Effect of charging ratio on thermal performance of a miniaturized two-phase super-heat-spreader

International Journal of Heat and Mass Transfer 104 (2017) 489–492

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Technical Note

Effect of charging ratio on thermal performance of a miniaturized twophase super-heat-spreader Lucang Lv, Ji Li ⇑ College of Engineering Science, University of Chinese Academy of Sciences, 19A Yu-quan-lu Road, Shijingshan District, Beijing 100049, China

a r t i c l e

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Article history: Received 28 May 2016 Received in revised form 26 August 2016 Accepted 26 August 2016

Keywords: Flat heat pipe Charging ratio Temperature difference Gravity force Electronics cooling

a b s t r a c t The effects of charging ratio on the thermal performance of a thin novel flat heat pipe (FHP) heat spreader under natural air convection were investigated systematically. Temperature difference and temperature distribution along the central axis of the flat heat pipe were measured carefully. Results show that there exists an optimal charging range for the FHP and a very small temperature difference was achieved, demonstrating superior heat spreading capability. Furthermore, a temperature inverse increase was observed in the condensation section. In addition, within the range of optimal charging ratios the flat heat pipe was insensitive to the gravity force. All the results indicate this novel type of two-phase heat spreader is very suitable for mobile electronics cooling in view of its high-performance as well as its easy processing. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction With the fast development of integrated circuit chips, the performance of mobile electronics has been improved with reduced size, e.g., portable LED lightings, portable projectors, smartphones and tablet computers. Due to the miniaturization of electronics, the generated high heat flux may lower the operational stability, shorten the life cycle and even cause burnout if heat dissipation is not efficient [1,2]. Therefore, novel smaller cooling devices insensitive to gravity force are in demand urgently. Flat heat pipes, or vapor chambers, are widely used in thermal management systems for electronic components such as laptops and LEDs [3,4]. Nowadays, in view of their possible application in small mobile electronics, thin or ultra-thin flat plate heat pipes start to be a hot topic research [5]. In this area, several mathematical models were developed based on single grooved micro heat pipe [6–9] and some experimental investigations were carried out for different forms of flat heat pipes [10–15]. Charging amount of working fluid is of vital importance for a micro/mini flat heat pipe [2,3]. Generally, the liquid amount in a heat pipe is described by the charging ratio (U), defined as the volume share (Vl) that the working fluid takes of the total inner volume (Vtotal) of the heat pipe, as described in Eq. (1). Many works dealing with the effect of the charging ratio on the thermal effi-

⇑ Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.08.087 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.

ciency of a heat pipe have been published, but only few reports on systematic studies about micro/mini flat heat pipes exist.

U¼

Vl  100% V total

ð1Þ

In addition, investigations on the fill charge were usually conducted by cooling the heat pipe with circulating water or using heat sink. However, the use of additional cooling equipment (especially active cooling) is not appropriate for mobile electronic devices. In this work, the performance sensitivity to different charging ratios was analyzed carefully for a thin flat heat pipe with hybrid sintered powder wick structure consisting of a convex region for collecting condensate and a striped region for liquid and vapor flow. Considering the application for mobile electronics, the FHP was only cooled by the natural convection and the thermal radiation from its surface during the testing process. 2. Experiments The FHP mainly consisted of two plates both with sintered copper powder wick structure as shown in Fig. 1. The wick structure was 1 mm thick and the copper sheet was 0.5 mm thick, respectively. After bonding the two plates together, the total thickness of flat heat pipe was less than 2 mm, with an inner space height less than 1 mm. The detailed FHP structure and its fabrication process can be found in our early work [16]. After charging with deionized water, the thermal performance of the FHP was tested under natural convection condition as illustrated in Fig. 2. A DC

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Fig. 1. Principal dimensions of the two plates of the flat heat pipe (mm).

power supply provided heating power to a ceramic heater, whose active heating area was of 1 cm  1 cm. The data acquisition system mainly included a data acquisition unit (Agilent 34970A) and an infrared thermograph camera (Fluke Ti32, uncertainty of ±2 °C). Seven microscale T-type Omega thermocouples (uncertainty of ±0.5 °C) were firmly attached to the surface of the flat heat pipe, where the heater was placed. The thermocouple locations are shown in Fig. 2(b). An additional thermocouple (not shown) monitored the ambient temperature; all tests were conducted at an ambient temperature of 24 ± 1 °C. The flat heat pipe was exposed to the air and cooled by natural processes (i.e. convection and thermal radiation from its surface). Thus, to eliminate the discrepancy of the surface emissivity among different testing prototypes, electrical-insulating black tape was pasted on the both sides of all FHP prototypes as shown in Fig. 2 (a). The emissivity of the black tape measured by infrared camera was 0.95. Exposed to air, the heat losses of the heater by natural convection and thermal radiation were 1.9–2.3% varying the heating load from 1.62 W to 9.73 W. In addition, to evaluate the antigravity capability, the working performances of the flat heat pipes with different charging ratios were investigated under three operation modes as shown in Fig. 2(c).

Fig. 2. Schematic of testing system: (a) experimental setup; (b) positions of thermocouples on the FHP; (c) operation modes.

3. Results and discussion Temperature difference along the central axis, DT = T101  T106, was calculated. T101 is the mean value of T101_1 and T101_2. The uncertainty of DT can be determined by Eq. (2). For 37.6% fill charge and mode 1, the uncertainty was calculated in the range from 4.6% to 2.1% for heating input varying from 1.61 W to 9.73 W.

DðDTÞ ¼ DT

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi       DT 101 1 2 DT 101 2 2 DT 106 2 þ þ T 101 1 T 101 2 T 106

ð2Þ

From Fig. 3, an optimum charging range (30–40%) exists for this FHP under natural convection, in which the DT along the heat pipe is within 0.5 °C, indicating excellent heat spreading capability. When under-charged (<25%), the DT increases gradually with the heat load increasing. This is because that the working fluid is insufficient that partially dry-out occurs in the heater section, which increases T101 and thus DT. For over-charged state (>45%), both

the gravity and the heat load have important impact on the operation characteristics of the FHP. The relatively large DT is mainly caused by the excess water flooding the condenser side and lowering the T106. In the horizontal direction, the DT decreases gradually with increasing the heating input. The increasing heat load could promote the working fluid circulation inside a heat pipe, resulting in the decreasing DT. For mode 2, the gravity aids in the fluid circulation and excess water would gather in the heater section. After heating load is applied, the portion taken by the water near the heating section will reduce the space for two-phase flow, resulting in a temperature rise of T101. So the optimal charging range is little smaller than that for mode 1. For heating mode 3, the gravity impedes the fluid circulation and the flooded region is located far from the heating section, due to the specially designed wick structure in this work which leads the vapor to flow downstream along the wicked channels, the vapor cannot circulate effectively at the

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sensitive to the gravity force, and this condition is of vital significance for real application to the electronics cooling. The recommended charging range is from 26.7% to 37.6% from a compromise for all operation modes from the present study. More importantly, a temperature inverse increase was observed in the condensation section for some situations (e.g., under optimal charging ratios and comparatively high heating loads), which indicates that a negative temperature difference along the FHP exists. Possible reasons are discussed here: (1) the thermocouples 101_1 and 101_2 were fixed 5 mm far from the side of the heater as shown in Fig. 2(b), which means that the measured temperatures were lower than the real evaporator temperature. But, we suppose this is not the main reason since the temperature inverse increases along the axial length were also observed even at the surface opposite to the heating surface by the infrared camera, which will be

Fig. 3. Temperature difference along the central axial.

end of the striped wick region where the wick is flooded. Therefore, T106 and thus DT will drop quickly due to this flow blockage under overcharged state. Furthermore, there exists a wide range of charging ratios in which the temperature difference are quite small and not quite

Fig. 4. Infrared inspection of temperature distribution for the FHP positioned horizontally (mode 1) with a 37.6% charging ratio.

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presented in Fig. 4; (2) an uncertainty of the used thermocouple is ±0.5 °C, which will bring out some measurement errors. Unfortunately, it seems this is not the main reason yet. Even we changed the thermocouples randomly and carried out the measurements many times, the temperature data still exhibited the same trend; (3) a complicated thermodynamic mechanism is behind the observed phenomena. For the present thermal system, heat dissipation, just by natural convection and thermal radiation at the external surfaces, cannot take the heat away immediately, vapor could not condensate completely in the condensation section at first, inducing the interaction between the stagnant vapor and the incoming vapor steam. Therefore, a new quasi-equilibrium process with higher vapor temperature will be established to increase the heat transfer rate in the condensation section. To better demonstrate the temperature inverse increase along the flat heat pipe, temperature distribution of the surface opposite to the heating surface was detected by the infrared camera. Fig. 4 illustrates the detailed temperature distribution on the FHP surface from the infrared photographs for mode 1 and 37.6% charging ratio. It should be noted that an obvious temperature inverse increase was also observed even at the backside of the FHP when heat load was 5.15 W and 7.13 W. 4. Conclusions In this work, the effects of different charging ratios on the thermal performance of a miniaturized FHP heat spreader have been investigated experimentally under natural air convection and following conclusions can be drawn: (1) Firstly, there exist optimum charging ratios for the present novel flat heat pipe with hybrid wick structure, and the values were different for different testing conditions, but roughly a charging ratio between 27% and 37% is recommended through present study. (2) Secondly, the FHP performance was not sensitive to the gravity when the charging ratios varied from 27% to 37%, which is of vital significance for mobile electronics cooling.

(3) Lastly and more importantly, a temperature inverse increase was observed in the condensation section for charging ratios ranging from 26.7% to 48.1%, which results in negative temperature difference along the central axis.

Acknowledgement This work is supported by the National Natural Science Foundation of China (Project Nos. 51476161 and 51176202). References [1] P.H. Chen, S.W. Chang, K.F. Chiang, J. Li, High power electronic component: review, Recent Pat. Eng. 2 (2008) 174–188. [2] L.C. Lv, J. Li, Micro flat heat pipes for microelectronics cooling: review, Recent Pat. Mech. Eng. 6 (2013) 169–184. [3] G.P. Peterson, An Introduction to Heat Pipes, Wiley, New York, 1994. [4] D.A. Reay, P.A. Kew, R.J. McGlen, Heat Pipes, sixth ed., Butterworth-Heinemann, 2013. [5] X. Chen, H. Ye, X. Fan, T. Ren, G. Zhang, A review of small heat pipes for electronics, Appl. Therm. Eng. 96 (2016) 1–17. [6] A.B. Duncan, G.P. Peterson, Charging optimization for a triangular-shaped etched micro heat pipe, J. Thermophys. Heat Transfer 9 (1995) 365–367. [7] K.K. Tio, C.Y. Liu, K.C. Toh, Thermal analysis of micro heat pipes using a porousmedium model, Heat Mass Transfer 36 (2000) 21–28. [8] D. Sugumar, K.K. Tio, Thermal analysis of inclined micro heat pipes, Trans. ASME 128 (2006) 198–202. [9] B. Suman, On the fill charge and the sensitivity analysis of a V-shaped micro heat pipe, AIXhE J. 52 (2006) 3041–3054. [10] S. Launay, V. Sartre, M. Lallemand, Experimental study on silicon micro-heat pipe arrays, Appl. Therm. Eng. 24 (2004) 233–243. [11] M. Zhang, Z. Liu, G. Ma, The experimental and numerical investigation of a grooved vapor chamber, Appl. Therm. Eng. 29 (2009) 422–430. [12] S. Lips, F. Lefevre, J. Bonjour, Combined effects of the filling ratio and the vapour space thickness on the performance of a flat plate heat pipe, Int. J. Heat Mass Transfer 53 (2010) 694–702. [13] L.H. Chien, Y.C. Shih, An experimental study of mesh type flat heat pipes, J. Mech. 27 (2011) 167–176. [14] J.S. Chen, J.H. Chou, Cooling performance of flat heat pipes with different liquid filling ratios, Int. J. Heat Mass Transfer 77 (2014) 874–882. [15] K.V. Paiva, M.B.H. Mantelli, Wire-plate and sintered hybrid heat pipes: model and experiments, Int. J. Therm. Sci. 93 (2015) 36–51. [16] J. Li, L. Lv, Experimental studies on a novel thin flat heat pipe heat spreader, Appl. Therm. Eng. 93 (2016) 139–146.