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T3-3.3-P11


STUDIES OF A THERMOSYPHON SYSTEM WITH A HEAT SOURCE NEAR THE TOP AND HEAT SINK AT THE BOTTOM
Sadasuke Ito Kanagawa Institute of Technology Atsugi, 243-0292 Japan E-mail: ito@sd.kanagawa-

it.ac.jp Kenichi Tateishi Kanagawa Institute of Technology Atsugi, 243-0292 Japan

Naokatsu Miura Kanagawa Institute of Technology Atsugi, 243-0292 Japan

ABSTRACT Thermosyphon are heat transport devices that can transfer heat using gravitation. They can work without any external power supply, and have simple structures. Recently, a device which transferred heat from the hot reservoir near the top to the cold reservoir at the bottom was invented by Ippohshi et al.[1] In this study the same type of device was made and the performance was examined. Then, another type of device which was simpler to make was proposed and the performance was compared with each other. It was found that the one proposed in this study did not take time before the water in the tube circulated and that there was a possibility for applying the device to a solar water heating system which would work without a pump.

tank must be above the collectors in order to circulate the fluid naturally by thermosyphon. When a thermosyphon system is set on a roof of a house, it weighs the roof and the appearance of it would not be good. Morrison[2] digested problems which had encountered in thermosyphon systems in solar water heating. If a lot of heat can be transferred from a heat source at a level to a heat sink at a lower level naturally, the method would be able to be applied not only to solar water heaters but also many other fields such as space heating, road heating for melting snow, and distillation using solar heat. Maydanik[3] reported about a loop heat pipe which equipped with an evaporator in which a wick sintered from fine-grained particle to produce capillary pressure for circulating the working fluid in the loop. The vapor evaporated in the evaporator was condensed at the condenser where heat was extracted. The loop heat pipes had been most extensively employed in thermoregulation devises. In the paper, ideas to use the devices for solar collectors were described. Ippohshi et al.[1] developed a unique top-heat loop thermosyphon which used the gravitational force to transfer heat from a heat source at a level to a heat sink at a lower level. This system could operate successfully achieving high heat transport rate except in the beginning of operation.

1. INTRODUCTION There are mainly two types of systems for solar water heaters. One is an active type which uses a pump to circulate the working fluid such as water and polypropylene glycol through collectors. To operate the system appropriately, a pump and a control system for circulating the fluid are necessary. The other type is a passive or thermosyphon system which circulates the working fluid naturally by the effect of the gravitational force acting on the fluid. This system has a great advantage but the storage

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This system used a rather big condenser, in which there was a coil of a tube, above the evaporator and the system seemed a little complicated. In this study, we modified the system proposed by Ippohshi et al.[1] to make the system simpler and the performance of the two systems was compared .

2. EXPERIMENT A model of the top-heat loop thermosyphon proposed by Ipposhi and et al.[1] Apparatus No. 1, is shown in Fig. 1(a). The evaporator, condenser (heat exchanger) and cooler are connected by a tube. The condenser works also as the reservoir of the working fluid. The working fluid in the loop is supplied after evacuation so that there are only the saturated liquid and the saturated vapor in the condenser. When the working fluid is heated at the evaporator, the liquid evaporates and vapor is formed in the evaporator. Since the density of the mixture is smaller than the liquid in the other parts of the loop, a gravitational force is produced to circulate the working fluid in the direction shown by the arrows in the loop. If there is a flow in the loop, the vapor in the condenser is condensed on the surface of the tube in which the liquid chilled by the cooler flows. The liquid formed by condensation from the vapor flows to the outlet at the bottom of the condenser. The flow in the tube in the condenser is heated by the latent heat of condensation and the sensible heat of the saturated liquid in the condenser before it leaves for the inlet of the evaporator. The Apparatus No. 1 for the experiments is shown in Fig. 1(b). Water was used as the working fluid. A model of the top-heat loop thermosyphon proposed in the present study, Apparatus No.2, is shown in Fig. 2(a). The system equipped with a small reservoir of water at the end of the condenser as shown in the figure. The condenser located on the top of the system is consisted of two tubes soldered together. The flow of the mixture of the liquid and vapor in the upper tube is cooled by the lower tube in which the liquid from the cooler flows so that the vapor in the upper tube is condensed. When the volume of the vapor is increased, the total volume of the vapor and the liquid expands. It is aimed to have reservoir for storing extra liquid in the loop.

(a) Model

(b) Experiment

Fig. 1: Schematic of Apparatus No. 1.

(a) Model

(b) Experiment

Fig. 2: Schematic of Apparatus No. 2. The Apparatus No. 2 for the experiments is shown in Fig. 2(b). The inside and outside diameters of the loop tube which is made from copper are 8.0mm and 9.5mm, respectively except the tube for the evaporator (heater). The same tube is used in the Apparatus No. 1. The inside and outside diameters of the copper tube for the evaporator are 11.0mm and 12.7 mm, respectively. The length of the evaporator is 600mm. The length of the tube of the condenser is 2.7m for the apparatus No. 1 and 1.8m for the Apparatus No. 2. The length of the coil dipped in the cooler is 12m for each apparatus. The length between the centers of the evaporator and the cooler are 1460mm in Apparatus No. 1 and 1000mm in Apparatus No. 2. The evaporator of

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Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement vapor would not be condensed on the surface of the coil. Then, the vapor would increase the temperature of the water and the coil. Heat would be transferred from the coil to the inlet of the tube so that the temperature of the tube at the inlet of the condenser, T8, would increase as shown in Figs. 3 and 4. Because of the heat conducted through the coil to the tube at the inlet, the temperature on the surface of the coil would become lower than the vapor. The difference of the temperatures between the vapor and the coil became large with time and the rate of condensation would increase with time. It is assumed that when the rate of condensation became large enough, the circulation started.
140
3

Apparatus No. 1 or No. 2 is a tube wound with two ribbon electric heaters. The thickness, the width, and the length of one heater are 1mm, 20mm, and 500mm, respectively. The rated power consumption of one heater is 250W. The power of the two heaters is varied by resistors from 0 to 500W. In order to estimate the temperatures of the water in the tube, condenser, or reservoir, the temperatures T1, T2, T3, T4 and T5 are measured at the center of the evaporator, at the condenser inlet where the water in the two phases flows through, on the wall of the reservoir, at the condenser outlet where the condensed liquid flows out, and at the inlet of the cooler, respectively. T7 and T8 are measured between the cooler and condenser and at the inlet of the condenser, respectively. The temperature of the water in the cooler, T11, was also measured. Most of the experiment are carried on at T11 of about 20℃.

120 100
T ,℃
1

T T

80

0 0 60 120 Time ,min

Fig. 3: Variations of temperatures with time for Apparatus No. 1, Qh=200W.
140 120 100 T Q =400W
3

T T

T ,℃

80 60 40 20 0 0 T

60 Time ,min

Fig. 4: Variations of temperatures with time for Apparatus No. 1, Qh=400W. On the other hand, the temperature at the inlet of the cooler, T5, increased from the beginning in case of Apparatus No. 2 as shown in Figs. 5 and 6. When the heat input was 200W (cf: Fig. 5), the temperature at the inlet of the cooler , T5, went up and down during the time between 10 and 40 minutes and between 80 to 90 minutes. It is expected that the fluid

11

T

120

h

11

1

5

8

Figs. 3 and 4 show the variation of the temperatures with time when the heat inputs at the evaporator were 200W and 400W, respectively for the Apparatus No. 1. The temperatures of the water in the water bath, T11, were about 20C in the beginning of the experiments. The temperature of the water in the water bath did not controlled in these cases so that it increased with time. The temperature at the evaporator, T1, and the wall of the heat exchanger (condenser), T3, increased with time until around 70 minutes at the heat input Qh of 200W and 30 minutes at Qh of 400W. The temperatures became about 120C. It can be known from Fig. 3 and 4 that just before these times the temperatures at the inlet of the cooler, T5, started to increase just before these times. If there had been a flow in the loop, then the temperature of the working fluid at the inlet of the cooler, T5, should have been higher than the temperature of the water in the cooler, T11. Since T5 and T11 are about the same, there must be no flow-circulation before these times. There was no result about the variation of temperatures with time in Ref. 1, but it was described in the paper that the water in the loop did not circulate for a while in the beginning of the experiments and them the fluid temperature in the evaporator and the heat exchanger continued to rize slowly. Because the temperature of the coil of the tube in the condenser would be the same as the water in the condenser at the beginning of operation, the

20

7

40

8

T

T

8

2

3. RESULTS AND DISCUSSIONS

60

T

5

T

T

T

W002=hQ

180

180

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80 60

T ,℃

20 0

100
1

80
T ,℃

60 40 20 0 0
7

30

Time ,min

60

Fig. 5: Variations of temperatures with time for Apparatus No. 2, Qh=200W.
100
2 1

T ,℃

20 0 0 30 Time , min
Q =400W
h

Fig. 6: Variations of temperatures with time for Apparatus No. 2, Qh=400W.

7

40

T

11

3

60

5

80

T

T

T

T

T

11

7

T

T

3

5

T

T

T

h

2

T

T

Q =200W

0

30

Time , min

60

Fig. 7: Variations of temperatures with time Apparatus No. 2, Qh=400W, T11=52℃.

4. CONCLUSIONS A top–heat loop thermosyphon in which the condenser combined reservoir started transfer of the heat supplied at the evaporator to the cooler after the temperature of the water became 120C. The modified top-heat loop thermosyphon in which the condenser and the reservoir were separated operated successfully from the beginning of heating. The temperature on the outside wall of the evaporator was in the range between 60C and 90C. The evaporation temperature was lower when the flow was stable than when it was unstable. It would be possible to apply this device to solar water heating. Further investigations will be necessary for the application.

90

5. ACKNOWLEDGEMENT
60

The support of "High-Tech Research Center Project for Private Universities: matching fund subsidy from MEXT, 2007-2011" for this research is appreciated.

h

11

T

7

3

5

40

T

T

T

2

1

temperature in the tube between the cooler and the condenser, T7, would be close to the temperature of the water, T11, when there was a flow circulation. When the flow stopped the temperature on the tube between the cooler and the condenser, T7, would be influenced by the temperature of the condenser at the exit of condenser, T4, by heat conduction so that T7 increased when the flow stopped. The evaporation temperature was higher when the flow was unstable than when the flow was stable as seen in Fig. 5. Fig. 6 shows the experimental results when the heat input was 400W. The variations of the temperatures with time were not so large as the case of the heat input of 200W. Since the difference between the temperature at the inlet of the condenser, T2, and the temperature at the inlet of the cooler was about 5C, the length of the condenser was considered to be long enough. If the length of the condenser had been too short, the temperature at the inlet of difference would have been much larger.

Fig. 7 shows the experimental results when the temperature of the water in the bath was kept at about 52C and the heat input was 400W. The condensation temperature which would not be so different as the evaporation temperature in the evaporator was about 80C at the maximum. Using solar collectors as the heater, this type of top-heat loop thermosyphon would be able to use as the solar water heater without pumping.
100 T

T

Q =400W 90

for

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Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement (2) G. L. Morrison, "Solar Water Heating", Solar Energy edited by J. Gordon, 2001, International Solar Energy Society, p.223-289 (3) Yu. Maydanik, "Loop Heat Pipes-Highly Efficient Heat-Transfer Devices for Systems of Sun Heat Supply", Proceedings 1 of EuroSun 2004 Conference, 2004, p.470-476

6. REFERENCES (1) S. Ippohshi, "Development of a Top-Heat –Mode Loop Thermosyphon", Proceedings of the 6th ASME-JSME Thermal Engineering Joint Conference, 2003, TED-AJ03-578

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