H. Zhang, zhanghn26@mail.sysu.edu.cn; F. Li, lifch6@mail.sysu.edu.cn

Visualization of bubble mechanism of pulsating heat pipe with conventional working fluids and surfactant solution

Durga Basatakoti1, Hongna Zhang1 (), Xiaobin Li1, Weihua Cai2, Fengchen Li1 ()

1. Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-Sen University, Zhuhai 519082, China 2. School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Abstract Visualization experiment is a must in realizing functional characteristics of an operational pulsating heat pipe (PHP). So far there is no general formulation which can foretell the complex and chaotic flow nature for every working fluid. Different response of working fluids and their distinct flow nature as well as their behavior can be visualized which thereby helps to understand the operational mechanism of the PHP. In this experiment, tests were conducted in a transparent PHP with 3 conventional working fluids, viz. de-ionized (DI) water, methanol, ethanol, and 300 ppm cetyltrimethyl ammonium chloride (CTAC) solution, each with fill ratio (FR) of 50%. With the help of high speed camera, flow characteristics at different operational stages for each working fluid are captured. Difference in the generation, growth, movement, and flow direction of bubbles are observed and the consequence of combined effects of various thermal properties of the fluid rather than a dominating single property. Start-up characteristics and dominating flow pattern for each fluid are reported in this paper. Moreover, peculiar flow characteristics with 300 ppm CTAC like bubble detachment, movement of cluster of micro-bubbles, and swirling are also presented.

Keywords pulsating heat pipe (PHP)

aqueous surfactant solution

flow pattern

visualization

Article History Received: 10 April 2019

Revised: 17 May 2019

Accepted: 21 May 2019

Research Article © Tsinghua University Press 2019

1 Introduction

Pulsating heat pipe (PHP) has attracted a lot of interests since the first report of PHP in the early 1960s because of its promising potential in the heat management systems. Abundant investigations can be found where researchers have been working to find ways of enhancing the thermal performance and fully understand the working mechanism of PHP (Han et al., 2016; Bastakoti et al., 2018). Those experimental investigations bring out the difference in thermal performance of PHP with different parameters like geometric designs, working fluids, and operating conditions. However, there is not much explanation to varying responses of PHP to these parameters extracted from those experiments. To this end, visualization experiment is a must to understand the operating characteristics of PHP under different working conditions. It can reveal the underlying working mechanism which can delineate the features of operational PHP.

Several researchers are performing investigations on non-conventional operating fluids, like nano-fluids (Qu et al., 2010; Aly, 2014), refrigerants (Wang and Jia, 2016; Nazari et al., 2018), inert gases (Spinato et al., 2016) and ionic fluids

(Fonseca et al., 2015), surfactant solution (Bastakoti et al., 2018b; Liang et al., 2018), etc., to find novel operating fluids which might enhance the operational performance of PHP. Regarding on surfactant solutions, Liang et al. (2018) conjointly discovered thermal performance of PHP with sodium stearate surfactant chemical agent solutions which have better performance results compared with pure water yielded. Also, Bastakoti et al. (2018b) performed experimental investigation where cetyltrimethyl ammonium chloride, C19H42ClN (CTAC), surfactant chemical agent’s solution was thought as the operating fluid. The lower surface tension of CTAC resolution is believed to be the rationale behind the yielding of better thermal performance of PHP at completely different operational conditions compared to de-ionized (DI) water. However, that experimental set-up lacked the visualization facility (Bastakoti et al., 2018a).

Boiling is an inevitable process of PHP, so investigations on boiling are very obliging to be integrated in the boosting of its performance. Many studies have been carried out and many more are under going to enhance the liquid boiling inside PHP. Addition of surfactants to the base fluid results in decrease of surface tension. The rate of nucleate boiling

Vol. 2, No. 1, 2020, 22–30Experimental and Computational Multiphase Flow https://doi.org/10.1007/s42757-019-0033-y

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heat transfer of water considerably increases even if small amount of surfactants is added. Wu and Yang (1992) in their studies showed how the dynamics of bubble growth in aqueous surfactant solutions of sodium lauryl sulfate is bound to enhance the boiling mechanism due to the lessening surface tension. Another experimental study of Hetsroni et al. (2001) showed that the different bubble behaviors in surfactant solutions at saturated boiling conditions are the reason for better heat transfer performance. Wang et al. (2016) experimentally studied the effect of surfactants on pool boiling which concludes that heat transfer enhancement in surfactant solution is mainly due to the bubble explosion.

As researchers specialize in finding new and effective operating fluids, observing the operational development will certainly facilitate in understanding and more exploring the non-conventional operating fluids. Visualization is a key factor in understanding the flow regimes of two-phase working fluid inside an operational PHP. The most common technique to visualize the operation of PHP is to use the transparent tube. Such test rigs would allow visualizing the insight of flow regime within operational PHP. The start-up characteristics may be understood by precisely observance of the flow regime whereas the temperature fluctuation happens once the heat is supplied within the evaporator region. Likewise, the flow pattern with two-phase interaction, bubble formation, growth and its dissipation, flow direction, and chaotic flow may also be captured by high speed camera.

Results of visualization experiments are fortunate in predicting the flow pattern of two-phase operating fluid within PHP (Kearney et al., 2016; Sun et al., 2017). Such studies are primarily focused on understanding the mechanisms behind the operation of PHP and its thermal characteristics. The heat transfer mechanism in associated operational PHP is thus far well-known. The operation of PHP is principally ruled by the various stages of two-phase flow regimes of the operating fluid as totally different typical stages of flow embrace the generation, growth, dissipation, and slug/annular flow. Liquid-vapor slug-plug flow generally dominates the entire flow. Once the bubbles are produced within the evaporator region, they come in the condenser region in conjunction with liquid column. The flow persists continuous and periodical nature, upon the availability of heat at evaporator, as there exist lower pressure and tem-perature at condenser. These pulsating flow patterns are complicated, and sometimes terribly chaotic, so it is very troublesome to grasp the flow characteristics at varied operating conditions (Shafii et al., 2002). Fumoto et al. (2016) performed visualization for PHP test rig which is consisted of pyex glass PHP of inner diameter of 1.8 mm. Their results suggest that the thermal behavior of the PHP is very smitten by the liquid films and slugs. Within the tube, the performance is dominated by the fluid motions induced by

the heat supply. This experiment is abundant useful in understanding the flow mechanism of associated operational PHP. Likewise, Mangini et al. (2017) utilized high-speed camera and infra-red analysis to comprehend the flow pattern development of PHP while at operation. The bottom heated PHP was conjointly attached with two pressure transducers to seek out the peculiar slug/plug motion. This study reveals the intriguing operational mechanism of liquid film dynamics and paves some way to numerically analyze the performance of PHP at wide ranges of operational conditions. Liang et al. (2016) performed their visualization experiment on PHP with ionic liquids (ILs) as the operating fluid to analyze the start-up characteristics and flow pattern. New correlation was proposed which outlined the form of bubble flow within associated operational PHP at 60% FR from which it may be understood that the bubble dynamics and flow characteristics are not function of single or two fluid properties rather an advanced combination of varied physical and thermodynamic properties. And thus, it may be understood that the responses of PHP are completely different and vary with operational conditions.

In this research, a visualization experimental set-up of PHP is prepared as the continuous study of previously done experiment on copper PHP with CTAC solution (Bastakoti et al., 2018b). This paper reports the immediate results of visualization experiment of PHP with conventional and non-conventional working fluids. Basic start-up and flow characteristics are presented along with the unique bubble dynamics of surfactant solution in an operational PHP. These results are expected to pave a way forward for further revealing of general flow characteristics inside PHP.

2 Experimental set-up and procedure

2.1 Experimental set-up

This experiment used a closed PHP of glass tube (thermal conductivity: 1.2 W/(m·K)) with four turns. The inside and outside diameters of PHP were 2 and 3 mm, respectively, satisfying the criterion suggested by Akachi (1990):

[ ]0.5cr f g2 / ( )d d σ g ρ ρ£ = - (1)

The PHP with glass tube taken in this experiment had height of 18 cm and width of 7 cm with the total eight continuous capillary tubes, the schematic diagram of which is shown in Fig. 1.

Systems for filling the working medium and evacuating, heating and cooling, temperature sensors, data acquisition, and image capturing were integrated to PHP to complete the experimental system. Figure 2 shows the experimental facilities in this work. An extension of glass tube was provided in the upper region to facilitate the filling with syringe and

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Fig. 1 Schematic diagram of PHP used in this experiment (dimension is in cm).

Fig. 2 System of visualization experiment.

evacuate with the pump. Ni–Cr wire with 0.1 cm thickness was winded around the bottom of the PHP for heating, considering it as an evaporator region. Two ends of windings were connected to adjustable DC power supply (ZHAOXIN- RPS-3002D, voltage error ±0.01 V, current error ±0.001 A) which was the heating source. Totally, six thermocouples (K-type, ±0.1 °C) were attached at outside wall of the PHP and the other end of these thermocouples were connected to a Memory Hi-Logger (HIOKI-LR8402-21, multi-channel module) to record the temperature data.

This experiment aims to realize the difference in functioning of PHP with conventional and non-conventional working fluids including DI water, methanol, and ethanol (conventional working fluids), and 300 ppm CTAC solution (non-conventional working fluid). Tables 1 and 2 list the thermo-hydro properties of the related working fluids. Surface tensionmeter (BZY-2, Shanghai Hengping Instrument and Meter Factory, China) based on the Wihelmy Plate Method was used to measure the surface tension of these working fluids. Likewise, rotational rheometer (Kinexus Pro, Malvern instruments, UK) was used to measure the rheological properties. Moreover, the heat transfer pro-perties were measured by an instrument which employed transient hotwire method (TC3100L, Xi’an Xiatech Electronic Technology Co. Ltd., China) which has a measuring range

Table 1 Thermo-physical properties of DI water, methanol, and ethanol

Property DI water Methanol Ethanol

Boiling point Ts (°C) 100 64.7 78.3

Liquid density ρl at 25 °C (kg/m3 ) 996.5 788 787

Liquid specific heat Cpl at 25 °C (kJ/(kg·°C)) 4.180 2.48 2.39

Thermal conductivity λl at 25 °C (W/(m·°C)) 0.61 0.203 0.169

Latent heat of vaporization Hfg (kJ/kg) 2257 1101 846

(dP/dTsat) × 103 at 80 °C (Pa/°C) 1.92 6.45 4.23

Dynamic viscosity υl at 25 °C (10−3Pa·s) 0.87 0.58 1.12

Surface tension σ at 25 °C (mN/m) 72.8 22.3 22.6

Critical tube diameter Dmax at 25 °C (mm) 5 3.2 3.1

Table 2 Thermo-physical properties of 300 ppm CTAC

Concentration of CTAC solution

Dynamic viscosity

(Pa·s)

Surface tension (mN/m)

Thermal conductivity (W/(m·K))

Thermal diffusivity

(m2/s)

300 ppm 1.59 × 10−3 45.61 0.612 1.024 × 10−7

of 0.0005–5.0 W/(m·K), a resolution of 0.0005 W/(m·K), and an accuracy of ±2%.

2.2 Experimental procedures

Figure 3 shows the flow chart of the experimental process. Before the operation of PHP, it was exhausted to absolute pressure 90 kPa with the assistance of vacuum pump. Operating fluids with the corresponding volume were charged through the charging tube and syringe into the PHP once lowering the pressure within the PHP. The pipe was sealed and ensured that there was no leakage by endlessly checking the gauge for about 8 hours. Heating within the evaporator region was managed by winding many turns of Ni–Cr wire, whose resistance was calculated by the digital ohmmeter. Voltage and current regulated from the DC power were used to regulate the specified heat input. The operating fluids were filled into the PHP individually and also the PHP was cleansed by continuously drying and evacuating before changing the operating fluids. For all operating fluids, the experiments were conducted at 50% fill ratio (FR).

A common and simple technique to observe the flow patterns directly in an operating PHP is to perform the experiment on a glass-tube PHP, where the visualization technique can be very handy. Assisted by high-speed camera, the sizes, distributions, and movement of bubbles can be captured and known via image processing. To capture the flow behavior inside the operational PHP, a high-speed camera was equipped and arranged to capture the gas–liquid two-phase change from the beginning of functioning of the PHP to the stable operation stage. The image with 1280 ×

Visualization of bubble mechanism of pulsating heat pipe with conventional working fluids and surfactant solution

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Fig. 3 Flow chart of experimental process.

1024 resolution was taken into concern with the shooting speed 400 frames per second with the time interval of 25 ms. Moreover, to get high quality images, high power illuminating light bulb was placed behind the PHP. Through analyzing the images captured by high-speed camera, the movement and formation of bubbles, evaporation, and condensation were recorded and analyzed.

3 Results and discussion

This experiment aims to find the characteristics of the dominating flow pattern, various stages of bubble formation, generation and movement in each case, especially in the CTAC solution. It is also anticipated to get the reason behind the unusual thermal response of PHP with CTAC solution. For each test, temperature recordings were done for each case to make sure that dry-out condition supplemented with temperature shoot that may damage the experimental system is not met. The overall thermal performance of this PHP is not considered because of the lack of adiabatic region, low thermal conductivity of glass and uncovered evaporator region. The tests were piloted at lower heat input (20 W) to medium heat input (50 W). Check valve at the filling connection is opened before heating the evaporator region, and it is observed that working fluids rushed into the PHP from the syringe due to the lower pressure inside the PHP. As the surface tension force dominated over the

gravitational force, the working fluids indiscriminately distribute itself as liquid slugs and vapor plugs. Dominating flow pattern for each case was considered to be the one which persisted for much more time once the pseudo-stable condition is attained.

From all the cases of working fluids in the operational PHP, it is understood the formation of liquid slugs interposed with vapor bubbles is under the direct influence of surface tension. Also, the evaporation process which took place immediately after supplying heat to the evaporator region augmented the vapor pressure and thus induced the growth of bubbles and successively pushed the liquids towards the condenser. On the other hand, the lower temperature of condenser reduced the vapor pressure which affected the bubble dynamics by promoting the condensation of vapor. From this set of visualization experiment, it is now established that this course is continuous as long as the heat is supplied in the evaporator and temperature at condenser is kept at lower temperature. This cyclic phenomenon prompts the oscillating motion of two phases of working fluids. Subsequently, with the vapor phase, the heat is transferred at the expense of latent heat and with liquid slugs, sensible heat is transported. This concludes the observation of fundamental working mechanism of PHP which is very much worthwhile in determining the heat transfer mechanism inside PHP which is mainly accompanied by bubble’s characteristics like its generation, growth, and movement. The following sections discuss the different phases of bubbles like generation (formation), growth (development), and movement (dissipation) for the individual cases of working fluids.

3.1 Start-up state and dominating flow pattern

Due to the subsequent lower vapor pressure, nucleate boiling occurred at lower temperature of around 50 °C after the application of heat in the evaporator region of PHP. Once the thin film of liquid received enough heat from the heated wall, vapor bubbles started to form. As a large amount of vapor bubbles are formed and the increase in vapor pressure is adequate to push the upper liquid regime, which triggers the start-up phenomenon in PHP. The variation in pressure ( v lP P- ) occurring across the liquid–vapor interface is believed to drive the fluids in motion, which is mostly dependent on the surface tension ( σ ) of the working fluids (Qu and Ma, 2007), given as

v lglobe

2σP Pr

- = (2)

Equation (2) implies that the driving force for the bubbles to move in oscillation across the hotter and the colder regions within the PHP is mainly due to fluctuation in vapor pressure,

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which also affects the bubble size. During the experiments, the occurrence of start-up is clearly witnessed with clear variation in bubble sizes for fluids of different values of surface tension. However, as the outside wall of the glass cannot offer the precise temperature inside the glass tube in the considered set-up, the start-up is perceived as the two- phase flow starts to rapidly oscillate within the PHP in this paper. Heating is turned off once the temperature of wall reaches on the far side of 120 °C.

The start-up time was comparable for DI water and CTAC surfactant solution cases, and similar for cases of methanol and ethanol. Start-up time was in the variation of 60–70 s for DI water and CTAC surfactant solution cases, and shorter by 10–20 s for alcohols. The different start-up time may be settled by comparing the boiling point based on the values provided in Table 1 which is the dominating properties of the process. 300 ppm CTAC surfactant solution has preserved this property to certain extent. However, there exists major variance within the bubble formation manner close to the evaporator region and the nature of bubble movement as well. Different thermo-physical pro-perties such as surface tension, latent heat of phase change, and others listed in Tables 1 and 2 seem to control these phenomena. The flow of methanol and ethanol within the pipe was more rapidly oscillating compared to other operating fluids at this stage, implying that higher value of alteration in pressure and temperature at saturation condition occurs. This promoted speedy movement of the bubbles among the capillary; however, there was not any dissipation of bubbles ascertained. It is very difficult to isolate the effect of each

thermodynamic property of the operating fluids at this stage.

Random and chaotic flow was ascertained for every case after reaching the quasi-stable stage. The flow direction cannot be anticipated before attaining of this stage. Figure 4 shows a schematic diagram of typical flow directions for CTAC surfactant solution case. Here, the flow direction is changing along the time, though in unpredictable pattern, making the flow inside the PHP chaotic. This is mainly due to the transient variation in the pressure difference of two phases of working fluid prompted upon by evaporation and con-densation associated with phase change. Also, it is notable that the densities of these phases differ as there occurs continuous and rapid heating and cooling of the fluid. The flow direction is extremely unpredictable even when it attains the pulsating motion. The continual pulsating motion ceases for a short while before it regains the pattern however with alter in direction. The more pulsating nature is preserved, the more efficient heat transfer is expected.

When the PHP is operated for enough time to realize the quasi-steady state, outstanding dominating flow nature is ascertained. It is considerably troublesome to predict or outline the flow pattern for the total operational period of PHP since there exists advanced and complicated flow nature in two-phase flow within a tube. At an equivalent operating condition of 50% FR and 50 W heat input, four completely different operating fluids viz. DI water, methanol, ethanol, and 300 ppm CTAC surfactant solution yielded completely different start-up characteristics. It is learnt that the flow nature is completely different for various operating

Fig. 4 Schematic diagram of flow direction of 50% FR CTAC at 50 W after start-up at (a) 0.525 s, (b) 0.632 s, (c) 0.866 s, (d) 0.963 s, (e) 1.12 s, and (f) 1.43 s.

Visualization of bubble mechanism of pulsating heat pipe with conventional working fluids and surfactant solution

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fluids at an equivalent operational condition. The flow nature of DI water was largely dominated by the annular flow related to the liquid and vapor phases. At time, slug flow was additionally noticed. In methanol case, separated bubbly flow is found in plentiful quantity and also the size of bubbles is smaller compared to those in the cases with other conventional operating fluids. Similarly, once ethanol was used, the slug flow was dominating the entire flow field. The distributions of bubbles within the condenser region for all the standard fluids and CTAC solution often seen are shown in Fig. 5. The variation of bubble size for different fluids is principally due to the surface tension of those operating fluids. Surface tension of those fluids is in decreasing order for DI water, 300 ppm CTAC surfactant solution, methanol, and ethanol (as shown in Tables 1 and 2), and so is the order of the dominating bubble sizes for those fluids. Similarly, small bubbles dominate PHP with methanol. Likewise, medium sized bubbles are found in abundant amount with the case of PHP with ethanol. However, it is very different for CTAC solution case, as not a single category was dominating; rather all three sizes of bubbles were found to be distributed along the capillary. A glimpse of state of bubble distribution for each case can be observed in Fig. 6 that represents the dominating flow pattern at similar working conditions. The observation of bubble sizes with these fluids is also supported by Eq. (2), which clarifies the direct relation between radius of globe and surface tension.

Fig. 5 Flow pattern at condenser region at 40 W of heat for (a) DI water, (b) methanol, (c) ethanol, and (d) CTAC solution (each image for four fluids are of one of the turns of condenser section).

Methanol and ethanol had quite stable distribution of bubble size but for DI water and CTAC solution cases, the distribution varied with time as the heat was continuously supplied from the bottom wall. For DI water and CTAC solution cases, the smaller bubbles were generated from the capillary wall near the evaporator region and they retained their shapes while moving up in the cooler region.

3.2 Peculiar characteristics of PHP with CTAC

3.2.1 Detachment and movement of bubble clusters

Although bubble dynamics and thermal performance of the boiling within the capillary system are substantially tough to measure, the behavior of bubble detachment can reflect the important information, as this is often the point where the flow nature of the PHP starts to get the form for the complete flow field throughout the operation time. Figure 7 shows the bubble detachment within the evaporator region of the PHP with DI water. Bubble detachment phenomenon is quite rare with alcohols. Not a lot of con-tinuous bubble forming and dissipation is discovered in the cases of methanol and ethanol. The bubbles shaped at the first stage are largely maintained within the same range and size while the PHP is continuously operated. Compared with that in DI water case, different behaviors of the bubble detachment are observed in CTAC solution case. Micro- bubbles are formed and detached from the region near to evaporator and then move upwards to the condenser region where they again form a cluster as shown in Fig. 8. It is thought that this phenomenon is key factor in defining the flow nature and bubble distribution within the capillary of PHP with CTAC solution, and it is also the reason behind the nature of dominating flow regime with CTAC solution case, which has a high potential in giving out better thermal response of PHP.

After detaching of the bubble clusters, completely different phenomena and nature of flow pattern compared with DI water are developed and witnessed in CTAC surfactant case, which is the movement of bubbles within the cluster as shown in Fig. 9. In fact, in CTAC solution, clusters of micro-bubbles tend to be formed before the operation due to the low pressure, right away when filling. During the filling process, clusters enter the condenser region and descend all the way down to evaporator once again and those small bubbles coalesce to make cluster of comparatively larger bubbles later. Therefore, it is greatly expected that in contrast to other cases of non-conventional operating fluids, the heat is transferred in bulk quantity of bubble clusters in PHP with CTAC surfactant solution. This can be extremely doubtless to support the heat dissipation from the bottom hotter region to upper colder region.

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Fig. 7 Bubble detachment at the evaporator of PHP with DI water (images at time interval of x + 0, 0.02, 0.04, 0.12, 0.18, 0.22, and 0.24 s).

Fig. 8 Time series of bubble detachment in PHP with 300 ppm CTAC solution (images at time interval of x + 0, 0.08, 0.18, 0.28, 0.32, 0.40, 0.50. 0.58, 0.66, 0.74, 0.82, and 0.92 s).

Fig. 9 Time series of location of bubble clusters in PHP with CTAC solution (order: from top left to bottom right, images at the time interval of x + 0, 0.1, 0.2, and 0.3 s).

3.2.2 Swirling motion of bubbles in CTAC solution

In addition, unique development of indications of bubble swirling was also observed in the case of CTAC surfactant solution, where new vapor bubbles were formed close to the evaporator region and attached among themselves before

Fig. 6 Dominating flow patterns at 40 W of heat for (a) DI water, (b) methanol, (c) ethanol, and (d) CTAC solution.

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they apparently began to swirl. Figure 10 shows a temporal series of this peculiar development of bubble swirling in CTAC solution. At this stage of investigation, the images from the front side of PHP can only indicate the signs of probable swirling. It is highly likely that the swirl motion accumulates with the continual supply of heat and the bubbles start to detach at certain time and dissipate into the upper region. Consequently, the sizes of the formed bubbles show very tiny compared to that in DI water case. It has been reported that swirling motion is helpful to the heat transfer performance. Bergles (1985) suggested that swirl flow device is effective in actively inducing coiling swirl flow to enhance heat transfer. There are also researchers who work on to search for different strategies, such as by using twisted tube within circular tubes of thermal management devices, to induce swirl flow motion and thereby enhance heat transfer. This concept implies that swirling (whirling) motion of bubbles, found within the case of PHP with CTAC solution, can be one in every of the key factors to yield higher thermal performance of PHP. However, the swirling phenomenon on PHP can only be confirmed by the images from the three dimensions and its validation can be done with the assistance of more cameras to capture 3D images.

4 Conclusions

Through visualization experiments, the distinction in operational mechanism is quite clearly detected from this visualization experiment of PHP with conventional (DI water, methanol, and ethanol) and non-conventional operating fluids (300 ppm CTAC surfactant solution). The following phenomena are observed.

The sizes of the bubbles and their flow characteristics are guaranteed to alter the thermal performance of PHP which is confirmed by the images captured by high speed camera. The flow is extremely dominated by the annular flow and it supports the efficient transfer of heat from the evaporator region in the operating PHP with DI water. In methanol and ethanol cases, supported by lower values of surface tensions than that of DI water, it provides rise to smaller sizes of bubbles along with early bubble movement due to lower boiling point. And, credited to high dP/dTsat

Fig. 10 Swirl and dissipation of bubbles in CTAC solution case (images at the time interval of x + 0, 0.02, 0.04, 0.06, 0.08, and 0.1 s).

of methanol and ethanol, bubbles in PHP with these fluids move more rapidly. Quite distinctive flow characteristics are observed in operating PHP with CTAC surfactant solution. The swirling motion of bubbles is possibly the cause behind the efficient transfer of heat from the evaporator region. Moreover, the movement of micro-bubble cluster within the condenser region is confirmed as the special feature of PHP with CTAC surfactant solution.

As a further exploration, an experimental set-up with operational PHP which will realize the velocity of flow field is for certain to reveal additional explanations to the various characteristics of flow field. Also, to offer out additional accurate thermal response, laser penetration technique to insert the thermocouples into the glass PHP is recommended after commencing this visualization experiment.

Acknowledgements

This paper is supported by the National Natural Science Foundation of China (Grant Nos. 51576051, 51606054, and 51776057). Zhang would like to thank the financial support of “Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education (ARES-2018-01)”.

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