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Operational characteristics of oscillating heat pipe with long heat transport distance for solar energy application
Experimental Thermal and Fluid Science 98 (2018) 137–145
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Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Operational characteristics of oscillating heat pipe with long heat transport distance for solar energy application Jiateng Zhao, Wei Jiang, Zhonghao Rao
T
⁎
School of Electrical and Power Engineering, China University of Mining and Technology, Xuzhou 221116 China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oscillating heat pipe Thermal performance Long-distance heat transport Pressure spectrum
In order to further understand the operational characteristic of long heat transport distance oscillation heat pipe (OHP) for solar energy application, an experimental investigation on the thermal performance of OHP with higher initial vacuum and pressure fluctuation behavior inside under two kinds of inner diameters, two kinds of working media, and different inclination angles and filling ratios (FR) were carried out. The main result shows that the OHPs exhibit excellent heat transfer performance. The effective thermal conductivity is about two orders of magnitude larger than that in the previous work and some other researchers’ work. The start-up temperature of the OHP with 3 mm inner diameter increases from 45 °C to 58 °C with the growing of FR from 30% to 70% when injected with SRWF and placed vertically. The start-up temperature is inversely related to the inclination angle under the same condition. The working status of the OHP with 3 mm inner diameter and SRWF is hardly effected by the inner pressure. However, the effect is contrary for OHP with critical inner diameter. It can be inferred that the movement law of the liquid medium in the OHP is random and chaotic through the pressure frequency spectrum analysis.
1. Introduction Oscillation or pulsation heat pipe (OHP or PHP) is being widely investigated by many researchers all over the world since it was first introduced in the 1990s [1,2]. Like other types of heat pipes, it has been studied in many application fields, such as the electronic device cooling [3–5], industrial heat exchange [6,7] and other areas [8,9]. Solar energy is clean and abundant and can play a significant role in the battle of energy-saving and emission-reduction if it can be well developed [10–12]. The application investigation of OHP in the solar thermal energy field has been explored by some researchers due to its great potential for heat transfer and structural flexibility. Rittidech et al. [13,14] designed two kinds of solar collector based on closed-loop OHP with check valve and closed-end OHP, respectively. In the both works, the detailed dimension of the solar collectors were given and the focus is on the thermal efficiency of the collectors. The heat transfer performance characteristic and potential of those OHPs is not presented. Kargarsharifabad et al. [15] proposed a flat plate solar collector based on closed-loop OHP, the structure of which is similar to the former. Many parameters were investigated and the results indicated that the optimal filling ratio and inclination angle was 0.3 and 20°, respectively. Similarly, the detailed thermal performance of OHPs is not provided. Xian et al. [16] also designed two kinds of solar collector respectively
⁎
Corresponding author. E-mail address: [email protected] (Z. Rao).
https://doi.org/10.1016/j.expthermflusci.2018.05.026 Received 12 February 2018; Received in revised form 23 April 2018; Accepted 28 May 2018 Available online 29 May 2018 0894-1777/ © 2018 Elsevier Inc. All rights reserved.
based on tube OHP and flat plate OHP. The heat transfer rate and thermal conductivity for those two types were compared under different filling ratios and inclination angles. The OHPs show good heat transfer performance and the largest effective thermal conductivity is above 35,000 W/(m °C). However, the movement law of the internal working fluids was not investigated to further understand the working mechanism of OHP. Xu et al. [17] proposed a newly solar collector that combines the OHP with compound parabolic concentrator and their research view is mainly concerned on the performance of collectors in actual conditions. The results implies that the absorber can work well with a thermal resistance of about 0.26 °C/ W. In the previous work [18], aiming to face large-scale heat transfer applications, such as solar energy utilization and industrial waste heat recovery, the thermal performance of a OHP with 4 mm inner diameter (DI) at critical range and long heat transport distance under different conditions including different working medium, condensation capability and filling ratios were investigated experimentally. As a continuation of the previous work, the further study on thermal performance and potential of long heat transport distance OHP with different DIs under different inclination angles, filling ratios (FRs) heat inputs and higher initial vacuum were carried out. The start-up characteristic, pressure fluctuation behavior, pressure frequency spectrum under different operation conditions were analyzed in this paper.
Experimental Thermal and Fluid Science 98 (2018) 137–145
J. Zhao et al.
Nomenclature R ṁ ΔT Q̇ k α p
s e c w eff
thermal resistance [°C W−1] flow rate[kg/s] temperature difference [°C ] heat transport [W] thermal conductivity [W m−1 °C−1] inclination angle[°] pressure [Pa]
Acronyms EF DI FR SRWF OHP
Subscripts ave adia
average adiabatic
2. Experiment set up and procedure
Q̇ = Cpṁ ΔTw
(1)
t
ΔTw =
∫t12 (T18 (t )−T17 (t )) dt (2)
(t2−t1)
where ṁ and ΔTw are the mass flow rate and the time-averaged temperature difference of the circulating water for a thermal equilibrium period of time, respectively. The thermal resistance (ROHP ) could be calculated using the following formulas (3)–(5) [19,20]:
ROHP = ΔTohp/ Q̇ = (Te −Tc )/ Q̇
(3)
Relevant Parameters and conditions
Liquid Injection System Pressure Sensor
Low-constant Temperature Bath
enhancement factor inner diameter filling ratio self-rewetting fluid oscillating heat pipe
different views, different evaluation parameters were derived from the original data, including the time-averaged temperature difference between the evaporation and condensation section (considering the temperature only), the thermal resistance (considering the heat transported further), the effective thermal conductivity (considering the heat transport distance further) and the enhanced factor (for direct comparison). The heat (Q̇ ) transferred by OHP can be calculated through formula (1) and (2), respectively.
Two long heat transport distance oscillating heat pipes with the DI of 3 mm and 4 mm were fabricated and the corresponding testing platforms were established, respectively. Fig. 1 shows the photograph of experimental platform configuration, relevant parameters and operational conditions. The structure and dimension of the OHPs can be found clearly. The heating system, cooling system and data acquisition system are similar to those in the previous work. The measuring frequency of the data acquisition instrument is 0.333 Hz. Fig. 2 presents the schematic of temperature measuring points, which includes eight measuring points in the evaporation section (T1 ∼ T8), eight in the condensation section (T9 ∼ T16), and each one in the entrance and exit of circulating water box (T17, T18), respectively. Deionized Water (DW) and Hept. DW Sol. 0.1 w% (a kind of self-rewetting fluid, SRWF) are employed as working medium, and the FR varies among 30%, 40%, 50%, 60% and 70%. The inclination angles (α ) varies among 0°, 30°, 60° and 90°. All the experiments under different conditions were performed long enough to obtain suitable data in the period of thermal equilibrium. In order to evaluate to thermal performance of the OHP from
DC Power
substrate material evaporation condensation water effective
Thermal transport distance
Heating Wire
Working fluid
Deionized Water (DW), Hept. DW Sol. 0.1w%
Tube material
Copper
Inner Diameter (DI)
3, 4[mm]
Evaporator Height
200[mm]
Condenser Height
150[mm]
Adiabatic Height
640[mm]
Width
480[mm]
Filling ratio (FR)
30%, 40%, 50%, 60%, 70%
Inclination angle ( )
0 , 30 , 60 , 90
Temperature/rate of cooling Water
15 , 3.5 mL/s
Fig. 1. The photograph of experimental platform and relevant parameters and conditions. 138
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keff =
Pressure Transducer
T16
T15
T14
EF =
T17
T18 T1
T10 T11
T12
T13
(6)
keff ks
(7)
where Ladia is the heat transfer length (length of adiabatic section), Do is the outer diameter of OHP, n is the number of parallel tube. ks is the thermal conductivity of the substrate material (copper in this paper). The uncertainty analysis has been taken into account and the uncertainties of the indirect measurement parameters, like heat output, temperature difference, and thermal resistance were obtained according to error propagation principle. The error propagation law and results of uncertainty analysis can be seen in the previous work [18].
Circulating Water
Adiabatic Section
Ladia
4Ladia Q̇ 2 n·π·Do (Te −Tc )
3. Results and discussion
Heating Wire
T 1 T2 T3
T4 T5 T6
In the medium or low temperature solar thermal utilization, copperwater OHP meets the actual needs. Water is cheap, stable and safe and has excellent thermodynamic performance, which is suitable as working medium. The SRWF is based on water whose property is modified by adding high carbon alcohols. It has lower surface tension which can reduce the capillary resistance and is beneficial to the movement of work fluid. The property of work fluid determines the range of the DI. In the previous study, the DI range for these two work fluid were listed. Many researches [21,22] indicate that the OHP mainly depends on the sensitive heat transfer of the working fluid. The larger DI can increase the quality of the work fluid, so as to improve the theoretical heat transfer limit which meets the demand of large-scale heat transfer utilization. Therefore, in order to improve the heat transfer potential and explore the heat transfer characteristic of OHP with near-critical DI at the same time, OHP with 3 mm DI and 4 mm DI were studied, respectively. Firstly, the long heat transport distance OHPs with 3 mm DI and 4 mm DI filled with SRWF at 50% FR were investigated comparatively. Subsequently, the OHP with 3 mm DI injected with SRWF at 50% FR were tested under the inclination angle of 90°, 60°, 30° and 0° in turn. Then, placed vertically, the OHP with 3 mm DI injected with SRWF and water under different FRs were studied, respectively. At last the influence of DI, FR, inclination angle on the start-up temperature, pressure fluctuation inside and overall thermal performance of OHP were discussed and analyzed comprehensively.
T7 T8
Fig. 2. The schematic of measuring points. t
Te =
∫t12 ( 51 ∑i8= 1 Ti (t ) ) dt (t2−t1)
(4)
t
Tc =
T (t ) ) dt ∫t12 ( 51 ∑16 i=9 i (t2−t1)
(5)
where Te and Tc are the time-averaged temperature between evaporation section and condensation section, respectively. i is the number of thermocouple. Effective thermal conductivity (keff ) and the EF of OHP could be calculated using the following two formulas.
Fig. 3. Temperature variation of OHP and cooling water under different operation conditions. 139
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J. Zhao et al.
and they will return to the evaporation section spontaneously. Larger driving force is needed to maintain the internal oscillation. In brief, it can be concluded from above that the 3 mm-DI long-distance heat transport OHP filled with SRWF have better thermal performance under larger inclination angle and proper FR. Table 1 summaries the working status of the OHP under all different operation conditions, including the start-up temperature, the working stability status and Δ Tave in the stable condition concisely. Those conditions not presented in Fig. 3 can be found in this table. It can be reflected that thermal crisis is easy to occur even at low heat input when the inclination angle or the FR is lower. For instance, as shown in the second row, the OHP cannot work stably and turns to thermal crisis at the heat input of 500 W when the FR is as low as 30%. The OHP can be stable at large range of heat input if the FR and the angle are suitable. The start-up temperature of the OHP filled with SRWF and placed vertically increases from 45 °C to 58 °C with the growing of FR from 30% to 70%, which shows that along with the increase of FR pressure inside the OHP after rejection also increases. It also can be observed that the start-up temperature is inversely related to the inclination angle under the same working fluid, DI and FR. At 50% FR and 3 mm DI, the start-up temperature under the inclination angel of 90°, 60° and 30° is 50 °C , 55 °C and 60 °C , respectively. The maximum heat input only reaches 1100 W and the heat transfer limit of the long heat transport distance OHP has not been tested due to the limitations of existing experimental instrument parameters and performance.
3.1. Start-up characteristic and temperature variation behavior Fig. 3(a)–(f) exhibit the temperature variation of OHP and cooling water for 6 sets of representative operation conditions, respectively. Fig. 3(a) depicts that the OHP with 3 mm DI placed 30° starts to work as the temperature of evaporation section (Th) reaches about 60 °C and the work state lasts for 1000 s, during which period the ΔTohp varies between 22 °C and 30 °C and the ΔTwater increases gradually. Followed by a rapid deteriorating work state under the same operation conditions, the ΔTohp exceeds 120 °C and the ΔTwater decreases obviously, resulting in extreme deterioration of heat transfer performance. During this period, the liquid slug may be concentrated in the adiabatic section and the condensation section as shown in Fig. 4(b) and the pressure difference of one turn (P1-P2) is zero. It may be the reason that the driving pressure difference is small at low heat input which result in small inertia force of liquid slug and the liquid slug is easier to reach the static equilibrium state compared with the high heat input case. Another reason is that gravity has less effect on the back flow of liquid slug at a low inclination angle. When the equilibrium state is broken, the distribution of liquid slug can be depicted in Fig. 4(a) and the force direction of liquid slug in each tube will change randomly for the reason that the location and quality differences of liquid slug in the evaporation section can generate different evaporated driving force in each turn. With the increase of inclination angle, the stable working state at lower heat input can sustain for a long time and the start-up temperature goes down slightly. For instance, the start-up temperature at 60° and 90° are 52 °C and 50 °C, respectively. It also can be concluded from Fig. 3(a)–(c) that the ΔTohp at the same heat input becomes smaller with the rising of inclination angle, indicating that the OHP has lower heat transport limit placed at lower inclination. The same phenomenon has also been drawn for the macro small scale OHP in the previous work. When placed vertically and heated at 900 W, the ΔTohp of OHP is as low as 1.6 °C and the temperature of the OHP is extremely uniform. When filled with water, seen in Fig. 3(d), excellent thermal performance of the OHP is demonstrated. However, the ΔTohp shows obvious improvement over the SRWF case at the same heat input. The result of the case when increasing the FR to 70% can be seen in Fig. 3(e). The ΔTohp is low similarly with only slight growth and its pulsation amplitude appears to be larger. It can be drown from Fig. 3(f) that as the DI turns to 4 mm, the ΔTohp becomes larger obviously at the same heat input, such as 16.5 °C at 900 W which is about 10 times larger than that of the 3 mm case. The possible reason is that the liquid slugs cannot distribute in the tube with 4 mm DI randomly and steadily due to low surface tension
3.2. Pressure fluctuation characteristic and its spectral analysis
Force direction
Liquid slug
Fig. 5 shows the pressure variation inside the OHPs under different operations which corresponds to the previous Fig. 3. The whole variation trend of pressure and the influence of pressure fluctuation on temperature changes are clearly reflected. As shown in Fig. 5(a), the pressure rises gradually with the increase of heat input and reaches about 80 kPa (absolute pressure) at 900 W. The pressure fluctuation can be maintained stable even there occurs the instability or thermal crisis among the OHP. With the rising of inclination angle, the pulsation amplitude of pressure becomes smaller under the same operation condition depicted in Fig. 5(a)–(c) and the average pressure magnitudes of OHP placed at 60°and 90°are almost the same, about 60 kPa at 900 W. As for the water case in Fig. 5(d), the change regulation of pressure is also similar to the SRWF case. When rising the FR to 70%, the pulsation amplitude and average magnitude of pressure both grow up compared with the 50% case which can be found in Fig. 5(e). However, the fluctuation amplitude of pressure has obvious influence on the variation
P1
P1
P2
(a)
Evaporation section
P2
(b)
Fig. 4. Distribution of working medium and the probable force direction under two different working statuses. 140
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Table 1 Summary of working status under different operation conditions. DI
Medium
α
FR
100 W
300 W
500 W
700 W
900 W
1100 W
3 mm
SRWF
90°
30% 40% 50% 60% 70% 50% 70% 50% 70% 30–70% 50% 50% 50% 50% 70%
SU(45)/IS SU(46)/S (20.5) SU(50)/S (24.3) SU(52)/S (35.7) SU(58)/S (27.5) SU(55)/S (24.5) SU(60)/S (32) SU(60)/IS SU(67)/S (43) CR SU(58)/IS SU(59)/IS SU(63)/S (21.3) –
IS S (15.4) S (14) S (28.3) S (18.9) S (12.2) S (24.4) IS IS – S (7.9) S (21) S (7.3) SU(58)/S (23.5) SU(60)/S (21.3)
CR S (6) S (3.2) S (2.8) S (2) S (12.6) S (9.4) S (23.8) S (4.7)
– S S S S S S S S
– S S S S S S S S
–
S (7.3) S (12.9) S (19.3) –
S (5) S (18) S (48)
60° 30° Water
4 mm
SRWF
0° 90° 60° 30° 90°
(7.6) (2.7) (1.8) (2.2) (9.3) (9.7) (28) (1.6)
(6) (1.6) (1.5) (3.5) (7.8) (14.4) (58) (2.7)
S (5) – S (16.5) S (17)
S (14.4) –
SU (T): Start-up; S (Δ Tave): Stable; IS: instable; CR: Thermal Crisis Temperature unit: °C .
relationship between pressure amplitude and frequency in this work. Fig. 6 exhibits the pressure frequency spectrum under different heat inputs when the working medium, FR, DI and inclination angle are SRWF, 50%, 3 mm and 90°, respectively. It can be seen that there is no distinguishable characteristic frequency for the pressure signal at different heat inputs. The large amplitude pressure signal is mainly concentrated in the low frequency band from 100 W to 300 W, and the average pulsation amplitude in different frequency band increases obviously. With the continuous increase of heating input to 500 W, 700 W and 900 W, depicted in Fig. 6(c)–(e), the average fluctuation amplitude in most frequency range is basically the same. Fig. 7 presents the pressure frequency spectrum under different inclination angles when the working medium, FR, DI and heat input are SRWF, 50%, 3 mm and 700 W, respectively. The frequency corresponding to relative larger fluctuation amplitude at 30° is larger than that of the 60° case and 90° case. With the increase of inclination angle, the amplitude of pressure pulsation gradually decreases and the amplitude of the low frequency signal is close to that of the high frequency signal. Fig. 8 summaries the pressure frequency spectrum under different operation conditions at 900 W, including two inclination angles, two working media, two FRs and two DIs. Fig. 8(a) is the reference object for
trend of ΔTohp at 900 W and 1100 W as the DI turns to 4 mm. It can be obtained from Fig. 5(f) that the pressure and ΔTohp almost have the same variation trend in both fluctuation frequency and amplitude, namely the internal pressure pulsation and apparent temperature fluctuation of OHP has good synchronization. The large amplitude moments of both pressure pulsation and temperature fluctuation are basically coincide and the pressure fluctuation law can be reflected in the temperature fluctuation in time. In summary briefly, OHP can be maintained in stable pressure fluctuation state under different operation conditions when the DI is 3 mm and its working status is hardly effected by the inner pressure. As the DI increases to the range of critical diameter, such as 4 mm in the SRWF case, the pressure status determines the working status of OHP. Besides, the FR and inclination angle have great influence on the fluctuation amplitude of OHP. Spectral characteristic is one of the basic characteristics of the dynamic signal. Pressure frequency spectrum analysis method is an effective way to study the dynamic characteristic of OHP. In order to obtain the frequency characteristics of pressure fluctuation inside the OHP, the Fast Fourier Transform (FFT) analysis with a sampling time of 3 s (0.33 Hz) is performed on the pressure test value for all the conditions to obtain the pressure spectrum characteristic, that is, the
Fig. 5. Pressure variation inside the OHP under different operation conditions. 141
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J. Zhao et al.
Fig. 6. Pressure frequency spectrum under different heat inputs (SRWF, 50%, 3 mm, 90°).
and the difference becomes smaller when the heat input is large enough. For the OHP with different DIs and considering the influence of heat transport distance, the effective thermal conductivity can be used to measure the heat transfer performance more clearly and objectively. It can be concluded from Fig. 9(b) that the thermal conductivity increases gradually and gets close to extreme value before the OHP reaches the heat transfer limit. The effective thermal conductivity of the OHP with 3 mm DI can reaches the order of 1 × 106 with the unit of W/ (m °C ) at 900 W, which is one order of magnitude larger than 4 mm case. As shown in section 3.1, though the OHP has relative long heat transport distance, the Δ Tave is quite low. Large heat can be transported though the OHP immediately resulting in unbelievable effective thermal conductivity. At 100 W, the EF is only 15 approximately. However, the effective thermal conductivity of the OHP with 3 mm DI can be over 1000 times larger than that of copper at 500 W, 700 W and 900 W. Fig. 10 compares the thermal resistance, effective thermal conductivity and EF under two kinds of working media and different angles when the FR and DI are 50% and 3 mm, respectively. As shown in it, the inclination angle has different effects on the thermal resistance of OHP with different working media, especially at low heat input, such as 100 W and 300 W. However, if the heat input becomes large enough, for example from 500 W to 900 W, the thermal resistances of OHP with different work fluid both decrease with the rising of inclination angle, which can be obtained in the enlarged view. As discussed in the previous section, when the inclination angle is 30°, the OHP is easy to be
Fig. 8(b)–(d). There exhibits little difference between the water case and the SRWF case in the amplitude frequency characteristic. With proper increase of FR, the amplitude of each frequency band increases. However, as the DI rises to critical diameter, presented in Fig. 8(d) and (e), the large amplitude of pressure signal is mainly concentrated in the low frequency range and the amplitude in the low frequency range decreases to a certain extent as the FR increases from 50% to 70%. Spectrum of periodic signal or quasi periodic signals is mostly composed of multi peak. Spectrum analysis method can distinguish between periodic and chaotic phenomenon. As seen from Figs. 6–8, the pressure spectrum figures all show broad peaks with different degrees and prominent narrowband and spikes, which shows obvious chaos characteristic. Thus, it can be concluded that the motion state of the working medium in OHP is random and chaotic. 3.3. Thermal resistance, thermal conductivity and enhance factor under different conditions Fig. 9 presents the thermal resistance, effective thermal conductivity and EF under different DIs and FRs when the OHP is filled with SRWF and placed vertically. The OHP under the listed operation conditions all exhibit excellent performance. The thermal resistance is lower than 0.2 °C/ W at 300 W. Comparing to the 4 mm case, the OHP with 3 mm DI has better heat transfer performance. Along with the rising of heat input, the thermal resistance all decrease fast at first and then almost remain the same. The 50% FR case has advantage under low heat input
Fig. 7. Pressure frequency spectrum under different inclination angles (SRWF, 50%, 3 mm). 142
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Fig. 8. Pressure frequency spectrum at 900 W under different operation conditions.
conductivity reaches 1.43 × 106 W/(m °C ) and 1.22 × 106 W/(m °C ) when the FR is 60% and 50%, respectively. In general, the OHP has high adaptability to the FR. When the FR is larger than 40%, the thermal conductivity of OHP with different FRs are basically within the same order of magnitude, especially at the range from50% to 70%. The heat transfer performance is even closer under this region. The heat transfer ability of OHP is affected by the quality of working medium and movement state of liquid slug at the same time. Quality of working medium determines the sensitive heat transfer capacity. Large FR can increase the sensitive heat transfer capacity, but it may also increase the friction resistance and reduce evaporation driving force, which is not conducive to the movement of working fluid. At different heat input, the FR has different influence on these two aspects that needs further investigation. However, the influence of thermal conductivity on the thermal efficiency of the system is not obvious as it is high to a certain extent, which can be concluded from the variation trend of ΔTwater in Fig. 3. Table 2 compares the thermal resistance, effective thermal conductivity and EF in this work with those in several different literatures. It can be found that the long distance heat transport OHP in this work shows much more excellent performance comparing to the
instable or fall across thermal crisis. At this time, the thermal resistance turns to rise at certain heat input, among them that of the water case has much lower inflection point. The reason might be that the general driving force should consider the combined effects of gravity and capillary resistance caused by surface tension. The smaller the surface tension, the greater the effect of gravity on the inverse flow of working medium. Fig. 11 exhibits the thermal resistance, effective conductivity and EF under different FRs when the DI, working medium and inclination angle are 3 mm, SRWF and 90°, respectively. It is obvious that the OHP with 40% has the lowest thermal resistance when the heat input is less than 300 W. What is also obvious is the minor difference of thermal resistance from 40% to 70% as the heat input varies from 500 W to 900 W. As the FR is 30%, heat transfer limit is easy to be reached and the thermal resistance rises by two orders of magnitude when the heat input is increased from 300 W to 500 W. The effective thermal conductivity can directly reflect the difference of heat transfer capacity under different FRs. As shown in Fig. 11(b) that the OHP with the FR range from 50% to 60% might has the best thermal performance in the stable working region at large heat input. The effective thermal
Fig. 9. Thermal resistance, effective thermal conductivity and EF under different DIs and FRs. 143
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J. Zhao et al.
Fig. 10. Thermal resistance, effective thermal conductivity and EF under different working media and angles.
experimental result provided by some other researchers. In the previous work, the effective thermal conductivity of OHP with the same size and 4 mm DI has the same order of magnitude as the others due to the lower initial vacuum. Perhaps the limit of heat transfer capacity of macro large-scale OHP is far from being explored, especially in the direction of long distance heat transport.
Table 2 Comparison of thermal resistance, effective thermal conductivity and EF of different OHPs.
4. Conclusions In this paper, a further experimental investigation on thermal performance of long heat transport distance OHP with higher initial vacuum under the condition of two DIs, two working media, and different inclination angles and FRs were carried out and then the start-up characteristic, pressure fluctuation behavior, pressure frequency spectrum and heat transport performance under different operation conditions were compared and analyzed. The main conclusions can be drawn as follows:
Author/Group
Akachi
Mameli[23]
Sun[24]
Previous study[18]
This study
Do/Di
3 mm
4 mm/2mm
k eff (kW m−1 °C−1)
0.46 160 2000 0.045 9.038
0.2 4 100 0.84 5.92
6 mm/ 4mm 0.64 10 700 0.0399 5.676
5 mm/3mm
Ladia (m) n (–) Qh (W) ROHP (°C W−1)
3 mm/ 2.4 mm 0.075 8 120 – 51.4
EF (–)
22.6
14.8
128.5
14.19
3721
0.64 10 900 0.0024 1430
rising of inclination angle. When the inclination angle is low, the OHP is easy to be instable or fall across thermal crisis and the thermal resistance shows the trend of turning to rise under lower heat input and the water case has much lower drop point. (3) The pressure inside OHP with 3 mm DI and SRWF can be maintained stable fluctuation under different operation conditions and the working status is hardly effected by the inner pressure. However, the pressure status determines the working status of OHP with critical DI. The FR and inclination angle have great influence on the fluctuation amplitude. Through the pressure frequency spectrum analysis, the motion state of the working medium in OHP is random. (4) The start-up temperature of the OHP injected with SRWF and placed vertically increases from 45 °C to 58 °C with the growing of FR from 30% to 70%. The start-up temperature is inversely related
(1) The long distance heat transport OHPs show much more excellent performance. The OHP with 3 mm DI under the FR of 50% to 60% might has the most considerable thermal performance in the stable working area and at large heat load. The thermal resistance is as low as 0.0024 °C/ W and the effective thermal conductivity reaches the order of 1 × 106 with the unit of W/(m °C ) which is about two orders of magnitude larger than that in some other researchers’ work. (2) The inclination angle has different effects on the thermal resistance of OHP with two kinds of working media especially at low heat input. As the heat input becomes large enough, the thermal resistances of OHP with different work fluid both decrease with the
Fig. 11. Thermal resistance, effective thermal conductivity and EF under different FRs. 144
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to the inclination angle under the same working fluid, DI and FR. At 50% FR and 3 mm DI, the start-up temperature under the inclination angel of 90°, 60° and 30° is 50 °C , 55 °C and 60 °C, respectively. The maximum heat input only reaches 1100 W and the heat transfer limit of the long heat transport distance OHP is worth of further exploration.
[11] [12]
[13]
Acknowledgements
[14]
This work was supported by the Fundamental Research Funds for the Central Universities (No. 2017XKZD05).
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Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Operational characteristics of oscillating heat pipe with long heat transport distance for solar energy application Jiateng Zhao, Wei Jiang, Zhonghao Rao
T
⁎
School of Electrical and Power Engineering, China University of Mining and Technology, Xuzhou 221116 China
A R T I C LE I N FO
A B S T R A C T
Keywords: Oscillating heat pipe Thermal performance Long-distance heat transport Pressure spectrum
In order to further understand the operational characteristic of long heat transport distance oscillation heat pipe (OHP) for solar energy application, an experimental investigation on the thermal performance of OHP with higher initial vacuum and pressure fluctuation behavior inside under two kinds of inner diameters, two kinds of working media, and different inclination angles and filling ratios (FR) were carried out. The main result shows that the OHPs exhibit excellent heat transfer performance. The effective thermal conductivity is about two orders of magnitude larger than that in the previous work and some other researchers’ work. The start-up temperature of the OHP with 3 mm inner diameter increases from 45 °C to 58 °C with the growing of FR from 30% to 70% when injected with SRWF and placed vertically. The start-up temperature is inversely related to the inclination angle under the same condition. The working status of the OHP with 3 mm inner diameter and SRWF is hardly effected by the inner pressure. However, the effect is contrary for OHP with critical inner diameter. It can be inferred that the movement law of the liquid medium in the OHP is random and chaotic through the pressure frequency spectrum analysis.
1. Introduction Oscillation or pulsation heat pipe (OHP or PHP) is being widely investigated by many researchers all over the world since it was first introduced in the 1990s [1,2]. Like other types of heat pipes, it has been studied in many application fields, such as the electronic device cooling [3–5], industrial heat exchange [6,7] and other areas [8,9]. Solar energy is clean and abundant and can play a significant role in the battle of energy-saving and emission-reduction if it can be well developed [10–12]. The application investigation of OHP in the solar thermal energy field has been explored by some researchers due to its great potential for heat transfer and structural flexibility. Rittidech et al. [13,14] designed two kinds of solar collector based on closed-loop OHP with check valve and closed-end OHP, respectively. In the both works, the detailed dimension of the solar collectors were given and the focus is on the thermal efficiency of the collectors. The heat transfer performance characteristic and potential of those OHPs is not presented. Kargarsharifabad et al. [15] proposed a flat plate solar collector based on closed-loop OHP, the structure of which is similar to the former. Many parameters were investigated and the results indicated that the optimal filling ratio and inclination angle was 0.3 and 20°, respectively. Similarly, the detailed thermal performance of OHPs is not provided. Xian et al. [16] also designed two kinds of solar collector respectively
⁎
Corresponding author. E-mail address: [email protected] (Z. Rao).
https://doi.org/10.1016/j.expthermflusci.2018.05.026 Received 12 February 2018; Received in revised form 23 April 2018; Accepted 28 May 2018 Available online 29 May 2018 0894-1777/ © 2018 Elsevier Inc. All rights reserved.
based on tube OHP and flat plate OHP. The heat transfer rate and thermal conductivity for those two types were compared under different filling ratios and inclination angles. The OHPs show good heat transfer performance and the largest effective thermal conductivity is above 35,000 W/(m °C). However, the movement law of the internal working fluids was not investigated to further understand the working mechanism of OHP. Xu et al. [17] proposed a newly solar collector that combines the OHP with compound parabolic concentrator and their research view is mainly concerned on the performance of collectors in actual conditions. The results implies that the absorber can work well with a thermal resistance of about 0.26 °C/ W. In the previous work [18], aiming to face large-scale heat transfer applications, such as solar energy utilization and industrial waste heat recovery, the thermal performance of a OHP with 4 mm inner diameter (DI) at critical range and long heat transport distance under different conditions including different working medium, condensation capability and filling ratios were investigated experimentally. As a continuation of the previous work, the further study on thermal performance and potential of long heat transport distance OHP with different DIs under different inclination angles, filling ratios (FRs) heat inputs and higher initial vacuum were carried out. The start-up characteristic, pressure fluctuation behavior, pressure frequency spectrum under different operation conditions were analyzed in this paper.
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Nomenclature R ṁ ΔT Q̇ k α p
s e c w eff
thermal resistance [°C W−1] flow rate[kg/s] temperature difference [°C ] heat transport [W] thermal conductivity [W m−1 °C−1] inclination angle[°] pressure [Pa]
Acronyms EF DI FR SRWF OHP
Subscripts ave adia
average adiabatic
2. Experiment set up and procedure
Q̇ = Cpṁ ΔTw
(1)
t
ΔTw =
∫t12 (T18 (t )−T17 (t )) dt (2)
(t2−t1)
where ṁ and ΔTw are the mass flow rate and the time-averaged temperature difference of the circulating water for a thermal equilibrium period of time, respectively. The thermal resistance (ROHP ) could be calculated using the following formulas (3)–(5) [19,20]:
ROHP = ΔTohp/ Q̇ = (Te −Tc )/ Q̇
(3)
Relevant Parameters and conditions
Liquid Injection System Pressure Sensor
Low-constant Temperature Bath
enhancement factor inner diameter filling ratio self-rewetting fluid oscillating heat pipe
different views, different evaluation parameters were derived from the original data, including the time-averaged temperature difference between the evaporation and condensation section (considering the temperature only), the thermal resistance (considering the heat transported further), the effective thermal conductivity (considering the heat transport distance further) and the enhanced factor (for direct comparison). The heat (Q̇ ) transferred by OHP can be calculated through formula (1) and (2), respectively.
Two long heat transport distance oscillating heat pipes with the DI of 3 mm and 4 mm were fabricated and the corresponding testing platforms were established, respectively. Fig. 1 shows the photograph of experimental platform configuration, relevant parameters and operational conditions. The structure and dimension of the OHPs can be found clearly. The heating system, cooling system and data acquisition system are similar to those in the previous work. The measuring frequency of the data acquisition instrument is 0.333 Hz. Fig. 2 presents the schematic of temperature measuring points, which includes eight measuring points in the evaporation section (T1 ∼ T8), eight in the condensation section (T9 ∼ T16), and each one in the entrance and exit of circulating water box (T17, T18), respectively. Deionized Water (DW) and Hept. DW Sol. 0.1 w% (a kind of self-rewetting fluid, SRWF) are employed as working medium, and the FR varies among 30%, 40%, 50%, 60% and 70%. The inclination angles (α ) varies among 0°, 30°, 60° and 90°. All the experiments under different conditions were performed long enough to obtain suitable data in the period of thermal equilibrium. In order to evaluate to thermal performance of the OHP from
DC Power
substrate material evaporation condensation water effective
Thermal transport distance
Heating Wire
Working fluid
Deionized Water (DW), Hept. DW Sol. 0.1w%
Tube material
Copper
Inner Diameter (DI)
3, 4[mm]
Evaporator Height
200[mm]
Condenser Height
150[mm]
Adiabatic Height
640[mm]
Width
480[mm]
Filling ratio (FR)
30%, 40%, 50%, 60%, 70%
Inclination angle ( )
0 , 30 , 60 , 90
Temperature/rate of cooling Water
15 , 3.5 mL/s
Fig. 1. The photograph of experimental platform and relevant parameters and conditions. 138
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keff =
Pressure Transducer
T16
T15
T14
EF =
T17
T18 T1
T10 T11
T12
T13
(6)
keff ks
(7)
where Ladia is the heat transfer length (length of adiabatic section), Do is the outer diameter of OHP, n is the number of parallel tube. ks is the thermal conductivity of the substrate material (copper in this paper). The uncertainty analysis has been taken into account and the uncertainties of the indirect measurement parameters, like heat output, temperature difference, and thermal resistance were obtained according to error propagation principle. The error propagation law and results of uncertainty analysis can be seen in the previous work [18].
Circulating Water
Adiabatic Section
Ladia
4Ladia Q̇ 2 n·π·Do (Te −Tc )
3. Results and discussion
Heating Wire
T 1 T2 T3
T4 T5 T6
In the medium or low temperature solar thermal utilization, copperwater OHP meets the actual needs. Water is cheap, stable and safe and has excellent thermodynamic performance, which is suitable as working medium. The SRWF is based on water whose property is modified by adding high carbon alcohols. It has lower surface tension which can reduce the capillary resistance and is beneficial to the movement of work fluid. The property of work fluid determines the range of the DI. In the previous study, the DI range for these two work fluid were listed. Many researches [21,22] indicate that the OHP mainly depends on the sensitive heat transfer of the working fluid. The larger DI can increase the quality of the work fluid, so as to improve the theoretical heat transfer limit which meets the demand of large-scale heat transfer utilization. Therefore, in order to improve the heat transfer potential and explore the heat transfer characteristic of OHP with near-critical DI at the same time, OHP with 3 mm DI and 4 mm DI were studied, respectively. Firstly, the long heat transport distance OHPs with 3 mm DI and 4 mm DI filled with SRWF at 50% FR were investigated comparatively. Subsequently, the OHP with 3 mm DI injected with SRWF at 50% FR were tested under the inclination angle of 90°, 60°, 30° and 0° in turn. Then, placed vertically, the OHP with 3 mm DI injected with SRWF and water under different FRs were studied, respectively. At last the influence of DI, FR, inclination angle on the start-up temperature, pressure fluctuation inside and overall thermal performance of OHP were discussed and analyzed comprehensively.
T7 T8
Fig. 2. The schematic of measuring points. t
Te =
∫t12 ( 51 ∑i8= 1 Ti (t ) ) dt (t2−t1)
(4)
t
Tc =
T (t ) ) dt ∫t12 ( 51 ∑16 i=9 i (t2−t1)
(5)
where Te and Tc are the time-averaged temperature between evaporation section and condensation section, respectively. i is the number of thermocouple. Effective thermal conductivity (keff ) and the EF of OHP could be calculated using the following two formulas.
Fig. 3. Temperature variation of OHP and cooling water under different operation conditions. 139
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J. Zhao et al.
and they will return to the evaporation section spontaneously. Larger driving force is needed to maintain the internal oscillation. In brief, it can be concluded from above that the 3 mm-DI long-distance heat transport OHP filled with SRWF have better thermal performance under larger inclination angle and proper FR. Table 1 summaries the working status of the OHP under all different operation conditions, including the start-up temperature, the working stability status and Δ Tave in the stable condition concisely. Those conditions not presented in Fig. 3 can be found in this table. It can be reflected that thermal crisis is easy to occur even at low heat input when the inclination angle or the FR is lower. For instance, as shown in the second row, the OHP cannot work stably and turns to thermal crisis at the heat input of 500 W when the FR is as low as 30%. The OHP can be stable at large range of heat input if the FR and the angle are suitable. The start-up temperature of the OHP filled with SRWF and placed vertically increases from 45 °C to 58 °C with the growing of FR from 30% to 70%, which shows that along with the increase of FR pressure inside the OHP after rejection also increases. It also can be observed that the start-up temperature is inversely related to the inclination angle under the same working fluid, DI and FR. At 50% FR and 3 mm DI, the start-up temperature under the inclination angel of 90°, 60° and 30° is 50 °C , 55 °C and 60 °C , respectively. The maximum heat input only reaches 1100 W and the heat transfer limit of the long heat transport distance OHP has not been tested due to the limitations of existing experimental instrument parameters and performance.
3.1. Start-up characteristic and temperature variation behavior Fig. 3(a)–(f) exhibit the temperature variation of OHP and cooling water for 6 sets of representative operation conditions, respectively. Fig. 3(a) depicts that the OHP with 3 mm DI placed 30° starts to work as the temperature of evaporation section (Th) reaches about 60 °C and the work state lasts for 1000 s, during which period the ΔTohp varies between 22 °C and 30 °C and the ΔTwater increases gradually. Followed by a rapid deteriorating work state under the same operation conditions, the ΔTohp exceeds 120 °C and the ΔTwater decreases obviously, resulting in extreme deterioration of heat transfer performance. During this period, the liquid slug may be concentrated in the adiabatic section and the condensation section as shown in Fig. 4(b) and the pressure difference of one turn (P1-P2) is zero. It may be the reason that the driving pressure difference is small at low heat input which result in small inertia force of liquid slug and the liquid slug is easier to reach the static equilibrium state compared with the high heat input case. Another reason is that gravity has less effect on the back flow of liquid slug at a low inclination angle. When the equilibrium state is broken, the distribution of liquid slug can be depicted in Fig. 4(a) and the force direction of liquid slug in each tube will change randomly for the reason that the location and quality differences of liquid slug in the evaporation section can generate different evaporated driving force in each turn. With the increase of inclination angle, the stable working state at lower heat input can sustain for a long time and the start-up temperature goes down slightly. For instance, the start-up temperature at 60° and 90° are 52 °C and 50 °C, respectively. It also can be concluded from Fig. 3(a)–(c) that the ΔTohp at the same heat input becomes smaller with the rising of inclination angle, indicating that the OHP has lower heat transport limit placed at lower inclination. The same phenomenon has also been drawn for the macro small scale OHP in the previous work. When placed vertically and heated at 900 W, the ΔTohp of OHP is as low as 1.6 °C and the temperature of the OHP is extremely uniform. When filled with water, seen in Fig. 3(d), excellent thermal performance of the OHP is demonstrated. However, the ΔTohp shows obvious improvement over the SRWF case at the same heat input. The result of the case when increasing the FR to 70% can be seen in Fig. 3(e). The ΔTohp is low similarly with only slight growth and its pulsation amplitude appears to be larger. It can be drown from Fig. 3(f) that as the DI turns to 4 mm, the ΔTohp becomes larger obviously at the same heat input, such as 16.5 °C at 900 W which is about 10 times larger than that of the 3 mm case. The possible reason is that the liquid slugs cannot distribute in the tube with 4 mm DI randomly and steadily due to low surface tension
3.2. Pressure fluctuation characteristic and its spectral analysis
Force direction
Liquid slug
Fig. 5 shows the pressure variation inside the OHPs under different operations which corresponds to the previous Fig. 3. The whole variation trend of pressure and the influence of pressure fluctuation on temperature changes are clearly reflected. As shown in Fig. 5(a), the pressure rises gradually with the increase of heat input and reaches about 80 kPa (absolute pressure) at 900 W. The pressure fluctuation can be maintained stable even there occurs the instability or thermal crisis among the OHP. With the rising of inclination angle, the pulsation amplitude of pressure becomes smaller under the same operation condition depicted in Fig. 5(a)–(c) and the average pressure magnitudes of OHP placed at 60°and 90°are almost the same, about 60 kPa at 900 W. As for the water case in Fig. 5(d), the change regulation of pressure is also similar to the SRWF case. When rising the FR to 70%, the pulsation amplitude and average magnitude of pressure both grow up compared with the 50% case which can be found in Fig. 5(e). However, the fluctuation amplitude of pressure has obvious influence on the variation
P1
P1
P2
(a)
Evaporation section
P2
(b)
Fig. 4. Distribution of working medium and the probable force direction under two different working statuses. 140
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Table 1 Summary of working status under different operation conditions. DI
Medium
α
FR
100 W
300 W
500 W
700 W
900 W
1100 W
3 mm
SRWF
90°
30% 40% 50% 60% 70% 50% 70% 50% 70% 30–70% 50% 50% 50% 50% 70%
SU(45)/IS SU(46)/S (20.5) SU(50)/S (24.3) SU(52)/S (35.7) SU(58)/S (27.5) SU(55)/S (24.5) SU(60)/S (32) SU(60)/IS SU(67)/S (43) CR SU(58)/IS SU(59)/IS SU(63)/S (21.3) –
IS S (15.4) S (14) S (28.3) S (18.9) S (12.2) S (24.4) IS IS – S (7.9) S (21) S (7.3) SU(58)/S (23.5) SU(60)/S (21.3)
CR S (6) S (3.2) S (2.8) S (2) S (12.6) S (9.4) S (23.8) S (4.7)
– S S S S S S S S
– S S S S S S S S
–
S (7.3) S (12.9) S (19.3) –
S (5) S (18) S (48)
60° 30° Water
4 mm
SRWF
0° 90° 60° 30° 90°
(7.6) (2.7) (1.8) (2.2) (9.3) (9.7) (28) (1.6)
(6) (1.6) (1.5) (3.5) (7.8) (14.4) (58) (2.7)
S (5) – S (16.5) S (17)
S (14.4) –
SU (T): Start-up; S (Δ Tave): Stable; IS: instable; CR: Thermal Crisis Temperature unit: °C .
relationship between pressure amplitude and frequency in this work. Fig. 6 exhibits the pressure frequency spectrum under different heat inputs when the working medium, FR, DI and inclination angle are SRWF, 50%, 3 mm and 90°, respectively. It can be seen that there is no distinguishable characteristic frequency for the pressure signal at different heat inputs. The large amplitude pressure signal is mainly concentrated in the low frequency band from 100 W to 300 W, and the average pulsation amplitude in different frequency band increases obviously. With the continuous increase of heating input to 500 W, 700 W and 900 W, depicted in Fig. 6(c)–(e), the average fluctuation amplitude in most frequency range is basically the same. Fig. 7 presents the pressure frequency spectrum under different inclination angles when the working medium, FR, DI and heat input are SRWF, 50%, 3 mm and 700 W, respectively. The frequency corresponding to relative larger fluctuation amplitude at 30° is larger than that of the 60° case and 90° case. With the increase of inclination angle, the amplitude of pressure pulsation gradually decreases and the amplitude of the low frequency signal is close to that of the high frequency signal. Fig. 8 summaries the pressure frequency spectrum under different operation conditions at 900 W, including two inclination angles, two working media, two FRs and two DIs. Fig. 8(a) is the reference object for
trend of ΔTohp at 900 W and 1100 W as the DI turns to 4 mm. It can be obtained from Fig. 5(f) that the pressure and ΔTohp almost have the same variation trend in both fluctuation frequency and amplitude, namely the internal pressure pulsation and apparent temperature fluctuation of OHP has good synchronization. The large amplitude moments of both pressure pulsation and temperature fluctuation are basically coincide and the pressure fluctuation law can be reflected in the temperature fluctuation in time. In summary briefly, OHP can be maintained in stable pressure fluctuation state under different operation conditions when the DI is 3 mm and its working status is hardly effected by the inner pressure. As the DI increases to the range of critical diameter, such as 4 mm in the SRWF case, the pressure status determines the working status of OHP. Besides, the FR and inclination angle have great influence on the fluctuation amplitude of OHP. Spectral characteristic is one of the basic characteristics of the dynamic signal. Pressure frequency spectrum analysis method is an effective way to study the dynamic characteristic of OHP. In order to obtain the frequency characteristics of pressure fluctuation inside the OHP, the Fast Fourier Transform (FFT) analysis with a sampling time of 3 s (0.33 Hz) is performed on the pressure test value for all the conditions to obtain the pressure spectrum characteristic, that is, the
Fig. 5. Pressure variation inside the OHP under different operation conditions. 141
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J. Zhao et al.
Fig. 6. Pressure frequency spectrum under different heat inputs (SRWF, 50%, 3 mm, 90°).
and the difference becomes smaller when the heat input is large enough. For the OHP with different DIs and considering the influence of heat transport distance, the effective thermal conductivity can be used to measure the heat transfer performance more clearly and objectively. It can be concluded from Fig. 9(b) that the thermal conductivity increases gradually and gets close to extreme value before the OHP reaches the heat transfer limit. The effective thermal conductivity of the OHP with 3 mm DI can reaches the order of 1 × 106 with the unit of W/ (m °C ) at 900 W, which is one order of magnitude larger than 4 mm case. As shown in section 3.1, though the OHP has relative long heat transport distance, the Δ Tave is quite low. Large heat can be transported though the OHP immediately resulting in unbelievable effective thermal conductivity. At 100 W, the EF is only 15 approximately. However, the effective thermal conductivity of the OHP with 3 mm DI can be over 1000 times larger than that of copper at 500 W, 700 W and 900 W. Fig. 10 compares the thermal resistance, effective thermal conductivity and EF under two kinds of working media and different angles when the FR and DI are 50% and 3 mm, respectively. As shown in it, the inclination angle has different effects on the thermal resistance of OHP with different working media, especially at low heat input, such as 100 W and 300 W. However, if the heat input becomes large enough, for example from 500 W to 900 W, the thermal resistances of OHP with different work fluid both decrease with the rising of inclination angle, which can be obtained in the enlarged view. As discussed in the previous section, when the inclination angle is 30°, the OHP is easy to be
Fig. 8(b)–(d). There exhibits little difference between the water case and the SRWF case in the amplitude frequency characteristic. With proper increase of FR, the amplitude of each frequency band increases. However, as the DI rises to critical diameter, presented in Fig. 8(d) and (e), the large amplitude of pressure signal is mainly concentrated in the low frequency range and the amplitude in the low frequency range decreases to a certain extent as the FR increases from 50% to 70%. Spectrum of periodic signal or quasi periodic signals is mostly composed of multi peak. Spectrum analysis method can distinguish between periodic and chaotic phenomenon. As seen from Figs. 6–8, the pressure spectrum figures all show broad peaks with different degrees and prominent narrowband and spikes, which shows obvious chaos characteristic. Thus, it can be concluded that the motion state of the working medium in OHP is random and chaotic. 3.3. Thermal resistance, thermal conductivity and enhance factor under different conditions Fig. 9 presents the thermal resistance, effective thermal conductivity and EF under different DIs and FRs when the OHP is filled with SRWF and placed vertically. The OHP under the listed operation conditions all exhibit excellent performance. The thermal resistance is lower than 0.2 °C/ W at 300 W. Comparing to the 4 mm case, the OHP with 3 mm DI has better heat transfer performance. Along with the rising of heat input, the thermal resistance all decrease fast at first and then almost remain the same. The 50% FR case has advantage under low heat input
Fig. 7. Pressure frequency spectrum under different inclination angles (SRWF, 50%, 3 mm). 142
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Fig. 8. Pressure frequency spectrum at 900 W under different operation conditions.
conductivity reaches 1.43 × 106 W/(m °C ) and 1.22 × 106 W/(m °C ) when the FR is 60% and 50%, respectively. In general, the OHP has high adaptability to the FR. When the FR is larger than 40%, the thermal conductivity of OHP with different FRs are basically within the same order of magnitude, especially at the range from50% to 70%. The heat transfer performance is even closer under this region. The heat transfer ability of OHP is affected by the quality of working medium and movement state of liquid slug at the same time. Quality of working medium determines the sensitive heat transfer capacity. Large FR can increase the sensitive heat transfer capacity, but it may also increase the friction resistance and reduce evaporation driving force, which is not conducive to the movement of working fluid. At different heat input, the FR has different influence on these two aspects that needs further investigation. However, the influence of thermal conductivity on the thermal efficiency of the system is not obvious as it is high to a certain extent, which can be concluded from the variation trend of ΔTwater in Fig. 3. Table 2 compares the thermal resistance, effective thermal conductivity and EF in this work with those in several different literatures. It can be found that the long distance heat transport OHP in this work shows much more excellent performance comparing to the
instable or fall across thermal crisis. At this time, the thermal resistance turns to rise at certain heat input, among them that of the water case has much lower inflection point. The reason might be that the general driving force should consider the combined effects of gravity and capillary resistance caused by surface tension. The smaller the surface tension, the greater the effect of gravity on the inverse flow of working medium. Fig. 11 exhibits the thermal resistance, effective conductivity and EF under different FRs when the DI, working medium and inclination angle are 3 mm, SRWF and 90°, respectively. It is obvious that the OHP with 40% has the lowest thermal resistance when the heat input is less than 300 W. What is also obvious is the minor difference of thermal resistance from 40% to 70% as the heat input varies from 500 W to 900 W. As the FR is 30%, heat transfer limit is easy to be reached and the thermal resistance rises by two orders of magnitude when the heat input is increased from 300 W to 500 W. The effective thermal conductivity can directly reflect the difference of heat transfer capacity under different FRs. As shown in Fig. 11(b) that the OHP with the FR range from 50% to 60% might has the best thermal performance in the stable working region at large heat input. The effective thermal
Fig. 9. Thermal resistance, effective thermal conductivity and EF under different DIs and FRs. 143
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Fig. 10. Thermal resistance, effective thermal conductivity and EF under different working media and angles.
experimental result provided by some other researchers. In the previous work, the effective thermal conductivity of OHP with the same size and 4 mm DI has the same order of magnitude as the others due to the lower initial vacuum. Perhaps the limit of heat transfer capacity of macro large-scale OHP is far from being explored, especially in the direction of long distance heat transport.
Table 2 Comparison of thermal resistance, effective thermal conductivity and EF of different OHPs.
4. Conclusions In this paper, a further experimental investigation on thermal performance of long heat transport distance OHP with higher initial vacuum under the condition of two DIs, two working media, and different inclination angles and FRs were carried out and then the start-up characteristic, pressure fluctuation behavior, pressure frequency spectrum and heat transport performance under different operation conditions were compared and analyzed. The main conclusions can be drawn as follows:
Author/Group
Akachi
Mameli[23]
Sun[24]
Previous study[18]
This study
Do/Di
3 mm
4 mm/2mm
k eff (kW m−1 °C−1)
0.46 160 2000 0.045 9.038
0.2 4 100 0.84 5.92
6 mm/ 4mm 0.64 10 700 0.0399 5.676
5 mm/3mm
Ladia (m) n (–) Qh (W) ROHP (°C W−1)
3 mm/ 2.4 mm 0.075 8 120 – 51.4
EF (–)
22.6
14.8
128.5
14.19
3721
0.64 10 900 0.0024 1430
rising of inclination angle. When the inclination angle is low, the OHP is easy to be instable or fall across thermal crisis and the thermal resistance shows the trend of turning to rise under lower heat input and the water case has much lower drop point. (3) The pressure inside OHP with 3 mm DI and SRWF can be maintained stable fluctuation under different operation conditions and the working status is hardly effected by the inner pressure. However, the pressure status determines the working status of OHP with critical DI. The FR and inclination angle have great influence on the fluctuation amplitude. Through the pressure frequency spectrum analysis, the motion state of the working medium in OHP is random. (4) The start-up temperature of the OHP injected with SRWF and placed vertically increases from 45 °C to 58 °C with the growing of FR from 30% to 70%. The start-up temperature is inversely related
(1) The long distance heat transport OHPs show much more excellent performance. The OHP with 3 mm DI under the FR of 50% to 60% might has the most considerable thermal performance in the stable working area and at large heat load. The thermal resistance is as low as 0.0024 °C/ W and the effective thermal conductivity reaches the order of 1 × 106 with the unit of W/(m °C ) which is about two orders of magnitude larger than that in some other researchers’ work. (2) The inclination angle has different effects on the thermal resistance of OHP with two kinds of working media especially at low heat input. As the heat input becomes large enough, the thermal resistances of OHP with different work fluid both decrease with the
Fig. 11. Thermal resistance, effective thermal conductivity and EF under different FRs. 144
Experimental Thermal and Fluid Science 98 (2018) 137–145
J. Zhao et al.
to the inclination angle under the same working fluid, DI and FR. At 50% FR and 3 mm DI, the start-up temperature under the inclination angel of 90°, 60° and 30° is 50 °C , 55 °C and 60 °C, respectively. The maximum heat input only reaches 1100 W and the heat transfer limit of the long heat transport distance OHP is worth of further exploration.
[11] [12]
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Acknowledgements
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This work was supported by the Fundamental Research Funds for the Central Universities (No. 2017XKZD05).
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