Operation of a Two-Phase Reverse Loop Thermosyphon

Transcription

1 Journal of Applied Science and Engineering, Vol. 18, No. 3, pp (2015) DOI: /jase Operation of a Two-Phase Reverse Loop Thermosyphon Meng-Chang Tsai 1, Shung-Wen Kang 2 *, Heng-Yi Li 1 and Wen-Fa Tsai 1 1 Institute of Nuclear Energy Research, Taoyuan, Taiwan 325, R.O.C. 2 Department of Mechanical and Electro-Mechanical Engineering, Tamkang University. Tamsui, Taiwan 251, R.O.C. Abstract A prototype two-phase reverse-loop thermosyphon (RLT) was fabricated to transport thermal energy downwards to a thermal storage tank. The effect the filling ratio exerted on the thermal performance of the RTL was investigated by conducting experiments that were performed by a range between 47% and 63%. The prototype device comprised an evaporator, a condenser, a reservoir (preheater), and pipes connecting each of the components. The heat transport height was 1500 mm from the evaporator to the bottom of the condenser. Methanol with a concentration of 95% was used as the working fluid. The temperature distribution, temperature difference between the evaporator and condenser, and thermal resistance of the thermosyphon were measured. In addition, a cyclic case, in which the power in coils was alternately switched on and off according to a square wave function was studied. The results indicated that the maximal thermal performance of the RLT occurred when the filling ratio was 60% and the input power was 660 W. Key Words: Reverse-loop, Thermosyphon, Heat Transfer Downward, Cyclic Operation 1. Introduction A thermosyphon is a two-phase heat transfer device that features highly effective thermal conductivity. When heat is applied to the evaporator, saturated liquid boils and evaporates. The heat from the evaporator is released into the condenser section, which is positioned above the evaporator to ensure that the condensate is pulled back to the evaporator by gravity. Looped thermosyphons improve the heat transfer efficiency of single-tube thermosyphons, because the design reduces the collision of condensed liquids and gases that flow in opposite directions. An advanced loop thermosyphon comprises an evaporator (in which liquid boils) and a condenser (in which the vapour condenses back into liquid) that are connected by a riser and downcomer. Loop thermosyphon cooling is a *Corresponding author. swkang@mail.tku.edu.tw promising technology because it can dissipate high heat fluxes with minimal difference in temperature [1]. Loop thermosyphons, which have numerous industrial applications because they are simple, have high heat transfer capability, and are passive in nature, have received widespread attention from academic and technical researchers [2]; however, loop thermosyphons only transfer heat upwards. Ensuring spontaneous downwards heat transfer is difficult, yet achievable. Koito et al. [3] explored the operational characteristics of a top-heat-type long heat transport loop using a heat exchanger. The loop consisted of a heated section, a cooled section, a reservoir, valves, and tubes connecting the components. Experiments verified that the temperatures and pressures inside the loop varied cyclically according to the valve operation, and the heat was transported downwards continuously from the heated section to the cooled section. Dobriansky and Yohanis [4] developed a heat exchanger

2 260 Meng-Chang Tsai et al. that operated on reverse thermosyphon action and comprised an automatic and self-contained liquid circulation loop that transferred heat downwards, the direction opposite that of natural convection. The pressure difference of the saturated vapour was used to move the heated liquid downwards. Dobriansky [5] reviewed scientific and technical literature related to general automatic flow loops that transmit heat upwards, and automatic reverse flow loops that transmit heat downwards. These systems, which are unlike natural convection loops, may be termed reverse loops, and can potentially be incorporated into solar installations, which are currently used worldwide. Despite the urgency of the problem, reverse loops remain to be applied widely to solar installations because of technical imperfections. Compared to studies on heat pipes and thermosyphons, fewer reports pertaining to the technical flaws of reverse loops have been published. In this study, a prototype two-phase reverse-loop thermosyphon (RLT) without a valve was developed to transfer heat downwards, and the effect the working fluid filling ratio exerted on the thermal performance was investigated. In addition, the cyclic temperature variations required for a step change in the power length and the cyclic on off operation were examined. 2. Description of the Device and Operation Principle Figure 1 depicts a schematic of the two-phase RLT design of the device. The RLT comprised an evaporator, a condenser, a preheat reservoir, and pipes connecting each of the components. The evaporator featured a 72 mm diameter copper slug in which 10 electronic resistance heaters were installed. The condenser comprised a 620 mm long ½ in (1.27 cm) copper coil and a thermostatic bath used to exchange and store heat, respectively. The reservoir comprised a 3½ in (8.89 cm) stainless steel tube that was 200 mm in height and 2 caps (50 mm in height). The lines from the evaporator to the reservoir, from the reservoir to the condenser, from the condenser to the reservoir, and through the reservoir to the evaporator (in which liquid was preheated by vapour and hot liquid) were named the vapour line, the high temperature liquid line, the low temperature liquid line, and the return line, respectively. Figure 2 illustrates the temperature measurement points and the operation principle of the RLT. When the evaporator was heated, the temperature of the heating surface increased until the boiling point was achieved. Figure 1. Schematic of the two-phase reverse loop thermosyphon.

3 Operation of a Two-Phase Reverse Loop Thermosyphon 261 The liquid vaporised in the evaporator and flowed to the reservoir. A density dissimilarity between the return fluid in the preheated liquid line and the fluid in the heating section implied a difference in force, resulting in a circulation flow throughout the loop. Hot fluid in the reservoir was pushed downwards through the high temperature liquid line to the condenser, releasing heat into the heat sink. The cold fluid in the condenser then flowed upwards through the low temperature liquid line into the reservoir, which was acting as a simple tube-within-atube cross-flow heat exchanger. The returned low-temperature liquid was preheated by the vapour and hot liquid, and then flowed back into the evaporator to be boiled. When a saturated vapour came into contact with a surface that had a lower temperature, condensation occurred inside the reservoir. Thus, the RLT transferred heat downwards from the evaporator to the condenser without requiring any valve setting or external power supply. Methanol with a concentration of 95% was used as the working fluid. Type-T thermocouples (Omega TT-photograph and T-040) were installed (as shown in the figure), and all temperatures outputs were connected to a data logger and continuously recorded; temperature measurement uncertainty was 0.1 C. To ensure thermal insulation, insulated fiberglass foam (k < 0.04 W/mK) was used to cover the evaporator, reservoir, and the entire pipe line. Experiments were conducted using filling ratios (defined as the ratio of the volume of fill liquid to the RLT volume) between 47% and 63%. AC power and a digital power regulator were used to supply steady heating power to the evaporator. The temperature distribution and thermal resistance of the RLT were measured and calculated. The cyclic temperature variations and the liquid flow direction were inspected when the heat was on and off. Figure 2 depicts the flow direction when the heat was in the on and off modes. 3. Results and Discussion 3.1 Temperature Variation Over Time When heat was supplied by the electronic heater to the heating section, circulation occurred and the fluid temperature in the evaporator and reservoir gradually increased. Figure 3 shows the temperature variations over time when 660 W of heat was input according to a 60% filling ratio (900 ml). The heater turned off after 3600 sec. As anticipated, the temperatures in the evaporator (Ch1 and Ch2) were higher than the temperatures elsewhere. According to Figure 3, during the heating process, the temperatures fluctuated in the evaporator (Ch1), and hot liquid flowed out of the reservoir (Ch3) and condenser (Ch4 and Ch6) because of the two-phase flow. The fluctuation amplitudes of Ch4 and Ch1 were approximately 1.5 C and1 C, respectively, whereas the fluctuation amplitudes at other measurement points were less than 1 C. The temperature trend of the condenser outlet (Ch6) Figure 2. Measurement points and operation principle of the two-phase RLT.

4 262 Meng-Chang Tsai et al. was nearly linear, indicating that the thermal energy stored in the water pool bath was stable. After the power shut down after 3600 sec, the reservoir became the heater of two-phase RLT. Figure 3 suggests that the apparatus reverted to natural convection, during which time the bubble flows returned through Ch5 and Ch7 and the temperature increased quickly. By contrast, the Ch4 temperature declined rapidly because the saturated vapour that had been driving the pressure suddenly dissipated. Temperature Ch5 exhibited fluctuations during the thermodynamic equilibration process that occurred between the evaporator and the reservoir. 3.2 Effect of Different Filling Ratios Figures 4 and 5 illustrate the evaporator vapour temperature (Ch1) and condensation temperature (average temperature of Ch6 and Ch7), respectively, when the filling rate increased from 47% to 63% according to the time step. As shown in Figure 4, the lowest vapour temperature during the heating process was obtained when the filling ratio was 60% and the temperature fluctuation began after 1700 sec. Figure 5 shows the condensation temperatures varied during the heating process. Figure 6 depicts the temperature difference between the vapour and the condensing temperature ( T = T vapor T condensing ) of the two-phase RLT when 6 working fluid filling ratios were employed. The working fluid that had a 60% filling ratio exhibited the lowest thermal resistance of approximately 0.04 C/W at the 3600 th sec. The thermal resistance of the two-phase RLT can be measured as given by equation (1), Figure 4. Vapour temperature variations over time with different filling ratio. Figure 5. Condensing temperature variations over time with different filling ratio. Figure 3. Temperature variations over time with 660 W of heat input and 60% filling ratio. Figure 6. Temperature difference variations over time with different filling ratio.

5 Operation of a Two-Phase Reverse Loop Thermosyphon 263 (1) where T vapor T condensing and Q are temperature difference between the vapour and the condenser inlet temperature, and the heat input. 3.3 Ten Minute Square Wave Heating Test A delta function of square wave heat pulse was input to the two-phase RLT according to a heating frequency of 10 min on and 10 min off. Figure 7 shows the results of vapour temperature (Ch1), reservoir outlet surface temperature (Ch3), and condenser inlet surface temperature (Ch4); heat was conducted from the top to the condenser in the two-phase RLT. The working fluid carried thermal energy from the evaporator to condenser rapidly. By contrast, the thermal energy was released from the working fluid into the water inside the water pool bath slowly. The thermal energy accumulated at the condenser inlet. It was observed that the temperature fluctuation at the condenser inlet was larger than the fluctuation in the evaporator and at the reservoir inlet surface. When the power was turned off at the 8000 th sec., temperature fluctuation ceased immediately. Figure 8 shows the temperature variations of the preheated liquid (Ch5) and returned liquid (Ch7). When the power was on, both temperatures decreased immediately and the temperature at Ch7 was lower than the temperature at Ch5. Both temperatures increased soon after the power was turned off, and the temperature at Ch7 was higher than the temperature at Ch5. The start-up behaviour of the two-phase RLT operating during a square wave cyclic on off process without valves was efficient. 4. Conclusions RLTs are a unique type of automatic reverse circulation loops that are capable of transferring heat downwards. In this study, a prototype of an RLT was developed, fabricated, and tested. Operating characteristics were examined during the cyclic power on off modes. The results can be summarised as follows: 1. The filling ratio of working fluid affected the thermal performance of the thermosyphon when methanol was employed as the working fluid. The lowest thermal Figure 7. Three temperature variations over time in the square wave heating process. Figure 8. Working fluid alternately reversed and observed by Ch5 and Ch7. resistance, 0.04 C/W, was observed in when using a 60% filling ratio. 2. The RLT could transfer 660 W of heat downwards from a height of 1500 mm. 3. During the power on off processes, the device exhibited favourable start-up behaviour, and the temperature, pressure, and flow directions inside the loop varied cyclically without valve operation; the heat was transported downwards continuously from the evaporator to the condenser. 4. The two-phase RLT was reliable, exhibiting antigravity capability and the potential to perform efficiently in high-power energy transport applications such as solar energy installations. References [1] Kang, S. W., Tsai, M. C., Hsieh, C. S. and Chen, J. Y.,

6 264 Meng-Chang Tsai et al. Thermal Performance of a Loop Thermosyphon, Tamkang Journal of Science and Engineering, Vol. 13, No. 3, pp (2010). doi: /jase [2] Franco, A. and Filippeschi, S., Closed Loop Two-Phase Thermosyphon of Small Dimensions a Review of the Experimental Results, Microgravity Science and Technology, Vol. 24, pp (2012). doi: / s [3] Koito, Y., Ahamed, M. S., Harada, S. and Imura, H., Operational Characteristics of a Top-Heat-Type Long Heat Transport Loop through a Heat Exchanger, Applied Thermal Engineering, Vol. 29, No. 2 3, pp (2009). doi: /j.applthermaleng [4] Dobriansky, Y. and Yohanis, Y. G., Cyclical Reverse Thermosiphon, Archives of Thermodynamics, Vol. 31, No. 1, pp (2010). doi: /v [5] Dobriansky, Y., Concepts of Self-acting Circulation Loops for Downward Heat Transfer (Reverse Thermosiphons), Energy Conversion and Management, Vol. 52, pp (2011). doi: /j.enconman Manuscript Received: Mar. 20, 2015 Accepted: Jul. 9, 2015