outcome of the combination of the flows.

Furthermore, the temperature of the liquid increases in faster rates with the increase of the level of immersion, regardless of the increase in the evaporation rate verified during the experiments.

Another common point between the three levels of immersion is the temperature measured by T7. It remains practically unchanged, suggesting the presence of a stagnation zone in the bulk liquid [⁹], characteristic of combined buoyant and thermo-capillary flow.

Nevertheless, the high temperature gradient in the vicinity of the tube and liquid free surface over time, the bulk liquid average temperature is likely to remain subcooled during the liquid film evaporation at low heat flux density. At this stage, it is important to recall that the rising liquid film flow is present during the whole duration of the experiment at all levels of immersion.

For a relatively low heat flux density, the rising liquid film flow is not triggered instantaneously or during a short period of time. It usually takes about 1 to 3 min for the robust rising liquid film flow to be triggered. The liquid film obtained on the surface of metallic mesh tube is very thin and almost invisible to the naked eye. Figure 5 shows the time variation of the temperature at different circumferential position of the metallic tube. At initial stages, a drastic change in the temperature variation at the thin film region matching the time of the initial robust evaporation is noticed, revealing the formation of the rising liquid film flow.

Different profiles of temperature variation over time are the characteristic of changes in the heat and mass transfer mechanism, thus different forms of convection. The results presented induce the possibility of occurrence of a combined buoyant and thermo-capillary flow in the vicinity of the tube [9], prior to the rising liquid film flow. This combined buoyant and thermo-capillary flow would play an important role in the trigger mechanism of the rising liquid film flow.

Conclusions

Experiments have been conducted to determine the temperature distribution and variations inside a rectangular liquid container filled with distilled water. The influences of the initial level of immersion of the metallic mesh tube in the liquid have been analysed, along with the presence of rising liquid film flow. In order to clarify the influences of the level of immersion, three different levels of immersion have been set and tested. A relatively low average heat flux density has been applied on the surface of the heating horizontal metallic tube. The work is mostly substantiated by temperature measurements and the following conclusions can be drawn:

Oscillations in temperature measurements are detected prior to the rising liquid film flow formation in the bulk and active region of the liquid. Upon the formation of the rising liquid film flow, increasing amplitude in the oscillations of circumferential temperature variation of the metallic tube is observed.

The temperature field changes with the increase of the level of immersion, thus the flow field changes. The thermo-capillary effect on the liquid free surface, that can induce a convective motion of the heated liquid away from the heating tube, can be avoided at a lower level of immersion.

Two different flows patterns, along with the rising liquid film flow, are identified after the formation of rising liquid film flow. One is at a low level of immersion which is due to buoyant forces and forces involved in the rising liquid film flow, where the bulk liquid flow is directed radially towards the immersed part of the metallic mesh tube. The other is at higher levels of immersion, due to buoyant and thermo-capillary forces, where the so called thermo-capillary (Marangoni) flow is being induced on the liquid free surface. The bulk region of the liquid, however, stays in subcooled conditions, while the active regions of the liquid show a high temperature gradient.

The circumferential temperature difference in the rising liquid film region of the tube decays with the increase of the level of immersion, affecting the rising liquid film flow. Nonetheless, the rising liquid film flow is still present in the system.

The oscillatory behaviour in rising liquid film flow shows possible connection of the latter flow to the oscillatory behaviour of buoyant force acting on the immersed region. This is substantiated by the oscillations in measured circumferential temperature variations.

The presence of the combined buoyant and thermo-capillary flow is identified in the formation of the rising liquid film flow. Initially, the combined flow is present in the meniscus formed during the contact between the liquid and the metallic mesh tube.

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