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Effect of wick on the performance of heat pipe with water and methanol as a working fluids.

Introduction

The conventional flat plate solar water heater is simple and reasonably efficient and automatic in operation, but it is a frost resistance. Efforts to overcome this serious drawback resulted in closed loop systems where, suitable fluid is circulated in the heat circuit. The collector in closed loop system operates at higher temperature than in the conventional open system. The heat pipe is self-contained device for transferring heat in which the latent heat of vaporization is exploited to transfer heat over long distance with a corresponding small temperature difference (1). The wickless heat pipe, which is essentially a two phase closed thermosyphon tube, is a highly efficient device for heat transmission. It utilizes the evaporation and condensation of its internal working fluid to transport heat. It has the ability to transfer high heat rates in a latent form over considerable distances and the extremely small temperature drops between the heated and cooled region resulting in small qualitative degradation of energy, make the heat pipe an attractive option in solar systems (2). In the past several years, the heat pipe has been receiving increased attention for possible use in the space environment (3). Some of the potential applications can be found in the heat recovery systems are solar collectors and thermal storage (4). In solar application systems use of heat pipe appears more appropriate in domestic hot water heating. Hot water in the residential sector accounts for about 10 per cent of the national energy consumption, hence the amount of energy required to heat domestic water is significant as compared to that required for space heating. The present concern of environmental issues linked to the use of fossil fuels gives great intensive to harness alternate energies wherever possible. The working fluid used in the heat pipes should be compatibility with the wick material used to provide necessary flow passage for the return of the condensed liquid (5). Inclination of heat pipes and the source of temperature play an important role to improve the performance of heat pipes (6). The aim of present study was to investigate experimentally the effect of using wick on the performance of heat pipe and its working fluid at different inclination from the horizontal

Experimental Set Up

The experimental set up consists of heat pipes made of copper having 20 mm and 22 mm inside and outside diameters respectively with 1000 mm length. The pipes were initially evacuated using vacuum pump after a series of cleaning to remove possible contamination (7). After evacuation the working fluid was sucked into the heat pipe through a separate arrangement. Figure1 shows the schematic diagram of the experimental setup. Heat pipe comprises of three sections. The bottom section of the heat pipe called evaporator-is having length of 140 mm to which heat is added through electric heater (150W) and the heater is wrapped around the lower end of the heat pipe with constant heat flux. The middle section of the heat pipe called adiabatic having length of 735 mm which is externally insulated by a glass wool. The upper section of the heat pipe called condenser- is having length of 125 mm from which the heat is extracted out of the system. The experiment was conducted to evaluate the performance of heat pipe containing wick and no wick. The wick used in the experiment is made of stainless steel wire mesh with 100 openings. The heat pipe filled with water and methanol as working fluids were placed at three levels of tilt angles (30[degrees], 60[degrees] & 90[degrees]) from horizontal. When the heat is added to the evaporator section, the same is absorbed in the form of latent heat through the vaporization process of the working fluid and converts it into vapour. This vapour moves upwards through the pipes, enters into the adiabatic section of the heat pipe and then to the condenser section and gets condensed due to partial pressure build up. The condensed returns back to the evaporator section along the walls of the heat pipe by gravity force penetrating a stainless steel wick. The latent heat released from the condenser section is taken away by means of water-cooling or air-cooling with fins. Heat pipe is capable of transmitting heat as high as 40 times more than that transmitted by ordinary heat conductor (8).

[FIGURE 1 OMITTED]

Results and Discussion

Variation of evaporator temperature

The variation of evaporator temperature in the heat pipes drawn for both water and methanol as working fluids at different tilt angles from horizontal is illustrated in Figure 2 & 3. It is clear from Figure 2 that the higher evaporator temperature (97.8[degrees]C) was recorded when the heat pipes with wick were positioned at 30[degrees] from horizontal using water as working fluid and 90[degrees] from horizontal using methanol as working fluid (97.4[degrees]C). On the other hand from Figure 3, it is observed that the heat pipes with no wick recorded higher steady--state evaporator temperature using water (105.4[degrees]C at 60[degrees] inclination) and methanol (109[degrees]C at 90[degrees] inclination) as working fluids as compared to that recorded in the heat pipes with wick. Further, the data reveals that there is slightly higher temperature gradients recorded when the system used with wick (0.37[degrees]C / min to 0.45[degrees]C / min) than with no wick (0.29[degrees]C / min to 0.44[degrees]C / min). This indicates that drawing heat from the evaporator section is comparatively faster in heat pipes with no wick than that with wick.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Variation of condenser temperature

The variation of condenser temperature for both water and methanol as working fluids used at different tilt angles of heat pipes is illustrated in Figure 4 & 5. The data indicates that condenser temperature in the heat pipes using water as working fluid with wick (Figure 4) and with no wick (Figure 5) is significantly higher as compared to the temperature recorded using methanol as working fluid for a given tilt angle from horizontal. On the other hand the data in Figure 5 show that higher condenser temperature was recorded when the heat pipes were positioned at 60[degrees] from horizontal using water as working fluid (92.4[degrees]C) and 90[degrees] from horizontal using methanol as working fluid (84[degrees]C) with no wick. Further, it was observed that there was no significant variation as far as the condenser temperature gradient per unit time (varying between 0.25[degrees]C/min to 27[degrees]C/min) is concerned between the working fluids and tilt angles with or without using wick.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Variation of temperature between evaporator and condenser

To compare the variation of temperature difference between the evaporator and condenser in the heat pipes using water and methanol as working fluids at three levels of tilt angles with wick and no wick, Figure 6 & 7 are plotted. The data in Figure 6 show that lower temperature difference is observed using water as working fluid (15[degrees]C to 15.8[degrees]C) as compared to the temperature difference observed using methanol as working fluid (23.6[degrees]C to 24[degrees]C) with wick for the same amount of heat rate input of 150 W supplied.

The data in Figure 7 show that lower temperature difference was observed using water as working fluid (13[degrees]C to 15.2[degrees]C) as compared to the temperature difference observed using methanol as working fluid (19.8[degrees]C to 25[degrees]C) with no wick for the same amount of heat rate input of 150 W supplied. This indicates that for the same amount of heat transfer rate, higher values of the overall heat transfer coefficient can be obtained using water as working fluid without using wick. Further, it is clear from the data that for the same amount of heat transfer rate, higher values of the overall heat transfer coefficient can be obtained using water as working fluid either using wick or no wick.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The overall heat transfer coefficients of heat pipes evaluated for two working fluids at three tilt angles using wick or no wick, Figure 8 & 9 are plotted. As stated earlier the performance of the heat pipes is significantly better using water as working fluid with no wick as compared to that recorded using methanol as working fluid. When wick is used (Figure 8) the overall heat transfer coefficient of heat pipes ranged from 24974 W/[m.sup.2] / oC to 26300W/[m.sup.2] /[degrees]C with water as working fluid as compared to 16440 W/[m.sup.2]/ [degrees]C to 16720 W/[m.sup.2] / [degrees]C with methanol as working fluid. When no wick was used (Figure 9) the overall heat transfer coefficient of heat pipes ranged from 25370 W/[m.sup.2] / [degrees]C to 30350W/[m.sup.2] / [degrees]C with water as working fluid as compared to 15784 W/[m.sup.2] / [degrees]C to 19929 [m.sup.2] / [degrees]C with methanol as working fluid.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Conclusions

The performance of heat pipe evaluated on two working fluids at three levels of inclinations from horizontal using wick and no wick, indicates that the average overall heat transfer co-efficient was higher when the heat pipe containing no wick and filled with water as working fluid than the heat pipes filled with methanol as working fluid. The performance of heat pipe filled with water as working fluid was improved significantly using no wick, especially when they are positioned at 60[degrees] and 90[degrees] from horizontal.

Acknowledgment

The authors gratefully acknowledge the financial support from the Institution of Engineers (India) and facilities provided by the management of S.J.C.I.T for the research work.

References

[1] Faghri, A., Dogineni, S. and Cao, Y., 1995, "Heat transfer-Part A", 28, pp. 723-737.

[2] Terpestra, M. and Van Veen, J., 1987, "Heat pipes: Construction and application a study of patents and patent applications", Elsevier Applied Science, New Yark.

[3] Leonard L. Vasiliev, 2005,"Heat pipes in modern heat exchangers", Applied

[4] Thermal Engineering, 25, pp.1-19. Azad, E., Bahar, F. and Moztarzadeh, 1987, "Solar water heater using gravity assisted heat pipe", Heat Recovery System & CHP, 7(4), pp. 343-350.

[5] Seshan, S. and Vijayalakshmi, D., 1986, "Heat pipes, materials and application". Energy Conservation Management, 26(1), pp. 1-9.

[6] Ali Cetin Gurses and Cannistraro, 1991, "The inclination effect on the performance of water filled heat pipes", Renewable Energy, 1 (5/6), pp. 667-674.

[7] Dunn, P.D. and Reay, D.A., 1978, "Heat pipes", Pergamon Press, London.

[8] Sridhara, S.N., Sreenivasa, T.N. and Rama Narasimha, K., 2004, "A study on the recent developments in the design of heat pipes," Proc. National Con. on Advances in Mech Engg. J.N.N. College, Shimoga, Karnataka (India).

G.V. Gnanendra Reddy, P. Sankaran Kutty, M. Chowde Gowda and C. Padmanaba

B.G.S. R&D Center, Department of Mechanical Engineering S.J.C. Institute of Technology, Chickballapur-562 101, Karnataka (India) E-mail: gvgrmed@rediffmail.com
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Article Details
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Author:Reddy, G.V. Gnanendra; Kutty, P. Sankaran; Gowda, M. Chowde; Padmanaba, C.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2009
Words:1863
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