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Heat transfer enhancement in a three-phase closed thermosyphon
Applied Thermal Engineering 65 (2014) 495e501
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Heat transfer enhancement in a three-phase closed thermosyphon Feng Jiang*, Wen-jing Chen, Ze Liu, Jin-tao Shi, Xiu-lun Li School of Chemical Engineering, Tianjin University, Tianjin 300072, PR China
h i g h l i g h t s Three-phase fluidized bed technology and the TPCT were combined. Performances of the THPCT were investigated and compared to the TPCT. Effects of filling ratio and solid holdup on heat transfer performance were examined. The new THPCT improves evaporator heat transfer coefficient up to 28.5%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 May 2013 Accepted 21 January 2014 Available online 31 January 2014
Taking advantage of the heat transfer enhancing effect of a three-phase flow, a three-phase closed thermosyphon (THPCT) was designed and built to improve the heat transfer performance of the conventional two-phase closed thermosyphon (TPCT). Experimental investigations on the effect of various parameters on the thermal performance of the THPCT were carried out. The results were compared with those of the TPCT, focusing on the evaporation heat transfer coefficient and overall thermal resistance. Filling ratio (FR), solid holdup (εs), and input power were considered. The THPCT with 20% carbon fiberreinforced nylon 66/water showed superior thermal performance compared with the TPCT with pure water. Evaporation heat transfer coefficient was improved by up to 28.5%, and overall thermal resistance was reduced by 11% with FR ¼ 55%, εs ¼ 50%, and an input power of 1000 W. The overall thermal resistance decreased as the solid holdup increased. For tested FR ¼ 45%, 55%, and 65%, an optimal filling ratio (i.e., 55% in this study) is required for the THPCT to obtain the maximum heat transfer enhancing effect. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Heat transfer performance Three-phase closed thermosyphon 20% PA66 Heat transfer enhancement
1. Introduction Two-phase closed thermosyphons (TPCTs) have received considerable attention in many industrial and energy applications, such as in heat exchangers, electronics cooling, chemical engineering, power generation, and solar heating systems. The most important advantages of thermosyphons are high effective thermal conductivity, simple structure, and low fabrication cost. A TPCT can be divided into three distinct sections, namely, the evaporator, adiabatic, and condenser sections. A thermosyphon tube consists of an evacuated sealed tube that contains a small amount of liquid. Heat is supplied through the evaporator section, the liquid in which reaches its boiling temperature and begins to boil. The vapor is under higher pressure in the evaporator section than in the condenser section, causing the vapor to flow upward. The vapor condenses in the condenser section, releasing the latent
* Corresponding author. Tel.: þ86 22 87401722. E-mail address: [email protected] (F. Jiang). 1359-4311/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2014.01.043
heat absorbed in the evaporator section. The condensate is then returned by gravity. No external driving force is required. The thermal performance of a TPCT is affected by several key factors, namely, aspect ratio, filling ratio, inclination angle, input power, and working fluid. These factors have been examined by numerous studies [1e8]. Numerous investigations that aim to develop novel methods for enhancing the thermal performance of working fluids have been conducted in the past few decades. One of these methods involves the use of a new kind of working fluid (see Refs. [3,6,7,9,10] ). Ong et al. [6] used R-134a as working fluid in a thermosyphon and stated that the heat flux transfer increased with the increase in coolant mass flow rate, fill ratio, and temperature difference between the bath and the condenser sections. Savino et al. [9] investigated the thermal performance of heat pipes charged with different working fluids and observed that water/alcohol binary mixtures showed superior thermal performance compared with pure water. Another effective method is the incorporation of particulate solids with high thermal conductivity, such as nanoparticles [11e18], into the base liquid. Noie et al. [12] used Al2O3/
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water nanofluid of various volume concentrations (1%e3%) in a TPCT. Their results showed that for different input powers, the efficiency of the thermosyphon filled with nanofluid increased by up to 14.7% compared with that of the water thermosyphon. Huminic et al. [15,16] utilized iron oxide nanoparticles with mass concentrations of 0%, 2%, and 5.3% in a thermosyphon heat pipe. They also investigated the effects of inclination angle, operating temperature, and concentration level of nanoparticles on the heat transfer characteristics of the thermosyphon. Their experimental results showed that the heat transfer coefficient increased with the increase in inclination angle, whereas the thermal resistance decreased with the increase in nanoparticle concentration. Shanbedi et al. [17] studied the effect of adding multiwalled carbon nanotubes functionalized with ethylenediamine to deionized water on the heat transfer performance of a TPCT. Their results indicated that when the input powers were <90 W, and the weight concentrations were >1%, the thermal efficiency was maximized. The distribution of the wall temperature of a thermosyphon was found to be the key parameter in calculating thermal resistance. However, the thermal performance of a thermosyphon using nanofluids may greatly differ depending on the type and the preparation method of nanofluids. For instance, Leong et al. [19] found that 23% overall heat transfer enhancement is observed when the hot air velocity increases from 2.0 m/s to 4.75 m/s for water based (7%) alumina and (4%) titanium dioxide nanofluids. But Khandekar et al. [13] investigated the overall thermal resistance of closed two-phase thermosyphon using pure water and various water based nanofluids (of Al2O3, CuO and laponite clay) as working fluids. They observed that all these nanofluids show inferior thermal performance than pure water. In addition, the complex preparation of nanofluids and their lack of stability in the long run are
the key issues preventing their commercial application [20]. Recently, a promising heat-transfer enhancing and self-cleaning technology has emerged, that is, the three-phase (vaporeliquide solid) fluidized bed technology. This technology has been successfully applied in evaporators, boilers, and preheaters [21,22] because of its capability to enhance heat transfer. In the current work, this new technology is combined with a TPCT to obtain a three-phase closed thermosyphon (THPCT), which has not been reported in the literature. Appropriate inert solid particles are added to the liquid pool in the evaporator section of the TPCT. The low-density and small-sized particles are fluidized because of liquid vaporization during the process. The shear stress and collision between the fluidized particles and the heat transfer wall have a significant effect on the thermal efficiency of a thermosyphon. In a THPCT, the thermophysical properties of the liquid working fluid along with the physical characteristics of the inert solid particles play an important role in heat transfer enhancement. In the present study, 20% PA66 is employed because of its good abrasion resistance, low density, and high thermal conductivity. The thermal performance of the THPCT filled with 20% PA66/water at different filling ratios (FR) and solid holdups (εs) is investigated. The results are compared with the thermal performance of the TPCT charged with pure water at various input powers. 2. Experiments 2.1. Experimental apparatus To study the heat transfer performance of the THPCT, a special experimental system was designed (Fig. 1). This experimental system, which can also be used for thermal performance measurement
Fig. 1. Schematic of experimental apparatus.
F. Jiang et al. / Applied Thermal Engineering 65 (2014) 495e501 Table 1 Experimental configuration.
Table 2 Physical characteristic of particles.
Thermosyphon container
Carbon steel
Thermosyphon length Evaporator length Condenser length Adiabatic length Outer diameter Wall thickness
1635[mm] 665[mm] 735[mm] 235[mm] 42.3[mm] 3.5[mm]
of TPCTs, consisted of a vertical gravity thermosyphon, a data acquisition system, and a vacuum pumping unit. The thermosyphon was made of carbon steel tube with an outer diameter of 42.3 mm and a length of 1635 mm. All the other geometric parameters for the thermosyphon are presented in Table 1. The heating method involved direct heating via the resistance wire around the evaporator section. The condenser section was cooled by water circulating in a cooling jacket. The thermosyphon was insulated by insulation cotton to prevent heat loss. Fifteen pairs of copper-constantan thermocouples, i.e., seven pairs at the evaporator section, two at the adiabatic section, and six at the condenser section, were used to measure the local temperature. The thermocouples were mounted symmetrically on both sides of the THPCT wall (Fig. 2). One of the thermocouples in the adiabatic section was inserted into the pipe to measure the temperature of the adiabatic steam. A pressure transducer was installed at the top of the condenser section to measure the system operating pressure. The inlet and outlet temperature from the condenser section and the volume flow rate of the cooling water were also measured.
2.2. Particle characteristics Particle characteristics are the basic factors affecting the fluidization of particles in the three-phase flow. Before carrying out the experiments, the sedimentation velocities of particles (listed in Table 2.) that may be related to the fluidization of particles were estimated. The sedimentation velocity was estimated as follows:
K ¼ de
rðrs rÞg m2
1=3 (1)
where de is the equivalent diameter of particles, r is the density of liquid, rs is the density of particles, m is the viscosity of liquid. When the value of K is between 2.62 and 69.1, the sedimentation velocity (ut) can be calculated by Eq. (2)
ut ¼ 0:154
1:4
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gd1:6 e ðrs rÞ
r0:4 m0:6
497
(2)
When the value of K is >69.1 (under the experimental conditions, the value of K was not <2.62.), the sedimentation velocity can be calculated by Eq. (3)
Particle
Equivalent diameter (mm)
Density (kg m3)
Thermal conductivity (W m1 C1)
20% PA66 (cylindrical) POM (spherical) Resin (cylindrical)
V2.5 V3.15 V2.5
1020 1390 1270
0.79 0.23 0.1
de ðrs rÞg 1=2 ut ¼ 1:74
r
(3)
The estimation of sedimentation velocity of different particles can provide a relative value, irrespective of the interaction between particles and the wall effect. The calculated sedimentation velocity of polyoxymethylene (POM) is larger than that of Resin, while the sedimentation velocity of 20% PA66 is the minimum one. Through the analyses of preliminary testing and the comprehensive consideration of sedimentation velocity, particle conductivity, and particle shape, 20% PA66 was used in the subsequent experiments. 2.3. Experimental procedure In this study, the experiments were designed to be in the allowable operational range of a two-phase closed thermosyphon. Thermosyphon dry out does not occur for a setup with a filling ratio above 40% [23]. The tested filling ratios were 45%, 55%, and 65%. Dry out did not appear in the TPCT and the THPCT, according to the experiments. The supplied heat flux values were 11.3 kW/m2, 17 kW/m2, 22.6 kW/m2 and 28.3 kW/m2. 20% PA66 particles (the physical characteristics of which are presented in Table 2) were used in this study. The experiment was initiated by adding a certain amount of the liquid working fluid and 20% PA66 to the THPCT. Then the experiments were carried out at four input powers: 1000 W, 1500 W, 2000 W and 2500 W. The exterior wall surface temperature was recorded after steady state was confirmed at each level of input power (each experiment was evacuated to the same degree of vacuum from 99 kPa to 100 kPa). All the tests were performed at a constant coolant volume flow rate. Two cases, i.e., for particles in water with εs ¼ 20% and 50%, were examined. The results were compared with those of the TPCT. The heat rejected to the cooling water can be calculated by Eq. (4) as follows:
Q ¼ Wcp ðto ti Þ;
(4)
where to is the outlet temperature, and ti is the inlet temperature of the cooling water. In the absence of any heat loss from the pipe to the surroundings, the amount of heat absorbed in the evaporator section is equal to the amount of heat rejected at the condenser. Thus, the heat transfer coefficients at the evaporator section can be calculated by Eq. (5) as follows:
a ¼
Q
pdi LðTe Ts Þ
Fig. 2. Location of thermocouples.
;
(5)
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where Te is the interior wall temperature of the evaporation section and the value can be obtained using the steady conduction equation of the one-dimensional cylindrical wall, as illustrated by Eq. (6).
Q lnðdo =di Þ Te ¼ T 2plL
(6)
The overall thermal resistance can be determined by the following equation:
R ¼
Te Tc : Q
(7)
Furthermore, enhancement factor (E) was adopted to evaluate the heat transfer enhancement efficiency of the THPCT. This factor is defined as
E ¼
avls ; avl
(8)
where avls and avl are the evaporation heat transfer coefficients of the THPCT and TPCT, respectively. 2.4. Experimental uncertainties Uncertainty of the experimental data may have resulted from measuring errors of parameters such as wall temperature of thermosyphon, inlet and outlet temperature of cooling water, volume flow rate. The uncertainty in temperature measurements was 0.1 C. The flow of cooling water was 87 L/h with an uncertainty of
4.6% in the measurement of results. The derived uncertainty was calculated from:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n 2 X v4 1u ¼ t Dxi 4 4 i ¼ 1 vxi
D4
(9)
where 4 is the derived parameter and x1, x2,., xn are the measured variables. The maximum uncertainty of heat rejected at the condenser was calculated at 4.9%. In addition, the maximum uncertainty in the evaporator heat transfer coefficients was estimated to 4.8% while the maximum uncertainty in the overall thermal resistances is 5.3%. The maximum experimental uncertainty value is acceptable in engineering applications. 3. Experimental results and discussions 3.1. Evaporation heat transfer coefficient Fig. 3 presents the evaporation heat transfer coefficient versus the input power for different solid holdups and filling ratios. The filling ratio changes from 45% to 65% with a 10% increase per step. The evaporation heat transfer coefficient increases as input power increases in both the THPCT and the TPCT. Compared with that for the case of the TPCT, the evaporation heat transfer coefficient for the case of the THPCT remarkably increases for all operating parameters under the different conditions mentioned above. The reason for this phenomenon can be explained as follows. The degree of superheat in the heating wall surface increases as input power increases. A large number of nucleation sites, which
Fig. 3. (aec) Evaporation heat transfer coefficient for different filling ratios and solid holdups.
F. Jiang et al. / Applied Thermal Engineering 65 (2014) 495e501
mainly contribute to the intensive pool boiling, are obtained because of a high degree of superheat. Furthermore, an increasing amount of particles are fluidized because of boiling, thereby promoting collision and shearing of the fluidized particles and heat transfer wall, respectively. These behaviors significantly destroy the boundary layer of flow and heat transfer, leading to the enhancement effect of heat transfer. Therefore, an increase in the input power causes the evaporation heat transfer coefficient to rise considerably. As shown in Fig. 3, the evaporation heat transfer coefficients of the THPCT with εs ¼ 50% are not always higher than those of the THPCT with εs ¼ 20%. At relatively high filling ratios (FR ¼ 55%, 65%), the evaporation heat transfer coefficients for εs ¼ 50% is higher than that for εs ¼ 20%, but this is reversed at FR ¼ 45%. Thus, the effect of solid holdup on the evaporation heat transfer coefficients is not independent of filling ratio and input power. This conclusion can be supported by the fact that at a low filling ratio, the rate of water vaporization is relatively low. Therefore, the amount of generated bubble is small and insufficient to drive a large number of particles away from the heating surface. In addition, the degree of fluidization in the THPCT with εs ¼ 50% is lower than that with εs ¼ 20% at FR ¼ 45%, which has a direct impact on the evaporation heat transfer coefficients. The above analysis shows that an optimal operating condition is required to achieve a high evaporation heat transfer coefficient. The enhancement factors (E) of the THPCT with an input power of 1000 W at three filling ratios mentioned above are illustrated in Fig. 4. The highest enhancement factors are obtained for the middle filling ratio of 55%. The same phenomenon can be observed at other input powers. At FR ¼ 55%, the presence of 50% particles in water results in an increase of evaporation heat transfer coefficient of up to 28.5%; for 20% particles in water, the evaporation heat transfer coefficient increases by up to 24.3%. The effect of the filling ratio on the enhancement factor can be explained as follows. First, at a low filling ratio, the immersed area of the internal wall surface and the amount of generated bubble are both small. Therefore, the fluidized particles cannot be driven fully, and the enhancement effect is not obvious. However, the resistance of bubbles escaping from the liquid pool increases as filling ratio increases because of the increments of depth of the liquid pool within the evaporator section. Consequently, the enhancement effect is reduced. Therefore, an optimal filling ratio is required for the THPCT (55% in the present study). For tested parameters, the optimal heat transfer
Fig. 4. Enhancement factor for different filling ratios.
499
Fig. 5. The overall thermal resistance versus input power at different solid holdups.
enhancement effect of the evaporator section is obtained at FR ¼ 55%, εs ¼ 50%, and an input power of 1000 W. 3.2. Overall thermal resistance The influence of different solid holdup levels on the thermosyphon heat transfer resistance is shown in Fig. 5. Increasing the input power causes a decrease in the overall thermal resistance. The results of this study demonstrate that with FR ¼ 55%, the heat transfer resistance of the THPCT is always less than that of the TPCT irrespective of the solid holdup and input power. The comparison of thermal resistance between the THPCT filled with 20% and 50% particles reveals that on the one hand, an increase in the solid holdup results in a decrease in the overall thermal resistance, thus proving an enhanced performance. The maximum reduction rate of the thermal resistance of the THPCT with εs ¼ 50% and an input power of 1000 W is 11%. As for the THPCT with εs ¼ 20% and an input power of 1000 W, the reduction rate of its thermal resistance is 7.2%. On the other hand, an increase in the solid holdup causes a decrease in the average temperature of the evaporator wall, which has a direct relationship with thermal resistance. A steady state distribution of the average temperature of the internal wall of the evaporator section at different input powers is shown in Fig. 6. Increasing solid holdup decreases the average temperature of the evaporator wall. A sharp drop in wall temperature is observed in the THPCT containing 50% particles and with an input power of 1000 W. Based on Eq. (7), the average temperature of the evaporator plays an indispensable role in the calculation of the overall thermal resistance. However, the average temperature of the evaporator section is significantly dependent on the solid holdup. The reason for the reduction of thermal resistance can be explained as follows. A major cause of thermal resistance is the formation of a vapor bubble at the liquidesolid interface. A larger bubble nucleation size creates a higher thermal resistance that prevents the transfer of heat from the solid surface to the liquid [24]. The particles tend to bombard the vapor bubble during bubble formation, and this action can substantially reduce the nucleation size of the vapor bubble. As depicted in Figs. 3e6, the addition of inert particles is conducive to achieving high evaporation heat transfer coefficients and reducing overall thermal resistance. An optimal operating condition is achieved for tested parameters in the study. The THPCT
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F. Jiang et al. / Applied Thermal Engineering 65 (2014) 495e501
(c) Increasing the solid holdup decreased the overall thermal resistance and the average temperature of the evaporator, thus ensuring an enhanced performance. The results indicate that 20% PA66 can be potentially applied in many industries as a solid working medium for THPCTs of high thermal performance. Although further investigations are needed, this study is the first one to make a substantial breakthrough in the subject of the association between the TPCT and the three-phase (vaporeliquidesolid) fluidized bed technique in considerably improving heat transfer efficiency by establishing a THPCT system. Acknowledgements This work was supported by the Municipal Science and Technology Commission of Tianjin, China under Contract No. 2009ZCKFGX01900. Fig. 6. Average temperature of evaporator versus input power for different solid holdups.
loaded with 20% PA66/water with FR ¼ 55%, εs ¼ 50%, and an input power of 1000 W obtains the highest factor of evaporator heat transfer enhancement (1.285) and the maximum reduction rate of the overall thermal resistance (11%). In the THPCT, The main heat transfer enhancement mechanism is destroying the boundary layer of flow and heat transfer by motion of particles. And the degree of particle fluidization and impact of collision forces and wall shear are both important. Small sized, low density particles can be easily fluidized. However, the larger particles could have the higher potential to contact and attack the boundary layer around the heater surface by contacting it, since the larger particles can have the larger inertia force during fluidization [22]. This issue is complicated and needs further research. In the present study, basing on theoretical analysis and preliminary testing, 20% PA66 was used to verify heat transfer enhancing effect of three-phase closed thermosyphon. Particle size is one of the factors affecting the heat transfer enhancement effect of three-phase flow. The others including particle density, conductivity and shape are of equal importance. These factors have a joint influence on the heat transfer performance of three-phase flow [25]. An investigation of these factors could also provide information regarding bubble behavior. Systematical study on the role of particle size and other characteristics in the heat transfer enhancement of three-phase flow are currently under consideration. 4. Conclusions This study designed and tested a THPCT to study the effects of filling ratio, solid holdup, and input power on heat transfer performance. The results are as follows. (a) The THPCT with 20% PA66/water demonstrated an obvious heat transfer enhancement effect under different operation parameters tested in the research. It improved the evaporation heat transfer coefficient by up to 28.5% and reduced the overall thermal resistance by 11% with filling ratio of 55%, solid holdup of 50% and an input power of 1000 W. (b) The filling ratio was found to have a significant impact on the evaporator heat transfer coefficient. Evaporator heat transfer coefficients increased with solid holdup at relatively high filling ratios (55%, 65%). For tested filling ratios in the present study, the optimal filling ratio is 55%.
Nomenclature cp specific heat of cooling water [J kg1 C1] de equivalent diameter of particles [m] di internal diameter of thermosyphon [m] do outer diameter of thermosyphon [m] E enhancement factor FR filling ratio g gravitational acceleration [m s2] K dimensionless number L length of evaporator section [m] Q heat transfer rate [W] R overall thermal resistance [ C W1] ti inlet temperature of cooling water [ C] to outlet temperature of cooling water [ C] T average temperature on external surface of evaporator [ C] Tc temperature on internal surface of condenser [ C] Te temperature on internal surface of evaporator [ C] Ts temperature of saturated vapor [ C] ut sedimentation velocity [m s1] W mass flow rate of cooling water [kg s1] Greek letters r density of liquid [kg m3] rs density of particles [kg m3] m viscosity of liquid [Pa s] a evaporation heat transfer coefficient, W m2 C1 avls evaporation heat transfer coefficient of THPCT, W m2 C1 avl evaporation heat transfer coefficient of THCT, W m2 C1 l thermal conductivity of tube wall, W m1 C1 εs solid holdup References [1] M.H.M. Grooten, C.W.M. van der Geld, Predicting heat transfer in long R-134a filled thermosyphons, J. Heat Transf. 131 (5) (2009) 1e9. [2] B. Jiao, L.M. Qiu, X.B. Zhang, Y. Zhang, Investigation on the effect of filling ratio on the steady-state heat transfer performance of a vertical two-phase closed thermosyphon, Appl. Therm. Eng. 28 (11e12) (2008) 1417e1426. [3] H. Jouhara, A.J. Robinson, Experimental investigation of small diameter twophase closed thermosyphons charged with water, FC-84, FC-77 and FC3283, Appl. Therm. Eng. 30 (2e3) (2010) 201e211. [4] I. Khazaee, R. Hosseini, A. Kianifar, S.H. Noie, Experimental consideration and correlation of heat transfer of a two-phase closed thermosyphon due to the inclination angle, filling ratio, and aspect ratio, J. Enhanced Heat Transf. 18 (1) (2011) 31e40. [5] S.H. Noie, Heat transfer characteristics of a two-phase closed thermosyphon, Appl. Therm. Eng. 25 (4) (2005) 495e506.
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Heat transfer enhancement in a three-phase closed thermosyphon Feng Jiang*, Wen-jing Chen, Ze Liu, Jin-tao Shi, Xiu-lun Li School of Chemical Engineering, Tianjin University, Tianjin 300072, PR China
h i g h l i g h t s Three-phase fluidized bed technology and the TPCT were combined. Performances of the THPCT were investigated and compared to the TPCT. Effects of filling ratio and solid holdup on heat transfer performance were examined. The new THPCT improves evaporator heat transfer coefficient up to 28.5%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 May 2013 Accepted 21 January 2014 Available online 31 January 2014
Taking advantage of the heat transfer enhancing effect of a three-phase flow, a three-phase closed thermosyphon (THPCT) was designed and built to improve the heat transfer performance of the conventional two-phase closed thermosyphon (TPCT). Experimental investigations on the effect of various parameters on the thermal performance of the THPCT were carried out. The results were compared with those of the TPCT, focusing on the evaporation heat transfer coefficient and overall thermal resistance. Filling ratio (FR), solid holdup (εs), and input power were considered. The THPCT with 20% carbon fiberreinforced nylon 66/water showed superior thermal performance compared with the TPCT with pure water. Evaporation heat transfer coefficient was improved by up to 28.5%, and overall thermal resistance was reduced by 11% with FR ¼ 55%, εs ¼ 50%, and an input power of 1000 W. The overall thermal resistance decreased as the solid holdup increased. For tested FR ¼ 45%, 55%, and 65%, an optimal filling ratio (i.e., 55% in this study) is required for the THPCT to obtain the maximum heat transfer enhancing effect. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Heat transfer performance Three-phase closed thermosyphon 20% PA66 Heat transfer enhancement
1. Introduction Two-phase closed thermosyphons (TPCTs) have received considerable attention in many industrial and energy applications, such as in heat exchangers, electronics cooling, chemical engineering, power generation, and solar heating systems. The most important advantages of thermosyphons are high effective thermal conductivity, simple structure, and low fabrication cost. A TPCT can be divided into three distinct sections, namely, the evaporator, adiabatic, and condenser sections. A thermosyphon tube consists of an evacuated sealed tube that contains a small amount of liquid. Heat is supplied through the evaporator section, the liquid in which reaches its boiling temperature and begins to boil. The vapor is under higher pressure in the evaporator section than in the condenser section, causing the vapor to flow upward. The vapor condenses in the condenser section, releasing the latent
* Corresponding author. Tel.: þ86 22 87401722. E-mail address: [email protected] (F. Jiang). 1359-4311/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2014.01.043
heat absorbed in the evaporator section. The condensate is then returned by gravity. No external driving force is required. The thermal performance of a TPCT is affected by several key factors, namely, aspect ratio, filling ratio, inclination angle, input power, and working fluid. These factors have been examined by numerous studies [1e8]. Numerous investigations that aim to develop novel methods for enhancing the thermal performance of working fluids have been conducted in the past few decades. One of these methods involves the use of a new kind of working fluid (see Refs. [3,6,7,9,10] ). Ong et al. [6] used R-134a as working fluid in a thermosyphon and stated that the heat flux transfer increased with the increase in coolant mass flow rate, fill ratio, and temperature difference between the bath and the condenser sections. Savino et al. [9] investigated the thermal performance of heat pipes charged with different working fluids and observed that water/alcohol binary mixtures showed superior thermal performance compared with pure water. Another effective method is the incorporation of particulate solids with high thermal conductivity, such as nanoparticles [11e18], into the base liquid. Noie et al. [12] used Al2O3/
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water nanofluid of various volume concentrations (1%e3%) in a TPCT. Their results showed that for different input powers, the efficiency of the thermosyphon filled with nanofluid increased by up to 14.7% compared with that of the water thermosyphon. Huminic et al. [15,16] utilized iron oxide nanoparticles with mass concentrations of 0%, 2%, and 5.3% in a thermosyphon heat pipe. They also investigated the effects of inclination angle, operating temperature, and concentration level of nanoparticles on the heat transfer characteristics of the thermosyphon. Their experimental results showed that the heat transfer coefficient increased with the increase in inclination angle, whereas the thermal resistance decreased with the increase in nanoparticle concentration. Shanbedi et al. [17] studied the effect of adding multiwalled carbon nanotubes functionalized with ethylenediamine to deionized water on the heat transfer performance of a TPCT. Their results indicated that when the input powers were <90 W, and the weight concentrations were >1%, the thermal efficiency was maximized. The distribution of the wall temperature of a thermosyphon was found to be the key parameter in calculating thermal resistance. However, the thermal performance of a thermosyphon using nanofluids may greatly differ depending on the type and the preparation method of nanofluids. For instance, Leong et al. [19] found that 23% overall heat transfer enhancement is observed when the hot air velocity increases from 2.0 m/s to 4.75 m/s for water based (7%) alumina and (4%) titanium dioxide nanofluids. But Khandekar et al. [13] investigated the overall thermal resistance of closed two-phase thermosyphon using pure water and various water based nanofluids (of Al2O3, CuO and laponite clay) as working fluids. They observed that all these nanofluids show inferior thermal performance than pure water. In addition, the complex preparation of nanofluids and their lack of stability in the long run are
the key issues preventing their commercial application [20]. Recently, a promising heat-transfer enhancing and self-cleaning technology has emerged, that is, the three-phase (vaporeliquide solid) fluidized bed technology. This technology has been successfully applied in evaporators, boilers, and preheaters [21,22] because of its capability to enhance heat transfer. In the current work, this new technology is combined with a TPCT to obtain a three-phase closed thermosyphon (THPCT), which has not been reported in the literature. Appropriate inert solid particles are added to the liquid pool in the evaporator section of the TPCT. The low-density and small-sized particles are fluidized because of liquid vaporization during the process. The shear stress and collision between the fluidized particles and the heat transfer wall have a significant effect on the thermal efficiency of a thermosyphon. In a THPCT, the thermophysical properties of the liquid working fluid along with the physical characteristics of the inert solid particles play an important role in heat transfer enhancement. In the present study, 20% PA66 is employed because of its good abrasion resistance, low density, and high thermal conductivity. The thermal performance of the THPCT filled with 20% PA66/water at different filling ratios (FR) and solid holdups (εs) is investigated. The results are compared with the thermal performance of the TPCT charged with pure water at various input powers. 2. Experiments 2.1. Experimental apparatus To study the heat transfer performance of the THPCT, a special experimental system was designed (Fig. 1). This experimental system, which can also be used for thermal performance measurement
Fig. 1. Schematic of experimental apparatus.
F. Jiang et al. / Applied Thermal Engineering 65 (2014) 495e501 Table 1 Experimental configuration.
Table 2 Physical characteristic of particles.
Thermosyphon container
Carbon steel
Thermosyphon length Evaporator length Condenser length Adiabatic length Outer diameter Wall thickness
1635[mm] 665[mm] 735[mm] 235[mm] 42.3[mm] 3.5[mm]
of TPCTs, consisted of a vertical gravity thermosyphon, a data acquisition system, and a vacuum pumping unit. The thermosyphon was made of carbon steel tube with an outer diameter of 42.3 mm and a length of 1635 mm. All the other geometric parameters for the thermosyphon are presented in Table 1. The heating method involved direct heating via the resistance wire around the evaporator section. The condenser section was cooled by water circulating in a cooling jacket. The thermosyphon was insulated by insulation cotton to prevent heat loss. Fifteen pairs of copper-constantan thermocouples, i.e., seven pairs at the evaporator section, two at the adiabatic section, and six at the condenser section, were used to measure the local temperature. The thermocouples were mounted symmetrically on both sides of the THPCT wall (Fig. 2). One of the thermocouples in the adiabatic section was inserted into the pipe to measure the temperature of the adiabatic steam. A pressure transducer was installed at the top of the condenser section to measure the system operating pressure. The inlet and outlet temperature from the condenser section and the volume flow rate of the cooling water were also measured.
2.2. Particle characteristics Particle characteristics are the basic factors affecting the fluidization of particles in the three-phase flow. Before carrying out the experiments, the sedimentation velocities of particles (listed in Table 2.) that may be related to the fluidization of particles were estimated. The sedimentation velocity was estimated as follows:
K ¼ de
rðrs rÞg m2
1=3 (1)
where de is the equivalent diameter of particles, r is the density of liquid, rs is the density of particles, m is the viscosity of liquid. When the value of K is between 2.62 and 69.1, the sedimentation velocity (ut) can be calculated by Eq. (2)
ut ¼ 0:154
1:4
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gd1:6 e ðrs rÞ
r0:4 m0:6
497
(2)
When the value of K is >69.1 (under the experimental conditions, the value of K was not <2.62.), the sedimentation velocity can be calculated by Eq. (3)
Particle
Equivalent diameter (mm)
Density (kg m3)
Thermal conductivity (W m1 C1)
20% PA66 (cylindrical) POM (spherical) Resin (cylindrical)
V2.5 V3.15 V2.5
1020 1390 1270
0.79 0.23 0.1
de ðrs rÞg 1=2 ut ¼ 1:74
r
(3)
The estimation of sedimentation velocity of different particles can provide a relative value, irrespective of the interaction between particles and the wall effect. The calculated sedimentation velocity of polyoxymethylene (POM) is larger than that of Resin, while the sedimentation velocity of 20% PA66 is the minimum one. Through the analyses of preliminary testing and the comprehensive consideration of sedimentation velocity, particle conductivity, and particle shape, 20% PA66 was used in the subsequent experiments. 2.3. Experimental procedure In this study, the experiments were designed to be in the allowable operational range of a two-phase closed thermosyphon. Thermosyphon dry out does not occur for a setup with a filling ratio above 40% [23]. The tested filling ratios were 45%, 55%, and 65%. Dry out did not appear in the TPCT and the THPCT, according to the experiments. The supplied heat flux values were 11.3 kW/m2, 17 kW/m2, 22.6 kW/m2 and 28.3 kW/m2. 20% PA66 particles (the physical characteristics of which are presented in Table 2) were used in this study. The experiment was initiated by adding a certain amount of the liquid working fluid and 20% PA66 to the THPCT. Then the experiments were carried out at four input powers: 1000 W, 1500 W, 2000 W and 2500 W. The exterior wall surface temperature was recorded after steady state was confirmed at each level of input power (each experiment was evacuated to the same degree of vacuum from 99 kPa to 100 kPa). All the tests were performed at a constant coolant volume flow rate. Two cases, i.e., for particles in water with εs ¼ 20% and 50%, were examined. The results were compared with those of the TPCT. The heat rejected to the cooling water can be calculated by Eq. (4) as follows:
Q ¼ Wcp ðto ti Þ;
(4)
where to is the outlet temperature, and ti is the inlet temperature of the cooling water. In the absence of any heat loss from the pipe to the surroundings, the amount of heat absorbed in the evaporator section is equal to the amount of heat rejected at the condenser. Thus, the heat transfer coefficients at the evaporator section can be calculated by Eq. (5) as follows:
a ¼
Q
pdi LðTe Ts Þ
Fig. 2. Location of thermocouples.
;
(5)
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where Te is the interior wall temperature of the evaporation section and the value can be obtained using the steady conduction equation of the one-dimensional cylindrical wall, as illustrated by Eq. (6).
Q lnðdo =di Þ Te ¼ T 2plL
(6)
The overall thermal resistance can be determined by the following equation:
R ¼
Te Tc : Q
(7)
Furthermore, enhancement factor (E) was adopted to evaluate the heat transfer enhancement efficiency of the THPCT. This factor is defined as
E ¼
avls ; avl
(8)
where avls and avl are the evaporation heat transfer coefficients of the THPCT and TPCT, respectively. 2.4. Experimental uncertainties Uncertainty of the experimental data may have resulted from measuring errors of parameters such as wall temperature of thermosyphon, inlet and outlet temperature of cooling water, volume flow rate. The uncertainty in temperature measurements was 0.1 C. The flow of cooling water was 87 L/h with an uncertainty of
4.6% in the measurement of results. The derived uncertainty was calculated from:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n 2 X v4 1u ¼ t Dxi 4 4 i ¼ 1 vxi
D4
(9)
where 4 is the derived parameter and x1, x2,., xn are the measured variables. The maximum uncertainty of heat rejected at the condenser was calculated at 4.9%. In addition, the maximum uncertainty in the evaporator heat transfer coefficients was estimated to 4.8% while the maximum uncertainty in the overall thermal resistances is 5.3%. The maximum experimental uncertainty value is acceptable in engineering applications. 3. Experimental results and discussions 3.1. Evaporation heat transfer coefficient Fig. 3 presents the evaporation heat transfer coefficient versus the input power for different solid holdups and filling ratios. The filling ratio changes from 45% to 65% with a 10% increase per step. The evaporation heat transfer coefficient increases as input power increases in both the THPCT and the TPCT. Compared with that for the case of the TPCT, the evaporation heat transfer coefficient for the case of the THPCT remarkably increases for all operating parameters under the different conditions mentioned above. The reason for this phenomenon can be explained as follows. The degree of superheat in the heating wall surface increases as input power increases. A large number of nucleation sites, which
Fig. 3. (aec) Evaporation heat transfer coefficient for different filling ratios and solid holdups.
F. Jiang et al. / Applied Thermal Engineering 65 (2014) 495e501
mainly contribute to the intensive pool boiling, are obtained because of a high degree of superheat. Furthermore, an increasing amount of particles are fluidized because of boiling, thereby promoting collision and shearing of the fluidized particles and heat transfer wall, respectively. These behaviors significantly destroy the boundary layer of flow and heat transfer, leading to the enhancement effect of heat transfer. Therefore, an increase in the input power causes the evaporation heat transfer coefficient to rise considerably. As shown in Fig. 3, the evaporation heat transfer coefficients of the THPCT with εs ¼ 50% are not always higher than those of the THPCT with εs ¼ 20%. At relatively high filling ratios (FR ¼ 55%, 65%), the evaporation heat transfer coefficients for εs ¼ 50% is higher than that for εs ¼ 20%, but this is reversed at FR ¼ 45%. Thus, the effect of solid holdup on the evaporation heat transfer coefficients is not independent of filling ratio and input power. This conclusion can be supported by the fact that at a low filling ratio, the rate of water vaporization is relatively low. Therefore, the amount of generated bubble is small and insufficient to drive a large number of particles away from the heating surface. In addition, the degree of fluidization in the THPCT with εs ¼ 50% is lower than that with εs ¼ 20% at FR ¼ 45%, which has a direct impact on the evaporation heat transfer coefficients. The above analysis shows that an optimal operating condition is required to achieve a high evaporation heat transfer coefficient. The enhancement factors (E) of the THPCT with an input power of 1000 W at three filling ratios mentioned above are illustrated in Fig. 4. The highest enhancement factors are obtained for the middle filling ratio of 55%. The same phenomenon can be observed at other input powers. At FR ¼ 55%, the presence of 50% particles in water results in an increase of evaporation heat transfer coefficient of up to 28.5%; for 20% particles in water, the evaporation heat transfer coefficient increases by up to 24.3%. The effect of the filling ratio on the enhancement factor can be explained as follows. First, at a low filling ratio, the immersed area of the internal wall surface and the amount of generated bubble are both small. Therefore, the fluidized particles cannot be driven fully, and the enhancement effect is not obvious. However, the resistance of bubbles escaping from the liquid pool increases as filling ratio increases because of the increments of depth of the liquid pool within the evaporator section. Consequently, the enhancement effect is reduced. Therefore, an optimal filling ratio is required for the THPCT (55% in the present study). For tested parameters, the optimal heat transfer
Fig. 4. Enhancement factor for different filling ratios.
499
Fig. 5. The overall thermal resistance versus input power at different solid holdups.
enhancement effect of the evaporator section is obtained at FR ¼ 55%, εs ¼ 50%, and an input power of 1000 W. 3.2. Overall thermal resistance The influence of different solid holdup levels on the thermosyphon heat transfer resistance is shown in Fig. 5. Increasing the input power causes a decrease in the overall thermal resistance. The results of this study demonstrate that with FR ¼ 55%, the heat transfer resistance of the THPCT is always less than that of the TPCT irrespective of the solid holdup and input power. The comparison of thermal resistance between the THPCT filled with 20% and 50% particles reveals that on the one hand, an increase in the solid holdup results in a decrease in the overall thermal resistance, thus proving an enhanced performance. The maximum reduction rate of the thermal resistance of the THPCT with εs ¼ 50% and an input power of 1000 W is 11%. As for the THPCT with εs ¼ 20% and an input power of 1000 W, the reduction rate of its thermal resistance is 7.2%. On the other hand, an increase in the solid holdup causes a decrease in the average temperature of the evaporator wall, which has a direct relationship with thermal resistance. A steady state distribution of the average temperature of the internal wall of the evaporator section at different input powers is shown in Fig. 6. Increasing solid holdup decreases the average temperature of the evaporator wall. A sharp drop in wall temperature is observed in the THPCT containing 50% particles and with an input power of 1000 W. Based on Eq. (7), the average temperature of the evaporator plays an indispensable role in the calculation of the overall thermal resistance. However, the average temperature of the evaporator section is significantly dependent on the solid holdup. The reason for the reduction of thermal resistance can be explained as follows. A major cause of thermal resistance is the formation of a vapor bubble at the liquidesolid interface. A larger bubble nucleation size creates a higher thermal resistance that prevents the transfer of heat from the solid surface to the liquid [24]. The particles tend to bombard the vapor bubble during bubble formation, and this action can substantially reduce the nucleation size of the vapor bubble. As depicted in Figs. 3e6, the addition of inert particles is conducive to achieving high evaporation heat transfer coefficients and reducing overall thermal resistance. An optimal operating condition is achieved for tested parameters in the study. The THPCT
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(c) Increasing the solid holdup decreased the overall thermal resistance and the average temperature of the evaporator, thus ensuring an enhanced performance. The results indicate that 20% PA66 can be potentially applied in many industries as a solid working medium for THPCTs of high thermal performance. Although further investigations are needed, this study is the first one to make a substantial breakthrough in the subject of the association between the TPCT and the three-phase (vaporeliquidesolid) fluidized bed technique in considerably improving heat transfer efficiency by establishing a THPCT system. Acknowledgements This work was supported by the Municipal Science and Technology Commission of Tianjin, China under Contract No. 2009ZCKFGX01900. Fig. 6. Average temperature of evaporator versus input power for different solid holdups.
loaded with 20% PA66/water with FR ¼ 55%, εs ¼ 50%, and an input power of 1000 W obtains the highest factor of evaporator heat transfer enhancement (1.285) and the maximum reduction rate of the overall thermal resistance (11%). In the THPCT, The main heat transfer enhancement mechanism is destroying the boundary layer of flow and heat transfer by motion of particles. And the degree of particle fluidization and impact of collision forces and wall shear are both important. Small sized, low density particles can be easily fluidized. However, the larger particles could have the higher potential to contact and attack the boundary layer around the heater surface by contacting it, since the larger particles can have the larger inertia force during fluidization [22]. This issue is complicated and needs further research. In the present study, basing on theoretical analysis and preliminary testing, 20% PA66 was used to verify heat transfer enhancing effect of three-phase closed thermosyphon. Particle size is one of the factors affecting the heat transfer enhancement effect of three-phase flow. The others including particle density, conductivity and shape are of equal importance. These factors have a joint influence on the heat transfer performance of three-phase flow [25]. An investigation of these factors could also provide information regarding bubble behavior. Systematical study on the role of particle size and other characteristics in the heat transfer enhancement of three-phase flow are currently under consideration. 4. Conclusions This study designed and tested a THPCT to study the effects of filling ratio, solid holdup, and input power on heat transfer performance. The results are as follows. (a) The THPCT with 20% PA66/water demonstrated an obvious heat transfer enhancement effect under different operation parameters tested in the research. It improved the evaporation heat transfer coefficient by up to 28.5% and reduced the overall thermal resistance by 11% with filling ratio of 55%, solid holdup of 50% and an input power of 1000 W. (b) The filling ratio was found to have a significant impact on the evaporator heat transfer coefficient. Evaporator heat transfer coefficients increased with solid holdup at relatively high filling ratios (55%, 65%). For tested filling ratios in the present study, the optimal filling ratio is 55%.
Nomenclature cp specific heat of cooling water [J kg1 C1] de equivalent diameter of particles [m] di internal diameter of thermosyphon [m] do outer diameter of thermosyphon [m] E enhancement factor FR filling ratio g gravitational acceleration [m s2] K dimensionless number L length of evaporator section [m] Q heat transfer rate [W] R overall thermal resistance [ C W1] ti inlet temperature of cooling water [ C] to outlet temperature of cooling water [ C] T average temperature on external surface of evaporator [ C] Tc temperature on internal surface of condenser [ C] Te temperature on internal surface of evaporator [ C] Ts temperature of saturated vapor [ C] ut sedimentation velocity [m s1] W mass flow rate of cooling water [kg s1] Greek letters r density of liquid [kg m3] rs density of particles [kg m3] m viscosity of liquid [Pa s] a evaporation heat transfer coefficient, W m2 C1 avls evaporation heat transfer coefficient of THPCT, W m2 C1 avl evaporation heat transfer coefficient of THCT, W m2 C1 l thermal conductivity of tube wall, W m1 C1 εs solid holdup References [1] M.H.M. Grooten, C.W.M. van der Geld, Predicting heat transfer in long R-134a filled thermosyphons, J. Heat Transf. 131 (5) (2009) 1e9. [2] B. Jiao, L.M. Qiu, X.B. Zhang, Y. Zhang, Investigation on the effect of filling ratio on the steady-state heat transfer performance of a vertical two-phase closed thermosyphon, Appl. Therm. Eng. 28 (11e12) (2008) 1417e1426. [3] H. Jouhara, A.J. Robinson, Experimental investigation of small diameter twophase closed thermosyphons charged with water, FC-84, FC-77 and FC3283, Appl. Therm. Eng. 30 (2e3) (2010) 201e211. [4] I. Khazaee, R. Hosseini, A. Kianifar, S.H. Noie, Experimental consideration and correlation of heat transfer of a two-phase closed thermosyphon due to the inclination angle, filling ratio, and aspect ratio, J. Enhanced Heat Transf. 18 (1) (2011) 31e40. [5] S.H. Noie, Heat transfer characteristics of a two-phase closed thermosyphon, Appl. Therm. Eng. 25 (4) (2005) 495e506.
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