Effect of Inclination Angle on the Thermal Performance of Closed Loop Pulsating Heat Pipe

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 03 | August 2016 ISSN (online): 2349-6010

Effect of Inclination Angle on the Thermal Performance of Closed Loop Pulsating Heat Pipe Pramod R. Pachghare Assistant Professor Department of Mechanical Engineering Government College of Engineering, Amravati-444 604, India

Abstract This paper presents an experimental study on the effect of inclination angle effect on the thermal performance of closed loop pulsating heat pipe (CLPHP), which consists of 10 turns of copper tubes having inner and outer diameter 2.0 mm and 3.6 mm respectively. The equal lengths of 50 mm were maintained for evaporator, condenser and adiabatic sections. Different working fluids are used as R-134a, Methanol and Water; at optimum filling ratio of 50% by volume. The thermal performance is scrutinized at various inclinations (viz. 0o, 20o, 40o, 60o and 90o) for different heat input. It is concluded that, the thermal performance up to 25W heat input, is more sensitive to the inclination angle; whereas above 25W heat input, less independent as pressure force dominates the inclination angle force. In comparison, vertical bottom heat mode position gives best thermal performance due to presence of inclination angle.

Keywords: pulsating heat pipe; heat input; inclination angle; inclination angle _______________________________________________________________________________________________________ I.

INTRODUCTION

A new member of wickless heat pipe known as the pulsating heat pipe (PHP) was proposed and patented by Akachi et al. [1], in 1990s. Its operation is based on the principle of oscillation for the working fluid and a phase change phenomena in a capillary tube. The diameter of the tube must be small enough such that liquid and vapor plugs exist. A CLPHP consist of meandering capillary tubes having no internal wick structure, as shown in Fig.1, which has been evacuated, and partially filled with the working fluid. When one end of the capillary tube is heated (the evaporator), the working fluid evaporates and increase the vapour pressure, thus causing the bubbles in the evaporator zone to grow. This pushes the liquid towards the low-temperature end (the condenser). Cooling of the condenser results in a reduction of vapour pressure and condensation of bubbles in that section of the heat pipe. The growth and collapse of bubbles in the evaporator and condenser sections, respectively, results in an oscillating motion within the tube. Heat is transferred through latent heat in the vapour and sensible heat by the liquid slugs [2].

As a passive heat transfer device, PHP has potential applications in electronic cooling because it may dissipate high heat fluxes created by the next generation electronics.

Zhang and Faghri [3] presented a review on the effects of various parameters such as working fluids, charge ratios, inclination angles, etc. on the thermal performance. Shafii et al. [5; 6] concluded that the inclination angle force has an insignificant effect on the PHP performance. Khandekar and Groll [7] demonstrate a series of experiments and concluded that, in 2mm ID CLPHP, the thermal performance (i.e., overall thermal resistance) is strongly dependent on the flow pattern existing inside the tubes.

Inclination angle does affect the thermal performance, at least for systems with low number of turns. Yang et al. [8] obtained the best thermal performance and maximum performance limitation when they operate in the vertical bottom heat mode CLPHP with 50% filling ratio. As the inner diameter decreases, performance differences due to the different heat modes (i.e. the effect of inclination angle) become relatively small and even insignificant, as can be seen for the CLPHP with 1 mm ID tubes. Also pointed that [9], PHP spreaders could operate well in all orientations; the inclination angle vector does augment the driving forces, improving performance in the bottom heat orientation as compared to other orientations. More recently, Mameli et al.

[10] demonstrated that, the device with 2mm ID is still sensitive to the effect of inclination angle and the bottom heat mode is more efficient then the horizontal mode. Test with nine turns are still sensitive to the effect of inclination angle and the bottom heat mode is more efficient then the horizontal mode as confirmed by experimental evidence.

One of the aims of good CLPHP design is to make the thermal performance, as far as possible, independent of the operating orientation. At a first glance, two physical phenomena affect the CLPHP performance with respect to orientation. The first is of course, the effect of inclination angle on slug flow and the second is the effect of total number of meandering turns on the level of internal temporal and spatial dynamic pressure perturbations. In addition to these two, the heat input is also a strong parameter, which affects dynamic instability, especially in density wave oscillations, and is therefore believed to affect the thermal performance of CLPHPs with respect to orientation [11, 12]. This aspect remains to be further explored and will not be highlighted in this paper. Also various experimental works on the pulsating heat pipe has been explored by Pachghare et al. [1723]. The objective of this work is to study the effect of inclination angle for different working fluids on the thermal performance of CLPHP.

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Effect of Inclination Angle on the Thermal Performance of Closed Loop Pulsating Heat Pipe (IJIRST/ Volume 3 / Issue 03/ 012)

II. EXPERIMENTAL SET-UP The experimental setup of CLPHP was developed and tested in the laboratory in controlled conditions. The photograph of CLPHP is shown in Fig. 1. The setup consists of a CLPHP, temperature recorder, power supply unit, and water tank cooling system for condenser. The overall dimension of the CLPHP is 300mm x 170mm has 10 turns, and the inner radius of each turn is 5.7mm. The pitch distance between tubes was 15mm. The equal lengths of evaporator, adiabatic and condenser section are maintained 50 mm. The heat load is applied by two heating coil of 500W each inserted on either sides in the oil bath. The heat is supplied through an oil bath using dielectric oil to the outer wall surface of the tube, and it is dissipated from the condenser by cooling water with a constant inlet temperature of 20 oC. The input heat load is measured by a power meter with an accuracy of 0.2%. Both the evaporator and adiabatic section are well thermally insulated by spray foam insulations. According to the analysis of heat balance between the evaporator and the condenser, the heat loss from the evaporator and adiabatic section to the ambience is varied from 5 to 10%, depending on the heat load. The inclination angle can be adjusted from horizontal plane (θ = 0o) to vertical plane (θ = 90o). The temperature of the CLPHP is measured by thermocouples (diameter of 0.125 mm, OMEGA ‘K’ type with ±0.1oC) and recorded by the data-collecting instrument. Ten thermocouples are used in the CLPHP located symmetrically (T1~T3 for evaporator, T4~T5 for adiabatic section, T6~T8 for condenser, T9~T10 for inlet and outlet temperatures of cooling water) as depicted in Fig. 2. Due to the high thermal conductivity and small wall thickness (0.8mm) of the copper tube, the temperature difference between the inner and outer walls of tube is negligible. Therefore, all the thermocouples are attached on the outer walls of the tube. The heating configuration was bottom heat mode (+90 o). Flow meter is used to record the mass flow rate of the cooling water. The detail of arrangement was made for inclination angles and positions, shown in Fig. 3.

Fig. 1: (a) photograph; (b) schematic of experimental set-up; (c) arrangement and positions of CLPHP .

About the tube design, the most important condition of PHP is the creation of the liquid slug. According to the literature review, Akachi et al. [13] proposed that the appearance and movement of bubbles are affected by surface tension and buoyancy in the channel. Relation of surface tension and buoyancy could be explained by the dimensionless formula shown in Eq. (1): gD2 (ρliq − ρvap ) Eö = (1) σ When Eö ≈ 4, the bubble will get seized on both side of the wall. At this condition, the terminal velocity becomes zero and the liquid slug flow is formed. Therefore, the inner diameter must be small enough so that the working fluid will distribute itself inside the tube length and form the liquid slugs and vapor plugs due to the effect of surface tension. The theoretical maximum inner diameter Dmax for a CLPHP is suggested as [13]. The dimension formula for the inner diameter ‘D’ is given in Eq. (2). σ D < Dmax ≤ 2 √ (2) (ρliq − ρvap )g In order to study the influence of thermophysical properties on the thermal performance of CLPHP, three working fluids such as water, methanol and R-134a are used here. The heat input was selected from 5 to 50W, according to operating temperature limits for different working fluids. The PHP was fabricated by bending a copper capillary tube with the inner diameter of 2 mm and outer diameter of 3.6 mm. The effect of orientation on the thermal performance was studied. The experiment was started by switching on the coolant circulation system and setting the flow rate. Heating at the evaporator section was produced by turning on the power supply. Heat was added to the evaporator section in stages and removed at the condenser section by cooling water.

Each time the heat input was changed, time was given for the system to reach a steady state. Then the temperature, flow rate and input power data was recorded.

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Effect of Inclination Angle on the Thermal Performance of Closed Loop Pulsating Heat Pipe (IJIRST/ Volume 3 / Issue 03/ 012)

III. RESULT AND DISCUSSION The CLPHP performance data for all working fluids at different orientation were obtained according to the following procedure: Heat input was stepwise increased until a quasi thermal equilibrium was established. Then, the spatial temperatures and heat input were recorded, so the thermal resistances could be determined [14]. The thermal resistance is defined by Eq. (3), as: ̅e − T ̅c ) (T R th = (3) Q̇ ̅e and T ̅c are the average of three evaporator and condenser surface temperatures, calculated by using Eq. (4), as: where, T ̅ ̅i,2 + T ̅i,3 ) (T + T ̅i = i,1 T where, i = e or c (4) 3 Q̇ is heat input power to the PHP. After considering thermal losses, Q̇ can be determined by Eq. (5), as: Q̇ = P − Q̇ loss (5) where, P is the input electrical power (measured with accuracy of ±1.5%). Q̇ loss is the heat loss, which was verified to be about 5 to 10%, depending on the heat load. In order to keep a good operation of PHP, 50% filling ratio was chosen for comparison with different working fluid at various inclination angle in the following test.

Effect of inclination angle on thermal performance of CLPHP The various graphs are plotted, to study the effect of inclination angle on the thermal resistance with heat input at various inclination angles, see Figs. 5-8. For all the experimentations, data was recorded at all inclination angles for different working fluids i.e. R134a, methanol and water. All the CLPHPs are used inline designs in which the tube sections are in one plane. When such a design is made horizontal, the inclination angle vector is nonexistent on all sections of the tube simultaneously. The CLPHP performance was affected by inclination angle on the system with respect to orientation. In the present study, the effect of inclination angle is scrutinized. For that, the CLPHP was tested from horizontal to vertical (BHM) position, at low heat input (upto 50W). `In Fig. 5, at all inclination angles of R134a PHP, the thermal resistance are decreases suddenly upto 25W and thereafter gradually decreases and then steady. As heat input is increase, the evaporator temperature rise resulting in a greater density gradient in the tubes. Simultaneously the liquid viscosity also drops diminishing the wall friction. Therefore, the thermal performance of PHP is very sensitive with the inclination angle. The difference between the thermal resistances of PHP for different inclination angle is minimizing over the heat input, upto 25W. Thereafter, no considerable effect of inclination angle is recorded. At all heat input to the device, the thermal resistance is decreases from horizontal (0 o) to vertical (90o) position of CLPHP, which shows that there is a significant role of inclination angle. The difference in the thermal resistance at various heat inputs for different inclination angle is also reducing consistently. Therefore, the device is more sensitive to inclination angle.

In Fig. 6, the mixed trends of thermal resistance of methanol CLPHP is shown at various inclination angles. During starting upto 40W heat input, no significant effect of inclination angle on the thermal performance of PHP is recorded. The behavior o f thermal resistance at low heat input is chaotic. Above 40W heat input, the thermal performance shows considerable effect of inclination angle. The thermal resistance is increases from horizontal (0 o) to vertical (90o) position. Whereas in Fig. 7, water is used as working fluid, shows negligible effect of inclination angle on the performance of CLPHP. At all inclination angles, the thermal resistance is randomly behaved over heat input, because the circulation is not initiated at low heat input. From 20 to 35W heat input, slight variations are observed for different orientations.

Fig. 5: Variation of thermal resistance for R-134a.

Fig. 6: Variation of thermal resistance for Methanol.

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Effect of Inclination Angle on the Thermal Performance of Closed Loop Pulsating Heat Pipe (IJIRST/ Volume 3 / Issue 03/ 012)

Fig. 7: Variation of thermal resistance for Water.

Fig. 8: Variation of thermal resistance for all working fluid.

In Fig. 8, variations of thermal resistance at various inclinations and different working fluids are plotted. The thermal resistances are increases in the sequence of R134a, methanol and water, which is according to the boiling point and latent heat of vaporizations. During starting initially upto 25W, the variations of thermal resistance is chaotic. In all working fluid, the best thermal performance is given by R134a, particularly for low heat input range. Also vertical position of CLPHP shows the better thermal performance.

IV. CONCLUSIONS The experimental set-up for the CLPHP was developed, to analyze the effect of inclination angle. Various researchers are demonstrated the effect of inclination angle on the thermal performance. But still this issue is contemporary unsolved. The conclusions from present work are summarized as follows: 1) As the heat input increases the thermal resistances are decreases smoothly, in all working fluids. In comparison, R134a working fluid PHP is shown better thermal performance than methanol and water, at all orientation. There is a significant role of inclination angle effect on the thermal performance of R134a PHP. In contrast, water PHP shows negligible inclination angle effect, whereas in methanol, considerable effect is observed. Therefore, effect of inclination angle is depending on the properties of working fluid used in CLPHP. 2) At low heat input i.e. upto 25W, the thermal resistance decreases rapidly and the PHP performance is more sensitive to the inclination angle whereas high heat input i.e. above 25W, the thermal resistance decreases smoothly and less independent to the inclination angle.

3) The thermal performance is considerably improved from horizontal to vertical position. Better thermal performance is shown in vertical position, due to presence of inclination angle.

4) From present research, it is suggested that R-134a PHP has given the better thermal performance for particularly low heat application devices. The operating range of CLPHP is from 60 o to 90o, but maximum thermal performance is observed in vertical position.

Nomenclature        

CP Specific heat (kJ/kgoC) D Diameter (m) Eö Eötvös number = (Bo)2 P Electrical input power (W) Q̇ Heat input (W) Rth Thermal resistance (oC/W) ̅ Average temperature (oC) T hfg Latent heat of vaporization (kJ/kg)

Subscripts a b c e liq sat vap

adiabatic section boiling condenser section evaporator section liquid saturation vapor

Greek symbols    

ρ μ υ σ

Density (kg/m3) Dynamic viscosity (Ns/m2) Kinematic viscosity (Pa.s) Surface tension (N/m)

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Effect of Inclination Angle on the Thermal Performance of Closed Loop Pulsating Heat Pipe (IJIRST/ Volume 3 / Issue 03/ 012)

ACKNOWLEDGEMENTS The research has been supported by Government College of Engineering, Amravati, (M.S.) India under Technical Education Quality Improvement Program (TEQIP) of Government of India. The authors express their sincere appreciation for all of the support provided.

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