Effect of Variable Adiabatic Length on a Heat Pipe Performance

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN (P): 2249–6890; ISSN (E): 2249–8001 Vol. 10, Issue 2, Apr 2020, 377–384 © TJPRC Pvt. Ltd.

EFFECT OF VARIABLE ADIABATIC LENGTH ON A HEAT PIPE PERFORMANCE RAQEEB HUMMADI RAJAB Lecture, Department of Agricultural Machines and Equipment, University of Mosul, Iraq ABSTRACT The study was conducted to investigate the effects of length ratio [LR=Levap./Ladia., the relationship between two parts, the evaporator length (Levap.) and the adiabatic length (Ladia.)] on the thermal performance of heat pipe. In the current study, purified water was used as an operating fluid and charged in the evaporator through a power supply of 250Watt. A series of experiments were conducted using five length ratios (LR=250/150=1.7, 225/175=1.3, 200/200=1, 175/225=0.8, 150/250=0.6). This was designed to investigate the impact of the studied factors on steady state heat transfer characteristics in the heat pipe (HP). The results derived from the experiments showed that the positive performance of heat pipe was recorded near the length ratio LR=1, where the maximum coefficient of heat transfer (HTC) was 1695W/m2.ºC. In addition, HTC gradually increased and decreased response to the fluctuated reactions of length ratio.

KEYWORDS: Heat Pipe, Thermosypon, Finned Condenser & Length Ratio

1. INTRODUCTION AND LITERATURE REVIEW Conventional heat pipes or also known as two-phase closed heat pipe is a device that can extremely effective in heat transfer implementation, which is used in several fields in engineering. The heat pipe (HP) is a heat transfer device,

Original Article

Received: Jan 15, 2020; Accepted: Feb 05, 2020; Published: Mar 09, 2020; Paper Id.: IJMPERDAPR202034

capable of high heat transfer with small space and small temperature difference over the large distance to reduce the moisture in the system. The HP is a sealed container, consists of three parts which are evaporator, adiabatic and condenser. These parts are normally ordered as following from the bottom to the top, evaporator-adiabaticcondenser. The adiabatic region is utilizing as a separator between the other two parts [1].

The operation begins at the section of evaporator as the liquid pool exists, which was active with a few quantity of operating fluid into the HP before preparing its vacuum condition. The working fluid was saturated through a heat source. The working fluid was turning into vapor, and the vapor flows from evaporator to the condenser zone via changing of phase. As a result, the vapor in the condenser section condensed to a liquid and returns to the evaporator due to its gravity. A schematic operation basis of the heat pipes is shown in Figure1.

Based on the high tool-use-efficiency, reliability and the low cost that established using this device, heat pipes has been used and widely applied. The applications include preservation of deforesting of the ice from the roads, turbine blade cooling, heat exchangers, humidity control, food industry, electronics cooling, chemical engineering, waste heat recovery, power generation, air conditioning systems, water heater, solar collector systems, and reactors [1, 2, 3, 4, 5, and 6].

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Figure 1: Schematic of Typical Heat Pipe, (Construction and Operation) Lin et al, (2002) developed the new capillary structures which were made of folded copper sheet fin for effective cooling of high heat flux electronics on the miniature heat pipes. The development includes two systems of capillary structure (fully and partially opened groove wick) with amounts of fill were tested in the horizontal orientation. The comparing results of the two systems showed that the partially opened groove wick is better than that with fully opened heat pipe. Song et al, (2008) investigated experimentally the driven mechanism in the evaporator of non-stepped rotating heat pipes at rotational speeds, that was in range 2000-4000 rpm, and heat flux rate >100KW/m2. The distribution of axial temperatures for the flow in the core zone was used to measure the thermal resistance of the evaporator and condenser.

Under these factors, the results showed that the heat transfer by natural convection happened in the liquid layer of the evaporator part. Chen et al, (2009) studied experimentally the heat transfer characteristics and fluid flow in a heat pipe with axial shaped grooves. The results of the study showed that the capillary radius in evaporator section increases slowly (non-linearly) along the axial direction, while it begins to increase intensely in the condenser section. Carbajal et al, (2006) investigated the temperature distribution across a flat HP sandwich structure was through experimental and computational studies. This study includes the application of the intense localized thermal flux for a propane torch on the evaporator section. Whereas, the natural convective and radiative heat transfer were cooling the condenser zone. The experimental data were found to be in good alignment with the computational results. AL-Saadi et al, (2007) performed an experimental study to measure the heat transfer performance of a heat pipe for residential heating with water as a working fluid. The performance of the heat pipe was investigated experimentally under a variety of operation conditions: different evaporator power levels, different water inventories, different angles of inclination. Noie (2005) studied experimentally the influence of three different factors considered in test: heat flow rates applied was in range 100-900Watt, fill charge ratios 30-90%, and three ratios (AR) which has relationship between the length of evaporator and its diameter, (AR=7.45, AR=9.8 and AR=11.8). The above parameters were studied to find the impact of them on steady state heat transfer performance of a vertical two phase closed heat pipe (without wick). The results showed that the maximum heat transfer takes place at different filling ratio for each aspect ratio. Ahmad (2009) study carried out experimentally to investigate the effect of the parameters which including heat power supply rates was in range 125 to 250Watt, the operating fluid types within many Impact Factor (JCC): 8.8746

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mixture ratios (water/acetone 30%, 50%, 70%, 85%, 100% water and 100% acetone) on the performance of the heat pipe (without wick). The study indicated that pure fluid is more effective than using the mixture.

In the literature, many investigations have been made to characterize the thermal performance of heat pipes to improve the features of the heat transfer [2, 3, 4, 7, 11 and 12]. There are a quite few research studies on effect of length ratio in heat pipes. However, studies on effect of length ratio are rare to find in literature. Therefore, it can be identified as a gap of research to achieve the best length of the evaporator /adiabatic to improve the performance of the heat pipes.

The objective of the present work paper is to investigate the impacts of variable evaporator /adiabatic length ratio on a heat pipe performance. Various experiments were conducted to achieve the best length ratio between the adiabatic and the evaporator lengths.

2. MATERIALS AND METHODS The aim of this research is to investigate the performance of a heat pipe which is shown in the schematic diagram of the experimental apparatus in Figure (2). The laboratory device was manufactured using copper tube sized of 15.5mm and 13.5mm for outside and inside diameters respectively, at a length of 600mm. The length of the condenser was 200mm, which was framed as a helical copper finned tube, where a finned heat sink on the condenser section can be air-cooled. The fin dimensions details were as following: Thickness =1mm, Inside diameter =15.5mm, Outside diameter =140mm and high =200mm. The details are more described in Figure (2) with all components of the instrument [13].

The rest of the length of the HP was utilized for the evaporator and adiabatic sections. Based on the length ratios (LR), the evaporator length was changed from 250 to 150mm to control the adiabatic section length. The evaporator lengths were varied by varying the length of the electrical coil heaters according to the heat source of the three clamps.

The electrical heaters can be used as one, two or three heaters in time to achieve the length of the evaporator that mention above (250mm, 150mm). The heaters were fixed tightly around the surface of the evaporator to ensure good contact with its outer surface. To prevent the heat losses, the whole length of the evaporator and adiabatic were insulated by asbestos in addition covered with glass wool having a thickness of 30mm as shown in Figure. The measurement of outside temperature that distributed along the same line on the pipe through ten thermo couples were inserted into the outer surface of HP within grooves that were machined. The power supplied was calculated using an accurate wattmeter which was connected in the heater circuit.

The steady-state behavior of the HP components was taken and included in the temperature profiles of the evaporator, adiabatic zone and the condenser respectively. For each component, the average value was computed. The behavior of the heat pipe was investigated experimentally in relation to length ratios of heat pipe.

The heat transfer coefficients in the evaporator and condenser were calculated by an Engineering Equation Solver program (EES) through temperatures data which were recorded during the experiments.

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Figure 2: Schematic of the Test Apparatus.

3. RESULTS AND DISCUSSIONS 3.1: Behavior of the Heat Pipe with Startup Operation The startup results indicated by thermo couples on the evaporator surface and the transient behavior of the heat pipewere are presented in Figure (3). These results were observed after the heaters were switched on, the temperature of the evaporator body was increased, and the slope of the temperature traces for these locations was significantly smaller compared with the other studied ratios (LR>1). The time taken to reach a state of stability was proportional to the length ratio. The response time was around 20-40 minutes, which it was stabled at this stage, during the results were measured.

The ratio (LR<1)was accelerated of the fluid's process has speed-up formation of steam bubbles of the operating fluid in the evaporator and leads to a faster heating surface temperature.The process caused an acceleration in the occupation of the

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vaporin the adiabatic zone.The working fluid to reach the steady-state inside the heat pipe at the time 20 minute. Moreover, the stability of the evaporator surface temperatures were used length ratio (LR=1.7 >1), at the 40 minute which it reported longer time compared with the time intervals approach to 20minute (Figure 3). The wall temperature shows a distinctive fluctuation along with the evaporator, which was enabled at low length ratios (LR<1). The menisci form more quickly begins to act a very fast response to the evaporator in order to improve the efficiently perform evaporation coefficient.

On the other hand, at high length ratios (LR>1), the menisci formation was a delayed generation, because it takes a long time to heat and also transfer slowly to the working fluid. Based on the transient time of the heat pipe presented in this section, all steady-state temperatures were recorded after steady state is assumed to be reached when successive readings were within 1ºC.

Figure 3: Evaporator Surface Temperature Varies with Time at the Startup Process for Different Length Ratio.

3.2: Effect of Length Ratio on Temperature Distribution Figure (4) shows the variation of wall temperature distribution along the heat pipe at all the conditions of study. It shows the transient behavior of a typical run for the evaporator, adiabatic and condenser, the temperature distribution with distance from the bottom end of the heat pipe. The temperature starts increasing gradually to reach a maximum value, and then it begins to decrease through the rest part of the evaporator and adiabatic section. The temperature decreases sharply at the condenser section and remains almost constant along with it due to cooling by convection. The temperature profile is the same for all length ratios (LR), but the values of temperature increase with an increase in the length ratio for the evaporator. The maximum temperature at the evaporator is (126˚C) while the minimum temperature (115˚C) at the same point when the length ratio is (1.7).

In current investigations, as indicated in a figure for length ratio, is 0.8 and 0.6, (LR<1) the temperature of the evaporator is almost isothermal with an average value variation were within 4ºC. Also, when the length ratio raises above (LR>1), the temperature increases at the middle of the evaporator and then decreases continuously with an increase in the distance from the bottom of the evaporator. The wall temperature of the condenser was approximately uniform around 1ºC.

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Figure 4: Wall Temperature Distribution Along Heat Pipe with Various Length Ratios.

3.3: Effect of Length Ratio on the Heat Transfer Coefficient (HTC) Figure (5) represents the behavior of length ratios on the heat transfer coefficient of the evaporator. The heat transfer coefficient responds to an inverse relationship with the length ratio. The reason for this response was due to increase the adiabatic length to ensure changing the working liquid to the vapor before being raised toward the condenser. The length ratios (0.6 and 0.8, respectively, LR<1) the heat transfer coefficient was obtained from 1667W/m2. ºC to 1678W/m2. ºC. As shown in this figure, the amount of difference between the ratios (LR≤1) was slight with a higher increase at (LR≥1).

On the other hand, the effect of the length ratios (LR>1) was inversely proportional to the heat transfer coefficients (HTC). In, the case of the LR=1.3 and 1.7, the HTC decreases sharply to a certain value, while the length ratio 1.7 was recorded to the minimum effect on the heat transfer coefficient (HTC) 1584W/m2.ºC and 1475W/m2.ºC, respectively. As a result, the effect of length ratios LR>1 on the heat transfer coefficient was clearly decreasing more, while it shows less than LR=1 has a slight effect on the thermal performance with HTC 1695W/m2.ºC. Finally, the good ratio was LR=1, this was enough length in evaporator/adiabatic sections. It may be due to the quick formation of menisci. It began to act at fast response to the heat load applied to the working fluid in order to change it into vapor to improve the efficiently perform the heat transport.

Figure 5: Length Ratios and Heat Transfer Coefficient (HTC).

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4. CONCLUSIONS As was found of the effect of length ratios on the thermal performance of the heat pipes, the conclusions are as follows: •

The temperature increased positively with an increase in the length ratio. The variation of the temperature has oscillated slightly along the evaporator surface.

•

The length ratio had an effect on the thermal performance of the heat pipes. The heat transfer coefficient (HTC) was increased with the decreased length ratio (LR<1).

•

It was found that the minimum thermal performance of heat pipe at the highest length ratio (LR>1) without denoted to the dry out limitation in the evaporator section.

•

The wall surface temperature nearly uniforms along the condensation section.

•

The startup time to reach a state of stability was proportional to the length ratio, for the ratio (LR>1) highest length was longer time than the lowest ratio (LR<1).

ACKNOWLEDGMENT The author is thankful for the University of Mosul/ faculty of Engineering for their contribution and support rendered to this work.

REFERENCES 1.

Reay, D. A., and Kew, P., "Heat Pipes Theory, Design and Applications,"5th Edition,Britain,(2006).

2.

P. Amatachaya, W. Srimuang," Comparative heat transfer characteristics of a flat two-phase closed thermosyphon (FTPCT) and a conventional two-phase closed thermosyphon (CTPCT)", International Communications in Heat and Mass Transfer 37 (2010) 293–298.

3.

H. Farsi, J.L. Joly, M. Miscevic, V. Platel, N. Mazet, An experimental and theoretical investigation of the transient behavior of a two-phase closed thermosyphon, Applied Thermal Engineering 23, (2003).

4.

Kumari, J. A., & Rao, P. C. (2017). Effect of Temperature on Soil Enzyme Urease Activity-productivity. International Journal of Humanities, Arts, Medicine and Science, 5, 65-72.

5.

B. Jiao, L.M. Qiu, X.B. Zhang, Y. Zhang, Investigation on the effect of filling ratio on the steady state heat transfer performance of a vertical two-phase closed thermosyphon, Applied Thermal Engineering 28, (2008).

6.

K.S. Ong, M.D. Haider, E. Alalhi, Experimental investigation on the hysteresis effect in vertical two-phase closed thermosyphons, Applied Thermal Engineering 19 (1999) 399–408.

7.

Chandran, S. R. (2018). Impact of Agro Machinery Service Centres on Labour Cost in Paddy Cultivation. IMPACT: International Journal of Research in Humanities, Arts and Literature, 6(01), 265-268.

8.

M. Zhang, Z. Liu, G. Ma, The experimental investigation on thermal performance of a flat two-phase thermosyphons, International Journal of Thermal Sciences 47(2008) 1195–1203.

9.

Lin, L., Ponnapan, R., and Leland, J.," Heat Performance Miniature Heat Pipe," Int. J. of Heat and Mass Transfer 45, (2002).

10. Song, F., Ewing, D., and Ching, C.Y.," Heat Transfer in the Evaporator Section of Moderate-Speed Rotating Heat Pipe", Int. J. of Heat and Mass Transfer 51, (2008).

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Raqeeb Hummadi Rajab 11. Kalbande, S. R., Deshmukh, S., & Khambalkar, V. P. (2017). Evaluation study of solar water desalination system for saline track area of Vidharba region. Int. J. Appl. Nat. Sci.(IJANS), 6(1), 55-64.

12. Chen, Y., Zhange, C. , Shi, M., Wu, J., and Peterson, G. P.,

" Study on Flow and Heat Transfer Characteristics of Heat Pipe

With Axial" â&#x201E;Ś " Shaped Microgrooves," Int. J. of Heat and Mass Transfer 52, (2009).

13. Carbajal, G., Sobhan, C. B., Peterson, G. P., Queheillat, D.T., and Wadley, H.N.G.," Thermal Response of a Flat Heat Pipe Sandwich Structure to a Localized Heat Flux", Int. J. of Heat and Mass Transfer 49, (2006).

14. AL-saadi Z., Mikdam M. saleh, "Design And Implementing A Heat Pipe Experimental System For Residential Heating. ," Journal of Engineering 27, (2007).

15. Judal, A., & Bhadania, A. Performance Assessment of a Solar Hot Water Generating System with Heat Pipe Evacuated Tube Solar Collectors.

16. S.H. Noie, Heat transfer characteristics of a two-phase closed thremosyphon, Applied Thermal Engineering 25, (4), (2005).

17. Ahmad, H., Hammed, "Heat transfer characteristics in a heat pipe using water-hydrocarbon mixture as a working fluid (An Experimental Study)", Al-Rafidain Engineering Vol.20 No. 3 June (2012).

AUTHOR PROFILE

Mr. Raqeeb Hummadi Rajab, Academic Lecturer (Senior) working in the Department of Agricultural Machines and Equipment at University of Mosul, IRAQ, since 2005. He was qualified in his MSc (Heat transfer, Thermal power) in the Department of Mechanical Engineering at University of Mosul, IRAQ, in the year 2009. He did his BSc (General Mechanics) in the same University in the year 1998. He has been lecturing undergraduate (Fluid Mechanics, Thermodynamics, Mathematics, Internal Combustion Engines) and supervising many undergraduates in the department.

He has published many articles within Iraq, about six articles in various journals. In addition, he has been participating and attending many conferences. He is a member of academic staff, department council member of Agricultural Machines & Equipment at Mosul University- College of Agriculture and Forestry and Iraqi engineers association. University address: Almajmoa, Mosul, 41002, Nineveh - IRAQ.

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